Experimental Study On Loss Of Prestress Caused By Characteristics Of Concrete Under Low Temperatures

The experimental study on loss of prestress caused by characteristics of concrete under low temperatures is the sub-project of experimental study of material and structural behaviour of prestressed concrete at cryogenic temperature(Natural Science Foundation 51478309).There are many reasons for loss of prestress.This research direction is loss of prestress caused by characteristics of concrete under low temperatures,including thermal strain tests under ultra-low temperature and shrinkage & creep tests under low temperatures of concrete.The variables of the thermal strain tests included moisture content,water cement ratio and cooling rate.Furthermore,the cooling curves and the heating curves were compared.The temperature ranged from 0 ? to-165 ?.The conclusions are summarized as follows: Based on the change law of strain-temperature curves,the thermal strain curves of concrete during the cooling process can be divided into four stages.The four stages are called room temperature stage(more than 0?),low temperature unstable stage(0?~-70?),low temperature stable stage(-70?~-120?)and ultra-low temperature stage(below-120?)respectively and the variables as above mainly affect the low temperature unstable stage.The phenomenon of “inverse expansion” of concrete becomes significant with the increase of water content,water cement ratio and cooling rate.By comparing the heating curves and the cooling curves,the heating curves tend to lag and be moderate especially for the high moisture content and high water cement ratio specimens.In addition,the residual deformation was found when specimens returned to room temperature.In the preliminary stage of shrinkage & creep tests of concrete under low temperatures,research group was in cooperation with the enterprise,developing the extensometer which could be used in ultra-low temperature(-165?).Based on the above,the influence of low temperatures on shrinkage & creep of concrete was studied.Through the expression of creep model of CEB-FIP2010,we could use the short-term creep test results to predict the nominal creep coefficient of concrete and to obtain the development law of creep.The results show that the influence of low temperatures on shrinkage of concrete is not obvious.But with the decrease of temperature,the creep of concrete decreases obviously and stabilizes faster.