Building materials such as cement and aggregate are in high demand as the construction sector increases. “As is well known, concrete mixes comprise 60 to 80 percent aggregate by volume, despite the fact that aggregate is a non-renewable resource. Continuous quarrying has a negative environmental impact as well as a scarcity of aggregate. As a result, aggregate replacement is preferred to reduce demand for quarries. PET, on the other hand, is a type of PW derived from human waste. As a result, there will be fewer landfills in the coming years. However, because recycling PET into concrete reduces quarrying operations and is a big step toward sustainability due to its economic and environmental benefits, replacing PET for aggregates is a mutualistic solution to both concerns”. Thus, the goal of the study is to understand the performance of concrete that contains a considerable amount of recycled PW of type PET as a fine aggregate.

Ordinary Portland cement (OPC) Type I which conformed with the Iraqi standard specification (I.O.S.) (No. 5/1984) (COSQC, 1984a) was used as the only binder in this study. The chemical properties and physical and mechanical properties of the OPC are presented in Table 1 and Table 2, respectively.

For all concrete mixtures, potable water free of impurities was utilized as the mixing solution.

Superplasticizer (SP) with trading name “Sika ViscoCrete Hi-Tech 1,316 conforming to ASTM C494 (ASTM-International, 2015) type D and G was added into the concrete mixtures to improve the workability. Furthermore, several testing were carried out in order to determine the optimal dosage of SP, which impacts the fresh and mechanical properties of concrete mixes” (Qaidi et al., 2022a). The properties of the SP are presented in Table 4.

The concrete mixtures made in this study were designed by ACI 211.1-91-R-02 (Aci-211-1-91, 2002) with w/c of .40, .45, and .55. The PET aggregates were used to replace the natural fine sand up to 50% at an increment of 25% (i.e., 0%, 25%, and 50%). Table 5 presents a detailed composition of the nine concrete mixtures made and evaluated. The mixture ID in Table 5 represents the content of PET aggregate used as a replacement of the fine aggregate and the w/c used.

The concrete mixtures were prepared as per ASTM-C192/C192M (2009) by using an electrical rotary tilting drum mixer with a capacity of .1 m³. The concrete mixtures were prepared by mixing the aggregates and OPC for 3 minutes followed by the addition of potable water premixed with the SP. After the mixing process was completed, the slump of the concrete mixtures was evaluated followed by casting of the samples for the properties to be evaluated. After casting the samples, the surfaces of the moulds were covered with a plastic sheet to avoid the loss of water from the samples. At approximately 24 h after casting the concrete samples, the samples were removed from the moulds and cured in water until the testing age. Furthermore, Superplasticizer (Polycarboxylic acid water-reducing agent) complying to ASTM C494 type D and G was added to the concrete mixtures to increase workability and eliminate the problem of PET floating in the mix.

The details of the tests carried out are outlined in Table 6.

Sandpaper was used to “smooth the surface of the cylinders and then cleaned with isopropyl alcohol. Then, two strain gauges (Model PL-60-11-3LJC-F) were installed, one horizontally and one vertically as T-shaped and equally spaced in the mid-height of all samples. The strain gauges were connected to the digital collector (data logger) for data collection during the compression test” (Qaidi et al., 2022a) as shown in Figure 5.

The workability of the mixture was evaluated in terms of the slump. Figure 6 shows the effect of PET content and w/c on the resulting workability of the concrete mixtures. “It can be noted that the slump reduced with higher content of PET regardless of the w/c ratio used. For example, compared to the control mix, at the 25% replacement, the reduction rate is 33.33% (w/c of .40), 38.1% (w/c of .45), and 41.28% (w/c of .55). While, at 50% replacement, the reduction rate is 79.9% (w/c of .40), 82.86% (w/c of .45), and 90.83% (w/c of .55). Nevertheless, all mixtures evaluated were still workable to some extent. The reduction in the workability of the concrete with the incorporation of PET can be attributed to the flat and irregular shape of PET particles which embodied them with a larger surface area compared to that of the natural sand. As a result, there would be further friction among the particles resulting in less workability of the concrete mixtures” (Qaidi et al., 2022a).

The detrimental impact of the PET aggregate on the workability of the concrete mixture is more significant in mixtures made with higher w/c. This significant impact of the PET aggregate at higher w/c can be linked to bleeding as the interface between PET particles and the hydrated Portland cement becomes more porous at higher w/c. This behavior occurred as a result of the inability to water absorption and the flat form of the PET particles (Xie et al., 2021; Khan et al., 2022c). Thus, lowering the workability of the mixture. Similar observations have been reported by other studies where PET aggregates were used (Albano et al., 2009; Rahmani et al., 2013).

