Experimental Study On Freeze-thaw Damage Law Of Concrete With Equal Strength Under Constant Negative Temperature Curing

With the establishment and advancement of the "One Belt And One Road" national strategy,the foundation construction of the Qinghai-Tibet Railway,which is located in the high cold,high altitude and high latitude permafrost region in China,is flourishing day by day.Special climate,geological condition and various environmental factors,determine the Qinghai-tibet railway,highway,Sichuan-tibet railway engineering construction structure the early strength of concrete frost resistance must have good physical and mechanical properties,durability and the ability to adapt to the environment,and how to ensure that the region of concrete structures can have the qualified performance has become a problem to be solved.This article is based on the Tibetan region of the bored piles foundation of negative temperature concrete construction and maintenance,as well as the national natural fund "frozen season activity layer of bored piles,strength of concrete pore structure evolution of frost resistance durability degradation control research" as the background to carry out the constant negative temperature lower strength concrete freeze-thaw damage law.Under the test condition of simulating the typical environment in Qinghai-Tibet railway area,the concrete performance tests with different concrete mix ratios and different concrete influence factors were carried out by using the method of multiple groups of contrast tests.By changing the dosage of water/cement ratio,air-entraining agent and other factors,on the frost resistance of concrete,early strength properties,such as law of development,and in accordance with the standard curing condition of the same concrete comparison,analysis,cold high altitude environmental conditions affect the performance of concrete strength and frost resistance,clear,cold high altitudes and the law of development of the performance of the concrete of different influence factors on the properties of concrete frost resistance are studied.The main research contents and results of this paper are as follows:(1)In the case of the same amount of air engraving agent and water-binder ratio,the compressive strength of concrete under the two curing conditions of constant negative temperature(-3℃)curing and standard curing showed an increasing trend with age.Compared with the standard curing,the compressive strength of concrete curing at-3℃constant temperature at each age is lower than that of concrete curing at standard curing.The concrete cured at a constant negative temperature(-3℃)with different water-binder ratios and different amount of air entraining agent all reached the strength of the same standard curing for 28 days in about 42 days,that is,the concrete with equal strength.The pore size distribution of concrete under negative temperature(-3℃)curing is worse than that of concrete under standard curing,but the compressive strength of concrete is equal.Therefore,by extending the age,the concrete under negative temperature(-3℃)curing can achieve the "equal strength" on the macro level,but not the "equal pore structure" on the micro level.(2)An experimental study on the deterioration of macro indexes of freeze-thaw concrete under the condition of constant strength was carried out,aiming to determine the strength of frost resistance of several groups of different concrete from a macro perspective through the variation law of the four indexes of mass and relative dynamic modulus,peak stress and peak strain of the macro indexes of freeze-thaw concrete with the cycles of freeze-thaw.The results show that a proper amount of air entrain agent is beneficial to improve the frost resistance of concrete.The freezing resistance of concrete decreases with the increase of water-binder ratio with or without the addition of air entraining agent.The frost resistance durability of concrete with constant negative temperature curing under constant strength is weaker and more obvious than that under standard curing.(3)According to the results of the cube compression test,the stress-strain curves of concrete with different amount of air entraining agent tend to flatten with the increase of the number of rapid freeze-thaw cycles.The peak stress of concrete decreases with the increase of freeze-thaw times.The peak strain increases with the increase of the number of freeze-thaw cycles.At the same time,the fitting formula for the change of peak stress and peak strain in freeze-thaw cycle of concrete with 0.28 water-binder ratio and different initial gas content under standard curing and constant-3℃ curing was obtained.(4)Nuclear magnetic resonance(NMR)instrument was used to test the microstructure parameters of concrete under different curing temperatures and different freeze-thaw cycles,and the variation rule of concrete microstructure parameters on freeze-thaw cycles was studied.The influences of water-binder ratio and curing temperature on the microstructure parameters of freeze-thaw concrete were analyzed.The influence of water-binder ratio and curing temperature on freezing resistance of concrete can be determined from microscopic point of view.The results show that the frost resistance durability of concrete can be improved by the appropriate amount of air entrain agent.Make concrete more "frost resistant".Freeze-thaw cycles make concrete develop in the direction of increasing aperture.(5)Based on the grey correlation method,the correlation degree of macro index and micro parameter in the freeze-thaw process was evaluated,and the quantitative analysis of the correlation degree of macro index damage and micro parameter was conducted.The results show that the correlation degree of strength loss,relative dynamic modulus loss and porosity of concrete,as well as the proportion of harmful holes and multiple harmful holes in the theory of pore separation is about 0.8～0.9 for the freeze-thaw cycle of constant negative temperature curing concrete and standard curing concrete at the same strength.According to the theory of correlation degree judgment,the correlation degree is high and the two have good correlation.The increase of concrete porosity and the proportion of harmful pores and multiple harmful pores in pore separation theory have important effects on the deterioration of frost resistance durability of concrete.(6)According to the regional location selection that the different latitude and yichang region in Xining area as an example,combining with the relative loss modulus in the process of freezing and thawing data 0.28 water-binder ratio concrete frost resistance life prediction of the trial,the results found that the standard curing of the concrete use fixed number of year is about 2 ～ 3 times under the constant negative temperature curing,use fixed number of year is much higher than in Yichang region in Xining region of use fixed number of year.