| A large number of foundation projects have been built in the high-altitude and cold regions of northwestern China,With the implementation of the important strategy of the western development.Bored piles are usually used as the foundation for railway and highway construction.Concrete freezes and hardens in frozen soil.With the increase of age,the strength of the concrete at a certain time and the strength of the standard curing 28 d reached a state of "equal strength".After pouring the concrete of the cast-in-place pile,the frozen soil will be melted and the pile concrete will be affected by the freezing of the frozen soil.During the strength increase of the concrete,the free water in the pores will freeze to form an ice core.Between the low-energy frozen water and the high-energy supercooled water the generated thermal imbalance drives the subcooled water to continuously flow to the icing area,and the ice crystals swell to form an ice expansion force,which causes the concrete holes structure to deteriorate.Deterioration of the concrete pore structure reduces the durability of the concrete and shortens the service life.Therefore,it is necessary to carry out in-depth research on the problems of bored cast-in-situ pile concrete in this special geological environment in the western frozen soil region,so as to provide a reference for the design and construction of bored cast-in-place pile concrete in the frozen soil region.In this paper,the effects of different hydration characteristics of cement under different curing systems,different mold temperatures,and different amounts of air-entraining agents on the number of days for concrete to reach equal strength,as well as the pore structure and impermeability of concrete under the same strength conditions were studied.Study on the evolution law of the performance degradation of isostrength concrete under-5? curing conditions.The main research contents and experimental conclusions are as follows:(1)Through constant-5? curing and standard curing environment,the effect of hydration heat of cement paste with different mold inlet temperatures at different mold temperatures was analyzed,and the influence of mold inlet temperature and curing temperature on cement hydration reaction was analyzed.The results show that the negative temperature curing environment has a significant inhibitory effect on the heat release of the four mold injection temperature cement pastes of 5?,10?,15?,and 20? at different ages.The degree of hydration is significantly lower than the standard curing environment.The lower the mold injection temperature,the lower the hydration heat and the degree of hydration in the early stage of the cement paste.The negative temperature environment prolongs the hydration age of the cement and makes the hydration reaction "lag" phenomenon".(2)The standard curing and constant-5? curing mold injection temperature and the amount of air-entraining agent have the same effect on the compressive strength of concrete.The negative temperature curing is significantly lower than the standard curing concrete compressive strength;the air-entraining concrete is at the same age the compressive strength of the concrete gradually decreases with the increase of the amount of air-entraining agent,and the compressive strength of the concrete at the same age gradually increases with the temperature of the mold.(3)By analyzing the influence of air-entraining agent dosage and mold injection temperature on the impermeability of concrete under standard curing and-5? curing,with the increase of air-entraining agent dosage,the electrical flux of concrete appears to increase first and then decrease.There is a reasonable amount of air-entraining agent for the impermeability of concrete.Outside the reasonable amount,the impermeability of concrete will show a negative growth.Add 0.00%,0.05%,0.10%,0.15%,0.20% When forming air-entraining agent,the concrete with 0.05% air-entraining agent has the smallest electrical flux and the best resistance to chloride ion penetration;the higher the mold temperature,the lower the concrete electrical flux and the higher the resistance to chloride ion penetration.(4)Through the analysis of the influence of air entraining agent content on the pore structure of concrete under different molding temperature,the results show that when the air entraining agent content is 0.00% and the air entraining agent content is 0.05%,the pore size distribution of concrete can be effectively improved,and the large pore size can be reduced,and the pore size can be increased;the irrational air-entraining agent will cause the deterioration of the pore structure of the concrete to different degrees.The more the amount of air-entraining agent,the more serious the deterioration.The porosity and average pore diameter will gradually increase with the increase of the air-entraining agent;the distribution of small pores in concrete pore structure increases,the distribution of large pores decreases,and the porosity and average pore size gradually decrease.(5)The strength of the air entrained concrete with the temperature of 20?,15?,10? and 5? under the constant curing temperature of-5? is equal at 42 d,49d,56 d and 63 d,respectively.With the decrease of the temperature of the formwork,the age of equal strength appears obvious "lag phenomenon".(6)Under the condition of equal strength,the micro pore structure and chloride penetration resistance of concrete in the negative temperature curing environment are not up to the standard curing age of 28 days.This shows that the equal strength of macro strength does not represent the "equal pore structure" and "equal resistance to chloride ion penetration" durability of micro pore structure. |
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