Effect of Particle Sizes and Dosages of Rubber Waste on the Mechanical Properties of Rubberized Concrete Composite

Abstract

The utilization of waste rubber in concrete composites has gained more attention nowadays owing to its enhanced engineering properties and eco-friendly viability. This study explored the effect of waste rubber sizes and its contents on the mechanical properties of developed concrete composites. Rubber waste with various particle sizes (R1, R5 and R10) was replaced with 10%, 20% and 30% of aggregates by volume, and the workability, compressive, splitting tensile and flexural strengths and impact resistance of the developed composite were investigated. An increase in the waste rubber contents decreased the slump of the composite due to the rougher surface of the rubber particles. The reduction in the slump was more pronounced for mixtures with smaller rubber sizes. Similarly, an increase in rubber contents decreased the compressive strength, tensile strength and flexural strength because of the lower stiffness of the used rubber waste and the poor bond between the rubber particles and the matrix. For instance, an approximately 27% decrease in compressive strength was observed for the mixture incorporating 20% of R1 rubber compared to that of the control mixture without rubber. It was observed that the incorporation of rubber waste in the concrete composite led to an enhanced resilience toward impact loading due to the improved energy dissipation mechanism offered by the rubberized concrete composite. For example, 13 blows in the case of 30% of the rubber replacement were required for the final crack as compared to 5 blows for the control mixture without rubber. It can be concluded that the choice of the optimal replacement ratio and the size of the rubber yield the developed rubberized concrete composite with a desirable strength and impact resistance.

1. Introduction

An important concern for the building sector is to modify the construction materials for achieving the desired properties, sustainability and economy. For cost-effective construction and a reduction of the dumping problems, it is always preferable to use the waste material such as tire rubber, fly ash and glass fibers, among others. Concrete is the largest user of natural resources and the most widely used construction material. It is bonded by cement, water and aggregate [1,2,3]. The use of traditional materials in concrete leads to running through the resources unless there is an acceptable replacement. Rubber can be an appropriate replacement with fine aggregates, as it contains some properties that can make it useful in concrete structures [4].

With the increasing population of the world, the usage of vehicles also increased. As a result, the annual rubber production attained its utmost stage. So, the easy and affordable solution for the disposal of waste tire rubber is by burning or dumping it into landfills [5]. However, burning causes smoke pollution, toxic emission and global warming, which are dangerous for health and other ecosystems. Other problems such as the contamination of rubber and environmental hazards occur if the waste rubber tires are being dumped into sanitary landfills. The disposal of waste tire rubber has been banned in many countries because of their bulkiness, their tendency to flow on the surface and other hazardous effects. That is why the recycling of rubber waste has been promoted worldwide without considering the economic or ecological problems [6]. The most efficient method for the disposal of this non-decaying rubber material is its use in concrete-based additives.

Rubber is widely used in engineering structures because the rubber materials offer many advantages and it has a major effect on the strength parameters and durability performance of reinforced concrete structures. It can be used in the construction of rigid pavements, steel-concrete composite beams, airport pavements and earthquake-resistant structures, among others. The utilization of the waste rubber in concrete improves the energy absorption, crack resistance, strain capacity for micro cracking, fracture toughness and impact resistance. The brittle failure of concrete can also be improved with the addition of rubber. Rubberized concrete shows ductile behavior and a plastic failure mode. Crumb rubber provided a better resistance to blasts and sudden explosions. Unlike normal concrete, rubberized concrete has a good appearance, an acceptable workability and a lesser unit weight [7,8,9].

