Mechanical Properties of Concrete Using Recycled Aggregates Obtained from Old Paving Stones

: Nowadays, construction, maintenance, reparation, rehabilitation, retroﬁtting, and demolition from infrastructure and buildings generate large amounts of urban waste, which usually are inadequately disposed due to high costs and technical limitations. On the other hand, the increasing demand for natural aggregates for concrete production seriously affects mountains and rivers as they are the source of these nonrenewable goods. Consequently, the recycling of aggregates for concrete is gaining attention worldwide as an alternative to reduce the environmental impacts caused by the extraction of nonrenewable goods and disposal of construction and demolition waste (C&DW). Therefore, this article describes the effect on the mechanical properties of new concrete using recycled aggregates obtained from old paving stones. Results show that replacing 50% by weight of the ﬁne and coarse aggregate fractions in concrete with recycled aggregate does not meaningfully affect its mechanical behavior, making the use of recycled aggregates in new precast paving stones possible. Therefore, the latter can reduce environmental impacts and costs for developing infrastructure and building projects.


Introduction
The consumer society, together with the technological revolution, has led to the most massive production of waste in humanity's entire history. This problem has led to most countries seeking solutions to decrease pollution rates on the planet [1]. According to John [2], the construction industry produces 40% of the different world economic sectors' waste. On the other hand, with the pivotal role that the construction industry has in developing countries, it is convenient to adopt urgent measures to achieve sustainable development [3].
The construction industry is still one of the largest waste generators today [4][5][6]. However, historically it has been a necessary pillar for the development of our communities. In general, the construction sector's pollution occurs in most stages: from the extraction of raw materials, the manufacture of materials, to the different activities carried out during the construction, operation, and end of the life cycle of buildings and infrastructure. This causes the depletion of various nonrenewable resources, as well as water and air pollution, in addition to excessive energy consumption [7].

Global Context
In other industrial sectors, recycled materials are typically competitive when there is difficulty in obtaining virgin raw materials and suitable locations for storage. Therefore, the proposal for sustainable concrete is based on the substitution of stone aggregates of natural origin for construction and demolition waste (C&DW), and it proves to be very interesting for infrastructure and buildings, enabling the implementation of circular economic models [8][9][10]. The government requirement to mitigate the environmental impact of the different construction projects through the proper management of the C&DW potentiates their reincorporation into the construction production chain through recycling. However, to use this waste in new projects, it is necessary to evaluate the physical, chemical, mechanical, and durability characteristics of the C&DW [11][12][13]. The use of C&DW as aggregates of concrete mixtures without knowing their properties can generate projects with concrete properties in a fresh and hardened state that are undesirable and unsafe for its users.
In the construction sector, a large amount of waste of different types is generated, but only a part of it can be reincorporated in the same sector, either by reusing or recycling it. C&DW must be inert and uncontaminated. Although there are significant fluctuations in Colombia, it can be estimated that the usable waste is 80% of the CDW, which is made up of materials such as bricks or blocks, concrete, rock, excavation material, steel, wood, and others [14]. The remaining 20% that is not usable in the construction sector (wood, plastics, packaging, and inert materials with organic matter) should be sent to specific recycling plants for industrial symbiosis or disposed in landfills [15].
To recycle concrete waste into new building products, intensive research worldwide, demonstrating that in general, the use of recycled aggregates from concrete in new fabrication of concrete has resulted in enhanced improvement of mechanical properties [16][17][18][19]. Perez-Benedicto et al. [20] studied the mechanical behavior of concrete made with recycled aggregate coming from discarded concrete prefabricated units, demonstrating that including coarse aggregate replacement can offer excellent quality for structural applications providing similar compressive strength respect conventional concrete. Cakir and Dilbas [21] demonstrated that durability of concrete can be increased up to 60% using recycled aggregate and an optimized ball mill method. The study included comparison of several aggregates such as natural aggregate, recycled aggregate, silica fume, basalt fiber and recycled aggregate and optimized ball mill method.
Vedrtnam et al. [22]. studied the response of cement-based composites under direct flame conditions, demonstrating improvement in the residual compressive strength compared to conventional concrete after the thermal exposure. In addition, it was found minimum damage in the microstructure of the material when residues of PET bottles are included in the mixture. Zareei et al. [23]. analyzed the combination of recycled waste ceramic aggregates and waste carpet fibers to produce high strength concrete. They used combinations from 20% up to 60% of recycled waste ceramic to replace natural coarse aggregate. As result it was observed an increase of compressive, splitting tensile, flexural and tensile strengths by 13%, 15%, 3% and 21% respectively. Zahid-Hossain et al. [24]. also studied the inclusion of recycled material as rubber and polypropylene fibers into concrete mixtures to evaluate their mechanical behavior. As result it was demonstrated that compressive strength, splitting tensile strength and flexural strength decreases as the crumb rubber content increases, and increase with the fiber content. In addition, it was observed a reduction in the propagation speed of failures in the material, providing a more gradual failure propagation.
On the other hand, several studies have demonstrated decrease in mechanical properties when replacing virgin aggregate with recycled aggregate. Alam et al. [25] found that replacements of 25% using recycled aggregate can reduce compressive strength of concrete in 15%. Similarly, Meherier [26] studied the replacement of aggregates using crumb rubber and obtained reductions of 20% in concrete compressive strength. Limbachiya et al. [27] found that replacements with recycled concrete higher than 30% affect drastically compressive strength. However, all previous studies did not follow a standard procedure and therefore results can vary from region to region and according to environmental and methodological tasks.

