Study On Mechanical Properties And Damage Model Of Cracked Concrete Under Freeze-Thaw Conditions

With the continuous development of the national economy,our country’s construction industry continues to challenge complex fields.Among them,infrastructure construction in cold regions has received extensive attention.As concrete is one of the most widely used materials in the field of infrastructure construction,its importance to the safe use of engineering construction and economic development has also attracted more and more attention.In actual engineering,most of the concrete structures have prefabricated cracks or cracks during service.After the cracked concrete structure is subjected to freezing and thawing,its strength will be lower than that of the ordinary concrete structure,and different forms of cracks will have different effects on the mechanical properties of the concrete structure.Significant research results have been made in concrete damage and fracture at home and abroad,but most of the research ignores the impact of environmental factors on the durability of concrete,especially for cracked concrete structures under freeze-thaw cycles.In view of the above situation,in-depth study of the changes in the mechanical properties of cracked concrete structures under the action of freeze-thaw cycles and the failure modes of concrete structures under different crack distribution conditions has important theoretical value and practical significance.In this paper,aiming at the failure of concrete with prefabricated cracks under the action of freeze-thaw cycles,four groups of concrete specimens with different forms of cracks and a group of specimens without cracks are designed for comparison.The number of freeze-thaw cycles is 0,25,50,75,and 100,respectively,and the test pieces of each group pass the uniaxial compression test after reaching the predetermined number of freeze-thaw cycles.According to the test results,analyze the strength changes of concrete specimens with different forms of cracks after undergoing freeze-thaw cycles.At the same time,the effects of different prefabricated crack forms and different freeze-thaw cycles on the mechanical properties of concrete such as compressive strength and elastic modulus are obtained.On this basis,based on the WEIBULL distribution and the principle of equivalent strain,the damage model of cracked concrete under the action of freeze-thaw and load is established.The main conclusions of this paper are as follows:1.The damage degree of concrete specimens increases with the increase in the number of freeze-thaw cycles.After the freeze-thaw cycles,the surface damage and deterioration of concrete specimens with prefabricated cracks is more serious than that of concrete specimens without prefabricated cracks.When the number of freeze-thaw cycles reached 25 times,the test found that the quality of the concrete specimens increased slightly.After the number of freeze-thaw cycles reached 100 times,the mass loss rate of the concrete specimens in the non-crack group reached1.12%.The mass loss rate of concrete specimens with different forms of prefabricated cracks ranged from 1.52% to 1.65%.As the number of freeze-thaw cycles continues to increase,the ultrasonic velocity of concrete shows a downward trend.When the number of freeze-thaw cycles reaches 100 times,the highest value of the wave velocity loss rate of concrete specimens with precast cracks is 51.3%,and the ultrasonic velocity loss rate of concrete specimens without pre-crack is 47.4%.2.The compression failure of concrete materials is mainly caused by the local tensile stress concentration in the compression process,and the tensile cracks are caused.When the number of freeze-thaw cycles is 25,the compression failure of the specimen is mainly reflected in the lump drop at the end.With the increase of the number of freeze-thaw cycles,the surface concrete bulges and falls when the specimen is damaged.For the concrete specimens with prefabricated cracks,the failure modes can be divided into tensile failure modes and mixed failure modes according to the different forms of prefabricated cracks.3.After the number of freeze-thaw cycles reached the maximum 100 times of the experimental design,the strength loss rate of concrete specimens with prefabricated cracks was up to 63.3%,and the strength loss rate of concrete specimens without prefabricated cracks was 54.5%.Among them,the strength loss rate of the specimen with the pre-crack inclination angle of 60° is higher.4.The compressive strength and elastic modulus of concrete have an inverse relationship with the number of freeze-thaw cycles.When 50 to 75 freeze-thaw cycles are experienced,the decline rate of the two is the largest,indicating that the internal damage of the concrete caused by the freeze-thaw cycle is serious at this time.After the number of freeze-thaw cycles reached 75 times,the rate of decline was eased.At this time,the original cracks in the specimen had penetrated,and the rate of new cracks was slow.When the number of freeze-thaw cycles reached 100 times,the elastic modulus of the five groups of concrete specimens decreased to 0.682 times,0.603 times,0.633 times,0.651 times and 0.675 times of the elastic modulus before freezing and thawing,respectively.Among them,the two groups with a crack inclination angle of 60° have the largest decrease in elastic modulus.5.Based on the Weibull distribution function and the principle of equivalent strain,the damage model of concrete under the coupling action of freeze-thaw and load is established.By introducing the crack damage variables,the damage model of the cracked concrete under the coupling action of freeze-thaw and load is derived,and the reliability of the model is verified by comparison with the test results.