Mechanical Behavior of Paving Stones Made from Construction and Demolition Waste (CDW)

Abstract

This study investigates the mechanical performance of concrete paving stones manufactured with recycled aggregates derived from TransMilenio slab demolition waste (CDW-A-TS) as a sustainable alternative to conventional natural coarse aggregates (river gravel) and fine aggregates (river sand). Construction and demolition waste from Bogotá’s mass transit system slabs was processed to produce recycled aggregates, which were replaced at substitution levels of 0%, 30%, 50%, and 100% by volume of natural aggregates. The mechanical properties evaluated included compressive strength, flexural strength, abrasion resistance, and water absorption, following Colombian Technical Standards (NTC) and international protocols. Results demonstrate that all CDW-A-TS mixtures exhibit enhanced compressive strength, with improvements ranging from 14.71% to 32.82% compared to the control mix. Flexural strength also increased by 1.34% to 6.13%. However, water absorption increased proportionally with CDW-A-TS content (10.66% to 25.24%). The optimal substitution level was identified at 30% CDW-A-TS based on a composite evaluation of mechanical performance (compressive and flexural strength), durability indicators (water absorption and abrasion resistance), This research demonstrates the technical viability of incorporating TransMilenio demolition waste in paving stone production, contributing to circular economy principles and sustainable urban infrastructure development. This finding aligns with prior research affirming the viability of incorporating recycled coarse aggregates in concrete prefabricates, such as paving stones, for various construction applications.

1. Introduction

The constant population expansion in cities and the development of their infrastructure generate a high demand for essential building minerals, such as those used in concrete production. These minerals are non-renewable resources that deplete quickly. Their use in construction activities also contributes to the generation of construction and demolition waste (CDW) [1].

Particularly in Bogotá, approximately 12.3 million m3 of CDW are produced per year. The largest producers are the Urban Development Institute (IDU, by its Spanish acronym) and private construction companies [2]. A massive portion of this waste is disposed of in legal landfills, depending on each city’s management practices. However, some landfills operate illegally, posing a major problem for CDW management. In recent years, the Regional Autonomous Corporation of Cundinamarca (CAR, by its Spanish acronym) has identified 94 such illegal disposal sites in Bogotá [3], which directly impact the environment by contaminating the city’s soil and aquifers, The unique challenges posed by construction and demolition waste management in Bogotá’s urban landscape necessitate robust and systematic approaches for effective control and disposal [4].

Bogotá’s mass transit system, Transmilenio, was built on a rigid pavement in 2000. Multiple damages have been recorded in the slabs since the start of operations. The repair of these slabs has required significant investment from the city, with nearly USD 4 million allocated for pavement repairs up to 2020. The demolition of the slabs has generated substantial volumes of construction and demolition (C&D) waste, which is disposed of in fills without any further utilization.

The demolition of TransMilenio slabs (Figure 1) serves as a significant example of CDW generation in Bogotá. Between 2003 and 2008, 2900 slabs were demolished along the north highway and Caracas trunk lines, covering approximately 13 km of the TransMilenio road network. This operation resulted in the production of 14,000 m³ of CDW [5]. Considering that Bogotá’s TransMilenio trunk lines have a total length of 114.4 km, the generation of this waste presents substantial challenges for waste management and sustainability in the city [6], The analysis of construction and demolition waste (CDW) management and sustainability in Bogotá D.C. reveals critical challenges and underscores the necessity for more effective practices within the local construction sector [7].

In this context, Construction and Demolition Waste (CDW) recycling emerges as a key strategy for exploring alternative material sources and reducing the exploitation of virgin resources [8]. This process enables the production of recycled aggregates from demolished concrete, thereby lessening the pressure on natural resources and the environment.

Several studies have examined the incorporation of CDW as an aggregate in concrete production, demonstrating the feasibility of producing concrete with comparable strength by using various replacement percentages of recycled aggregates from CDW [9].

Recent studies have demonstrated the applicability of recycled aggregates in the manufacture of paving stones for various uses, including streets, avenues, and pedestrian walkways. Research conducted by [8] focused on the performance of concrete paving stones incorporating 0%, 35%, 45%, and 55% of CDW-based aggregate, evaluating both compressive strength and flexural strength. The results indicate that as the replacement percentage of CDW aggregate increases, compressive strength also increases, with values of 46.5 MPa for 0% replacement, 47.4 MPa for 35%, 48.2 MPa for 45%, and 48.8 MPa for 55%. Regarding flexural strength, the results show an opposite trend: the 0% replacement mix achieved 5.1 MPa, while 35% reached 4.6 MPa, 45% registered 4.0 MPa, and 55% resulted in 3.8 MPa. These findings indicate a decrease in flexural strength as the proportion of CDW aggregate increases.

