2.2.2. Shredded Rubber (SR)

The density of SR used in this study is 566 kg/m3. SR obtained from the recycling plant has been sieved, and particles smaller than 12.5 mm are used to make sure that similarly-sized SR particles are used in the partial replacement of the coarse aggregates. The size distribution of the SR, which is also compared with that of the coarse aggregates (9.5 mm and 12.5 mm), is shown in Figure 3. The particle size distribution for both sizes of coarse aggregate as well as the SR are more or less well-distributed, and interestingly, the particle size distribution of the SR fits in between the two sizes of coarse aggregates used in this study. Therefore, a direct replacement of both sizes of coarse aggregates with SR is used.

**Figure 3.** Size distribution of shredded rubber and coarse aggregates used in this study.

#### 2.2.3. Steel Fibers (SF)

The density of SR used in this study is 7850 kg/m3. The length and diameter of 100 randomly selected steel fibers have been measured, and the results are shown in Table 2.


#### *2.3. Concrete Mixes Containing Recycled Tire Products*

A total of 24 concrete mixes are cast, including the benchmark. The mix codes and descriptions are shown in Table 3. SR, CR, and SF in the concrete mix codes show the presence of shredded rubber, crumb rubber, and steel fiber in the concrete mix respectively. When combinations of SF, CR, and SF are used in a concrete mix, the code starts with an "H."

For each concrete mix, a total of 14 cylinders (100 mm diameter by 200 mm height) and 3 beams (100 mm × 100 mm × 400 mm) are cast and sulfur-capped as per ASTM C617 [38]. A minimum of 3 cylinders are tested to calculate the tensile strength at 28 days as per ASTM C496 [39]. Similarly, a minimum of 3 cylinders are used to calculate the compressive strength at 7 and 28 days (Figure 4) as per ASTM C39 [40]. For the benchmark concrete only, 3 cylinders are tested at 3 days under compression.


**Table 3.** Concrete mixes and descriptions.

**Figure 4.** Test of a concrete cylindrical sample (**a**) Compression; and (**b**) Tension.

Third-point loading tests (Figure 5) are performed on the beams to obtain the modulus of rupture of the concrete mix as per ASTM C78 [41].

**Figure 5.** Third-point loading test to obtain the modulus of rupture.

#### **3. Results**

#### *3.1. Slump Test Results*

For a pavement-grade concrete mix in a hot climate, a target slump of 70 mm ± 30 mm is set. The slump of the benchmark concrete, BM, was 75 mm, and the slump of all other concrete mixes were within the targeted range. The slump test results are compared in Figure 6. It is worth mentioning that in all mixes, oven-dried sand and aggregates are used to ensure consistency in water-to-cement ratio. In addition, to improve the workability of concrete mixes, superplasticizer is used in all mixes (2.0% to 2.5% of the weight of cement). Since 2.0% is used for the benchmark, a minimum amount of 2.0% is used in all other mixes; if the workability is not desirable, an extra 0.5% is added to the mix.

(**a**)

**Figure 6.** *Cont*.

**Figure 6.** Comparing the slump test results of benchmark (75 mm) with (**a**) shredded rubber (SR); (**b**) crumb rubber (CR); (**c**) steel fibers (SF); and (**d**) Hybrid (i.e., H) mixes.

#### *3.2. Density of Concrete*

The BM's density is 2390 kg/m3. Replacing the coarse aggregates and sand by SR and CR respectively reduces the density. For example, the density of SR-4 and CR-4 is 2317 kg/m<sup>3</sup> and 2362 kg/m3, respectively. On the other hand, the addition of SF increases the density; for example, SF-3 has a density of 2425 kg/m3. In the hybrid mixes (i.e., H-1 to H-12), the maximum density is seen in H-5 (2402 kg/m3) and the minimum is seen in H-4 (2314 kg/m3).

#### *3.3. Compressive Strength*

A minimum of 3 cylinders are tested in order to obtain the compressive strength. Figure 7 shows the compressive strength of BM mix at 3, 7, and 28 days. All other mixes are tested at 7 and 28 days. The compressive strength of SR, CR, and SF mixes at these two ages are compared with those of the BM mix in Figure 8. Similarly, the compressive strength of hybrid mixes (H-1 to H-12) are compared with BM in Figure 9.

