Symmetry–Asymmetry Framework for Rubberized Concrete: Correlations Between Mixture Design and Rubber Properties and Concrete Flowability and Mechanical Characteristics, and Three-Stage Transition of Compressive Strength

This study systematically investigated the effects of mix design conditions (water–cement ratio) and rubber properties (particle size, surface area, and mixing ratio) on the flowability and mechanical characteristics of rubberized concrete, in which rubber particles were incorporated as part of the fine aggregate. The fresh properties (slump and air content) and hardened properties (compressive strength and Young’s modulus) were measured, and their correlations with rubber surface area and mixing ratio were analyzed. The results showed that slump and air content converged to constant values with increasing rubber surface area, exhibiting symmetric behavior. These characteristics were accurately approximated using logistic and exponential functions. In contrast, compressive strength did not decrease monotonically with increasing rubber content but could be divided into three distinct regions: a low-substitution region (Region I), an intermediate transition region (Region II), and a high-substitution region (Region III). Particularly in Region II, where the rate of strength reduction increased sharply, the logistic function was found to describe the asymmetric behavior more appropriately than the conventional exponential function. Furthermore, an estimation formula incorporating a correction term into the logistic function was proposed to account for the influence of the W/C ratio on compressive strength. This two-stage estimation model achieved higher predictive accuracy than conventional equations, eliminating the 0.88 bias observed in previous models. Finally, a practical design methodology based on this two-stage model was presented, demonstrating its applicability to concrete with various mixture ratios and water–cement ratios.