Natural Rubber Latex-Modified Concrete with PET and Crumb Rubber Aggregate Replacements for Sustainable Rigid Pavements

: There are ongoing research challenges for the addition of the blend of PET and crumb rubber in polymer-modiﬁed concretes, which aims to leverage the beneﬁts of both materials. In this study, various percentage combinations of waste aggregates, such as PET and crumb rubber, were used to replace coarse and ﬁne aggregates in natural rubber latex (NRL)-modiﬁed concrete. Engineering properties such as compressive and ﬂexural strengths, modulus of elasticity, and toughness obtained from compressive-and ﬂexural stress-strain curves were investigated. Scanning electron microscopy (SEM) analysis was performed to examine the microstructural properties and study the strength development of the studied concretes. The results revealed that the compressive and ﬂexural strengths of NRL-modiﬁed concretes with PET and crumb rubber aggregate replacements decreased with increasing replacement ratios. SEM analysis indicated that PET and crumb rubber (hydrophobic and non-polar materials) can affect the microstructure of the studied concrete by creating a weak interface between the aggregate and cement pastes, leading to reduced strength development. With the addition of the NRL additive, the ﬁlm formation was found to act as a bridge and improve the bond strength of aggregates and hydration products in NRL-modiﬁed concrete. Furthermore, the integration of PET and crumb rubber aggregate can enhance the ability of the concrete to absorb energy and improve ductility. It was found that 10% of PET and crumb rubber aggregate replacement can be used for NRL-modiﬁed concrete as a rigid pavement, as its mechanical strengths satisfy the requirements set by the Department of Highways (DOH) in Thailand. This research helps repurpose waste materials and reduce the environmental footprint of concrete production.


