Experimental Investigation On Axial Compression Behavior Of Reinforced Concrete Columns After Freeze-thaw Cycles

The freeze-thaw damage of concrete is a common and serious phenomenon,which will bring huge safety risks and economic losses to the project.At present,researchers both at home and abroad have conducted a large number of studies at material-level on freeze resistance of concrete,including adding additives and improving pore structure,and the basic mechanical properties of concrete after freeze-thaw damage have also been widely studied.However,there are relatively few studies on the mechanical properties of reinforced concrete members which are subjected to freeze-thaw cycles.Therefore,the thesis carried out the experimental investigation on axial compression behavior of reinforced concrete columns after freeze-thaw cycles.The specific work includes:(1)Three groups of reinforced concrete columns with reinforcement ratios of 1.40%,2.74%,and 4.52% were freeze-thawed 0 to 300 times,and the changes of test specimens surface morphology,freeze-resistant test specimens mass loss,dynamic elastic modulus,and cube compressive strength after freeze-thaw cycles were investigated.(2)The axial compression tests were carried out on reinforced concrete columns after freeze-thaw cycles.The cracks development process and the failure forms of reinforced concrete columns were observed,and the effects of different freeze-thaw cycles and different reinforcement ratios on the ultimate bearing capacity of the reinforced concrete columns were studied,the changes of the load bearing capacity of concrete and steel bars in reinforced concrete columns after freeze-thaw cycles were analyzed,the pros and cons of evaluating the frost resistance index of concrete and the compressive strength of concrete cubes on the bearing capacity of reinforced concrete columns under axial compression are analyzedThe main conclusions of this thesis are:(1)The freeze-thaw cycles caused significant damage to the surface of test specimens,the quality of the freeze-resistant test specimens increased slightly in the early stage of freeze-thaw cycles,and the quality decreased rapidly in the later stage,relative dynamic elastic modulus has been declining,the compressive strength of 150 mm cubes showed a downward trend with the increase of freeze-thaw cycles.(2)Freeze-thaw cycles accelerated the development of cracks in the axial compression test of reinforced concrete columns,resulting in increased cracks at the end of the test specimens,and the speed of cracks through the surface of the test specimens were accelerated,high reinforcement ratios can effectively reduce the damage degree of test specimens after axial compression.(3)Freeze-thaw cycles caused the ultimate bearing capacity of the three groups of reinforced concrete columns to show a drop of different magnitudes.The higher the reinforcement ratios,the greater the drop of the test specimens.In the same group of test specimens,the loads bearing of the steel bars under different freeze-thaw cycles remain basically unchanged,and the freeze-thaw damage of concrete is the main factor affecting the ultimate bearing capacity of the test specimens.According to the test results,the formulas for the ultimate bearing capacity of reinforced concrete columns with different reinforcement ratios after freeze-thaw cycles are given.(4)The analysis results of the test data show that the frost resistance index of concrete dynamic elastic modulus and mass loss rate cannot directly reflect the degradation of the axial compression bearing capacity of the reinforced concrete columns after freeze-thaw cycles,and the degradation law of the concrete cube compressive strength with the increase of the freeze-thaw cycles can reflect the degradation of the bearing capacity of reinforced concrete columns axial compression specimens to a certain extent.