## 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  
Figure 19: Cracking pattern for specimens with various dosages of R10 rubber: (**a**) initial crack for 10% R10, (**b**) initial crack for 20% R10, (**c**) initial crack for 30% R10, (**d**) final crack for 10% R10, (**e**) final crack for 20% R10, (**f**) final crack for 30% R10. 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.