**Figure 7.** Compressive strength of the benchmark mix at 3, 7, and 28 days.

(**c**)

**Figure 8.** Comparing the compressive strength of the benchmark mix with (**a**) SR; (**b**) CR; and (**c**) SF mixes.

**Figure 9.** Comparing the compressive strength of the benchmark mix with hybrid mixes (**a**) H-1 to H-4; (**b**) H-5 to H-8; and (**c**) H-9 to H-12.

#### *3.4. Tensile Strength*

A splitting tensile test is performed on 3 cylinders for each mix, and the average tensile strength of the benchmark mix is compared with that of all other mixes in Figure 10.

**Figure 10.** *Cont*.

**Figure 10.** Comparing the tensile strength of the benchmark mix (3.11 MPa) with (**a**) SR; (**b**) CR; (**c**) SF; and (**d**) Hybrid (i.e., H) mixes.

#### *3.5. Modulus of Rupture*

A third-point loading test is performed on 3 beams for each mix, and the average value relating to the modulus of rupture of the benchmark mix is compared with that of all other mixes in Figure 11.

**Figure 11.** *Cont*.

**Figure 11.** Comparing the modulus of rupture of the benchmark mix (5.04 MPa) with (**a**) SR; (**b**) CR; (**c**) SF; and (**d**) Hybrid (i.e., H) mixes.

#### **4. Discussion**

#### *4.1. Slump Test*

Figure 6a,c show the slump test results for mixes containing SR and SF, respectively. The results shown in the figures are inconclusive, as they do not clearly indicate a trend with the introduction of either of the materials. However, the reduction in slump due to the incorporation of SF into the concrete mix was corroborated by findings by other researchers [33]. Additionally, all SR and SF mixes presented acceptable slump values that are within the target slump range for pavements (70 mm ± 30 mm).

Figure 6b shows the slump test results for mixes containing CR. As shown in the figure, the introduction of CR initially decreased the slump but then increases with additional doses. This trend is the case for doses up to and including 15%. The introduction of 20% CR reverses the trend but still maintains a slump value similar to the benchmark. The conclusions of this family of concrete are similar to that of other researchers [34].

Figure 6d shows the slump test results for the hybrid mixes, containing combinations of SR, CR, and SF. The figure shows four "families" of hybrid mixes: combinations of SR and SF are shown in red, combinations of CR and SF are shown in blue, combinations of SR, CR, and SF are shown in yellow, and combinations of SR and CR are shown in green. The results in red show that with the exception of H2, there is a clear and increasing trend in the slump of concrete. Initially, the slump significantly decreased but then increases, with the last mix reaching the benchmark's slump. The results in blue show that the slump remained somewhat consistent, hovering in the range of 85–100 mm. This excludes the results of H8, which can be considered anomalous. The results in yellow show that there is a slight increase in the slump at the higher dosage of H10, but this remains significantly

lower than the benchmark's slump. The results in green show that there is a slight increase in the slump at the higher dosage of H12, but this remains lower than the benchmark's slump and outperforms the results in yellow. All slump test results were within the target slump range for pavements.

#### *4.2. Compressive Strength*

Figure 8a shows the variation of compressive strength of samples containing SR compared with the benchmark at the 7- and 28-day marks. As shown in the figure, compressive strength is directly affected by the introduction of SR into the mix, where larger quantities of SR present in the mix lead to a decrease in the 7- and 28-day compressive strength in an almost proportional fashion. The samples also exhibit similar rates of compressive strength gain to the benchmark mix, as shown by the slope of the lines. Comparable studies stated that the introduction of SR would reduce the compressive strength by up to 23% [27,29], where the compressive strength reduction in the results of this study ranged between 13% and 36%. There is some correlation between the results of the study and existing literature, with variations in the upper/lower bounds of compressive strength reduction. However, the results stand in contrast to the findings of some other researchers [13] who found that compressive strength, among other properties, was improved by the introduction of SR into the mix.

