Self-healing concrete-What Is it Good For?

1. INTRODUCTION

Reinforced concrete structures are the most used system in buildings and infrastructure constructions. Concrete is an affordable material, easy to produce, which allows variable consistency for application (from dry to self-compacting) and can take different forms and strengths. According to the CEB-FIB Code Model 2010, as well as other Codes, the specified design service life of a structure is derived from the requirements given by the stakeholders and the technical implications for structural analysis, maintenance, and quality management. Service life is usually decided depending on the importance of a structure. For common structures, the most usual design service life is 50 years, while in more complex structures design service life is usually increased to more than 100 years. Because of that, materials and construction systems need enough performance to endure these long lifespans, but they also need precise evaluations and knowledge of their long-term mechanical and durability performances, in order to plan realistic maintenance interventions.

In many reinforced concrete structures, cracks are frequent, and they can be acceptable in the structural design as a result of actions considered in the design. In general, concrete elements will suffer combined axial, shear, and bending stresses. These elements will be designed in a way that the compression stresses will be endured by the concrete matrix and the tensile stresses by the reinforcement (Figure 1). The size of acceptable cracks is generally controlled through the material properties, cover, and section design, but mostly through the reinforcement content.

In concrete codes, allowed crack width will depend on the exposure classes, which are chosen depending on the aggressivity of the environment in terms of the risks of corrosion, carbonation, freeze-thaw, erosion, or chemical attacks. In the case of reinforced concrete, values of 0.3 mm of allowed crack width are frequent. In the case of prestressed concrete, cracks of 0.2 mm can be accepted in less aggressive environments. In contrast, no-decompression (and thus, no cracks) is the requirement for the elements under more aggressive conditions. These limits are considered to guaranty that, in the expected service conditions, the structure can maintain its service requirements.

Figure 1. General diagram of a reinforced concrete element under bending stresses and some aggressive agents.

However, it is true that even if the structural conditions are not significantly affected by the crack opening limits allowed for each aggressive class, the durability of concrete and reinforcement can be affected by the mobility of fluids through the open surface cracks. Gaseous materials such as CO₂, water, and acid vapour can be transported even in cracks of up to a few tenths of micrometres (11. Damgaard Jensen, A.; Chatterji, S. (1996) State of the art report on micro-cracking and lifetime of concrete - Part 1. Mater. Struct. 29 [1], 3-8. https://doi.org/10.1007/BF02486001. ). Thus, liquids and gasses with aggressive substances can lead to partial deterioration of concrete and corrosion of the reinforcement, affecting the durability and service life of the structure (Figure 1). If cracking overpasses certain limits, either because the design evaluation was wrong or because the expected service conditions were exceeded, deterioration in specific structures could imply high costs for inspection, monitoring, maintenance, and repair.

Self-healing of concrete can be defined as the process in which the material regenerates itself repairing its own cracks, similarly to what happens in some natural materials, such as bones or trees (22. Speck, T.; Bauer, G.; Flues, F.; Oelker, K.; Rampf, M.; Schüssele, A.C.; et al. (2013) Chapter 16. Bio-inspired self-healing materials. In: Peter, Fratzl.; John, W.C. Dunlop; Weinkamer, R.; (Eds). Materials Design Inspired by Nature: Function Through Inner Architecture. The Royal Society of Chemistry. 359-389. https://doi.org/10.1039/9781849737555-00359., 33. Speck, O.; Speck, T. (2019) An overview of bioinspired and biometric self-repairing materials. Biomimetics. 4 [1], 26. https://doi.org/10.3390/biomimetics4010026. ). Increasing the self-healing properties of the concrete can lead to mitigate the potential decrease in durability produced by cracking. The immediate objective of self-healing concrete is promoting a partial or total recovery of their physical, mechanical and/or durability properties. Its final objectives are to increase service life or to be able to design more competitive structures. With that purpose, the extent of the properties recovered needs to be quantified accurately as well as the implications in their long-term performance and service life.

Every year, new advances are being researched in order to obtain new ways of producing self-healing in concrete, as well as new methodologies to quantify those improvements produced, in which properties, and under which conditions. Up to now, hundreds of articles have been published related to the self-healing capacity of different types of cement-based materials, including several reviews (4-104. Van Tittelboom, K.; De Belie, N. (2013) Self-healing in cementitious materials-A review. Mater. 6 [6], 2182-2217. https://doi.org/10.3390/ma6062182.
5. Hearn, N. (1998) Self-sealing, autogenous healing and continued hydration: What is the difference? Mater. Struct. 31 [8], 563-567. https://doi.org/10.1007/BF02481539.
6. Souradeep, G.; Kua, H.W. (2016) Encapsulation technology and techniques in self-healing concrete. J. Mater. Civ. Eng. 28 [12], 1-15. https://doi.org/10.1061/(ASCE)MT.1943-5533.0001687.
7. Ferrara, L.; Van Mullem, T.; Alonso, M.C.; Antonaci, P.; Borg, R.P.; Cuenca, E.; et al. (2018) Experimental characterization of the self-healing capacity of cement based materials and its effects on the material performance: A state of the art report by COST Action SARCOS WG2. Constr. Build. Mater. 167, 115-142. https://doi.org/10.1016/j.conbuildmat.2018.01.143.
8. Sánchez, M.; Faria, P.; Ferrara, L.; Horszczaruk, E.; Jonkers, H.M.; Kwiecień, A.; et al. (2018) External treatments for the preventive repair of existing constructions: A review. Constr. Build. Mater. 193, 435-452. https://doi.org/10.1016/j.conbuildmat.2018.10.173.
9. De Belie, N.; Gruyaert, E.; Al-Tabbaa, A.; Antonaci, P.; Baera, C.; Bajare, D.; et al. (2018) A review of self-healing concrete for damage management of structures. Adv. Mater. Inter. 5 [17], 1800074. https://doi.org/10.1002/admi.201800074.
10. Vijay, K.; Murmu, M.; Deo, S.V. (2017) Bacteria based self healing concrete - A review. Constr. Build. Mater. 152, 1008-1014. https://doi.org/10.1016/j.conbuildmat.2017.07.040. ). Despite this high number of works published, most of the publications follow a descriptive approach of the results achieved in the different papers, not putting the focus on the questions that still need to be discussed.

This review presents a critical examination of the advances performed in the field of self-healing concrete. The document describes briefly the main processes involved in autogenous and autonomous agents and the methods used for its evaluation to provide a general framework to the reader. Additionally, large-scale tests and the evaluation of long-term improvements have also been reviewed, which are topics of very recent development that are critical to understanding the feasibility of the self-healing systems. Afterwards, the authors critically discuss the levels of efficiency obtained, pointing out some doubts, weak points, and some of the challenges that need to be addressed in the next years.

2. BACKGROUND - BASIC CONCEPTS

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Concrete self-healing is the process in which the material regenerates itself repairing inner cracks by an intrinsic (autogenous) or an extrinsic (autonomous) process (77. Ferrara, L.; Van Mullem, T.; Alonso, M.C.; Antonaci, P.; Borg, R.P.; Cuenca, E.; et al. (2018) Experimental characterization of the self-healing capacity of cement based materials and its effects on the material performance: A state of the art report by COST Action SARCOS WG2. Constr. Build. Mater. 167, 115-142. https://doi.org/10.1016/j.conbuildmat.2018.01.143.). Thus, self-healing in concrete can be divided into (see also diagram in Figure 2):

Autogenous healing: does not require the use of specific agents added to the matrix and is produced by hydration of unhydrated cementitious material particles or by precipitation of calcium carbonate (CaCO₃) (44. Van Tittelboom, K.; De Belie, N. (2013) Self-healing in cementitious materials-A review. Mater. 6 [6], 2182-2217. https://doi.org/10.3390/ma6062182., 55. Hearn, N. (1998) Self-sealing, autogenous healing and continued hydration: What is the difference? Mater. Struct. 31 [8], 563-567. https://doi.org/10.1007/BF02481539., 99. De Belie, N.; Gruyaert, E.; Al-Tabbaa, A.; Antonaci, P.; Baera, C.; Bajare, D.; et al. (2018) A review of self-healing concrete for damage management of structures. Adv. Mater. Inter. 5 [17], 1800074. https://doi.org/10.1002/admi.201800074. , 1111. Edvardsen, C. (1999) Water permeability and autogenous healing of cracks in concrete. ACI Mater J. 96 [4], 448-454. https://doi.org/10.14359/645.);
Autonomous healing: produced by reactions of a specific agent intentionally added in the concrete mix design to produce self-healing. This agent can be added directly, embedded in capsules, or introduced through vascular networks. Some examples of the most used systems are superabsorbent polymers (12-1512. Snoeck, D.; Van den Heede, P.; Van Mullem, T.; De Belie, N. (2018) Water penetration through cracks in self-healing cementitious materials with superabsorbent polymers studied by neutron radiography. Cem. Concr. Res. 113, 86-98. https://doi.org/10.1016/j.cemconres.2018.07.002.
13. Sun, B.; Wu, H.; Song, W.; Li, Z.; Yu, J. (2019) Design methodology and mechanical properties of Superabsorbent Polymer (SAP) cement-based materials. Constr. Build. Mater. 204, 440-449. https://doi.org/10.1016/j.conbuildmat.2019.01.206.
14. He, Z.; Shen, A.; Guo, Y.; Lyu, Z.; Li, D.; Qin, X.; et al. (2019) Cement-based materials modified with superabsorbent polymers: A review. Constr. Build. Mater. 225, 569-590. https://doi.org/10.1016/j.conbuildmat.2019.07.139.
15. Mignon, A.; De Belie, N.; Dubruel, P.; Van Vlierberghe, S. (2019) Superabsorbent polymers: A review on the characteristics and applications of synthetic, polysaccharide-based, semi-synthetic and ‘smart’ derivatives. Eur. Polym. J. 117, 165-178. https://doi.org/10.1016/j.eurpolymj.2019.04.054. ), crystalline admixtures (16-1816. Ferrara, L.; Krelani, V.; Carsana, M. (2014) A “fracture testing” based approach to assess crack healing of concrete with and without crystalline admixtures. Constr. Build. Mater. 68, 535-551. https://doi.org/10.1016/j.conbuildmat.2014.07.008.
17. Roig-Flores, M.; Pirritano, F.; Serna, P.; Ferrara, L. (2016) Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests. Constr. Build. Mater. 114, 447-457. https://doi.org/10.1016/j.conbuildmat.2016.03.196.
18. Cuenca, E.; Tejedor, A.; Ferrara, L. (2018) A methodology to assess crack-sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles. Constr. Build. Mater. 179, 619-632. https://doi.org/10.1016/j.conbuildmat.2018.05.261. ), microencapsulated sodium silicate (19-2119. Kanellopoulos, A.; Giannaros, P.; Palmer, D.; Kerr, A.; Al-Tabbaa, A. (2017) Polymeric microcapsules with switchable mechanical properties for self-healing concrete: synthesis, characterisation and proof of concept. Smart. Mater. Struct. 26, 045025. https://doi.org/10.1088/1361-665X/aa516c.
20. Beglarigale, A.; Seki, Y.; Demir, N.Y.; Yazıcı, H. (2018) Sodium silicate/polyurethane microcapsules used for self-healing in cementitious materials: Monomer optimization, characterization, and fracture behavior. Constr. Build. Mater. 162, 57-64. https://doi.org/10.1016/j.conbuildmat.2017.11.164.
21. Al-Tabbaa, A.; Litina, C.; Giannaros, P.; Kanellopoulos, A.; Souza, L. (2019) First UK field application and performance of microcapsule-based self-healing concrete. Constr. Build. Mater. 208, 669-685. https://doi.org/10.1016/j.conbuildmat.2019.02.178. ), tubes with adhesives (22-2422. Dry, C.M. (2000) Three designs for the internal release of sealants, adhesives, and waterproofing chemicals into concrete to reduce permeability. Cem. Concr. Res. 30 [12], 1969-1977. https://doi.org/10.1016/S0008-8846(00)00415-4.
23. Van Tittelboom, K.; Adesanya, K.; Dubruel, P.; Van Puyvelde, P.; De Belie, N. (2011) Methyl methacrylate as a healing agent for self-healing cementitious materials. Smart Mater. Struct. 20, 125016. http://doi.org/10.1088/0964-1726/20/12/125016.
24. Gilabert, F.A.; Van Tittelboom, K.; Van Stappen, J.; Cnudde, V.; De Belie, N.; Van Paepegem, W. (2017) Integral procedure to assess crack filling and mechanical contribution of polymer-based healing agent in encapsulation-based self-healing concrete. Cem. Concr. Compos. 77, 68-80. https://doi.org/10.1016/j.cemconcomp.2016.12.001. ), and bacteria (1010. Vijay, K.; Murmu, M.; Deo, S.V. (2017) Bacteria based self healing concrete - A review. Constr. Build. Mater. 152, 1008-1014. https://doi.org/10.1016/j.conbuildmat.2017.07.040., 25-2825. Wiktor, V.; Jonkers, H.M. (2011) Quantification of crack-healing in novel bacteria-based self-healing concrete. Cem. Concr. Compos. 33 [7], 763-770. https://doi.org/10.1016/j.cemconcomp.2011.03.012.
26. Palin, D.; Wiktor, V.; Jonkers, H.M. (2016) A bacteria-based bead for possible self-healing marine concrete applications. Smart. Mater. Struct. 25, 084008. https://doi.org/10.1088/0964-1726/25/8/084008.
27. Pawar, S.S.; Parekar, P.S.R. (2018) Bacteria based Self-Healing Concrete : Review. Inter. Res. J. Engi. Tech. 5 [3], 1001-1004.
28. Li, L.; Zheng, Q.; Li, Z.; Ashour, A.; Han, B. (2019) Bacterial technology-enabled cementitious composites: A review. Compos. Struct. 225, 111170. https://doi.org/10.1016/j.compstruct.2019.111170. ).

Figure 2. Self-healing concrete mechanisms and potential extent of healing of some of the most used self-healing systems.

2.1. Autogenous healing

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Autogenous healing is a natural process in concrete. The most important mechanisms involved are carbonation of calcium hydroxide (portlandite) and continuing hydration of partially unhydrated cement grains (2929. De Rooij, M.R.; Schlangen, E.; Joseph, C. (2013) Introduction. In: de Rooij M., Van Tittelboom K., De Belie N., Schlangen E. (eds) Self-healing phenomena in cement-based materials. RILEM State-of-the-Art Reports, vol 11. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-6624-2_1.). In general, high humidity environments are not able to produce effective autogenous healing, and direct contact with water is required (99. De Belie, N.; Gruyaert, E.; Al-Tabbaa, A.; Antonaci, P.; Baera, C.; Bajare, D.; et al. (2018) A review of self-healing concrete for damage management of structures. Adv. Mater. Inter. 5 [17], 1800074. https://doi.org/10.1002/admi.201800074. , 30-3230. Snoeck, D.; De Belie, N. (2012) Mechanical and self-healing properties of cementitious composites reinforced with flax and cottonised flax, and compared with polyvinyl alcohol fibres. Biosyst. Eng. 111 [4], 325-335. https://doi.org/10.1016/j.biosystemseng.2011.12.005.
31. Snoeck, D.; De Belie, N. (2016) Repeated autogenous healing in strain-hardening cementitious composites by using superabsorbent polymers. J. Mater. Civ. Eng. 28 [1], 1-11. https://doi.org/10.1061/(ASCE)MT.1943-5533.0001360.
32. Roig-Flores, M.; Serna, P. (2020) Concrete early-age crack closing by autogenous healing. Sustain. 12 [11], 4476. https://doi.org/10.3390/su12114476. ).

In the case of carbonation, calcium carbonate crystals (CaCO₃) precipitate in the crack surfaces due to the chemical reactions between the Ca²⁺ (present in the hydrated concrete matrix) and the CO₂ available in the water of the crack. The precipitation of CaCO₃ is able to reduce water permeability in cracked concrete elements, with higher healing speed during the first 3-5 days of healing in water immersion (1111. Edvardsen, C. (1999) Water permeability and autogenous healing of cracks in concrete. ACI Mater J. 96 [4], 448-454. https://doi.org/10.14359/645.). However, it has to be noted that some studies (3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005.) reported that the highest concentration of CaCO₃ precipitation occurred only at the crack surface.

One factor of significant influence on self-healing is the size of the crack. Smaller cracks have better chances of healing due to the smaller volume that needs to be filled. The maximum crack width reported achieving complete closing through autogenous healing range from up to 0.06 mm (3434. Wang, X.F.; Yang, Z.H.; Fang, C.; Han, N.X.; Zhu, G.M.; Tang, J.N.; et al. (2019) Evaluation of the mechanical performance recovery of self-healing cementitious materials - its methods and future development: A review. Constr. Build Mater. 212, 400-421. https://doi.org/10.1016/j.conbuildmat.2019.03.117.) or up to 0.20 mm (1010. Vijay, K.; Murmu, M.; Deo, S.V. (2017) Bacteria based self healing concrete - A review. Constr. Build. Mater. 152, 1008-1014. https://doi.org/10.1016/j.conbuildmat.2017.07.040., 3232. Roig-Flores, M.; Serna, P. (2020) Concrete early-age crack closing by autogenous healing. Sustain. 12 [11], 4476. https://doi.org/10.3390/su12114476., 3535. Sangadji, S. (2017) Can self-healing mechanism helps concrete structures sustainable? Procedia Eng. 171, 238-249. https://doi.org/10.1016/j.proeng.2017.01.331.) or 0.30 mm after healing during one year (3636. Suleiman, A.R.; Nehdi, M.L. (2018) Effect of environmental exposure on autogenous self-healing of cracked cement-based materials. Cem. Concr. Res. 111, 197-208. https://doi.org/10.1016/j.cemconres.2018.05.009.). Under immersion in CO₂-water for 90 days, autogenous healing could heal cracks up to 0.45 mm (3737. Yıldırım, G.; Khiavi, A.H.; Yeşilmen, S.; Şahmaran M. (2018) Self-healing performance of aged cementitious composites. Cem. Concr. Compos. 87, 172-186. https://doi.org/10.1016/j.cemconcomp.2018.01.004.).