Various studies have been conducted in the past on the behavior of concrete incorporating rubber waste. For instance, Khatib and Bayomy [10] investigated the slump variation of rubberized concrete and found a reduction in the slump with increasing percentages of rubber content. It was also reported that the slump was zero when the rubber was more than 40%. The workability of mixtures with fine rubber particles was more than that of the mixture with coarser rubber particles [10]. Fedroff et al. [11] reported that air content was increased with the addition of rubber particles. Eldin and Senouci [12] worked on the crumb rubber concrete (CRC) mixtures and compared the results with plain concrete without rubber. It was reported that rubberized concrete has a lower compressive strength than conventional concrete. An approximately 85% decrease in compressive strength was recorded when the rubber was completely replaced with coarse aggregates. The loss of compressive strength was lesser with smaller-sized rubber aggregates compared to that of the coarse aggregate. Rubber has good energy absorbing capacity; therefore, rubberized concrete showed a ductile failure when subjected to compressive forces [12]. Olivares et al. [13] used plastomers and other materials along with 3.5% to 5% of rubber. They performed various tests and concluded that the compressive strength of rubber plastic was 23 MPa, while the compressive strength of their control mixture was 36 MPa [13]. Shu and Huang [14] concluded that all parameters of strength were reduced in rubber concrete as compared to plain concrete [14]. Al-Tayeb et el. [15] used different percentages of rubber aggregates—5%, 10% and 15% by volume replacement of fine aggregate. They tested 180 samples for compressive strength, and the results indicated that compressive strength was reduced by adding 30% of crumb rubber. The strength properties of CRC vary with the size of the rubber aggregate. The results showed that large size of rubber particles in concrete mixtures attained a lower strength as compared to the smaller rubber particles [15,16,17].

Eldin and Senouci [18] investigated different concrete mixtures incorporating rubber for determining the tensile strength in comparison with conventional concrete. The results indicated that approximately 50% of the split tensile strength was reduced when course and fine aggregates were completely replaced with rubber particles. However, the rubberized concrete mixtures showed ductile behavior, and a large amount of energy was absorbed under tensile loads [18]. Farhan et al. [19] reported that rubber particles stabilize the cement matrix. The tensile strength was reduced because of the low tensile strength of the rubber particles. It was also reported that the stiffness increased due to the partial replacement of rubber particles [19]. Eldin and Senouci [12] replaced the coarse aggregate in a large volume with rubber particles (75% and 100% replacement). They reported that after 7 and 28 days of curing, there was the least change in the tensile and compressive strength [12]. Ismail and Hassan [20] worked on the self-consolidating rubberized concrete and evaluated the ductility and internal microcracking of tested mixtures [20]. Rubber particles bridge the crack propagation in rubber concrete; thus, flexural strength should be higher in CRC members. However, the results showed the opposite behavior, as investigated by Bing and Liu [21]. They investigated the flexural strength for two mixtures with different w/c ratios of 0.40 and 0.60. The results indicated that the flexural strength was reduced in both of the tested mixtures. For example, 25%, 50%, 75% and 100% replacement of the course aggregate with 0.40 w/c showed 18%, 44%, 52% and 63% reductions in flexural strength, respectively. One main reason for the lower flexural strengths of CRC is the weaker bond of the cement matrix and rubber particles [21].

Raghvan et al. [22] worked on two types of rubber (rubber aggregates comprised of two sizes (5 mm and 10 mm), and granular rubber particles of about 2 mm in diameter) in concrete mixtures. The results indicated that rubber shreds withstand higher loads even after the ultimate load. Rubber shreds bridge the cracks under the failure load in such a manner that the specimen does not separate into pieces. The results also showed that the post-cracking strength increased for shredded rubber particles rather than for granular rubber particles. Hong et al. [23] investigated the failure pattern and cracking behavior of rubberized concrete. Eldin and Senouci [18] investigated the failure pattern of concrete with and without the addition of rubber and found the ductile behavior of rubberized concrete [18]. Topcu [24] suggested that rubberized concrete has a good energy absorbing property and can control the vibration damping. The results indicated that, with higher rubber contents, the impact resistance of rubberized concrete increases. Ataria and Wang [25] studied the mechanical and durability performance of crumb rubber concrete. It was observed that the mechanical properties of rubberized concrete reduced, but the durability performance improved [25]. Similarly, Abbas et al. [26] studied the effect of rubber contents on the alkali-silica reactivity of concrete incorporating rubber particles and found promising results [26]. Choi et al. [27] conducted an experimental study using destructive and non-destructive tests on the concrete specimens incorporating fine rubber particles. Swaminathan et al. [28] and Ashraf et al. [29] studied the impact behavior of concrete incorporating rubber particles. Similarly, various literature reviews have been conducted to evaluate the mechanics, durability, dynamic behavior and applications of rubberized concrete [30,31,32,33].