Colombian Context
In Colombia, large amounts of construction and demolition waste produced by the construction industry are inadequately disposed (Figure 1a,b). Similarly, large amounts of nonrenewable resources are converted illegally, by industrial processing, into construction materials (Figure 2a,b).  Based on those above-mentioned environmental and social problems, a circular economy strategy from the national government has been launched. Particularly for C&DW, more than 22 million tons generated yearly in major cities should be waste managed [32]. On the other hand, the most widely used materials in the construction industry have historically been: aggregates, wood, concrete, steel, and glass. Except for aggregates and wood, the rest are composite materials that are made from nonrenewable raw materials. They are also the predominant materials in the last hundred years in large, intermediate cities and, unfortunately, even in the most remote rural areas [33].
So far, the construction of infrastructure and buildings has become one of the country's main economic activities. Despite the social benefits that the previous process has brought, cities and rural environments face significant environmental challenges due to the demand for nonrenewable resources generated by urban centers. For example, the problem of consuming nonrenewable natural resources for concrete production in Colombia is growing, with increases in concrete production of around 6% per year [34]. This generates critical environmental problems derived from the manufacture of concrete, such as the requirement of large quantities of stone material used in the natural stage as fine and coarse aggregates, and for producing Portland cement [35]. Commonly sand from the riverbed is used as a fine aggregate, while extraction and crushing of rock from quarries, usually rocky mountains, is carried out for the coarse aggregate. This situation poses significant technological challenges such as reduction, reuse, and recycling applied to all the country's productive sectors for the genuinely sustainable development of Colombian cities and municipalities.
In the case of Colombia for example, Diosa [36] performed an experimental analysis of mechanical properties of high-strength concrete from recycled sources, obtaining significant improvements in terms of mechanical resistance and durability. Hurtado [37] studied the effect of partial replacement of Portland cement by ash from the paper industry in the manufacture of mortar samples; results demonstrated that it is possible to obtain better mortar workability but less mechanical resistance. Moreover, Londoño [38] developed an analysis to determine the technical and financial suitability of using in situ recycled aggregates to fabricate prefabricated elements for construction.
In summary, the high demand for concrete, the natural resources for its production, and the generation of construction and demolition waste motivate the development of studies in which some waste is used and incorporated into the production of materials such as concrete [39,40]. Therefore, in this article, the mechanical performance of new concrete paving stones made with recycled aggregates from old paving stones in the municipality of Almaguer (Cauca, Colombia) were evaluated to be used for the same application in the municipality. The novelty of this research is focused in the use of aggregate to recirculate material within the same application and location, without incur in additional costs and using conventional crushing and laboratory equipment. The proposed method provides useful insights to recycle C&DW in the Colombian context and similar countries to fabricate added value products and lower environmental impact. The subsequent sections of this paper are organized as follows: Section 2 summarizes the materials and methods employed to conduct the experimentation and analyze the mechanical performance of concrete mixtures. Results are presented in Section 3, while Sections 4 and 5 correspond to discussion and conclusions.

