The use of recycled aggregates in the construction sector: a scientific bibliometric analysis

2.2.1. Bibliometric performance analysis

In particular, bibliometric performance studies use several bibliometric indicators to analyse and quantify the influence of diverse aspects relating to scientific productivity (5555. Gutiérrez-Salcedo, M.; Martínez, M.A.; Moral-Munoz, J.A.; Herrera-Viedma, E.; Cobo Martín, M.J. (2018) Some bibliometric procedures for analyzing and evaluating research fields. Appl. Intell. 48 [5], 1275-1287. https://doi.org/10.1007/s10489-017-1105-y.
, 6464. Hirsch, J.E. (2005) An index to quantify an individual´s scientific research output. Proc Natl. Acad. Sci. 102 [46], 16569-16572. https://doi.org/10.1073/pnas.0507655102.
, 6565. Martínez Sánchez, M.A.; Herrera Díaz, M.; Lima Fernández, A.I.; Herrera Gómez, M.; Herrera-Viedma, E. (2014) Un análisis bibliométrico de la producción académica española en la categoría de Trabajo Social del “Journal Citation Report”. Cuader. Trab. Soc. 27 [2], 429-438. https://doi.org/10.5209/rev_CUTS.2014.v27.n2.44662.
). It should be noted that bibliometric indicators objectively and numerically assess the production, activity and quality or impact in the field of science and technology, using scientific knowledge published. These indicators are grouped into three main categories (6363. Torres-salinas, D. (2007) Diseño de un sistema de información y evaluación científica. Análisis ciencimétrico de la actividad investigadora de la Universidad de Navarra en el área de ciencias de la salud. 1999-2005. Tesis Doctoral. 396. Retrieved from http://eprints.rclis.org/10545/1/Tesis_Daniel_Torres.pdf.
): (i) production indicators, which measure the amount of results or publication count (number of publications or percentage of works indexed in a repository); (ii) visibility and impact indicators, either based on the number of citations received by documents (h-index (6464. Hirsch, J.E. (2005) An index to quantify an individual´s scientific research output. Proc Natl. Acad. Sci. 102 [46], 16569-16572. https://doi.org/10.1073/pnas.0507655102.
), g-index (6666. Eggue, L. (2006) Theory and practise of the g-index. Scientometrics. 69, [1] 131-152. Retireved from https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.395.9064&rep=rep1&type=pdf.
), hg-index (6767. Alonso, S.; Cabrerizo, F.J.; Herrera-Viedma, E.; Herrera, F. (2010) hg-index: A new index to characterize the scientific output of researchers based on the h- and g-indices. Scientometrics. 82 [2], 391-400. https://doi.org/10.1007/s11192-009-0047-5.
), q²-index (6868. Cabrerizo, F.J.; Alonso, S.; Herrera-Viedma, E.; Herrera, F. (2010) q2-Index: Quantitative and qualitative evaluation based on the number and impact of papers in the Hirsch core. J. Informetr. 4 [1], 23-28. Retrieved from http://hdl.handle.net/10481/5679.
), etc.) or by journal in which they were published (Impact Factor of Journal Citation Reports - JCR); and (iii) collaboration indicators that measure the collaboration of the authors or institutions in scientific production (co-authorship index, similarity measures, etc.).

The bibliometric analysis carried out in this research evaluated the performance of the field “Applications of recycled aggregate in the construction sector” from an objective and quantitative perspective, taking into account the scientific output produced by authors and journals; in addition, the highly cited papers in this research field have been studied.

2.2.2. Bibliometric content analysis

Other types of bibliometric indicators are known as relational indicators, which generate graphic scientific representations through the use of relational information (3434. Cobo-Martín, M.J. (2011) SciMAT: Herramienta software para el análisis de la evolución del conocimiento científico. propuesta de una metodología de evaluación. Universidad de Granada. 2011. Retrieved from http://hdl.handle.net/10481/20201.
). In this case, scientific mapping has been developed using VOSviewer and SciMAT, which are considered two of the most widely used tools in scientific articles (55-5755. Gutiérrez-Salcedo, M.; Martínez, M.A.; Moral-Munoz, J.A.; Herrera-Viedma, E.; Cobo Martín, M.J. (2018) Some bibliometric procedures for analyzing and evaluating research fields. Appl. Intell. 48 [5], 1275-1287. https://doi.org/10.1007/s10489-017-1105-y.
56. Castillo-Vergara, M.; Alvarez-Marin, A.; Placencio-Hidalgo, D. (2018) A bibliometric analysis of creativity in the field of business economics. J. Bus. Res. 85, 1-9. https://doi.org/10.1016/j.jbusres.2017.12.011.
57. Hosseini, M.R.; Martek, I.; Zavadskas, E.K.; Aibinu, A.A.; Arashpour, M.; Chileshe, N. (2018) Critical evaluation of off-site construction research: A Scientometric analysis. Autom. Constr. 87, 235-247. https://doi.org/10.1016/j.autcon.2017.12.002.
). The most important facets of the analysis of scientific mapping have been described below.

2.2.2.1. VOSviewer

VOSviewer software (version 1.6.15) is a free-to-use tool developed by the Center for Science and Technology Studies of Leiden University (Netherlands) in 2010. It has been designed for constructing and visualising bibliometric networks through the use of the VOS mapping technique, where VOS stands for visualisation of similarities (3535. van Eck, N.J.; Waltman, L. (2010) Software survey: VOSviewer, a computer program for bibliometric mapping. Scientometrics. 84 [2], 523-538. https://doi.org/10.1007/s11192-009-0146-3.
). The main advantage of this program over most other available bibliometric mapping software is that it focuses on the graphic representation of large maps, due to its powerful graphic interface, which allows examination of the created maps in an intuitive way.

The analysis developed in this study has been aimed at understanding the collaboration among researchers who have worked on RA in construction. To do this, the most cited authors have been considered, i.e. authors with more than three published documents. Based on this information, a two-dimensional map of authors was created based on a co-occurrence data matrix and using the author entity as the analysis unit. According to the indicated methodology (3535. van Eck, N.J.; Waltman, L. (2010) Software survey: VOSviewer, a computer program for bibliometric mapping. Scientometrics. 84 [2], 523-538. https://doi.org/10.1007/s11192-009-0146-3.
), the frequency of occurrence of a particular term is represented by a circle and it is defined by its size on the map, so that the more important the element, the greater the associated circle. Due to an intelligent algorithm at each zoom level, only the most important (or most frequent) elements are displayed, thus avoiding overlap. In addition, VOSviewer selects a different and random colour for each group of elements (according to the strength of association) and the circles in the same group should be displayed in the same colour. Likewise, the distance between two elements is related to their degree of relationship or similarity; in consequence, the shorter distance, the stronger the author’s relationship is.