Introduction
Highways are important components of a nation's infrastructure and play an indispensable role in stimulating economic growth, enhancing connectivity, encouraging regional development, and elevating the standard of living [1][2][3][4].The construction of pavements involves a series of layered components, such as a subbase, base, and surface course, each with a specific role.The subbase, which sits directly atop the prepared natural ground level or subgrade, is usually composed of locally sourced materials or stabilized soil.It is designed to evenly distribute the load from the base to the subgrade, effectively safeguarding the impact of aggressive moisture.The base layer above the subbase often consists of crushed aggregate designed to further distribute the load, thereby providing a stable surface for the subsequent pavement layer and protecting the underlying subbase from detrimental environmental effects.The surface course is constructed using highquality materials such as asphalt or concrete, ensuring its ability to resist traffic loads and provide a durable and skid-resistant surface, making it critical for safety and efficiency in transportation.Based on the type of surface course, the pavement can be classified into two categories: flexible pavement (asphalt surface layer) and rigid pavement (concrete surface layer).Because of their rigidity, the load on rigid pavements is distributed over a wide area of the subgrade.This reduces the stress on the subgrade and makes the pavement more tolerant to weaker or less uniformly compacted subgrades when compared with the flexible pavement.In addition, rigid pavements are resistant to rutting and weathering and require less maintenance over time [5][6][7].
Rigid pavements are primarily made from a mix of cement, coarse and fine aggregates, and water.In addition, the admixture is incorporated into the mix to modify concrete properties.Portland cement is the binding agent in the concrete mix.When mixed with water, it reacts chemically to form a hard, strong, and durable matrix, which binds the aggregate particles together.In concrete paving materials, a significant source of greenhouse gas emissions is the cement manufacturing process [8,9].The production of Portland cement is a process that emits a lot of greenhouse gases, as it relies on burning fossil fuels for its energy.In a cement kiln, limestone (calcium carbonate) and clays are heated to very high temperatures (about 1400-1450 • C) to produce clinker, the main component in Portland cement.Throughout the manufacturing process, calcination can generate approximately 7% of CO 2 [10].This has led to an increasing interest in developing alternative cementitious materials and production methods that are more sustainable.Because of the low-temperature production methods, hydraulic cement is reported to have a lower carbon footprint than conventional Portland cement [11].
Coarse and fine aggregates are crucial components in the concrete mix.The coarse aggregate provides bulk, strength, and wear resistance to the pavement, while the fine aggregate (typically sand) fills the spaces between the coarse aggregate, makes it more cohesive, and prevents the concrete from shrinking and cracking.In recent years, environmental and sustainable considerations have led to the inclusion of recycled materials and additives in the concrete layer to improve performance and reduce the environmental impact [12][13][14].The environmental issues associated with the disposal of used tires and PET plastic are substantial, which cause negative impacts on the environment and may result from improper management of these materials.The production of plastic waste is a significant issue, with millions of tons produced annually [15].Due to their slow decomposition and incorrect disposal, PET plastic bottles add to the rising problem of plastic waste by taking up landfill space and increasing the risk of microplastic pollution [16,17].Similarly, a large amount of waste tires is generated every year and poses long-term environmental risks since they take up valuable landfill space and do not biodegrade [18][19][20].Poorly managed waste tires can pose safety issues, attract bugs, and discharge harmful materials into the ground and water [21][22][23].
In recent years, there have been significant developments in the field of sustainable construction materials, such as innovation involving the incorporation of PET plastics and waste tires in concrete pavement [24,25].Waste can be made into crumb rubber particles and used in the concrete mix as a sand replacement [26].Utilizing crumb rubber reduces the need for sand mining, protects natural resources, and lessens its negative effects on the environment.PET plastic bottles can also be broken down into small particles and utilized as alternative aggregates in the concrete mix [27].However, the utilization of PET and waste tires in the concrete mix resulted in a reduction of its flexural strength, which is a vital property for concrete pavement.Therefore, additional additives might enhance the properties of concrete in rigid pavement applications.A synthetic latex polymer, namely Styrene-Butadiene Rubber (SBR), is highly regarded for its good resistance to abrasion and durability.When added to concrete, SBR latex can enhance the adhesive strength between the aggregate and cement paste, making the concrete more resistant to mechanical stress and reducing the risk of cracking.Basalt fibers are a type of reinforcement that can prevent the post-cracking of concrete.The combination of SBR latex and basalt fibers in concrete can therefore create a highly durable material with excellent post-cracking behavior [28,29].In terms of sustainable road construction, Yaowarat et al. proposed an innovative idea of value-added NRL, a polymer additive and studied the effect of various rubber-to-cement (r/c) ratios on the strength development of normal concrete [30].It indicated that the optimum r/c ratio could improve the modulus of rupture of the concrete, resulting in the durability of the roadway.
To the authors' best knowledge, the evaluation of using NRL to improve the strength characteristics of concrete with crumb rubber (CR) and PET as aggregate replacement is still limited; hence, it is the aim of this research work.The experimental testing, including compressive and flexural strength, was conducted to investigate the mechanical strength development of concretes with various PET and CR replacement ratios and an optimum r/c ratio.The toughness and resilient modulus of the studied mixtures were also evaluated.Scanning electron microscopy (SEM) was used to examine the microstructure and evaluate the strength development of the studied concrete.SEM was used to investigate the particle interaction within the concrete matrix in order to gain knowledge about the composite's overall efficacy and pinpoint areas for development.
The results of this study will advance understanding of the mechanical characteristics of concretes containing PET plastic and crumb rubber.It will offer useful knowledge for the development of sustainable construction methods and the incorporation of recycled aggregates in the concrete mixture.This study's ultimate goals are to encourage recycling, lessen trash production, and propose a more circular and environmentally friendly method of designing and building rigid pavements.

Materials
The research used hydraulic cement with a specific gravity of 3.15.The properties of the studied polymer additive are illustrated in Table 1.It was obtained from the Rubber Authority of Thailand and contained 52% dry rubber, 48% water, and water-soluble components.The pH was tested to ensure the acidity content in the rubber controlled the property for the other tests, which equaled 10.4.Table 2 shows the properties of coarse aggregate (crushed rock) and fine aggregate (natural river sand).The concrete specimens were prepared in accordance with ASTM C127 and ASTM C128 [31,32].The nominal maximum size of 19 mm of the coarse aggregate was used in this study.The specific gravity of the coarse aggregate was 2.70, while that of the fine aggregate was 2.66.The coarse and fine aggregates had water absorptions of 1.85% and 0.74%, respectively.The fineness modulus and the percent of voids in the fine aggregate were 2.70 and 38.24, respectively.The coarse aggregate had a Los angles abrasion loss, a flakiness index, and an elongation index of 22.2%, 27.69%, and 22.35%, respectively.The waste tires were crushed by a tire granulator crusher machine, resulting in a granular size between 2 and 3 mm to replace the sand with a specific gravity of 1.02.While PET bottles were collected, pre-washed, and shredded by a plastic shredder machine into granules with an average size of 10 mm to replace the crushed stone, the specific gravity was 1.20 [33].Thus, there was a mixture between the crumb rubber and PET in concrete proportions to reduce the fine and coarse aggregates.The particle sizes were sieved and controlled by ASTM C33 and ASTM C136 [34,35], which limited the lower and upper boundaries in Figure 1.The figure showed the mixture between sand and crushed stone and the combination between PET and crumb rubber in various percentage mixtures.

