Performance Study of Sustainable Concrete Containing Recycled Aggregates from Non-Selected Construction and Demolition Waste

Abstract

In the present study, we investigated the mechanical performance of concrete composed of non-selected construction and demolition waste (C&DW) sourced from both old and new sections of an inactive waste landfill site in Karaj, Iran. Initially, we determined the composition of the coarse and fine C&DW used in concrete production. Subsequently, we meticulously examined the physical and chemical properties of both the C&DW and virgin materials to enable thorough comparisons of the results. We then conducted experimental analyses on 33 concrete mixtures containing recycled C&DW, utilizing various tests, including a compressive strength test (CST) for cylindrical and cubic samples, modulus of elasticity (MOE), wide wheel abrasion test (Capon test), British pendulum number (BPN), and ultrasonic pulse velocity (UPV) test. We considered both non-separated fine and coarse C&DW at different replacement ratios in the recycled concrete (RC). Our findings indicate that using non-separated coarse and fine C&DW in concrete yielded satisfactory results, leading to significant savings in virgin materials required for concrete preparation and promoting sustainable development. Furthermore, non-selected C&DW proved to be a viable sustainable material for similar concrete applications. The results revealed a decrease in brick material consumption in various constructions over the past 20 years in Karaj, contributing to the enhanced strength of C&DW concrete. However, the presence of clay minerals in aged landfill sites can adversely affect concrete performance as a potential destructive factor. Despite the possible negative impact of incorporating fine recycled C&DW materials on concrete mechanical performance, the Capon test results demonstrated that the presence of coarse C&DW can enhance concrete’s wear resistance.

1. Introduction

Construction and demolition wastes (C&DW) currently pose significant global challenges, with approximately 10–30% of total waste being disposed of in landfill sites [1]. One of the most prevalent methods of C&DW disposal remains burial [2,3,4,5]. However, the excessive extraction of natural resources can have profound environmental repercussions [6,7,8]. Therefore, recycling such waste is imperative to reduce its volume, and utilizing these materials as sustainable resources can contribute to sustainable development. One effective strategy to increase recycling rates is by incorporating these wastes into concrete production.

Considering the preservation of ecosystems for future generations, integrating C&D wastes with virgin aggregates (VAs) in concrete manufacturing can yield substantial economic benefits. Unfortunately, a conservative stance or lack of supportive regulations regarding the use of recycled C&DW in concrete products has hindered their widespread adoption, leading to their underutilization in other applications. Addressing this issue, the utilization of waste as a sustainable construction material emerges as a crucial and viable endeavor [2,3,6,9,10].

Due to the increasing number of constructions in Karaj over the past decade, the generation rate of construction and demolition waste (C&DW) has significantly escalated. According to statistics released by the Karaj Municipality, approximately 6000 tons of C&DW are generated daily in this city. Against this backdrop, the present study aimed to analyze recycled concrete mixes using C&D aggregates sourced from different regions of the inactive waste landfill site in Line 4 of Hesar. As shown in Figure 1, Region 1 of this landfill contains materials accumulated from 1990 to 2005, representing old landfills in developing countries. Meanwhile, Region 2 showcases waste composition from 2005 to the present, representing new landfills in developing countries. Notably, approximately five million cubic meters of C&DW have accumulated at this landfill site since 1990. In Region 1, a relatively small amount of waste accumulated over 15 years, while in Region 2, C&DW accumulation exceeded that of Region 1 by more than 2.5 times over a period of 22 years, indicating a significant increase in waste production in recent years.

The extraction of C&DW samples from the landfill was conducted through four boreholes (BHs) and four test pits (TPs) with depths of 12 m, drilled into the aforementioned waste landfill site, as illustrated in Figure 1.

Regarding the impact of landfill age, no chemical evaluation was reported. However, this study presents the results of common essential chemical tests on aggregates used in concrete, considering old and new C&DW as well as virgin materials. Furthermore, rare experiments such as the Capon and British pendulum number (BPN) tests were conducted on concrete produced from the fine and coarse C&DW, with the obtained outcomes compared against acceptable criteria outlined in certain regulations for conventional concrete. Ultimately, the ultrasonic pulse velocities (UPVs) in the recycled concrete samples were compared with those of reference samples, and some correlations were proposed.

Numerous studies investigating the utilization of construction and demolition waste (C&DW) in concrete have encountered a range of limitations, as outlined in this section. Various factors, including people’s preferences, resource availability, and abundance, significantly influence the selection of materials for construction projects. Consequently, the age of a landfill can profoundly impact material selection [11,12]. Understanding the properties of C&DW in both old and new landfill sites is crucial for analyzing the mechanical performance of recycled concrete. To the best of the authors’ knowledge, there have been no recent reports on changes in these materials over the past 40 years or the effects of using old and new waste in concrete. Therefore, comprehending the effects of landfill age on material selection is essential.

Most researchers [13,14,15,16,17] have conducted common tests such as slump, density, compressive strength tests (CST), and so on, on conventional and recycled concrete. However, the use of some complementary tests, such as non-destructive ultrasonic pulse velocity (UPV), British pendulum number (BPN) for skid friction resistance, and Capon wide wheel abrasion tests for determining the characteristics of C&DW concrete made from fine and coarse non-selected C&D wastes, has been limited. Moreover, understanding the chemical properties of C&DW before their use in recycled concrete is essential, a factor often neglected in many studies.

Recent research has explored the utilization of various wastes in concrete, including rubber particles [18,19], palm oil clinker (POC) [20], glass powder, aluminum fibers [21], recycled concrete, brick aggregates [22], glass and carbon-fiber-reinforced polymers [23], reclaimed asphalt pavement [24], newspaper ash [25], and more. However, most studies have only examined a single type of selected C&D aggregates, often separated in recycling plants. Other mentioned wastes may be found in small amounts in specialized plants. Nevertheless, research on concrete composed of non-separated C&D aggregates (with different compositions), considering the physical–chemical properties prevalent in all industrialized countries, remains scarce.