2.2.2.2. SciMAT

Science Mapping Analysis software Tool - SciMAT (version 1.1.04) is an open source software tool (GPLv3) developed by a group of researchers from the University of Granada (3434. Cobo-Martín, M.J. (2011) SciMAT: Herramienta software para el análisis de la evolución del conocimiento científico. propuesta de una metodología de evaluación. Universidad de Granada. 2011. Retrieved from http://hdl.handle.net/10481/20201.
). SciMAT integrates everything necessary to analyse the scientific maps generated under a longitudinal framework and study the evolution of the conceptual aspects of a research field over consecutive time periods. In addition, this tool improves the maps with the integration of visibility and impact bibliometric measures (citations), such as h-index, g-index, hg-index, q2-index, etc. As result, the conceptual subdomains (particular topics or general thematic areas) of a specific field can be detected and visualised (2929. Cobo Martín, M.J.; Lõpez-Herrera, A.G.; Herrera-Viedma, E.; Herrera, F. (2012) SciMAT: A new science mapping analysis software tool. J. Am. Soc. Inf. Sci. Technol. 63 [8], 1609-1630. https://doi.org/10.1002/asi.22688.
).

On the basis of the methodology developed by Cobo and his collaborators (2929. Cobo Martín, M.J.; Lõpez-Herrera, A.G.; Herrera-Viedma, E.; Herrera, F. (2012) SciMAT: A new science mapping analysis software tool. J. Am. Soc. Inf. Sci. Technol. 63 [8], 1609-1630. https://doi.org/10.1002/asi.22688.
, 3434. Cobo-Martín, M.J. (2011) SciMAT: Herramienta software para el análisis de la evolución del conocimiento científico. propuesta de una metodología de evaluación. Universidad de Granada. 2011. Retrieved from http://hdl.handle.net/10481/20201.
), the applied analysis has been developed according to the following steps:

Finally, the performance measures obtained for the most important themes detected in each period are shown. Among all the quality measures that can be selected in SciMAT, the most commonly used ones were chosen, according to Cobo´s methodology (2828. Cobo Martín, M.J.; López-Herrera, E.; Herrera-Viedma, E.; Herrara, F. (2011) Science mapping software tools: review, analysis, and cooperative study among tools. J. Am. Soc. Inf. Sci. Technol. 62 [7], 1382-1402. https://doi.org/10.1002/asi.21525.
, 2929. Cobo Martín, M.J.; Lõpez-Herrera, A.G.; Herrera-Viedma, E.; Herrera, F. (2012) SciMAT: A new science mapping analysis software tool. J. Am. Soc. Inf. Sci. Technol. 63 [8], 1609-1630. https://doi.org/10.1002/asi.22688.
): sum of citations and h-index. Accordingly, the quantitative values obtained from the analysis were the total number of core documents analysed, the sum of citations received, the corresponding h-index, and values of centrality and density. These bibliometric measures give information about the interest in and impact on the specialised research community of each detected cluster or evolution area (2929. Cobo Martín, M.J.; Lõpez-Herrera, A.G.; Herrera-Viedma, E.; Herrera, F. (2012) SciMAT: A new science mapping analysis software tool. J. Am. Soc. Inf. Sci. Technol. 63 [8], 1609-1630. https://doi.org/10.1002/asi.22688.
).

3.1.1. Article quantification by year and period

Although the first indexed paper in Web of Science and Scopus that published the results of research on applications of RA (“Recycled concrete” by Buck (7272. Buck, A.D. (1973) Recycle concrete. Highw Res Rec. [430], 1-8. Retrieved from https://www.scopus.com/inward/record.uri?eid=2-s2.0-0015560896&partnerID=40&md5=1dad6016cb43cf976ff6a5af3574da05.
)) dates back to 1973, the number of scientific documents published on this subject has grown substantially since 2000, particularly in the last ten years (2009-2019). For this reason, the study has been conducted in 3 periods: before 2000, 2000-2009 and 2010-2019. Figure 3 shows the number of documents published per year (Figure 3a) and for each period, including the sum citations received for documents published by period (Figure 3b). An exponential growth of the number of publications, both per year and per period, has been observed; however, in the case of the number of citations, a lineal growth has been noticed. It can be seen that over 75% of total citations are concentrated in less than 16% of total analysed papers and located in the second period; this is because the assessment of the impact of works through the citations they receive is not an immediate measure, but can only be applied years after the publication of the documents. In 2019, 192 documents were identified with a total of 616 citations; this number of papers represents almost doubling of all documents published in the previous period (112 articles published between 2000 - 2009 have been cited 15174 times). Specifically, 85% of the associated literature with the analysed field was published in the study’s last time period; this leads to the conclusion that this is a literature that is continuing to grow, and whose recognition by the scientific community will increase over time (as the number of citations received increases). This is due to the fact that more attention is paid to this topic in social, economic, and scientific contexts, in an attempt to reduce the use of natural resources, enhance economic benefits, and encourage conscientious respect for the environment (73-7573. Comisión Europea. (2016) Protocolo de gestión de residuos de construcción y demolición en la UE. Com Eur y ECORYS Ref Ares (2016) 6914779 - 12/12/2016. [septiembre] 61.
74. Oliveira Neto, R.; Gastineau, P.; Cazacliu, B.G.; Le Guen, L.; Paranhos, R.S.; Petter, C.O. (2017) An economic analysis of the processing technologies in CDW recycling platforms. Waste Manag. 60, 277-289. http://doi.org/10.1016/j.wasman.2016.08.011.
75. Parlamento Europeo; Consejo de la Unión Europea. (2018) Directiva (UE) 2018/851 del Parlamento europeo y del Consejo de 30 de mayo de 2018 por la que se modifica la Directiva 2008/98/CE sobre los residuos. DOUE. L 150, [14 de junio] 109-140.
). All of this is associated with the approach of the construction sector to the paradigm of sustainable development which has been integral to the European Strategy 2020 (since 2010) to promote a Circular Economy.

3.1.2. Contribution of authors and collaborations

The top 10 authors (from a list of 1828) with the highest contribution to the field studied have been summarised in Table 1. This table also includes the number of documents published, the total and average value of citations received, the average publications per year, the university and the country where they came from, and the author links according to the VOSviewer data. In this ranking, the Portuguese researcher Jorge de Brito leads the list with 110 documents and a total of 5745 citations received; he is followed at an important distance by the Hong Kong-born Chi-Sun Poon and his compatriot Luis Evangelista, with 57 and 35 publications and with 5086 and 2439 citations, respectively. It is important to point out that Ravindra K. Dhir held 10th position on the list, with only 11 articles published in the field under investigation, but received a large number of citations (1286), with an average of 117 citations per document. This author’s impact is associated with collaboration with highly productive scientists, which helps to increase his scientific output, such as by his collaboration with the Portuguese Jorge de Brito. As a consequence, it can be asserted that the measure of the impact of the number of citations is not always related to the quantity of production itself. Likewise, the average period of distribution of the most productive authors’ research time has been estimated between 2011 and 2016, showing a greater concentration of published documents in the third period studied (2010-2019) (Figure 3b).

