Frost Resistance Durability Of Concrete In High Altitude Cold Area

High altitude cold areas have low temperature,high altitude and large temperature difference in one day.Concrete will experience freezing and thawing cycle in rainy and snowy weather,which will cause damage to the structure,affect the service life,and even threaten the safety of buildings.In this paper,through the physical and mechanical tests and freeze-thaw cycle tests of concrete mixed with different admixtures in high altitude cold area,the working performance,mechanical properties and frost resistance of concrete in high altitude cold area are studied.On this basis,the nuclear magnetic resonance tests of concrete in high altitude cold area under different freeze-thaw cycles were carried out to reveal the freeze-thaw damage mechanism of concrete in high altitude cold area.The freeze-thaw damage model of concrete in high altitude cold area is studied by combining theoretical derivation with experimental results.The main work includes:(1)The raw materials of concrete in high altitude cold area are tested,and the mix proportion of concrete in high altitude cold area is calculated.The performance of concrete in high altitude cold area is improved by adding admixtures.The working performance and mechanical performance of concrete in high altitude cold area mixed with different admixtures are tested.The results show that both air entraining agent and water reducing agent can improve the slump and air content of concrete in high altitude and cold area.Compared with ordinary concrete,the mechanical properties of air entrained concrete in high altitude and cold area are decreased,while the mechanical properties of water reducing concrete and air entrained concrete in high altitude and cold area are improved.The compressive and flexural properties of four kinds of concrete meet the requirements of the specification.(2)The freeze-thaw cycle tests were carried out on the ordinary concrete,air-entraining concrete,water-reducing concrete and air-entraining water-reducing concrete.The changes of ultrasonic wave velocity,mass,dynamic elastic modulus and flexural strength of four kinds of high cold concrete in the process of freeze-thaw cycle were tested,and the freeze-thaw durability of high cold air entrained concrete was the best.(3)The nuclear magnetic resonance(NMR)tests were carried out on four kinds of high cold concrete specimens after different freeze-thaw cycles,and the variation of internal pores of high cold concrete during freeze-thaw cycles was analyzed.The results show that the admixture can improve the frost resistance of concrete by changing the internal pore structure of high cold concrete,and the internal pores and mesopores of high cold concrete are beneficial to its frost resistance durability.The greater the total proportion of the two,the better the frost resistance durability of concrete.(4)Combined with the damage theory,the freeze-thaw damage model of the high altitude cold area concrete is deduced,and the macro and micro models of freeze-thaw damage of the ordinary concrete in high altitude cold area are studied.By using MATLAB software,the fitting accuracy of the micro model is higher,up to 0.88～0.90.The established model can be used to infer the freeze-thaw cycle damage degree of concrete structure in high altitude cold area.