The Early-age Deformation And Frost Damage Of Concrete Under Sub-zero Temperature

In the process of the concrete construction, low temperature and poor curing conditions may cause frost. This research reveals the regularity of concrete early-age deformation from sub-zero temperature to room temperature, also investigates the frozen progress of fresh concrete when it suffers from sub-zero temperature time. And the paper analyses the mechanism of frost and provides theory and basic data about concrete frost damage in early age.First, an equipment is designed to measure the deformation of fresh concrete at varying temperature, which adopts the eddy current displacement sensor. Before conducting the measurement of deformation, the accuracy and temperature deformation coefficient are revised to get accurate data. The experiment results indicate that the equipment can measure concrete early-age deformation precisely when temperature ranges from-40°C to 50°C.Then, the equipment designed is used to measure the early deformation of concrete and mortar with different mixture ratio. In chapter 3, the paper discusses the influence of temperature, w/c, antifreeze, curing time on deformation of concrete and mortar as well as puts forward the model of early deformation when concrete or mortar is frozen. Meanwhile, mechanical properties(-7d,-7+7d,-7+28d) and dynamic elasticity modulus of concrete and mortar are measured.The results show that the curve of deformation and the characteristic value can reflect the process of fresh concrete(mortar) by the equipment designed, furthermore the curve can be divided into six stage, which includes(1) shrinkage at sub-zero temperature(2) frost heave(3) stable stage at sub-zero temperature(4) shrinkage deformation at room temperature(5) expansion(6) stable stage at room temperature. And the results indicate that frost occurs at the first two stages.The deformation characteristic values at the first two stages show a good correlation with compressive strength loss and dynamic elasticity modulus loss respectively. The mechanisms of frost are different when concrete is frozen at varying temperature, which means that concrete has larger frost deformation during quick freezing pattern while the compressive strength of specimens can still recover effectively after returning to the positive temperature. Anti-freeze admixtures decrease the value of deformation and the rate of frost heave obviously, at the same time they also increase-7d compressive strength, but anti-freeze admixtures cause a larger compressive strength loss compared with the control concrete when they are cured after 28 d at standard curing condition. According to results of the concrete without anti-freeze admixture, there is a better linear correlation between compressive strength loss and dynamic elasticity modulus loss, while this correlation would not show on the concrete with admixtures. Extending curing time in room temperature can dramatically decrease the frost heave and residual deformation, which is an effective way to prevent concrete frost.