The aim of this study was to reveal the macroscopic and mesoscopic deterioration behaviors of concrete under the coupling effect of chlorine salt erosion and the freezing–thawing cycle. The rapid freezing–thawing test was carried out in a 5% chlorine salt environment. The macroscopic characteristics of concrete were analyzed by testing the mass, the relative dynamic modulus of elasticity, and the compressive strength of concrete under different freezing–thawing cycles. Using CT scanning technology and three-dimensional reconstruction technology, the pore structure, CT value, and surface deviation of concrete before and after freezing–thawing were analyzed. Based on the changes of solid volume, pore volume, and solid CT value of concrete, the calculation method of relative CT value was proposed, and the damage model was established with relative CT value as the damage variable. The results demonstrate that the mass loss rate decreases in the beginning and then increases in the process of chlorine salt erosion and freezing–thawing, and the smaller the concrete size, the greater the mass loss rate. The relative dynamic modulus of elasticity decreases gradually, slowly at the initial stage and then at a faster rate, and the compressive strength loss rate increases gradually. The pore quantity, porosity, and volume loss rate of concrete increase in a fluctuating manner, whereas the relative CT value decreases. The comprehensive analysis shows that the chlorine salt frost resistance of concrete is negatively related to the water-cement ratio when the freezing–thawing cycle is fixed. The damage model could better reflect the freezing–thawing damage degree of concrete with different water cement ratios, and the damage evolution process is well described by the Weibull function.

To investigate the deterioration laws of concrete durability under the coupling action of salt erosion and drying–wetting cycles, this study evaluated the macro–performance evolution of concrete under different environments (NaCl, Na₂SO₄, and MgSO₄). Several measuring methods including nuclear magnetic resonance, computerized tomography tests, and microscopic morphology analysis are employed to investigate the damage mechanism of concrete. Experimental results show that with the action of salt erosion and drying–wetting cycles, the mass loss rate of concrete decreased first and then increased, and the relative dynamic modulus of elasticity and the compressive strength increased first and then decreased. The porosity and the pore quantity fluctuation increased. During the whole erosion process, needle-like attack products were formed in the pores, the crystals gradually filled the pores, and generated expansion pressure on the pore wall. As a result, concrete damage gradually occurred. Consequently, the damage degree was greater. The degrees of concrete damage under drying–wetting cycles induced were arranged as MgSO₄>Na₂SO₄>NaCl.

The usage of carbon fiber–reinforced polymer (CFRP) to strengthen cracked steel structures can effectively improve its bear capacity, so it has been extensively used in recent years. The degradation of interfacial bonding is one of the most important factors affecting the durability of CFRP–steel structures under a freeze–thaw(F–T)/wet–dry (W–D) environment. In this study, epoxy resin adhesive (ERA) dog-bone specimens and CFRP–steel double-lap joints (bonded joints) were prepared. F–T/W–D cycles experiment and tensile tests of the ERA specimens and the bonded joints were also performed. Under F–T/W–D cycles, the main properties of the ERA specimens and the bonded joints were examined. Results indicated that fracture failure occurred in all ERA specimens. The hybrid failure modes of fiber peeling on the surface of CFRP plate and the bonded interface peeling between the CFRP plate and ERA layer primarily occurred in the bonded joints. The failure of both of them can be considered to be brittle, which was unaffected by the F–T/W–D cycles. With increased F–T/W–D cycles, the ultimate load and tensile strength of the ERA specimens initially increased and then decreased, whereas the elastic modulus initially increased and then remained unchanged. The ultimate load of the bonded joints decreased gradually. Based on the relationship between the interfacial bond-slip parameters and the number of F–T/W–D cycles, the bond–slip model of the bonded joints was established. The proposed relationship was validated by comparing with the experimental bond-slip relationships and the predicted relationships under the F–T/W–D cycles.

Freeze–thaw damage and salt erosion are important factors that influence the durability of concrete. In this study, degradation laws of concrete in salt freeze–thaw environment were discussed from the microscopic perspective based on...

In order to investigate the influence of basalt fiber (BF) on the mechanical properties of concrete and its inhibitory effect and mechanism on the degradation process under chloride salt attack and freeze-thaw coupling environment. A combined approach of macro-scale mechanical property tests and microstructural analysis was employed to systematically examine the effects of different BF volume fractions on the workability, mechanical properties, and durability of concrete. The results suggest that the optimum comprehensive performance is attained when the BF volume fraction is 0.2%. This is evidenced by an augmentation in compressive strength of 6.3% to 24.6% in comparison to the control group, a 10.8% to 24.8% enhancement in splitting strength, a maintenance of flexural strength that is equivalent to the control group, and a substantial enhancement in crack resistance, thereby the failure mode have a change from a brittle to a somewhat ductile nature. In the context of chloride salt attack and freeze-thaw cycle, BF can suppress microcrack propagation through a three-dimensional network structure, resulting in a 18.2% decrease in mass loss rate and a 22.4% increase in dynamic elastic modulus retention rate of 0.2% BF volume fraction specimens after 200 cycles compared to the control group. Microstructural analysis reveals that BF enhances durability through three mechanisms: optimizing pore structure, inhibiting chloride ion penetration, and reducing consumption of hydration products. Research indicates that the optimal dosage of BF forms a uniform fiber-matrix interface that balances mechanical reinforcement and pore filling effects, providing theoretical basis and technical support for the design of concrete structures in cold regions with chloride salt attack and freeze-thaw coupling environment.