Based on the literature review, it can be argued that various studies have been published on the mechanical performance of rubberized concrete with partial and complete percentage replacements of rubber with aggregates. However, very limited studies are available in the open literature on different sizes of rubber particles at various dosages. Therefore, this experimental investigation has been conducted to examine the mechanical performance of rubberized concrete made with different percentage replacements of fine aggregates for different rubber sizes (R1, R5 and R10). Moreover, the performance of rubberized concrete under impact loading was also investigated. The performance of concrete mixtures with rubber particles was assessed in light of the crack pattern and failure modes. This study will pave a path for construction stakeholders in utilizing the waste rubber materials in concrete mixtures with desired properties, leading to the elimination of the environmental overburden and other related adverse issues.

2. Materials and Mixture Proportions

Ordinary Portland cement (OPC) was used.

Table 1. Chemical properties of the used cement.

Compounds	LOI	MgO	SO3	Fe2O3	Al2O3	SiO2	CaO
Percentage	1.31	1.30	2.12	3.98	5.08	21.75	64.45

shows the chemical properties of the used cement, as determined through X-ray fluorescence (XRF). Locally available crush with various size fractions was used for the preparation of the concrete mixtures. The maximum size of the coarse aggregates was 20 mm. Figure 1 shows the sieve analysis results of the used fine and coarse aggregates and the ASTM C33 [34] minimum and maximum percentage passing.

Crumb rubber (Figure 2) of various sizes has been used in this study and replaced with aggregates. Three types of rubber sizes were used: R1 represented the size of crumb rubber less than 1 mm, R5 showed the size of rubber particles in the range of 3 to 5 mm and R10 indicated the rubber particle size in between 7 and 10 mm.

Table 2. Chemical composition of the used rubber.

Compounds	Values (%)
Metal	0.07
Fiber	0.55
Water	0.68
Ash	0.87
Isoprene	11.34
Acceton extract	13.80
Carbon black	26.20
Rubber hydrocarbon	45.21
Others	1.18

shows the chemical composition of the used rubber. A similar chemical composition of rubber has also been reported in a previous study [35]. The specific gravity of the used crumb rubber was 0.90, and the density was 590 kg/m³. Figure 3 shows the sieve analysis results of the used rubber. The third-generation superplasticizer was also used.

Table 3. Technical details of the used superplasticizer.

Physical Form	Appearance	Ph Value	Relative Density	Viscosity
Viscous liquid	Char light brown	6.7	1.2 at 21 °C	130 cps at 20 °C

shows the technical data of the used superplasticizer. Ordinary clean tap water which was free from organic matter was used for mixing and curing purposes.

Table 4. Concrete mixture design (mass/cement mass).

Mixtures	Cement	Fine Aggregates	Coarse Aggregates	Water	Superplastizer
Control	1.00	1.51	2.56	0.35	0.01

Rubber contents were replaced with fine aggregates. (Mass/cement mass) was 0.15, 0.30 and 0.45 for 10%, 20% and 30% of rubber, respectively.

shows the used concrete mixture design. The mixture design was optimized to achieve a compressive strength of 50 MPa to replicate the strength requirement used in constructing major infrastructure in the local construction industry. The rubber contents were replaced with 10%, 20% and 30% of fine aggregates by mixture volume. The superplasticizer was kept constant for all the tested mixtures.

3. Specimen Preparation and Test Methodologies

After measuring the exact quantities of materials, the concrete mixture was prepared in a rotatory drum concrete mixer with high-speed vertical shaft rotating. Firstly, the mixer was cleaned and sprayed with water to moisten the inner surface of the mixer pan. Initially, the course and fine aggregates were added and mixed for one minute. Afterwards, the crumb rubber and cement were added, and mixing continued for another two minutes. The superplasticizer mixed with water was added stepwise while mixing. The complete mixing was concluded within seven to ten minutes until a homogenous mixture was achieved.

Before pouring the fresh concrete mixture into the specimen molds, a slump test was performed to examine the mixture’s workability. Figure 4 shows the specimen preparation. The specimen molds were cleaned and lubricated before pouring the concrete mixture into them. The concrete was filled into the molds in three layers on a vibratory table. After 24 h, the specimens were removed from their respective molds, and water curing was performed by immersing them in a container filled with water. Compressive, splitting tensile and flexural strength were determined at 14, 28 and 56 days. A drop weight impact test was performed at 28 days. Three specimens were tested for each mixture design at various ages.

