Sustainable Concrete with Recycled Aggregate from Plastic Waste: Physical–Mechanical Behavior

Featured Application

The use of recycled plastic aggregates up to 20% is recommended for use in active mobility (sidewalks, bike lanes and curbs).

Abstract

In Mexico, approximately 6.5 million tons of plastic waste is generated, of which 38–58% is improperly managed and has the potential to leak into the environment. Furthermore, producing natural aggregates is associated with the unsustainable use of non-renewable resources. In this sense, this work aimed to evaluate the influence that recycled aggregates from plastic waste have on the behavior of concrete. Coarse aggregates of thermoplastic paint (TP) from paving waste were prepared and incorporated into four mixes, with concentrations of 5 to 20%. In addition, three mixes with fine aggregates from PET were evaluated as one reference mix. The studied properties were slump, compressive strength, flexural strength, rebound number, density, absorption, and porosity. The results indicate that both aggregates have significant potential for use in concrete, including structural use, when replacement percentages of around 5% are considered, with property losses not exceeding 8%. Their use is proposed for active mobility infrastructure, with percentages of up to 20% analyzed in this study. Finally, it is necessary to analyze the influence that the incorporation of plastic waste has on mitigating environmental impacts, as well as the durability properties.

1. Introduction

In recent years, the growing volume of plastic pollution has prompted a global call to action, demanding solutions from governments, organizations, civil society, industries, and academies [1] for the innovative management and disposal of concrete [2]. In Mexico, approximately 6.5 million tons of plastic waste is generated, of which 38–58% is improperly managed and has the potential to leak into the environment [3]. According to Lamba et al. (2022), plastic waste is clogging our water resources and waterways, overflowing the landfills, leaching into the soil, and transferring through the air, thus polluting every natural resource in our environment [4].

On the other hand, producing natural aggregates is associated with the unsustainable use of non-renewable resources and the generation of environmental liabilities, such as exposure to the natural soil layer, erosion processes, the deforestation of extraction areas, river sedimentation, and exposure to the water table. The world’s production of natural aggregates is estimated to be around 32 billion tons per year [5]; the most pervasive and permanent environmental impacts derived from mining involve changes in land cover and landforms, water over-extraction, soil pollution, and surface and groundwater pollution [6].

In the search for a solution to these problems, the use of recycled plastic aggregates (RPAs) added to concrete [7,8], asphalt [5,9], or as base or sub-base fill for roads [10] has been studied. Among the most studied polymeric materials as plastic aggregates are polyethylene terephthalate (PET) [11,12,13,14,15,16,17,18,19,20,21], high-density polyethylene (HDPE) [15,22,23], polypropylene (PP) [15,18,22], PVC [23,24,25,26,27,28], rubber [11,16], and e-waste [29,30,31,32,33,34] (Table 1).

Table 1. Types of recycled plastic aggregates used in previous studies.

PET	PET	HDPE	Rubber	PP	PVC	E-waste	Fine	Coarse	Fibers	Others
Mousavimehr and Nematzadeh, 2020 [35]							X			Mixed PET/rubber
Abdal Qadir and Noaman, 2024 [11]	X		X				X		X	Mixed with steel fibers
Bamigboye et al., 2022 [12]	X							X
Abu-Saleem et al., 2021a [22]	X	X		X				X
Skibicki et al., 2022 [14]	X						X
Tayeh et al., 2021 [15]	X	X					X
Abu-Saleem et al., 2021b [17]	X							X		Mixed PET/PP/PEAD
Supit and Priyono, 2022 [16]	X		X					X	X	Mixed fibers; PP and PET
Chan et al., 2022 [18]	X			X				X	X
Babafemi et al., 2022 [13]	X									Mixed plastics
Abu-Saleem et al., 2021c [36]								X		Mixed PET, HDPE, PP
Lazorenko et al., 2022 [21]	X						X
Kangavar et al., 2022 [37]	X						X
Jaskowska-Lemańska et al., 2022 [20]	X						X
Farina et al., 2022 [19]	X						X
Askar et al., 2023 [7]	X	X	X	X			X	X	X	Review
Mohammed et al., 2021 [24]					X		X
El-Seidy et al., 2023 [25]					X		X			Mixed PVC/glass
Senhadji et al., 2015 [26]					X		X	X
Arnaud et al., 2024 [23]		X			X		X	X
Noor-Azline et al., 2023 [27]					X					Review
Mohammed et al., 2019 [28]					X
Ullah et al., (2021) [29]						X		X
Ahmad, Qureshi, et al., (2022) [30]						X		X		Mixed nanographite
Makri et al., (2019) [31]						X	X			Mortar
Ahmad, Jamal, et al., (2022) [32]						X		X
Abbas et al., (2022) [33]						X		X
Chang-Chi et al., (2025) [34]						X	X			Mortar

