Introduction
De-icing salt has been a common way to melt snow accumulated on concrete structures. But this often leads to the deterioration of concrete structures. Therefore, frost resistance durability of concrete subjected to de-icing salt is crucial. Conventionally, concrete is allowed to freely deform during single-side freezing and thawing (SSFT) tests. However, a concrete specimen is only equivalent to a unit of the concrete component in actual, and its deformation subjected to SSFT is restrained by the surrounding concrete, leading to alterations in salt frost damage. The differences in the evolution of concrete’s frost damage between the laboratory and actual engineering are highlighted. The different internal damage of concrete subjected to freezing and thawing with pure water and de-icing salt is usually related to the water migration and phase transformation inside the concrete. However, the only difference between restrained concrete and unrestrained concrete is the restraint effect. The significant difference in internal damage of the two concretes subjected to SSFT cannot be explained by classical theory. Additionally, the influence of restraint effect on water migration in concrete also needs to be clarified. Thus, it is crucial to study the effect of restraint on the internal damage of concrete subjected to SSFT, which will provide guidance for the mix design of high frost resistant concrete in cold regions. In this paper, restraint ring and anchors were designed to confine the deformation of concrete to simulate the serving condition during SSFT tests. The relative dynamic elastic modulus and water absorption in volume of restrained and unrestrained concrete with a water cement ratio of 0.60 were tested after every 4 SSFT cycles. The temperature and humidity sensors were used to monitor the evolution of the relative humidity (UIRH) inside the concrete during the SSFT test. The influence and mechanism of restraint effect on the internal damage of concrete subjected to SSFT were clarified from the perspective of moisture migration.
Methods
P·I 42.5 cement complying GB 8076—2024, 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 with water to cement ratios of 0.60. The slump and air content of fresh concrete are 120 mm and 1.4%, respectively, tested according to GB/T 50080—2016. The 28 d cube compressive strength of the concrete specimen is 28.3 MPa, tested according to GB/T 50081—2019.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 designed, which is used as the mould to cast a restrained concrete specimen meanwhile used as the restraining device to limit concrete deformation in SSFT tests. The device consists of a restraint ring, 24 restraint anchors, a counter-force ring, and a flange. All parts of the device are made of 304 stainless steel.The curing and pre saturation of concrete specimens were carried out according to the salt freezing method in CIF test and GB/T 50082—2009. The CDF/CIF TESTER produced by Schleibinger Geräte was used to provide SSFT cycles. After every 4 SSFT cycles, the relative dynamic elastic modulus (rn) and water absorption in volume (Vn) of the concrete specimens were tested. The evolution of internal relative humidity (UIRH) of 28 SSFT cycles was monitored, and the pore size ratios of restrained and unrestrained concrete during the first cooling cycle were calculated.
Results and discussion
The results show that after every one freeze-thaw cycle, the UIRH of both restrained and unrestrained concrete increased, resulting in hysteresis of the UIRH-temperature curves, which reflects the process of water migration inside concrete subjected to SSFT cycles. The hysteresis of the UIRH-temperature curves of unrestrained concrete is more significant than that of restrained concrete. After 28 SSFT cycles, the UIRH increment, rn loss and Vn of restrained concrete are 30%, 32% and 28% less than those of unrestrained concrete, respectively.The calculated results based on the UIRH-temperature curves show that the radius of pores (rpr) in restrained and unrestrained concrete increases with SSFT cycles. After 28 SSFT cycles, the rpr of restrained concrete is 58% less than that of unrestrained concrete. A model for the internal water migration of concrete subjected to SSFT is established to analyze the influence of restraint on the water migration. The ratio of pore radius of the restrained and unrestrained concrete before freezing point during the cooling process in first SSFT cycle was calculated. The results indicate that under the influence of restraint, the water migration inside the concrete driven by SSFT is effectively weakened.
Conclusions
The main conclusions of this paper are summarized as following. The conventional SSFT test of unrestrained concrete overestimates the internal damage. The restraint effect weakens the water migration before the freezing point of concrete during the cooling process, which is the basis for reducing internal damage induced by SSFT cycles. Moreover, the restraint effect also reduces the pore size expansion of concrete subjected to SSFT cycles, which is manifested macroscopically as less loss of relative dynamic elastic modulus and less increase of water absorption in volume.