Concerning the use of coarse C&DW in concrete production instead of coarse virgin aggregates (VAs), previous studies have shown that low replacement ratios do not significantly alter the physical and chemical properties of concrete [15,26,27]. However, limited research has explored the effects of fine recycled aggregates from C&DW on concrete. Previous studies have identified excessive water absorption as the primary hindrance to the use of fine C&DW in new concrete production.

Ultrasonic pulse velocity (UPV) is a recognized non-destructive testing method to estimate the mechanical properties and internal cracks of concrete materials. Despite this, studies implementing this technique to determine the characteristics of recycled concrete made from fine and coarse non-selected C&DW are scarce. This situation is even more pronounced for the British pendulum number (BPN) for skid friction resistance and wide wheel abrasion (Capon) tests, as illustrated in

Table 1. Some previous studies about concrete tests containing recycled aggregates.

Reference	Tests on Fresh Concrete	Tests on Hardened Concrete
Poon et al. [28]		compressive strength in cubes, SEM
de Brito et al. [29]	slump, density	compressive strength in cubes, flexural strength, abrasion resistance (grinding wheel method)
Poon et al. [13]	slump, bleeding test	compressive strength in cubes
Evangelista and de Brito [14] and Medina et al. [30]	slump, density	compressive strength in cubes, splitting tensile strength, MOE, abrasion resistance (grinding wheel method)
Medina et al. [31]		splitting tensile strength, compressive strength in cylinders, SEM
Xiao et al. [32]		nanoindentation tests, SEM, AFM
Barbudo et al. [33] and Bravo et al. [34]	water reduction, slump, density	splitting tensile strength, compressive strength in cubes, MOE, abrasion resistance (grinding wheel method)
Medina et al. [15]	slump	density, compressive strength in cylinders, splitting tensile strength, total water absorption, sorptivity
Alves et al. [35]	slump, density	compressive strength in cubes, splitting tensile strength, MOE, abrasion resistance (grinding wheel method)
Zong et al. [36]		water permeability, air permeability, chloride ion permeability, SEM
Andre et al. [37], Bravo et al. [10]	slump, density	water absorption by immersion, and chloride ions, capillarity, carbonation resistance, penetration resistance
Zhou and Chen [38]		compressive strength in prisms and cubes, flexural strength, MOE, energy absorption capacity
Alexandridou [39]		compressive strength in cubes, water absorption, sorptivity, freeze-thaw resistance, carbonation
Kazmi et al. [40]	slump	splitting tensile strength, flexural strength, MOE
Ghorbani et al. [41]	slump	water absorption, compressive strength in cylinders, splitting tensile strength, abrasion resistance (wide wheel method), freeze-thaw resistance
Shaban et al. [42]		compressive strength in cubes, flexural strength, MOE, rapid chloride ion migration, SEM
Saha et al. [43]	slump, density	compressive strength in cylinders, flexural strength, SEM, electrical resistivity
Sohail et al. [44]		compressive strength in cylinders, SEM, UPV
Wu e al. [45]		density, compressive strength in cylinders, XRD, SEM, thermal tests, leaching toxicity
Revilla-Cuesta et al. [16]	slump, density	density, compressive strength in cylinders and cubes, splitting tensile strength, MOE, UPV, shrinkage tests
Zhang et al. [46]	slump	compressive strength in cubes, MOE, shrinkage tests, X-ray diffraction
Wang et al. [47]		compressive strength in cubes, dynamic MOE, resistivity, chloride penetration

. Moreover, understanding the chemical properties of C&DW before its use in recycled concrete is crucial, a consideration often overlooked in many studies.

To address the above limitations, the current study investigated non-separated C&D materials for new concrete production with different mix compositions. Recycled materials and virgin aggregates were employed in the present study.

Nowadays, construction waste has significantly increased due to urbanization and economic development. Consequently, the production of construction waste has reached a critical juncture where landfills near urban areas are overflowing with these materials, emphasizing the urgent need for recycling on a global scale.

With the rise in construction material prices and the recognition of the limitations of natural resources, there is a growing awareness of the importance of preserving national capital for future generations. Additionally, participation in recycling programs, while safeguarding the environment, has become paramount. Therefore, the management and recycling of construction waste have become increasingly important and necessary worldwide.

Given the significance of this issue, researchers worldwide are dedicating considerable efforts to studying the recycling of construction waste and its integration into construction projects, aiming to reintroduce these materials into the construction sector and mitigate the unsustainable depletion of natural resources. In this context, several research gaps and innovations are being explored, including:

• Utilizing aggregates from non-selected construction and demolition waste (C&DW) as a sustainable material.
• Exploring the potential of concrete made from non-selected C&DW to contribute to sustainable development.
• Investigating the effects of landfill age on construction waste properties.
• Assessing the impact of fine aggregates and addressing issues related to excessive water absorption.
• Understanding the properties of C&D waste in both old and new landfill sites.

Addressing these research gaps and advancing our understanding of construction waste recycling will play a crucial role in promoting sustainability in the construction industry and mitigating environmental degradation.

2. Experimental Methodology

2.1. C&DW Materials

Figure 1 depicts the location of the construction and demolition waste (C&DW) material preparation site. The recycled C&DW aggregates were sourced from the waste landfill site in Line 4 of the Hesar zone, situated in the city of Karaj, Alborz Province, Iran. A mini-crusher was employed to crush these low-quality wastes. Subsequently, the materials in the boreholes (BHs) and test pits (TPs) were thoroughly mixed, and the coarse and fine wastes were separated.

Figure 2 presents the results of the C&DW compositions for the fine and coarse materials, respectively. Based on the C&DW compositions depicted in Figure 2, TP3 and BH8, located in Region 1, exhibited a significant amount of clays. This suggests a decline in the use of bricks and tiles (i.e., clay structure materials) in the city of Karaj from 1990–2005 to the present. According to this figure, the coarse component consists of concrete masonry units, unbound aggregates, clay masonry units, bituminous mixtures, and a few other materials. The fine component comprises masonry units, clay structures, bituminous mixtures, and other materials.