Table 1. Types of recycled plastic aggregates used in previous studies.

PET	PET	HDPE	Rubber	PP	PVC	E-waste	Fine	Coarse	Fibers	Others
Mousavimehr and Nematzadeh, 2020 [35]							X			Mixed PET/rubber
Abdal Qadir and Noaman, 2024 [11]	X		X				X		X	Mixed with steel fibers
Bamigboye et al., 2022 [12]	X							X
Abu-Saleem et al., 2021a [22]	X	X		X				X
Skibicki et al., 2022 [14]	X						X
Tayeh et al., 2021 [15]	X	X					X
Abu-Saleem et al., 2021b [17]	X							X		Mixed PET/PP/PEAD
Supit and Priyono, 2022 [16]	X		X					X	X	Mixed fibers; PP and PET
Chan et al., 2022 [18]	X			X				X	X
Babafemi et al., 2022 [13]	X									Mixed plastics
Abu-Saleem et al., 2021c [36]								X		Mixed PET, HDPE, PP
Lazorenko et al., 2022 [21]	X						X
Kangavar et al., 2022 [37]	X						X
Jaskowska-Lemańska et al., 2022 [20]	X						X
Farina et al., 2022 [19]	X						X
Askar et al., 2023 [7]	X	X	X	X			X	X	X	Review
Mohammed et al., 2021 [24]					X		X
El-Seidy et al., 2023 [25]					X		X			Mixed PVC/glass
Senhadji et al., 2015 [26]					X		X	X
Arnaud et al., 2024 [23]		X			X		X	X
Noor-Azline et al., 2023 [27]					X					Review
Mohammed et al., 2019 [28]					X
Ullah et al., (2021) [29]						X		X
Ahmad, Qureshi, et al., (2022) [30]						X		X		Mixed nanographite
Makri et al., (2019) [31]						X	X			Mortar
Ahmad, Jamal, et al., (2022) [32]						X		X
Abbas et al., (2022) [33]						X		X
Chang-Chi et al., (2025) [34]						X	X			Mortar

RPAs have been studied as fine fractions, coarse fractions, and fibers. Furthermore, they have been incorporated in both hybrid and isolated forms. The most common form of incorporation is as fine aggregates, since it does not require a pre-treatment procedure but usually only a simple mechanical cleaning and grinding process [11,14,15,19,20,21,35,37]. A similar process occurs for the incorporation of fibers [11,16,18,35]. However, when coarse aggregates are made, a thermal pre-treatment procedure is usually applied, where the polymer is melted and cooled to generate a paste with homogeneous thickness. Subsequently, it is crushed to meet the parameters established for gravel [12,16,17,18,22,36].

Regarding the degree of settlement, some studies [38,39,40] have suggested that as the percentage of RPA increases, the workability tends to decrease. This can be explained by different reasons: First, plastic residues affect the viscosity of the mixture and increase its consistency and its sharper and non-uniform shape [7], which can lead to poor bonding between the interfacial transition zone of the cement paste and the RPAs [12,22]. In this sense, some investigations have studied applications for self-compacting concrete (SCC). Jaskowska-Lemańska et al. (2022) found that when 5% RPA was added, the slump decreased by only 2% concerning the reference mixture [20]; similar results were found by Abdal Qadir and Noaman (2024) when they added 10% fine rubber aggregate, reporting losses of 2% [11].