On the other hand, the “intellectual structure” of the studied field knowledge base has been represented with the help of VOSviewer software. Figure 4 shows the connections between the 45 authors with the most significant associations in this network (with more than seven published articles), distributed into ten clusters with 113 links; and it reveals two predominant different but interrelated schools of thought in the used of RA. The largest group of collaborators is led by the Portuguese researcher Jorge de Brito (blue) who, in turn, shows the relationship with other smaller groups, amoung a total of 23 author links, such as José Ramón Jiménez, Jesús Ayuso and Francisco Agrela (red), Iris Gónzalez-Taboada and Belén González-Fonteboa (purple) or María I. Sánchez de Rojas and Moises Frias, among others (green). The subject investigated by Jorge De Brito and his colleagues focused on the use of RA in a wide variety of applications, including structural concrete (76-7876. Evangelista, L.; De Brito, J. (2007) Mechanical behaviour of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 29 [5], 397-401.
77. Gomes, M.; de Brito, J. (2009) Structural concrete with incorporation of coarse recycled concrete and ceramic aggregates: Durability performance. Mater. Struct. 42 [5], 663-675. https://doi.org/10.1617/s11527-008-9411-9.
78. Evangelista, L.; de Brito, J. (2010) Durability performance of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 32 [1], 9-14. http://doi.org/10.1016/j.cemconcomp.2009.09.005.
), non-structural use (7979. de Brito, J.; Pereira, A.S.; Correia, J.R. (2005) Mechanical behaviour of non-structural concrete made with recycled ceramic aggregates. Cem. Concr. Compos. 27 [4], 429-433. https://doi.org/10.1016/j.cemconcomp.2004.07.005.
, 8080. López-Uceda, A.; Ayuso, J.; Jiménez, J.R.; Agrela, F.; Barbudo, A.; De Brito, J. (2016) Upscaling the use of mixed recycled aggregates in non-structural low cement concrete. Materials. 9 [2], 91. https://doi.org/10.3390/ma9020091.
), self-compacting (81-8381. Carro-López, D.; González-Fonteboa, B.; Martínez-Abella, F.; González-Taboada, I.; de Brito, J.; Varela-Puga, F. (2017) Proportioning, microstructure and fresh properties of self-compacting concrete with recycled sand. Procedia Eng. 171, 645-657. https://doi.org/10.1016/j.proeng.2017.01.401.
82. Esquinas, A.R.; Álvarez, J.I.; Jiménez, J.R.; Fernández, J.M.; de Brito, J. (2018) Durability of self-compacting concrete made with recovery filler from hot-mix asphalt plants. Constr. Build. Mater. 161, 407-419. https://doi.org/10.1016/j.conbuildmat.2017.11.142.
83. Barroqueiro, T.; da Silva, P.R.; de Brito, J. (2019) Fresh-state and mechanical properties of high-performance self-compacting concrete with recycled aggregates from the precast industry. Materials. 12 [21], 3565. https://doi.org/10.3390/ma12213565.
) and high strength concrete (8484. Pedro, D.; de Brito, J.; Evangelista, L. (2018) Durability performance of high-performance concrete made with recycled aggregates, fly ash and densified silica fume. Cem. Concr. Compos. 93, 63-74. https://doi.org/10.1016/j.cemconcomp.2018.07.002.
), mortars (85-8785. Silva, J.; de Brito, J.; Veiga, R. (2009) Incorporation of fine ceramics in mortars. Constr. Build. Mater. 23 [1], 556-564. https://doi.org/10.1016/j.conbuildmat.2007.10.014.
86. Jiménez, J.R.; Ayuso, J.; López, M.; Fernández, J.M.; de Brito, J. (2013) Use of fine recycled aggregates from ceramic waste in masonry mortar manufacturing. Constr. Build. Mater. 40, 679-690. https://doi.org/10.1016/j.conbuildmat.2012.11.036.
87. Silva, J.; de Brito, J.; Veiga, R. (2010) Recycled red-clay ceramic construction and demolition waste for mortars production. J. Mater. Civ. Eng. 22 [3], 236-244. https://doi.org/10.1061/(ASCE)0899-1561(2010)22:3(236).
) and geotechnical applications (2020. Cardoso, R.; Silva, R.V.; Brito, J. de; Dhir, R.K. (2016) Use of recycled aggregates from construction and demolition waste in geotechnical applications: A literature review. Waste Manag. 49, 131-145. https://doi.org/10.1016/j.wasman.2015.12.021.
). The benefits of incorporating different types of chemical (88-9088. Pereira, P.M.; Evangelista, L.; de Brito, J. (2012) The effect of superplasticisers on the workability and compressive strength of concrete made with fine recycled concrete aggregates. Constr. Build. Mater. 28 [1], 722-729. http://doi.org/10.1016/j.conbuildmat.2011.10.050.
89. Barbudo, M.A.; de Brito, J.; Evangelista, L.; Bravo, M.; Agrela, F. (2013) Influence of water-reducing admixtures on the mechanical performance of recycled concrete. J. Clean. Prod. 59, 93-98. http://doi.org/10.1016/j.jclepro.2013.06.022.
90. Cartuxo, F.; de Brito, J.; Evangelista, L.; Jiménez, J.R.; Ledesma, E.F. (2015) Rheological behaviour of concrete made with fine recycled concrete aggregates - Influence of the superplasticizer. Constr. Build. Mater. 89, 36-47. http://doi.org/10.1016/j.conbuildmat.2015.03.119.
) or mineral additives (8484. Pedro, D.; de Brito, J.; Evangelista, L. (2018) Durability performance of high-performance concrete made with recycled aggregates, fly ash and densified silica fume. Cem. Concr. Compos. 93, 63-74. https://doi.org/10.1016/j.cemconcomp.2018.07.002.
, 91-9391. Kurda, R.; de Brito, J.; Silvestre, J.D. (2017) Influence of recycled aggregates and high contents of fly ash on concrete fresh properties. Cem. Concr. Compos. 84, 198-213. https://doi.org/10.1016/j.cemconcomp.2017.09.009.
92. Kurda, R.; de Brito, J.; Silvestre, J.D. (2018) Indirect evaluation of the compressive strength of recycled aggregate concrete with high fly ash ratios. Mag. Concr. Res. 70 [4], 204-16. https://doi.org/10.1680/jmacr.17.00216.
93. Kurda, R.; de Brito, J.; Silvestre, J.D. (2019) Water absorption and electrical resistivity of concrete with recycled concrete aggregates and fly ash. Cem. Concr. Compos. 95, 169-182. https://doi.org/10.1016/j.cemconcomp.2018.10.004.
) were analysed in many of these studies, with the objective of improving the properties of recycled aggregate concrete (RAC). The replacement of natural sand by recycled material has also been a recurring theme for this author and collaborators, both in concrete (7676. Evangelista, L.; De Brito, J. (2007) Mechanical behaviour of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 29 [5], 397-401.
, 8888. Pereira, P.M.; Evangelista, L.; de Brito, J. (2012) The effect of superplasticisers on the workability and compressive strength of concrete made with fine recycled concrete aggregates. Constr. Build. Mater. 28 [1], 722-729. http://doi.org/10.1016/j.conbuildmat.2011.10.050.
, 9494. Evangelista, L.; de Brito, J. (2004) Criteria for the use of fine recycled concrete aggregates in concrete production. Int. RILEM Conf. Use Recycl. Mater. Build. Struct. [November] 503-510.
, 9595. Evangelista, L.; Guedes, M.; de Brito, J.; Ferro, A.C.; Pereira, M.F. (2015) Physical, chemical and mineralogical properties of fine recycled aggregates made from concrete waste. Constr. Build. Mater. 86, 178-188. http://doi.org/10.1016/j.conbuildmat.2015.03.112.
) and in masonry mortars (8585. Silva, J.; de Brito, J.; Veiga, R. (2009) Incorporation of fine ceramics in mortars. Constr. Build. Mater. 23 [1], 556-564. https://doi.org/10.1016/j.conbuildmat.2007.10.014.
, 8686. Jiménez, J.R.; Ayuso, J.; López, M.; Fernández, J.M.; de Brito, J. (2013) Use of fine recycled aggregates from ceramic waste in masonry mortar manufacturing. Constr. Build. Mater. 40, 679-690. https://doi.org/10.1016/j.conbuildmat.2012.11.036.
, 9696. Neno, C.; de Brito, J.; Veiga, R. (2014) Using fine recycled concrete aggregate for mortar production. Mater. Res. 17 [1], 168-77. https://doi.org/10.1590/S1516-14392013005000164.
).

