3.1. Fresh Properties
Figure 9 illustrates the required HRWRA dosage to reach the slump flow target, 650 ± 25 mm, in each mix. However, the horizontal free flow was changed for flowable concrete mixtures with recycled aggregates. The incorporation of 50% of recycled aggregate caused the highest demand for HRWRA. There was a greater water absorption by using more waste plastic aggregate. For flowable concrete specimens with 30% or more plastic content, the water absorption percentage was significantly greater than those of other specimens with no waste materials. Therefore, samples with greater replacement percentages tended to absorb more water on their surfaces and consequently increased the water demand of the mixtures. Therefore, the lubricant effect of water decreased and the cohesion of flowable concrete mixes increased, which required a higher level of HRWRA to yield the anticipated slump flow. The average slump flows for 10%–20% recycled aggregate replacements were equal to or greater than that of the control sample. This shows the advantages of using recycled aggregate to improve the workability of flowable concrete by only adding up to 20% waste materials. However, 30%–50% replacement decreased the slump flow in comparison with that of the control sample. Al-Hadithi and Hilal reported that slump flow diameters ranging from 650 to 780 mm were obtained for the Self Compacting Concrete (SCC) with plastic waste replacement [
32].
For all the mixes, although the HRWRA percentage increased with an increased amount of waste aggregate to maintain an acceptable slump, a reduction in the slump flow was seen with increasing waste material content. In addition, the time that the concrete took to achieve the slump flow of 500 mm was measured (T500).
Figure 10 shows that the T500 increased from 2.02 seconds for the control flowable concrete to 2.12, 2.26, 2.15, 2.73, and 3.34 for flowable concrete with 10%–50% replacements. Although the plastic waste aggregate did not change the slump flow excessively, the homogeneity of the concrete decreased. The comparison between the T500 and V-funnel flow times of the control sample and flowable concrete at different replacement ratios confirmed that adding waste aggregate significantly increased the flow time.
The viscosity of flowable concrete was measured by the V-funnel test. Based on the specifications, a longer V-funnel flow time indicates a flowable concrete with greater viscosity. Moreover, those mixtures with shorter V-funnel flow times (i.e., low viscosities) are prone to having segregation [
33]. As shown in
Figure 10 with a dashed line, using waste aggregates increased the V-funnel flow time of flowable concrete; thus, the viscosity of SCC would be increased. The minimum value for the V-funnel was 8 seconds, which corresponded with a 10% aggregate replacement. Therefore, all of the flowable concrete mixtures were greater than the minimum EFNARC requirement (i.e., 6 seconds). However, using waste aggregates for more than 30% kept the V-funnel values greater than 12 seconds, which indicated an inappropriate flowability and a viscosity too high for being flowable. Al-Hadithi and Hilal found that the addition of waste plastic increases both T500 slump flow and V-funnel flow times [
32].
Figure 11 shows the results of the L-box test. In this test, the L-box values of the mixes, indicating the flowabilities of the mixes, ranged between 0.8 and 1. The mixes with the lower waste aggregate replacement ratios had higher L-box ratio than those with the higher replacement ratios, indicating higher flowability and workability of the mixtures. Addition of waste aggregate at a 30%–50% replacement level decreased the L-box ratio in comparison to that of the control specimen. According to Albano et al., having a higher absorption capacity in mixtures with plastic aggregates can influence the porosity [
34]. This behavior can cause an increase in viscosity, as is evident from the reduced L-box ratio magnitudes. The L-box ratio of the mix 10R was greater than those of other mixtures, which indicates that 10% replacement of waste aggregate was more successful in improving workability in comparison with other replacements. Al-Hadithi and Hilal also reported the same trend in L-box testing [
32].
The results of the J-ring test also confirmed the results obtained by the L-box, V-funnel, and slump flow tests (see
Figure 12). The maximum reduction of mixtures in slump flow in the J-ring test was not higher than 50 mm, except with 50R. Brameshuber and Uebachs [
35] reported that a flowable concrete mixture with an acceptable passing ability should have a blocking index (difference between J-ring flow and slump flow) of less than 50 mm to not see any blockage.