Regarding the basic mechanical properties, such as compressive strength, flexural strength, tension strength, and modulus of elasticity, some studies have found that they decrease as the percentage of RPA substitution increases. However, favorable results have been obtained with 5–10% substitutions [14,15,22], with outcomes remaining similar to the reference mixture [20]. In response to this loss of properties, some researchers have studied the addition of cementitious materials [21], heat treatments [17], and fibers [11] to reverse this trend and generate concretes that can compete for structural use or that resemble conventional concrete.

Contrary to the above results, some studies have found that the properties of the reference mixture can be slightly improved. Chan et al. (2022) found that the compressive strength increased when 5% PET gravel was used [18]; meanwhile, by using 10% PET aggregate, Babafemi et al. (2022) found gains of 3% [13], and Kangavar et al. (2022) found gains of 9% [37].

In addition to the basic mechanical and physical properties, thermal properties have been studied. Farina et al. (2022) found that adding PET aggregates reduced concrete’s thermal conductivity by up to 33% [19], while Lazorenko et al., (2022) found reductions of up to 59% when 100% RPA was added [21]. On the other hand, when evaluating the durability properties, Abu-Saleem et al. (2021) found that resistance to sulfate attack decreased slightly when plastic mixture comprising 30% (10% PET, 10% PP, 10% HDPE) was added. The mixtures presented greater absorption of up to 9% for the most unfavorable case [36].

Other RPA studies in recent years that can serve as a basis for understanding the results of the current work are those on electronic waste (e-waste) and PVC waste. On the one hand, e-waste is constantly increasing, causing damage to people’s health and the environment [29]. General trends have shown that these types of RPAs, like other more common ones, such as PET, PP, and HDPE, reduce concrete’s mechanical properties, limiting its application to non-structural uses [29,30,32,33,34] without reinforcement from treatments or additions. In this sense, Ahmad et al. (2022) used 25% RPAs and incorporated 5% nano-graphite platelets (NGPs), reporting gains in mechanical strengths (compression: 13.56%; traction: 15.53%; bending: 31.42%). This was explained by the stacking of the hydration products, which made the paste more massive and thick [30]. Most e-waste studies have incorporated replacement percentages not exceeding 30% [29,30,31,33,34]. Other properties, such as performance against alternative wetting and drying cycles [29], durability [32], sulfate resistance, and resistivity [34], have been improved with the incorporation of these types of plastic aggregates.

Regarding PVC waste, it has been found that substitutions of up to 30% can be used mainly for non-structural applications [27]. Some studies have proposed its use in lightweight concrete [23,25,26], while others have proposed its use as structural concrete, provided it is reinforced with additional fibers and treatments [24]. Marginal losses have also been found in mechanical strength (no greater than 8% with substitutions of up to 30% RPA), absorption, and workability [28]. Other authors have reported gains in flexural and abrasion resistance [23,28]. Likewise, various investigations have explored substituting high percentages of PVC RPA (75%). For example, Mohammed et al. (2021) added polyvinyl alcohol, which increases the water viscosity in concrete and reduces the desiccation phenomenon [24]. El-Seidy et al. (2023) replaced 100% of natural sand with recycled PVC in combination with glass waste, finding promising properties, such as thermal insulation, but limiting its use as structural concrete [25]. Senhadji et al. (2015) evaluated coarse and fine aggregates in proportions of less than 70% [26]. Arnaud et al. (2024) reported increases of up to 10% in flexural strength when replacing up to 10% NA with PVC RPA [23]. Meanwhile, Mohammed et al. (2019) found that up to 30% coarse PVC RPA can be used without any significant losses in the mechanical properties [28].

