Strength Development and Microstructural Observations of Concrete Incorporating Corn Straw Ash and KH570-Treated Crumb Rubber

Strength losses and poor compatibility between recycled crumb rubber (CR) and cement paste constrain the use of waste-tire polymers in concrete. This study investigated the combined use of corn straw ash (CSA) and KH570-treated crumb rubber (CR) through mechanical testing, microstructural characterization, and compressive strength prediction. Fifteen mixtures were examined, with compressive strength measured at 3, 7, and 28 d and flexural strength at 28 d. Among the investigated CSA replacement levels at 10% CR, 8% CSA produced the highest 28 d compressive and flexural strengths of 39.28 and 7.1 MPa, respectively. These values were 15.1% and 20.3% higher than those of the corresponding rubberized concrete without CSA, and 15.2% and 16.4% higher than those of the untreated-rubber mixture. The combined treatment therefore partially compensated for rubber-induced strength losses. X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy provided complementary evidence consistent with matrix refinement and improved rubber–paste interfacial compatibility. Backpropagation (BP), genetic-algorithm-optimized BP, and particle-swarm-optimized BP (PSO-BP) models were compared using 135 experimental records. PSO-BP yielded the lowest test-set root-mean-square error of 2.1713 MPa under the adopted data partition. These findings demonstrate the potential of combining agricultural ash incorporation with waste-rubber surface treatment to improve the strength performance of rubberized concrete and provide an experimental basis for composition selection.