2.2. Concrete Mix Proportions

To investigate the behavior of the new concrete, eight mixtures were prepared for each test pit (TP), comprising four recycled concretes with replacement ratios ranging from 25% to 100% of the overall weight of the coarse virgin aggregates (VAs), and four mixes with replacement ratios ranging from 25% to 100% of the overall weight of the fine VAs. A total of 33 different cases, including four TPs and one reference concrete with 100% VA, were further analyzed. The concrete mixes were proportioned without any admixtures or additions, aiming for a strength class of C20/25 concrete. C20/25 concrete, typically with a slump of 50 ± 15 mm, is commonly used for light traffic requirements. Therefore, it was chosen as the target concrete for this research. To ensure a fair comparison between the results, the slump was kept constant and adjusted accordingly for each mix. The material content, based on the mixture design commonly employed by ready-mix companies, is detailed in

Table 2. Mixture data in kg/m³ for reference and recycled concrete (RC).

Reference	Cement	Sand	Gravel	C&D Wastes
Reference concrete	330	998	788	0
RC-C25	330	998	591	197
RC-F25	330	748	788	250
RC-C50	330	998	394	394
RC-F50	330	499	788	499
RC-C75	330	998	197	591
RC-F75	330	250	788	748
RC-C100	330	998	0	788
RC-F100	330	0	788	998

. In this table, the letters C, F, and # denote the coarse aggregates, fine aggregates, and replacement ratios, respectively. For example, C75 represents recycled concrete, where construction and demolition (C&D) materials replace 75% (by weight) of the coarse VAs.

The analysis results for the reference and recycled concrete are presented in Figure 3 and then compared with the upper and lower limit plots defined by the technical specifications, as well as the grading of both materials suitable for RC production. It was observed that the density of fine particles was slightly lower than that of the coarse C&DW aggregates. The fine particles comprised a mixture of cement mortar and crushed brick, potentially resulting in a lower density. This study also noted that the density of the C&DW particles was lower than that of the natural materials.

2.3. Laboratory Tests

Eight continuous machine core boreholes (BHs) were drilled along Line 4 of the waste landfill in Karaj city to extract C&DW (the locations are shown in Figure 1). The properties of the C&DW materials were compared with the “technical specifications of typical road and highway administrations” for various applications. This comparison of properties is crucial for contractors, state road authorities, consultants, operators, and designers to assess the potential use of these materials in different civil engineering projects.

Basic laboratory tests were conducted to evaluate the characteristics of both the virgin and C&DW materials (see

Table 3. Summary of the main laboratory tests.

Experiment	Standard	Output Characteristics	Description
Sieve analysis	ASTM D 422 [48]	Physical characteristics of natural materials and C&DW	All tests were performed four times for each borehole (one test for each batch) and virgin materials to verify the repeatability of the test data.
particle density	AS 1141.5 [49] and AS 1141.6.1 [50]	Physical characteristics of natural materials and C&DW
water absorption	AS 1141.5 [49] and AS 1141.6.1 [50]	Physical characteristics of natural materials and C&DW
Atterberg limits	ASTM D 4318 [51]	Physical characteristics of natural materials and C&DW
flakiness index	BS 812-105.1 [52]	Physical characteristics of natural materials and C&DW
Los Angeles abrasion	ASTM C 131 [53]	Physical characteristics of natural materials and C&DW
modified compaction	ASTM D 1557 [54]	Compaction characterization of natural materials and C&DW
CBR	ASTM D 1883 [55]	Load-displacement curves of C&DW and natural materials
direct shear (305 × 305 × 204 mm)	ASTMD 5321 [56]	Shear strength performance and behavior of natural and C&DW materials

). The experiments were conducted in accordance with the ASTM standards, common technical highway specifications, and British Standards (BS), which included tests such as direct shear tests, California bearing ratio, Los Angeles loss degradation, modified Proctor compaction effort, Atterberg limits, water absorption, flakiness index, Pycnometer test, and sieve analysis.

The particle size distribution was determined for both the virgin and C&DW aggregates in accordance with ASTM D422 [48]. The flakiness index test was carried out according to BS 812-105.1 [52]. Additionally, particle density and water absorption tests were performed on both the fine and coarse fractions. The Atterberg tests served as indicators of water content [57]. Specifically, the fine parts were used to determine liquid and plastic limits in accordance with ASTM D4318 [51].

The Los Angeles abrasion test, a suitable method for assessing resistance to forces and abrasion, was used to compare the relative quality of both the virgin and waste aggregates. Following the ASTM C 131 [53] guidelines, the materials were placed in a drum containing steel spheres and rotated a specified number of times to assess the abrasion values.

Modified Proctor compaction tests, as per ASTM D1557, were utilized to determine the optimum moisture content and maximum dry density of the materials. The C&DW were compacted in a mold using five layers and 25 blows with a 4.5 kg rammer.

CBR tests, commonly employed to determine load-displacement behavior, were carried out at optimum water content and under modified compaction conditions. To simulate worst-case scenarios, specimens were soaked for four days, in line with ASTM D1883 standards [55]. The tests involved inserting a steel piston into the compacted sample at a specified rate.

Direct shear tests were conducted at the optimum moisture content and maximum dry density of the samples to evaluate the shear strength characteristics of both types of materials. The samples were prepared and compacted in the shear box in three equal layers and the test was conducted according to ASTM D5321 [56] for both types of materials. Different normal stresses of 120, 60, and 30 kPa were applied in three direct shear tests, corresponding to stresses of a back-fill wall at varying heights of 6.7 m, 3.3 m, and 1.6 m, respectively, with a unit weight of 18 kN/m³.

To simulate the initial stress, the samples underwent a consolidation process for 30 min. The shearing displacement step rate was set at 0.025 mm/min. The dimensions of the specimens were 305 mm × 305 mm × 204 mm, and the device had a maximum horizontal shear displacement capability of 75 mm. This study presented horizontal displacement-shear stress curves, as well as the peak shear strength, residual strength, and ultimate strength of the C&DW aggregates [58]. A summary of the main laboratory tests is listed in Table 3.