Generally, when RPAs are incorporated into concrete, the trend indicates a loss in the overall quality of the most important properties regardless of the plastic used; however, low replacement percentages can provide adequate quality. To reverse the losses in the properties of recycled concrete, some studies have used supplementary cementitious materials and/or additions [41,42,43,44,45,46] or gamma or microwave radiation treatments in the aggregates [17,47] to use them in structural applications. Some of the additions that have been incorporated include nano-silica or silica fume [43,44,45], fly ash [45], nano-iron oxide [42], nano-graphite platelets [30], and metakaolin [46], among others. These studies have found that incorporating these additions reverses the initial losses in properties when using RPAs and increases the mechanical, physical, and durability properties [41,42,43,44,45], suggesting microstructural densification [30,42].

Other researchers have measured the environmental impacts of incorporating plastic aggregates in concrete. Gravina et al. (2021) assessed the potential of climate change in a gate-to-gate life cycle assessment. Their study suggests that using up to 25% aggregate can generate environmental gains while maintaining the same compressive strength [48]. In this sense, it is necessary to continue exploring the environmental impacts in scenarios considering all life cycle stages, including stage D, i.e., the benefits and loads after the system’s life cycle has ended.

Although research on RPA has been previously conducted, it is necessary to continue looking for alternatives using different types of plastics and new thermal treatments, expanding durability studies, and performing life cycle assessments, among other tests [40,49], especially in countries such as Mexico, where structural regulations are limited regarding the use of recycled materials. Furthermore, it should be noted that the most studied plastic to date is PET. In this sense, the current work seeks to explore additional alternatives that are usually off the radar and have not been reported in the literature, such as thermoplastic paint residues used in paving, which come from horizontal signage procedures and do not comply with current regulations due to their color or consistency. Much of the contribution of this research to the current knowledge lies in the above since, according to the Scopus database, only thermoplastic paint and its relationship with concrete are mentioned in the work of Chaudhary and Akhtar (2024) [50], where it is not discussed as a recycled aggregate that is part of a concrete mix but as part of the LCA carried out on road construction projects in India.

In addition to the above, further research is needed to achieve homogenous results to create regulations for the regulated use of RPAs. The Mexican complementary technical standard for the design and construction of concrete structures [51] establishes that only recycled aggregates from construction and demolition waste can be used in their coarse fraction and limits them to a maximum substitution of 20% for use in structural concrete. The use of fine aggregate is not permitted. On the other hand, polymeric fibers, such as PP, HDPE, acrylic, nylon, or polyester, are permitted for non-structural use in compliance with ASTM D7508D/7508M regulations [51]. In other words, more research is needed to support the use of fine aggregates and widen the permissible percentages of substitution using coarse aggregates and fibers.

In order to address this problem, which would allow for a reduction in the extraction of raw materials and their associated impacts, as well as the reincorporation of urban solid waste and construction and demolition waste, which tend to have limited reuse and/or recycling rates in the Mexican context, the objective of this work is to evaluate the physical and mechanical properties of concrete that incorporate recycled aggregates from two plastics with different natures. First, four mixtures were made, where natural gravel was replaced by recycled gravel from thermoplastic paint (TP) residues used for horizontal paving, in percentages of 5%, 10%, 15%, and 20%. Then, three mixtures of recycled PET sand were made with replacement percentages of 5%, 9%, and 14% to support the use of sand in structural concrete and vary the percentages used in studies previously reported in the literature. Both types of mixture were compared with a reference mixture.

2. Materials and Methods

For the process of obtaining the recycled fine aggregate, PET waste was used (Figure 1c), which was crushed to a maximum particle size of 0.475 cm (No. 4 mesh) and a minimum size of 0.015 cm (No. 100 mesh). The particle size curve was adjusted using a partial crushing process that allowed for a grain distribution similar to natural sand and within the limits of the regulations [52].

The recycled coarse aggregate came from thermoplastic paint residues used in horizontal signage for pavement construction (Figure 1a). TP residues resemble artificial rocks, with surface hardness and color variability. These rocks were mechanically crushed with a demolition hammer and then manually crushed until reaching sizes that complied with ASTM C33 regulations and were consistent with the sizes of natural coarse aggregate. No. 4 mesh (0.475 cm) was considered the lower limit, and the 1” mesh (2.54 cm) was the upper limit.

