Study On The Evolution Law Of Micro-properties On The Equal Strength Concrete Of Bored Pile Under-3? Curing Condition

With the rapid development of economic construction in our country,our country needs better transportation,transportation must not only be economical in terms of cost,but also be guaranteed in terms of safety and quality.Concrete is widely used as the main building material for road traffic,However,In the north of China,it is cold in winter especially the average low temperature along the road is 0~-3.5? in the Qinghai Tibet area and the foundation is mostly frozen soil area.In order to cross the frozen soil area,the road mostly adopts the bridge with bored piles.Therefore,the concrete of bored piles is mostly maintained under negative temperature.So in order to improve the frost resistance of concrete,air-entrained concrete is used.But Compressive strength should not be ignored in order to improve frost resistance,otherwise it will bring safety problems.Based on this problem,the concrete with different water binder ratio(0.28,0.32,0.36,0.40)and different air entraining agent content(0,0.05%,0.08%,0.12%)is used as the research object in this experiment,and two maintenance methods of standard maintenance and negative temperature(-3?)maintenance are used to explore the evolution law of compressive strength and pore structure.The compressive strength of concrete is cube compressive strength,the pore structure is measured by nuclear magnetic resonance and mercury intrusion test,the hydration degree is obtained by simultaneous thermal analysis test,in addition the compressive strength and pore structure of the two curing methods were compared.The research results of concrete compressive strength show that the compressive strength of concrete increases with the age under the negative temperature(-3?),but there is a certain "age lag" phenomenon.The negative temperature environment makes the compressive strength of concrete develop slowly;the compressive strength of concrete decreases with the increase of air entraining agent,At the same age,the compressive strength of concrete decreases with the increase of water binder ratio which is similar to the growth rule of the compressive strength of standard curing concrete;the comparison of the compressive strength of concrete under the two curing conditions,negative temperature(-3?)restrains the development of concrete compressive strength,so negative temperature concrete needs to extend the curing age to achieve equal compressive strength.The age of concrete with water binder ratio reaching equal strength is the same and the age of concrete reaching equal strength is 35 d.Therefore,the negative temperature has the same effect on the compressive strength of concrete.In this paper,the differences of equal strength concrete under the two curing methods are explored from the aspects of pore structure and hydration degree.The pore structure is studied by MRI and MIP.Under the two maintenance methods,the pore structure of concrete isgradually optimized with the curing age.The larger the air entraining agent content is,the worse the pore structure of concrete is,although the concrete has reached the equal compressive strength,the pore structure is not similar.The pore structure of the concrete cured at negative temperature(-3?)curing is inferior to that of the standard curing concrete,The main difference is that the pore diameter is larger and the macropore is more under negative temperature curing.The degree of hydration is characterized by the content of calcium hydroxide and chemical combined water by TG.The results show that the hydration degree of cement paste at negative temperature(-3?)is lower than that of standard curing cement paste.The above results show that the performance of negative temperature curing concrete is worse than that of standard curing concrete.Therefore,Measures should be taken to reduce the influence of negative temperature on the performance of concrete under negative temperature curing.