A compression test was performed in accordance with ASTM C39 [36] on concrete cylinders that were 300 mm × 150 mm in size. The loading rate was kept at 0.14 MPa/s during the test, and the axial compressive load was applied until failure occurred. This loading rate was chosen after several trails in order to examine the complete failure behavior and cracking pattern of rubberized concrete. Before the test performance, the date of casting and testing and the diameter of the specimen were noted. The cylindrical specimen was then placed in the machine to make sure that the platforms of the machine touched the cylinders before applying the load. A rubber pad was placed at the top and bottom of the specimens for the uniform application of the loading. For calculating the compressive strength, the maximum load achieved was divided by the cross-sectional area of the specimen. The splitting tensile strength was also determined on cylinder specimens following the ASTM C496 [37]. Steel strips were placed at the top and bottom of the cylindrical specimen (Figure 5b). Small beams that were 450 mm × 150 mm × 150 mm in size were casted for determining the flexural strength in accordance with ASTM C78 [38]. The beam was marked at span/3 from both sides and placed at the loading position. The head of the testing machine should be in contact with the specimen. After the test performance, the cross section of the specimen at each end and the center was measured, and the ultimate load values were recorded. The modulus of rupture (flexural strength) was calculated using the ultimate applied load, span length, average width and depth of the specimen at fracture. The loading rate was 0.017 MPa/s and was applied through a universal testing machine for the testing of both splitting tensile and flexural strength. Figure 5 shows the performance of various conducted tests.

To investigate the impact resistance of rubberized concrete, a drop weigh impact test was performed following the ACI 544 guidelines [39]. The impact test was performed on specimens that were 150 mm × 63 mm in size. Figure 6 shows the impact testing performance. In the impact load test, the impact energy is transferred repeatedly by falling weight. The number of cracks was noted by examining the first and last failure cracks. The impact test load apparatus (Figure 6) consisted of a 4.5 kg ball which was dropped at the specimen from a height of 0.25 m with a hammer weight of 15 kg. The specimen was placed at the baseplate between the lugs. The total number of blows was calculated corresponding to the first visible crack and last visible cracks. The number of blows for the first crack was designated as N₁, which is the initial crack resistance factor. Similarly, the number of blows for the last crack was N₂, which is the ultimate crack resistance factor. The ultimate crack resistance factor is calculated when the specimen starts to toughen the sides of the positioning lugs. After visualizing the initial and final cracks, the total absorbed energy was calculated as follows:

$$Total energy absorbed=\frac{N_2-N_1}{N_1}\times 100$$

(1)

4. Results and Discussion

4.1. Slump Flow

Figure 7 shows the variation of the slump flow of fresh concrete incorporating various rubber sizes (R1, R5 and R10) and dosages (10%, 20% and 30%). It was observed that the workability reduced with increasing dosages of rubber. For instance, the mixture incorporating R1 showed approximately 36%, 40% and 44% reductions in the slump for 10%, 20% and 30% dosages, respectively, compared to that of the similar mixture without the addition of rubber. This decrease in the slump with the higher dosage of rubber was mainly attributed to the rougher surface of the rubber particles, which might increase the friction between the fresh concrete ingredients [40].

On the other hand, the slump loss becomes significant by increasing the fineness of the rubber aggregates. It was observed that the larger rubber particles (R5 and R10) did not significantly influence the workability compared to that of the very fine rubber particles (R1). The results showed that, for the mixture incorporating 10% of rubber, an approximately 53% increase in the slump was observed for the mixture incorporating R5 compared to that of the identical mixture with the R1 rubber size. However, the slump was reduced by approximately 5% for the mixture incorporating R10 as compared to the mixture incorporating R5. The higher slump reduction for the mixture incorporating the R1 rubber size might be due the higher water absorption of the finer rubber particles compared to that of the larger rubber particles [41].

Table 5. Results of the concrete mixture incorporating R1 rubber.

Properties	Rubber Contents
	0%	10%	20%	30%
Slump (mm)	136	88	81	76
Compressive strength (MPa)	55.1	40.5	33.7	22.4
Splitting tensile strength (MPa)	9.6	7.5	6.9	6.3
Flexural strength (MPa)	7.4	5.9	4.4	3.3

shows the results of various properties for the concrete mixture incorporating R1 rubber at various dosages at 28 days.