Concrete Mix Design
Earlier research reported that the w/c ratio is a key determinant of the strength of concrete, i.e., a lower w/c ratio can lead to higher-strength concrete [30,36].However, a lower w/c ratio often requires a higher cement content, leading to an increase in the cost

Concrete Mix Design
Earlier research reported that the w/c ratio is a key determinant of the strength of concrete, i.e., a lower w/c ratio can lead to higher-strength concrete [30,36].However, a lower w/c ratio often requires a higher cement content, leading to an increase in the cost of the concrete mix and an environmental burden.Therefore, a balanced approach considering cost, performance, and a sustainable solution is crucial in the concrete mix design.The w/c ratio was fixed at 0.5 for all studied samples, whereas the r/c ratio was fixed at 0.58 for NRL-modified concrete and NRL-modified concrete with PET and crumb rubber replacements.
Natural Rubber Latex, NRL = ((r/c)/100) C (1) where r/c = rubber-to-cement ratio (%), C = cement (kg), W NRL (kg), and W t (kg).These ratios were selected to study based on previous research and practical work, which were reported to be the optimum ratios for producing concrete pavement that meets the minimum required compressive and flexural strengths set by the DOHs in Thailand [36].The amount of NRL can be determined following Equations ( 1)-( 3), which relate to a water content of 48% in total natural rubber latex, according to Yaowarat et al. [30].
In this study, the slump value for the studied concrete mixes was in control at 75 mm ± 25 mm for the concrete pavement application.To examine the influence of PET and CR on the compressive and flexural strengths of the studied mixes, the combination of PET and CR was designed to substitute coarse and fine aggregates, respectively, in percentages of 5%, 10%, and 15%.Therefore, a total of 4 mixes, as shown in Table 3, were studied in this research (i.e., normal concrete, NRL-modified concrete, NRL10WP = a blend of 5% crumb rubber and 5% PET, NRL20WP = a blend of 10% crumb rubber and 10% PET, and NRL30WP = a blend of 15% crumb rubber and 15% PET).The fresh concrete was prepared following ASTM C192 [37] and then placed in the cylindrical (150 mm × 300 mm) molds for the compressive strength test as per ASTM C39 [38] and prismatic molds (150 mm × 150 mm × 600 mm) for the flexural strength test as per AASHTO T97 [39].The mixture was placed in three separate stages, and each layer was compacted using a standard tamping rod with 25 strikes.After being cured at room temperature for a day, the samples were then demolded and placed in water maintained at a temperature of 27 • C ± 2 • C.They were kept submerged for periods of 7, 14, and 28 days prior to the compressive and flexural tests.The procedure for the compressive strength test was implemented as per the ASTM C39 standard, while the flexural strength test adhered to the AASHTO T97, employing the third-point loading technique.The flexural strength (f f ) was computed using the following expression: where P represents the maximum load applied to the sample; L denotes the length of the support; and b and d are the average width and depth of the beam specimen, respectively.The tested data were analyzed in the mean value of test specimens within the same testing condition in compression and flexural strengths to secure reliable data, followed by a low mean standard deviation, SD (SD/x < 10%, where x is the mean strength value).

Scanning Electron Microscopy (SEM)
The effect of various replacement materials, such as NRL, PET, and crumb rubber, on the strength developments was examined using SEM analysis.Fragments from the specimen strength tested after 28 days were used to perform SEM analysis.To prevent the hydration of the samples, they were first frozen and dried, and then coated with gold before conducting SEM analysis.SEM morphological detection at 1000 and 5000 times magnification was performed.