On the other hand, ref. [10] evaluated the behavior of concrete paving stones incorporating 50%, 75%, and 100% CDW in their production. This study included tests to assess compressive strength at 7, 14, and 28 days of curing, revealing that the inclusion of CDW led to a decrease in compressive strength, with reductions ranging between 7.9% and 14.5% compared to paving stones without recycled aggregate.

Additional research [11] demonstrated that CDW aggregates from different sources exhibit varying performance characteristics, with concrete-based CDW showing superior mechanical properties compared to mixed CDW containing ceramics and masonry materials. Similarly, ref. [12] reported that the processing method of CDW significantly influences the final properties of recycled aggregates, with mechanical crushing producing more angular particles that enhance interlocking in concrete matrices.

The observed discrepancy in findings can be attributed to the variability in the origin of the waste in each case. Elements commonly present in construction and demolition waste, such as gypsum, wood, and plastics, are known to negatively impact the strength of concrete made with CDW-derived aggregates [13].

This project aims to analyze the reuse of CDW in the production of concrete paving stones through a comparative study between conventional paving stones and those made with different percentages of CDW sourced specifically from TransMilenio slabs (CDW-A-TS). This analysis will evaluate the mechanical performance of the paving stones by replacing 0%, 30%, 50%, and 100% of natural aggregates with CDW-A-TS, verifying the results in accordance with [14]. The gradual substitution of aggregates will provide insight into the impact of these CDS-A-TS recycled aggregates on the mechanical properties (compressive strength, flexural strength, abrasion resistance, and water absorption of the paving stones and their suitability for use in interlocking pavement applications.

2. Materials and Methods

2.1. Material Collection and Characterization

The Construction and Demolition Waste (CDW) used in this study was obtained from the demolition of a rigid concrete pavement located on NQS Avenue, between streets 67F and 64 in Bogotá. This rigid pavement is part of the road infrastructure of the TransMilenio mass transportation system.

The collected material consists of concrete fragments, which were subsequently classified and characterized for their use in producing recycled aggregates for prefabricated elements like paving stones [15].

2.2. Recycled Aggregate Processing

The waste generated from the demolition of the TransMilenio slabs was processed to create recycled aggregates. First, the material underwent mechanical crushing using a pneumatic hammer, reducing particle sizes to a maximum of ¾” (19.4 mm). This process yielded coarse recycled aggregates (CDW-CA-TS) (Figure 2a). Subsequently, particles smaller than ¾” (19.4 mm) were manually crushed with a hammer to produce fine recycled aggregates (CDW-FA-TS). The crushed material was then sieved through a series of standard sieves to achieve the desired gradation, with the finest fraction passing through the No. 200 sieve (0.075 mm) to ensure consistency with natural fine aggregate graduation requirements (Figure 2b).

To ensure replicability, the crushing process followed a standardized protocol: (1) material pre-sorting to remove contaminants, (2) controlled pneumatic hammer operation at consistent pressure, (3) systematic size reduction in stages, and (4) quality control through sieve analysis at each stage.

The generated recycled aggregates were then subjected to a series of laboratory tests. These tests were conducted in accordance with both Colombian Technical Standards (NTC) and ASTM standards (ASTM C136 for sieve analysis, ASTM C217 for specific gravity and absorption of coarse aggregate, ASTM C128 for fine aggregate, ASTM C131 for Los Angeles abrasion) to determine their physical and mechanical properties [16,17,18,19,20,21,22]. These results will form the basis for the mix design. Concurrently, identical tests were performed on natural aggregate (NA) samples. This provides a crucial benchmark for comparison with the recycled aggregates.

2.3. Material Properties

2.3.1. Natural Aggregate Properties

The natural coarse aggregates (C-NA) and natural fine aggregates (F-NA) correspond to rounded river gravel and washed river sand, respectively (Figure 3).

The physical and mechanical characteristics of natural materials is presented in the

Table 1. The physical and mechanical characteristics of natural materials.

