Introduction
Experimental Investigation
Materials
Concrete Mix Design
Sample Preparation
Test Procedure
Test Results & Discussions
Slump
Unit Weight
Compressive Strength
Split Tensile Strength
Flexural Strength
Axial Load Capacity of Column
Summary of the Findings
Conclusions
Introduction
Rubber tires are a serious environmental concern, with over 1 billion tires reaching the termination of their life each year, more than half of these are discarded without treatment [1]. Discarded tires contribute greatly to solid waste and pose risks to human health, the atmosphere, and the economy due to the desecration of soil, water, and air [2]. Their non-porous nature allows for water accumulation, creating ideal breeding grounds for pests, while burning tires releases harmful pollutants and poses fire hazards. Recycling tire waste in construction promotes sustainability by decreasing the need for raw materials and helping to prevent land degradation [3, 4, 5].
It was observed that using tire rubber in concrete can reduce its strength and stiffness [6, 7, 8, 9, 10, 11]; however, it also enhances hardness, ductility, and damping capacity [12, 13, 14], fulfilling essential requirements for construction materials [15]. Rajagopal et al. [16] tested M20 and M30 concrete mixes with varying CR levels and found improved workability and strength for CRC pre-treated with NaOH and micro-silica, with optimal results at 5% CR replacement. Although CR reduces compressive strength and elastic modulus, CRC demonstrated an 11.8% higher toughness index at 18% CR replacement [17]. According to another study by Hasan et al. [18], human hair showed potential in the improvement of CRC. They observed a declining phenomenon in compressive and tensile strengths with increasing CR content; however, a mix of 5% CR and 1% HHF increased compressive strength by 3.6% and tensile strength by 8.56%. Concrete properties can also be improved by using fibers, such as steel fiber [19, 20, 21, 22], polypropylene fiber [20, 23, 24, 25], glass fiber [22, 26], date palm fibers [27], with cement mortar. Some studies assessed treatment methods using cement coating, potassium permanganate (KMnO4), and NaOH, and observed improved properties compared to untreated CR [28]. Bilema et al. [29] discovered that all modified warm mix asphalt mixtures exhibited superior mechanical properties regarding indirect tensile strength and stiffness. The use of different fibers such as carbon fiber [30], glass fiber [22], polypropylene fiber [31, 32], and steel fiber [33, 34, 35, 36] was studied to ameliorate the mechanical characteristics of CRC, and positive outcomes were observed.
Although different CR treatment methods and the use of fibers enhance the strength of CRC, they are expensive compared to normal concrete (NC), which limits widespread application in construction works. In contrast, the use of water-reducing admixtures has been gaining popularity due to their contribution to strength enhancement by reducing water content. However, there is limited research that focuses solely on the use of such admixtures in CRC and their structural application. This study addresses this gap using only a water-reducing admixture dose of 0.5% of the weight of cement by evaluating and comparing the mechanical characteristics, such as compressive, split tensile, and flexural strength, of concrete with and without CR and admixtures. Furthermore, it investigates the effect of CR on the load-carrying capacity of reinforced columns under static concentric load to address the limitations in structural applications. In this study, the CRC was prepared and tested by substituting recycled CR aggregate as a replacement for fine aggregate at 0%, 5%, 10%, and 15% levels. This study offers novel insights into the structural performance, cost efficiency, and sustainable potential of CRC for practical construction applications by using a cost-effective admixture with systematically varying CR content.
Experimental Investigation
The study was completed by following several procedural steps, such as selecting appropriate materials, determining their properties, concrete mix design, casting the concrete samples, curing them for 28 days, and testing the prepared samples to obtain the outcomes.
Materials
This research employed Portland Composite Cement, CEM II/A-M, as the binding material. It contains 80-94% clinker, 6-20% fly ash, slag, and limestone, and 0-5% gypsum [37]. The physical characteristics of the cement were ascertained as per ASTM C187 [38] and ASTM C191 [39]. Test results presented a consistency of 28 mm, a specific gravity of 3.15, an initial setting time of 170 minutes, and a final setting time of 275 minutes for cement. Concrete materials were mixed using potable water for better workability and durability. The photographs and physical properties of the aggregates are given in Figure 1 and Table 1, respectively. Sieve analysis of fine and coarse aggregate was conducted as per ASTM C136 [40], and the particle size distribution curve is presented in Figure 2. Tests on fineness modulus yielded results of 2.54 for sand as fine aggregate and 7.50 for stone as coarse aggregate. The fineness of CR was found to be 3.62, which was higher than that of the fine aggregate and lower than that of the coarse aggregate. The water- reducing admixture (Type A), as per ASTM C494 [41], was incorporated into the mixture at a dose of 0.5% by weight of cement to maintain the desired workability.