Figures 7, 8 present the fresh and dry densities results with different proportions of PET particles and w/c ratios. The results showed that the fresh and dry densities of all mixtures decreased with higher PET content. For fresh density, a maximum reduction of 17.75% was observed for concrete made with w/c of .55 and PET aggregate as a 50% replacement of the natural sand. For the dry density, the maximum reduction was 21.63% when the concrete was made with a w/c of .55 and PET as a 50% replacement of the natural sand. The reduction in the density of the concrete when PET aggregates were incorporated can be ascribed to the lower density of PET aggregates compared to that of natural sand.

The decline in concrete density is “more pronounced when higher w/c especially for the dry density, as shown in Figures 7, 8. This has been related to the excess water in the concrete samples that does not take part in the water and cement reactions. Thus, so small canals are formed that can create pores after drying. Therefore, lower unit weights are achieved for higher w/c ratios, as confirmed by Choi et al. (2005). Moreover, values of dry density for mixtures including 0% of PET PAs decreased from 2,388 kg/m³ (w/c of .40), 2,376 kg/m³ (w/c of .45), and 2,347 kg/m³ (w/c of .55) to 1993 kg/m³, 1964 kg/m³, and 1839 kg/m³, respectively, for mixtures including 50% of PET PAs. Thus, all dry density values, at the replacement ratio of 50%, were below 2000 kg/m³ [minimum dry density needed for lightweight concrete as per the classification of RILEM-LC2 (1978)]. Accordingly, these concretes are classified as lightweight concretes” (Qaidi et al., 2022a). This result is also within the scope of the results obtained by Azhdarpour et al. (2016).

Water absorption refers to the gradation of material porosity by measuring the ratios of water absorbed under a particular state (Farhana et al., 2015; Babafemi et al., 2018). Figures 9, 10 show the influence of w/c and PET aggregate content on the water absorption and the porosity of concrete mixtures evaluated. The values presented for each mixture represent the average value. From the data presented, the water absorption and porosity of concrete, irrespective of the w/c ratio, increased significantly with higher PET aggregate content. For example, compared to the reference mixture, at the 50% replacement, the increased rate of water absorption and porosity was, respectively, 101.25% and 69.32% (w/c of .40), 143.6% and 89.96% (w/c of .45), and 181% and 95.85% (w/c of .55). This behavior occurred because of the fact that natural aggregates and plastic are not sufficiently combined in the concrete matrix, leading to a porous matrix. This could be attributed to two factors: (i) the smooth surface and irregular shape of most PAs weakens the bond between the aggregates and the matrix of cement; and (ii) PAs almost have no absorption capacity for water, this will cause water to accumulate in the ITZ. Consequently, after these samples have dried, they create blank cavities. Accordingly, allowing the water absorption to increase when the specimens are exposed to water (Ahmed et al., 2021; Cao and Khan, 2021; Maglad et al., 2022).

From the data presented in Figures 9, 10, “it can be observed that the water absorption and porosity increased with higher w/c. This is because the coated surface of the aggregate (surface-coating pozzalanics materials around aggregate) is smaller in case of a higher w/c ratio. So, as the volume of paste is decreased, the pores in the concrete increase, and the water absorption increases. Moreover, as this study mentioned before, the extra water in the concrete samples that has not reacted with the cement and has not been absorbed by the waste PET particles creates cavities after drying” (Qaidi et al., 2022a). Therefore, higher water absorption and porosity are achieved for higher w/c ratios, as confirmed by Albano et al. (2009); Heredia (2018).

The influence of the PET aggregate content and w/c on the compressive strength of concrete is presented in Table 7. “The presented results are the average of three specimens whose individual values are (±2.0 MPa) relevant to the average mentioned in Table 7. Generally, and at all test ages, while the substitution rate of PET particles increases, the trend of compressive strength is further reduced. For example, compared to the control mix, at the 25% replacement at 90-day, the reduction rate in strength was 43.46% (w/c of .40), 40.96% (w/c of .45) and 25.2% (w/c of .55). While, at the 50% replacement, the reduction rate was 76.12% (w/c of .40), 76.82% (w/c of .45), and 74.41% (w/c of .55). This strength lessening can be labelled by the following reasons: The bonding strength between cement paste and PW surfaces is diminished as a consequence of the smooth surface and flat shape of the plastic particles acting as a barrier and preventing cement paste from adhering to the natural aggregate. As a consequence, the ITZ in concrete containing PET particles is weaker than in control concrete, lowering the resulting compressive strength. Also, the water that did not get absorbed by the PET and did not take part in the reaction between water and cement surrounded these aggregates. This made the bonding weaker and led to the formation of tiny channels that, when dry, could turn into pores” (Qaidi et al., 2022a).

