Compressive behavior deterioration mechanism of concrete under carbonization and freeze-thaw cycles and time-varying structural reliability evaluation

Typical environmental factors in seasonal frozen areas inevitably lead to the deterioration of concrete materials, posing a threat to the service performance and life of concrete structures. However, the deterioration mechanism and law of concrete materials under typical environmental factors such as carbonation and freeze-thaw cycle are still unclear, and the concrete structural reliability is difficult to grasp. In this study, a combination of experiment and theoretical analysis is adopted to address the above challenges. Experimentally, the pore size distribution within concrete based on transverse relaxation time spectrum was analyzed for deterioration mechanism by low field nuclear magnetic resonance (LF NMR) technology, which has advantages of high sensitivity and fast data acquisition speed. The variation in the leaching solution pH values of concrete samples is a macroscopic reflection of de-alkalinization mechanism. Additionally, the deterioration law of concrete based on uniaxial compressive strength (UCS) was analyzed comparatively under different conditions of carbonization and freeze- thaw cycles. In terms of theoretical analysis, a time-varying model of UCS was established with deviations within 20 % by comparison with literature, which has important application potential in concrete strength prediction. Combined with the previous time-varying model of steel corrosion and load uncertainty, the time-varying reliability method of bridge structural ultimate bearing capacity in seasonal frozen areas was proposed. The results showed that after 28-d carbonation, the capillary pores and macropores within concrete decreased, UCS increased by 14.81 %. However, the freeze-thaw cycle has the opposite effect. After 150 freeze-thaw cycles, micropores and mesopores were transformed to capillary pores and macropores, UCS decreased by 28.57 %. Carbonization and freeze-thaw cycles will lead to de-alkalinization of concrete. Especially, carbonation enhances the early freeze-thaw resistance. However, with the accumulation of freeze-thaw damage, the positive effect of carbonation on the freeze-thaw resistance was gradually replaced by the negative effect of freeze-thaw damage. Finally, taking a three-span continuous concrete box girder bridge as an example, the influence of negative bending moments attenuates the load-induced bending moments, leading to the precedence of alterations in the reliability and failure probability of the side spans over the middle span. In conclusion, this study aims to better understand the deterioration of concrete under carbonization and freeze-thaw cycles. Research results could be used to establish a maintenance decision model and provide an effective reliability guidance for structural maintenance.