The cement used for the mixtures was Ordinary Portland Cement, strength class 30 (CPO 30), corresponding to type 1 according to ASTM C150 [53]. The coarse and fine natural aggregates used were limestone from the Metropolitan Area of Monterrey. The maximum nominal size for the gravel was 3/4”. The basic physical properties were obtained for both the natural and recycled aggregates (Table 2), which allowed for the design of the mixtures.

Table 2. Basic physical properties of natural and recycled aggregates.

PROPERTY	REGULATIONS	CNA	CRPA	FNA	FRPA
Density	ASTM C128/C127 [54,55]	2.69	2.01	2.62	1.14
Absorption (%)	ASTM C128/C127 [54,55]	0.53	6.09	2.41	NA
Dry compact volumetric weight (kg/m3)	ASTM C29 [56]	1565.5	1182	NA
Fineness modulus	ASTM C33 [52]	NA		2.4
Resistance to degradation (%)	ASTM C131 [57]	23.2	42.8	NA
Granulometry	ASTM C33 [52]	See Figure 2

CNA: coarse natural aggregate. CRPA: coarse recycled plastic aggregate. FNA: fine natural aggregate. FRPA: fine recycled plastic aggregate.

Table 2. Basic physical properties of natural and recycled aggregates.

PROPERTY	REGULATIONS	CNA	CRPA	FNA	FRPA
Density	ASTM C128/C127 [54,55]	2.69	2.01	2.62	1.14
Absorption (%)	ASTM C128/C127 [54,55]	0.53	6.09	2.41	NA
Dry compact volumetric weight (kg/m3)	ASTM C29 [56]	1565.5	1182	NA
Fineness modulus	ASTM C33 [52]	NA		2.4
Resistance to degradation (%)	ASTM C131 [57]	23.2	42.8	NA
Granulometry	ASTM C33 [52]	See Figure 2

CNA: coarse natural aggregate. CRPA: coarse recycled plastic aggregate. FNA: fine natural aggregate. FRPA: fine recycled plastic aggregate.

The recycled coarse and fine aggregates were adjusted to the natural gravel and sand granulometries to ensure proper distribution in the new concrete mixes. Although this procedure was considered adequate for preparing the mixes, it leads to increased environmental impact due to the higher consumption of energy required to screen the aggregates. Therefore, when developing an industrial process, the extent to which the screening process can be considered adequate must be established.

Once the aggregates were characterized, the mixtures were designed following the ACI 211.1 methodology for mixture design [58]. The volume percentages of the RPAs corresponding to each mixture varied due to the difference between the densities of the recycled and natural aggregates, both for fine and coarse aggregates. A w/c ratio of 0.48, a design strength of 35 MPa, and a slump of 10 cm were used.

Eight mixtures were prepared in total: one conventional (CON); three that replaced the fine aggregate with PET sand in percentages of 5% (PE5), 9% (PE9), and 14% (PE14), keeping the natural coarse aggregates constant; and four that replaced the natural coarse aggregate with thermoplastic paint (TP) gravel in percentages of 5% (TP5), 10% (TP10), 15% (TP15), and 20% (TP20), keeping the natural fine aggregates constant.

In Mexico, the complementary technical standards for the design and construction of concrete structures limit the use of recycled fine aggregates and only allow the addition of up to 20% recycled coarse aggregates from concrete waste [51]. The percentages of substitution in the mixtures were planned considering this limitation, with the objective of closely examining variations that can serve as a reference framework for their application in the local industry or for updating the current regulations. The values for a cubic meter of concrete are illustrated in

Table 3. Dosage by weight (kg) of each mixture for 1 m³.

			Coarse Aggregate		Fine Aggregate
Mix	Cement	Water	Natural	TP	Natural	PET
CON	427.08	226.49	1028.99	0.00	646.65	-
TP5	427.08	228.84	988.86	36.75	646.65	-
TP10	427.08	231.24	948.96	74.45	646.65	-
TP15	427.08	233.71	908.02	113.15	646.65	-
TP20	427.08	236.25	865.98	152.87	646.65	-
PE5	427.08	226.49	1028.99	-	633.72	12.93
PE9	427.08	226.49	1028.99	-	620.79	25.87
PE14	427.08	226.49	1028.99	-	607.86	38.80

.