Restricting crack width can be used as a method to improve the self-healing of concrete. Two main methods have been analysed in order to improve self-healing: the use of fibres (3838. Desmettre, C.; Charron, J.P. (2012) Water permeability of reinforced concrete with and without fiber subjected to static and constant tensile loading. Cem Concr Res. 42 [7], 945-952. https://doi.org/10.1016/j.cemconres.2012.03.014.) and the application of compression stresses. The type of fibre can also influence the formation of precipitates inside the crack (3939. Nishiwaki, T.; Kwon, S.; Homma, D.; Yamada, M.; Mihashi, H. (2014) Self-healing capability of fiber-reinforced cementitious composites for recovery of watertightness and mechanical properties. Mater. 7 [3], 2141-2154. https://doi.org/10.3390/ma7032141.), and crack control (4040. Plagué, T.; Desmettre, C.; Charron, J.P. (2017) Influence of fiber type and fiber orientation on cracking and permeability of reinforced concrete under tensile loading. Cem. Concr. Res. 94, 59-70. https://doi.org/10.1016/j.cemconres.2017.01.004.). The application of compressive stresses in the element to close the crack and bring the two faces closer also improves self-healing, either if this stress is applied externally (4141. Schlangen, E.; ter Heide, N.; van Breugel, K. (2007) Crack healing of early age cracks in concrete. In: Konsta-Gdoutos M.S. (eds) Measuring, monitoring and modeling concrete properties. Springer, Dordrecht. 273-284 https://doi.org/10.1007/978-1-4020-5104-3_32.) or internally through tendons made of shape-memory materials (4242. Jefferson, A.; Joseph, C.; Lark, R.; Isaacs, B.; Dunn, S.; Weager, B. (2010) A new system for crack closure of cementitious materials using shrinkable polymers. Cem. Concr. Res. 40 [5], 795-801. https://doi.org/10.1016/j.cemconres.2010.01.004.).

Continuing hydration is always proposed as an essential mechanism with effects on autogenous healing; however, this process has not received as much attention as carbonation (55. Hearn, N. (1998) Self-sealing, autogenous healing and continued hydration: What is the difference? Mater. Struct. 31 [8], 563-567. https://doi.org/10.1007/BF02481539.). Healing by continuing hydration effect is generally evaluated comparing the response of young matrices with well-hydrated matrices (4141. Schlangen, E.; ter Heide, N.; van Breugel, K. (2007) Crack healing of early age cracks in concrete. In: Konsta-Gdoutos M.S. (eds) Measuring, monitoring and modeling concrete properties. Springer, Dordrecht. 273-284 https://doi.org/10.1007/978-1-4020-5104-3_32., 4343. Jaroenratanapirom, D.; Sahamitmongkol, R. (2011) Self-crack closing ability of mortar with different additives. J. Met. Mater. Miner. 21 [1], 9-17. , 4444. Hearn, N.; Morley, C.T. (1997) Self-sealing property of concrete - Experimental evidence. Mater. Struct. 30, 404-411. https://doi.org/10.1007/BF02498563.), obtaining better healing for younger matrices (4141. Schlangen, E.; ter Heide, N.; van Breugel, K. (2007) Crack healing of early age cracks in concrete. In: Konsta-Gdoutos M.S. (eds) Measuring, monitoring and modeling concrete properties. Springer, Dordrecht. 273-284 https://doi.org/10.1007/978-1-4020-5104-3_32., 4343. Jaroenratanapirom, D.; Sahamitmongkol, R. (2011) Self-crack closing ability of mortar with different additives. J. Met. Mater. Miner. 21 [1], 9-17. ) due to the higher availability of the calcium hydroxide and humidity in the matrix (3434. Wang, X.F.; Yang, Z.H.; Fang, C.; Han, N.X.; Zhu, G.M.; Tang, J.N.; et al. (2019) Evaluation of the mechanical performance recovery of self-healing cementitious materials - its methods and future development: A review. Constr. Build Mater. 212, 400-421. https://doi.org/10.1016/j.conbuildmat.2019.03.117.). However, the influence of continuing hydration appears to be smaller than that of carbonation. One study isolated continuing hydration from carbonation by working into sealed rooms to avoid the entrance of CO₂ in the water (4545. Huang, H.; Ye, G.; Damidot, D. (2013) Characterization and quantification of self-healing behaviors of microcracks due to further hydration in cement paste. Cem. Concr. Res. 52, 71-81. https://doi.org/10.1016/j.cemconres.2013.05.003.), and cracks of size up to 15 μm were filled only between 5 and 40%.

One sub-topics that drawn attention is the incorporation of supplementary cementitious materials (SCC) to improve autogenous healing, such as silica fume, fly ash, metakaolin, or blast furnace slag. These materials can improve autogenous healing by taking advantage of their delayed hydration, depending on the remaining reaction capacity of the material at the moment of cracking (4343. Jaroenratanapirom, D.; Sahamitmongkol, R. (2011) Self-crack closing ability of mortar with different additives. J. Met. Mater. Miner. 21 [1], 9-17. , 46-4946. Van Tittelboom, K.; Gruyaert, E.; Rahier, H.; De Belie, N. (2012) Influence of mix composition on the extent of autogenous crack healing by continued hydration or calcium carbonate formation. Constr. Build. Mater. 37, 349-359. https://doi.org/10.1016/j.conbuildmat.2012.07.026.
47. Darquennes, A.; Olivier, K.; Benboudjema, F.; Gagné, R. (2016) Self-healing at early-age, a way to improve the chloride resistance of blast-furnace slag cementitious materials. Constr. Build Mater. 113, 1017-1028. https://doi.org/10.1016/j.conbuildmat.2016.03.087.
48. Zhou, Z.H; Li, Z.Q.; Xu, D.J.; Yu, J.H. (2011) Influence of slag and fly ash on the self-healing ability of concrete. Advan. Mater. Research. 306-307, 1020-1023. https://doi.org/10.4028/www.scientific.net/AMR.306-307.1020.
49. Na, S.H.; Hama, Y.; Taniguchi, M.; Katsura, O.; Sagawa, T.; Zakaria, M. (2012) Experimental investigation on reaction rate and self-healing ability in fly ash blended cement mixtures. J. Adv. Concr. Technol. 10 [7], 240-253. https://doi.org/10.3151/jact.10.240. ). Mixes with higher cement content and/or SCC will have potentially better autogenous healing (1818. Cuenca, E.; Tejedor, A.; Ferrara, L. (2018) A methodology to assess crack-sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles. Constr. Build. Mater. 179, 619-632. https://doi.org/10.1016/j.conbuildmat.2018.05.261., 3535. Sangadji, S. (2017) Can self-healing mechanism helps concrete structures sustainable? Procedia Eng. 171, 238-249. https://doi.org/10.1016/j.proeng.2017.01.331.).

All in all, autogenous healing can occur in any type of cement-based material but requires certain conditions to be effective, such as crack widths below 0.15 mm and the presence of water and/or CO₂.

2.2. Autonomous healing

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Autonomous healing systems use agents engineered to be directly mixed in the cementitious matrix or introduced embedded in an encapsulation system, which protects them until the moment when they are released (5050. Han, N.X.; Xing, F. (2017) A comprehensive review of the study and development of microcapsule based self-resilience systems for concrete structures at Shenzhen University. Mater. 10 [1], 2. https://doi.org/10.3390/ma10010002.). Their activation can be produced by cracking itself or by the contact with the air or water in the crack, or by other agents introduced into the matrix. If the agent is incorporated in the mix without protection, its efficiency must remain latent until the generation of a crack. If the agent is encapsulated, the capsules must resist the mixing process and the collision with aggregates or mixer blades (5151. Sidiq, A.; Setunge, S.; Gravina, R.J.; Giustozzi, F. (2020) Self-repairing cement mortars with microcapsules: A microstructural evaluation approach. Constr. Build. Mater. 232, 117239. https://doi.org/10.1016/j.conbuildmat.2019.117239.), and the healing material must have good mobility to ensure an adequate release (99. De Belie, N.; Gruyaert, E.; Al-Tabbaa, A.; Antonaci, P.; Baera, C.; Bajare, D.; et al. (2018) A review of self-healing concrete for damage management of structures. Adv. Mater. Inter. 5 [17], 1800074. https://doi.org/10.1002/admi.201800074. ).

This section reviews self-healing agents used to produce autonomous healing in cracks, organised by controlled release of water, inorganic chemical agents, reactive adhesives, and biological agents. Subsequent section 2.3. will shortly describe some encapsulation methods proposed in the literature.

2.2.1. Controlled release of water

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The encapsulation of water has been studied to promote autogenous healing in concrete, using absorbent materials that can act as water pockets, such as superabsorbent polymers, vegetal fibres (5252. Toledo Filho, R.D.; Ghavami, K.; Sanjuán, M.A.; England G.L. (2005) Free, restrained and drying shrinkage of cement mortar composites reinforced with vegetable fibres. Cem. Concr. Compos. 27 [5], 537-546. https://doi.org/10.1016/j.cemconcomp.2004.09.005., 5353. Singh, H.; Gupta, R. (2020) Influence of cellulose fiber addition on self-healing and water permeability of concrete. Case Stud. Constr. Mater. 12, e00324. https://doi.org/10.1016/j.cscm.2019.e00324.), or nanoclays (5454. Qian, S.Z.; Zhou, J.; Schlangen, E. (2010) Influence of curing condition and precracking time on the self-healing behavior of Engineered Cementitious Composites. Cem. Concr. Compos. 32 [9], 686-693. https://doi.org/10.1016/j.cemconcomp.2010.07.015.). These materials have high water absorption and produce a controlled release afterwards if a crack appears in concrete. The main benefit of these systems is an increase in the self-healing speed compared to autogenous healing. In fact, using cellulose microfibers increased healing rate during the initial days of healing as compared to reference concrete (5353. Singh, H.; Gupta, R. (2020) Influence of cellulose fiber addition on self-healing and water permeability of concrete. Case Stud. Constr. Mater. 12, e00324. https://doi.org/10.1016/j.cscm.2019.e00324.), but reaching the same final results. Similarly, vegetal fibres of higher absorption displayed faster healing (5252. Toledo Filho, R.D.; Ghavami, K.; Sanjuán, M.A.; England G.L. (2005) Free, restrained and drying shrinkage of cement mortar composites reinforced with vegetable fibres. Cem. Concr. Compos. 27 [5], 537-546. https://doi.org/10.1016/j.cemconcomp.2004.09.005.).

Superabsorbent polymers (SAPs) are a particular case that experiences combined effects. SAPs are cross-linked polymers with a high capacity for absorbing fluids (1414. He, Z.; Shen, A.; Guo, Y.; Lyu, Z.; Li, D.; Qin, X.; et al. (2019) Cement-based materials modified with superabsorbent polymers: A review. Constr. Build. Mater. 225, 569-590. https://doi.org/10.1016/j.conbuildmat.2019.07.139.). When they absorb liquids, they swell to form a soft and insoluble gel (5555. Lee, H.X.D.; Wong, H.S.; Buenfeld, N.R. (2010) Potential of superabsorbent polymer for self-sealing cracks in concrete. Adv. Appl. Ceram. 109 [5], 296-302. https://doi.org/10.1179/174367609X459559., 5656. Tsuji, M.; Shitama, K.; Isobe, D. (1999) Basic studies on simplified curing technique, and prevention of initial cracking and leakage of water through cracks of concrete by applying superabsorbent polymers as new concrete admixture. Jour. Soci. Mater. Sci., Japan. 48 [11], 1308-1315. https://doi.org/10.2472/jsms.48.1308.). Their swelling capacity is highly dependent on their environment (5757. Kang, S-H.; Hong, S-G.; Moon, J. (2018) Importance of monovalent ions on water retention capacity of superabsorbent polymer in cement based solutions. Cem. Concr. Compos. 88, 64-72. https://doi.org/10.1016/j.cemconcomp.2018.01.015.). SAPs have high absorption on neutral/acid water, but hardly absorb alkaline water in fresh/hardened concrete (5656. Tsuji, M.; Shitama, K.; Isobe, D. (1999) Basic studies on simplified curing technique, and prevention of initial cracking and leakage of water through cracks of concrete by applying superabsorbent polymers as new concrete admixture. Jour. Soci. Mater. Sci., Japan. 48 [11], 1308-1315. https://doi.org/10.2472/jsms.48.1308.). The estimated absorption capacity of SAPs in relation to its own mass can vary from 500 to 1080 times in distilled water, from 10 to 30 times during the mortar mixing (1414. He, Z.; Shen, A.; Guo, Y.; Lyu, Z.; Li, D.; Qin, X.; et al. (2019) Cement-based materials modified with superabsorbent polymers: A review. Constr. Build. Mater. 225, 569-590. https://doi.org/10.1016/j.conbuildmat.2019.07.139., 5757. Kang, S-H.; Hong, S-G.; Moon, J. (2018) Importance of monovalent ions on water retention capacity of superabsorbent polymer in cement based solutions. Cem. Concr. Compos. 88, 64-72. https://doi.org/10.1016/j.cemconcomp.2018.01.015., 5858. Snoeck, D.; Schaubroeck, D.; Dubruel, P.; De Belie, N. (2014) Effect of high amounts of superabsorbent polymers and additional water on the workability, microstructure and strength of mortars with a water-to-cement ratio of 0.50. Constr. Build. Mater. 72, 148-157. https://doi.org/10.1016/j.conbuildmat.2014.09.012.). Swelling time also depends on the properties of the SAPs and can vary from seconds to minutes (1212. Snoeck, D.; Van den Heede, P.; Van Mullem, T.; De Belie, N. (2018) Water penetration through cracks in self-healing cementitious materials with superabsorbent polymers studied by neutron radiography. Cem. Concr. Res. 113, 86-98. https://doi.org/10.1016/j.cemconres.2018.07.002., 5959. Hong, G.; Choi, S. (2017) Rapid self-sealing of cracks in cementitious materials incorporating superabsorbent polymers. Constr. Build. Mater. 143, 366-375. https://doi.org/10.1016/j.conbuildmat.2017.03.133.).

When SAPs are added into concrete, during the concrete casting and setting, they do not experience a high water absorption nor volume increase because of the concrete high pH. If the matrix cracks and is put in contact with water, two reactions related to self-healing occur:

Physical blocking produced by the gel: SAPs absorb water and form gels that can fill the cracks. This process can recover water-tightness in the concrete element. This effect is temporary: if the concrete surface is dried, water is released, and SAPs shrink. SAPs are available for reswell if put in contact again with water (5555. Lee, H.X.D.; Wong, H.S.; Buenfeld, N.R. (2010) Potential of superabsorbent polymer for self-sealing cracks in concrete. Adv. Appl. Ceram. 109 [5], 296-302. https://doi.org/10.1179/174367609X459559.).
Promoting autogenous healing: The water absorbed in SAPs is gradually released, reacting with unhydrated cement particles and hydration byproducts, promoting autogenous healing.

SAPs are added in dry conditions with typical dosages of 0.3-0.6% by the cement weight (1313. Sun, B.; Wu, H.; Song, W.; Li, Z.; Yu, J. (2019) Design methodology and mechanical properties of Superabsorbent Polymer (SAP) cement-based materials. Constr. Build. Mater. 204, 440-449. https://doi.org/10.1016/j.conbuildmat.2019.01.206., 1414. He, Z.; Shen, A.; Guo, Y.; Lyu, Z.; Li, D.; Qin, X.; et al. (2019) Cement-based materials modified with superabsorbent polymers: A review. Constr. Build. Mater. 225, 569-590. https://doi.org/10.1016/j.conbuildmat.2019.07.139., 5858. Snoeck, D.; Schaubroeck, D.; Dubruel, P.; De Belie, N. (2014) Effect of high amounts of superabsorbent polymers and additional water on the workability, microstructure and strength of mortars with a water-to-cement ratio of 0.50. Constr. Build. Mater. 72, 148-157. https://doi.org/10.1016/j.conbuildmat.2014.09.012., 5959. Hong, G.; Choi, S. (2017) Rapid self-sealing of cracks in cementitious materials incorporating superabsorbent polymers. Constr. Build. Mater. 143, 366-375. https://doi.org/10.1016/j.conbuildmat.2017.03.133.). Dry SAPs can consume part of the water during the mixing process, increasing the plastic viscosity in the fresh state if no additional water is introduced (1414. He, Z.; Shen, A.; Guo, Y.; Lyu, Z.; Li, D.; Qin, X.; et al. (2019) Cement-based materials modified with superabsorbent polymers: A review. Constr. Build. Mater. 225, 569-590. https://doi.org/10.1016/j.conbuildmat.2019.07.139.). Additions of 1% SAPs by the cement weight can demand an increase in the water/cement ratio from 0.35 to 0.43 (1313. Sun, B.; Wu, H.; Song, W.; Li, Z.; Yu, J. (2019) Design methodology and mechanical properties of Superabsorbent Polymer (SAP) cement-based materials. Constr. Build. Mater. 204, 440-449. https://doi.org/10.1016/j.conbuildmat.2019.01.206.). The size of SAPs is also an essential factor. Typically, SAPs of size around 500 μm provide good self-healing response, better than sizes around 200 μm or 80 μm (6060. Park, B.; Choi, Y.C. (2018) Self-healing capability of cementitious materials with crystalline admixtures and super absorbent polymers (SAPs). Constr. Build. Mater. 189, 1054-1066. https://doi.org/10.1016/j.conbuildmat.2018.09.061. ), and less reaction time.