4.2. Compressive Strength

Figure 8 depicts the results of the compressive strength of all the tested mixtures incorporating various rubber sizes (R1, R5 and R10) and their dosages (10%, 20% and 30%) at 28 days. Figure 9 shows the compressive strength results at 14, 28 and 56 days for various tested mixtures.

All the results reported in Figure 8 and Figure 9 showed a coefficient of variation (CV) of less than 1.2%. The results showed that the compressive strength increased for the mixture incorporating the R5 rubber size compared to that of the R1 for all the tested dosages of rubber (10%, 20% and 30%). For instance, for the mixture incorporating 10% of rubber, an approximately 7% increase in compressive strength was observed for the mixture incorporating R5 rubber compared to that of the similar mixture with the R1 rubber size. Furthermore, a relatively higher increase in compressive strength (i.e., approximately 11%) was observed in between the R5 and R1 rubber sizes for the 30% replacement level. This increase in compressive strength from the R1 to R5 rubber size mixtures was mainly due to the similarity of the gradation curve of the R5 rubber size with the sand particles. Moreover, due to the excessive grinding of rubber to obtain the desired size, (R1) resulted in irregular and flakier shapes of rubber, which might generate flexibility, spring a like effect in the mixture and allow the concrete to deform more under an applied load. The low stiffness and large pore volume attributed to the small rubber size (R1) also decreased the mechanical strengths of the rubberized concrete [42].

On the other hand, compressive strength reduction was observed for the mixture incorporating the R10 rubber size compared to that of the R1 and R5 rubber sizes. For example, for the mixture incorporating 20% of rubber, a decrease in compressive strength of approximately 13% was observed for the R10 rubber size compared to that of the R1 rubber size. A similar decreasing trend was observed between the R5 and R10 rubber sizes. This reduction in compressive strength for R10 compared to the R1 rubber size was mainly attributed to the fact that the aggregates are considered as the strongest load-carrying material in concrete. Rubber is a softer material and is not as strong as aggregates; consequently, it reduces the strength. Moreover, the bonding between the rubber and the other concrete ingredients is weak; therefore, rubberized concrete faces more of a stress concentration at rubber aggregates, causing a reduction in the strength properties [43]. It was observed that the 28-day compressive strength decreased with the higher dosages of rubber (Figure 8). For example, the mixture incorporating the R1 rubber showed approximately 27% and 39% decreases in the 28-day compressive strength for the 10% and 20% dosages, respectively, compared to that of the similar mixtures without the addition of rubber. Similarly, a higher reduction in compressive strength was observed for the mixture incorporating 30% of rubber. The weaker bond between the crumb rubber and the cement promotes the strength reduction properties. The distortion of rubber aggregates compared to the neighboring cement paste initiated the crack immediately. Furthermore, a decreased density of rubber particles also contributed towards the decreased strength [40]. The compressive strength reduction in the mixture incorporating the R1 and R10 rubber sizes was comparable (the difference in reduction was less than 2%) for all the tested dosages. The minimum reduction in compressive strength due to the addition of rubber was observed for the mixture incorporating R5 rubber compared to the R1 and R10 rubber sizes.

For all the tested mixtures with various rubber sizes and their dosages, the compressive strength increased at later stages (Figure 9). For instance, the mixture incorporating 10% of the R5 rubber size showed approximately 16% and 23% higher compressive strengths at 28 and 56 days, respectively, compared to that of 14 days. The continuous hydration increased the strength of concrete with time [44]. The concrete specimens without rubber particles showed brittle failure. However, the concrete specimens incorporating rubber exhibited somewhat ductile behavior owing to the high deformability capacity of the used waste rubber. Diagonal failure patterns with small concrete fragments de-attaching from the surface were observed for the control specimen without rubber. On the other hand, transverse cracks along the longitudinal direction were observed in the concrete specimens incorporating rubber particles. Furthermore, no concrete chipping off near the failure was observed for the rubberized concrete, indicating the improved energy dissipation capacity. It should be noted that rubber particles are softer materials than the sand particles, leading to them exhibiting a flexible behavior under compression. Therefore, the concrete specimens with rubber particles exhibited a progressive and gradual failure [33].