Materials and Methods
This section summarizes the materials used to manufacture the paving stones and the methodology to analyze and compare mechanical properties. Materials mainly consist of used pavers, cement, coarse and fine aggregates (from natural sources and recycled). The proposed methodology started performing an aggregate sampling to classify materials, followed by a characterization of components, mixture design, evaluation of mechanical properties, and selecting the most suitable mixture concentrations to manufacture new paving stones. Table 1 shows the materials employed to manufacture and analyze concrete samples. It included used paving stones converted into recycled aggregates, natural aggregates, and Portland cement. Such materials are processed and converted into new concrete samples, which are later analyzed and tested to measure and compare conventional values of mechanical properties (flexural and compressive strength).

Method
The proposed methodology for this study consisted of five phases, which are oriented to generate and establish suitable concrete mixtures. Figure 3 shows the overall methodology followed in this article.  As a first stage, the process started with collecting used paving stones, which corresponded later to recycled aggregates (coarse and fine). Such components were triturated until obtaining the desired granulometry according to the NTC 174 standard [41]. In a parallel process, natural aggregates were also obtained from a quarry to compare and create different concrete mixtures, varying the percentage of recycled material. The second stage was denominated characterization of components, and it aimed to determine the most relevant properties of all materials used during the experimentation process; such materials comprise cement and aggregates. Here, physical characterization was performed analyzing granulometry, specific weight and absorption, the unitary mass of CDW and natural materials, shape, elongation, compressive indexes, and density and consistency of Portland cement. Besides, a chemical characterization that included sulfide resistance and organic material analysis. Later, mechanical characterization covered the response under wear conditions. The third state covers the mixture design, the manufacturing of concrete specimens for mechanical tests after curing processes. Evaluation of mechanical properties was performed as the fourth stage, and it included experimental tests for compression and flexural strength of concrete specimens. Lastly, step five concludes the proposed methodology by manufacturing the paving stones using the most suitable mixture obtained from tests and analysis of previous phases. Table 2 summarizes the overall composition of the nine concrete samples selected for analyzing and performing mechanical tests. To develop the five phases previously shown in Figure 3 it was necessary to perform several task and analysis activities to obtain a proper mixture of concrete including recycled aggregates. Such phases were performed using as reference the Colombian normative Phase 3 included mixture design, which is performed using the American Concrete Institute ACI 211 method (also equivalent to NTC 2017 standard). This method consists of selecting amounts of cement, aggregates, water and additives to produce cost-effective concretes able to obtain a desired mechanical strength, durability, stability, unitary mass, and appearance. In this case nine mixtures were proposed varying the percentage of natural and recycled aggregates. Phase 3 also comprised the curing process of concrete specimens following the standard INV E-402. Then, when specimens were fabricated, it is necessary to perform mechanical tests to validate and compare flexural and compressive strength (NTC 673 / INV E-414) respect to conventional concrete mixtures in Phase 4. Lastly, Phase 5 consisted of selecting the best performance mixtures in terms of mechanical properties to fabricate new paving stones. Such fabrication was developed using the standard NTC 2017.

Results
This section comprises the results of five phases previously mentioned in the methodology. Section 4 is later presented to elucidate findings and interesting topics identified from the obtained results.

Aggregate Sampling
Natural aggregate samples were obtained from the company Canteras de Ingeocc S.A. (Yumbo, Colombia). On the other hand, used paving stones for recycled aggregates were obtained from the Almaguer's municipality (Cauca, Colombia). One hundred used paving stones were processed three times using a jaw crusher until acceptable sizes were obtained for an aggregate. The natural and recycled aggregates were classified into coarse and fine according to standard NTC 174 related to prefabricated paving stones. Approximately 530 kg of both natural and recycled aggregates were used during this study.

Characterization of Components
Physical and chemical attributes of interest were analyzed to determine the suitability of components to manufacture new concrete mixes. This subsection includes physical, chemical, and mechanical characterization. Physical characterization included several measurements and tests in determining key parameters in aggregates and cement employed: granulometric analysis, specific weight, water absorption, unitary mass, flattening, and elongation indexes for aggregates. In the case of cement, we determined density and consistency. Secondly, chemical characterization was dedicated to analyzing the chemical response of aggregates to sulfides (solidness) and analyzing the organic matter. Lastly, mechanical characterization was the first stage and consisted of a wear analysis for natural and recycled aggregates. Each characterization result is described in detail, as follows in Table 3. Additional properties of materials are summarized in Appendix B. Natural: 1460 g/cm 3 for fine (l) and 1530 g/cm 3 for fine (c) 1600 g/cm 3 for coarse (l) and 1710 g/cm 3 for coarse (c) Recycled: 1250 g/cm 3 for fine (l) and 1422 g/cm 3 for fine (c) 1123 g/cm 3 for coarse (l) and 1232 g/cm 3