The chemical characterization of the C&D materials, including water solubility, light contaminant content, water-soluble chlorides and sulfate ions, acid-soluble sulfates and chlorides, and total sulfur content, was investigated for all BH and TP materials based on the EN 1744-1 [59] requirements [60]. For brevity, the results of the physical tests on the C&DW for all eight BHs can be found in a previous study [61]. Following the chemical analysis of the C&DW in various BHs and TPs, the mechanical performance of the new recycled concrete with C&D wastes was determined.

To streamline the presentation and enhance its clarity for readers, only the results of the novel concrete made with materials from TPs 1, 3, 5, and 7 were examined here. Basic tests such as density (EN12350-6 [62]), slump (EN 12350-2 [63]) tests, concrete sample tests (CSTs) (in cubic and cylindrical samples, EN 12390-3 [64]), and modulus of elasticity (MOE) (ASTM C469 [65]) were conducted. Additionally, complementary tests including scanning electron microscopy (SEM), Capon abrasion resistance (DIN EN 1338 [66]), skid friction resistance (BS 6717 [67]), and ultrasonic pulse velocity (UPV) (ASTM C215 [68]) tests were performed.

3. Comparison of Some Waste Aggregates with C&DW Aggregates in Previous Studies

The geotechnical characterization of C&DW in the present article is compared with a previous study and presented in

Table 4. The geotechnical characterization of C&DW in the present article compared with previous studies.

Other Cases	The Contents of Recycled Aggregates	Geotechnical Characterization of C&DW
		Cu	Cc	γs (kN/m3)	WA (%)	LA (%)	FL (%)
Arulrajah et al., 2013 [69]	RCA	31.25	0.94	27.1 c, 26 f	4.7 c, 9.8 f	44	11
Arulrajah et al., 2014 [70]	RCA	30.42	1.12	27.6 c, 26.5 f	4.66 c, 9.75 f	28	11
Gabr et al., 2012 [71]	RCA	46.67	2.14	26	8.9	39	-
Herrador et al., 2012 [72]	RCA-RAP-CB	30	1.29	-	-	34	12
Blankengel et al., 2005 [73]	RCA	23.75	4.87	-	5.2	31	-
Cerni et al., 2012 [74]	Non selected C&DW	85.71	1.93	25.7	7.72	37	26.5
Park et al., 2003 [75]	RCA	105	3.44	25.27	1.43	32.9	-
Jimenez et al., 2012 [76]	Non selected C&DW	43.18	2.53	21.38	8.8	34	8
This study	Non selected C&DW	28.43	1.23	19.69–21.3	4.6 c, 7.8 f	35–42	16–17

^c: Coarse; ^f: fine.

. This table outlines the geotechnical characteristics of C&D waste, including scaling index, grading, specific gravity, water absorption, and Los Angeles abrasion of the recycled aggregates.

The geotechnical properties of the studied C&DW depot in Karaj city were found to be similar to the average results of various C&DW studies. Moreover, the Los Angeles abrasion loss of the C&DW in the present study was found to be between 35–42%, while the flakiness index value ranged from 16–17%. This indicates that the particles have relatively fewer elongated aggregates, resulting in higher efficiency under compression. Therefore, these materials are still acceptable for usage as a base material, with a maximum value of 35, in road construction according to normal state road authorities and pavement applications. The water absorption of natural materials does not exceed 3%, while it varies from 1 to 10% for the C&DW materials. Thus, the existing conditions and values remain consistent with this study.

Considering that C&DW includes concrete and brick bits in specific proportions for each BH, the water absorption in this study is roughly 4.6 times higher than that of natural materials for coarse aggregates and 7.8 times higher for fine aggregates. According to the specific weight of natural materials, the dry unit weight of C&DW ranges from 19.69–21.3 kg/m³.

Given that the density of different materials is affected by the amount of water absorption, the optimal humidity in this study is approximately 7.8%, which is similar to other studies. The comparison of some geotechnical parameters of C&DW with other studies is shown in Table 4. Therefore, in the current study, the geotechnical parameters of C&DW are comparable to the recycled aggregates of other studies. According to Figure 4, the maximum unit weight of C&DW is lower than that of virgin materials, while its optimal water content is higher than that of virgin materials.

In the present study, the comparison of geotechnical characteristics such as scaling index, grading, Los Angeles abrasion, specific gravity, and water absorption of the C&D waste and recycled aggregates with previous studies is shown in Table 4.

4. Results and Analysis

4.1. Chemical Properties of C&DW

The results of chemical tests on the fine and coarse VAs and C&D wastes from different BHs and TPs are presented in

Table 5. Chemical properties in percentage for C&DW and VA materials.

Chemical Tests	Water Solubility	Light Contaminants Content	Total Sulfur Content	Acid Soluble Sulfates Content	Water Soluble Sulfates Content	Acid Soluble Chlorides Content	Water Soluble Chlorides Content
CTP1	1.5	<0.1	0.2	0.3	0.06	0.013	0.006
CBH2	0.7	0.3	<0.1	0.15	0.04	0.011	0.005
CTP3	0.9	1.2	0.1	0.15	0.03	0.015	0.007
CBH4	1.7	0.3	<0.1	0.1	0.03	0.013	0.006
CTP5	1.6	0.5	0.1	0.2	0.04	0.011	0.005
CBH6	0.8	0.2	0.1	0.2	0.04	0.012	0.005
CTP7	1.1	0.2	<0.1	0.1	0.03	0.011	0.005
CBH8	0.7	0.9	<0.1	0.1	0.02	0.004	0.004
FTP1	1.5	0.2	0.3	0.7	0.08	0.018	0.007
FBH2	1.5	0.6	0.3	0.6	0.08	0.015	0.007
FTP3	1.4	1.5	0.3	0.6	0.04	0.02	0.009
FBH4	1.9	0.7	0.2	0.4	0.06	0.011	0.006
FTP5	1.5	0.5	0.1	0.3	0.05	0.013	0.006
FBH6	1.1	1	0.1	0.2	0.05	0.014	0.006
FTP7	1.2	0.9	0.1	0.2	0.05	0.013	0.006
FBH8	1	1.3	<0.1	0.1	0.03	0.003	0.004
Gravel (CVA)	insoluble	not detected	<0.05	<0.1	not detected	<0.01	<0.001
Sand (FVA)	insoluble	not detected	<0.05	<0.1	not detected	<0.01	0.002
Threshold	10 ¥	0.5 *	1 *	0.8 *	0.20 *	0.015 §	0.010 *

*: EN 12620 [77]; ^§: ASTM C 1152 [78]; ^¥: EN 1744 [60].

