Experimental Study On Mechanical Properties Change Mechanism Of Concrete At Cryogenic Temperatures

With increasingly concrete application in cryogenic environment,its mechanical performance under cryogenic temperature and enhancement mechanism is worth studying.This research is one of subjects of the research of national natural science fund — "Experimental study of material and structural behavior of prestressed concrete at cryogenic temperature"(no.: 51478309).In order to explore the concrete mechanics performance enhancement mechanism under cryogenic and mechanical damage mechanism after freeze-thaw cycle,our tests focus on the influence of moisture content and water-cement ratio on compressive strength of concrete at cryogenic temperatures,as well as the effects of the admixture of resistance to ultra-low temperature freezing and thawing cycle.Mainly includes the following aspects:1)System preparation before the test: cooling insulation device is designed and made;methods of keeping moisture content are prepared.2)188 different water-cement ratio cube concrete blocks are made,the compressive strength of low and ultra-low temperatures test is conducted,the influence of moisture content and water-cement ratio on compressive strength of concrete at cryogenic temperatures is studied of which results are compared with relevant academic research results.3)198 cube concrete blocks which added different dosage of air-entraining agent and antifreeze are made.With the full water component in different temperature,we conduct freezing and thawing cycle test,record every specimen weight as well as the specimen before and after freezing and thawing phenomenon,then get the freezing and thawing compressive strength of the specimens,finally summarize the relationship between different dosage of admixture and performance of concrete after freeze-thaw cycle.4)We observe the concrete microstructure and describe the internal damage and micro cracks developing process by using SEM technology,and explore the mechanism of concrete mechanical performance change from the micro level.Through the experiment and analysis following conclusion can be concluded:1)With the reduction of temperature,concrete compressive strength increase.With lower temperature,moisture content has greater contribution to the compressive strength increment of concrete.With increase of water cement ratio,the trend of concrete strength increment increasing with the increase of water content is more obvious.Lower temperature lead water in the pores freeze according to the radius size ice gradually and the strength of the ice increase with the temperature reduction,which enhance strength of concrete.2)After low temperature freezing and thawing cycle of concrete compressive strength is lower than which without experience of concrete freezing and thawing cycle.The lower the temperature is,the lower the compressive strength is.3)The mixed air-entraining agent loses the original mechanical strength of concrete,bubbles will eliminate or reduce evenly stress due to the frost heave and the temperature stress due to alternating plus or minus temperature evenly,which make concrete more resistant to-80 ℃ cryogenic freezing and thawing cycle of damage.4)With the increase of antifreeze,concrete compressive strength is a downward trend after rising first.Proper dosage of antifreeze can increases the compressive strength of concrete.Too much antifreeze can reduce the compressive strength of concrete.Antifreeze agent content of 2% can make the compressive strength of concrete improved.