One of the drawbacks of SAPs and the additional water to compensate for their absorption is the formation of voids, which reduces compressive strength (5858. Snoeck, D.; Schaubroeck, D.; Dubruel, P.; De Belie, N. (2014) Effect of high amounts of superabsorbent polymers and additional water on the workability, microstructure and strength of mortars with a water-to-cement ratio of 0.50. Constr. Build. Mater. 72, 148-157. https://doi.org/10.1016/j.conbuildmat.2014.09.012., 5959. Hong, G.; Choi, S. (2017) Rapid self-sealing of cracks in cementitious materials incorporating superabsorbent polymers. Constr. Build. Mater. 143, 366-375. https://doi.org/10.1016/j.conbuildmat.2017.03.133.). Another drawback is that SAPs will not work in marine environments, since their swelling capacity is hugely reduced (5555. Lee, H.X.D.; Wong, H.S.; Buenfeld, N.R. (2010) Potential of superabsorbent polymer for self-sealing cracks in concrete. Adv. Appl. Ceram. 109 [5], 296-302. https://doi.org/10.1179/174367609X459559., 6161. Snoeck, D.; Van Tittelboom, K.; Steuperaert, S.; Dubruel, P.; De Belie, N. (2014) Self-healing cementitious materials by the combination of microfibres and superabsorbent polymers. J. Intell. Mater. Syst. Struct. 25 [1], 13-24. https://doi.org/10.1177/1045389X12438623.).

2.2.2. Inorganic chemical agents

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In this group, several types of healing agents have been proposed. Some of these agents can be added without encapsulation, but others need to be encapsulated to be protected until the moment they are required to act. The self-healing agents included in this category are crystalline admixtures, expansive agents and silica-based agents.

Crystalline admixtures (CA) are commercial admixtures that react with the humidity of fresh concrete and with the products of cement hydration, producing non-soluble crystals that promote self-healing of cracks. The term “crystalline admixtures” is a label used in commercial admixtures not necessarily reflecting functionality or molecular structure (99. De Belie, N.; Gruyaert, E.; Al-Tabbaa, A.; Antonaci, P.; Baera, C.; Bajare, D.; et al. (2018) A review of self-healing concrete for damage management of structures. Adv. Mater. Inter. 5 [17], 1800074. https://doi.org/10.1002/admi.201800074. ). Some proprietary CA are Penetron Admix and Xypex Admix. CA have been reported to slightly enhance self-healing in terms of crack closing and water tightness (1717. Roig-Flores, M.; Pirritano, F.; Serna, P.; Ferrara, L. (2016) Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests. Constr. Build. Mater. 114, 447-457. https://doi.org/10.1016/j.conbuildmat.2016.03.196., 1818. Cuenca, E.; Tejedor, A.; Ferrara, L. (2018) A methodology to assess crack-sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles. Constr. Build. Mater. 179, 619-632. https://doi.org/10.1016/j.conbuildmat.2018.05.261., 3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005., 6262. Roig-Flores, M.; Moscato, S.; Serna, P.; Ferrara, L. (2015) Self-healing capability of concrete with crystalline admixtures in different environments. Constr. Build. Mater. 86, 1-11. https://doi.org/10.1016/j.conbuildmat.2015.03.091 ), and less frequently, also recovery of stiffness and bearing capacity (1616. Ferrara, L.; Krelani, V.; Carsana, M. (2014) A “fracture testing” based approach to assess crack healing of concrete with and without crystalline admixtures. Constr. Build. Mater. 68, 535-551. https://doi.org/10.1016/j.conbuildmat.2014.07.008.). However, some studies did not found recovery of mechanical properties (6363. Escoffres, P.; Desmettre, C.; Charron, J.P. (2018) Effect of a crystalline admixture on the self-healing capability of high-performance fiber reinforced concretes in service conditions. Constr. Build. Mater. 173, 763-774. https://doi.org/10.1016/j.conbuildmat.2018.04.003., 6464. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2013) Effect of exposure conditions on self healing behavior of strain hardening cementitious composites incorporating various cementitious materials. Constr. Build. Mater. 42, 217-224. https://doi.org/10.1016/j.conbuildmat.2013.01.012.). CA are effective only in direct contact with water and for cracks below 0.30 mm (1717. Roig-Flores, M.; Pirritano, F.; Serna, P.; Ferrara, L. (2016) Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests. Constr. Build. Mater. 114, 447-457. https://doi.org/10.1016/j.conbuildmat.2016.03.196., 6262. Roig-Flores, M.; Moscato, S.; Serna, P.; Ferrara, L. (2015) Self-healing capability of concrete with crystalline admixtures in different environments. Constr. Build. Mater. 86, 1-11. https://doi.org/10.1016/j.conbuildmat.2015.03.091 ).

Several expansive additions have been proposed in the literature, such as calcium sulfoaluminate (CSA), (Ca₄(AlO₂)₆SO₄), magnesium oxide (MgO), calcium oxide (CaO) and calcium sulphate (CaSO₄), due to their expansive behaviour. Some authors studied CSA in 10% by the weight of the binder, obtaining better self-healing for cracks up to 0.3 mm (3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005., 4343. Jaroenratanapirom, D.; Sahamitmongkol, R. (2011) Self-crack closing ability of mortar with different additives. J. Met. Mater. Miner. 21 [1], 9-17. , 6464. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2013) Effect of exposure conditions on self healing behavior of strain hardening cementitious composites incorporating various cementitious materials. Constr. Build. Mater. 42, 217-224. https://doi.org/10.1016/j.conbuildmat.2013.01.012.) in terms of visual closure and water permeability. CSA with a lower amount of CaO and higher Al₂O₃ and SO₃ displayed better results (3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005., 6464. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2013) Effect of exposure conditions on self healing behavior of strain hardening cementitious composites incorporating various cementitious materials. Constr. Build. Mater. 42, 217-224. https://doi.org/10.1016/j.conbuildmat.2013.01.012.). In the case of mechanical properties, only slight improvements were detected.

CA and CSA have been combined in some studies to investigate possible synergies, obtaining improved self-healing (3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005., 6060. Park, B.; Choi, Y.C. (2018) Self-healing capability of cementitious materials with crystalline admixtures and super absorbent polymers (SAPs). Constr. Build. Mater. 189, 1054-1066. https://doi.org/10.1016/j.conbuildmat.2018.09.061. ), with contents of 10% CSA and 1.5% CA by the cement weight, closing cracks up to about 0.4 mm (3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005.). Even though these admixtures are designed to be added directly to the concrete mix, their controlled activation when self-healing is required is a concern. The combined use of SAPs and CA has also been proposed (6060. Park, B.; Choi, Y.C. (2018) Self-healing capability of cementitious materials with crystalline admixtures and super absorbent polymers (SAPs). Constr. Build. Mater. 189, 1054-1066. https://doi.org/10.1016/j.conbuildmat.2018.09.061. , 6565. Chindasiriphan, P.; Yokota, H.; Pimpakan, P. (2020) Effect of fly ash and superabsorbent polymer on concrete self-healing ability. Constr. Build. Mater. 233, 116975. https://doi.org/10.1016/j.conbuildmat.2019.116975.), to provide controlled water release for the reactions and showed that CAs produced more self-healing products around SAPs. Similarly, SAPs with CSA agents have also been proposed (6060. Park, B.; Choi, Y.C. (2018) Self-healing capability of cementitious materials with crystalline admixtures and super absorbent polymers (SAPs). Constr. Build. Mater. 189, 1054-1066. https://doi.org/10.1016/j.conbuildmat.2018.09.061. ), but further research is still needed.

Magnesium oxide has also been used to promote self-healing due to its expansive properties. Without encapsulation, MgO has been reported to close cracks up to 0.5 mm after 28 days (6666. Qureshi, T.S.; Al-Tabbaa, A. (2016) Self-healing of drying shrinkage cracks in cement-based materials incorporating reactive MgO. Smart Mater. Struct. 25 [8], 1-16. https://doi.org/10.1088/0964-1726/25/8/084004.), with increasing healing when increasing MgO content. Similarly, bentonite and lime have also been proposed (6767. Qureshi, T.; Kanellopoulos, A.; Al-Tabbaa, A. (2018) Autogenous self-healing of cement with expansive minerals-I: Impact in early age crack healing. Constr. Build. Mater. 192, 768-784. https://doi.org/10.1016/j.conbuildmat.2018.10.143., 6868. Qureshi, T.; Kanellopoulos, A.; Al-Tabbaa, A. (2019) Autogenous self-healing of cement with expansive minerals-II: Impact of age and the role of optimised expansive minerals in healing performance. Constr. Build. Mater. 194, 266-275. https://doi.org/10.1016/j.conbuildmat.2018.11.027.), as well as their combinations. MgO encouraged in the formation of brucite and other magnesium hydro-carbonates, while bentonite influenced in the formation of ettringite, and quicklime produced additional portlandite, calcite and calcium-based hydration products (6767. Qureshi, T.; Kanellopoulos, A.; Al-Tabbaa, A. (2018) Autogenous self-healing of cement with expansive minerals-I: Impact in early age crack healing. Constr. Build. Mater. 192, 768-784. https://doi.org/10.1016/j.conbuildmat.2018.10.143.). Since the addition of MgO can lead to undesired expansions and stresses, its encapsulation has been studied to control its activation (6969. Alghamri, R.; Kanellopoulos, A.; Al-Tabbaa, A. (2016) Impregnation and encapsulation of lightweight aggregates for self-healing concrete. Constr. Build. Mater. 124, 910-921. https://doi.org/10.1016/j.conbuildmat.2016.07.143., 7070. Qureshi, T.S.; Kanellopoulos, A.; Al-Tabbaa, A. (2016) Encapsulation of expansive powder minerals within a concentric glass capsule system for self-healing concrete. Constr. Build. Mater. 121, 629-643. https://doi.org/10.1016/j.conbuildmat.2016.06.030.).

Silica-based agents generally promote the formation of CSH gels, similarly to pozzolanic reactions. The most frequently used agent is sodium silicate (Na₂SiO₃), but also colloidal silica (mSiO₂.nH₂O) has been proposed (7171. Tan, N.P.B.; Keung, L.H.; Choi, W.H.; Lam, W.C.; Leung, H.N. (2015) Silica-based self-healing microcapsules for self-repair in concrete. J. Appl. Polym. Sci. 133 [12], 43090. https://doi.org/10.1002/app.43090. ). Sodium silicate is one of the most used agents in the self-healing microencapsulated systems. Sodium silicate reacts with hydrated cement pastes in complex interactions with calcium hydroxide (Ca(OH)₂), calcium aluminate (CaAl₂O₄) and non-hydrated C₃S/C₂S phases to develop hydration products as CSH or CASH (1818. Cuenca, E.; Tejedor, A.; Ferrara, L. (2018) A methodology to assess crack-sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles. Constr. Build. Mater. 179, 619-632. https://doi.org/10.1016/j.conbuildmat.2018.05.261., 72-7472. Li, G.; Huang, X.; Lin, J.; Jiang, X.; Zhang, X. (2019) Activated chemicals of cementitious capillary crystalline waterproofing materials and their self-healing behaviour. Constr. Build. Mater. 200, 36-45. https://doi.org/10.1016/j.conbuildmat.2018.12.093.
73. Irico, S.; Bovio, A.G.; Paul, G.; Boccaleri, E.; Gastaldi, D.; Marchese, L.; et al. (2017) A solid-state NMR and X-ray powder diffraction investigation of the binding mechanism for self-healing cementitious materials design: The assessment of the reactivity of sodium silicate based systems. Cem. Concr. Compos. 76, 57-63. https://doi.org/10.1016/j.cemconcomp.2016.11.006.
74. Restuccia, L.; Reggio, A.; Ferro, G.A.; Tulliani, J.M. (2017) New self-healing techniques for cement-based materials. Procedia Struct. Integr. 3, 253-260. https://doi.org/10.1016/j.prostr.2017.04.016. ). Most studies that use sodium silicate as a self-healing agent include this product encapsulated, so it reacts after being released out of a microcapsule (2020. Beglarigale, A.; Seki, Y.; Demir, N.Y.; Yazıcı, H. (2018) Sodium silicate/polyurethane microcapsules used for self-healing in cementitious materials: Monomer optimization, characterization, and fracture behavior. Constr. Build. Mater. 162, 57-64. https://doi.org/10.1016/j.conbuildmat.2017.11.164.). Microencapsulated sodium silicate has displayed positive impact, healing cracks up to 0.20 mm (7575. Sidiq, A.; Gravina, R.J.; Setunge, S.; Giustozzi, F. (2019) Microstructural analysis of healing efficiency in highly durable concrete. Constr. Build. Mater. 215, 969-983. https://doi.org/10.1016/j.conbuildmat.2019.04.233.), improving water impermeability through sorptivity (7676. Mao, W.; Litina, C.; Al-Tabbaa, A. (2020) Development and application of novel sodium silicate microcapsule-based self-healing oil well cement. Mater. 13 [2], 456. https://doi.org/10.3390/ma13020456.), and promoting mechanical recovery (2121. Al-Tabbaa, A.; Litina, C.; Giannaros, P.; Kanellopoulos, A.; Souza, L. (2019) First UK field application and performance of microcapsule-based self-healing concrete. Constr. Build. Mater. 208, 669-685. https://doi.org/10.1016/j.conbuildmat.2019.02.178.). However, increasing the content of microcapsules can decrease compressive strength (7676. Mao, W.; Litina, C.; Al-Tabbaa, A. (2020) Development and application of novel sodium silicate microcapsule-based self-healing oil well cement. Mater. 13 [2], 456. https://doi.org/10.3390/ma13020456.) because of the increase in porosity. Sodium silicate is also being studied as pore blocking surface treatment (88. Sánchez, M.; Faria, P.; Ferrara, L.; Horszczaruk, E.; Jonkers, H.M.; Kwiecień, A.; et al. (2018) External treatments for the preventive repair of existing constructions: A review. Constr. Build. Mater. 193, 435-452. https://doi.org/10.1016/j.conbuildmat.2018.10.173.) to repair structures or to improve water tightness.

2.2.3. Reactive adhesives

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Reactive adhesives have been used encapsulated as self-healing systems due to their ability to bond surfaces. Specifically, two types have been used: one-part and multi-component adhesives.

One-part adhesives harden via radiation, heat, or moisture. Because of that, adhesives that harden when exposed to light (7777. Lv, L.; Guo, P.; Liu, G.; Han, N.; Xing, F. (2020) Light induced self-healing in concrete using novel cementitious capsules containing UV curable adhesive. Cem. Concr. Compos. 105, 103445. https://doi.org/10.1016/j.cemconcomp.2019.103445.), high temperature (2222. Dry, C.M. (2000) Three designs for the internal release of sealants, adhesives, and waterproofing chemicals into concrete to reduce permeability. Cem. Concr. Res. 30 [12], 1969-1977. https://doi.org/10.1016/S0008-8846(00)00415-4.), or moisture (cyanoacrylates (7878. Selvarajoo, T.; Davies, R.E.; Freeman, B.L.; Jefferson, A.D. (2020) Mechanical response of a vascular self-healing cementitious material system under varying loading conditions. Constr. Build. Mater. 254, 119245. https://doi.org/10.1016/j.conbuildmat.2020.119245., 7979. Du, W.; Yu, J.; Gu, Y.; Li, Y.; Han, X.; Liu, Q. (2019) Preparation and application of microcapsules containing toluene-di-isocyanate for self-healing of concrete. Constr. Build. Mater. 202, 762-769. https://doi.org/10.1016/j.conbuildmat.2019.01.007.) or polyurethanes (8080. Hu, Z.X.; Hu, X.M.; Cheng, W.M.; Zhao, Y.Y.; Wu, M.Y. (2018) Performance optimization of one-component polyurethane healing agent for self-healing concrete. Constr. Build. Mater. 179, 151-159. https://doi.org/10.1016/j.conbuildmat.2018.05.199., 8181. Anglani, G.; Tulliani, J-M.; Antonaci, P. (2020) Behaviour of pre-cracked self-healing cementitious materials under static and cyclic loading. Mater. 13 [5], 1149. https://doi.org/10.3390/ma13051149.)) have been proposed as healing agents. Once a crack appears in concrete, the interior of the matrix and the adhesive will be exposed to ambient conditions, activating the healing reaction.

Multi-component systems need the addition of two encapsulated elements inside the concrete. Some examples of systems used to produce self-healing are epoxy resin (8282. Perez, G.; Erkizia, E.; Gaitero, J.J.; Kaltzakorta, I.; Jiménez, I.; Guerrero, A. (2015) Synthesis and characterization of epoxy encapsulating silica microcapsules and amine functionalized silica nanoparticles for development of an innovative self-healing concrete. Mater. Chem. Phys. 165, 39-48. https://doi.org/10.1016/j.matchemphys.2015.08.047.) or one-part adhesives (such as a pre-polymer of polyurethane) combined with an accelerator to improve the reaction (2424. Gilabert, F.A.; Van Tittelboom, K.; Van Stappen, J.; Cnudde, V.; De Belie, N.; Van Paepegem, W. (2017) Integral procedure to assess crack filling and mechanical contribution of polymer-based healing agent in encapsulation-based self-healing concrete. Cem. Concr. Compos. 77, 68-80. https://doi.org/10.1016/j.cemconcomp.2016.12.001., 8383. Van Tittelboom, K.; De Belie, N.; Van Loo, D.; Jacobs, P. (2011) Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent. Cem. Concr. Compos. 33 [4], 497-505. https://doi.org/10.1016/j.cemconcomp.2011.01.004.).