Compressive Strength
The compressive strengths of the studied samples cured at all curing periods are shown in Figure 2. The solid red line demonstrates the minimum 28-day compressive strength requirement (f c ≥ 32 MPa) for concrete pavement set by the DOH, Thailand [40].For normal concrete (a controlled sample), the 7, 14, and 28-day compressive strengths were 27.6, 34.3, and 38.8 MPa, respectively, which is a common strength development of concrete.It implies that the compressive strength developed by increasing curing time.As expected, when NRL additive was added to the concrete, the compressive strength of the NRL-modified concrete was reduced when compared with the control concrete, which was similar to the previous research [30,36].Umasabor and Daniel [41] examined the influence of various percentages of PET replacement on the compressive strength of concrete.It was found that PET at 5% by weight was the optimum value and provided the highest compressive strength of concrete.Chong and Shi [42] performed a statistical analysis based on 100 data sets to study the use of PET plastics as concrete fine and coarse aggregates and revealed that concrete containing up to 30% PTE replacement can be sustainable and have minimal strength reduction.The 7, 14, and 28-day compressive strengths of NRL-modified concrete were 24.6, 29.6, and 34.3 MPa, respectively.The compressive strength of the NRLmodified concrete with waste aggregate replacements decreased with increasing aggregate replacement ratios and was lower than that of normal concrete and NRL-modified concrete, respectively.The findings of this study are consistent with the results of previous research on the effects of waste PET bottles and crumb rubber aggregates on the hardened properties of concrete [27,43,44].However, the NRL10WP sample (NRL-modified concrete with 5% PET + 5% crumb rubber aggregate) had compressive strength at 28 days greater than the requirement for a concrete pavement designated by DOHs in Thailand.The compressive strength of NRL10WP was 21.9 MPa, 28.77 MPa, and 32.5 MPa at 7, 14, and 28 days, respectively.In other words, the compressive strength was found to be reduced by 8.3% compared to that of NRL-modified samples.The compressive strength of NRL20WP was approximately 15% lower than that of NRL-modified concrete.The compressive strength was found to be remarkably reduced when the waste aggregate replacement in the NRLmodified concrete was 30%.The 7, 14, and 28-day compressive strengths of NRL30WP were 13.6, 22.3, and 24.5 MPa, respectively.The 28-day compressive strength of NRL30WP indicated 28.5% and 36.9% in strength reduction when compared to the NRL-modified concrete and normal concrete, respectively.
NRL-modified concrete.The compressive strength was found to be remarkably reduced when the waste aggregate replacement in the NRL-modified concrete was 30%.The 7, 14, and 28-day compressive strengths of NRL30WP were 13.6, 22.3, and 24.5 MPa, respectively.The 28-day compressive strength of NRL30WP indicated 28.5% and 36.9% in strength reduction when compared to the NRL-modified concrete and normal concrete, respectively.

Flexural Strength
The flexural strengths of the control samples, NRL-modified concrete, and NRLmodified waste polymer concrete samples cured at 7, 14, and 28 days are shown in Figure 3