4.3. Splitting Tensile Strength

Figure 10 shows the splitting tensile strength results of concrete mixtures incorporating various rubber sizes (R1, R5 and R10) and their dosages (10%, 20% and 30%) at 14, 28 and 56 days. All the results showed a CV of less than 1.3%. It was observed that the splitting tensile strength increased with testing stages with various dosages of rubber and their sizes. For example, approximately 13% and 27% increases in tensile strength were observed for the mixture incorporating 10% of the R5 rubber at 28 and 56 days, respectively, compared to 14 days due to the continuous hydration process [44].

For the mixture incorporating 10% of rubber, an approximately 13% increase in tensile strength was observed for the mixture with R5 rubber compared to that of the similar mixture with the R1 rubber size. This increase in tensile strength from the R1 rubber size to the R5 rubber size was mainly due to the lower stiffness and the larger pore volume of the smaller rubber particles (R1), leading to a decrease in the mechanical strength of the mixture incorporating the R1 rubber aggregates [35].

On the other hand, a reduction in tensile strength was observed for the mixture incorporating the R10 rubber size compared to that of the R1 and R5 rubber sizes. For example, for the mixture incorporating 20% of rubber, an approximately 10% decrease in tensile strength was observed for the R10 rubber size compared to that of the R1 rubber size. A similar decreasing trend was observed between R5 and R10. This reduction in tensile strength for the R10 rubber size compared to the R1 rubber size was mainly due to the softer nature of the rubber particles. Larger rubber particles produce more tensile stresses in the cement paste vicinity. These tensile stresses produce premature cracking in the concrete, leading to a decrease in the loading carrying capacity [40].

Figure 11 shows the effect of rubber dosages on the 28-day tensile strength of the concrete mixtures. It was observed that the tensile strength at 28 days decreased with the higher dosages of rubber (Figure 11). For example, the mixture incorporating the R5 rubber showed approximately 11% and 23% decreases in the 28-day splitting tensile strength for 10% and 20% dosages, respectively, compared to that of the similar mixtures without rubber. Similarly, a higher reduction in tensile strength was observed for the mixture incorporating 30% of rubber.

4.4. Flexural Strength

The effects of using various rubber sizes (R1, R5 and R10) and dosages (10%, 20% and 30%) on the flexural strength at 14, 28 and 56 days are illustrated in Figure 12. All of the results showed a CV of less than 1.2%. For all the tested mixtures with various rubber sizes and their dosages, an increased flexural strength was observed at later stages. For instance, the mixture incorporating 20% of R5 rubber showed approximately 19% and 33% higher flexural strengths at 28 and 56 days, respectively, compared to that of 14 days. This increase in flexural strength at later stages is mainly attributed to the continuous hydration process [44].

Figure 13 shows the results of the 28-day flexural strength for mixtures incorporating various dosages of rubber and their sizes. It was observed that the flexural strength increased for the mixture incorporating the R5 size of rubber compared to that of the R1 for all dosages of rubber (10%, 20% and 30%). For example, for the mixture incorporating 10% of rubber, an approximately 9% increase in flexural strength was observed for the mixture incorporating R5 rubber compared to that of the similar mixture with the R1 rubber size.

It was observed that the flexural strength was reduced for the mixture incorporating the R10 rubber size compared to that of the R1 and R5 sizes. For example, for the mixture incorporating 20% of rubber, approximately 8% and 14% decreases in flexural strength were observed for the R10 rubber size compared to that of the R1 and R5 rubber sizes, respectively. This decreasing trend is similar to the compressive and splitting tensile strengths and is mainly related to the weaker bond between concrete ingredients and rubber aggregates [45].

