Introduction
Concrete structures serving in cold regions are often affected by freeze－thaw damage, which is the main reason for the loss of durability in cold region concrete. Additionally, cold region concrete is also subject to salt erosion due to factors such as soil salinity and winter salt application. The coupled action of freeze－thaw and salt attack causes more severe damage to the internal structure of concrete materials, greatly impacting their service life. The deterioration of concrete durability under the coupled action of freeze–thaw and salt attack is primarily manifested as degradation of macroscopic properties such as mass, dynamic elastic modulus, and strength, accompanied by changes in the microstructure. The deterioration process is a complex physical-chemical process involving multiple scales. Merely focusing on the evolution of macroscopic properties often hinders a deep understanding of the degradation mechanisms. Therefore, it is of great significance to clarify the evolution process of microscopic features of concrete under the coupled action of freeze–thaw and salt attack, quantitatively characterize the damage evolution laws from the perspective of pore structure, spalling depth, and compactness, in order to gain a profound understanding of the coupling mechanism and degradation mechanisms of both phenomena.
Methods
Concrete specimens were prepared using volume-stable P·O 42.5 ordinary Portland cement, river sand with a fineness modulus of 2.5, and coarse aggregates ranging from 5 mm to 10 mm. The dimensions of the concrete specimens were 100 mm× 100 mm × 100 mm. NaCl solutions with four different concentrations, namely 0%, 3.5%, 5.0%, and 8.0%, were used as the corrosive medium in the freeze–thaw process. A freeze–thaw cycling tests according to the fast freezing method specified in GB/T50082—2009.Concrete specimens were scanned using a LightSpeed 64-layer helical CT scanner with a resolution of 512×512 pixels and an X-ray tube voltage of 120 kV. To prevent water evaporation and loss of moisture during CT scanning, the specimens were wrapped in plastic film. After obtaining the CT data, three-dimensional reconstruction analysis was performed to investigate microstructural parameters such as delamination depth, volumetric loss rate, pore structure, and CT value.
Results and discussion
The depth and volume of concrete spalling under the coupling effect of freeze–thaw and salt erosion gradually increase with the increase of freeze–thaw cycles, and the damage is mainly manifested as surface spalling. The degree of spalling in specimens with different concentrations of chloride salt solution is significantly different, increasing first and then decreasing with the increase of chloride salt concentration, and the spalling depth is the largest when the chloride salt concentration is 3.5%. The volume loss rate of concrete gradually increases with the increase of freeze–thaw cycles, and first increases and then decreases with the increase of chloride salt concentration. Among the four concentrations used in this experiment, the volume loss rate is the highest at a chloride salt concentration of 3.5%, followed by 5%, 8%, and 0%. The ultrasonic wave amplitude of concrete gradually decreases with the increase of freeze–thaw cycles, and there is a good exponential relationship between the two. The mass loss rate of concrete gradually increases with the increase of freeze–thaw cycles, and first increases and then decreases with the increase of concentration.The coupling effect of freeze–thaw and salt erosion leads to dynamic changes in the pore structure of concrete, mainly occurring on the outer side of the specimens. With the increase of freeze–thaw cycles, the overall porosity shows a fluctuating increasing trend. With the aggravation of freeze–thaw damage, some closed pores gradually transform into open pores and then disappear with the peeling off of mortar. The damage evolution model based on the relative CT value definition has a good linear relationship with the damage characterization of amplitude and mass, and has a higher agreement with the mass loss rate. The damage degree of concrete gradually increases with the increase of freeze–thaw cycles, and first increases and then decreases with the increase of chloride salt concentration. A certain range of chloride salt concentration can accelerate the freeze–thaw damage of concrete, but when it exceeds this range, the development of damage is slowed down, and the concrete freeze–thaw damage degree reaches the maximum value at a chloride salt concentration of 3.5%.
Conclusions
Chloride salt has two effects on the freeze–thaw damage of concrete. On one hand, chloride salt increases the concentration difference between the inside and outside of the concrete pores during the freeze–thaw process, increases the liquid absorption rate of the specimen, and generates more pore ice during freezing, which accelerates the damage of concrete within a certain concentration range. On the other hand, the freezing point of chloride salt solution is lower than that of pure water. High concentration chloride salt solution has a slower freezing rate during the cooling process and a faster melting rate during the warming process. Compared with low concentration solution, the specimen has a shorter freezing time, which reduces the extent of concrete damage. Based on the analysis of indicators such as peeling depth, volume loss rate, amplitude, and mass loss rate in this study, it's found that a certain range of chloride salt concentration can accelerate the freeze–thaw damage of concrete, but the development rate of damage slows down when the range is exceeded. Among the four chloride salt concentrations used in this experiment, the concrete damage reaches the maximum value when the concentration is 3.5%.