Recycled concrete aggregates (RCAs) have been demonstrated as an alternative source to exhaustible natural aggregates. In this paper, we focus on the enhancement treatments for coarse and fine RCAs and the effect of these treatments on the durability of recycled concrete (RC) made from treated RCAs by the weakening and strengthening of the cement paste adhered to their surface. We conducted comparative analyses on the improvement of durability and mechanical properties. The results showed that the reduction of water absorption in RCAs increases the compressive strength of RC and that there is a strong linear trend of improvement in the carbonation depth, chloride ion penetration, and compressive strength of RC. For fine and coarse RCAs, the treatment mechanism and particle size are determinants in efficiently improving RC durability.
El uso de áridos de hormigón reciclado (RCA) ha demostrado ser una fuente alternativa a los áridos naturales agotables. Este trabajo se centra en los tratamientos de mejora de los RCA, tanto gruesos como finos, y su efecto en la durabilidad del hormigón reciclado (RC) a partir de tratamientos por debilitamiento y fortalecimiento de la pasta de cemento adherida a su superficie de los RCA. Se llevaron a cabo análisis comparativos sobre la mejora de la durabilidad y las propiedades mecánicas, permitiendo identificar que la reducción de la absorción de agua de los RCA conduce a un aumento en la resistencia a la compresión del RC, y que existe una fuerte tendencia lineal de mejora de la profundidad de carbonatación, la penetración de iones cloruro y la resistencia a la compresión, donde el mecanismo de tratamiento y el tamaño de partícula son determinantes para lograr una mejora eficiente de la durabilidad del RC.
The demand of the construction industry regarding urban development is associated with the increasing use of building materials. Concrete is the most widely used artificial material in the world; thus, abundant construction and demolition concrete waste is produced (
Several studies (
Previous studies have mainly focused on the mechanical and microstructural behavior of RC made with treated RCAs. Studies have investigated the effect of each treatment on the durability of RC. Therefore, there is still a long road ahead in terms of the relationship between the mechanical properties and the durability performance of concretes made with treated RCA. In this paper, we review the previous results of the existing improvement treatments (weakening or strengthening of the residual bonded cement layer to the coarse and fine RCAs) and their effect on the durability of RC made with treated RCA. We construct graphs that provide a comparative analysis to illustrate the effect of each treatment.
This study is expected to contribute to the selection of improvement treatments effective enough to achieve enhanced replacement of NA in concrete and mortar mixes. The work was conducted following a systematic literature review, where 117 research articles from 2003 to 2022 were reviewed, using Scopus, ScienceDirect, and SpringerLink as sources.
The use of RCA in concrete has been limited due to its high water absorption, elevated porosity, and poor microstructure properties, especially at the interface transition zone (ITZ) between a new cement matrix and RCAs (ITZ1) and a new cement matrix and the old cement paste surrounding natural RCAs (ITZ2) (
Cement paste bonded to the surface of RCAs has been reported to be extremely porous (up to 20 times higher than that of NA), which depends on the volume of cement paste bonded to the RCA (30). Thus, the RCA exposed to moisture can absorb more water than NA. Therefore, water absorption and the degree of porosity have been indicated as key parameters in determining the mixing performance, mechanical performance, and durability of RC made from RCA (
In this context, several methods have been proposed to treat the cracks resulting from the transformation processes, and the cement paste adhered to the surface of RCAs. Treatments have been reported from chemical, thermal, and mechanical processes (
Treatment1 | In-depth studied2 | |
---|---|---|
Fine RCA | Coarse RCA | |
Soaking | +++ | +++ |
CO2 curing | ++ | +++ |
Coating | ++ | +++ |
Biodeposition | + | +++ |
Mixing/Rehydrating | + | ++ |
Heating | + | ++ |
Rehydrating | − | +++ |
Coating/Heating | − | ++ |
Mixing/Coating | − | ++ |
Heating | − | ++ |
Crushing | − | + |
Cleaning/Heating | − | + |
Soaking/ Impregnating | − | + |
Grinding | + | − |
Heating/Grinding | + | − |
Heating/Rehydrating | + | − |
1 All treatments are referenced by groups in
2 (+++): highly studied; (++): moderately studied; (+): lowly studied; (−): not studied.