Flexural Strength
The flexural strengths of the control samples, NRL-modified concrete, and NRLmodified waste polymer concrete samples cured at 7, 14, and 28 days are shown in Figure 3 Figure 3 demonstrates that the NRL polymer can enhance the flexural strength of NRL-modified concrete, and its 28-day flexural strength was about 3.3% higher than that of normal concrete.The flexural strength of NRL-modified concrete was 2.9, 4.3, and 4.86 MPa at 7, 14, and 28 days, respectively.This is because the NRL modifies the microstructure of the concrete, making it denser and reducing the gaps between particles, which prevents water alteration in the matrix and modifies the chemical reaction phases [30,36].The influence of the PET and crumb rubber aggregate replacement was found to reduce the flexural strength when compared with normal concrete.
The flexural strength of concrete with PET and crumb rubber aggregate replacement decreased with increasing replacement ratios.Although the flexural strength was reduced, the 28-day flexural strength of NRL10WP was 4.4 MPa, which met the minimum requirement of 4.2 MPa.
Furthermore, at the age of 28 days, the flexural strength of NRL10WP, NRL20WP, and NRL30WP concretes was respectively 5.2%, 24.9%, and 36.9%lower than that of normal concrete.The percentage reduction in flexural strength of NRL10WP was lower than that of concrete using PET or crumb rubber aggregate as a rigid pavement previously reported [22].This implies that NRL additives can improve the flexural strength development of NRLmodified concrete with PET and crumb rubber aggregate replacement.
The influence of the PET and crumb rubber aggregate replacement was found to reduce the flexural strength when compared with normal concrete.
The flexural strength of concrete with PET and crumb rubber aggregate replacement decreased with increasing replacement ratios.Although the flexural strength was reduced, the 28-day flexural strength of NRL10WP was 4.4 MPa, which met the minimum requirement of 4.2 MPa.Furthermore, at the age of 28 days, the flexural strength of NRL10WP, NRL20WP, and NRL30WP concretes was respectively 5.2%, 24.9%, and 36.9%lower than that of normal concrete.The percentage reduction in flexural strength of NRL10WP was lower than that of concrete using PET or crumb rubber aggregate as a rigid pavement previously reported [22].This implies that NRL additives can improve the flexural strength development of NRL-modified concrete with PET and crumb rubber aggregate replacement.
The modulus of elasticity (Young's modulus) of the studied concrete in Table 4 was obtained from the compressive stress-strain curve at 28 days, as shown in Figure 4.In The modulus of elasticity (Young's modulus) of the studied concrete in Table 4 was obtained from the compressive stress-strain curve at 28 days, as shown in Figure 4.In general, the Young's modulus of the normal concrete was in the range of 30 and 50 GPa, and the Young's modulus of the studied control sample was 33.91 GPa.The Young's modulus of the NRL-modified sample was approximately 28.4% lower than the control sample.While the Young's modulus of NRL-modified samples with PET and crumb rubber aggregate replacement remarkably decreased with the increase in replacement ratios, the modulus of elasticity of NRL10WP, NRL20WP, and NRL30WP was 20.4,16.2, and 13.04 GPa, respectively.The compressive toughness was used to demonstrate the energy absorption in the compressive mode.The compressive toughness (measured in MJ/m 3 ) of the studied samples can be determined from their compressive stress-strain curve depicted in Figure 4.The compressive toughness of the control sample was 5.35 MJ/m 3 , while the NRL-modified sample had a compressive toughness of 4.89 MJ/m 3 , or about 8.6% lower than the normal concrete.The compressive toughness of NRL10WP, NRL20WP, and NRL30WP was 4.74, 4.30, and 4.22 MJ/m 3 , respectively.In other words, the toughness of the NRL-modified sample was about 11.4%, 19.6%, and 21.1% with 10%, 20%, and 30% of PET and crumb rubber aggregate replacement, respectively, lower than the control sample.When the concrete has a higher energy absorption capacity, it can deform more be fore fracturing.This deformation distributes stresses over a large volume of the materia which can prevent or delay the initiation of cracks.Furthermore, even when cracks d start, the higher energy absorption capacity can help prevent the rapid growth of thes cracks [46].As such, it increases the pavement's durability and resilience against variou forms of distress, potentially extending its service life.
The values of the flexural toughness of the studied concrete samples summarized i Table 4 were determined in MPa.m. and obtained from their flexural stress-strain curves as depicted in Figure 5.It clearly indicated that the energy absorption of the NRL-modi fied concrete (1.65 MPa•m) was higher than that of normal concrete (1.29 MPa•m).Thi confirms that the addition of an NRL additive can enhance the flexural strength and en ergy absorption capacity of normal concrete.Even though the flexural strength values o the NRL-modified concrete with PET and crumb rubber aggregate replacement wer lower than those of normal concrete, their flexural toughness values were higher tha those of normal concrete.In addition, the energy absorption capacity of the NRL-modified concrete with waste aggregate replacement increased with the increase in replacemen ratios.It implies that the NRL-modified concrete with PET and crumb rubber aggregat replacement had better ductility behavior than that of the normal concrete.On the other hand, the flexural toughness characteristics of NRL-modified concrete with and without PET and crumb rubber replacement were found to be different from their compressive toughness.The flexural toughness is used to indicate the energy absorption of the studied concrete when subjected to bending stress, such as traffic loading on the concrete slab.The bending stress generated by repeated traffic loading can cause distress in the rigid pavement, which is considered the major mode of failure of concrete pavement.Therefore, the concrete's ability to absorb energy (fracture energy), also known as flexural toughness, can positively impact the service life of a rigid pavement.This characteristic allows the pavement to resist crack propagation better, which is particularly important considering the dynamic loads and environmental impacts pavements are subjected to [45].
When the concrete has a higher energy absorption capacity, it can deform more before fracturing.This deformation distributes stresses over a large volume of the material, which can prevent or delay the initiation of cracks.Furthermore, even when cracks do start, the higher energy absorption capacity can help prevent the rapid growth of these cracks [46].As such, it increases the pavement's durability and resilience against various forms of distress, potentially extending its service life.
