Effect of Recycled Rubber Particle Size and Replacement Level on the Properties of Rubberized Alkali-Activated Concrete

Urban construction relies heavily on Portland cement, contributing to substantial CO 2 emissions and consumption of natural resources. Meanwhile, end-of-life tire waste continues to accumulate, with over 1 billion units discarded annually. This study develops rubberized alkali-activated concrete (RuAAC) as a circular construction material that simultaneously diverts recycled crumb rubber (CR) from urban waste streams and replaces Portland cement with industrial by-products, namely fly ash (FA) and slag. CR of three gradations, fine (0-0.8 mm), medium (1-3 mm), and wide-range (0-3 mm), was incorporated at 0%, 5%, 10%, and 20% by volume, replacing fine aggregates. Untreated CR was deliberately used to eliminate the environmental and economic burdens associated with chemical pretreatment, thereby enhancing circularity. The test program covered physical, mechanical, and durability properties. Direct shear strength, load-slip response, and shear toughness, which have not previously been reported for RuAAC at the material level, were characterized. CR increased slump by up to 73% and reduced 28-day compressive strength by 21-56%; the extent of the strength loss depended on gradation. At 20% replacement, fine-graded CR was the only gradation to increase ductility and resistivity while reducing water absorption and abrasion mass loss relative to the control. All medium-graded CR mixes disintegrated before 500 abrasion cycles, and the wide-range CR mixes had the lowest compressive and flexural strengths at every replacement level. Splitting tensile strength remained within a narrow range, with the fine-graded mixes matching or exceeding the control. Empirical relationships between compressive strength and the other mechanical properties were fitted to the ten mixes and are valid only within the tested range.