To understand the influence of fly ash (FA) content on the microstructure and strength of concrete under freeze–thaw cycles (FTCs), four groups of concrete with different FA contents (0–30%) were tested under FTC condition. Mass loss rate, relative dynamic modulus of elasticity (RDME), splitting tensile strength and other damage indicators were selected to describe the impact of macro properties. The micro physical changes, porosity and pore size distribution parameters were obtained through scanning electron microscope (SEM) and nuclear magnetic resonance (NMR) experiments. The influence mechanism of FA content on the frost resistance durability of concrete under FTC was discussed from the macro and micro perspectives. The results show that under the action of FTC, the addition of FA fills the pores, reduces the pore spacing, improves the strength of concrete, and makes the RDEM and splitting tensile strength of concrete increase first and then decrease. Among them, 20% FA concrete has the best frost resistance. The pore structure parameters show that the content of pores smaller than 100 nm has a great impact on the frost resistance durability of FA concrete, and increasing the content of these pores can improve the frost resistance durability of concrete.

Freeze–thaw cycles (FTCs) and steel bar corrosion (SBC) are the most common service conditions of hydraulic concrete and have significant impacts on its durability. Using pullout and microscopic tests of different FTC and SBC rates, we selected the mass loss rate, ultrasonic velocity, bond strength and bond slip in order to describe the changes in the macro-properties, and also selected the porosity and pore size distribution as micro-parameters in order to explore the influence of FTCs and SBC on the mechanical properties of hydraulic concrete. The results showed that the bond strength decreased as the FTCs increased due to the microstructure damage caused by FTC and SBC, which affects the mechanical properties. A corrosion rate of ≤3% offset the damage caused by 50 FTCs. FTCs and SBC resulted in superimposed damage effects on the concrete. In addition, we established a bond strength damage model based on the joint FTCs and SBC and quantitatively described the degradation law of the macro-mechanical properties. The analysis shows that the influence of FTCs on the bond strength was greater than that of the SBC. These research results can provide a reference and experimental support for the frost-resistant design and durability prediction of hydraulic concrete structures in cold environments.

The existing studies of the corrosion of reinforced concrete have mainly focused on the interface area and chemical ion erosion, ignoring the specific service environment of the reinforced concrete. In this study, the effect of freeze‐thaw damage was investigated via corrosion experiments under different freeze‐thaw cycle conditions. Steel reinforcement corrosion mass, ultimate pull‐out force, corrosion rate, and bond slippage were chosen as characteristic parameters in the experiments, and scanning electron microscopy (SEM) analysis was used to explain the mechanism of action of freeze‐thaw damage on corrosion. The results showed that, under identical corrosion conditions, the mass of steel reinforcement corrosion and corrosion rate increased by 39.6% and 39.7% when comparing 200 freeze‐thaw cycles to 0 cycles, respectively. The ultimate pull‐out force and bond slippage after 200 freeze‐thaw cycles decreased by 73% and 31%, respectively, compared with 0 freeze‐thaw cycles. In addition, SEM analysis indicated that microstructure damage caused by freeze‐thaw cycles accelerated the corrosion reaction and decreased cementitious properties, leading to decreasing ultimate pull‐out force and bond slippage. The effect of freeze‐thaw cycles and steel reinforcement corrosion on the macro mechanical properties of concrete is not a simple superposition.

Concrete structures in cold regions are affected by freeze–thaw cycles (FTCs) and carbonation, which lead to the premature failure of concrete structures. The carbonation depth, relative dynamic elastic modulus (RDEM), compressive strength, porosity, and pore size distribution of concrete under FTC conditions were tested through an accelerated carbonation experiment to study the carbonation performance evolution. The freeze–thaw effect mechanism on concrete carbonation was further analyzed via the obtained relationship between carbonation depth and pore structure. The results showed that the FTC, as a powerful source of concrete damage, accelerates the carbonation reaction. Carbonization products fill some microcracks caused by the freeze–thaw process, improve the compressive strength and dynamic elastic modulus, and alleviate the damage to concrete caused by the FTC. After carbonization under freeze–thaw damage conditions, the content of macropores with d > 1000 nm decreases, while the content of transition pores with d ≤ 10 nm increases, which is the direct reason for the decrease in porosity and the improvement in strength. Therefore, the carbonation durability of concrete under freeze–thaw conditions can be improved by controlling the content of macropores with d > 1000 nm and increasing the content of transition pores with a pore size of 10 nm ≤ d < 100 nm. In addition, the relationship between carbonation depth and pore structure under freeze–thaw conditions was established, and the research results can provide a reference for the study of the carbonation performance of concrete under freeze–thaw conditions.

Water and water transfer are the keys of the concrete durability problem; the non-uniform moisture transfer caused by the concrete micro–meso structure has a great effect on the drying shrinkage crack, transfers of inimical ions, etc. For the non-uniform moisture transfer problem, a multi-scale concrete moisture diffusion coefficient model which can consider the effect of Knudsen diffusion was established and verified based on the moisture transfer mechanism of porous medium and the concrete micro–meso structure characteristics. The effects of pore structure, the interfacial transition zone, and aggregate on the concrete moisture diffusion coefficient were studied based on the model, and the non-uniform moisture transfer characteristics and differences in concrete wetting and drying were analyzed via simulations. The results show that the moisture transfers more easily via the pores ranging from 10 nm to 100 nm, the effect of Knudsen diffusion increases with the increasing water-to-cement ratio and decreases with the increasing relative moisture, and Knudsen diffusion is also an effect factor which causes the moisture diffusion coefficient to increase with the increase in moisture. Moisture transfers more easily via the interfacial transition zone at the meso-level and causes a “flow around” phenomenon. The “S” growth relation between the moisture diffusion coefficient and relative moisture can consider the differences in the moisture diffusion coefficient under wetting and drying conditions to a certain extent, which makes concrete wet faster than dry. In addition, the jumping growth of the moisture diffusion coefficient in the relation also leads to an “inflection point” in the concrete moisture distribution.