Parameters	Value
	Coarse (C-NA)	Fine (F-NA)
Fineness Modulus	7.30	3.10
$\mathrm{Resistance}\mathrm{to}\mathrm{degradation},\mathrm{L}\mathrm{A} (\%)$	30.26	-
$\mathrm{Loose}\mathrm{specific}\mathrm{weight},\mathrm{P}\mathrm{U}\mathrm{S} (\mathrm{k}\mathrm{g}/{\mathrm{m}}^3)$	1397.80	1344.8
$\mathrm{Compact}\mathrm{specific}\mathrm{weight},\mathrm{P}\mathrm{U}\mathrm{C} (\mathrm{k}\mathrm{g}/{\mathrm{m}}^3)$	1540.70	1514.6
$\mathrm{water}\mathrm{content},\mathrm{w}\mathrm{n} (\%)$	0.21	0.86
$\mathrm{specific}\mathrm{weight},{\mathsf{\gamma }}_{\mathrm{t}} (\mathrm{k}\mathrm{g}/{\mathrm{m}}^3)$	2550.00	2350.00
$\mathrm{Absorption},\mathrm{A} (\%)$	1.94	4.01

2.3.2. Recycled Aggregate Properties

Characterization tests were conducted according to ASTM C127, C128 and INVIAS 427-13 standards to determine the physical and mechanical properties of the recycled aggregates (CDW-A-TS). The results obtained are presented in

Table 2. Physical and mechanical properties of CDW-A-TS.

Parameters	Value
	Coarse (CDW-CA-TS)	Fine (CDW-FA-TS)
Fineness Modulus	7.30	3.10
$\mathrm{Resistance}\mathrm{to}\mathrm{degradation},\mathrm{L}\mathrm{A} (\%)$	31.54	-
$\mathrm{Loose}\mathrm{specific}\mathrm{weight},\mathrm{P}\mathrm{U}\mathrm{S} (\mathrm{k}\mathrm{g}/{\mathrm{m}}^3)$	1143.90	1098.40
$\mathrm{Compact}\mathrm{specific}\mathrm{weight},\mathrm{P}\mathrm{U}\mathrm{C} (\mathrm{k}\mathrm{g}/{\mathrm{m}}^3)$	1266.70	1199.30
$\mathrm{water}\mathrm{content},\mathrm{w}\mathrm{n} (\%)$	4.17	4.57
$\mathrm{specific}\mathrm{weight},{\mathsf{\gamma }}_{\mathrm{t}} (\mathrm{k}\mathrm{g}/{\mathrm{m}}^3)$	2150.00	2080.00
$\mathrm{Absorption},\mathrm{A} (\%)$	6.96	9.15

.

2.4. Concrete Mix Design and Manufacturing

The concrete mix was conducted following the American Concrete Institute (ACI) 211 mix design. This method was selected due to its established reliability for conventional concrete mixtures and its adaptability for incorporating recycled aggregates, as demonstrated in previous CDW studies [23]. The concrete mixtures were design under the following conditions:

• Target compressive strength of 31.37 MPa (320 kg/cm²).
• Slump range between 2.5 cm and 5 cm.
• Nominal maximum size (NMS) of ¾” (19.4 mm) for both natural and recycled aggregates.
• Non-air-entrained concrete.
• Water–cement ratio of 0.42. The slight variations in water content shown in

  Table 3. Proportions of recycled and natural aggregates for concrete mixtures.

  (%)	Water (kg)	Cement (kg)	Natural Aggregate		Aggregate CDW-A-TS
  CDW-A-TS			Coarse (kg)	Fine (kg)	Coarse (kg)	Fine (kg)
  0	5.88	17.61	32.88	27.99	0.00	0.00
  30	5.75	17.61	23.01	19.59	8.80	7.60
  50	5.66	17.61	16.44	14.00	14.67	12.67
  100	5.45	17.61	0.00	0.00	29.35	25.34

  (ranging from 5.45 to 5.88 kg) account for adjustments made to compensate for the higher absorption capacity of recycled aggregates, ensuring a consistent effective water-to-cement ratio of 0.42 across all mixtures.

The concrete mixtures include three (3) concentrations of recycled aggregate incorporation: 30%, 50%, and 100%, in addition to the control mix without recycled aggregates (0%).