These two systems have the capability of healing large cracks, even up to 0.3-0.5 mm (7878. Selvarajoo, T.; Davies, R.E.; Freeman, B.L.; Jefferson, A.D. (2020) Mechanical response of a vascular self-healing cementitious material system under varying loading conditions. Constr. Build. Mater. 254, 119245. https://doi.org/10.1016/j.conbuildmat.2020.119245., 8080. Hu, Z.X.; Hu, X.M.; Cheng, W.M.; Zhao, Y.Y.; Wu, M.Y. (2018) Performance optimization of one-component polyurethane healing agent for self-healing concrete. Constr. Build. Mater. 179, 151-159. https://doi.org/10.1016/j.conbuildmat.2018.05.199.), and in several studies, the healed elements even recovered their mechanical properties (8080. Hu, Z.X.; Hu, X.M.; Cheng, W.M.; Zhao, Y.Y.; Wu, M.Y. (2018) Performance optimization of one-component polyurethane healing agent for self-healing concrete. Constr. Build. Mater. 179, 151-159. https://doi.org/10.1016/j.conbuildmat.2018.05.199., 8383. Van Tittelboom, K.; De Belie, N.; Van Loo, D.; Jacobs, P. (2011) Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent. Cem. Concr. Compos. 33 [4], 497-505. https://doi.org/10.1016/j.cemconcomp.2011.01.004.).

2.2.4. Biological agents: bacteria-based self-healing

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Microbially-induced calcite precipitation (MICP) has become an area of interest for self-healing of cement-based materials. Bacteria can precipitate CaCO₃ with different metabolic pathways, such as photosynthesis, sulphate reduction, urea hydrolysis or denitrification. For each of these pathways, bacteria also need the appropriate nutrients, which can be, yeast extract, urea, calcium lactate or other compounds. The two processes proposed to introduce self-healing in concrete by MICP are (2828. Li, L.; Zheng, Q.; Li, Z.; Ashour, A.; Han, B. (2019) Bacterial technology-enabled cementitious composites: A review. Compos. Struct. 225, 111170. https://doi.org/10.1016/j.compstruct.2019.111170.): urea hydrolysis by ureolytic bacteria or the respiration process.

Ureolytic bacteria can use carbon sources to produce CO₂ or CO₃ ^2-, which will react with Ca²⁺ to form bacterial precipitation of CaCO₃. Ureolytic bacteria have the ability to produce a urease enzyme, which hydrolyses urea into ammonia and CO₂, inducing a rapid increase of pH and the precipitation of CaCO₃. The alkali-tolerant ureolytic strains most commonly investigated for their application in cement-based materials are Sporosarcina pasteurii, Sporosarcina ureae, Bacillus sphaericus and Bacillus megaterium (1010. Vijay, K.; Murmu, M.; Deo, S.V. (2017) Bacteria based self healing concrete - A review. Constr. Build. Mater. 152, 1008-1014. https://doi.org/10.1016/j.conbuildmat.2017.07.040., 2727. Pawar, S.S.; Parekar, P.S.R. (2018) Bacteria based Self-Healing Concrete : Review. Inter. Res. J. Engi. Tech. 5 [3], 1001-1004. , 84-8684. Nain, N.; Surabhi, R.; Yathish, N. V.; Krishnamurthy, V.; Deepa, T.; Tharannum, S. (2019) Enhancement in strength parameters of concrete by application of Bacillus bacteria. Constr. Build. Mater. 202, 904-908. https://doi.org/10.1016/j.conbuildmat.2019.01.059.
85. Huynh, N.N.T.; Phuong, N.M.; Toan, N.P.A.; Son, N.K. (2017) Bacillys subtilis HU58 immobilized in micropores of diatomite for using in self-helaing concrete. Procedia Eng. 171, 598-605. https://doi.org/10.1016/j.proeng.2017.01.385.
86. Shaheen, N.; Khushnood, R.A.; Khaliq, W.; Murtaza, H.; Iqbal, R.; Khan, M.H. (2019) Synthesis and characterization of bio-immobilized nano/micro inert and reactive additives for feasibility investigation in self-healing concrete. Constr. Build. Mater. 226, 492-506. https://doi.org/10.1016/j.conbuildmat.2019.07.202. ).

In the respiration process, non-ureolytic bacteria act as nucleation sites for the precipitation of CaCO₃ when oxygen is present. The cell wall of bacteria is negatively charged, and, because of that, bacteria can extract cations from the environment, including Ca²⁺ ions that are deposited on the surfaces of the cell wall. Ca²⁺ ions react with CO₃ ^2-, leading to bacterial precipitation on the cell surface (8787. De Belie, N. (2016) Application of bacteria in concrete: a critical evaluation of the current status. RILEM Tech. Lett. 1, 56-61. https://doi.org/10.21809/rilemtechlett.2016.14. ). Bacteria are activated and proliferate after the ingress water and oxygen through the cracks. Then, they metabolise organic nutrients (e.g. calcium lactate) instead of urea as the electron donor to produce calcium carbonate (8888. Zhang, J.; Liu, Y.; Feng, T.; Zhou, M.; Zhao, L.; Zhou, A.; Li, Z. (2017) Immobilizing bacteria in expanded perlite for the crack self-healing in concrete. Constr. Build. Mater. 148, 610-617. https://doi.org/10.1016/j.conbuildmat.2017.05.021.). The non-ureolytic species most common are Bacillus subtilis, Bacillus cohnii, Bacillus pseudofirmus, Bacillus thuringiensis, Bacillus alkalinitrilicus (8585. Huynh, N.N.T.; Phuong, N.M.; Toan, N.P.A.; Son, N.K. (2017) Bacillys subtilis HU58 immobilized in micropores of diatomite for using in self-helaing concrete. Procedia Eng. 171, 598-605. https://doi.org/10.1016/j.proeng.2017.01.385., 8686. Shaheen, N.; Khushnood, R. A.; Khaliq, W.; Murtaza, H.; Iqbal, R.; Khan, M. H. (2019) Synthesis and characterization of bio-immobilized nano/micro inert and reactive additives for feasibility investigation in self-healing concrete. Constr. Build. Mater. 226, 492-506. https://doi.org/10.1016/j.conbuildmat.2019.07.202., 8888. Zhang, J.; Liu, Y.; Feng, T.; Zhou, M.; Zhao, L.; Zhou, A.; Li, Z. (2017) Immobilizing bacteria in expanded perlite for the crack self-healing in concrete. Constr. Build. Mater. 148, 610-617. https://doi.org/10.1016/j.conbuildmat.2017.05.021., 8989. Zhang, J.; Zhao, C.; Zhou, A.; Yang, C.; Zhao, L.; Li, Z. (2019) Aragonite formation induced by open cultures of microbial consortia to heal cracks in concrete: Insights into healing mechanisms and crystal polymorphs. Constr. Build. Mater. 224, 815-822. https://doi.org/10.1016/j.conbuildmat.2019.07.129.).

Inside concrete, after the formation of cracks, once bacteria are in contact with their nutrients, they are awakened from their hibernation stage (1010. Vijay, K.; Murmu, M.; Deo, S.V. (2017) Bacteria based self healing concrete - A review. Constr. Build. Mater. 152, 1008-1014. https://doi.org/10.1016/j.conbuildmat.2017.07.040.). Once bacteria are activated, their metabolism starts producing CaCO₃ in the crack. When the crack is completely filled, the bacteria return to hibernation due to the low availability of water or oxygen and the formation of a mineral layer (CaCO₃) covering the bacterial cells. If new cracks form, bacteria will be activated, and the crack will heal again. Therefore, bacteria act like as a catalyst, since they transform a precursor to a suitable filler material (1010. Vijay, K.; Murmu, M.; Deo, S.V. (2017) Bacteria based self healing concrete - A review. Constr. Build. Mater. 152, 1008-1014. https://doi.org/10.1016/j.conbuildmat.2017.07.040.), but they remain in the matrix.

There are three methods of applying bacteria, each with different efficiency: direct application as spores disperse in the matrix (9090. Liu, S.; Bundur, Z. B.; Zhu, J.; Ferron, R. D. (2016) Evaluation of self-healing of internal cracks in biomimetic mortar using coda wave interferometry. Cem. Concr. Res. 83, 70-78. https://doi.org/10.1016/j.cemconres.2016.01.006., 9191. Jadhav, U.U.; Lahoti, M.; Chen, Z.; Qiu, J.; Cao, B.; Yang, E.H. (2018) Viability of bacterial spores and crack healing in bacteria-containing geopolymer. Constr. Build. Mater. 169, 716-723. https://doi.org/10.1016/j.conbuildmat.2018.03.039.), by immobilisation in porous particles like porous aggregates (8585. Huynh, N.N.T.; Phuong, N.M.; Toan, N.P.A.; Son, N.K. (2017) Bacillys subtilis HU58 immobilized in micropores of diatomite for using in self-helaing concrete. Procedia Eng. 171, 598-605. https://doi.org/10.1016/j.proeng.2017.01.385., 8686. Shaheen, N.; Khushnood, R. A.; Khaliq, W.; Murtaza, H.; Iqbal, R.; Khan, M. H. (2019) Synthesis and characterization of bio-immobilized nano/micro inert and reactive additives for feasibility investigation in self-healing concrete. Constr. Build. Mater. 226, 492-506. https://doi.org/10.1016/j.conbuildmat.2019.07.202., 8989. Zhang, J.; Zhao, C.; Zhou, A.; Yang, C.; Zhao, L.; Li, Z. (2019) Aragonite formation induced by open cultures of microbial consortia to heal cracks in concrete: Insights into healing mechanisms and crystal polymorphs. Constr. Build. Mater. 224, 815-822. https://doi.org/10.1016/j.conbuildmat.2019.07.129.), in cellulose microfibers (9292. Singh, H.; Gupta, R. (2020) Cellulose fiber as bacteria-carrier in mortar: Self-healing quantification using UPV. J. Build. Eng. 28, 101090. https://doi.org/10.1016/j.jobe.2019.101090.) or encapsulation (9393. Wang, J.Y.; Soens, H.; Verstraete, W.; De Belie, N. (2014) Self-healing concrete by use of microencapsulated bacterial spores. Cem. Concr. Res. 56, 139-152. https://doi.org/10.1016/j.cemconres.2013.11.009.). Immobilisation or encapsulation are methods recommended to protect and prolong the life of the bacteria since their survivability is decreased for increasing hydration of concrete (due to the decreasing size of the pores) (1010. Vijay, K.; Murmu, M.; Deo, S.V. (2017) Bacteria based self healing concrete - A review. Constr. Build. Mater. 152, 1008-1014. https://doi.org/10.1016/j.conbuildmat.2017.07.040., 9494. Jonkers, H.M.; Thijssen, A.; Muyzer, G.; Copuroglu, O.; Schlangen, E. (2010) Application of bacteria as self-healing agent for the development of sustainable concrete. Ecol. Eng. 36 [2], 230-235. https://doi.org/10.1016/j.ecoleng.2008.12.036.).

Non-ureolytic bacteria are thought to be less efficient than ureolytic bacteria as they do not produce such a rapid increase in pH (9595. Reeksting, B.J.; Hoffmann, T.D.; Tan, L.; Paine, K.; Gebhard, S. (2020) In-depth profiling of calcite precipitation by environmental bacteria reveals fundamental mechanistic differences with relevance to application. Appl. Environ. Microbiol. 86 [7], e02739-19. https://doi.org/10.1128/AEM.02739-19. ). A non-ureolytic bacteria (8686. Shaheen, N.; Khushnood, R. A.; Khaliq, W.; Murtaza, H.; Iqbal, R.; Khan, M. H. (2019) Synthesis and characterization of bio-immobilized nano/micro inert and reactive additives for feasibility investigation in self-healing concrete. Constr. Build. Mater. 226, 492-506. https://doi.org/10.1016/j.conbuildmat.2019.07.202.) was reported to heal cracks up to 0.45 mm and to recover 65% of strength. Some ureolytic bacteria also showed good self-healing responses, recovering compressive strength (9090. Liu, S.; Bundur, Z. B.; Zhu, J.; Ferron, R. D. (2016) Evaluation of self-healing of internal cracks in biomimetic mortar using coda wave interferometry. Cem. Concr. Res. 83, 70-78. https://doi.org/10.1016/j.cemconres.2016.01.006., 9393. Wang, J.Y.; Soens, H.; Verstraete, W.; De Belie, N. (2014) Self-healing concrete by use of microencapsulated bacterial spores. Cem. Concr. Res. 56, 139-152. https://doi.org/10.1016/j.cemconres.2013.11.009.) and closing cracks up to 0.85-0.97 mm (9393. Wang, J.Y.; Soens, H.; Verstraete, W.; De Belie, N. (2014) Self-healing concrete by use of microencapsulated bacterial spores. Cem. Concr. Res. 56, 139-152. https://doi.org/10.1016/j.cemconres.2013.11.009.). Bacteria are able to heal larger cracks than autogenous healing (at least two times larger). Bacterial spores for concrete are starting to be commercialised, with few small companies and start-ups providing them, such as Avecom (9696. Da Silva, F. B.; De Belie, N.; Boon, N.; Verstraete, W. (2015) Production of non-axenic ureolytic spores for self-healing concrete applications. Constr. Build. Mater. 93, 1034-1041. https://doi.org/10.1016/j.conbuildmat.2015.05.049., 9797. Silva, F.B.; Boon, N.; De Belie, N.; Verstraete, W. (2015) Industrial application of biological self-healing concrete: Challenges and economical feasibility. J. Commer. Biotechnol. 21 [1], 31-38. https://doi.org/10.5912/jcb662.) or Basilisk (9898. Basilisk - Basilisk self-healing concrete. [consulted 2020 May 29]. Available from: https://www.basiliskconcrete.com.), but its commercial availability at competitive prices is still very limited.

2.3. Encapsulation methods

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One of the main challenges for achieving self-healing is to protect the healing agents inside the concrete and activate them only at the required moment. With this purpose, several encapsulation techniques are still being developed, either for bacteria or for chemical agents. The encapsulation systems can be comprised by a one-part component embedded (if the healing agent reacts with radiation, heat, water, or air) or multi-component.

The types of encapsulation system can be gathered in two groups: disperse capsules (mostly microcapsules between 20-800 μm (1919. Kanellopoulos, A.; Giannaros, P.; Palmer, D.; Kerr, A.; Al-Tabbaa, A. (2017) Polymeric microcapsules with switchable mechanical properties for self-healing concrete: synthesis, characterisation and proof of concept. Smart. Mater. Struct. 26, 045025. https://doi.org/10.1088/1361-665X/aa516c., 9999. Yang, Z.; Hollar, J.; He, X.; Shi, X. (2011) A self-healing cementitious composite using oil core/silica gel shell microcapsules. Cem. Concr. Compos. 33 [4], 506-512. https://doi.org/10.1016/j.cemconcomp.2011.01.010.), but also porous vessels up to 8 mm (6969. Alghamri, R.; Kanellopoulos, A.; Al-Tabbaa, A. (2016) Impregnation and encapsulation of lightweight aggregates for self-healing concrete. Constr. Build. Mater. 124, 910-921. https://doi.org/10.1016/j.conbuildmat.2016.07.143.)) and located capsules (mostly glass or ceramic tubes between 10-100 mm length (8383. Van Tittelboom, K.; De Belie, N.; Van Loo, D.; Jacobs, P. (2011) Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent. Cem. Concr. Compos. 33 [4], 497-505. https://doi.org/10.1016/j.cemconcomp.2011.01.004., 100100. Van Tittelboom, K.; Tsangouri, E.; Van Hemelrijck, D.; De Belie, N. (2015) The efficiency of self-healing concrete using alternative manufacturing procedures and more realistic crack patterns. Cem. Concr. Compos. 57, 142-152. https://doi.org/10.1016/j.cemconcomp.2014.12.002., 101101. Escobar, M.M.; Vago, S.; Vázquez, A. (2013) Self-healing mortars based on hollow glass tubes and epoxy-amine systems. Compos. Part B: Eng. 55, 203-207. https://doi.org/10.1016/j.compositesb.2013.06.023.)). Both methods need a physical breakage or an increase of the porosity to be activated. The first group is thought to be added to the concrete matrix as an additional compound and is a suitable method for unpredictable or dispersed cracking; while the second needs to be placed in a specific location in a similar way to reinforcement bars, thus being optimal for predictable cracks. The two types of capsules can be filled with different types of healing agents. Figure 3 shows a diagram with a classification of the self-healing agents, introduction methods and their combinations. Previously published reviews discuss the benefits of different types of microcapsules (44. Van Tittelboom, K.; De Belie, N. (2013) Self-healing in cementitious materials-A review. Mater. 6 [6], 2182-2217. https://doi.org/10.3390/ma6062182., 99. De Belie, N.; Gruyaert, E.; Al-Tabbaa, A.; Antonaci, P.; Baera, C.; Bajare, D.; et al. (2018) A review of self-healing concrete for damage management of structures. Adv. Mater. Inter. 5 [17], 1800074. https://doi.org/10.1002/admi.201800074. ).

Figure 3. Classification of the self-healing agents and methods.

An efficient microcapsule should be chosen such that the wall of the microcapsule is strong enough to resist mechanical impacts without breaking during the concrete mixing, with good bond with the matrix and a weak enough shell that it breaks when a crack occurs, and the crack path is not diverged around the capsule (99. De Belie, N.; Gruyaert, E.; Al-Tabbaa, A.; Antonaci, P.; Baera, C.; Bajare, D.; et al. (2018) A review of self-healing concrete for damage management of structures. Adv. Mater. Inter. 5 [17], 1800074. https://doi.org/10.1002/admi.201800074. ). Dispersed encapsulation answers to dispersed cracks, so it is more effective for random cracking, but the damage must hit the capsules. Most of the successful studies performed with microcapsules use high amounts (around 4-7% in volume), which can be detrimental to other concrete properties, such as strength.