. The thresholds correspond to the maximum allowable values for employing the aggregates in various concrete productions. According to this study, the water soluble chloride ions were lower than the threshold. However, the acid soluble chlorides exceeded the maximum limit for the fine C&D wastes in TPs 1 and 3. To ensure safety, EN 12620 [77] recommends that acid-soluble chloride should be used for calculating the chloride ion content of concrete. Therefore, in this scenario, the application of epoxy-coated steels (or corrosion-resistant ones) or reducing the proportion of C&D materials in the designed compositions of reinforced recycled concrete could be beneficial.

As expected, the water- (or acid-) soluble contents of the fine materials were higher compared to those of the coarse ones, as the fine fragments had a larger surface area exposed to the solvent. The water-soluble sulfates with relatively low percentages were obtained for the C&DW (ranging from 0.02% to 0.08%) and virgin materials, satisfying the threshold limits of 0.2 set in the EN 12620 standard [77]. However, according to the acid-soluble sulfate content results, the relatively high amounts of sulfates (close to the maximum ranges) for the fine aggregates in TP 1 might increase the vulnerability of concrete to chemical attacks.

The percentage of light contaminant content in different C&D wastes poses a major problem. Based on EN 12620 [77], the threshold limit for organic contaminants is 0.5%, which could be reduced to 0.25% by mass when a particular concern exists about the concrete surface finishing. It should be noted that corresponding values ranging from 0.3% to 17.1% (very high contents) have been reported in previous studies for eight types of C&D wastes in Portugal. The water solubility test could provide quick indications of aggregate mass loss in water, and an increase in solubility could jeopardize concrete performance. However, for this chemical test on the C&DW and virgin materials in this study, the results fell within the threshold values.

4.2. Basic Tests on the Fresh State of Recycled Concrete

All mixtures were prepared with a cone Abrams slump of 50 ± 15 mm, as depicted in Figure 5. The composition and shape of different C&D wastes could irregularly affect the workability of the mixes, consistent with the findings reported in [79]. Due to the larger voids of the C&D materials (more porous) compared to VAs, additional lubrication was needed to achieve workability, similar to the reference concrete. A notable increase in the water-to-cement ratio was observed for mixes with fine C&D wastes from TP 3, as this TP contained a significant amount of fine clay particles in the crushed bricks that absorbed more water. The old concrete and mortar masonry in some TPs had gained strength from hydration by absorbing a significant amount of water.

Figure 6 illustrates the fresh-state densities for the recycled concrete composed of fine and coarse C&D wastes in TPs 1, 3, 5, and 7. As the C&DW replacement ratios increased, the densities approximately decreased linearly, with coefficients of determination (R2) ranging from 0.94 to 0.99. This reduction in density could be attributed to the lower density of the C&D materials compared to the VA density. It is worth noting that these linear relationships were qualitatively consistent with the experiments reported in [80] on concrete incorporating coarse recycled aggregates.

4.3. Basic Tests on the Hardened State of Recycled Concrete (CSTs)

Figure 7a showcases various recycled concrete samples in cubic and cylindrical shapes before and after the CST. Figure 7b,c reveal the shear plane and hourglass modes of failure observed in cylindrical and cubic recycled concrete samples, respectively. All of the reference and recycled concrete samples exhibited similar brittle behavior at the end of the tests. In the cubes, cracks initiated from the corners and propagated vertically along the length of the sample. For the cylinders, shallow cracks dispersed randomly in various directions, with a deeper crack at an approximately 45° angle observed in the middle of the cylinder.

Table 6. Compressive strength of the recycled concrete mixes in cubes and cylinders with compressive strength classes.