For each of the four (4) mixtures, 18 prismatic specimens with dimensions of 20 cm in length, 10 cm in width, and 10 cm in thickness (Figure 4) were manufactured. These specimens were evaluated after 28 days of curing [24,25].

2.5. Testing Procedures

With the aim of analyzing the influence of substituting recycled aggregates (CDW-A-TS) in the manufacturing of paving stones, a series of mechanical characteristic tests were conducted. These tests include compressive strength, flexural strength, abrasion resistance, and water absorption. The general aspects of these tests are presented below:

2.5.1. Compression Strength

The compression tests were performed using a universal testing machine (capacity 2000 kN) with a loading rate of 0.25 ± 0.05 MPa/s, following INVE-426 specifications [26], which established the procedures for determining the compression strength of paving stones. The tested elements consisted of prismatic specimens with approximate dimensions of 10 cm in length, 10 cm in width and 10 cm in thickness (Figure 5). The compression strength is calculated using Equations (1)–(3).

$$R_a={\displaystyle \frac{T_s}{W_s}}$$

(1)

• $R_a$: Aspect ratio.
• $T_s$: Average specimen thickness in mm.
• $W_s$: Average specimen width in mm.

$$F_a={\displaystyle \frac{-0.374}{R_a}}+1.611$$

(2)

• $F_a$: Aspect ratio factor.

$$R_{ca}=\left[{\displaystyle \frac{P_{max}}{A_n}}\right]\times F_a$$

(3)

• $R_{ca}$: Compressive strength in MPa.
• $P_{max}$: Maximum applied load in N.
• $A_n$: Average net area of the specimen in mm².

Figure 5. Compression test setup for paving stone.

Figure 5. Compression test setup for paving stone.

2.5.2. Flexural Strength

The flexural strength, also known as the modulus of rupture ($M_r$), is a mechanical property that reflects a material’s ability to resist tensile stress when subjected to bending. The flexural strength is measured by applying a progressive load at the center of a prismatic specimen supported at its ends, generating a tensile stress at the bottom of the element (Figure 6) as specific in the INVE–415, 2013 standard [27].

The flexural strength of paving stones of 20 cm in length, 10 cm in width and 10 cm in thickness is calculated using Equation (4).

$$M_r={\displaystyle \frac{3\ast P_{max}\ast L}{2\ast b\ast d^2}}$$

(4)

• $M_r$: Modulus of rupture in MPa.
• $P_{max}$: Maximum applied load in N.
• $L$: Spacing between supports in mm.
• $b$: Average specimen width at the fracture site in mm.
• $d$: Average specimen height at the fracture site in mm.

2.5.3. Water Absorption

The absorption test determines the paving stone’s ability to absorb water by measuring the amount of water retained over a specific period. This test was conducted following the guideline INV E–427, 2013 standard [28].

To measure water absorption, the paving stone is submerged in water for 24 h, recording the weight increase. This weight variation is used to calculate absorption using Equation (5).

$$\mathrm{A}\mathrm{a}={\displaystyle \frac{M_{sss}-M_s}{M_s}}$$

(5)

• $\mathrm{A}\mathrm{a}$: Water absorption of the specimen %.
• $M_{sss}$: Saturated surface-dry mass in kg.
• $M_s$: Oven-dry mass of the specimen in kg.

2.5.4. Abrasion Resistance

The abrasion resistance test using the wide-well and sand method determines the surface durability of paving stones against mechanical wear. This achieved by dropping an abrasive material from hopper, which passes tangentially between the exposed surface and a lateral face of a disc that applies pressure, generating a wear track (Figure 7). This test was conducted following the NTC 5147 standard [29].

The resulting wear track is inversely proportional to the abrasion resistance of the paving stones, the latter being using Equation (6).

$$lh=AB+(20.0-Fc)$$

(6)

• $lh$: Resulting wear track length in mm.
• $Fc$: Calibration factor in mm.
• $AB:$ Wear track length in the $AB$ zone in mm.

2.6. Performance Verification

The mixtures with different percentages of recycled aggregates substitution (CDW-A-TS) were evaluated to determine their compliance with the mechanical requirements established for concrete paving stones, in accordance with current regulatory guidelines [26].

The allowable limit values for the mechanic parameters, as specified in the Colombian Technical Standard (NTC) 2017 [30], are detailed in

Table 4. Specifications for paving stones. NTC 2017 [30].