The values of the flexural toughness of the studied concrete samples summarized in Table 4 were determined in MPa.m. and obtained from their flexural stress-strain curves, as depicted in Figure 5.It clearly indicated that the energy absorption of the NRLmodified concrete (1.65 MPa•m) was higher than that of normal concrete (1.29 MPa•m).This confirms that the addition of an NRL additive can enhance the flexural strength and energy absorption capacity of normal concrete.Even though the flexural strength values of the NRL-modified concrete with PET and crumb rubber aggregate replacement were lower than those of normal concrete, their flexural toughness values were higher than those of normal concrete.In addition, the energy absorption capacity of the NRL-modified concrete with waste aggregate replacement increased with the increase in replacement ratios.It implies that the NRL-modified concrete with PET and crumb rubber aggregate replacement had better ductility behavior than that of the normal concrete.The flexural and compressive strengths are crucial parameters in concrete materia selection for rigid pavement application because they determine how much load a mate rial or pavement structural layers can withstand without deforming or failing in both com pressive and tensile modes.The relationship between the two is important because it help pavement engineers or designers predict how a material will behave under different type of stress.In other words, a normal concrete material may have high compressive strengt but low flexural strength, meaning it can withstand being compressed but may easily frac ture when subjected to tensile stress.This is significant in applications such as rigid pave ment, where both compression and bending forces are present; hence, the relationshi between the flexural and compressive strengths of normal concrete is generally plotted.
The relationship between the flexural strength (ff) and compressive strength (fc) of th NRL-modified concrete with PET and crumb rubber aggregate replacement is plotted i Figure 6.The dashed red line shows the normalize of the studied concrete mixes.Th relationship can help pavement engineers or designers choose waste materials such a PET and crumb rubber for a specific application and design safer, more efficient struc tures.It can also be used in quality control, failure analysis, and the development of new waste materials.In addition, a common assumption in concrete pavement design is tha the modulus of elasticity of concrete is directly related to its compressive strength.Th implies that as the compressive strength of the concrete increases, so does its stiffnes Therefore, the relationship between the modulus of elasticity and the compressiv strength can be used to predict the performance of concrete under various load condition This relationship is not always linear or direct, and it can be influenced by several factor such as the type and quantity of aggregate used in the mix.As such, the actual tests on th concrete are required to determine these properties accurately for pavement structura design applications.The flexural and compressive strengths are crucial parameters in concrete material selection for rigid pavement application because they determine how much load a material or pavement structural layers can withstand without deforming or failing in both compressive and tensile modes.The relationship between the two is important because it helps pavement engineers or designers predict how a material will behave under different types of stress.In other words, a normal concrete material may have high compressive strength but low flexural strength, meaning it can withstand being compressed but may easily fracture when subjected to tensile stress.This is significant in applications such as rigid pavement, where both compression and bending forces are present; hence, the relationship between the flexural and compressive strengths of normal concrete is generally plotted.
The relationship between the flexural strength (f f ) and compressive strength (f c ) of the NRL-modified concrete with PET and crumb rubber aggregate replacement is plotted in Figure 6.The dashed red line shows the normalize of the studied concrete mixes.This relationship can help pavement engineers or designers choose waste materials such as PET and crumb rubber for a specific application and design safer, more efficient structures.It can also be used in quality control, failure analysis, and the development of new waste materials.In addition, a common assumption in concrete pavement design is that the modulus of elasticity of concrete is directly related to its compressive strength.This implies that as the compressive strength of the concrete increases, so does its stiffness.Therefore, the relationship between the modulus of elasticity and the compressive strength can be used to predict the performance of concrete under various load conditions.This relationship is not always linear or direct, and it can be influenced by several factors, such as the type and quantity of aggregate used in the mix.As such, the actual tests on the concrete are required to determine these properties accurately for pavement structural design applications.
Figure 7 shows the relationship between modulus of elasticity (E c ) and 28-day compressive strength (f c ) of NRL-modified concrete with PET and crumb rubber aggregate replacement.The relationship between these two properties can help in optimizing the NRL-modified concrete mix design when waste PET and crumb rubber are used as aggregate replacements.By adjusting the ingredients and proportions in the mix, one can control both compressive strength and the modulus of elasticity to meet the specific requirements of a rigid pavement design.The dashed red line indicates the boundary between the normal concrete and the NRL-modified concrete with PET and crumb rubber aggregates replacement.Figure 7 shows the relationship between modulus of elasticity (Ec) and 28-day compressive strength (fc) of NRL-modified concrete with PET and crumb rubber aggregate replacement.The relationship between these two properties can help in optimizing the NRL-modified concrete mix design when waste PET and crumb rubber are used as aggregate replacements.By adjusting the ingredients and proportions in the mix, one can control both compressive strength and the modulus of elasticity to meet the specific requirements of a rigid pavement design.The dashed red line indicates the boundary between the normal concrete and the NRL-modified concrete with PET and crumb rubber aggregates replacement.