Mix Type	Cylinder Sample		Cube Sample		Compressive Strength Class (28 Day)
	fcm (MPa)	∆ (%)	fcm (MPa)	∆ (%)
Reference concrete	25.5 ± 0.8	-	35.1 ± 1.5	-	C20/25
TP1-C25	25.4 ± 1.1	−0.4	33.9 ± 0.9	−3.4	C20/25
TP1-C50	23.6 ± 0.6	−7.5	30.3 ± 0.4	−13.7	C20/25
TP1-C75	22.4 ± 1	−12.2	27 ± 1.4	−23.1	C20/25
TP1-C100	17.3 ± 0.7	−32.2	23.3 ± 1.8	−33.6	C16/20
TP3-C25	22.4 ± 1	−12.2	31.2 ± 2	−11.1	C20/25
TP3-C50	20.7 ± 1.4	−18.8	27.9 ± 0.7	−20.5	C20/25
TP3-C75	17.7 ± 0.9	−30.6	24.5 ± 1.4	−30.2	C16/20
TP3-C100	14.8 ± 1.9	−42	20.4 ± 1.9	−41.9	C16/20
TP5-C25	24.9 ± 1.7	−2.4	34.3 ± 0.7	−2.3	C20/25
TP5-C50	23.6 ± 1.4	−7.4	31.8 ± 0.4	−9.4	C20/25
TP5-C75	21.2 ± 1	−16.8	28.7 ± 1.1	−18.2	C20/25
TP5-C100	18.2 ± 0.8	−28.6	25.1 ± 1.6	−28.5	C16/20
TP7-C25	23.9 ± 0.4	−2.4	34.5 ± 1.1	−1.7	C20/25
TP7-C50	22.7 ± 0.6	−11	30.9 ± 1	−12	C20/25
TP7-C75	19.3 ± 1.7	−24.3	29.2 ± 2.1	−16.8	C16/20
TP7-C100	16.8 ± 0.4	−34.1	27.1 ± 1.8	−22.8	C16/20
TP1-F25	24.2 ± 1.6	−5.1	31 ± 0.5	−11.7	C20/25
TP1-F50	22 ± 1.2	−13.7	27.6 ± 0.6	−21.4	C20/25
TP1-F75	21.4 ± 0.9	−16.1	25.7 ± 1.3	−26.8	C20/25
TP1-F100	18 ± 0.8	−29.4	20 ± 1.6	−43	C16/20
TP3-F25	20.5 ± 1.1	−19.6	27.3 ± 0.9	−22.2	C20/25
TP3-F50	19.2 ± 0.8	−24.7	24.5 ± 1.1	−30.2	C16/20
TP3-F75	16. 4 ± 1.3	−35.7	21. 5 ± 0.4	−38.7	C16/20
TP3-F100	13.2 ± 1.2	−48.2	17.6 ± 0.8	−49.9	C12/15
TP5-F25	23.3 ± 0.7	−8.6	32.1 ± 0.6	−8.5	C20/25
TP5-F50	22.9 ± 0.3	−14.1	28.8 ± 0.8	−17.9	C20/25
TP5-F75	20.8 ± 0.5	−22.4	24.9 ± 0.1	−29.1	C20/25
TP5-F100	17.8 ± 0.8	−31.4	22.8 ± 0.7	−35	C16/20
TP7-F25	22.4 ± 0.9	−12.2	31.7 ± 1.4	−9.7	C20/25
TP7-F50	20.4 ± 1.1	−20	27.4 ± 2	−21.9	C20/25
TP7-F75	19 ± 2	−25.5	25.5 ± 1.8	−27.4	C16/20
TP7-F100	15.2 ± 0.6	−40.4	23.3 ± 1	−33.6	C12/15

presents the mean compressive strength (fcm) of the recycled concrete after 28 days and its strength variations (∆) compared to the reference concrete in terms of cylindrical and cubic concrete samples, considering the different incorporation ratios of the fine and coarse C&D wastes. The 28-day compressive strength for the reference concrete in cylindrical and cubic samples was 25.5 ± 0.8 and 35.1 ± 1.5 MPa, respectively, corresponding approximately to the C20/25 strength class, as targeted. The compressive strength of mixtures with coarse and fine C&D wastes reached the strength classes of C16/20 and C12/16, respectively. However, if only the strength levels of the cubic samples were considered, the recycled concrete would need to be categorized in a higher class. Mixtures with fine recycled C&D aggregates exhibited a much higher compressive strength loss compared to those with coarse aggregates. For instance, using 100% recycled coarse and fine C&D wastes from TP 5 resulted in a 28-day compressive strength decrease of 28.5% and 35% in cubes, respectively. Regardless of the C&D aggregate size, the compressive strength was influenced by the replacement ratios of the C&D materials due to the more porous nature of the waste. As the incorporation of C&D wastes increased, the effective water-to-cement ratio also increased to maintain a constant target slump. These results are quantitatively consistent with the findings in [81], which investigated the compressive strength of concrete in cubes with recycled fine and coarse C&D aggregates from different recycling plants. They demonstrated that 100% replacement ratios could decrease the 28-day compressive strength by around 28%, assuming a constant concrete slump. Additionally, [39] reported compressive strength reductions of recycled concretes ranging from 8% to 37% for cubic samples at a 75% replacement ratio, based on various C&DW sources in Greece.

Figure 8 illustrates that the compressive strength maturity for mixtures with recycled C&D wastes is similar to that of the reference concrete. In other words, as the concrete ages, the increase in strength over time decreases. These general trends for recycled concrete were more pronounced compared to reference concrete. Therefore, the compressive strength of recycled concrete continued to increase after 28 days but with a lower slope than conventional concrete.

4.4. Basic Tests on the Hardened State of Recycled Concrete (MOE)

Figure 9 and Figure 10 compare the measured MOE (Gpa) values determined in the current research for the recycled concrete with different replacement ratios of the coarse and fine C&D wastes with those mentioned in some previous studies and the relevant standards, as provided in

Table 7. Practical relationships for determining MOE of conventional concrete.

Reference	Equation
ACI 318-11 [83]	$MOE=4.73\sqrt{f_{cm}}$
ACI 318-08 [82]	$MOE=4.3\times {10}^{-5}{d}^{1.5}\sqrt{f_{cm}}$
Ravindrarajah et al., 1987 [84]	for conventional concrete	$MOE=5.31\sqrt{f_{cm}}+5.83$
	for recycled concrete	$MOE=3.02\sqrt{f_{cm}}+10.67$
Eurocode 4-04 [85]	$MOE=9.5f_{cm}{ }^{0.33}$
Liu 2007 [86]	$MOE=3.9\times {10}^{-5}{d}^{1.5}\sqrt{f_{cm}}+3.338$

.

In these figures, the substitution of VA with C&D wastes resulted in a decrease in MOE. Regardless of the incorporation ratios, one of the primary factors influencing the concrete’s MOE was the composition of the C&DW. Mixes with C&DW from TP3, characterized by a high clay content, yielded the poorest results, irrespective of aggregate sizes. Variations in incorporation ratios also led to a reduction in the MOE of 2–34% and 9–38% compared to reference concrete for coarse and fine C&D aggregates, respectively. These differences were attributed to the significant effects of C&DW stiffness and size variations in each TP on MOE. Notably, these results closely aligned with findings from some references [87,88], indicating that 100% replacement of VAs with C&D wastes could result in a 28-day MOE loss of 18–40%.