Mixture Design
This phase consisted of nine steps, which are summarized as follows in Table 4. According to Sánchez de Guzmán [42], calculations and analysis during this phase were performed meeting the requirements of the standard NTC 2017 [43], which implies a compressive strength of 50 MPa and flextraction of 5 MPa. The mixture design did not consider severe conditions (e.g., freeze-thaw cycles). Table 4. Steps and their outputs during the mixture design phase.

Task
Results and Outputs

(i) Settling selection
An average value of 10 mm was selected. A very dry consistency is recommended for paving stones and implies the use of extreme vibration and possible pressure for achieving the desired compaction (recommended settlement: 0-20 mm).
(iii) Air content estimation Since paving stones are not exposed to extreme conditions (freeze-thaw cycles), the concrete's air content is zero.
(iv) Estimation of mixing water content This was performed using a linear regression between values provided, and using a value of settlement of 10 mm, which establishes that for settlement of 0 mm and 25 mm are required 201 kg and 208 kg of mixing water content respectively per 1 m 3 of concrete. The resulting value of mixing water content was 203.8 kg per 1 m 3 .
(v) Determination of design resistance According to standard NTC 2017, a paving stone unit must provide a minimum modulus of rupture of 4.2 MPa after 28 days. Such modulus of rupture commonly varies between 10% and 20% of the compressive resistance. Therefore, as an indirect measurement, minimum compressive resistance of the mixture of 42 MPa is required. To compensate for possible fluctuations, it is desirable to include a safety factor. Following the standard NTC 2017, 100 kg/cm 2 were added to the compressive resistance, thus an approximate compressive resistance of 50 MPa (520 kg/cm 2 ).
(vi) Selection of water/cement ratio This was established according to the water/cement ratio values provided by. This establishes that for a value of 520 kg/cm 2 the corresponding water/cement ratio is equal to 0.36.

(vii) Calculation of cement content
This was calculated using the value of mixing water content and the water/cement ratio. Thus, it was required 565 kg per m 3 of concrete.
(viii) Estimation of aggregate proportions They were determined using a graphical method. The recommended value is 52% for fine aggregate and 48% for coarse aggregate.
(ix) Adjustment of water content Due to aggregate moisture, this was performed assuming a volume of the concrete mixture of 0.033 m 3 to perform a test for one slump and two beams. Moisture was determined for both coarse and fine aggregates.

Evaluation of Mechanical Properties
Flexural and compression resistance tests were carried out at 28 days of the age of beams and cylinders with different contents of recycled aggregates to evaluate the concrete's mechanical properties (Tables 5 and 6). In terms of flexural strength, it was observed that compared to the reference sample (M1), made with aggregates of natural origin and considering the minimum strength required for paving stones (5 MPa), the mixtures M2, M3 and M4 met with established mechanical requirements. However, given the high deviation found for the M3 mixture and the low average compressive strength, it was not considered suitable in this project. Considering the above, M2 and M4 mixtures, which replace 50% of the fine natural aggregates and 50% of the natural coarse aggregates, respectively, were the only viable options in terms of flexural strength. On the other hand, in terms of compressive strength, mixtures M2 and M4 met with the established design resistance (50MPa) and are comparable to the reference mixture (M1) resistance that used fine and coarse aggregates of natural origin.   Figure 4 summarizes the results obtained for flexural and compressive tests, while Figure 5 shows the relationship between flexural and compressive strengths obtained from mechanical tests for all nine specimens.
Finally, when analyzing the correlation between compressive and flexural strength of the results (Figure 5), it was observed that flexural strength represented between 8 and 10% of the compressive strength, such output is similar to the relationship that conventional concrete offers (using natural aggregates).

Performance-Based Selection of Mixture
The M2 (50% replacement of fine natural aggregate) and M4 (50% replacement of natural coarse aggregate by recycling) mixtures were selected based on results for flexural (modulus of rupture) and compressive strengths of the concrete mixtures concerning the behavior of the reference mixture and the established design strengths. Figure 6 shows the new paving stones from the M2 mixture, and Table 7 summarizes the most relevant parameters for selected mixtures (M2 and M4) compared to the reference sample M1.