Three types of specimens were made. Rectangular prismatic specimens of 45 × 15 × 15 cm to measure the rebound index at 7, 14, and 28 days of curing and the flexural strength at 28 days of curing; cylinders of θ15 × 30 cm to measure the simple compressive strength; and cylinders of θ10 × 20 cm to measure the porosity, absorption, and density (see Figure 3).

The properties of the hardened concrete were evaluated using the procedures standardized by ASTM (

Table 4. Standards used to evaluate the properties obtained in hardened concrete.

Property in Hardened State	Regulations
Rebound rate	ASTM C805 [61]
Compressive strength	ASTM C39 [62]
Flexural resistance	ASTM C293 [63]
Porosity, absorption, and density	ASTM C642 [64]

). In the fresh state, ASTM C192 [59] was followed to prepare the specimens, while the slump was obtained following ASTM C143 [60]. The recycled concrete specimens with both coarse and fine aggregates had similar behavior to conventional concrete, acquiring rigidity 24 h after their manufacture; therefore, they were removed from the mold and introduced into a curing chamber at a temperature of 23 ± 2° and relative humidity of 100%.

The compressive and flexural strengths were assessed with a Davi hydraulic press (Figure 4). For the indirect tension test, an ALCON brand beam testing frame was used, with support placed at the thirds of the span. The compressive strength was measured on θ15 × 30 cm test pieces taken to failure. For the flexural strength and rebound index, 45 × 15 × 15 cm specimens were used. All tests were carried out after 28 days of curing, except for the rebound index, which was also tested at 7 and 14 days. For the porosity, absorption, and density tests, three θ10 × 20 specimens were used, which were cut in half with a diamond-tipped cutter, and the mass measurement processes were carried out following the methodology of the regulations.

3. Results

The results of the physical and mechanical tests allowed for establishing potential applications for recycled plastic concrete and to visualize future scenarios for plastic aggregates. The slump test results of the mixtures (Figure 5) ranged between 15 and 19 cm which is appropriate for the size of the specimens produced; this is consistent with previously reported studies [12,15]. However, regarding this property, there are still discrepancies in terms of the influence that RPA has on new concrete. Askar et al. (2023) [7] reviewed different works, and some of them reported gains in slump, while others reported losses [11,20], although some of these losses were not significant.

The slumps of the eight mixtures reached 17 cm, with a standard deviation of 1.76, which is 23.5% higher than the conventional mixture (13 cm). In the mixtures with TP, it was possible to control the water, resulting in similar slumps between them (16–18 cm). The PE mixtures showed a tendency for the slump to decrease as the replacement percentage increased, ranging from 19 cm (PE5) to 15 cm (PE14). However, it did not reach the value of the conventional mixture (13 cm). This is due to the poor adhesion between the cementitious matrix and the RPA [12,22], in this case, PET sand and TP gravel, which caused a reduction in the rigidity of the matrix linked to the settlement gain [12]. In addition, the RPA and the cementitious matrix present a chemical incompatibility that lead to losses in the mechanical properties, such as the compressive strength, tension, and modulus of elasticity, since the smooth surface of the plastic, its impermeability, lower resistance, and hydrophobic nature weaken the cohesion forces at the interface between the cementitious matrix and the aggregates [17,65,66], causing a weak adhesion [67].

A high degree of propagation was found on the failure surface between the cementitious matrix and the plastic aggregate, showing detachment of the RPA from the cementitious matrix, which is a consequence of the weaker adhesion and incompatibility between these two components [67].

On the other hand, higher compressive strength was reported in the mixtures where FRPA was used compared to the mixtures with CRPA (Figure 6). In addition, it was observed that, as the percentage of RPA substitution increased, both for PET sands and TP gravels, the compressive strength decreased when compared to the conventional mixture. The PE5 (−6.7%), PE9 (−7%), and PE14 (−8%) mixtures presented maximum losses of 8% compared to the conventional mixture.

