Discussion And Application Of The Relationship Between Fast Freeze-thaw And Natural Freeze-thaw

Hydraulic concrete will gradually deteriorate under the influence of freeze-thaw and temperature fatigue.Therefore,in the design of hydraulic concrete structures,the frost resistance of concrete is an important durability indicator.At present,the frost-resistance of concrete is mainly determined based on rapid freeze-thaw in the laboratory,slow freezing and thawing,or unilateral freezing and thawing.Because the deteriorating mechanism of the actual hydraulic concrete structure under natural freeze-thaw is very complicated,the temperature freeze-thaw temperature amplitude and circulation time in the room are different from natural freeze-thaw.Therefore,the relationship between the damage caused by rapid freeze-thaw and natural freezing and thawing of the hydraulic concrete is established,so that the indoor freeze-thaw principle can effectively guide the deterioration of the actual hydraulic concrete structure under natural freeze-thaw conditions.In order to judge the service life of hydraulic concrete structures,it is of great significance to propose an economic and reasonable maintenance program.To this end,this paper first designed and conducted freeze-thaw temperature freeze-thaw tests.Then analogy to the equivalent age theory,an equivalent damage age calculation formula is proposed,and a freeze-thaw damage evolution model of concrete considering the temperature history of freeze-thaw cycles is established.Finally,based on the proposed new model,the relationship between natural freeze-thaw and indoor rapid freeze-thaw was discussed.The specific research content and main conclusions are as follows:(1)Experimental study on rapid thawing and thawing cycles in concrete with different freezing and thawing temperatures.Firstly,three indoor freeze-thaw experiments with different freeze-thaw cycles(-18℃,-10℃,-5℃)were designed and carried out to study the effect of the lowest temperature of freeze-thaw cycles on the freeze-thaw damage of concrete.At the same time,the influence of air-entraining agent on the freeze-thaw damage of concrete was discussed.The experimental results show that with the increase of the number of cycles and the decrease of the circulating temperature,the appearance of concrete specimens deteriorates gradually,and the loss rate of compressive strength and the rate of mass loss increase gradually,indicating that the freeze-thaw damage of concrete increases gradually.Under the same conditions,the freeze-thaw damage was significantly reduced in the test piece with air-entraining agent.(2)Establishment of freeze-thaw damage evolution model for concrete considering temperature history of freeze-thaw cycle.Based on the experimental data and analysis results of the rapid freezing and thawing cycles of concrete with different freeze-thaw temperature amplitudes above,the equivalent ageing theory of equivalent concrete,the equivalent damage age calculation formula considering the temperature history of the freeze-thaw cycle is proposed for the first time.Based on the combination of exponential and fractional calculus theory,a concrete freeze-thaw damage evolution model based on equivalent damage age was established.At the same time,the freeze-thaw damage model of air-entrained concrete was explored.Finally,based on the freeze-thaw test data described above,an optimization algorithm was used to optimize the parameters of the new model.The fractional freeze-thaw damage model that adopts the global fitting of parameters has the highest accuracy.(3)The Establishment of freeze-thaw relationship and rapid freezing and thawing in Indoors based on large aqueduct.Firstly,the aqueduct concrete strength parameters are obtained by using on-site inspection technology.Then the mechanical parameters of the aqueduct concrete materials are obtained based on the optimized aqueduct reinforcement steel stress inversion,and then the current damage degree of the aqueduct is calculated,and then obtained from the new freeze-thaw damage evolution model established in this paper.The equivalent damage age corresponding to rapid freezing and thawing in the room;At the same time,combined with the environmental temperature data of the large-scale aqueduct over the years,the equivalent damage age corresponding to natural freeze-thaw cycles was calculated,and the relationship between rapid freezing and thawing and natural freezing and thawing was established.The study shows that the aqueduct body suffered 30.4% strength damage after undergoing many years of freeze-thaw cycles.By comparison and analysis,it can be obtained that the ratio of indoor and natural equivalent damage age is 1:6.34(reference temperature is 20℃,freeze-thaw temperature).