Likewise, the research developed by Chi-Sun Poon’s group (related by means of 12 author links) have also been focused on different applications of RA, such as structural concrete types (9797. Li, W.; Xiao, J.; Shi, C.; Poon, C.S. (2015) Structural behaviour of composite members with recycled aggregate concrete - An overview. Adv. Struct. Eng. 18 [6], 919-938. https://doi.org/10.1260/1369-4332.18.6.919.
, 9898. Zhang, J.; Shi, C.; Li, Y.; Pan, X.; Poon, C-S.; Xie, Z. (2015) Influence of carbonated recycled concrete aggregate on properties of cement mortar. Constr. Build. Mater. 98 [Supplement C], 1-7. https://doi.org/10.1016/j.conbuildmat.2015.08.087.
), self-compacting concrete (9999. Kou, S-C.; Poon, C-S. (2009) Properties of self-compacting concrete prepared with coarse and fine recycled concrete aggregates. Cem. Concr. Compos. 31 [9], 622-627. http://doi.org/10.1016/j.cemconcomp.2009.06.005.
) and high strength concrete (100100. González-Corominas, A.; Etxeberria, M.; Poon, C-S. (2017) Influence of the quality of recycled aggregates on the mechanical and durability properties of high performance concrete. Waste Biomass Valor. 8 [5], 1421-1432.
) or previous concrete using waste glass and RA (101101. Lu, J.X.; Yan, X.; He, P.; Poon, C.S. (2019) Sustainable design of pervious concrete using waste glass and recycled concrete aggregate. J. Clean. Prod. 234, 1102-12. https://doi.org/10.1016/j.jclepro.2019.06.260.
). However, the production of precast concrete using different types of RA has been the most important contribution of this author’s research (102-106102. Poon, C.S.; Kou, S.C.; Lam, L. (2002) Use of recycled aggregates in molded concrete bricks and blocks. Constr. Build. Mater. 16 [5], 281-9. https://doi.org/10.1016/S0950-0618(02)00019-3.
103. Poon, C.S.; Chan, D. (2006) Paving blocks made with recycled concrete aggregate and crushed clay brick. Constr. Build. Mater. 20 [8], 569-577. https://doi.org/10.1016/j.conbuildmat.2005.01.044.
104. Poon, C-S.; Chan, D. (2007) Effects of contaminants on the properties of concrete paving blocks prepared with recycled concrete aggregates. Constr. Build. Mater. 21 [1], 164-75. https://doi.org/10.1016/j.conbuildmat.2005.06.031.
105. Poon, C-S.; Lam, C.S. (2008) The effect of aggregate-to-cement ratio and types of aggregates on the properties of pre-cast concrete blocks. Cem. Concr. Compos. 30 [4], 283-289. https://doi.org/10.1016/j.cemconcomp.2007.10.005.
106. Poon, C-S.; Kou, S-C.; Wan, H. wen; Etxeberria, M. (2009) Properties of concrete blocks prepared with low grade recycled aggregates. Waste Manag. 29 [8], 2369-77. http://doi.org/10.1016/j.wasman.2009.02.018.
). The manufacture of concrete using RA with different humidity (107107. Kou, S-C.; Poon, C-S.; Chan, D. (2004) Properties of steam cured recycled aggregate fly ash concrete. Int. RILEM Conf. use Recycl. Mater. Build. Struct. Barcelona. [1], 590-9.
, 108108. Poon, C.S.; Shui, Z.; Lam, L.; Fok, H.; Kou, S.C. (2004) Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete. Cem. Concr. Res. 34 [1], 31-6. https://doi.org/10.1016/S0008-8846(03)00186-8.
), even, using carbonated RAs to understand their effects on the durability of RAC (109109. Zhan, B.J.; Xuan, D.X.; Zeng, W.; Poon, C.S. (2019) Carbonation treatment of recycled concrete aggregate: Effect on transport properties and steel corrosion of recycled aggregate concrete. Cem. Concr. Compos. 104, 103360. https://doi.org/10.1016/j.cemconcomp.2019.103360.
) or the study of the environmental consequences of the production of RA through the analysis of the life cycle (110110. Hossain, U.; Poon, C-S.; Lo, I.M.C.; Cheng, J.C.P. (2016) Comparative environmental evaluation of aggregate production from recycled waste materials and virgin sources by LCA. Resour. Conserv. Recycl. 109, 67-77. http://doi.org/10.1016/j.resconrec.2016.02.009.
) are other topics addressed by these authors.