Studies have suggested that the difference in the mechanical strength values of mixtures with RPAs is attributed to higher exuded water content in the concrete mixes; this water is mainly found around the PET aggregate particles, leading to weaker bonds between the cement matrix and the RPAs. In addition, large air bubbles were observed in the cement paste prepared with plastic aggregates, contributing to the losses in the mechanical strength [67].

These results are consistent with those of previous research on different types of RPAs, such as PET, HDPE, and PP [14,15,21,37]. Tayeh et al. (2021) reported that although the compressive strength decreased as the percentage of PET sand substitution increased, when 10% was added, it was similar to the conventional mixture (−1.63%) [15]. Jaskowska-Lemanska et al. (2022) found that when 5% PET sand was used, the mechanical strength and modulus of elasticity were maintained [20]. Other studies even found that when 10% PET sand was added, the mechanical strength increased by 9% and was maintained by adding 30% PET sand [37]. The above benefits highlight the potential use of RPAs in percentages of about 10% for non-structural applications and could even represent an opportunity in structural applications (if additional studies are carried out, such as on the modulus of elasticity and durability).

The results of previous studies and this one demonstrate an opportunity for using sand in non-structural and structural concrete and establish the foundation for exploring the advantages of plastic aggregates in sustainable concrete. Regarding the flexural resistance (Figure 7), it was found that with a replacement of up to 10% PET sand, the resistance decreased by about20%. In comparison, when the replacement was increased to 15%, the resistance increased, with a loss of 13%. Given these fluctuating losses and gains in flexural strength, it is recommended to increase the percentages of PET sand substitution to analyze an adequate trend and infer whether higher percentages lead to gains in this property. This could be attributed to a potential increase in the deformability, which the more ductile nature of PET could provide.

Compressive strength losses (Figure 8) were obtained for the TP gravel mixtures, ranging from 13% for PE5 (12.8%) and PE10 (13.8%). Meanwhile, for PE20, a loss of 27% was obtained concerning the conventional mixture. Regarding flexural strength, the TP5 and TP10 mixtures exhibited similar behavior, with a loss of about 21.5% compared to the conventional mixture. Based on these results, their use in non-structural uses is recommended. Hardened thermoplastic paint residues have not been previously explored as a potential RPA in new concrete. Therefore, further research is recommended to evaluate new substitution percentages, durability tests, and other gravel production procedures. In addition, its behavior should be evaluated by incorporating additives or additions that reverse these losses in properties. However, TP gravel is compromised at 50 °C, so if it is used in infrastructure for active mobility or non-structural concrete, it should be used in temperate or cold climates.

The rebound number graph (Figure 9) shows hardness behaviors similar to those of conventional concrete, which increased as the curing age increased. Except for the TP15 and PE14 mixtures, all mixtures exhibited a drop in hardness ranging from 4.6% to 17.5%. Previous studies have found hardness losses of up to 15% with replacements of up to 20% PET sand. In addition, they showed non-linear variations in behavior, with this test presenting the most variations in the mechanical results [20]. In the case of mixtures with TP gravel, as expected, there was a tendency for the rebound number to decrease as the percentage of RPA replacement increased. Meanwhile, as the percentage of PET sand increased, the hardness increased, going from a loss of 17.5% in the PE5 mixture to a gain in surface hardness of 3.5 compared to the reference mixture. The above can be attributed to a rearrangement of particles on the surface as the percentage of PET sand increased, which should be corroborated in future studies with image analysis.

Starting at 50 °C, the TP gravel began to show a decrease in hardness and an increase in plasticity. Therefore, the results of the ASTM C642 tests regarding porosity, absorption, and density were discarded for this methodology. This phenomenon could partly explain why the absorption after immersion was around 1% for all mixtures, which is 85% less than that of the conventional mixture (absorption after immersion: 6.9%). This can be attributed to a possible impermeable layer formed by the TP on the surface of the new concrete, which could be beneficial in terms of durability or in concretes that require high impermeability. However, as there is no comparison of this type of RPA with other similar ones in the literature, it is necessary to replicate these studies with larger quantities of test specimens, perform statistical analysis, or develop a methodology adapted to this type of aggregate.