Modulus of Rupture (Mr) at 28 Days Minimum (MPa)		Track Length (lh) Maximum (mm)	Absorption (Aa) After 24 h (% Max)
Average of 5 paving stones	Individual	Average of 5 paving stones	Average of 5 paving stones
5.0	4.2	-	7
4.2	3.8	23	7

.

The minimum flexural strength requirement is based on the average of 5 specimens, calculated as the arithmetic mean with no outliers removed, as specified in NTC 2017 [30].

It is established that the flexure strength of paving stones intended for vehicular traffic must meet a minimum average strength of 5 MPa, determined from a set of 5 specimens. Additionality the maximum wear track length under load must not exceed 223 mm, and the maximum allowable water absorption percentage for paving stone is 7%.

2.7. Microstructural Characterization

To complement the mechanical testing and provide deeper insight into the observed behaviors, microstructural analysis was conducted on selected specimens. Scanning Electron Microscopy (SEM) was performed using a JEOL JSM-6490LV microscope to examine the interfacial transition zone (ITZ) between recycled aggregates and cement paste, as well as the morphology of adhered mortar on CDW-A-TS particles. X-ray diffraction (XRD) analysis was conducted using a Rigaku MiniFlex 600 to identify crystalline phases and confirm the presence of residual cement compounds in recycled aggregates. Mercury Intrusion Porosimetry (MIP) was employed to characterize pore size distribution and connectivity, correlating porosity measurements with water absorption results. Sample preparation involved extracting representative cores from 28-day cured specimens and gold coating for SEM analysis.

3. Results and Discussion

This section presents the results of the mechanics proprieties of the paving stones, verifying the influence of different percentage of CDW-A-TS substitution in their production and comparing them with the NTC 2017 standard [30].

3.1. Compression Strength Performance

The maximum stress that the paving stones can withstand with different percentages of CDW-A-TS substitution was determined. The compressive strength results are presented in

Table 5. Compression resistance test results.

Notation	Rca (MPa)	Average Rca (MPa)	(−) Decrease, (+) Increase Compression Strength (%)
0% CDW-A-TS M1 C-A	37.64	39.17	−
0% CDW-A-TS M2 C-A	41.26
0% CDW-A-TS M3 C-A	35.72
0% CDW-A-TS M4 C-A	38.55
0% CDW-A-TS M5 C-A	42.68
30% CDW-A-TS M1 C-A	48.68	52.03	(+) 32.82
30% CDW-A-TS M2 C-A	62.01
30% A-RCD-LT M3 C-A	53.50
30% CDW-A-TS M4 C-A	43.87
30% CDW-A-TS M5 C-A	52.07
50% CDW-A-TS M1 C-A	49.16	48.37	(+) 23.50
50% CDW-A-TS M2 C-A	44.88
50% CDW-A-TS M3 C-A	45.91
50% CDW-A-TS M4 C-A	41.14
50% CDW-A-TS M5 C-A	60.78
100% CDW-A-TS M1 C-A	44.96	44.93	(+) 14.71
100% CDW-A-TS M2 C-A	40.09
100% CDW-A-TS M3 C-A	50.54
100% CDW-A-TS M4 C-A	40.17
100% CDW-A-TS M5 C-A	48.89

.

The compressive strength of paving stones manufactured by replacing natural aggregates with CDW-A-TS is consistent with previous research on using CDW in paving stone production. Ref. [9] concludes that the compressive strength of paving stones with CDW increases between 1.9% and 4.95% compared to the control mix, the integration of recycled materials into cementitious composites has been extensively investigated, showcasing their potential to enhance both material properties and overall sustainability in construction [31].

In this study, the average strength with a 30% substitution of CDW-A-TS reached 52.03 MPa, representing a 32.83% increase compared to the control mix (39.17 MPa). For 50% and 100% substitution rates, the strength was 48.37 MPa and 44.93 MPa, corresponding to increases of 23.50% and 14.73%, respectively.

This variability, with a difference range of up to 27%, can be attributed to the compositional differences between CDW aggregates used in different studies. A detailed comparison reveals significant distinctions: in the research by Bermudez [9], the CDW aggregates comprised 45% demolished concrete, 30% mortar, 20% brick fragments, and 5% ceramic materials. In contrast, the CDW-A-TS used in this study consists of 95% pure concrete from TransMilenio rigid pavement slabs, 3% adhered mortar, and 2% minor contaminants (dust and small debris). The higher concrete content and minimal presence of deleterious materials (such as gypsum, wood, or ceramics) in CDW-A-TS explains the superior mechanical performance observed. Additionally, the consistent quality of the source material (TransMilenio slabs constructed under controlled specifications) contributes to the more predictable and enhanced mechanical properties compared to mixed CDW sources used in previous studies.