At the collaboration level, Jorge De Brito has mainly collaborated with Portuguese authors, such as Luis Evangelista (7676. Evangelista, L.; De Brito, J. (2007) Mechanical behaviour of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 29 [5], 397-401.
, 7878. Evangelista, L.; de Brito, J. (2010) Durability performance of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 32 [1], 9-14. http://doi.org/10.1016/j.cemconcomp.2009.09.005.
, 111111. Fonseca, N.; de Brito, J.; Evangelista, L. (2011) The influence of curing conditions on the mechanical performance of concrete made with recycled concrete waste. Cem. Concr. Compos. 33 [6], 637-643. https://doi.org/10.1016/j.cemconcomp.2011.04.002.
) or Rui V. Silva (8585. Silva, J.; de Brito, J.; Veiga, R. (2009) Incorporation of fine ceramics in mortars. Constr. Build. Mater. 23 [1], 556-564. https://doi.org/10.1016/j.conbuildmat.2007.10.014.
, 112-114112. Silva, R.V.; de Brito, J.; Dhir, R.K. (2014) Properties and composition of recycled aggregates from construction and demolition waste suitable for concrete production. Constr. Build. Mater. 65, 201-217.http://doi.org/10.1016/j.conbuildmat.2014.04.117.
113. Silva, R.V.; de Brito, J.; Dhir, R.K. (2016) Establishing a relationship between modulus of elasticity and compressive strength of recycled aggregate concrete. J. Clean. Prod. 112 [4], 2171-2186. http://doi.org/10.1016/j.jclepro.2015.10.064.
114. Barroqueiro, T.; da Silva, P.R.; de Brito, J. (2019) Fresh-state and mechanical properties of high-performance self-compacting concrete with recycled aggregates from the precast industry. Materials. 12 [21], 3565. http://doi.org/10.3390/ma12213565.
), and he is also linked to Spanish research, such as that of José Ramón Jiménez, Francisco Agrela and Jesús Ayuso group (8080. López-Uceda, A.; Ayuso, J.; Jiménez, J.R.; Agrela, F.; Barbudo, A.; De Brito, J. (2016) Upscaling the use of mixed recycled aggregates in non-structural low cement concrete. Materials. 9 [2], 91. https://doi.org/10.3390/ma9020091.
, 8686. Jiménez, J.R.; Ayuso, J.; López, M.; Fernández, J.M.; de Brito, J. (2013) Use of fine recycled aggregates from ceramic waste in masonry mortar manufacturing. Constr. Build. Mater. 40, 679-690. https://doi.org/10.1016/j.conbuildmat.2012.11.036.
, 115115. Fernández-Ledesma, E.; Jiménez, J.R.; Ayuso, J.; Fernández, J.M.; de Brito, J. (2015) Maximum feasible use of recycled sand from construction and demolition waste for eco-mortar production - Part-I: ceramic masonry waste. J. Clean. Prod. 87, 692-706. https://doi.org/10.1016/j.jclepro.2014.10.084.
) or with Belén González Fonteboa group (1313. González-Fonteboa, B.; Seara-Paz, S.; de Brito, J.; González-Taboada, I.; Martínez-Abella, F.; Vasco Silva, R. (2018) Recycled concrete with coarse recycled aggregate. An overview and analysis. Mater. Construcc. 68 [330], e151. https://doi.org/10.3989/mc.2018.13317.
, 8181. Carro-López, D.; González-Fonteboa, B.; Martínez-Abella, F.; González-Taboada, I.; de Brito, J.; Varela-Puga, F. (2017) Proportioning, microstructure and fresh properties of self-compacting concrete with recycled sand. Procedia Eng. 171, 645-657. https://doi.org/10.1016/j.proeng.2017.01.401.
). In the case of Chi-Sun Poon, he has collaborated with Asian authors, such as Shi-Cong Kou (9999. Kou, S-C.; Poon, C-S. (2009) Properties of self-compacting concrete prepared with coarse and fine recycled concrete aggregates. Cem. Concr. Compos. 31 [9], 622-627. http://doi.org/10.1016/j.cemconcomp.2009.06.005.
, 102102. Poon, C.S.; Kou, S.C.; Lam, L. (2002) Use of recycled aggregates in molded concrete bricks and blocks. Constr. Build. Mater. 16 [5], 281-9. https://doi.org/10.1016/S0950-0618(02)00019-3.
, 116116. Poon, C-S.; Kou, S-C.; Lam, L. (2007) Influence of recycled aggregate on slump and bleeding of fresh concrete. Mater. Struc. 40 [9], 981-988.
) and Jian-Zhuang Xiao (9797. Li, W.; Xiao, J.; Shi, C.; Poon, C.S. (2015) Structural behaviour of composite members with recycled aggregate concrete - An overview. Adv. Struct. Eng. 18 [6], 919-938. https://doi.org/10.1260/1369-4332.18.6.919.
, 117117 117. Xiao, Z.; Ling, T-C.; Kou, S-C.; Wang, Q.Y.; Poon, C-S. (2011) Use of wastes derived from earthquakes for the production of concrete masonry partition wall blocks. Waste Manag. 31 [8], 1859-1866. https://doi.org/10.1016/j.wasman.2011.04.010.
), as well as with the Spaniards Miren Etxeberria (106106. Poon, C-S.; Kou, S-C.; Wan, H. wen; Etxeberria, M. (2009) Properties of concrete blocks prepared with low grade recycled aggregates. Waste Manag. 29 [8], 2369-77. http://doi.org/10.1016/j.wasman.2009.02.018.
, 118118. Kou, S-C.; Poon, C-S.; Etxeberria, M. (2014) Residue strength, water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures. Cem. Concr. Compos. 53, 73-82. http://doi.org/10.1016/j.cemconcomp.2014.06.001.
) and Francisco Agrela (119119. Barbudo, M.A.; Agrela, F.; Ayuso, J.; Jiménez, J.R.; Poon, C-S. (2012) Statistical analysis of recycled aggregates derived from different sources for sub-base applications. Constr. Build. Mater. 28 [1], 129-38. http://doi.org/10.1016/j.conbuildmat.2011.07.035.
).

3.1.3. Distribution of journals

In this study, a total of 843 documents were analysed in the field under investigation; due to its interdisciplinary nature these documents had been published in 219 international journals, conferences and handbooks. The 10 top sources of information (Table 2), in respect of the number of documents published, include approximately 55% of all the articles collected in the database. The Construction and Building Materials journal stands out from the rest with 221 documents published in the research field, 26% of the total of the documents. The Cement and Concrete Composites and Journal of Cleaner Production ranked second and third, respectively with 49 (6%) and 41 (5%) documents from each one. Likewise, most of the journals have been included in the first quartile (Q1) of the following four JCR categories: Multidisciplinary, Construction and Building Technology, Civil Engineering and Materials Science; some of them are also included in the Engineering, Environmental and Environmental Science categories.

In relation to the cites received as an indicator of the impact or visibility, Construction and Building Materials is also the journal with a higher number of citations; a total of 12432 citations and an average value of citations per document of 56 was quantified. In contrast, journals such as Cement and Concrete Research (position 7) and Cement and Concrete Composites (position 2) with a lower number of documents published in the studied field (20 and 49, respectively), have received the next highest numbers of citations (4302 and 4806 citations, respectively - 215 and 98 average citations per document - Table 2). It can be seen that the number of publications and the total number of citations have not been related, it follows that the most prolific sources do not always have the greatest impact in the field of research.