Located capsules do not need to resist the mixing process since they are tubes or capsules placed in the moulds before pouring the concrete (in a similar way to the reinforcement). However, they still need to endure the impacts produced during pouring. Effective use of located encapsulation to produce self-healing requires crack prediction. This method has high potential, since many authors using adhesive-type products as healing agents encapsulated in tubes achieved excellent recoveries, including mechanical regain (8383. Van Tittelboom, K.; De Belie, N.; Van Loo, D.; Jacobs, P. (2011) Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent. Cem. Concr. Compos. 33 [4], 497-505. https://doi.org/10.1016/j.cemconcomp.2011.01.004.). However, in some cases, the healing agent released from the tube has been reported as being only a small fraction of the volume embedded (102102. Joseph, C.; Jefferson, A. D.; Isaacs, B.; Lark, R.; Gardner, D. (2010) Experimental investigation of adhesive-based self-healing of cementitious materials. Mag. Concr. Res. 62 [11], 831-843. https://doi.org/10.1680/macr.2010.62.11.831. ). The viscosity of the healing agent, the diameter of the tubes, tube slenderness, and the reaction speed will be critical for an adequate release of the agent (8383. Van Tittelboom, K.; De Belie, N.; Van Loo, D.; Jacobs, P. (2011) Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent. Cem. Concr. Compos. 33 [4], 497-505. https://doi.org/10.1016/j.cemconcomp.2011.01.004., 102102. Joseph, C.; Jefferson, A. D.; Isaacs, B.; Lark, R.; Gardner, D. (2010) Experimental investigation of adhesive-based self-healing of cementitious materials. Mag. Concr. Res. 62 [11], 831-843. https://doi.org/10.1680/macr.2010.62.11.831. , 103103. Dry, C. (1994) Matrix cracking repair and filling using active and passive modes for smart timed release of chemicals from fibers into cement matrices. Smart Mater. Struct. 3 [2], 118-123. https://doi.org/10.1088/0964-1726/3/2/006.).

Vascular networks are a concept similar to located tubes (104-107104. Davies, R.; Teall, O.; Pilegis, M.; Kanellopoulos, A.; Sharma, T.; Jefferson, A.; et al. (2018) Large scale application of self-healing concrete: Design, construction, and testing. Front. Mater. 5. https://doi.org/10.3389/fmats.2018.00051.
105. Teall, O.; Davies, R.; Pilegis, M.; Kanellopoulos, A.; Sharma, T.; Paine, K.; et al. (2016) Self-healing concrete full-scale site trials. Proc. 11th fib Int. PhD Symp. Civ. Eng. FIB 2016.
106. Li, Z.; Souza, L.R; Litina, C.; Markaki, A.E.; Al-Tabbaa, A. (2019) Feasibility of using 3D printed polyvinyl alcohol (PVA) for creating sef-healing vascular tunnels in cement system. Mater. 12 [23], 3872. https://doi.org/10.3390/ma12233872.
107. Minnebo, P.; Thierens, G.; De Valck, G.; Van Tittelboom, K.; De Belie, N.; Van Hemelrijck, D.; et al. (2017) A novel design of autonomously healed concrete: Towards a vascular healing network. Mater. 10 [1], 49. https://doi.org/10.3390/ma10010049. ), which consists of artificially created channels through which self-healing agents can be pumped inside the concrete element. This system will also answer to predictable cracks but has the advantage of an increase in the amount of material that can be released into the crack. This concept can also be considered a self-healing system even if it is necessary to use a sensor that shows the results to a technician, who would trigger the healing process (108108. Schlangen, E.; Joseph, C. (2013) Modelling of self-healing cementitious materials. In: de Rooij M., Van Tittelboom K., De Belie N., Schlangen E. (eds) Self-Healing Phenomena in Cement-Based Materials. RILEM State-of-the-Art Reports, vol 11, 217-240. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-6624-2_5. ). The difference would lie on the level of intelligence of the system. In contrast, if external material is needed, or the matrix needs to be replaced, the process would not be considered self-healing but repair.

2.4. Evaluation of self-healing

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Several methods have been proposed in the last years for the evaluation of self-healing. Besides the specific method used in each research group, some international efforts have been made towards the standardisation, in the context of RILEM committees (TC 221-SHC active from 2005 to 2013), the HealCON project, finished in 2016, and the COST Action SARCOS (started in 2016). However, until date, there are no standards published to test self-healing in concrete.

Different methodologies have been used to evaluate the improvements in properties produced by self-healing. An in-depth review of the methods to evaluate self-healing was published by COST Action Sarcos (77. Ferrara, L.; Van Mullem, T.; Alonso, M.C.; Antonaci, P.; Borg, R.P.; Cuenca, E.; et al. (2018) Experimental characterization of the self-healing capacity of cement based materials and its effects on the material performance: A state of the art report by COST Action SARCOS WG2. Constr. Build. Mater. 167, 115-142. https://doi.org/10.1016/j.conbuildmat.2018.01.143.). These evaluation methods can be classified depending on the type of property of interest:

Filling of cracks: Crack filling or closure of a crack most straightforward consequence of self-healing, as in the human body is the closure of a wound. The techniques used to evaluate this crack closure include methods to evaluate both, surface cracks, which is the most common method (1717. Roig-Flores, M.; Pirritano, F.; Serna, P.; Ferrara, L. (2016) Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests. Constr. Build. Mater. 114, 447-457. https://doi.org/10.1016/j.conbuildmat.2016.03.196., 3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005., 4343. Jaroenratanapirom, D.; Sahamitmongkol, R. (2011) Self-crack closing ability of mortar with different additives. J. Met. Mater. Miner. 21 [1], 9-17. , 5252. Toledo Filho, R.D.; Ghavami, K.; Sanjuán, M.A.; England G.L. (2005) Free, restrained and drying shrinkage of cement mortar composites reinforced with vegetable fibres. Cem. Concr. Compos. 27 [5], 537-546. https://doi.org/10.1016/j.cemconcomp.2004.09.005.) and internal cracks through CT-scans (2424. Gilabert, F.A.; Van Tittelboom, K.; Van Stappen, J.; Cnudde, V.; De Belie, N.; Van Paepegem, W. (2017) Integral procedure to assess crack filling and mechanical contribution of polymer-based healing agent in encapsulation-based self-healing concrete. Cem. Concr. Compos. 77, 68-80. https://doi.org/10.1016/j.cemconcomp.2016.12.001., 5959. Hong, G.; Choi, S. (2017) Rapid self-sealing of cracks in cementitious materials incorporating superabsorbent polymers. Constr. Build. Mater. 143, 366-375. https://doi.org/10.1016/j.conbuildmat.2017.03.133.).
Transport properties: One of the main objectives of crack healing is to improve the durability of concrete structures. The more important fluids that can enter concrete and that are relevant to durability are water (which may carry aggressive ions) as well as gases, such as carbon dioxide and oxygen. Studies performed in this matter may refer to permeability (flow under a pressure differential) (1111. Edvardsen, C. (1999) Water permeability and autogenous healing of cracks in concrete. ACI Mater J. 96 [4], 448-454. https://doi.org/10.14359/645., 1717. Roig-Flores, M.; Pirritano, F.; Serna, P.; Ferrara, L. (2016) Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests. Constr. Build. Mater. 114, 447-457. https://doi.org/10.1016/j.conbuildmat.2016.03.196., 3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005., 6262. Roig-Flores, M.; Moscato, S.; Serna, P.; Ferrara, L. (2015) Self-healing capability of concrete with crystalline admixtures in different environments. Constr. Build. Mater. 86, 1-11. https://doi.org/10.1016/j.conbuildmat.2015.03.091 , 8383. Van Tittelboom, K.; De Belie, N.; Van Loo, D.; Jacobs, P. (2011) Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent. Cem. Concr. Compos. 33 [4], 497-505. https://doi.org/10.1016/j.cemconcomp.2011.01.004., 100100. Van Tittelboom, K.; Tsangouri, E.; Van Hemelrijck, D.; De Belie, N. (2015) The efficiency of self-healing concrete using alternative manufacturing procedures and more realistic crack patterns. Cem. Concr. Compos. 57, 142-152. https://doi.org/10.1016/j.cemconcomp.2014.12.002.), diffusion (flow under a concentration differential) (4747. Darquennes, A.; Olivier, K.; Benboudjema, F.; Gagné, R. (2016) Self-healing at early-age, a way to improve the chloride resistance of blast-furnace slag cementitious materials. Constr. Build Mater. 113, 1017-1028. https://doi.org/10.1016/j.conbuildmat.2016.03.087., 109109. Ismail, M.; Toumi, A.; François, R.; Gagné, R. (2008) Effect of crack opening on the local diffusion of chloride in cracked mortar samples. Cem. Concr. Res. 38 [8-9], 1106-1111. https://doi.org/10.1016/j.cemconres.2008.03.009.), and sorption (flow caused by capillary movement in the pores open to the environment) (1212. Snoeck, D.; Van den Heede, P.; Van Mullem, T.; De Belie, N. (2018) Water penetration through cracks in self-healing cementitious materials with superabsorbent polymers studied by neutron radiography. Cem. Concr. Res. 113, 86-98. https://doi.org/10.1016/j.cemconres.2018.07.002., 7676. Mao, W.; Litina, C.; Al-Tabbaa, A. (2020) Development and application of novel sodium silicate microcapsule-based self-healing oil well cement. Mater. 13 [2], 456. https://doi.org/10.3390/ma13020456., 110110. Snoeck, D.; Steuperaert, S.; Van Tittelboom, K.; Dubruel, P.; De Belie, N. (2012) Visualization of water penetration in cementitious materials with superabsorbent polymers by means of neutron radiography. Cem. Concr. Res. 42 [8], 1113-1121. https://doi.org/10.1016/j.cemconres.2012.05.005., 111111. Van Belleghem, B.; Montoya, R.; Dewanckele, J.; Van Den Steen, N.; De Graeve, I.; Deconinck, J.; et al. (2016) Capillary water absorption in cracked and uncracked mortar - A comparison between experimental study and finite element analysis. Constr. Build. Mater. 110, 154-162. https://doi.org/10.1016/j.conbuildmat.2016.02.027.).
Mechanical properties: A complete healing process aims to recover also mechanical properties. Flexural (1616. Ferrara, L.; Krelani, V.; Carsana, M. (2014) A “fracture testing” based approach to assess crack healing of concrete with and without crystalline admixtures. Constr. Build. Mater. 68, 535-551. https://doi.org/10.1016/j.conbuildmat.2014.07.008., 7474. Restuccia, L.; Reggio, A.; Ferro, G.A.; Tulliani, J.M. (2017) New self-healing techniques for cement-based materials. Procedia Struct. Integr. 3, 253-260. https://doi.org/10.1016/j.prostr.2017.04.016., 8383. Van Tittelboom, K.; De Belie, N.; Van Loo, D.; Jacobs, P. (2011) Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent. Cem. Concr. Compos. 33 [4], 497-505. https://doi.org/10.1016/j.cemconcomp.2011.01.004.) and tensile tests (112112. Cuenca, E.; Ferrara, L. (2020) Fracture toughness parameters to assess crack healing capacity of fiber reinforced concrete under repeated cracking-healing cycles. Theor. Appl. Fract. Mech. 106, 102468. https://doi.org/10.1016/j.tafmec.2019.102468.) have been proposed with this purpose. Compressive tests have also been used to produce distributed damage (4848. Zhou, Z.H; Li, Z.Q.; Xu, D.J.; Yu, J.H. (2011) Influence of slag and fly ash on the self-healing ability of concrete. Advan. Mater. Research. 306-307, 1020-1023. https://doi.org/10.4028/www.scientific.net/AMR.306-307.1020., 4949. Na, S.H.; Hama, Y.; Taniguchi, M.; Katsura, O.; Sagawa, T.; Zakaria, M. (2012) Experimental investigation on reaction rate and self-healing ability in fly ash blended cement mixtures. J. Adv. Concr. Technol. 10 [7], 240-253. https://doi.org/10.3151/jact.10.240.); however, it is difficult to link directly with a measure of crack healing.

Due to the difficulty of investigating the state of internal cracks, non-destructive tests have also been proposed to evaluate self-healing, since they can give an indirect measure of several properties. Some methods are Resonant Frequency (113-115113. Yang, Y.; Yang, E.H.; Li, V.C. (2011) Autogenous healing of engineered cementitious composites at early age. Cem. Concr. Res. 41 [2], 176-183. https://doi.org/10.1016/j.cemconres.2010.11.002.
114. Yang, Y.; Lepech, M.D.; Yang, E.H.; Li, V.C. (2009) Autogenous healing of engineered cementitious composites under wet-dry cycles. Cem. Concr. Res. 39 [5], 382-390. https://doi.org/10.1016/j.cemconres.2009.01.013.
115. Yildirim, G.; Sahmaran, M.; Ahmed, H.U. (2015) Influence of hydrated lime addition on the self-healing capability of high-volume fly ash incorporated cementitious composites. J. Mater. Civ. Eng. 27 [6]. https://doi.org/10.1061/(ASCE)MT.1943-5533.0001145. ), Ultrasonic Pulse Velocity (116-118116. Zhong, W.; Yao, W. (2008) Influence of damage degree on self-healing of concrete. Constr. Build. Mater. 22 [6], 1137-1142. https://doi.org/10.1016/j.conbuildmat.2007.02.006.
117. Shahid, K.A.; Jaafar, M.F.M.; Yahaya, F.M. (2014) Self-healing behaviour of pre-cracked POFA-concretes in different curing conditions. J. Mech. Eng. Sci. 7 [1], 1227-1235. http://dx.doi.org/10.15282/jmes.7.2014.22.0120.
118. Karaiskos, G.; Tsangouri, E.; Aggelis, D.G.; Van Tittelboom, K.; De Belie, N.; Van Hemelrijck, D. (2016) Performance monitoring of large-scale autonomously healed concrete beams under four-point bending through multiple non-destructive testing methods. Smart Mater. Struct. 25 [5], 055003. https://doi.org/10.1088/0964-1726/25/5/055003. ) and Acoustic Emissions (118-120118. Karaiskos, G.; Tsangouri, E.; Aggelis, D.G.; Van Tittelboom, K.; De Belie, N.; Van Hemelrijck, D. (2016) Performance monitoring of large-scale autonomously healed concrete beams under four-point bending through multiple non-destructive testing methods. Smart Mater. Struct. 25 [5], 055003. https://doi.org/10.1088/0964-1726/25/5/055003.
119. Granger, S.; Loukili, A.; Pijaudier-Cabot, G.; Chanvillard, G. (2007) Experimental characterization of the self-healing of cracks in an ultra high performance cementitious material: Mechanical tests and acoustic emission analysis. Cem. Concr. Res. 37 [4], 519-527. https://doi.org/10.1016/j.cemconres.2006.12.005.
120. Van Tittelboom, K.; De Belie, N.; Lehmann, F.; Grosse, C.U. (2012) Acoustic emission analysis for the quantification of autonomous crack healing in concrete. Constr. Build. Mater. 28 [1], 333-341. https://doi.org/10.1016/j.conbuildmat.2011.08.079. ).

The general methodology used to evaluate the effects of self-healing consists of the stages of: creation of controlled damage in the specimens, measurement of properties before healing (e.g. permeability or crack width), healing and evaluation of the properties after healing. In the case of the study of mechanical regain, the damaging stage is also the step of the study of initial properties. Most authors evaluate self-healing by direct comparison of the property of interest before and after healing (2323. Van Tittelboom, K.; Adesanya, K.; Dubruel, P.; Van Puyvelde, P.; De Belie, N. (2011) Methyl methacrylate as a healing agent for self-healing cementitious materials. Smart Mater. Struct. 20, 125016. http://doi.org/10.1088/0964-1726/20/12/125016., 6262. Roig-Flores, M.; Moscato, S.; Serna, P.; Ferrara, L. (2015) Self-healing capability of concrete with crystalline admixtures in different environments. Constr. Build. Mater. 86, 1-11. https://doi.org/10.1016/j.conbuildmat.2015.03.091 , 6464. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2013) Effect of exposure conditions on self healing behavior of strain hardening cementitious composites incorporating various cementitious materials. Constr. Build. Mater. 42, 217-224. https://doi.org/10.1016/j.conbuildmat.2013.01.012.), which can be enough in those cases where healing produces a significant change in the property (such as changing from being permeable to watertight). On the contrary, accompanying reference specimens are recommended when the changes expected in the property of interest are small, in order to distinguish the healing process from hydration or other processes taking place inside the material. Reference specimens can be cracked accompanying specimens stored in an environment where healing is not activated (such as a humidity chamber) or undamaged specimens undergoing the same healing process as the healing specimens (119119. Granger, S.; Loukili, A.; Pijaudier-Cabot, G.; Chanvillard, G. (2007) Experimental characterization of the self-healing of cracks in an ultra high performance cementitious material: Mechanical tests and acoustic emission analysis. Cem. Concr. Res. 37 [4], 519-527. https://doi.org/10.1016/j.cemconres.2006.12.005., 121121. Hannant, D.J.; Keer, J.G. (1983) Autogenous healing of thin cement based sheets. Cem. Concr. Res. 13 [3], 357-365. https://doi.org/10.1016/0008-8846(83)90035-2.) (Figure 4).

Figure 4. Options for accompanying reference specimens to evaluate self-healing.