Physical, Chemical and Mechanical Characterization
According to granulometric measurements, coarse and fine aggregates did not fully satisfy the reference standard (NTC 174). However, these aggregates were used in this study due to their high quality and commercial acceptance (mechanical properties and durability). For recycled aggregates, these satisfy acceptance ranges according to the reference standard (NTC 174). Except for the finest fraction of coarse aggregates (fringe sieve between #200 and #10). Thus, fine recycled aggregate is higher than the fine aggregate while maintaining the same maximum size regarding fineness modulus.
It is observed from the fineness modules that the recycled fine aggregate is higher than the natural one, while the maximum sizes for both types of aggregates are equal. This indicates that concrete with recycled aggregates may require higher water content to achieve workabilities, similar to conventional concrete. Regarding other interest parameters, it was possible to identify several important differences between recycled and virgin aggregates. In the case of absorption and density, recycled aggregates provided values 9 to 11 times higher and 32 to 35% lower than natural ones. Recycled aggregates have a lower density or specific gravity concerning the densities of natural aggregates. Thus, commonly the water content required for a concrete mix with the same workability is higher when recycled aggregates are used and may also be worsened; this is because although in this case the standards are met, the elongation and flattening index of recycled coarse aggregates is relatively low compared to natural ones [44].
Another interesting difference is related to mechanical properties, demonstrating that recycled aggregates proved three times less resistant to wear than natural aggregates (64% of loss). Lastly, unitary mass for both recycled and natural aggregates presented conventional values for concretes (950 to 1950 kg/m 3 ). Thus, both aggregate materials can reduce cement consumption, plastic deformation and contribute to concrete density once it is solidified. In terms of chemical properties, it was found that despite organic material not affecting the hydration of Portland cement, virgin fine aggregates (sand) provide a higher content compared to recycled ones. This can be explained by the fact that natural aggregates come directly from the mineral quarry. Regarding wear resistance under exposition to sodium sulfate, values of 68% and 86% were registered for recycled coarse and fine aggregates, respectively.

Mechanical Evaluation
According to Figure 4, when 50% fine aggregate is replaced, the flexural strength is reduced by approximately 20%. In contrast, with the replacement of the coarse aggregate, a reduction of 10% is presented concerning the reference mixture, noting that it gets better bending behavior of the mix when replacing coarse aggregates. Regarding mixtures with a 100% replacement in both fine and coarse, the resistance decreased by approximately 30% and did not satisfy the NTC 2017 standard (4.2 MPa). On the other hand, it is observed that when replacing 50% of the coarse aggregate, the resistance to compression is reduced by approximately 8%. It is a concrete that met the established mechanical requirements; by replacing 100% of the coarse aggregate, the compressive strength is decreased by approximately 30%. Therefore, this replacement was not considered viable due to its high reduction in mechanical performance.
When combining the recycled aggregate replacements between fine and coarse, a decrease of more than 30% and 25% of the mechanical resistance to compression and flexion, respectively, values that do not meet the initial requirement. Reductions of this concrete's mechanical properties are mainly since recycled materials from paving stones provided high absorptions greater than 14%, indirectly indicating that the aggregates also offered high porosity, which implied a considerable volume of voids in the internal structure of the aggregates. In addition, the pores of recycled material include small cracks that, within the concrete, reduce the mechanical properties of mixtures. Although there are different improvement techniques for the mechanical response (e.g., coatings and mineral fillers), the most economical and straightforward way is balancing the mechanical properties with the partial use of natural aggregates. In addition to the environmental benefits of using recycled aggregates, these have better adherence to the matrix, low content of organic matter, and typically low cost. Finally, by replacing 50% of fine natural aggregate with recycled aggregate, it was possible to observe a reduction in compressive strength by approximately 10%, a similar value to replacing 50% of coarse aggregate in the mix. One hundred percent replacement of the fine aggregate content decreases resistance by approximately 20%, which was an admissible value according to the requirement. Nevertheless, mechanical behavior in terms of flexure was less than 4.2 MPa. Therefore, the M7 mixture was not considered for the manufacture of paving stones.
According to Figure 4, mixes M2, M3, and M4 met the mechanical requirements established regarding flexural strength. However, given the high deviation found for the M3 mixture and the low average compressive strength, this study was not considered. Therefore, the M2 and M4 mixtures, which replace 50% of the fine natural aggregates and 50% natural coarse aggregates, respectively, were the two only viable options for flexural strength fulfillment.

