Study On Durability Of Hydraulic Concrete Under Dry-wet-salt Attack-freeze-thaw Coupling

In order to improve the fragile ecological environment in Northwest China,the state has successively built some large-scale water diversion,water-lifting irrigation areas and cross-basin water transfer projects in Northwest China,such as the project to divert water from Datong River to Qinwangchuan area,Jingtai Electric Power Lifting Irrigation project,project to divert water from Tao River,etc.However,in Northwest China,channels,tunnels and other water diversion structures have been exposed to harsh environments such as alternating dry and wet,salt attack,freeze-thaw cycles,etc.This environment has resulted in frequent damage,which has greatly threatened the safe operation of the project.In addition,under the complex effect,the damage mechanism of hydraulic concrete is still unclear,and the law of durability degradation is still unclear.There are fewer evaluation methods for the durability of hydraulic concrete,and the damage prediction has large errors.Therefore,in this paper,the project to divert water from Datong River to Qinwangchuan area in Gansu province,which has suffered severe damage in Northwest China,is taken as a typical project.Through field investigation and sampling,it is proved that the main erosion effect of the hydraulic structure damage of the typical project is the coupling effect of dry and wet cycles,salt attack and freeze-thaw cycles,and the main erosion ions are sulfate and chloride ions;according to construction requirements,design 9 groups with different water-cement ratio(S1,S2,S3),fly ash content(F1,F2),mineral powder content(K1,K2),and air-entraining agent content(Y1,Y2)concrete specimens.Carry out indoor accelerated tests in clear water and 5% sodium sulfate-sodium chloride solution respectively.Clarified the failure process of concrete and revealed the deterioration mechanism of concrete.Calculate the associate degree based on the grey associate analysis method,and evaluate the durability of concrete through the associate degree.The grey residual GM(1,1)model was constructed using the experimental data,and the Markov model was used to modify the sign to predict the durability damage of hydraulic concrete.The main conclusions are as follows:(1)By analyzing the test data,it can be known that the smaller the water-cement ratio,the stronger the durability,and the greater the water-cement ratio,the worse the durability.Adding 15% low-dosage fly ash can improve the durability of concrete,and adding30% high-dosage fly ash will accelerate the destruction of concrete and reduce the durability of concrete.Different dosages of mineral powder have no significant effect on the durability of concrete.Adding air-entraining agent can significantly improve the durability of concrete.And through nuclear magnetic resonance analysis,it can be known that the pore size distribution of concrete shows a "three-peak distribution".After four cycles test,the small pores of the benchmark concrete in the clear water and the double salt solution increase significantly,and the other two pores increase slightly.After 8 cycles test,the two rules showed differences.In clear water,the number of small pores in concrete decreases significantly,and the small pores continue to transition to mesopores and macropores,and the distribution values of mesopores and macropores increase significantly;in salt solution,the number of small pores still increases significantly,while less increase in mesopore and macropores distribution value.(2)Comparing the associate degree of each test group,we can find that in clear water,after 3 major cycles,the associate degree of each group is larger,and the durability of concrete is better;after 8 major cycles,the associate degree drops a lot,and the durability becomes worse.In the double salt solution,the maximum value of the associate degree appeared after the end of the first major cycle;after five major cycles,the associate degree value of each group decreased significantly,and the durability of the concrete deteriorated.And in the S group,for any number of cycles,the associate degree exists S1>S2>S3;in group F,there is F1>S2>F2;in group K,the associate degree of S2,K1,and K2 are basically the same;in group Y,associate degree always exist Y2>Y1>S2.(3)The relative dynamic elastic modulus is used to characterize the durability damage of concrete,and the gray residual model is constructed with the test data of four groups of S2,F1,K1 and Y1 from 0 to 5 tests,and the 6 to 8 test data are calculated through the model.The Markov process is used for symbol correction.Compared with the original data,the average error is only 3.507%,which verifies the applicability of the model.Based on the above model,it is predicted that the test times when the four groups of concrete specimens of S2,F1,K1,Y1 reach failure in clear water are 12,17,13,16,respectively.The test times when the four groups of concrete specimens of S2,F1,K1,Y1 reach failure in salt solution are 9,11,9,10,respectively.