In both Figure 9 and Figure 10, the models recommended in [84] and Eurocode 4-04 [85] produced the poorest results for both conventional and recycled concrete, respectively. According to the American Concrete Institute (ACI) 318-11 [83] guidelines for conventional concrete mixes, the MOE values for recycled concrete were overestimated. These results suggest that neglecting the concrete density in practical MOE formulas may lead to misleading MOE predictions for recycled concrete mixes. Liu’s model for calculating the MOE of recycled concrete with fine and coarse C&D aggregates is deemed acceptable.

Finally, as illustrated in Figure 9 and Figure 11a, the measured MOE of mixes composed of coarse C&D materials closely matched those predicted by ACI 318-08 [82], showing a relatively excellent R² = 0.98. However, in Figure 10, for mixes with fine C&D wastes, the experimental MOE values were lower than the corresponding values defined by the standards and previous research for conventional concrete mixes. Therefore, it was necessary to define a novel equation for concrete made with fine-grained C&D materials, as depicted in Figure 11b. The proposed model exhibited a relatively good fit for data with an R² of 90, covering a wide range of replacement ratios.

4.5. Complementary Tests, Capon Abrasion Resistance

Wide wheel (Capon) abrasion tests were conducted on 200 × 100 × 50 mm concrete block samples composed of C&D aggregates over a period of 28 days. The sample surfaces were meticulously smoothed and kept parallel to each other. During the test, a disk rotated at a constant speed of 75 revolutions per 60 ± 3 s, ensuring uninterrupted abrasion on the upper surface of the paving sample. At the conclusion of the test, the abrasion resistance was assessed by measuring the groove width produced on the sample surface, following the DIN EN 1338 (2003) standard [66]. Each test was replicated twice for different recycled C&DW and reference concrete samples, and the abrasion trace was examined using a precision digital caliper with 0.01 mm accuracy. Subsequently, the samples were observed under a magnifying glass (4× magnification) and placed on a faceplate to measure the groove width. Figure 12 provides a schematic illustration and a view of the wide wheel test device.

Figure 13 illustrates the average wear widths recorded for different concrete mixes, taking into account the impact of the C&DW replacement ratios. In the case of coarse aggregates, the groove width tended to decrease as the replacement ratio increased, consistent with the findings from previous studies [27] on superplasticizer mixes. This increase in abrasion resistance could be attributed to the improved distribution of the recycled concrete matrix and the enhanced microstructure at the interfacial transition zone. The porous surfaces of the C&D wastes allowed for easier penetration of cement into the aggregates, leading to increased internally hydrated cement paste. Moreover, the mini-cracks resulting from the crushing of C&D materials were reduced in the interfacial area. Additionally, the preferred orientations for calcium hydroxide crystals might decrease, resulting in a higher bond strength between the C&D wastes and the cement paste. As suggested by [27], this increase in bond strength could enhance wear resistance more than compressive strength. However, for fine particles, contradictory results were observed. Full replacement of coarse VA led to an increase in abrasion resistance by over 4.1%, regardless of the TP composition. Conversely, 100% replacement of the fine VA resulted in a decrease in abrasion resistance of about 13–21%.

4.6. Complementary Tests, Skid Friction Resistance, and UPV

In order to control the skid resistance and the surface friction of the pavements, the British pendulum skid-resistance tester is commonly used [89], as shown in Figure 14a. UPV is shown in Figure 14b.

In this study, the British pendulum number (BPN) for the blocks composed of C&D and virgin materials was determined. Figure 15 displays the average BPN for different concrete mixes, reflecting the effects of C&DW replacement ratios. The skid resistance of the recycled samples with coarse aggregates was generally higher than that of the reference samples made up of VAs. This was attributed to the sharper shape (after the crushing process) and the rough surface texture of the recycled coarse materials. However, the skid resistance of the recycled samples with fine C&D aggregates slightly declined compared to that of the reference concrete samples. In other words, the use of fine C&D aggregates, as a partial replacement of sands, provided a more homogeneous mix and a smoother surface layer. According to BS 6717 [67], a BPN of at least 45 is typically required for paving blocks. Previous research [90] also suggested a standard value of more than 40. Therefore, the skid resistance of all of the recycled and reference concrete samples met the aforementioned criteria.

A decreasing trend in the UPV (Figure 16) was observed with increasing C&D waste incorporation. As depicted in this figure, the 28-day UPV values ranged from 4.3 to 5 km/s (a reduction of 5 to 19% compared to the reference concrete). The reason for this was the increase in the air content of the recycled C&DW concrete, with a rise in the replacement ratio. The UPV instrument could detect large air voids, thereby influencing wave propagation through the recycled concrete samples due to the porous nature of C&D wastes. All of the recycled concrete samples also achieved UPV values of more than 3.5 km/s, falling within the good zone according to ASTM C215 [68]. The fastest pulse transmission of 5.1 km/s was observed for the reference concrete with a lower void content as the concrete denseness increased. Figure 17 plots the UPV values against compressive strengths for different concrete mixes during 28 days.

To predict the compressive strength of conventional concrete, a general exponential equation was expressed based on UPV values, as shown in

Table 8. Parameter estimations of compressive strength and UPV for conventional concrete.

Equation		Determination Coefficient	Reference
$f_{cm}=1.146.e^{0.77UPV}$		0.80	Turgut [91]
$f_{cm}=0.32.e^{0.9895UPV}$		0.51	Trtnik et al. [92]
$f_{cm}=3.38.e^{0.62UPV}$		0.61	Bogas et al. [93]
$f_{cm}=0.1662.e^{0.0014UPV}$		0.51	Mohammed et al. [94]
with coarse C&D wastes	$f_{cm}=0.8166.e^{0.6928UPV}$	0.84	This study (for recycled concrete)
with fine C&D wastes	$f_{cm}=0.1959.e^{1.0121UPV}$	0.50

, in various studies. The recycled concrete exhibited good correlation coefficients of 0.83 and 0.88 for the coarse and fine C&DW incorporation, as displayed in Figure 17.