Figure 8b shows the variation of compressive strength of samples containing CR compared with the benchmark at the 7- and 28-day marks. As shown in the figure, there is no clear correlation between the amount of CR present in the sample and the compressive strength at any age. However, the introduction of CR lowers both 7- and 28 day compressive strength. It is worth mentioning that all concrete mixes in this figure have acceptable 28-day compressive strengths, ranging from 26.4 MPa (for CR4) to 31.1 MPa (for CR2). Comparable studies stated that the introduction of CR would reduce the compressive strength by 15% [26], where the compressive strength reduction in the results of this study ranged between 8% and 30%. It is worth mentioning that the replacements considered in this study are within the bounds of acceptable replacement of conventional materials [29].

Figure 8c shows the variation of compressive strength of samples containing SF compared with the benchmark at the 7- and 28-day marks. As shown in the figure, an interesting observation occurs at SF2, where the 7-day compressive strength was higher than the benchmark and the 28-day compressive strength was very similar to the benchmark. Other than that, both SF1 and SF3 performed similarly to each other, both being slightly lower than the benchmark and having similar rates of compressive strength gain to each other. This comes at odds with the conclusions of other researchers who had success in increasing mechanical properties (such as compressive strength) with increasing dosages of SF [30].

Figure 9a shows the variation of compressive strength of samples containing a combination of SR and SF compared with the benchmark at the 7- and 28-day marks. As shown in the figure, higher doses of SR with the introduction of 0.1% SF into the mix decrease the 7 and 28-day compressive strength. However, there is no impact on the rate of compressive strength gain in any of the samples, exhibiting similar slopes to that of the benchmark.

Figure 9b shows the variation of compressive strength of samples containing a combination of CR and SF compared with the benchmark at the 7- and 28-day marks, and Figure 9c shows the variation of compressive strength of samples containing a combination of SR, CR, and SF compared with the benchmark at the 7- and 28-day marks. An interesting observation can be seen across the two figures above, where the compressive strengths of the samples converge to the same value at the 28-day mark. The rate of compressive strength gain is expected to decrease as the proportion of the replacement materials increase in the mixes.

All mixes used in this study exhibit acceptable levels of 28-day compressive strength, with a minimum of 21.3 MPa (H-4) and a maximum of 33.1 MPa (SF-2).

#### *4.3. Tensile Strength*

Figure 10a shows the variation of split tensile strength of samples containing SR compared with the benchmark. The results show a very clear decreasing trend as the percentage of SR increases in the mix. The results of SR3 can be considered anomalous.

Figure 10b shows the variation of split tensile strength of samples containing CR compared with the benchmark. The results show a very clear decreasing trend as the percentage of CR increases in the mix. Comparable studies stated that the introduction of CR would reduce the tensile strength by up to 43% [2,26], where the tensile strength reduction in the results of this study ranged around 22%. While this is a marked improvement over existing literature, it still represents a significant drop in the tensile strength of concrete.

Figure 10c shows the variation of split tensile strength of samples containing SF compared with the benchmark. The results show very consistent tensile strength results that are close to the benchmark's results, regardless of the dosage of SF present in the concrete. The results of this test indicate that there is no advantage in incorporating additional doses of steel fibers beyond 0.1%. This, coupled with the fact that higher dosages of SF led to a significant decrease in the slump, justifies the use of this dosage in the hybrid mixes.

Figure 10d shows the tensile strength test results for the hybrid mixes, containing combinations of SR, CR, and SF. The figure shows four "families" of hybrid mixes: combinations of SR and SF are shown in shades of green, combinations of CR and SF are shown in shades of orange, combinations of SR, CR, and SF are shown in shades of blue, and combinations of SR and CR are shown in shades of yellow. The results in green show that there is a clear decreasing trend in the tensile strength of the samples as the proportion of SR increases in the samples. The results in orange indicate a clear trendline showing that the values are almost consistent throughout the samples tested. However, the samples have tensile strength values that are lower than the benchmark's. The results in blue show that there is a slight increase in the tensile strength at the higher dosage of H10. The results in yellow show that there is a slight decrease in the tensile strength at the higher dosage of H12.

#### *4.4. Modulus of Rupture*

Figure 11a shows the variation of modulus of rupture of samples containing SR compared with the benchmark. The results show a very clear trend where the modulus of rupture remains consistent for all four dosages of SR tested.