The treatments can be classified into two groups. The first group focuses on weakening the residual bonded cement layer in the RCAs, and the second group focuses on strengthening the bonded cement layer and cracked surface of the RCAs, which has demonstrated a densification of the structure and an increase in their physical and mechanical performances.
Group | Treatment type | Approach/Products | Decreased |
Compressive strength |
RCA replacement |
w/b | |
---|---|---|---|---|---|---|---|
|
|
( |
Dehydration of cement hydration products (endothermic reactions) |
[3-60] | [1.9-2.3] | 100 | [0.34-0.5] |
|
( |
Mechanical actions, generating thermal and mechanical stress difference |
[0.3-32] | [10-17] | 100 | 0.50 | |
|
( |
Ultrasonic or water rubbing to remove the attached cement paste |
[0-50] | [0-29] | 100 | 0.45 | |
|
( |
Hydrochloric acid (HCl) Sulfuric acid (H2SO4)
Phosphoric acid (H3PO4)
Acetic acid (C2H4O2):
Al2O3 + C2H4O2
|
[6-40] | [1.3-25] | [25-100] | [0.38-0.5] | |
|
|
( |
Carbonation |
[6-32] | [6-33] | 100 | [0.48-0.6] |
|
( |
Sporosarcina pasteurii bacteria |
[15-20] | [20-35] | 100 | [0.4-0.5] | |
|
( |
Silane emulsion Pozzolans (
Nanomaterials
|
[15-84] | [0-58] | [25-100] | [0.38-0.50] | |
|
( |
Mixing in two or three stages, and the addition of pozzolanic materials (silica fume and blast furnace slag).
|
[32-8] | [6-45] | [20-100] | [0.4-0.55] | |
|
( |
Diammonium hydrogen phosphate -DAP Sodium silicate
Tanic acid |
[4-79] | [1.3-25] | [15-100] | [0.38-0.50] |
Considering that the improvement of the RCAs is mainly evidenced by the decrease in porosity, the results in
In this study, the results of reported treatments are obtained, specifically the percentage decrease in the water absorption of RCAs and the increase in the compressive strength of RC at 28 days, substituting 100% treated and untreated RCAs. The relationship between the water absorption of RCAs and the compressive strength of RC is shown in
Compared to fine RCAs, the improvement in coarse RCAs quality has been more relevant in terms of number of proposed treatments, especially treatments that weaken the cement paste that adheres to the RCA surface (see
For coarse RCAs treated with weakening methods (
For coarse RCAs treated with strengthening methods (
Finally, some treatments have reportedly shown no significant effect on compressive strength. Other studies with rehydration (FA) and coating (silane) have reported adverse effects in which compressive strength decreases. Thus, the study conditions should be investigated in detail to establish the feasibility of their application.
The scenario for fine RCA is promising with treatments that strengthen the cement layer attached to their surface (
Concrete durability is related to the serviceability and structural requirements over its expected life (
The effect of high water absorption, which affects the water/cement (w/c) ratio and hydration processes (
The weak microstructure due to all ITZs present in an RC (
The properties related to RC durability have been extensively studied (
In RC, an increase in the volume of cement paste adhered to the RCA surface and the number of cracks or fissures increases the porosity, promoting the transport of external agents and thus reducing mechanical and durability performances (
According to the results reported by D. Pedro et al. (2017) and D. Brito (2016), fine RCAs affect RC permeability more than coarse RCAs due to the increased number of capillary channels in the concrete system. Conversely, the superior size of coarse RCAs results in a reduced surface area, thus reducing the amount of water required and increasing RC permeability (
In contrast, related to the effect of fine RCA content on RC permeability, Basheer et al. (
The resistance of RC to carbonation is observed to be poorer than that of conventional concrete due to its porosity (
(a) RC with coarse RCAs. (b) RC with fine RCAs.
