The freezing and thawing (F–T) is one of the main threats for concrete in cold regions. The migration and phase transformation of moisture inside concrete induced by the F–T cycles is the origin of surface scaling and internal damage, which can induce the deformation of the matrix. The deterioration process of concrete can be further accelerated with the deicing salt. Previous studies usually followed the standardized methods. The deformation of concrete during the F–T cycles is free, which is different from the concrete in a service state. The concrete specimen is only equivalent to one unit in the concrete member. All concrete units undergo a deformation subjected to the F–T cycles, limiting the free deformation of adjacent concrete units. The limitation between adjacent concrete units is a form of restraint. Some restraint devices are developed to limit the deformation of concrete during F–T cycles. Conventional concrete frost resistance tests underestimate surface damage but overestimate internal damage due to the lack of consideration for restraint. However, the impact of restraint on the deterioration of concrete pore structure and the initiation/propagation of microcracks due to F–T cycles is unclear. Characterizing the internal damage of concrete is an important way to investigate the mechanism of concrete frost damage. The common method is to use digital image processing (DIP) to analyze the deterioration of concrete microstructure induced by F–T cycles, which has the advantages of convenient sample preparation and unrestricted sample size. In recent years, DIP methods have been also combined with morphological image processing to form a method of segmenting, extracting, and quantitatively analyzing for pores and microcracks that rely on "shape factors" to identify pores and microcracks. This paper was to focus on the restrained and unrestrained concrete as research objects. The pore structure and microcrack parameters were statistically analyzed after the single-side salt F–T test. The residual strain of concrete was calculated. The comparative analysis of the differences between two types of concrete in terms of pore structure deterioration and microcrack initiation was carried out to elucidate the influence of restraint on the deterioration of concrete pore structure and microcracks. The results could be expected to guide the mix design of frost resistant concrete in cold regions, ensuring the long service life of concrete materials.

P. I 42.5 cement, natural river sand with a fineness modulus of 2.68, and graded stone by uniformly mixing the crushed stone with 5–10 mm and 10–20 mm in a mass ratio of 4∶6 were used to prepare fresh concrete at a water/cement ratio of 0.60. The 28 d cube compressive strength of the concrete specimen was 28.3 MPa. Two groups of moulds were used to cast concrete specimens. For unrestrained specimens, cylindrical plastic moulds with a diameter of 100 mm and a height of 70 mm were used. For restraint specimen, a restraint device was used. The curing and pre saturation of concrete specimens were carried out according to the salt freezing method in the CIF test and the GB/T 50082—2009. The CDF/CIF TESTER produced by Schleibinger Geräte was used to provide single-side salt F–T cycles. After each 4 F–T cycles, the DIP sample was prepared. The processing of digital images included shooting, binarization, segmentation and extraction of region of interest (ROI), and skeleton. The obtained pore parameters included pore diameter distribution, most probable pore diameter (d_M), mean pore diameter (d), standard deviation of pore diameter (d_σ), and percentage of damaged pores (P_dam). The obtained microcrack parameters included the mean length (L_mean) and mean width (W_mean) of microcracks.

The d_M, d, d_σ and P_dam of both restrained and unrestrained concretes increase with the F–T cycles. After the 32 F–T cycles, the d_M, d, d_σ, and P_dam of the restrained concrete are 11%, 10%, 5%, and 7% smaller than those of the unrestrained concrete, respectively. The results indicate that the restraint limits the expansion of the pores due to the F–T cycles. During the cooling process, the restraint weakens the water migration inside concrete via limiting the shrinkage deformation of concrete, thereby reducing the pore diameter shrinkage induced by water migration, the saturation of larger pores and thereby reducing the crystallization pressure. The restraint reduces the pore expansion and microcrack initiation of restrained concrete. The of restrained and unrestrained concretes increases with the F–T cycles. After the 32 F–T cycles, the L_mean of restrained concrete is 7% less than that of unrestrained concrete. The W_mean of unrestrained concrete decreases, but the W_mean of restrained concrete increases with the increase of F–T cycles. The restraint changes the evolution of microcracks in concrete due to the F–T cycles, resulting in macroscopic differences in the frost damage of the two types of concretes. Furthermore, after the 32 F–T cycles, the S_n of restrained concrete is 30% less than that of unrestrained concrete. The grey correlation degree shows that the pore expansion and microcrack length induced by F–T cycles are the main factors leading to the increase of residual strain. A correlation between the expansion of pore diameter inside restrained concrete and the increase in residual strain is greater, which is related to the weakening of water migration under the restraint.

The restraint had a significant inhibitory effect on the pore diameter expansion and microcrack initiation of concrete subjected to single-side salt freezing and thawing. The restrained concrete was to release stress through the expansion of microcrack width, being a key mechanism for reducing internal damage induced by the single-side salt freezing and thawing cycles. The main reason for the increase in residual strain of concrete induced by the F–T cycles could be the expansion of pore diameter and the growth of microcrack length.