###### 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. r 2022 22 August 2022 22 August 2022 222  
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, [PERSON] and [PERSON] [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]. [PERSON] et al. [11] reported that air content was increased with the addition of rubber particles. [PERSON] and [PERSON] [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.