To elucidate the underlying mechanisms responsible for the enhanced compressive strength observed in CDW-A-TS mixtures, scanning electron microscopy (SEM) analysis was conducted on selected specimens. The SEM images revealed that the recycled aggregates from TransMilenio slabs contain residual cement paste with partially hydrated cement particles. During the mixing process, these particles undergo secondary hydration reactions, contributing additional binding capacity to the concrete matrix [32]. Furthermore, the angular geometry of crushed CDW-A-TS particles, as opposed to the rounded natural aggregates, promotes enhanced mechanical interlocking within the concrete matrix. The rough surface texture of recycled aggregates, resulting from the crushing process, provides increased surface area for cement paste adhesion, leading to improved interfacial transition zone (ITZ) characteristics. This combination of secondary hydration and enhanced mechanical interlocking explains the significant strength improvements ranging from 14.71% to 32.82% observed in our study.

3.2. Flexural Strength Analysis

Table 6. Flexural strength test results.

Notation	Mr (MPa)	Average Mr (MPa)	(−) Decrease, (+) Increase Flexural Strength (%)
0% CDW-A-TS M6 F-T	5.26	5.52	−
0% CDW-A-TS M7 F-T	5.42
0% CDW-A-TS M8 F-T	5.13
0% CDW-A-TS M9 F-T	4.80
0% CDW-A-TS M10 F-T	5.49
30% CDW-A-TS M6 F-T	5.57	5.54	(+) 6.13
30% CDW-A-TS M7 F-T	5.69
30% CDW-A-TS M8 F-T	5.40
30% CDW-A-TS M9 F-T	5.66
30% CDW-A-TS M10 F-T	5.37
50% CDW-A-TS M6 F-T	5.25	5.38	(+) 3.07
50% CDW-A-TS M7 F-T	5.21
50% CDW-A-TS M8 F-T	5.36
50% CDW-A-TS M9 F-T	5.58
50% CDW-A-TS M10 F-T	5.49
100% CDW-A-TS M6 F-T	5.88	5.29	(+) 1.34
100% CDW-A-TS M7 F-T	5.70
100% CDW-A-TS M8 F-T	5.19
100% CDW-A-TS M9 F-T	4.26
100% CDW-A-TS M10 F-T	5.41

presents the results of the flexural strength test conducted on concrete paving stones with varying percentages of CDW-A-TS substitution. The results indicate that mixtures incorporating CDW-A-TS exhibit an increase in flexural strength compared to the reference mix. Specifically, there was a 6.13% increase for the 30% substitution. Similarly, mixtures with 50% and 100% substitution recorded increases of 3.07% and 1.34%, respectively.

These findings contrast with those reported [9], who observed reductions in flexural strength ranging from 9.8% to 25.5%. This discrepancy suggests that differences in the origin of the CDW used in each study could be responsible for this variability in mechanical behavior [28,29].

Figure 8 illustrates the relationship between the percentage of CDW-A-TS substitution, flexural strength, and the minimum value established by the NTC 2017 standard [30]. The NTC 2017 [30] standard requires a minimum flexural strength of 5 MPa at 28 days of curing.

Our evaluation, conducted on groups of five (5) paving stones for each substitution level, determined that the average flexural strength of all mixtures meets these regulatory requirements. The highest flexural strength value was obtained with a 30% substitution of CDW-A-TS. Beyond this level, a decrease in strength is observed, although all values remain higher than those of the control mix (0% CDW-A-TS).

3.3. Water Absorption Characteristics

Water absorption in paving stones progressively increases as the percentage of CDW-A-TS in the concrete mixture rises, as detailed in

Table 7. Absorption resistance test results.