2.5. Moving towards greater challenges

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2.5.1. Large scale tests

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Laboratory testing is not enough to validate technologies and to introduce them into the construction industry. Most of the tests performed about self-healing concrete in the literature were performed at the laboratory level, and often in pastes or mortars, but not in concrete. This difference can be of particular importance since the percentage in the volume of cementitious materials and self-healing agents are higher in pastes and mortars than in concrete mixes. However, some research groups worked at the concrete level, and some pilots have been built to evaluate the performance of different self-healing concrete technologies at higher Technology Readiness Levels (TRL).

The first test at a larger scale demonstrating a self-healing technique was performed by the end of the 90s, in real scale models of a bridge deck (2222. Dry, C.M. (2000) Three designs for the internal release of sealants, adhesives, and waterproofing chemicals into concrete to reduce permeability. Cem. Concr. Res. 30 [12], 1969-1977. https://doi.org/10.1016/S0008-8846(00)00415-4.). In that study, four decks were built with different self-healing systems, all based in embedded glass tubes containing adhesive/sealants placed at various fixed locations. In all cases, tubes broke due to the crack formation releasing the self-healing agents. They also detected that after performing additional loadings, an additional release of adhesive occurred and that the adhesives survived for over one year in field conditions.

Self-healing concrete with crystalline admixtures has been used in several projects worldwide in the last decades, such as in Brazil (122122. Silva, E.F.; Moreira, M.; Manzano M.A.R.; Blanco, R. (2016) Case study of permeability-reducing admixture use in anti-flotation slabs: building in Brasilia, Brazil. J. Build Pathol. Rehabil. 2, 1. https://doi.org/10.1007/s41024-016-0014-5.): an anti-floating slab (1200 m³ of concrete), a wave-type coverage (320 m³), or one slab in the building basement and the reform of football Stadium complex. In these cases, visual inspections reported no cracks in the hardened concrete. However, no reports about the effectiveness of self-healing were published.

In 2015, a small part of a water channel in Ecuador was the first field self-healing concrete construction using lightweight aggregates containing a bacterial healing agent (123123. Sierra-Beltran, M.G.; Jonkers, H.M.; Mors, R.M.; Mera-Ortiz, M. (2015) Field application of self-healing concrete with natural fibres as linings for irrigation canals in Ecuador. ICSHM 2015: Proceedings of the 5th International Conference on Self-Healing Materials, Durham, USA, 22-24 June 2015. ). The channel cross-section had a size of 1×1 m and thickness of 100 mm. Five months after casting, no signs of cracking were detected. In 2016 bacterial self-healing concrete was used in two projects (124124. Mors, R.; Jonkers, H.M. (2019) Bacteria-based self-healing concrete: evaluation of full scale demonstrator projects. RILEM Tech. Lett. 40, 138-144. https://doi.org/10.21809/rilemtechlett.2019.93.): the construction of prefabricated parts of a wastewater purification tank and in two walls of a water reservoir. Monitoring in the following two years suggested that the repairs were effective. According to the authors, in the following time (3 years in the first case and one year in the second) both applications had not yet demonstrated possible benefits of using self-healing concrete for the specific application, but no adverse effects were observed.

Recently, in the context of Materials4Life project, six concrete wall panels of size 1.8 m high and 1 m wide with several self-healing technologies were tested at large-scale laboratory testing and onsite trials (104104. Davies, R.; Teall, O.; Pilegis, M.; Kanellopoulos, A.; Sharma, T.; Jefferson, A.; et al. (2018) Large scale application of self-healing concrete: Design, construction, and testing. Front. Mater. 5. https://doi.org/10.3389/fmats.2018.00051. , 105105. Teall, O.; Davies, R.; Pilegis, M.; Kanellopoulos, A.; Sharma, T.; Paine, K.; et al. (2016) Self-healing concrete full-scale site trials. Proc. 11th fib Int. PhD Symp. Civ. Eng. FIB 2016., 125125. Al-Tabbaa, A.; Lark, B.; Paine, K.; Jefferson, T.; Litina, C.; Gardner, D.; et al. (2018) Biomimetic cementitious construction materials for next-generation infrastructure. Proc. Inst. Civ. Eng. - Smart Infrastruct. Constr. 171 [2], 67-76. https://doi.org/10.1680/jsmic.18.00005.). The self-healing systems studied were microcapsules with sodium silicate, shape-memory tendons combined with vascular networks, perlite with embedded bacteria combined with vascular networks, and vascular networks. Sodium silicate was the agent chosen to be delivered by the networks. These panels were cracked by applying loads with a hydraulic jack. Crack width and permeability to air were evaluated after cracking and after healing. Their results showed good self-healing performance for the panel with microcapsules with sodium silicate (crack reduction of 60% in 1 month and permeability recovery of 2.5 orders of magnitude in 6 months). The other systems displayed reduced (but potential) healing capability, but the test allowed the researchers to detect those points that will need improvements of the systems in the subsequent studies to optimise their feasibility (125125. Al-Tabbaa, A.; Lark, B.; Paine, K.; Jefferson, T.; Litina, C.; Gardner, D.; et al. (2018) Biomimetic cementitious construction materials for next-generation infrastructure. Proc. Inst. Civ. Eng. - Smart Infrastruct. Constr. 171 [2], 67-76. https://doi.org/10.1680/jsmic.18.00005.).

In Belgium (126126. Van Mullem, T.; Gruyaert, E.; Caspeele, R.; De Belie, N. (2020) First large scale application with self-healing concrete in belgium: Analysis of the laboratory control tests. Materials. 13 [4], 997. https://doi.org/10.3390/ma13040997. ) a slab of an inspection pit was cast with bacterial self-healing concrete. The element has a quadrilateral section in its top view with its sides measuring 0.37 and 0.25 m, and a width of 30 mm. The element also had traditional top and bottom reinforcements with 12 mm rebars. During the inspection, no signs of cracking were detected. Self-healing efficiency was studied in the accompanying specimens for quality control. Over 90% of healing efficiency was reported in terms of crack closing and water permeability. However, no assessment of the self-healing was performed in the full-size structure.

Other pilot actions are currently being built for the H2020 ReSHEALience project. They are structures being of ultra-high durability concrete based on Ultra-High-Performance Fibre-Reinforced Concrete or Textile-Reinforced Concrete to guarantee very tight crack widths, and with crystalline admixtures as self-healing enhancers (127127. Serna, P.; Lo Monte, F.; Mezquida-Alcaraz, E.J.; Cuenca, E.; Mechtcherine, V.; Reichardt, M.; et al. (2019) Upgrading the concept of UHPFRC for high durability in the cracked state: the concept of ultra high durability concrete (UHDC) in the approach of the H2020 project ReSHEALience. Sustainable Materials Systems and Structures SMSS 2019, Rovinj, Croatia, 20-22 March 2019.). However, this project aims to monitor durability in cracked conditions, and the differentiation of self-healing expressly may not be guaranteed.

Most constructions that introduced self-healing systems verified self-healing through the lack of cracks, showing the difficulty of evaluating self-healing efficiency in real constructions. This difficulty is mainly produced by the fact that to evaluate self-healing, a crack needs to be developed first. Nevertheless, these pilot activities show promising results to add information in the field behaviour of self-healing concrete.

2.5.2 Service life improvements and life cycle analysis

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Self-healing concrete is, in general, more expensive than conventional concrete (some values are discussed in section 3.3). Therefore, it should present benefits that justify its use in structures, such as improved performance, reducing maintenance costs and/or increasing the service life of the structure. Several factors have to be considered to estimate the long-term behaviour and benefits of self-healing concrete in comparison to conventional concrete. Some of these factors are the time needed for the self-healing reaction, time that the self-healing agent will remain active, and its repeatability in the long term, the response in large-scale elements, etcetera.

The improvements in long-term behaviour (mechanical and durability properties) have to be investigated to verify if the extension of service life produced by self-healing is worth the inversion. That quantification is usually based on models on chemical and fluid transport processes, but also on specific aspects that are of interest when modelling self-healing, such as breakage and release of the healing agents in the different systems (128128. Jefferson, T.; Javierre, E.; Freeman, B.; Zaoui, A.; Koenders, E.; Ferrara, L. (2018) Research progress on numerical models for self-healing cementitious materials. Adv. Mater. Interfaces. 5 [17], 1701378. https://doi.org/10.1002/admi.201701378. ).

One recent publication investigated the service life improvement produced by the reduction of chloride diffusion generated by self-healing (129129. Van Belleghem, B.; Van den Heede, P.; Van Tittelboom, K.; De Belie, N. (2017) Quantification of the service life extension and environmental benefit of chloride exposed self-healing concrete. Mater. 10 [1], 5. https://doi.org/10.3390/ma10010005. ), using as a basis a modified version of Fick’s second law of diffusion. This study reported that the application of self-healing concrete with polyurethane encapsulated in embedded glass capsules could reduce the chloride concentration in a cracked zone by 75%. As a consequence of this, the service life of steel-reinforced concrete slabs in marine environments could amount to 60-94 years, as opposed to 7 years for ordinary cracked concrete. Another work with crystalline admixture (2% of cement weight) in concrete obtained a reduction up to 30% in chloride ion penetration, which could increase up to 34% the structure service life (130130. Helene, P.; Guignone, G.; Vieira, G.; Roncetti, L.; Moroni, F. (2018) Evaluation of the chloride penetration and service life of self-healing concretes activated by crystalline catalyst. Rev. IBRACON Estrut. Mater. 11 [3], 544-563. https://doi.org/10.1590/s1983-41952018000300007. ).

Another step to analyse the benefits of using self-healing concrete would be to perform life cycle assessments (LCA). In (131131. Van den Heede, P.; Mignon, A.; Habert, G.; De Belie, N. (2018) Cradle-to-gate life cycle assessment of self-healing engineered cementitious composite with in-house developed (semi-)synthetic superabsorbent polymers. Cem. Concr. Compos. 94, 166-180. https://doi.org/10.1016/j.cemconcomp.2018.08.017.) a self-healing slab made of engineered cementitious composites (ECC) with SAPs was compared to a traditional solution of steel-reinforced concrete slab suffering 300 μm wide and 25 mm deep cracks, and its associated repair actions, located in exposure class XS2. Their cradle-to-gate life cycle assessment showed that the self-healing slab reported lower impacts compared to those of a traditional concrete slab considering the required cover replacements. Similarly, LCA calculations in (129129. Van Belleghem, B.; Van den Heede, P.; Van Tittelboom, K.; De Belie, N. (2017) Quantification of the service life extension and environmental benefit of chloride exposed self-healing concrete. Mater. 10 [1], 5. https://doi.org/10.3390/ma10010005. ) indicated 56%-75% of environmental benefits when using encapsulated polyurethane as compared to ordinary cracked concrete. The reduction of repair actions principally produced these benefits.

Despite the promising results of these studies, the publications that cover this topic are only a few, and all of them are very recent. The authors believe that more research and discussion is needed to have a factual basis for the quantification of the potential long-term improvements that can be obtained through self-healing concrete.

3. CRITICAL DISCUSSION ABOUT THE CURRENT SITUATION

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Self-healing concrete is a topic in continuous improvement. Many investigations have sought to understand and enhance the autogenous capacity of concrete and to design new techniques to achieve this property through autonomous healing agents.

Ideally, a complete self-healing material would be able to heal itself infinite times and to recover its initial properties perfectly (132132. van der Zwaag, S. (2007) An introduction to material design principles: damage prevention versus damage management. In: van der Zwaag S. (eds) Self Healing Materials. Springer Series in Materials Science, vol 100, 1-18. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-6250-6_1. ). Van der Zwaag also indicates that current materials are closer to “minimal” self-healing materials than to “complete” or ideal self-healing materials. An “ideal” self-healing material not only should heal completely damage of any size, every time it is needed, it should also have similar or superior properties to current materials.

In the case of concrete, most self-healing designs can be considered to be at the halfway point between ideal and minimal self-healing materials (probably closer to minimal). Thus, it would be necessary to evaluate the convenience of the additional initial cost or to accept future repairs.

In this section, the authors expose a critical discussion, regarding the maturity level of the self-healing technologies for concrete as well as discussing, which elements and situations have interesting potential.

3.1. Maturity level of the technology

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Technology readiness level (TRL) is an indicator for estimating the maturity of technologies and follows a numerical scale from 1 (lowest score) to 9 (higher score).

In the case of self-healing concrete, most of the developed techniques reported in the literature were proven by experimental tests under ideal laboratory conditions (TRL 4) and some of them under relevant environments (TRL 5). Only a few technologies have been tested in upscaled tests, that is, pilots or demonstrators, with TRL levels between 6-7 depending on if the environment is relevant (TRL 6) or operating (TRL 7). Higher TRLs correspond to the commercial levels; these levels are not reached in the field of self-healing concrete. Significant research has also been performed in TRL levels 1-3, to test and prove concepts, and studies at this basic level should still be promoted to verify novel ideas or technologies that produce a more efficient self-healing concrete. The authors are considering as a requirement of a system to reach TRL 8-9 to have used its self-healing properties in an operational structure successfully. In the case of methods to evaluate self-healing, high TRL would mean that there are international standards that can be implemented in some labs and that construction sites could hire laboratories for a report using standards.

Figure 5 displays the TRL ranges of different technologies related to self-healing concrete, as interpreted by the authors, which has been performed for scientific discussion purposes.

Figure 5. TRL ranges for the self-healing agents, introduction methods and properties evaluated (light and dark colours: min and max values, respectively).

Regarding the self-healing agents:

Cement and Supplementary Cementitious Materials have demonstrated in operational environments that autogenous healing exists and, under certain conditions, seal cracks (99. De Belie, N.; Gruyaert, E.; Al-Tabbaa, A.; Antonaci, P.; Baera, C.; Bajare, D.; et al. (2018) A review of self-healing concrete for damage management of structures. Adv. Mater. Inter. 5 [17], 1800074. https://doi.org/10.1002/admi.201800074. , 4444. Hearn, N.; Morley, C.T. (1997) Self-sealing property of concrete - Experimental evidence. Mater. Struct. 30, 404-411. https://doi.org/10.1007/BF02498563.). However, they have not been used on purpose to produce controlled self-healing.
CA are commercially available and have shown their potential as waterproofing admixture (depending on the mix) (133133. Mohammadreza Hassani, E.; Vessalas, K.; Sirivivatnanon, V.; Baweja, D. (2017) Influence of permeability-reducing admixtures on water penetration in concrete. ACI Mater. J. 114 [6], 911-922. https://doi.org/10.14359/51701002.), with some criticisms due to the extent of the improvements (134134. Pazderka, J.; Hájková, E. (2016) Crystalline admixtures and their effect on selected properties of concrete. Acta Polytech. 56 [4], 306-311. https://doi.org/10.14311/AP.2016.56.0306. ), and has been used in several constructions. Improving self-healing is still in a lower TRL level, currently validated in relevant environments (16-1816. Ferrara, L.; Krelani, V.; Carsana, M. (2014) A “fracture testing” based approach to assess crack healing of concrete with and without crystalline admixtures. Constr. Build. Mater. 68, 535-551. https://doi.org/10.1016/j.conbuildmat.2014.07.008.
17. Roig-Flores, M.; Pirritano, F.; Serna, P.; Ferrara, L. (2016) Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests. Constr. Build. Mater. 114, 447-457. https://doi.org/10.1016/j.conbuildmat.2016.03.196.
18. Cuenca, E.; Tejedor, A.; Ferrara, L. (2018) A methodology to assess crack-sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles. Constr. Build. Mater. 179, 619-632. https://doi.org/10.1016/j.conbuildmat.2018.05.261. ).
Bacteria concept has been validated in laboratory and field pilots (123123. Sierra-Beltran, M.G.; Jonkers, H.M.; Mors, R.M.; Mera-Ortiz, M. (2015) Field application of self-healing concrete with natural fibres as linings for irrigation canals in Ecuador. ICSHM 2015: Proceedings of the 5th International Conference on Self-Healing Materials, Durham, USA, 22-24 June 2015. , 124124. Mors, R.; Jonkers, H.M. (2019) Bacteria-based self-healing concrete: evaluation of full scale demonstrator projects. RILEM Tech. Lett. 40, 138-144. https://doi.org/10.21809/rilemtechlett.2019.93.), but the products are not generally produced at large scales (9696. Da Silva, F. B.; De Belie, N.; Boon, N.; Verstraete, W. (2015) Production of non-axenic ureolytic spores for self-healing concrete applications. Constr. Build. Mater. 93, 1034-1041. https://doi.org/10.1016/j.conbuildmat.2015.05.049., 9797. Silva, F.B.; Boon, N.; De Belie, N.; Verstraete, W. (2015) Industrial application of biological self-healing concrete: Challenges and economical feasibility. J. Commer. Biotechnol. 21 [1], 31-38. https://doi.org/10.5912/jcb662.). Currently, bacteria production is limited, which complicates its application in concrete structures, but its production may improve in the next years.
Encapsulated silicates (19-2119. Kanellopoulos, A.; Giannaros, P.; Palmer, D.; Kerr, A.; Al-Tabbaa, A. (2017) Polymeric microcapsules with switchable mechanical properties for self-healing concrete: synthesis, characterisation and proof of concept. Smart. Mater. Struct. 26, 045025. https://doi.org/10.1088/1361-665X/aa516c.
20. Beglarigale, A.; Seki, Y.; Demir, N.Y.; Yazıcı, H. (2018) Sodium silicate/polyurethane microcapsules used for self-healing in cementitious materials: Monomer optimization, characterization, and fracture behavior. Constr. Build. Mater. 162, 57-64. https://doi.org/10.1016/j.conbuildmat.2017.11.164.
21. Al-Tabbaa, A.; Litina, C.; Giannaros, P.; Kanellopoulos, A.; Souza, L. (2019) First UK field application and performance of microcapsule-based self-healing concrete. Constr. Build. Mater. 208, 669-685. https://doi.org/10.1016/j.conbuildmat.2019.02.178. ) and adhesives (22-2422. Dry, C.M. (2000) Three designs for the internal release of sealants, adhesives, and waterproofing chemicals into concrete to reduce permeability. Cem. Concr. Res. 30 [12], 1969-1977. https://doi.org/10.1016/S0008-8846(00)00415-4.
23. Van Tittelboom, K.; Adesanya, K.; Dubruel, P.; Van Puyvelde, P.; De Belie, N. (2011) Methyl methacrylate as a healing agent for self-healing cementitious materials. Smart Mater. Struct. 20, 125016. http://doi.org/10.1088/0964-1726/20/12/125016.
24. Gilabert, F.A.; Van Tittelboom, K.; Van Stappen, J.; Cnudde, V.; De Belie, N.; Van Paepegem, W. (2017) Integral procedure to assess crack filling and mechanical contribution of polymer-based healing agent in encapsulation-based self-healing concrete. Cem. Concr. Compos. 77, 68-80. https://doi.org/10.1016/j.cemconcomp.2016.12.001. ) have shown excellent results in onsite trials (104104. Davies, R.; Teall, O.; Pilegis, M.; Kanellopoulos, A.; Sharma, T.; Jefferson, A.; et al. (2018) Large scale application of self-healing concrete: Design, construction, and testing. Front. Mater. 5. https://doi.org/10.3389/fmats.2018.00051. , 105105. Teall, O.; Davies, R.; Pilegis, M.; Kanellopoulos, A.; Sharma, T.; Paine, K.; et al. (2016) Self-healing concrete full-scale site trials. Proc. 11th fib Int. PhD Symp. Civ. Eng. FIB 2016., 125125. Al-Tabbaa, A.; Lark, B.; Paine, K.; Jefferson, T.; Litina, C.; Gardner, D.; et al. (2018) Biomimetic cementitious construction materials for next-generation infrastructure. Proc. Inst. Civ. Eng. - Smart Infrastruct. Constr. 171 [2], 67-76. https://doi.org/10.1680/jsmic.18.00005.). However, these agents are usually prepared in the laboratory (8181. Anglani, G.; Tulliani, J-M.; Antonaci, P. (2020) Behaviour of pre-cracked self-healing cementitious materials under static and cyclic loading. Mater. 13 [5], 1149. https://doi.org/10.3390/ma13051149.), and in the case of being produced industrially, they are still at developing stages. Therefore, no commercial information is readily available, and their feasibility in the construction field is very limited.
SAPs have been validated in laboratory conditions (12-1512. Snoeck, D.; Van den Heede, P.; Van Mullem, T.; De Belie, N. (2018) Water penetration through cracks in self-healing cementitious materials with superabsorbent polymers studied by neutron radiography. Cem. Concr. Res. 113, 86-98. https://doi.org/10.1016/j.cemconres.2018.07.002.
13. Sun, B.; Wu, H.; Song, W.; Li, Z.; Yu, J. (2019) Design methodology and mechanical properties of Superabsorbent Polymer (SAP) cement-based materials. Constr. Build. Mater. 204, 440-449. https://doi.org/10.1016/j.conbuildmat.2019.01.206.
14. He, Z.; Shen, A.; Guo, Y.; Lyu, Z.; Li, D.; Qin, X.; et al. (2019) Cement-based materials modified with superabsorbent polymers: A review. Constr. Build. Mater. 225, 569-590. https://doi.org/10.1016/j.conbuildmat.2019.07.139.
15. Mignon, A.; De Belie, N.; Dubruel, P.; Van Vlierberghe, S. (2019) Superabsorbent polymers: A review on the characteristics and applications of synthetic, polysaccharide-based, semi-synthetic and ‘smart’ derivatives. Eur. Polym. J. 117, 165-178. https://doi.org/10.1016/j.eurpolymj.2019.04.054. ); however, no results are available in large scale elements yet.