Table 1.
Physical properties of the aggregates
| Properties | CR | Stone | Sand |
|
Fineness modulus Specific gravity Bulk density (kg/m3) Surface moisture (%) Water absorption capacity (%) |
3.62 - 515 - - |
7.50 2.72 1777 0.93 1.45 |
2.54 2.69 1620 7.58 1.10 |
Concrete Mix Design
The mix design was conducted as per the mix design guideline of ACI 211.1 [42]. The water-cement ratio was used as 0.45 to obtain an M30 grade concrete. The quantity of each material for each batch of mixing is provided in Table 2.
Table 2.
Quantity of materials according to the mix design
Sample Preparation
The concrete ingredients were mixed by a rotary drum mixer for 5-7 minutes to ensure uniformity during mixing. After achieving uniform mixing, the slump test was conducted to determine the expected slump value. The mixed slurry was then poured into the steel molds to prepare concrete cylinders and compacted with a 12 mm diameter bar to eliminate air entrapment. A total of 48 pieces of concrete cylinders were prepared, among which 24 pieces were for compressive strength and 24 pieces for the split tensile strength test. The dimensions of the cylinders for the compressive and split tensile tests were 100 mm × 200 mm. Wooden molds measuring 150 mm × 150 mm × 500 mm were used to prepare 24 pieces of flexural test samples, and molds measuring 150 mm × 150 mm × 900 mm were used to cast the 12 pieces of small-scale column samples. In the column, 4-Φ12 mm diameter rebar was used as longitudinal and Φ8 mm diameter rebar at 100 mm intervals was used as lateral confinement rebar. The reinforcement detailing is presented in Figure 3. The samples were extracted from the molds after 24 hours from the casting time, and then immersed in water for a curing period of 28 days.
Test Procedure
The slump test was carried out following ASTM C143 [43] guidelines, and the unit weight of the concrete samples was estimated by dividing the weight of the concrete cylinders by their volume, as per ASTM C138 [44]. The test follows ASTM C39 standards [45] to find out the compressive strength of the concrete cylinders using a universal testing machine with a 1000 kN capacity. The split tensile strength test followed the ASTM C496 [46] standard to find out the tensile strength of concrete cylinders. The study adhered to the ASTM C293 [47] standard to evaluate the flexural strength of the concrete prisms. The test setup of each test is demonstrated in Figure 4. The column test samples were subjected to static concentric load to find out the axial load capacity using a universal testing machine.
Test Results & Discussions
CR reduces the mechanical characteristics of concrete due to weak adhesion at the aggregate-cement interface and increased air voids. Admixtures enhance the workability and reduce void content, restoring strength properties closer to conventional concrete.
Slump
The correlation between the slump values of concrete mixes with and without admixtures and the percentage of CR replacement is shown in Figure 5. The difference in slump values between the two mix types shows that adding more CR improves workability over time. The slump of unmixed concrete rose continuously from 55 mm at 0% CR to 85 mm at 15% CR. A larger slump value of 97 mm was achieved at 15% CR, compared to 73 mm for 0% CR, in the mixtures that contained admixture. Admixtures are generally useful for enhancing the flowability [48, 49] and convenience of placing CRC, as their constant improvement of slump shows. This pattern indicates that while CR on its own can improve workability to a moderate degree, the combined effects of CR and chemical additives provide far better results when it comes to the qualities of freshly mixed concrete. Previous studies also reported an increasing phenomenon of concrete workability due to an increase in CR concrete [50, 51]. CR may increase the concrete slump value because rubber particles are softer, lighter, and more elastic than natural aggregates like sand or gravel. As a consequence, when CR replaces a portion of fine aggregate, the overall mix becomes less dense and more flexible, allowing particles to move past each other more easily under gravity.