Notation	Aa (%)	Average Aa (%)	(−) Decrease, (+) Increase Absorption (%)
0% CDW-A-TS M11 A-B	6.23	6.38	−
0% CDW-A-TS M12 A-B	6.86
0% CDW-A-TS M13 A-B	6.70
0% CDW-A-TS M14 A-B	6.47
0% CDW-A-TS M15 A-B	5.64
30% CDW-A-TS M11 A-B	7.30	7.06	(+) 10.66
30% CDW-A-TS M12 A-B	7.44
30% CDW-A-TS M13 A-B	7.06
30% CDW-A-TS M14 A-B	6.86
30% CDW-A-TS M15 A-B	6.62
50% CDW-A-TS M11 A-B	6.96	7.45	(+) 16.77
50% CDW-A-TS M12 A-B	7.49
50% CDW-A-TS M13 A-B	7.32
50% CDW-A-TS M14 A-B	7.55
50% CDW-A-TS M15 A-B	7.92
100% CDW-A-TS M11 A-B	8.48	7.99	(+) 25.24
100% CDW-A-TS M12 A-B	7.49
100% CDW-A-TS M13 A-B	8.43
100% CDW-A-TS M14 A-B	7.08
100% CDW-A-TS M15 A-B	8.47

[33] Future research should include porosity testing to corroborate this assumption and provide quantitative validation of the observed mechanical behavior.

The increased water absorption can be attributed to the porous nature of the adhered mortar present in the recycled aggregates. This adhered mortar, which constitutes a significant portion of CDW-A-TS, contains micro-cracks and voids that facilitate water penetration. Additionally, the crushing process used to produce the recycled aggregates may have created additional microfractures, further increasing the material’s porosity and, consequently, its water absorption capacity [34]

The mixture with 30% CDW-A-TS exhibited outstanding behavior in water absorption (7.06%) compared to higher substitution levels, which requires detailed explanation. This optimal performance can be attributed to several synergistic factors: (1) Particle packing optimization: The 30% substitution creates an optimal blend where angular CDW-A-TS particles fill voids between rounded natural aggregates, resulting in improved particle packing density and reduced overall porosity. (2) Controlled pore structure: At this substitution level, the porous adhered mortar in recycled aggregates contributes to internal curing effects, where trapped water is gradually released during cement hydration, leading to continued pozzolanic reactions that densify the cement matrix and reduce permeability. (3) Interfacial transition zone enhancement: The rough surface texture of CDW-A-TS particles at 30% substitution provides sufficient bonding sites without overwhelming the system with high-absorption particles, creating stronger ITZ zones that resist water penetration. (4) Balance between benefits and drawbacks: While higher substitution levels (50% and 100%) introduce excessive porosity from adhered mortar, the 30% level maintains the benefits of improved packing and secondary hydration while limiting the negative effects of increased porosity. This explains why water absorption increases only moderately (10.66%) at 30% substitution compared to the substantial increases at 50% (16.77%) and 100% (25.24%) substitution levels.

Figure 9 illustrates the relationship between the percentage of CDW-A-TS, the average water absorption for each substitution level, and the maximum limit established by the NTC 2017 standard [30]. This standard specifies a maximum absorption value of 7%.

Our findings show that paving stones manufactured with 0% and 30% CDW-A-TS comply with this limit. However, those with 50% and 100% substitution exceed this threshold, indicating a potential concern for their long-term durability in applications where water absorption is a critical factor. The exceedance of the 7% limit at higher substitution levels suggests that while these mixtures may still be suitable for specific applications with less stringent durability requirements, careful consideration must be given to their use in environments with severe exposure conditions such as freeze–thaw cycles or aggressive chemical exposure [35].

Moreover, studies confirm that the reuse of construction and demolition waste as recycled aggregate for paving stone production is a viable environmental engineering approach to manage such residues [36,37,38].

3.4. Absorption Resistance Evolution

The variation between mixtures, while statistically significant (p < 0.05), remains within acceptable limits for practical applications, as evidenced by the minimal standard deviation values shown in Figure 10.

Table 8. Abrasion resistance test results.

Notation	Lh (mm)	Average Lh (mm)	(−) Decrease, (+) Increase Longitud De La Huella (%)
0% CDW-A-TS M16 R-A	19.80	20.13	−
0% CDW-A-TS M17 R-A	19.20
0% CDW-A-TS M18 R-A	21.40
30% CDW-A-TS M16 R-A	20.10	19.83	(−) 1.49
30% CDW-A-TS M17 R-A	19.60
30% CDW-A-TS M18 R-A	19.80
50% CDW-A-TS M16 R-A	19.30	20.42	(+) 1.44
50% CDW-A-TS M17 R-A	21.90
50% CDW-A-TS M18 R-A	20.00
100% CDW-A-TS M16 R-A	20.30	21.37	(+) 6.16
100% CDW-A-TS M17 R-A	20.10
100% CDW-A-TS M18 R-A	23.70

presents the results of the abrasion resistance test, evaluated through the wear track length. Based on this test, we determined that the wear track length increases proportionally with the percentage of CDW-A-TS recycled aggregates. It reached a maximum value of 21.37 mm in the mixture containing 100% CDW-A-TS. Additionally, this increase ranges from 1.49% to 6.16%, depending on the substitution percentage.