4.7. Discussion of Landfill Age Effects

TP3 and BH8 are located in Region 1 and represent old C&D wastes, while the other BHs and TPs are situated in Region 2, indicating younger wastes. Based on the waste compositions in Figure 2, TP3 and BH8 contain a significant amount of clays. This indicates a considerable decrease in the use of bricks and tiles (clay structure materials) in Karaj city from 2005 to the present compared to the period between 1990 and 2005.

As shown in Table 5, the water-soluble contents are generally lower than the acid-soluble contents for all old and new BHs and TPs. However, this phenomenon was not observed for chlorides in old materials in BH8. The reason for this can be attributed to the relatively low percentage of concrete and mortar in this borehole. According to EN 12620, chloride ions must originate from chlorides incorporated into the clay mineral structure (not in calcium and other phases of concrete), which are difficult to extract in water, even for the fine particles [39]. Conversely, in old wastes in TP3, which contain higher amounts of mortar and concrete, the opposite is true. Therefore, in old landfills containing a small percentage of cementitious materials, the amounts of acid-soluble chloride content may not differ much from the water-soluble chloride content, as shown in BH8. Based on Table 5, limitations of light organic contaminants and chloride ions were mostly observed in old landfills. To overcome them, C&D wastes can be mixed with other virgin aggregates in appropriate proportions in the mixture design to chemically improve their performance for use in concrete production.

The composition of C&D materials in different TPs is one of the main factors that affect the compressive strengths. For example, according to Table 6, in the mixes with the full incorporation of coarse aggregates from TPs 1 and 3 (TP1-C100 and TP3-C100) in cylinders, there was a reduction of 32.2% and 42% in compressive strength, respectively. This lower value for recycled concrete from TP3 (representing an old landfill) is mainly due to its low percentage of high-resistance unbound aggregate (e.g., natural stone) content relative to the other TPs. This phenomenon was also valid for fine particles, as seen in the TP1-F100 and TP3-F100 cylinder samples, where the compressive strength decreased by about 29.4% and 48.2%, respectively. This change is essentially due to the considerable increase in the water-to-cement ratio of the recycled mixes in TP3 due to its significant fine clay content [95]. The fine clay fragments coat the recycled grains and absorb the mixing water, preventing proper bonding between the cement paste materials and the recycled aggregates, consequently weakening the inner structure of the cement. This phenomenon leads to an increase in the porosity of the recycled concrete, resulting in a decrease in compressive strength. This was confirmed by the evidence in Figure 18 obtained with the SEM (scanning electron microscopy) method, where the micro-porosity of TP3-F75 is higher than that of TP1-F75 and the reference concrete. In this figure, the voids are visible in a dark black color. Additionally, the significant amount of old concrete and mortar in TP3 can lead to lower aggregate interlock, resulting in reduced compressive strength [96].

The nature and composition of recycled C&D wastes in different TPs can also influence the results of abrasion and skid resistance tests. For instance, according to Figure 13, concrete made with the old landfill wastes of TP 3 and the concrete made with the new wastes of TP 5 are compared. In the TP3-F100 and TP5-F100 samples, a reduction of 42% and 12% in abrasion resistance was observed, respectively. The main reason for this is the greater water-to-cement ratio for TP3-F100 (due to the presence of more clay particles in this TP) compared to TP5-F100. Similar results were qualitatively observed for the skid resistance of the fine grains for old and young wastes in TP 3 and TP 5, respectively. Nevertheless, the impacts of the size of the C&D wastes on abrasion and skid resistance were higher than their ages. Based on Figure 15, the lowest UPV was measured for the mixes made with coarse C&DW in old TP 3 with a high content of bricks, possibly due to the formation of micro-cracks in the particles of recycled brick aggregates caused by the manual crushing process. Also, some past studies [57,97] on concrete with selected C&D materials (e.g., recycled concrete aggregate) reported that the presence of adhered old mortars on the surface of recycled bricks can increase the travel time of the UPs (or decrease the UPV) in recycled concrete. However, by comparing the results of replacing fine C&D wastes in different ratios with VA, it can be said that there was no significant difference between the UPV values, which means they all help prevent significant void formation.

5. Conclusions

This study aimed to evaluate the performance of recycled concrete made from C&D aggregates sourced from two regions of unexploited waste landfill, representing both old and new landfills in developing countries. Based on the laboratory test results of the C&D materials and the recycled concrete, the following conclusions were drawn:

• The accumulation of waste in Region 1 (1990–2005) and Region 2 (after 2005) indicated a significant increase in waste production in recent years. Old landfills contained a notable amount of clay structure materials, which have decreased considerably in recent years.
• The construction wastes obtained from the demolition of old structures and deposits at the study site can serve as suitable substitutes for natural materials in various applications, particularly in concrete production, manufacturing concrete parts, and roller compacted concrete pavements.
• Considering the chemical threshold limits typically applied to the use of VAs in concrete products, it is possible to use non-selected C&D wastes in concrete with appropriate considerations.
• The replacement of coarse VAs with C&D wastes resulted in an increase in wide wheel abrasion resistance by up to 5.8%. The maximum loss in abrasion resistance was 13–21% at full replacement of the fine VA. The impacts of the size of the C&D wastes on abrasion and skid resistance were greater than their ages. An increase in BPNs ranging from 0–15% was recorded for samples with coarse C&D wastes in replacement ratios of 25 to 100% due to the sharper shape of crushed C&D aggregates and their rough surface texture.
• The UPV test method can be beneficially implemented to estimate the compressive strength of recycled concrete with coarse non-selected C&D wastes. Due to the formation of micro-cracks in particles of recycled brick aggregates caused by the manual crushing process, the lowest UPV was obtained for mixes made with coarse C&D wastes in the old landfill. Additionally, the presence of adhered old mortars on the surface of old recycled bricks decreased the UPV values.

Considering the satisfactory performance of concrete prepared from non-selected C&D wastes, this material can be confidently used in practice. Utilizing waste instead of new materials contributes to achieving sustainable development goals and offers significant cost savings in construction. Moreover, the use of recycled materials minimizes environmental destruction and preserves natural resources for future generations.