Enhancing flexural fatigue performance of recycled aggregate concrete under freeze–thaw cycles through aggregate and cement matrix modification: Effects, prediction models, and mechanisms

In this study, the enhancement of aggregate modification and cement matrix modification to the flexural fatigue performance of recycled aggregate concrete (RAC) subjected to freeze-thaw damage (F-T) was investigated. Experimental assessments were conducted at stress levels (i.e., the ratio of applied flexural stress to the material's flexural strength) of 0.6, 0.7, and 0.8 following 0, 70, and 140 freeze-thaw cycles. The underlying mechanisms were explored through microstructural observations, including quantitative analysis of ITZ porosity and hydration products using backscattered electron -imaging analysis techniques (BSE-IA). After 140 freeze-thaw cycles, the residual flexural fatigue life of the control concrete group (C) was reduced to 15 %, 21 %, and 32 % of its initial fatigue life under stress levels of 0.8, 0.7, and 0.6, respectively. In contrast, these values significantly increased to 44 %, 51 %, and 57 % for the modified group with aggregate treatment. Additionally, fatigue deterioration under freeze-thaw cycles can be efficiently mitigated by the proposed cement matrix modification. The Weibull distributions for fatigue life with different freeze-thaw cycles were established. Subsequently, fatigue life prediction models were developed using reliability theory, incorporating both stress levels and F-T cycles as parameters. The effects of aggregate modification and cement matrix modification on mechanical properties, water permeability and freeze-thaw resistance were also presented. ITZ strengthening resulted in a substantial reduction (33.25 %-50.68 %) in ITZ porosity between recycled aggregates and the new cement paste at varying distances from the aggregate surface, thereby confirming the enhancement of flexural fatigue performance under F-T cycles. Cement matrix modification significantly decreased the water permeability of the concrete by 51.1 %, improving its F-T resistance, which contributes to mitigating fatigue deterioration. The excellent flexural fatigue resilience and enhanced freeze-thaw resistance of modified RAC underscore the potential to advance sustainable transportation infrastructure by facilitating increased use of recycled aggregates in cold climate environments.