Experimental Study On The Frost Resistance Of Concrete Materials At Low Temperatures

The rapid development of the economy and the continuous progress of science and technology have promoted the study of building materials and structures at low temperatures such as polar regions,the area of high latitudes and high altitude and deep seas.The mechanical properties of normal concrete decreases after freezing-thawing cycles at low temperatures,and the applicability of ordinary concrete is obviously insufficient.In order to broaden the application of concrete structure at low temperatures,three series of tests including:1)effect of air-entraining agent on frost resistance of concrete;2)influence of water content on frost resistance of concrete;3)experimental study on mechanical properties of UHPC at low temperatures.140 cube specimens incorporating with different types of air-entraining agent were used to research the effect of air-entraining agent after freezing-thawing cycling test.Then the comparative analysis was used to study the relationship between the characteristic parameters of pore structure and macroscopic mechanical properties by the mercury injection test on several groups of specimens.The results showed that there is a close relationship between the macroscopic mechanical properties and internal pore structures of concrete at low temperatures.A certain amount of harmless pores generated in the concrete by the incorporation of air-entraining agent,and pore structure is optimized,the frost resistance is improved.Comparing the three different types of air-entraining agent used in the test,high-efficiency air-entraining agent in the form of powder shows a more excellent improvement effect.The influence of water content was analyzed by concrete with three different pore structures after freezing-thawing cycles by 180 cube specimens under the dry state,natural state and water-saturated state,respectively.Under the same freezing-thawing cycle conditions,the concrete with different pore structure shows a different frost resistance,and the corresponding optimal water conditions was also different.Compressive strength test and freezing-thawing cycle test at low temperatures were carried out to explore the influence of low temperature on the performance of ultra-high performance concrete(UHPC).The microscopic pore structure of UHPC before and after cycling was tested in order to be compared with normal concrete.The results showed that the compressive strength of UHPC increases with the decrease of temperatures,and the mechanical properties decreases obviously after freezing-thawing cycles.Due to the low porosity and a large part of harmless pore,UHPC is less affected by low temperature than ordinary concrete.In addition,the calculation formula obtained by a regression analysis can be used to predict the strength change of UHPC after freezing-thawing cycles,and provide a theoretical foundation for the application of UHPC structures in low temperature environments.