Regarding the introduction methods, agents of direct incorporation (CA in (122122. Silva, E.F.; Moreira, M.; Manzano M.A.R.; Blanco, R. (2016) Case study of permeability-reducing admixture use in anti-flotation slabs: building in Brasilia, Brazil. J. Build Pathol. Rehabil. 2, 1. https://doi.org/10.1007/s41024-016-0014-5.)), in porous aggregates (123123. Sierra-Beltran, M.G.; Jonkers, H.M.; Mors, R.M.; Mera-Ortiz, M. (2015) Field application of self-healing concrete with natural fibres as linings for irrigation canals in Ecuador. ICSHM 2015: Proceedings of the 5th International Conference on Self-Healing Materials, Durham, USA, 22-24 June 2015. ), and, very recently inside glass tubes (126126. Van Mullem, T.; Gruyaert, E.; Caspeele, R.; De Belie, N. (2020) First large scale application with self-healing concrete in belgium: Analysis of the laboratory control tests. Materials. 13 [4], 997. https://doi.org/10.3390/ma13040997. ), have been used in operational environments. Therefore, the efficiency of self-healing in this point has not been enough evaluated. The rest of the systems reach the level of system demonstration in relevant environments since they have been tested in pilots (104104. Davies, R.; Teall, O.; Pilegis, M.; Kanellopoulos, A.; Sharma, T.; Jefferson, A.; et al. (2018) Large scale application of self-healing concrete: Design, construction, and testing. Front. Mater. 5. https://doi.org/10.3389/fmats.2018.00051. , 105105. Teall, O.; Davies, R.; Pilegis, M.; Kanellopoulos, A.; Sharma, T.; Paine, K.; et al. (2016) Self-healing concrete full-scale site trials. Proc. 11th fib Int. PhD Symp. Civ. Eng. FIB 2016., 125125. Al-Tabbaa, A.; Lark, B.; Paine, K.; Jefferson, T.; Litina, C.; Gardner, D.; et al. (2018) Biomimetic cementitious construction materials for next-generation infrastructure. Proc. Inst. Civ. Eng. - Smart Infrastruct. Constr. 171 [2], 67-76. https://doi.org/10.1680/jsmic.18.00005., 127127. Serna, P.; Lo Monte, F.; Mezquida-Alcaraz, E.J.; Cuenca, E.; Mechtcherine, V.; Reichardt, M.; et al. (2019) Upgrading the concept of UHPFRC for high durability in the cracked state: the concept of ultra high durability concrete (UHDC) in the approach of the H2020 project ReSHEALience. Sustainable Materials Systems and Structures SMSS 2019, Rovinj, Croatia, 20-22 March 2019.).

Regarding the evaluation properties, there are no standards, and in full-scale constructions, self-healing has validated only visually (122122. Silva, E.F.; Moreira, M.; Manzano M.A.R.; Blanco, R. (2016) Case study of permeability-reducing admixture use in anti-flotation slabs: building in Brasilia, Brazil. J. Build Pathol. Rehabil. 2, 1. https://doi.org/10.1007/s41024-016-0014-5., 123123. Sierra-Beltran, M.G.; Jonkers, H.M.; Mors, R.M.; Mera-Ortiz, M. (2015) Field application of self-healing concrete with natural fibres as linings for irrigation canals in Ecuador. ICSHM 2015: Proceedings of the 5th International Conference on Self-Healing Materials, Durham, USA, 22-24 June 2015. ). At laboratory and pilot conditions (104104. Davies, R.; Teall, O.; Pilegis, M.; Kanellopoulos, A.; Sharma, T.; Jefferson, A.; et al. (2018) Large scale application of self-healing concrete: Design, construction, and testing. Front. Mater. 5. https://doi.org/10.3389/fmats.2018.00051. , 105105. Teall, O.; Davies, R.; Pilegis, M.; Kanellopoulos, A.; Sharma, T.; Paine, K.; et al. (2016) Self-healing concrete full-scale site trials. Proc. 11th fib Int. PhD Symp. Civ. Eng. FIB 2016., 125125. Al-Tabbaa, A.; Lark, B.; Paine, K.; Jefferson, T.; Litina, C.; Gardner, D.; et al. (2018) Biomimetic cementitious construction materials for next-generation infrastructure. Proc. Inst. Civ. Eng. - Smart Infrastruct. Constr. 171 [2], 67-76. https://doi.org/10.1680/jsmic.18.00005., 127127. Serna, P.; Lo Monte, F.; Mezquida-Alcaraz, E.J.; Cuenca, E.; Mechtcherine, V.; Reichardt, M.; et al. (2019) Upgrading the concept of UHPFRC for high durability in the cracked state: the concept of ultra high durability concrete (UHDC) in the approach of the H2020 project ReSHEALience. Sustainable Materials Systems and Structures SMSS 2019, Rovinj, Croatia, 20-22 March 2019.), some transport properties and mechanical tests have been validated. Six interlaboratory testing programs are being developed in the framework of the COST Action SARCOS. One program has already been finished (135135. Van Mullem, T.; Anglani, G.; Dudek, M.; Vanoutrive, H.; Bumanis, G.; Litina, C.; et al. (2020) Addressing the need for standardization of test methods for self-healing concrete : an inter-laboratory study on concrete with macrocapsules. Sci. Technol. Advanc. Mater. 21 [1], 661-682. https://doi.org/10.1080/14686996.2020.1814117. ), and the six laboratories involved obtained comparable sealing efficiencies, highlighting the potential of the methods used for further standardisation. However, further research is needed to obtain standard methods as well as accurate theoretical models, and, in a later stage, to evaluate the improved expected life span.

3.2. What is self-healing concrete good for?

⌅

In this section, the authors discuss several aspects of interest to discuss the potential of self-healing concrete, highlighting aspects that are still missing in current developments.

a) In what type of concrete element?

In the opinion of the authors, the use of self-healing concrete conceptually makes much sense for reinforced or prestressed concrete, since reinforcement is needed where the concrete matrix may work in cracked conditions. Self-healing of these cracks may protect from the entrance of aggressive agents, such as water or chlorides, towards the reinforcement or delaying carbonation (and thus, de-passivation). Structures with water-tightness requirements are also a potential niche for self-healing concrete since it could ensure the functional requirements of the structure (44. Van Tittelboom, K.; De Belie, N. (2013) Self-healing in cementitious materials-A review. Mater. 6 [6], 2182-2217. https://doi.org/10.3390/ma6062182., 136136. Schlangen, E.; Sangadji, S. (2013) Addressing infrastructure durability and sustainability by self healing mechanisms - Recent advances in self healing concrete and asphalt. Proce. Engin. 54, 39-57. https://doi.org/10.1016/j.proeng.2013.03.005. ).

Due to the importance of durability in reinforced concrete structures, some structures in aggressive exposure conditions are dimensioned following the condition of crack control for durability purposes instead of fulfilling the mechanical requirements solely. This condition represents an over-cost in the structure and could even represent an additional constructive difficulty. Therefore, there would be a potential benefit if self-healing concrete guarantees an improvement in durability.

b) For cracks at what construction stage?

Cracks can be produced in concrete during execution, during service conditions or after suffering accidental actions.

At the execution stage, adverse exposure conditions of the element and inadequate curing may produce cracks, such as those produced by shrinkage. A market study (137137. Gardner, D.; Lark, R.; Jefferson, T.; Davies, R. (2018) A survey on problems encountered in current concrete construction and the potential benefits of self-healing cementitious materials. Case Stud. Constr. Mater. 8, 238-247. https://doi.org/10.1016/j.cscm.2018.02.002.) indicated the presence of cracks was the main problem reported in constructions, with a total of 90% of the cases. Additionally, it reports that they considered poor execution to have led to cracking. This study also reports that in 73% of the cases, the problems were caused by water ingress. Unexpected construction cracks should be avoided entirely. The authors consider that investing in a better quality of the element and the construction process will be more cost-effective than investing in self-healing concrete as a system to avoid construction cracks.

In the case of concrete structures working under service conditions, codes on the design of reinforced concrete structures stipulate a maximum allowable width for surface cracks. These allowed cracks can have a range between 0.1 and 0.3 mm depending on the environment and the structure. This range of cracks is where most self-healing systems can produce efficient healing (1717. Roig-Flores, M.; Pirritano, F.; Serna, P.; Ferrara, L. (2016) Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests. Constr. Build. Mater. 114, 447-457. https://doi.org/10.1016/j.conbuildmat.2016.03.196., 3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005., 4343. Jaroenratanapirom, D.; Sahamitmongkol, R. (2011) Self-crack closing ability of mortar with different additives. J. Met. Mater. Miner. 21 [1], 9-17. , 6262. Roig-Flores, M.; Moscato, S.; Serna, P.; Ferrara, L. (2015) Self-healing capability of concrete with crystalline admixtures in different environments. Constr. Build. Mater. 86, 1-11. https://doi.org/10.1016/j.conbuildmat.2015.03.091 , 6464. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2013) Effect of exposure conditions on self healing behavior of strain hardening cementitious composites incorporating various cementitious materials. Constr. Build. Mater. 42, 217-224. https://doi.org/10.1016/j.conbuildmat.2013.01.012., 7575. Sidiq, A.; Gravina, R.J.; Setunge, S.; Giustozzi, F. (2019) Microstructural analysis of healing efficiency in highly durable concrete. Constr. Build. Mater. 215, 969-983. https://doi.org/10.1016/j.conbuildmat.2019.04.233., 7878. Selvarajoo, T.; Davies, R.E.; Freeman, B.L.; Jefferson, A.D. (2020) Mechanical response of a vascular self-healing cementitious material system under varying loading conditions. Constr. Build. Mater. 254, 119245. https://doi.org/10.1016/j.conbuildmat.2020.119245., 8080. Hu, Z.X.; Hu, X.M.; Cheng, W.M.; Zhao, Y.Y.; Wu, M.Y. (2018) Performance optimization of one-component polyurethane healing agent for self-healing concrete. Constr. Build. Mater. 179, 151-159. https://doi.org/10.1016/j.conbuildmat.2018.05.199.) and then, where most of the potential impact can be produced.

In some applications, cracks are predictable in location and difficult or expensive to eliminate. This situation happens in half-joints in precast concrete structures, or in certain elements sensitive to shrinkage. In this case, the authors believe that the use of located macro capsules can be a good alternative to increasing reinforcement content for some cases. This case was explored in the bridge deck prototype cracked by shrinkage, which had embedded tubes and reported the sealing of the crack (2222. Dry, C.M. (2000) Three designs for the internal release of sealants, adhesives, and waterproofing chemicals into concrete to reduce permeability. Cem. Concr. Res. 30 [12], 1969-1977. https://doi.org/10.1016/S0008-8846(00)00415-4.). When diffuse cracking is expected, and their location is not predictable, technologies based on distributed products (such as microcapsules or CA) would be more efficient.

Self-healing systems designed to heal damage produced by accidental or not frequent actions are difficult to justify economically and conceptually. In structures that experienced extreme actions that can be under extremely damaged conditions, repairing the element usually is not a priority, because the focus is usually given to maintain the stability of the structure during enough time to ensure people’s safety. Afterwards, these structures are repaired or retrofitted, but sometimes, the solution chosen is demolition and reconstruction of the damaged element.

c) For which environment?

Some self-healing agents do not work effectively under certain environments, such as SAPs in marine environments (5555. Lee, H.X.D.; Wong, H.S.; Buenfeld, N.R. (2010) Potential of superabsorbent polymer for self-sealing cracks in concrete. Adv. Appl. Ceram. 109 [5], 296-302. https://doi.org/10.1179/174367609X459559., 6161. Snoeck, D.; Van Tittelboom, K.; Steuperaert, S.; Dubruel, P.; De Belie, N. (2014) Self-healing cementitious materials by the combination of microfibres and superabsorbent polymers. J. Intell. Mater. Syst. Struct. 25 [1], 13-24. https://doi.org/10.1177/1045389X12438623.) or CA in environments without direct contact with water (1717. Roig-Flores, M.; Pirritano, F.; Serna, P.; Ferrara, L. (2016) Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests. Constr. Build. Mater. 114, 447-457. https://doi.org/10.1016/j.conbuildmat.2016.03.196.). Systems based on reactive adhesives can activate with different systems, moisture (7878. Selvarajoo, T.; Davies, R.E.; Freeman, B.L.; Jefferson, A.D. (2020) Mechanical response of a vascular self-healing cementitious material system under varying loading conditions. Constr. Build. Mater. 254, 119245. https://doi.org/10.1016/j.conbuildmat.2020.119245., 7979. Du, W.; Yu, J.; Gu, Y.; Li, Y.; Han, X.; Liu, Q. (2019) Preparation and application of microcapsules containing toluene-di-isocyanate for self-healing of concrete. Constr. Build. Mater. 202, 762-769. https://doi.org/10.1016/j.conbuildmat.2019.01.007.), light (7777. Lv, L.; Guo, P.; Liu, G.; Han, N.; Xing, F. (2020) Light induced self-healing in concrete using novel cementitious capsules containing UV curable adhesive. Cem. Concr. Compos. 105, 103445. https://doi.org/10.1016/j.cemconcomp.2019.103445.), high temperature (2222. Dry, C.M. (2000) Three designs for the internal release of sealants, adhesives, and waterproofing chemicals into concrete to reduce permeability. Cem. Concr. Res. 30 [12], 1969-1977. https://doi.org/10.1016/S0008-8846(00)00415-4.), or with their activator component (2424. Gilabert, F.A.; Van Tittelboom, K.; Van Stappen, J.; Cnudde, V.; De Belie, N.; Van Paepegem, W. (2017) Integral procedure to assess crack filling and mechanical contribution of polymer-based healing agent in encapsulation-based self-healing concrete. Cem. Concr. Compos. 77, 68-80. https://doi.org/10.1016/j.cemconcomp.2016.12.001., 8383. Van Tittelboom, K.; De Belie, N.; Van Loo, D.; Jacobs, P. (2011) Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent. Cem. Concr. Compos. 33 [4], 497-505. https://doi.org/10.1016/j.cemconcomp.2011.01.004.). For those self-healing systems that are activated with water, the presence of water can become at the same time a mechanism that transports aggressive agents that start some degradation phenomena as well as the activator for self-healing reactions, in a kind of love-hate relationship (138138. Neville, A. (2000) Water and concrete: a love-hate relationship. Concr. Int. 22 [12], 34-38. ). Each self-healing system has different optimal conditions for their reactions, and the system should be chosen depending on the specific situation.

d) What type of recovery?