Microstructural Analysis
SEM analysis is a powerful tool for examining the microstructural properties of concrete, including cement hydration products, aggregate properties, the Interface Transition Zone, microcracks, and the pore structure.These insights help understand concrete's performance, including its mechanical properties and durability [47,48].Figure 8 illustrates the SEM images of normal concrete, NRL-modified concrete, NRL10WP, and NRL30WP which were cured for 28 days at magnifications of 1000 and 5000 times, respectively.In Figure 8a, plenty of C-S-H gel, Ca(OH)2, and ettringite were evidently identified in the control sample (without NRL and waste aggregates).This confirmed that the strength of

Microstructural Analysis
SEM analysis is a powerful tool for examining the microstructural properties of concrete, including cement hydration products, aggregate properties, the Interface Transition Zone, microcracks, and the pore structure.These insights help understand concrete's performance, including its mechanical properties and durability [47,48].Figure 8 illustrates the SEM images of normal concrete, NRL-modified concrete, NRL10WP, and NRL30WP, which were cured for 28 days at magnifications of 1000 and 5000 times, respectively.In Figure 8a, plenty of C-S-H gel, Ca(OH) 2 , and ettringite were evidently identified in the control sample (without NRL and waste aggregates).This confirmed that the strength of the concrete was theoretically developed in accordance with the production of hydration products.It was evident that the higher rate of crumb rubber and PET replacement affected the microstructure by creating a brittle interface between the aggregate and the cement paste.PET and crumb rubber are hydrophobic and non-polar materials, which make them Figure 8b shows the SEM images of NRL-modified concrete, which demonstrated that its matrix became slightly loose compared to the normal concrete.This might be due to the effect of the NRL additive-generated film network blocking the growth of hydration products, which resulted in lower compressive strength.However, the film network was detected to be covering and bridging the C-S-H gels and aggregates.Furthermore, less ettringite was detected, resulting in enhanced adhesion or bonding between the aggregates by the film network.Therefore, the flexural strength of the NRL-modified concrete was increased [25,32].In addition, the NRL film network can absorb energy and help mitigate crack propagation in the concrete, which reduces the likelihood of a brittle failure.This confirmed the improvement of concrete's toughness and ductility, as demonstrated in Figure 5. Figure 8c illustrates the SEM images of NRL-modified concrete with 10% PET and crumb rubber aggregate replacement (NRL10WP).It clearly indicated that the connection between the waste aggregates and hydration products resulted in several pores and a loose matrix.This led to a reduction in its compressive and flexural strengths.An SEM image of NRL-modified concrete with 30% PET and crumb rubber aggregate replacement (NRL30WP) was demonstrated in Figure 8d.
It was evident that the higher rate of crumb rubber and PET replacement affected the microstructure by creating a brittle interface between the aggregate and the cement paste.PET and crumb rubber are hydrophobic and non-polar materials, which make them difficult for the cement paste to bond with.Subsequently, this can lead to a remarkable reduction in compressive strength.However, the inclusion of crumb rubber and PET can increase the concrete's ability to absorb energy, improving its toughness and resistance to impact and dynamic loads due to its plastic deformation characteristics [41].
The use of waste PET and crumb rubber in NRL-modified concrete is a sustainable practice that can alter the microstructure and mechanical properties of the concrete when used in rigid pavement applications.However, it is crucial to consider the specific requirements of the concrete and adjust the mix design accordingly to attest to the best possibility of using these waste aggregates in concrete pavement applications.

