Studies On Humidity Field And Shrinkage Stress Of Ordinary And Internally Cured Concrete

The relative humidity change in concrete is the driving force for the development of shrinkage.It is also the basis of the analysis and calculation of shrinkage stress under constraint conditions.In this paper,C30,C50,C80 and C100 four kinds of ordinary and internal curing concretes were selected as the research objects.From the experimental and theoretical aspects,the development of relative humidity from the beginning of pouring in concrete was studied.The content of the study includes,surface factor of concrete,internal moisture diffusion coefficient,calculation model of humidity field for ordinary and internal curing concretes,calculation of shrinkage stress of ordinary and internal curing concrete structures based on humidity field.The surface factors of four strength grades ordinary and internal curing concretes were tested by weighing tests.And a theoretical calculation model that can cover its main factors was also proposed.The experimental and theoretical results show that the surface factor is mainly influenced by three factors,such as temperature,wind speed and specimen size,the higher the temperature,the greater the wind speed,the greater the surface factor;the larger the size of the specimen,the smaller the surface factor.The concrete strength grade and internal curing have no effect on the surface factor.The humidity tests of four strength grades ordinary and internal curing concretes under drying and sealing conditions were carried out.Based on the results of humidity tests,the internal moisture diffusion coefficients of four strength grades ordinary and internal curing concretes were obtained by reversed calculation method.The results show that,the internal moisture diffusion coefficient of concrete is a function of relative humidity.In the high humidity region(H>0.7),the moisture diffusion coefficient is large,in the low humidity region(H<0.7),the moisture diffusion coefficient is small.The higher the concrete strength grade,the smaller the moisture diffusion coefficient is.In the high humidity region(H>0.7),the moisture diffusion coefficient of internal curing concrete is larger than that of the corresponding strength grade ordinary concrete.But in the low humidity region(H<0.7),it is the same with ordinary concrete.Based on the mechanism of water loss,a method of determining the time of humidity reduction by using the critical water loss was proposed.Based on the humidity tests under sealing condition,the water consumption or diffusion at the internal humidity drop time of four strength grades ordinary and internal curing concretes were calculated,which were defined as the critical water loss.A humidity field calculation model for internal curing concrete considering the influence of lightweight aggregate discharge was established.The model takes into account the influence of multiple factors such as cement hydration,water diffusion and lightweight aggregate discharge.The model can be used to calculate the development and distribution of the internal humidity of ordinary and internal curing concretes under different environmental conditions.Based on the established calculation model of humidity field,taking concrete pavement,concrete column and concrete continuous wall as examples,the shrinkage strain and shrinkage stress of the concrete structures were calculated.The influence of concrete strength grade,internal curing and using of permanent formwork on the shrinkage strain and shrinkage stress in the above structures were analyzed.The calculation results show that the using of internal curing concrete can effectively improve the internal humidity level,reduce the shrinkage stress and cracking risk of the concrete structures.