Properties of Selected Mixes
Average values for absorption mixtures M2 and M4 are higher than 7%, while the M1 mixture is within the parameter according to the NTC 176 standard [45], which is to be expected taking into account that M2 and M4 mixtures contain recycled material from old paving stones, which are more porous and less dense. In terms of wear, it must be less than 23 mm, according to the NTC 5147 standard [46]. Tested mixtures showed satisfactory results since all of them satisfy that reference parameter. However, it is noteworthy that new paving stones with M2 concrete presented lower wear than M1, contrary to the M4 sample that presented higher wear. Fifty percent replacement of fine aggregates in M2 did not affect the wear of the new paving stone, which indicates that the manufacture of paving stones with such a mixture will promote durability by friction over time. New paving stones made with recycled material decreased their resistance by approximately 50% compared to M1. This decrease due to the recycled material used is highly porous and has possible microcracks derived from the crushing process.
To summarize, it is observed that new paving stones manufactured with a 50% replacement of natural fine and coarse aggregates by recycled aggregates (M2 and M4) satisfactorily comply with the minimum mechanical resistance (4.2 MPa according to the NTC 2017 standard). However, the results in absorption and wear are above the permitted values for these applications. It is recommended to quantify in future research the practical impact on the lifecycle of paving stones, including the interaction between these high percentages of absorption and wear. The results obtained demonstrated that it is possible to fabricate paving stones with similar properties to those fabricated using 100% virgin material. Possible applications of concrete with recycled aggregates include structural applications and buildings, however, it is necessary to perform specific experiments to validate its suitability in such applications. The study summarized in this article did not consider extreme conditions such as exposure to fire or freezing environment.

Limitations of the Study
The experimentation and examination processes followed in this study involved several characterization, physical and chemical analysis, and laboratory tests. Therefore it is relevant to address several limitations regarding the methodology, materials and results obtained:

•
Properties of raw material for aggregates (old paving stones) can vary depending on the age, degree of wear and typical use conditions. Hence, it is possible to find differences in results for mechanical properties considering another paving stone with different age and use regime.

Conclusions
This article described first the evaluation of the mechanical properties of concrete using recycled aggregates obtained from old paving stones, and second the development of two suitable concrete mixtures for manufacturing new paving stones with important replacements of the fraction of natural coarse and fine aggregate. Based on the technical standards, the two concrete mixes with the replacement of 50% by weight of the fine and coarse fraction of natural origin presented adequate flexural strength when evaluated as new paving stones. These results are entirely satisfactory concerning the replacements found in the literature where recycled aggregates are typically used in fractions less than 50% by weight of the natural coarse or fine aggregates of a concrete mix. This is fundamentally due to the high quality and processing of the recycled aggregates used in this project. It is remarkable that the selected recycled aggregates were fundamentally constituted of concrete and did not present significant contamination.
On the other hand, in terms of the relative high absorption and low resistance to wear of the paving stones developed, future research is proposed to improve recycled aggregates against these variables, particularly wear caused by pedestrian. The correlation of existing standardized wear tests with actual applications still demands better approaches that open the possibility of using recycled aggregates without affecting construction safety. Finally, this study demonstrated that use of recycled aggregates has the potential to reduce the environmental impact and, when applied correctly, it can significantly reduce the costs of a construction project. Therefore, results from this project, together with more research results about recycling technology in building materials are the base of a circular economic model proposed for one of the major cities from Colombia, Santiago de Cali (Figure 7). In particular, the Circular Economy Model, a tool led by the municipal planning department of the mayor's office of Santiago de Cali for the construction sector, is composed of four stages that are: (I) production of construction materials, (II) construction, (III) use and operation and (IV) termination of the life cycle of buildings and infrastructure. It also includes the articulation with other productive sectors that generate waste of interest to the construction chain, a strategy known as industrial symbiosis. Furthermore, in the Circular Economy Model, it is very important to connect at all stages with the Environmental and Technological Park (Parque Ambiental y Tecnológico) of Santiago de Cali, a place conceived as a physical space that has infrastructure and shared human capital that reserves investment for sustainable construction companies. As they are inserted in flexible groups, they find support (in situ and in the park) to transform C&DW and other waste into sustainable materials for construction.