Figure 12 demonstrates the influences of recycled aggregate on the J-ring flow results; i.e., the passing ability was decreased for 20%–50% replacements as the J-ring flow decreased. On the contrary, the J-ring flow was improved at the 10% waste aggregate replacement level, which shows a greater passing ability compared to that of the control sample. The higher percentage of waste materials made the concrete less workable and increased the potential of blocking. Due to the angularity and rough surface texture of the waste aggregate, the passing ability of flowable concrete was decreased by its friction. Safiuddin et al. reported the same behavior when using construction waste aggregates [
36].
In order to verify the utility and efficacy of using waste plastic materials, the correlations between different rheological test measurements were calculated by employing the Pearson correlation method, as shown in
Table 5. The correlation coefficient is a number between −1 and +1. This number can specify how strongly two factors are correlated to each other. Coefficients of -1 and +1 designate great negative and positive correlations, respectively [
37]. In this study, an absolute value of a correlation coefficient of greater than 0.8 was considered as a robust correlation. In addition, a correlation coefficient of less than 0.5 was considered as a weak correlation.
The correlation coefficients between the slump flow, J-ring, L-box, and V-funnel were greater than 0.8, which shows a strong correlation. However, the T500 test was weakly correlated with other rheological factors. These relationships between the fresh properties were considered as a strong correlation. The effects of using the waste aggregate in flowable concrete by different levels of replacement were identical. In other words, the rheological properties (stability, mobility, and compactability) were improved in the same manner.
3.2. Mechanical Tests
The compressive strengths of samples were measured at the ages of 7, 28, and 91 days. As can be seen in
Figure 13, at the age of 7 days, samples containing recycled aggregates resulted in lower compressive strength values than that of the control sample. However, at the age of 28 days, using waste aggregates increased the compressive strengths of the samples beyond those of the control mix, except for the 30%–50% replacement percentages. Safi et al. reported that the compressive strength of self-compacting mortars decreased with the increase in plastic waste content at all curing times [
38]. At 30% and 50% substitution of waste, the percentages of reduction of compressive strength were 15% and 33%, respectively. Other authors found that, compared to control mixes, up to 72% reductions in compressive strength were observed for concrete with 20% replacement [
39,
40].
The same trend was seen in the tensile test. As shown in
Figure 14, using waste materials as aggregates reduced the splitting-tensile strength of SCC excessively. By using waste products instead of natural aggregates, in comparison with the control sample, the splitting-tensile strengths of samples decreased by 22.3% by adding 50% waste. In addition,
Figure 15 shows that the average value for the flexural strength test of the control samples was higher than for other samples except 40R after 7 days. However, after 28 days, for flexural strength, there was a great improvement in 20%–50% replacement percentages, as shown in
Figure 15. The highest increase was in the SCC samples by adding 40% waste materials, as the flexural strength increased by 25.8%. Adding 10% waste materials did not change the flexural strength and decreased it insignificantly. The flexural strength decreased by 35% by adding 10% waste products. Other authors reported that the flexural tensile strength decreases with the increase in plastic waste content. Authors found that this is due to the low resistance of the waste [
41].
The analysis of variance (ANOVA) is shown in
Table 6, which indicates whether the strength differences between samples containing waste replacement and the control sample are significant. As this evaluation was done within the samples in one group, it is called an omnibus test. Based on a defined level of 0.05, when the significance factor with waste materials is less than or equal to 0.05, there is a significant difference between this and the strength of the control sample. Otherwise, samples with significance factors of higher than 0.05 have an insignificant difference with the control sample [
42,
43]. When adding waste materials, the compressive strengths and splitting-tensile strengths of flowable concrete samples were decreased, but the magnitudes of strength did not change noticeably at small replacement levels.
Table 6 shows that the percentage reduction of 50R compared to the control sample was significant for both compressive and splitting-tensile strengths. Based on statistical analysis, all significance factors are larger than 0.05 for replacing waste materials up to 40%. This indicates that almost all compressive and splitting-tensile strength values were in the same range. Based on ANOVA, adding 10% waste materials decreased the flexural strength significantly compared to that of the control sample. While using waste materials to replace more than 10% of the fine aggregates had a slight impact on the flexural strength compared to that of control sample, no notable changes were observed in improving the flexural strength at 28 days.