On the other hand, it is very interesting to note that the number of articles published in Construction and Building Materials has increased dramatically in the last period (Table 2). In particular, the most cited documents in this journal were mainly focused on the influence of the mortar-adherence properties of coarse RA (depending on its applications (120120. Sánchez de Juan, M.; Gutiérrez Alaejos, P. (2009) Study on the influence of attached mortar content on the properties of recycled concrete aggregate. Constr. Build. Mater. 23, [2] 872-7. http://doi.org/10.1016/j.conbuildmat.2008.04.012.
)), and the influence of different types of RAs on the microstructure of the interfacial transition zone, besides the implications of the microstructure on the strength development of the RAC (121121. Poon, C-S.; Shui, Z.; Lam, L. (2004) Effect of microstructure of ITZ on compressive strength of concrete prepared with recycled aggregates. Constr. Build. Mater. 18, [6] 461-468. https://doi.org/10.1016/j.conbuildmat.2004.03.005.
). Instead, papers published in Cement and Concrete Composites were fundamentally focused on the effect of the partial or total replacement of natural sand by fine RA on the structural concrete behaviour, in mechanical terms (7676. Evangelista, L.; De Brito, J. (2007) Mechanical behaviour of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 29 [5], 397-401.
), or durability, in the long term (7878. Evangelista, L.; de Brito, J. (2010) Durability performance of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 32 [1], 9-14. http://doi.org/10.1016/j.cemconcomp.2009.09.005.
). Finally, Cement and Concrete Research published high impact scientific papers focusing on the study of the influence of the amount of RA on the mechanical properties of RAC for structural use (122122. Xiao, J.; Li, J.; Zhang, C. (2005) Mechanical properties of recycled aggregate concrete under uniaxial loading. Cem. Concr. Res. 35 [6], 1187-1194. https://doi.org/10.1016/j.cemconres.2004.09.020.
, 123123. Etxeberria, M.; Vázquez-Ramonich, E.; Marí, A.R.; Barra de Oliveira, M. (2007) Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete. Cem. Concr. Res. 37 [5], 735-742. https://doi.org/10.1016/j.cemconres.2007.02.002.
). The effect of partially hydrated residual concrete on the properties of the RAs and those of the concrete manufactured with these aggregates (124124. Katz, A. (2003) Properties of concrete made with recycled aggregate from partially hydrated old concrete. Cem. Concr. Res. 33 [5], 703-711. https://doi.org/10.1016/S0008-8846(02)01033-5.
) was also analysed.

3.1.4. Highly cited papers analysis

The analysis of citations could be used to ascertain the influence of a certain document in a field of research, as well as the importance that the authors have acquired through it. For each period studied, Table 3 includes the most cited publication among the 843 documents analysed by SciMAT.

For the period between 1973 and 1999, research about the use of RA in concrete was led by Torben C. Hansen; in fact his papers are still a scientific reference for researchers. In the first of the most cited articles (125125. Hansen, T.C. (1986) Recycled aggregates and recycled aggregate concrete second state-of-the-art report developments 1945-198. Mater. Struct. 19 [111], 201-46.
) was published a comprehensive and extensive work about the production of RA from mixed debris, its quality and the behavior of the RAC produced, both in the fresh and hardened state; the regulations applicable in different countries were also analysed and the article concluded with a total of 34 recommendations.

In the period 2000-2009, the first of the most cited papers (123123. Etxeberria, M.; Vázquez-Ramonich, E.; Marí, A.R.; Barra de Oliveira, M. (2007) Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete. Cem. Concr. Res. 37 [5], 735-742. https://doi.org/10.1016/j.cemconres.2007.02.002.
) had 730 citations and were published in Cement and Concrete Research by Miren Etxeberria and his collaborators in 2007. The article evaluated the influence of the amount of coarse concrete, the order of materials used in concrete production and the mechanical properties of the RAC, in order to check the numerical models proposed by several researchers.

Finally, during the third period studied (from 2010 to 2019), the document published by Rui Vasco Silva and his colleagues was highlighted (112112. Silva, R.V.; de Brito, J.; Dhir, R.K. (2014) Properties and composition of recycled aggregates from construction and demolition waste suitable for concrete production. Constr. Build. Mater. 65, 201-217.http://doi.org/10.1016/j.conbuildmat.2014.04.117.
). These authors focused on examining the factors affecting the physical, chemical, mechanical, permeation and compositional properties of RAs sourced from C&DW, intended for concrete production. The results obtained allowed producing a practical means of measuring the quality of RAs, which can be used to produce concrete with predictable performance.

The published researches mainly focused on the use of recycled granular materials in concrete applications and how this approach has evolved, in terms of material testing or the quantity and type of fraction replaced. However, knowledge gaps in other areas of the field, such as in applications where there are no restrictions on RA use, such as mortars, non-structural prefabricated concrete or green roofs.

3.2.1. Content analysis. Strategic diagrams

The most significant results obtained after the word analysis for each period studied are then developed and discussed.

3.2.1.1. Period before 2000

The first analysed period (from 1973 to 1999), the strategic diagram shows only two main themes, focusing on two areas: Physical Properties and Recycled Aggregate (Figure 5a). The first one is the motor theme and it refers to the physical properties of the aggregates as well as those of the concrete manufactured with them; its strong centrality and high density (1/1) indicates that it was a highly studied subject and co-appears quite frequently. The most cited articles related to this topic are focused on the investigation of RA properties and the concrete manufactured with this granular material; in consequence they serve as a guide for the production and evaluation of the RA, as well as for the design, manufacture and use of RAC (125-127125. Hansen, T.C. (1986) Recycled aggregates and recycled aggregate concrete second state-of-the-art report developments 1945-198. Mater. Struct. 19 [111], 201-46.
126. Buck, A.D. (1977) Recycled concrete as a source of aggregate. J. Am. Concr. Inst. 74, [5] 212-219.
127. Barra de Oliveira, M.; Vázquez-Ramonich, E. (1996) The influence of retained moisture in aggregates from recycling on the properties of new hardened concrete. Waste Manag. 16 [1-3], 113-117. https://doi.org/10.1016/S0956-053X(96)00033-5.
).

On the other hand, Recycled Aggregate is an issue that is becoming a motor theme and represents the beginning of the use of RA in construction sector applications, which slowly but certainly increasing in popularity. That is why the most cited studies pay special attention to RA research (125125. Hansen, T.C. (1986) Recycled aggregates and recycled aggregate concrete second state-of-the-art report developments 1945-198. Mater. Struct. 19 [111], 201-46.
, 126126. Buck, A.D. (1977) Recycled concrete as a source of aggregate. J. Am. Concr. Inst. 74, [5] 212-219.
, 128128. Hansen, T.C.; Narud, H. (1983) Strength of recycled concrete made from crushed concrete coarse aggregate. Concr. Int. 5 [01], 79-83. Retrieved form https://www.concrete.org/publications/internationalconcreteabstractsportal.aspx?m=details&ID=9140.
), focusing on the importance of RA quality (both physical and mechanical), especially in the strength of the original concrete, in order to obtain RAC of characteristic resistance suitable for a given use.

3.2.1.2. Period 2000-2009

During the second period, it can be seen how the scientific activity of the studied field has evolved, due to its growth compared to previous years in thematic diversity, as well as in the increase in its h-index and in the number of citations received for each new theme (Table 4). The strategic diagram in Figure 6a shows the detected issues, for example Recycling, Construction Materials and Masonry Mortars are well placed and have the largest number of associated documents (107107. Kou, S-C.; Poon, C-S.; Chan, D. (2004) Properties of steam cured recycled aggregate fly ash concrete. Int. RILEM Conf. use Recycl. Mater. Build. Struct. Barcelona. [1], 590-9.
, 3737. Mymoon, M.; Mahendran, S.; Lakshmi-Poorna, R.; Suryakala, S. (2016) Directions in self consolidating concrete research : A bibliometric study. J Struct Eng. 43 [4], 329-340. Retrieved from https://www.scopus.com/inward/record.uri?eid=2-s2.0-85016088593&partnerID=40&md5=e631247174590589df97657ce1839ed5.
and 4949. Jin, R.; Yuan, H.; Chen, Q. (2019) Science mapping approach to assisting the review of construction and demolition waste management research published between 2009 and 2018. Resour. Conserv. Recycl. 140, 175-188. https://doi.org/10.1016/j.resconrec.2018.09.029.
, respectively). For this reason, they have been categorised as motor issues. In addition, Aggregate Properties is an emerging topic that began to gain importance in the field studied; while Non-Structural Concretes Precast is a topic that in this period has not yet been developed.