On the other hand, “it is noticed that when increasing the ratio of w/c, the compressive strength decreases, similar to the conventional concrete mixtures. Because, in the case of higher ratios of w/c, the coated surface of the aggregates (surface-coating pozzalanics materials around aggregate) is smaller, and as a result of lower paste volume, higher bleeding water quantity occurs (Xie et al., 2021; Khan et al., 2022b). The latter (excess water), which is mainly located around PET particles, does not participate in the reaction with the cement, generates a weaker connection between the cement paste and the PET particles, also, generates some small channels which can form pores after drying, causing a decrease in strength” (Ahmad et al., 2022; Al-Tayeb et al., 2022; Tayeh et al., 2022c).

To understand the impact of replacing a natural aggregate with a PA with various w/c ratios on the behavior of splitting tensile strength of concrete at 90 days, the experimental results are shown in Table 7. Generally, the results show performance reductions of tensile strength for any pattern of substitution, like compressive strength but with less severe due to the flexible nature of plastic. Accordingly, decreases in split tensile strength can be related to the same reasons as decreases in compressive strength.

The stress-strain curves governed from compression tests, with different percentages of PET quantity and w/c, are plotted in Figure 11. From the data presented, “for a constant w/c ratio, it can be noticed that: (i) PET-concrete achieved the highest strain increase; and (ii) the peak compressive stress is less at a high PA level, but the corresponding strains are higher. In other words, due to the high flexibility of plastic, the ductility behavior will be enhanced at high PA substitutions. But, compared to the control concrete, the peak compressive stress was lower. Consequently, these reductions impact the stress-strain graph and lead to a decrease in the gradient of the graph during its linear elastic phase” (Qaidi et al., 2022a).

UPV is a non-destructive test to verify concrete uniformity and quality. Durability and concrete strength are evaluated via the concrete specimen by determining the speed of an ultrasonic pulse through the concrete. Mostly, pulse velocity is influenced by the moisture quantity, density, and elasticity of the material (Akçaözoğlu et al., 2013). Figure 12 presents the UPV performance at different curing ages (7, 28, 90 days) for concrete specimens with different PET quantity and w/c ratios. Each value shown is an average of three cubes. Results indicate that UPV decreases by increasing the PET quantity in the mix. Such behavior can be attributed to: (i) the incorporation of PET directly affects the porosity of the concrete (cavities are formed), which in turn attenuates the velocity of the ultrasonic waves: and (ii) plastic particles have a plate structure (flat and angular shape), which has a role in becoming the refractive limit of ultrasonic waves. In contrast, as shown in Figure 12, it can be gotten that the UPV raised with curing age due to improving the chemical and physical characteristics of concrete as a result of continuing the hydration reactions. On the other hand, one can recognize from Figure 12, that the UPV declines as the w/c ratio increases. This is attributable to extra water stored in the pores which leave empty holes in the concrete upon hardening. Such outcomes have already been confirmed by Albano et al. (2009); Rahmani et al. (2013).

To increase understanding of the impact of the substitute of normal fine aggregate with a PA with various w/c ratios on the thermal conductivity (k_c) behavior of concrete, the theoretical results obtained by the exponential equation provided by ACI-122R (2002) are presented in Table 8. Moreover, as this study mentioned earlier, the coefficient of Kc of this equation depends on the form of aggregate utilized in the mixtures of concrete. From the data presented, it can be found that with an increase in the quantity of PET aggregates, at a constant ratio of w/c, a significant decrease in Kc occurs. The reason for this decrease is attributed to (i) the PET aggregate had lower Kc (.15–.24 W/mK), compared with natural aggregate (2 W/mK), as stated by Hannawi et al. (2010). “Therefore, the PET aggregates act to slow thermal heat spread, causing a decline in Kc; and (ii) the presence of a large number of pores in the structure of PET-concrete, which reduces the Kc as a result of the low Kc of the air in these voids” (Akçaözoğlu et al., 2013).

On the other hand, from the data presented in Table 8, one can notice that the maximum reduction in Kc value is 46.99% (kc = .71 W/mK) for 50% replacement of PET (w/c ratio of .55), in comparison with control concrete (kc = 1.35 W/mK). Consequently, these composites (50% PET with .55 w/c) could be utilized as a bearing insulator according to RILEM-LC2 (1978) (kc sample <.75 W/mK,; f c ′ > 3.5 MPa).