4.5. Impact Resistance

Figure 14 shows the number of blows for the initial and final impact resistance of the concrete mixtures incorporating various rubber sizes (R1, R5 and R10) and their dosages (10%, 20% and 30%) at 28 days under a repeated drop weight test. Each result reported showed a CV of less than 0.25%. For all the tested mixtures, the impact resistance increased with larger size and the higher dosage of rubber. For example, the difference was 13 blows in the case of the 30% replacement as compared to 5 blows for the control mixture without rubber. The mixture incorporating R5 rubber exhibited 33% and 66% increases in the first crack impact strength and 31% and 81% increases in the ultimate impact strength for 20% and 30% rubber, respectively. Similarly, a higher impact strength was also reported by Zheng et al. [17] for a mixture with rubber. The increase in impact strength with a higher replacement of rubber was mainly attributed to the flexibility and energy absorption capacity of rubber particles. On the other hand, the joining capacity of rubber with concrete ingredients was also affected by a higher rubber replacement; therefore, an optimized dosage of rubber is recommended for desired properties. The change in impact resistance was found to be more in the case of the R10 size as compared to R1 and R5. For instance, there were almost 44% and 94% increases in impact resistance for the mixture incorporating R10 rubber as compared to R1 and R5, respectively (Figure 15). This increase in impact strength was mainly attributed to the enhanced ductility and crack arresting property of larger-sized rubber particles [46]. Figure 16, Figure 17, Figure 18 and Figure 19 show the initial and final crack patterns for mixtures incorporating various sizes of rubber and their dosages. The failure mechanism was cracking, crushing and compaction in the direction of impact loading.

5. Conclusions

This study examined the effect of waste rubber on the mechanical performance of concrete mixtures. Various rubber sizes (R1, R5 and R10) and their dosages (10%, 20% and 30% by volume of fine aggregates) were investigated. Initially, the workability of the mixtures incorporating the rubber waste were measured by performing a slump test. Mechanical properties including compressive, splitting tensile and flexural strengths and impact resistance were inspected on hardened concrete specimens with waste rubber using a drop weight test.

It was observed that the control mixture without rubber exhibited a slump of 136 mm. The slump values were decreased due to the incorporation of waste rubber in the concrete mixtures. For example, the slump was reduced to 88 mm, 81 mm and 76 mm for 10%, 20% and 30% of R1 rubber, respectively. It was also observed that, for mixtures with a larger size of rubber (R5 and R10), a relatively lesser reduction in the slump was observed compared to that of the control mixture. For instance, for the mixture with 20% of R5, a slump value of 131 mm was recorded. A decrease in compressive strength was observed due to the incorporation of waste rubber for all the tested mixtures. The compressive strength for the control specimen without rubber was around 55 MPa, and the specimen with 10% of R1 rubber showed a compressive strength of approximately 40 MPa at 28 days. This reduction in compressive strength may attributed to the softer behavior of rubber particles. Rubber particles may act as void in the cement matrix because of the weaker bond strength or the lower adhesion between the cement paste and rubber particles. Therefore, cracks start rapidly near the rubber particles in the matrix, causing failure or rupture upon the application of the load. The specimens incorporating rubber did not exhibit abrupt failure without concrete de-attachment or chipping off, indicating an improved energy absorption or dissipation capacity of rubber particles compared to that of the control specimen without rubber. Furthermore, it was observed that the specimens incorporating R5 showed a relatively higher compressive strength compared to that of the identical specimen with R1 rubber. This increase in compressive strength from R1 to R5 rubber size mixtures may be due to the similarity of the gradation curve of the R5 rubber size with the sand particles. Moreover, the excessive grinding of rubber to obtain the smaller size for R1 rubber resulted in an irregular and flakier shape, which might generate flexibility and a spring-like effect in the mixture, leading to earlier deformation under the applied load. The flexural strength was also reduced for the specimens with rubber particles. For example, the flexural strengths at 28 days were 7.4 MPa and 4.7 MPa for the specimen without rubber particles and the specimens with 20% of R5 rubber particles, respectively. This decrease in flexural strength due to the incorporation of rubber particles was because of the weaker bond between the cement matrix and rubber particles. The impact resistance of the concrete incorporating rubber particles showed improved behavior compared to that of the control specimen without rubber. The drop weight impact test data showed that the number of blows increased for the initial and final cracks and the ultimate failure for the specimen with rubber particles compared to the control sample. Furthermore, it was evident that the impact resistance was more significant for specimens incorporating larger-sized rubber particles in comparison with identical specimens with smaller-sized rubber particles.

It should be noted that the mechanical properties of the concrete mixtures incorporating rubber particles were decreased for all the tested mixtures compared to those of the identical mixtures without rubber particles. However, the rubberized concrete showed an improved performance against impact loading, which warrants its widescale application where energy dissipation due to a sudden load is a concern. Moreover, a significant improvement in durability performance may be observed due to the incorporation of various sizes and dosages of waste rubber particles in concrete mixtures, which demands a dedicated study in future research.