For coarse RCAs, the applied treatments showed two tendencies (
Regarding fine RCAs, which are widely used in practice, a reduction in mortar and concrete strength has been reported; therefore, replacing fine RCAs by not more than 30% has been proposed (
Chloride ion migration has been shown to be a determining factor in concrete durability, especially because it promotes the corrosion of steel in reinforced concrete (
The last two regions show an improvement in both chloride ion penetrability and compressive strength, where we observe that the SF mixing/rehydrating, SF coating, and nanomaterial coating treatments produce a mostly significant improvement. The improved reduction in chloride permeability for the SF and nanomaterial treatments is mainly due to the pore-filling effect of SF, owing to its small particle size compared with other supplementary materials. Additionally, Faysal et al. (
For fine RCAs ( (a) RC with coarse RCAs. (b) RM with fine RCAs.
Regarding the durability of RC from treated fine and coarse RCAs, it is relevant to establish the relationship between chloride penetration coefficient and carbonation depth with the compressive strength (28 days of curing), from which the effect of the treatments on RC permeability (directly related to porosity and weak ITZ) was identified. In both cases, a strong linear trend exists between durability and mechanical strength in most treatments. That is, an increase in compressive strength leads to a decrease in carbonation depth and chloride ion penetration; thus, the absorption results in the treated RCA strongly correlate with the durability results because a considerable improvement in compressive strength is obtained when the degree of absorption in the RCA is reduced. Thus, pozzolan coating and CO2 curing treatments are the most studied treatments regarding durability, where the former improves the bond between the RCA and the new mortar, achieving a stronger ITZ, and the latter has a pore-filling effect on the RCA surface to prevent the transport of aggressive agents, thus improving the durability of the material.
When comparing the weakening and strengthening treatments, a continuous improvement in the mechanical strength of RC made with RCAs improved by strengthening treatments was observed. This is consistent with the literature, reporting about six times more effectiveness for fine RCA and about two times more effectiveness for coarse RCA. The significant improvement in the RCA quality is mainly achieved from a balance between the type of treatment and the size of the RCA, as both the pore structure and the RCA surface are modified, which is noted in the strengthening of the hydration product reactions and in the creation of new products that allow the pore structure to be refined. Conversely, compared to fine RCA, the improvement in the quality of coarse RCAs has been more common, especially with the treatments that weaken the cement paste bonded to the surface. This result is explained by the fact that an increase in particle size eases the removal of the cement paste. For fine RCAs, the particle size complicates the weakening action so that strengthening treatments improve the surface and pore structure. In this sense, strengthening treatments are recommended for fine RCAs, and weakening treatments should be complemented with some strengthening treatments for coarse RCAs.
The study results validate the importance of the water absorption of the RCA in the mixture design of an RC since all mixes need to be corrected for moisture to achieve the design w/c ratio. Therefore, the different treatments in the RCA reduce the water absorption percentage, reducing the error associated with the moisture setting and, in turn, improving mechanical and durability performance. For coarse RCAs, the 30% decrease in water absorption for pozzolan coating is the optimal treatment, increasing compressive strength by over 50%. For fine RCAs, although CO2 treatment achieves a superior absorption reduction, the improvement in the concrete strength of pozzolan-treated RCAs is superior, suggesting further research on the combination of both treatments.
This review paper has presented a summary of RCA treatments and their effect on some durability properties of RC. The results will help future research to advance in this field. Although the durability performance of RC has been found to be generally inferior to that of conventional concrete, a possible improvement is identified by applying treatments based on strengthening the cement paste attached to the RCA surface.
Performance of RC made from RCA improved under hybrid methods.
Effect of treatments on different particle sizes of RCA and shredding types.
Environmental and cost assessment of RCA treatments.
Long-term behavior of steel corrosion in RC made from treated RCA and modification of the microstructure.
Regulations governing the incorporation of properly treated RCA.
This work was supported by [Universidad Nacional de Colombia] under Grant [HERMES research project 50091].
Conceptualization: Y.P. Arias-Jaramillo. Data curation: D. Gómez-Cano. Formal analysis: D. Gómez-Cano, Y.P. Arias-Jaramillo. Funding acquisition: R. Bernal. Investigation: D. Gómez-Cano. Methodology: J.I. Tobón. Project administration: R. Bernal. Supervision: J.I. Tobón. Validation: Y.P. Arias-Jaramillo. Visualization: R. Bernal. Writing, original draft: D. Gómez-Cano. Writing, review & editing: D. Gómez-Cano, J.I. Tobón.