Unit Weight
The variation in average unit weight of concrete with varying percentages of CR and admixture is illustrated in Figure 6. The unit weight values range from 23.91 kN/m³ to 21.92 kN/m³, with mix NC exhibiting the highest unit weight and mix CR15-A0.5 the lowest. The reference mix, which presumably contains no CR or admixtures, achieved the highest unit weight at 23.91 kN/m³, indicating a dense and compact concrete matrix. As the CR content increases, the overall density of the mix tends to decrease, contributing to a reduction in unit weight as observed in mixes CR5-A0, CR10-A0, and CR15-A0. This trend underscores the inverse relationship between CR content and unit weight of concrete. This reduction likely reflects a replacement level of fine aggregate with CR, which is inherently lighter and less dense than natural aggregates. Heylemelecot et al. [52] found that the unit weight was decreased steadily by 1.63, 3.67, 4.08, and 6.94% with CR content of 4%, 8%, 12%, and 16%, respectively, when compared to the NC. The concrete mix containing admixture for a specific content of CR showed a little decrease in unit weight compared to the mix without admixture, suggesting that incorporation of admixtures has no significant impact on concrete density. Overall, the results indicate that CR can be used to tailor concrete density based on performance requirements, though trade-offs in mechanical properties must be considered.
Compressive Strength
The compressive strength of concrete with varying percentages of CR is illustrated in Figure 7. Initially, concrete containing admixtures and CR exhibited a rise in compressive strength, but this trend reversed when no admixture was present. In the absence of admixtures, the maximum compressive strength reached 30.09 MPa at 0% CR for NC, and the minimum was 23.07 MPa at 15% CR. The NC-A0.5 mix, which had 0% CR and 0.5% admixture, recorded the highest compressive strength at 36.14 MPa, while the CR15- A0.5 mix, with 15% CR and 0.5% admixture, showed the lowest strength at 29.40 MPa. Therefore, the incorporation of CR in concrete mixtures has a negative influence on compressive strength; however, the admixture contributed to overcoming this consequence by improving the compressive strength. A study by Shofi [53] observed a gradual drop, with reductions of 3.09%, 13.66%, and 33.09% recorded at CR levels of 5%, 10%, and 15%, respectively. Previous studies also observed a similar trend of decreasing strength because of using CR. According to Son et al. [54], adding rubber particles to the mix reduced compressive strength by 8% and 32% for 0.5% and 1%, respectively, when compared to the reference sample that did not contain any CR. The use of CR led to the formation of voids [55, 56]; it is softer compared to the natural fine aggregate and possesses a lower density [57, 58], which decreased the compression strength of CRC.
Split Tensile Strength
Figure 8 represents the split tensile strength of concrete mixes incorporating varying levels of CR with and without admixture. The control mix without CR and admixture (i.e., NC) recorded a strength of 3.02 MPa. The inclusion of CR generally caused a reduction in tensile strength; however, the inclusion of admixtures helped to mitigate this effect. Among all mixes, NC-A0.5 showed the highest strength at 3.48 MPa, outperforming even the control mix, highlighting the positive influence of admixture on CR- modified concrete. In contrast, the CR15-A0 mix demonstrated the lowest strength at 2.32 MPa, emphasizing the negative impact of untreated CR. Other admixture-containing mixes, such as CR5-A0.5, CR10-A0.5, and CR15-A0.5, consistently exhibited higher strengths than their non-admixture counterparts (CR5-A0, CR10-A0, and CR15-A0, respectively). These findings indicate that while CR decreases split tensile strength, the use of admixtures can significantly improve the mechanical performance of CRC, especially at lower CR contents.
The causes of decline in split tensile strength are similar to the causes of decline in compressive strength. A previous study by Hasan et al. [59] revealed that as CR content rose from 5% to 15%, the split tensile strength showed a consistent decline, ranging from 17% to 33%. Banerjee et al. [60] found that the split tensile strength was greater after 7 days when admixtures were utilized compared to concrete without admixtures. They found that the split tensile strength with admixtures rose by 4.62% to 9.67% compared to concrete without admixtures.
Flexural Strength
The variation in flexural strength with different percentages of CR and the inclusion of admixtures is illustrated in Figure 9. The flexural strength values ranged from a high of 8.23 MPa to a low of 5.65 MPa across all mixes from NC to CR15-A0.5. The results represent a general decline in flexural strength with increasing CR percentage; however, the presence of admixtures mitigates this reduction to some extent.