Conclusions
This research investigates the effect of waste PET and crumb rubber aggregate replacement on the microstructure and strength characteristics of NRL-modified concrete.The combination of PET and crumb rubber in percentages of 10%, 20%, and 30% aggregate replacement ratios was studied.With the addition of the NRL additive, the compressive strength of the NRL-modified concrete was lower than that of the normal concrete.At the age of 28 days, the compressive strength of NRL10WP, NRL20WP, and NRL30WP was about 8.3%, 15%, and 28.5% lower than that of NRL-modified concrete.On the other hand, the flexural strength of NRL-modified concrete was approximately 3.3% higher than normal concrete.While the NRL-modified concrete with PET and crumb rubber aggregate replacement indicated a reduction in flexural strength with the increased replacement ratio, the flexural strength of NRL10WP was found to meet the minimum flexural strength requirement (f f ≥ 4.2 MPa) for rigid pavement specified by DOH, Thailand.Microstructural analysis indicated that the addition of NRL additive generated the film network to prevent the development of hydration products and resulted in a compressive strength reduction.However, this film network acts as a bridge mechanism to reinforce the adhesion between the aggregates in the concrete matrix, leading to improved flexural strength.PET and crumb rubber are hydrophobic and non-polar materials; hence, increasing the amount of these materials in the concrete mix can affect their microstructure by creating weaker interfaces between aggregates and cement paste.As such, the mechanical strength dropped.However, the PET and crumb rubber aggregate replacement can improve the ability to absorb energy in NRL-modified concrete and hence improve its toughness and resistance to impact and dynamic loads.
The research demonstrates that integrating waste PET and crumb rubber into concrete mixes is a sustainable practice.In terms of mechanical strength properties, it is confirmed that 10% of waste PET and crumb rubber aggregate replacement can be used in concrete mix for rigid pavement design.The output of this research can be used as a guideline for pavement researchers, engineers, designers, and end-users to understand the crucial microstructural and macro-characteristics of NRL-modified concrete with waste aggregates and adjust the mix design to meet the specification requirements of the concrete for rigid pavement applications.Ultimately, a combination of lab-scale research, field trials, and close collaboration between researchers and practitioners is required to successfully develop and implement this new material and techniques in the field of concrete technology.Hence, field trials to validate the lab results under more realistic conditions are recommended for future research.

Figure 1 .
Figure 1.Grain size distribution of coarse and fine aggregate.

Figure 1 .
Figure 1.Grain size distribution of coarse and fine aggregate.

Figure 2 .
Figure 2. Compressive Strength Development of Waste Polymer Concrete.
. The solid line indicates the minimum 28-day flexural strength requirement (ff ≥ 4.2 MPa) for concrete pavement specified by DOH, Thailand [40].For normal concrete, the

Figure 2 .
Figure 2. Compressive Strength Development of Waste Polymer Concrete.
. The solid line indicates the minimum 28-day flexural strength requirement (f f ≥ 4.2 MPa) for concrete pavement specified by DOH, Thailand [40].For normal concrete, the flexural strength increased with curing time as expected; flexural strength initially was 2.8 MPa at 7 days, 4.3 MPa at 14 days, and 4.7 MPa at 28 days, which met the specification.These values are commonly reported for normal concrete.

Figure 3 .
Figure 3. Flexural Strength Development of Waste Polymer Concrete.

Figure 3 .
Figure 3. Flexural Strength Development of Waste Polymer Concrete.

Figure 4 .
Figure 4. Compressive stress-strain curves of the studied concrete samples curing at 28 Days.

Figure 5 .
Figure 5. Flexural stress-strain curves of the studied concrete samples curing at 28 Days.

Figure 6 .
Figure 6.Relationship between compressive and flexural strength at 28 days.Sustainability 2023, 15, x FOR PEER REVIEW 13 of 18

Figure 7 .
Figure 7. Relationship between compressive strength and modulus of elasticity at 28 days.

Figure 7 .
Figure 7. Relationship between compressive strength and modulus of elasticity at 28 days.

Table 1 .
Physical properties of Natural Rubber Latex (NRL).

Table 2 .
Basic physical and engineering properties of NRL, fine, and coarse aggregate.

Table 3 .
Mixing proportion of fresh concrete.

Table 4 .
Mechanical properties of waste polymer concrete.