PET mixtures showed lower absorption and porosity (Figure 10) compared to the conventional mixture when up to 10% was used. As the replacement percentage increased, the absorption and porosity tended to rise (compared to the PET mixes, not the CON), with the maximum value reported for PE14 being 2% more porous and absorbent than the CON mixture. This is in agreement with previous studies that have found that absorption and porosity increased as the proportion of sand replaced with RPA increased. This is attributed to the fact that natural aggregates and plastic do not combine sufficiently in the cementitious matrix and, therefore, the concrete becomes porous [68]. The presence of these pores usually results in decreases in the compressive strength and the unit weight of composite concretes [66].

Despite this trend, this study’s absorption and porosity values remained similar. The low replacement contents and the similar granulometric curves between the natural and recycled aggregates explain this. Therefore, if RPA is adequately bonded to the cementitious matrix, it could be an excellent substitute for natural sand up to specific replacement percentages. Some studies for CRPAs have performed thermal [17,47] or chemical [65] treatments, achieving better contact between the particles and the cement paste [17,47] and increasing the roughness on the surface, which increases the adhesion strength of the RPAs with the cementitious matrix [65]. This could reduce this problem and allow for a higher percentage of RPA replacement.

Regarding the dry and wet densities of the PET specimens (Figure 10), a loss of less than 3% was observed for all mixtures; previous studies have reported decreases in density when RPAs were used [13,15]. The results showing slight losses in density are consistent with the results of the compressive and flexural strength (Figure 10), which show that the incorporation of RPAs did not lead to significant changes in the basic physical and mechanical properties of the concrete. In fact, an even better arrangement of particles is expected, due to a slight decrease in porosity in the PE5 and PE9 mixtures.

4. Conclusions

This work studied the incorporation of recycled plastic aggregates as substitutes for gravel (hardened thermoplastic paint waste) and sand (PET) in new concrete mixtures. Mechanical properties, such as the compressive strength, flexural strength, and rebound number were evaluated, as well as physical properties, such as the porosity, absorption, and density in hardened concrete, and slump in fresh concrete. This study analyzed a previously studied material (PET) and one that has not been previously explored (hardened thermoplastic paint residue), and guidelines and recommendations for its application were established. It should be noted that the most studied plastic to date is PET.

It was found that the compressive strength with up to 14% PET sand did not decrease by more than 8%, while for percentages of up to 10% thermoplastic paint gravel, the strength decreased by 14%. The density decreased slightly (less than 2%) for the mixtures with PET. At the same time, the porosity and absorption tended to increase as the percentage of substitution of fine PET aggregate increased compared to the PE5 mixture. When compared to the conventional mixture, the PE5 and PE9 mixtures exhibited lower porosity (−8% and −5%, respectively) and absorption. This suggests that the use of up to 9% PET sand could lead to improvements in the durability properties, and it is recommended to study this in future research.

It is concluded that, under the methodology followed, recycled plastic aggregates have the potential to be used in concrete with active mobility infrastructure applications, such as sidewalks, cycle paths, and curbs, which is in line with what has been pointed out by other authors when denoting the relevance of road infrastructure in the achievement of SDGs for a country [2,50]. In addition, new studies should be carried out to replicate these results and to include tests for the elasticity modulus, durability, and microscopy to establish their use in structural concrete. This study could be extended with optical or scanning microscope (SEM) investigations to observe the characteristics of the two materials that influence their chemical compatibility.

This work found that FRPAs have the potential to be used in structural concrete with replacement percentages not greater than 10%. Meanwhile, it is recommended that the use of CRPAs of thermoplastic paint for structural purposes is not ruled out as long as additives or additions are used that reverse the losses in mechanical properties. Establishing a methodology to evaluate the physical properties of thermoplastic aggregates is recommended. Finally, it is recommended that the potential to reduce environmental impacts generated by plastic aggregates in new concrete is evaluated through life cycle analysis [49,50].