The control mix without admixture (i.e., NC) recorded a flexural strength of 6.81 MPa, while its counterpart with admixture (i.e., NC-A0.5) achieved the highest overall value of 8.23 MPa, highlighting the positive impact of admixtures at 0% CR replacement. As the CR content increased, a downward trend was observed among both series of mixes. For example, the mix CR5-A0 dropped to 6.12 MPa, while the CR5-A0.5 mix still maintained a relatively high value of 7.68 MPa. This suggests that the admixture continued to enhance flexural performance even at moderate CR replacement. At higher CR levels, however, the reduction in strength became more pronounced. In addition, CR10-A0 and CR15-A0 mixes recorded flexural strengths of 5.65 MPa and 5.23 MPa, respectively. Similarly, the CR10-A0.5 and CR15-A0.5 mixes showed strengths of 6.72 MPa and 5.71 MPa, respectively. However, the CR15-A0.5 mix which contains the highest CR content and admixture, recorded one of the lowest values, suggesting that the beneficial effects of admixtures diminish at higher CR content.
Overall, the results indicate that admixtures improve flexural strength, especially at lower CR content. The flexural strength decreases as CR content increases, regardless of whether an admixture is present. Multiple studies assessed the effect of CR on flexural strength. According to Mustafa et al. [61], the control mix recorded the peak value (4.74 MPa), while the mix containing 16% CR indicated the lowest value (4.35 MPa), showing an 8.22% reduction. The study conducted by Banerjee et al. [60] revealed that the flexural strength of concrete after 28 days was higher with the use of admixture than without it. In all cases, the flexural strength with admixture showed an increasing trend from 4.31% to 4.91% as opposed to concrete without admixture.
Axial Load Capacity of Column
Table 3 summarizes the experimental load-carrying capacities of concrete columns incorporating various percentages of CR and compares the theoretical load with the experimental values. The column with CR5- A0.5 and CR10-A0.5 concrete mix showed an improvement in experimental axial load capacity of 15% and 14%, respectively, compared to the column with NC mix. However, although the CR10-A0.5 mix showed a lower load capacity than the CR5-A0.5 mix, which was obvious due to the presence of a higher content of CR, the difference is not significant. This suggests that the inclusion of CR, when combined with admixtures, may enhance the actual load capacity of the column up to a certain threshold. Furthermore, at 15% CR content (i.e., CR15-A0.5 mix), the experimental loads dropped by 5.84%, at 742.0 kN, compared to the NC, indicating a decline in structural efficiency at higher CR levels. It is obvious that the CRC column capacity may decrease compared to the NC column due to the reduced compressive strength of CRC. Previous studies also reported a decrease in column axial load capacity of CRC due to the concentric load [62, 63, 64]. Elsayed et al. [63] also reported the failure load reductions for columns modified with CR. Their study established that columns with 5%, 10%, and 20% CR showed strength losses of 3.3%, 7.0%, and 14.4%, respectively, in relation to the reference column.
Table 3.
Axial load capacity of the concrete column
The theoretical axial load capacity of the reinforced concrete column due to concentric load was determined by applying Eqn. (1) as per ACI 318-19 [65]. In this Eqn. (1), the yield strength of the steel rebar is 500 MPa, which was used in the samples, and the compressive strength was applied as the experimental result obtained for the specific concrete mix. The control mix NC with 0% CR had a theoretical load of 801.4 kN, which is 1.68% greater than the experimental load. In contrast, mixes with 5% and 10% CR (i.e., CR5-A0.5 and CR10-A0.5) exhibited lower theoretical loads than experimental ones, but the difference was below 5%. Additionally, theoretical capacity was 5.93% lower than that of experimental load for the mix with 15% CR (i.e., CR15-A0.5). This indicates that Eqn. (1) suitable for predicting the axial load capacity of CRC with up to 10% CR. Overall, moderate CR replacement combined with admixtures appears to improve the column’s load capacity, while excessive CR reduces performance.