Crack closing and the recovery of water tightness has been widely reported, such as in (1111. Edvardsen, C. (1999) Water permeability and autogenous healing of cracks in concrete. ACI Mater J. 96 [4], 448-454. https://doi.org/10.14359/645., 1717. Roig-Flores, M.; Pirritano, F.; Serna, P.; Ferrara, L. (2016) Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests. Constr. Build. Mater. 114, 447-457. https://doi.org/10.1016/j.conbuildmat.2016.03.196., 1818. Cuenca, E.; Tejedor, A.; Ferrara, L. (2018) A methodology to assess crack-sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles. Constr. Build. Mater. 179, 619-632. https://doi.org/10.1016/j.conbuildmat.2018.05.261., 3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005., 110110. Snoeck, D.; Steuperaert, S.; Van Tittelboom, K.; Dubruel, P.; De Belie, N. (2012) Visualization of water penetration in cementitious materials with superabsorbent polymers by means of neutron radiography. Cem. Concr. Res. 42 [8], 1113-1121. https://doi.org/10.1016/j.cemconres.2012.05.005.). However, consistent recovery of mechanical properties has only been reported when using embedded tubes with adhesives as self-healing agents, such as in (8181. Anglani, G.; Tulliani, J-M.; Antonaci, P. (2020) Behaviour of pre-cracked self-healing cementitious materials under static and cyclic loading. Mater. 13 [5], 1149. https://doi.org/10.3390/ma13051149., 8383. Van Tittelboom, K.; De Belie, N.; Van Loo, D.; Jacobs, P. (2011) Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent. Cem. Concr. Compos. 33 [4], 497-505. https://doi.org/10.1016/j.cemconcomp.2011.01.004.). It should be mentioned that visual closing does not imply necessarily improvements in other properties like durability or mechanical recovery (3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005.).

Reinforced concrete structures are designed considering cracked conditions in the service state; in fact, deflections are evaluated considering the cracked concrete stiffness. The contribution of concrete tensile strength is usually neglected and is only considered through its influence on tension stiffening. Given this premise, self-healing concrete thinking on the mechanical recovery of the cracked zones does not seem a promising concept. The authors consider that the highest potential for self-healing technologies is to recover durability-related properties. However, mechanical properties could be of high interest with the purpose of controlling crack opening rather than the purpose of mechanical regain of the structure itself.

Water tightness recovery may produce a reduction in the rate of deterioration of concrete and the reinforcement, and to produce an increase in the service life of the structure. Only a few studies are quantifying the improvements in durability and service life, such as (129129. Van Belleghem, B.; Van den Heede, P.; Van Tittelboom, K.; De Belie, N. (2017) Quantification of the service life extension and environmental benefit of chloride exposed self-healing concrete. Mater. 10 [1], 5. https://doi.org/10.3390/ma10010005. , 130130. Helene, P.; Guignone, G.; Vieira, G.; Roncetti, L.; Moroni, F. (2018) Evaluation of the chloride penetration and service life of self-healing concretes activated by crystalline catalyst. Rev. IBRACON Estrut. Mater. 11 [3], 544-563. https://doi.org/10.1590/s1983-41952018000300007. ). However, in the opinion of the authors, this is a key point, and improvements in terms of long-term behaviour need to be investigated to verify if the extension of service life produced by self-healing is worth the money inversion.

e) In-depth formation of the healing by-product and bond with the matrix

Some points related with the formation of the self-healing by-product are not enough developed in the opinion of the authors: such achieving self-healing in-depth, characterisation of the properties of the healing by-product (compacity, density, brittleness) and the bond between the healing by-product and the concrete matrix.

Several studies suggest that autogenous healing and some healing promoters are producing only surface blocking (3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005., 139139. Roig-Flores, M. (2018) Self-healing concrete : efficiency evaluation and enhancement with crystalline admixtures. Universitat Politècnica de València - PhD Thesis. https://doi.org/10.4995/Thesis/10251/100082.). It should be mentioned here that most studies reporting crack closing were performed analysing surface cracks. If complete self-healing is achieved in all the crack depth, it will be more likely that blocking of transport agents through the crack will be more effective. Similarly, healing by-products of higher density will provide better protection from the entrance of aggressive agents. Some options that could happen in the self-healing process of a crack are displayed in Figure 6.

Figure 6. Different levels of self-healing that can be produced in concrete.

A strong bond between both crack faces and the filling materials will allow the distribution of stresses in a larger contact area. Thus, assessing and improving the bond between the old matrix and the filling material would be a potential improvement step to obtain consistent durability improvements as well as a preliminary step before being able to recover effectively mechanical properties either completely or partially.

Another goal for self-healing concrete is to keep the self-healing products attached to the crack wall to maintain the crack closed. If the filling material can endure cracks’ movements (due to repeated loads or new actions), self-healing will be more efficient. To obtain this, healing products that allow plastic deformation may produce a benefit over rigid products.

f) Can current limits of crack opening be changed?

Crack limits stipulated in reinforced concrete structure codes depend on the environmental condition of exposure and the cover of the primary reinforcement. The size of allowed crack widths can reach up to 0.10 mm for highly aggressive environments, up to 0.20 mm for moderately aggressive environments, and up to 0.30 mm for non-aggressive environments. This limit is more restrictive for prestressed concrete, not allowing cracks nor decompression of the element.

In an element loaded with combined flexural and compression stresses, the crack will be developed in the tensioned layer and will be V-shaped, with the maximum opening in the most tensioned surface (11. Damgaard Jensen, A.; Chatterji, S. (1996) State of the art report on micro-cracking and lifetime of concrete - Part 1. Mater. Struct. 29 [1], 3-8. https://doi.org/10.1007/BF02486001. ). When the codes limit values of allowable crack width, the value is established assuming that at the reinforcement level, crack width will be even more reduced. For example, a depth of penetration of about 1 mm can be assumed for a crack with a surface opening of 10 μm (11. Damgaard Jensen, A.; Chatterji, S. (1996) State of the art report on micro-cracking and lifetime of concrete - Part 1. Mater. Struct. 29 [1], 3-8. https://doi.org/10.1007/BF02486001. ). There are still some unclear points regarding crack propagation and its internal geometry as well as regarding the durability degradation produced in concrete with small micro-cracks. Length, depth of penetration, density of microcracks and interconnectivity are other crack parameters considered of relevance that have not been widely studied in the literature (11. Damgaard Jensen, A.; Chatterji, S. (1996) State of the art report on micro-cracking and lifetime of concrete - Part 1. Mater. Struct. 29 [1], 3-8. https://doi.org/10.1007/BF02486001. ).

What is clear is that with the presence cracks, it is a matter of time that concrete suffers the degradation produced by the aggressive agents that access the matrix. If these cracks reach the reinforcement level, corrosion of the reinforcement will be accelerated. Therefore, it is essential to control the formation and propagation of cracks and, if possible, heal them as quickly as possible. In this way, the velocity of the healing reaction becomes of high importance.

A typical value for obtaining efficient self-healing effectiveness is one month if produced by mineral additions or crystalline admixtures (1717. Roig-Flores, M.; Pirritano, F.; Serna, P.; Ferrara, L. (2016) Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests. Constr. Build. Mater. 114, 447-457. https://doi.org/10.1016/j.conbuildmat.2016.03.196., 3333. Sisomphon, K.; Copuroglu, O.; Koenders, E.A.B. (2012) Self-healing of surface cracks in mortars with expansive additive and crystalline additive. Cem. Concr. Compos. 34 [4], 566-574. https://doi.org/10.1016/j.cemconcomp.2012.01.005.). During that time, no significant amount of aggressive agents should have entered the cracked matrix (neither to have reached the reinforcement). The velocity of the reaction is one of the critical advantages of using some resins as a self-healing agent, which in a matter of hours can seal the cracks efficiently (8080. Hu, Z.X.; Hu, X.M.; Cheng, W.M.; Zhao, Y.Y.; Wu, M.Y. (2018) Performance optimization of one-component polyurethane healing agent for self-healing concrete. Constr. Build. Mater. 179, 151-159. https://doi.org/10.1016/j.conbuildmat.2018.05.199., 8282. Perez, G.; Erkizia, E.; Gaitero, J.J.; Kaltzakorta, I.; Jiménez, I.; Guerrero, A. (2015) Synthesis and characterization of epoxy encapsulating silica microcapsules and amine functionalized silica nanoparticles for development of an innovative self-healing concrete. Mater. Chem. Phys. 165, 39-48. https://doi.org/10.1016/j.matchemphys.2015.08.047.), or SAPs (1212. Snoeck, D.; Van den Heede, P.; Van Mullem, T.; De Belie, N. (2018) Water penetration through cracks in self-healing cementitious materials with superabsorbent polymers studied by neutron radiography. Cem. Concr. Res. 113, 86-98. https://doi.org/10.1016/j.cemconres.2018.07.002., 5959. Hong, G.; Choi, S. (2017) Rapid self-sealing of cracks in cementitious materials incorporating superabsorbent polymers. Constr. Build. Mater. 143, 366-375. https://doi.org/10.1016/j.conbuildmat.2017.03.133.), which expand in few minutes.

Successful self-healing concrete designs open a discussion on the current limits of crack opening. Since self-healing could contribute to the closure of these cracks the codes could be more tolerant in situations of high environmental aggressiveness; or stipulate a longer minimum service life for the structures, if the current limits of crack opening are maintained. Nevertheless, these changes can only be considered if self-healing systems demonstrate their capabilities to improve the durability of cracked elements in the conditions of interest.

3.3. Commercial situation and potential applications

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Currently, the price of self-healing agents can vary a lot depending on several factors, mostly due to the novelty of the topic. The price of bacteria to produce self-healing concrete can vary between 714-5760 € per m³ of concrete (9797. Silva, F.B.; Boon, N.; De Belie, N.; Verstraete, W. (2015) Industrial application of biological self-healing concrete: Challenges and economical feasibility. J. Commer. Biotechnol. 21 [1], 31-38. https://doi.org/10.5912/jcb662.) depending on the production process. The price of crystalline admixtures in the recommended dosages varies between 50-100 € per m³ of concrete since no additional workforce is needed. These values may change substantially in the next years due to the expected new developments and improvements in production techniques.

The cost of self-healing concrete compared to the ordinary concrete is still high given their effectivity, a fact that has limited its application in civil constructions. Using self-healing would reduce the frequency and cost of maintenance during its life cycle, the need for monitoring, inspection, and repair of structures. That would promote greater sustainability, because of the fewer interventions, material resources and energy used, and lower emission of pollutants (140140. Li, V.C.; Herbert, E. (2012) Robust self-healing concrete for sustainable infrastructure. J. Adv. Concr. Technol. 10, [6], 207-218. https://doi.org/10.3151/jact.10.207. ).

One weakness of several methods that produce self-healing is that they also produce a decrease of compressive strength due to the introduction of voids, such as using SAPs (5858. Snoeck, D.; Schaubroeck, D.; Dubruel, P.; De Belie, N. (2014) Effect of high amounts of superabsorbent polymers and additional water on the workability, microstructure and strength of mortars with a water-to-cement ratio of 0.50. Constr. Build. Mater. 72, 148-157. https://doi.org/10.1016/j.conbuildmat.2014.09.012., 5959. Hong, G.; Choi, S. (2017) Rapid self-sealing of cracks in cementitious materials incorporating superabsorbent polymers. Constr. Build. Mater. 143, 366-375. https://doi.org/10.1016/j.conbuildmat.2017.03.133.) or microcapsules (141-143141. Shanmuga Priya, T.; Ramesh, N.; Agarwal, A.; Bhusnur, S.; Chaudhary, K. (2019) Strength and durability characteristics of concrete made by micronized biomass silica and Bacteria-Bacillus sphaericus. Constr. Build. Mater. 226, 827-838. https://doi.org/10.1016/j.conbuildmat.2019.07.172.
142. Nguyen, T.H.; Ghorbel, E.; Fares, H.; Cousture, A. (2019) Bacterial self-healing of concrete and durability assessment. Cem. Concr. Compos. 104, 103340. https://doi.org/10.1016/j.cemconcomp.2019.103340.
143. Siddique, R.; Singh, K.; Kunal, P.; Singh, M.; Corinaldesi, V.; Rajor, A. (2016) Properties of bacterial rice husk ash concrete. Constr. Build. Mater. 121, 112-119. https://doi.org/10.1016/j.conbuildmat.2016.05.146. ). This point should be considered since different contents of these self-healing materials will produce a different extent of self-healing, but also different decreases of compressive strength, and thus, a compromise needs to be reached fitted for each case.

Figure 7 shows a decision-making diagram to evaluate if the use of self-healing systems is of interest for solving a concrete construction problem considering the aforementioned aspects. Some of the questions that the constructor would need to consider are, “What is more cost-effective to use self-healing concrete or…

to improve the quality of a concrete system (such as better-quality concrete or appropriate joints in pavements)?
to increase the amount of reinforcement (to better control cracks)?
to repair the system once a certain damage threshold has been reached?”

A proper study of these alternatives needs a complete analysis, such as a cradle-to-grave analysis, to compare the cost of each alternative.

Current development and costs suggest that the application of self-healing concrete would be justified now only in high demanding applications (144144. Huseien, G.F.; Shah, K.W.; Sam, A.R.M. (2019) Sustainability of nanomaterials based self-healing concrete: An all-inclusive insight. J. Build. Eng. 23, 155-171. https://doi.org/10.1016/j.jobe.2019.01.032.), like tunnels and marine structures, where safety is a major problem, or in structures where the accessibility for repair and maintenance is limited. Other structures with a strict level of safety that may experience some benefits are some bridges, hydroelectric dams, buried reservoirs, retaining walls, raft foundations in contact with water containing chlorides or sulphates. Additionally, in prefabricated elements self-healing can also be beneficial to heal cracks produced due to accelerated production or to heal expected cracks in located points.

Due to the current high cost or low-moderate performance, self-healing concrete seems to be not justified in other structures such as small prefabricated elements or structures in low humidity environments. However, most of the current systems and materials for self-healing concrete are still young technologies, and a significant advance is expected to be achieved in the next years. Not only more efficient self-healing technologies are needed, but also standardisation in methodologies to evaluate self-healing efficiency will be useful to ensure a fair comparison of self-healing agents. All the advances produced in these points, and others mentioned in this review, will allow self-healing technologies to be progressively introduced in the construction field.

Figure 7. A self-healing concrete organisation chart for decision making.

4. CONCLUSIONS

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Autogenous healing of concrete is a natural phenomenon that heals very small cracks (under 0.15 mm), and that is produced by continuing hydration and carbonation. Autogenous healing can be enhanced with specific concrete compositions and by the introduction of supplementary cementitious materials, such as pozzolanas. Autonomous healing of concrete is based on the introduction of specific agents to produce self-healing. The most studied autonomous agents are SAPs, CA, encapsulated sodium silicate or adhesives, and bacteria. Each of these agents has a different functioning basis and different effectiveness under certain environmental conditions.

Dispersed agents have been reported to heal cracks with a small opening (especially at early ages), generally up to 0.30 mm. Cracks larger than 0.3 mm can only be healed efficiently by embedded tubes with adhesives in located positions and bacteria. These two latter systems are also those that showed higher efficiencies, being able to recover some transport and mechanical properties in several studies.

There are no standardised methods to assess the effects of healing on concrete. The methods used in the literature to assess the crack closing are widespread. However, they do not always show the level of internal healing nor the efficiency of the filling products against durability. Techniques for assessing permeability, diffusion, absorption in cracked conditions, as well as the recovery of mechanical properties, are still being discussed. In this sense, the standardisation of the methods to evaluate the self-healing effects is a great task that must be overcome in the near future.

There are still some challenges to overcome, such as producing some healing agents at an industrial scale or the evaluation of self-healing in a structure in operating conditions. Self-healing of concrete is still practically in laboratory scale, except for some remarkable reduced-scale pilots. These pilots demonstrated the potential of self-healing concrete, especially in the case of microencapsulated sodium silicate. Some self-healing agents have been used in high-volume constructions, especially CA, and bacteria. However, their self-healing effectiveness was not verified, and only the absence of cracks was verified. The authors believe that more experimental tests in larger-scale elements are necessary to evaluate the self-healing capabilities better, mainly thinking in durability properties, and covering the most diverse environmental conditions.

In the authors’ opinion, the use of self-healing concrete has potential for reinforced structural concrete elements, where the concrete matrix works under cracked conditions. Self-healing of these cracks can produce recovery of water tightness and protect against the entrance of aggressive agents such as chlorides, sulphates, or CO₂. This increased protection can improve the service life of the concrete structure, and thus, self-healing has a high potential in terms of durability recovery. Once enough efficiency is demonstrated, they could even modify allowable crack widths from the concrete structural codes.

Nowadays, self-healing concrete has a higher cost than conventional concrete, and its application is justified only in cases with high safety requirements, such as tunnels or marine structures, or in structures where accessibility for repair and maintenance is limited. However, their potential applications are likely to be widened in the next years, as long as self-healing agents are improved and developed at larger industrial scales. If the weak points that have been discussed throughout this review, were successfully developed in the upcoming years, self-healing systems could be a pillar for obtaining more durable reinforced concrete structures.