Characterization And Mechanism Of Frost Damage Of Restrained Concrete Under Single-Side Salt Freezing And Thawing

Concrete structures in cold regions suffer from cyclic freezing and thawing（F-T）in winter,so improving the frost resistance of concrete has become an important task in concrete durability research.The study on frost damage of restrained concrete based on its actual service state has achieved breakthroughs.However,there are still obvious areas for improvement in the quantitative characterization method,frost damage evolution and mechanism.This thesis investigated the single-side F-T test of restrained concrete,explored the new methods for characterizing concrete surface damage and internal damage,discussed the effect of restraint on frost damage evolution,and the influence of aggregate types on the frost resistance of restrained concrete.The frost damage mechanism of restrained concrete was explained by modifying multiple classic theories.The main results are as follows:（1）Based on the principle of laser triangle ranging,a new Multi-point Micro Displacement Laser Ranging（MMD-LR）method with high accuracy,fast response,and anti-interference was designed and invented.By analyzing the new parameters of mean vertical displacement(₉)),surface roughness(₉)),skewness（）,and kurtosis（）,the surface damage evolution process of concrete was clarified.Test results show that the hardened cement paste is removed first and then peels off the fine aggregates,leaving frost pits on the concrete surface.The frost pit first develops depth and then expands the edge.The start time of the fine aggregate removal and the frost pit edge expansion of restrained concrete is about 4 F-T cycles earlier than that of unrestrained concrete,indicating that the surface damage of restrained concrete is more serious than unrestrained concrete.（2）Based on the principle of mathematical morphology,the existing digital image processing（DIP）technology was improved,which overcomes the limitation of only taking the shape factor as the criterion.A new method of distinguishing pores and microcracks using modified DIP was proposed for the independent analysis of pores and microcracks,and the ROI recognition accuracy was improved from 30μm to 10.6μm.The new parameters,including the most probable pore diameter((9),the mean pore diameter(（9?）,the standard deviation of pore diameter(_（9）,the mean length of microcrack(₈0)（69）)),and the mean width of microcrack((2₈0)（69）))were used to characterize the microstructure evolution in concrete under F-T cycles.The restraint reduces the internal damage in frost concrete,including pore expansion and microcrack initiation.Accordingly,the residual strain(₉)),the water absorption in volume((1₉)),and the relative dynamic elastic modulus loss of restrained concrete is reduced by 163%,28%,and 33%than unrestrained concrete,respectively.（3）The difference in frost damage between restrained shale ceramsite lightweight aggregate concrete（LAC）and the restrained limestone normal aggregate concrete（NC）with the same water-to-cement ratio and the volume ratio of paste-to-aggregate were investigated.The reason for reducing surface damage lies in the shrinkage strain during the cooling process of sintered shale is 6%greater than that of limestone,and the shrinkage strain of LAC is 13%greater than that of NC,which reduces the deformation mismatch between ice and concrete.The reason for reducing internal damage lies in the air content of the fresh LCA increases by 150%to that of the fresh NC due to plenty of open pores of shale ceramsite,which increases the proportion of bubble pores in hardened concrete.（4）The relationship between concrete deformation coefficient()during temperature dropping phase and temperature was obtained,and then the reason for serious surface damage of restrained concrete was revealed through experiments.The results show the restraint causes the deformation mismatch between the concrete and the ice layer to increase by 38%.Consequently,the tensile stress in the ice layer and the shearing stress on the concrete surface are increased by 37%and 80%,respectively.The relationship among relative humidity,pore size and water migration in concrete was established,and the weakened water migration in concrete by the restraint was elucidated.To overcome the difficulty of using the Crystallization Pressure theory that needs to quantify the size of concrete pores,a thick-walled cylindrical pore structure model was established to calculate the tensile strain of the pore wall(₍（8（6（86）））.The results show that the restraint reduces the₍(8（6（86））by 10%,and reduces the ice formation rate in the pores.The weakened water migration and the reduced₍(8（6（86））decrease the internal damage of restrained concrete.Furthermore,it was clarified that after several F-T cycles,the restraint further exacerbates surface damage of restrained concrete.Based on the results above,a model for frost damage evolution of restrained concrete under F-T cycles was established.