Recycling has been the best developed and highest impact issue, with 107 associated items, a strong centrality and high density (1/1) (Table 4). It also has the highest biometric indexes (h-index of 66 and 14872 citations). This means that 107, of the 112 documents published in this period and related to the field of study (Figure 3b), were focused on recycling. In related researches, great interest was given to the environmental benefits of C&DW recycling, thus creating an viable alternative to NA (7676. Evangelista, L.; De Brito, J. (2007) Mechanical behaviour of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 29 [5], 397-401.
, 122122. Xiao, J.; Li, J.; Zhang, C. (2005) Mechanical properties of recycled aggregate concrete under uniaxial loading. Cem. Concr. Res. 35 [6], 1187-1194. https://doi.org/10.1016/j.cemconres.2004.09.020.
, 123123. Etxeberria, M.; Vázquez-Ramonich, E.; Marí, A.R.; Barra de Oliveira, M. (2007) Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete. Cem. Concr. Res. 37 [5], 735-742. https://doi.org/10.1016/j.cemconres.2007.02.002.
).

3.2.1.3. Period 2010-2019

Figure 7a shows the final time interval studied (2010 to 2019 inclusive), which includes a total of eight themes (Table 4). Three themes have been clearly identified as motor ones: Recycling, Sustainable Development and Durability Properties. In addition, the issue of Fine Aggregates has been considered as a basic theme that is becoming a well-developed one. In turn, several basic, peripheral and emerging or undeveloped themes have been identified in this period: these are Microstructural Analysis, Environmental Impact and Laboratory Tests, respectively. Finally, Road Pavement is a peripheral topic that, although relevant, has a poor relationship with the analysed field to date. The increase in the number of articles published in this period (Figure 3) is directly related to the development of a large number of legal and technical regulations that include the use of RA at a European level, with the publication of the Waste Framework Directive (129129. Parlamento Europeo; Consejo de la Unión Europea. (2008) Directiva 2008/98/CE del Parlamento Europeo y del Consejo de 19 de noviembre de 2008 sobre los residuos y por la que se derogan determinadas Directivas. DOUE. L 312 [22 de noviembre], 28.
), the national Royal Decree 105/2008 (130130. Ministerio de la Presidencia. (2008) Real Decreto 105/2008, de 1 de febrero, por el que se regula la producción y gestión de los residuos de construcción y demolición. BOE. 38, [13 de febrero] 11. Retrieved form http://www.boe.es/boe/dias/2008/02/13/pdfs/A07724-07730.pdf.
) and Code on Structural Concrete EHE-08 (55. Ministerio de Fomento. (2008) EHE-08. Instrucción de Hormigón Estructual. BOE. 203, 35176-8.
), which did not exist until the end of the second period studied. Most of these regulations, together with the publication of Law 22/2011 on Waste (131131. Jefatura del Estado. (2011) Ley 22/2011, de 28 de julio, de residuos y suelos contaminados. BOE. 181 [29 de julio], 56. Retrieved form http://www.minetur.gob.es/.
), expect the minimisation and control of the generation of C&DW in the UE (132132. Solís-Guzmán, J.; Marrero, M.; Montes-Delgado, M.V.; Ramírez-de-Arellano, A. (2009) A Spanish model for quantification and management of construction waste. Waste Manag. 29 [9], 2542-2548. https://doi.org/10.1016/j.wasman.2009.05.009.
, 133133. Corinaldesi, V.; Moriconi, G. (2009) Influence of mineral additions on the performance of 100% recycled aggregate concrete. Constr. Build. Mater. 23 [8], 2869-2876. http://doi.org/10.1016/j.conbuildmat.2009.02.004.
) to fulfil the objectives for 2020 to become an intelligent, sustainable and inclusive economy, through a series of policies and actions aimed at moving towards a low carbon economy with an efficient use of resources (33. Comisión Europea. (2014) COM 445. Comunicación de la comisión al parlamento europeo, el consejo, el comité económico y social europeo y el comité de las regiones. Oportunidades para un uso más eficiente de los recursos en el sector de la construcción. Bruselas. 1-29.
, 134134. Comisión Europea. (2013) Decisión N^o 1386/2013/UE del parlamento europeo y del consejo de 20 de noviembre de 2013 relativa al Programa general de acción de la Unión en materia de medio ambiente hasta 2020 “Vivir bien, respetando los límites de nuestro planeta” (VII PMA). DOUE. [28 diciembre], 30.
, 135135. Comisión Europea. (2012) COM 433. Comunicación de la comisión al parlamento europeo y al consejo. Estrategia para una competitividad sostenible del sector de la construcción y de sus empresas. Bruselas. [31 de julio], 1-16.
). A greater importance has been given to the use of these types of granular materials in different applications (1010. Behera, M.; Bhattacharyya, S.K.; Minocha, A.K.; Deoliya, R.; Maiti, S. (2014) Recycled aggregate from C&D waste & its use in concrete - A breakthrough towards sustainability in construction sector: A review. Constr. Build. Mater. 68, 501-516. http://doi.org/10.1016/j.conbuildmat.2014.07.003.
, 136136. Martín-Morales, M.; Zamorano, M.; Valverde-Palacios, I.; Cuenca-Moyano, G.M.; Sánchez-Roldán, Z. (2013) Quality control of recycled aggregates (RAs) from construction and demolition waste (CDW). In: Handbook of Recycled Concrete and Demolition Waste Woodhead Publishing Limited. 2013. 270-303.
, 137137. Tam, V.W.Y.; Soomro, M.; Evangelista, A.C.J. (2018) A review of recycled aggregate in concrete applications (2000-2017) Constr. Build. Mater. 172, 272-292. https://doi.org/10.1016/j.conbuildmat.2018.03.240.
) since the publication of this law.

Recycling has also been identified as the most important motor theme in this period; it has shown the highest number of publications (674), as well as a high density and strong centrality (1/1), meaning that it was a theme of great interest and impact, as reflected in Table 4, even more so than in the previous period. This fact indicates that recycling is not only a field of research but it is a practical reality (7878. Evangelista, L.; de Brito, J. (2010) Durability performance of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 32 [1], 9-14. http://doi.org/10.1016/j.cemconcomp.2009.09.005.
, 112112. Silva, R.V.; de Brito, J.; Dhir, R.K. (2014) Properties and composition of recycled aggregates from construction and demolition waste suitable for concrete production. Constr. Build. Mater. 65, 201-217.http://doi.org/10.1016/j.conbuildmat.2014.04.117.
, 138138. Marinković, S.B.; Radonjanin, V.; Malešev, M.; Ignjatović, I.S. (2010) Comparative environmental assessment of natural and recycled aggregate concrete. Waste Manag. 30 [11], 2255-2264. https://doi.org/10.1016/j.wasman.2010.04.012.
), and where the quality of the recycling process has a considerable influence on the quality of the recycled material obtained (139139. Pedro, D.; de Brito, J.; Evangelista, L. (2015) Performance of concrete made with aggregates recycled from precasting industry waste: influence of the crushing process. Mater Struct. 48, 3965-3978. http://doi.org/10.1617/s11527-014-0456-7.
).