Where, = Compressive strength of concrete
= Gross area of the column
= Area of longitudinal steel rebar
= Yield strength of steel rebar
The failure mode of the concrete columns is demonstrated in Figure 10. All tested specimens of columns constructed with NC and CRC (i.e., CR5-A0.5, CR10- A0.5, and CR15-A0.5) failed by concrete crushing, subsequently leading to internal buckling of the rebar. The NC column (i.e., M1) shows severe crushing at the top with spalling of concrete and vertical splitting cracks. This indicates brittle crushing failure combined with splitting due to axial load exceeding the compressive strength. In contrast, the column with 5% CR and admixture (i.e., M4) shows localized crushing near the top and a few vertical cracks, but much less spalling compared to M1. In addition, the column with the CR10-A0.5 mix (i.e., M6) displays extensive diagonal and vertical cracking over the height of the column without severe spalling. The column specimen with the 15% CR and 0.5% admixture mix (i.e., M8) exhibits significant vertical cracking and bowing (lateral deformation). The cracking pattern suggests early loss of stiffness and ductile deformation, consistent with higher CR content reducing compressive capacity and enhancing deformability. This cracking pattern of CRC suggests a more ductile crushing failure, possibly due to rubber particles providing some energy absorption, delaying fragmentation, and increasing deformability. Overall, increasing CR content changes the failure mode from brittle crushing with spalling (M1) to more ductile crushing and extensive cracking with deformation (M4, M6, and M8). The admixture helped retain cohesion and reduced spalling, but higher CR replacement clearly reduced load-bearing capacity and increased deformability.
Summary of the Findings
The slump value of the concrete mix increased with increasing CR percentage from 5% to 15%, whereas the unit weight showed a reverse trend. In addition, the admixture contributed to enhancing the slump value; however, it slightly reduced the unit weight of concrete compared to its counterpart without admixture. A significant enhancement was found when 0.5% admixture was added to the concrete mix with and without CR, increasing compressive strength by 20%-27%, split tensile strength by 10%-15%, and flexural strength by 6%-23%. Although the CR decreased the strength and thus the load-bearing capacity of the column, the 0.5% admixture contributed to improving load-bearing capacity by up to 15% when the CR content was 5% to 10%. However, more than 10% CR reduced the capacity by 6% compared to the NC. Therefore, limiting the CR content to 10% is optimal for producing structural and non-structural CRC.
Conclusions
Admixtures generally contribute to improving the strength of concrete by reducing the amount of water required in the fresh mix. This approach is cost- effective in comparison with other methods, such as CR treatment or fiber addition for enhancing strength. However, there is little research available to evaluate this potential in CRC, which has been addressed in this study. This study involved analysis and comparison of two sets of concrete of the same mix: one without admixture and the other with admixture to assess the effect of admixture on CRC and structures. The test findings and analysis have led to the following conclusions on the usage of admixture in CRC:
a)The application of water-reducing admixtures helped to preserve the appropriate workability in CRC mixes, ensuring improved compaction and fewer voids, both of which are essential for strength and longevity. CRC can be utilized as a lightweight building material because its weight per unit consistently decreases as the CR concentration increases. However, with greater CR percentages, this results in a decrease in mechanical strength.
b)In terms of compressive, split tensile, and flexural strength, the concrete mixes that contained admixtures continuously performed better than their counterparts without them. This demonstrates how admixtures can enhance the bonding and strength properties of CRC. The findings show that when mixed with admixtures, a moderate amount of CR, roughly 5% to 10%, can preserve or marginally enhance mechanical qualities. Compressive, split tensile, and flexural strengths noticeably decrease over this point, especially at 15%, indicating a limit to the positive effects of CR content.
c)Column tests revealed that CRC mixes containing admixtures (CR5-A0.5 and CR10-A0.5) and 5% and 10% CR had greater load capacities than NC in both experimental and theoretical values, indicating enhanced ductility and energy absorption due to increasing CR content. However, theoretical and experimental loads decreased and deviation rose to more than 5% when 15% CR was used in CR15-A0.5 mix, confirming that too much rubber the reduces load-bearing capacity of the column.
In summary, CRC modified with admixtures shows promising potential for sustainable construction applications, especially when CR content is kept within an optimal range (around 5-10%). Exceeding this range can negatively affect the strength and structural performance, warranting careful mix design considerations. Incorporating CR into concrete not only helps in managing waste tires sustainably but also offers a partial substitute for natural fine aggregates. This can contribute to more environmentally friendly construction practices when used within optimal limits.
