Compressive strength and impact performance of rubberised concrete as a sustainable construction material

Recycling waste rubber tyres presents a significant global challenge, with millions of new tyres being produced annually. To address this problem, it is crucial to find high-volume applications that effectively utilize recycled rubber. This research project focuses on modifying concrete properties by substituting rubber aggregates by volume, creating what is known as rubberised concrete. An extensive test schedule is found within, composed of 5 phases with each one focusing on developing the understanding of the best-performing mixes.

The study initially investigates the compressive strength of rubberised concrete and explores the effects of different rubber aggregate grading at 5%, 10%, 15%, 20%, 25%, 30%, 40%, 50%, 60% and 70% by volume. To enhance benefits and minimize drawbacks associated with rubber aggregate addition, plain rubber was treated either by coating it with cement paste or washing it with water or cleaning it with sodium hydroxide (NaOH). An initial control mix of C50MPa concrete was used with 7, 14 and 28 day compressive tests for 88 batches of 100mm cubes for phase 2. Following this select range were chosen for SEM analysis. Flexural testing and mid-scale reinforced beam testing A common strength retention across all aggregate ranges when 70% of natural aggregates are replaced is between 20%-30% only. This is increased between 40-60% on average when only 30% rubber is present.

Once the best-performing mixes are ascertained, a C60MPa mix will be used to enhance the strength further whilst durability aspects are analysed. Compressive testing at 1,3,7,14 and 28 days was evaluated, and 7,14 and 28 day splitting tensile tests. Schmidt Hammer, carbonation and water absorption testing was also conducted. Finally, a key focus of this research is the impact performance of rubberised concrete with the prospect of a high-volume application in highway crash barrier systems. This incorporates tests between C60MPa and C80MPa concrete. Deflection at its peak and post impact were obtained via highspeed telemetry, and cracking along the axes of the specimens were recorded.

The study revealed that the reduction in concrete strength was prevalent, depending on factors such as the grading of rubber aggregates, the type of surface treatment applied, and the percentage replacement of natural aggregates. However, when subjected to impact in the range of 1100 Joules, an increasing quantity of rubber is highly beneficial to the impact performance, energy absorption, displacement, and crack propagation of rubberised concrete. Depending on the type of mix, reduction in final displacement would often range between 30% to 95% relative to the control after a single impact. Average crack width in some instances reduced by more than 67% from the control value when 40% coarse rubber was used.