3.2.2. Analysis of most significative thematic networks

Recycling is a theme what is worth mentioning as it is one of the most characteristic topics for the field studied, even more so if it is analysed from the point of view of its thematic network. From the beginning, and until its consolidation in the third period, this topic maintains a common thread with Recycled Aggregate; this fact reflects its scientific evolution.

Figure 5b shows the network built with the keywords associated with the Recycled Aggregate theme during the first period; it reflects the connection between the most significant nodes (thicker relationship lines). It can be further observed that the strongest internal relationships are not related to the central theme of the cluster but are established between the recycled materials and the tests that must be performed for them (127127. Barra de Oliveira, M.; Vázquez-Ramonich, E. (1996) The influence of retained moisture in aggregates from recycling on the properties of new hardened concrete. Waste Manag. 16 [1-3], 113-117. https://doi.org/10.1016/S0956-053X(96)00033-5.
, 140140. Topcu, I.B.; Güncan, N.F. (1995) Using waste concrete as aggregate. Cem. Concr. Res. 25 [7], 1385-1390.
), or between the sustainable development of the construction sector and how this has been influenced by the replacement percentage of natural material by recycling (141141. Bairagi, N.K.; Vidyadhara, H.S.; Ravande, K. (1990) Mix design procedure for recycled aggregate concrete. Constr. Build. Mater. 4 [4], 188-193. https://doi.org/10.1016/0950-0618(90)90039-4.
, 142142. Bairagi, N.K.; Ravande, K.; Pareek, V.K. (1993) Behaviour of concrete with different proportions of natural and recycled aggregates. Resour. Conserv. Recycl. 9 [1-2], 109-126.
).

After analysing the network of the Recycling cluster during the second period (Figure 6b), a significant relationship was observed between almost all nodes; this means that the keywords co-appear in most documents, thus providing great similarity between the elements. Despite this, the strong internal connection between the RA from concrete and its recycling reflects the fact that the recycling of concrete waste is beneficial and necessary from the point of view of environmental preservation and the efficient use of resources available (7676. Evangelista, L.; De Brito, J. (2007) Mechanical behaviour of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 29 [5], 397-401.
, 122122. Xiao, J.; Li, J.; Zhang, C. (2005) Mechanical properties of recycled aggregate concrete under uniaxial loading. Cem. Concr. Res. 35 [6], 1187-1194. https://doi.org/10.1016/j.cemconres.2004.09.020.
, 123123. Etxeberria, M.; Vázquez-Ramonich, E.; Marí, A.R.; Barra de Oliveira, M. (2007) Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete. Cem. Concr. Res. 37 [5], 735-742. https://doi.org/10.1016/j.cemconres.2007.02.002.
).

3.2.3. Scientific evolution analysis

Evolution of the main themes in the scientific field are shown in Figure 8 and it is possible to observe that Recycling is the most dominant issue (in the period 2010-2019) as it has a larger sphere, according to documents count. In a conceptual way, the evolution of this thematic area can be understood to the extent that, until the year 2000, the few related documents (Table 4) focused on the recycling of concrete to be used as RA which, when evolving to the Recycling group in 2000-2009 and merging with Physical Properties, it became the most important topic in the entire field investigated in 2010-2019.

Between the first and second period it is observed that the conceptual nexus (greater thickness of continuous lines) is identified between Physical Properties - Recycling and between Recycled Aggregates - Recycling. While between 2000-2009 and 2010-2019 the strongest link is observed between the following topic: Construction Materials - Sustainable Development, in addition, Recycling remained the most predominant thematic area, maintaining the same designation in both periods, although with a greater number of documents published when passing from one period to another.

Among the groups in the second period, Construction Materials merged with Sustainable Development, based on a non-conceptual link with Recycled Aggregate, in addition to incorporating part of its content to Fine Aggregates. In other words, they share some keywords with others clusters but are not the main item (see Figure 8). In turn, Masonry Mortars, integrated with the Physical Properties theme (from the first period), merged with Physical-Mechanical Properties and Non-Structural Concretes Precast to evolve as a new motor theme, called Durability Properties, in the last period studied. On the other hand, Construction Materials, together with Masonry Mortars, created a new theme in the 2010-2019 period called Fine Aggregates, in addition to the influence of keywords provided by the Physical Properties and Recycled Aggregates groups. As for the Non-Structural Concretes Precast group, it gains little importance, since it incorporates terms of Recycled Aggregates but then it has evolved only a little. Nevertheless, this theme incorporated new elements from various groups in the last period, such as Durability Properties, Road Pavement and Environmental Impact, helping these themes to reach a better position (Figure 7).

In the last period, different groups emerged that share part of their elements with other themes from the previous period, such as Road Pavement and Microstructural Analysis. Finally, Environmental Impact appeared as a thematic group, with a weak contribution of several themes (Construction Materials, Masonry Mortars, Non-Structural Concretes Precast and Physical-Mechanical Properties), by means of a non-conceptual nexus (discontinuous lines); even, a new cluster is created, Laboratory Test, without visible participation from other nodes.

Figure 9 shows the evolution of the main themes according to their h-index, where the size of the spheres or higher h-index of the themes Recycling, Durability Properties and Sustainable Development should be highlighted. Recycling is considered the thematic area with the greatest impact, as we have already seen, being central to the development of the field studied. The evolution of its h-index shows an upward trend, whose development stands out above the rest of the topics, with an h-index of 66 in the second period and 76 in the last period (Table 4). Although it started with low impact (Recycled Aggregate), it progressively became the origin of other research areas that are closely linked to Recycling, mainly in the last period (2010-2019), such as Sustainable development (h-index= 52) or Environmental impact (h-index=17).

Finally, Figure 10 shows the continuity of keywords shared between consecutive periods using a longitudinal map; that the keywords have increased considerably over the years, in parallel with an increase in the number of documents published (Figure 3). In fact, the field evolved from 31 word groups in the first period (1973 to 1999) to 96 in the second and 127 in the third. The increase in the number of keywords testifies to the growing thematic diversity discussed under the use of RA in different applications. Likewise, it is noticeable that the first and second periods share all of their words with the following period (31 and 96, respectively). 65 of the 96 words of the period 2000-2009 belong to the previous period; furthermore this one adds 31 new words to the next one (2010-2019). In addition, the index of stability between periods, whose value is 1, indicates that keywords do not disappear over time; this is a quite remarkable fact, which indicates that the field under investigation evolved in a solid way and is well consolidated.