Figure 11b shows the variation of modulus of rupture of samples containing CR compared with the benchmark. The results show a very clear trend where the modulus of rupture decreases with increasing dosages of crumb rubber. The results of CR3 can be considered anomalous. The findings of the CR trials match closely with the findings of other researchers, who have stated that the incorporation of rubber products has a negative effect on mechanical properties [2].

Figure 11c shows the variation of modulus of rupture of samples containing SF compared with the benchmark. Initially, this does not have an impact on the modulus of rupture at a low dosage. However, with increasing dosages of SF, there is a clear trend of decreasing modulus of rupture. As was the case with the tensile test results, the results of this test also indicate that there is no advantage in incorporating additional doses of steel fibers beyond 0.1%. Again, this comes at odds with the conclusions of other researchers who had success in increasing mechanical properties (such as flexural strength) with increasing dosages of SF [30].

Figure 11d shows the test results for the modulus of rupture for the hybrid mixes, containing combinations of SR, CR, and SF. The figure shows four "families" of hybrid mixes: combinations of SR and SF are shown in shades of blue, combinations of CR and SF are shown in shades of green, combinations of SR, CR, and SF are shown in shades of yellow, and combinations of SR and CR are shown in shades of orange. The results in blue show that there is a clear decreasing trend in the modulus of rupture of the samples as the

proportion of SR increases in the samples. The results in green show that there is a clear decreasing trend in the modulus of rupture of the samples as the proportion of CR increases in the samples. However, results up to and including H7 are very close to the modulus of rupture of the benchmark, which is similar to the findings of other researchers [2,35]. The results in yellow show that there is a sharp drop in the modulus of rupture values as the dosage of replacement materials increases, while noting the improvement in the modulus of rupture for H-9 compared with the benchmark mix, which proves the benefits of including all replacement materials in the concrete mix. The results in orange show that there is a sharp increase in the modulus of rupture values as the dosage of replacement materials increases.

#### **5. Conclusions**

After analyzing the results presented above, and despite the reduction in fresh properties due to the introduction of recycled tire products in the concrete mix, it has been shown that multiple tire by-products (shredded/crumb rubber and recovered steel fibers) can be successfully hybridized to produce a novel pavement-grade green concrete that is suited for use in hot-weather climates. This is important, as it achieves two goals in one: to be able to produce a sustainable construction material that is tailored for use in Kuwait's climate and to also reduce the number of tires going to landfill.


All mixes, whether individual replacement or hybrid mixes, exhibited acceptable properties for pavement-grade concrete for use in hot climates. The conclusions of this study show that it is possible to hybridize all materials sourced from recycled tires in the production of a feasible pavement-grade concrete suited for hot weather. Possible extensions of the project could include the investigation of other properties to evaluate the usefulness of the hybrid concrete, such as measuring skid resistance and modulus of elasticity. Further, long-term effects of using this concrete could be modeled using finite-element analysis software packages, which could be used to predict the behavior of the concrete while it is being used at a selected intersection in Kuwait.

**Author Contributions:** Conceptualization, S.M.S., A.R.A., A.J., N.M. and A.F.; methodology, S.M.S.; formal analysis, S.M.S., A.R.A., A.J. and N.M.; investigation, A.R.A., A.J. and N.M.; resources, A.R.A.; data curation, A.J.; writing—original draft preparation, S.M.S., A.R.A., A.J., N.M. and A.F.; writing review and editing, S.M.S., A.R.A., A.J., N.M. and A.F.; supervision, S.M.S.; project administration, A.F.; funding acquisition, S.M.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was funded "partially" by Kuwait Foundation for the Advancement of Sciences under project code: PR19-15EV-02.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors would like to acknowledge Tahir Afrasiab for his contributions, hard work and dedication while preparing the concrete mixes for this study. The authors would like to thank Sameer Hamoush of North Carolina A&T State University for his input and advice throughout the course of the project. The authors would also like to express their gratitude to Green Rubber Recycling Co. for providing samples of their recycled tire products to be used in this study. The authors are grateful to Sika for providing the team with the superplasticizer used in the study.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**


#### **References**