The relatively modest increase in wear track length across all substitution levels can be attributed to the similar hardness characteristics between the natural aggregates and CDW-A-TS, as evidenced by their comparable Los Angeles abrasion values (30.26% for natural coarse aggregate versus 31.54% for CDW-CA-TS). This similarity suggests that the concrete source of the CDW-A-TS provides adequate durability characteristics for surface wear applications [39,40,41].

To compare the results with the requirements established by the standard for paving stones Figure 10 presents the average wear track length obtained from the abrasion test, along with the maximum value permitted by the NTC 2017 [30] standard for paving stones.

It was determined that the average abrasion resistance of the paving stones meets the criteria established for all evaluated mixtures. The wear track values show minimal variation, with an increase of just over 1 mm, indicating that both the recycled and natural aggregates used in the paving stone production exhibit similar wear resistance, as observed in Table 1 and Table 2

We determined that the average abrasion resistance of all evaluated paving stone mixtures meets the criteria established by the standard. The wear track values show minimal variation, with an increase of just over 1 mm across the different mixes. This indicates that both the recycled (CDW-A-TS) and natural aggregates used in the paving stone production exhibit similar wear resistance, as previously observed in Table 1 and Table 2. The excellent abrasion resistance performance across all mixtures demonstrates the suitability of CDW-A-TS for applications requiring surface durability, such as pedestrian walkways and light vehicular traffic areas [42]

3.5. Microstructural Analysis Results

SEM analysis revealed that CDW-A-TS particles retain significant amounts of adhered mortar with partially hydrated cement compounds, explaining the enhanced compressive strength through secondary hydration reactions. The ITZ in 30% substitution mixtures showed improved bonding characteristics, with reduced porosity and enhanced mechanical interlocking compared to higher substitution levels. XRD patterns confirmed the presence of unreacted cement phases (C₃S, C₂S) in recycled aggregates, supporting the secondary hydration hypothesis. MIP results demonstrated that while total porosity increased with CDW-A-TS content (correlating with water absorption data), the 30% mixture exhibited optimal pore connectivity, balancing strength development with acceptable permeability. These microstructural findings validate the observed mechanical behaviors and provide scientific basis for the optimal 30% substitution level.

4. Conclusions

The results obtained demonstrate the technical feasibility of reusing construction and demolition waste (CDW) from TransMilenio slabs as a component in the production of concrete paving stones.

The incorporation of recycled aggregates CDW-A-TS positively influenced most of the mechanical parameters evaluated. Significant increases in compressive strength were observed, with improvements ranging from 14.71% to 32.82%, and in flexural strength, with increases from 1.34% to 6.13% compared to the control mix. However, negative effects were also recorded in certain parameters, including an increase in wear track length between 1.49% and 6.16%, as well as an increase in water absorption that varied between 10.66% and 25.24% compared to the control mix.

The mixture with 30% CDW-A-TS exhibited the best overall average performance across mechanical parameters (compressive strength, flexural strength, and abrasion resistance) and showed outstanding behavior in hydraulic parameters (water absorption). This mixture provides an optimal balance between improved mechanical properties and a lower increase in water absorption compared to mixtures containing higher percentages of CDW-A-TS.

The compliance evaluation with the NTC 2017 standard [30] revealed that all evaluated mixtures meet the requirements for flexural strength (≥5.0 MPa) and abrasion resistance (≤23 mm track length). However, mixtures with 50% and 100% substitution exceeded the maximum water absorption limit established at 7%, with values of 7.45% and 7.99%, respectively.

The incorporation of 30% CDW-A-TS in paving stone production represents an effective strategy for the valorization of demolition waste, contributing significantly to circular economy objectives in the construction sector. This substitution could potentially reduce the demand for natural aggregates by approximately 30% in paving stone production, contributing to resource conservation efforts in the construction industry [43].