Experimental And Mechanism Study On The Correlation Between Internal Curing Water Content And Autogenous Shrinkage

Autogenous shrinkage is one of the primary factors resulting in early cracking of high performance concrete(HPC).The addition of saturated lightweight aggregate(LWA)into HPC has internal curing effect and can effectively inhibit the development of autogenous shrinkage.However,the incorporation of abundant LWA will reduce the mechanical properties and durability of concrete.In this paper,the correlation between the content of internal curing water and autogenous shrinkage is studied by combining the experimental parameters such as the properties of water absorption and desorption of LWA,temperature deformation and relative humidity.The purpose of the paper is to obtain the optimal content of light aggregate on the premise of eliminating autogenous shrinkage.The main work and conclusions are as follows:(1)The mixture proportion design of internal curing self-compacting concrete is completed,which solves floating problem of LWA in Vibration molding process.Four groups(0%,30%,70% and 100%)concrete respectively are set,where "100%" corresponds to the amount of saturated LWA for eliminating autogenous shrinkage calculated by Bentz formula.(2)The moisture and temperature in the concrete with different LWA contents under constant temperature are monitored.There are rising section,falling section and stationary section one after another in the temperature time-varying curve,which corresponds to the time-varying characteristics of the rate of heat release from cement hydration.The time-varying curve of humidity has a law of rising-falling-rising – falling,which is the result of the combined action of bleeding water absorption of fresh concrete,water absorption and desorption of LWA inside concrete and cement hydration process.(3)Free deformation tests under closed and dry conditions are carried out for concrete with different LWA content.The results showed that saturated LWA can eliminate shrinkage in both dry and closed conditions,and the effect increases with the increase of LWA content.For concrete with the same LWA content,the free shrinkage under dry condition is much higher than that under closed condition.In order to obtain the autogenous deformation,the temperature deformation is subtracted from the free deformation which is monitored under the closed condition.The data of 5-day autogenous deformation shows that the expansion deformation of 85?m/m and 119?m/m occur in the 70% and 100% LWA content groups respectively,and the shrinkage deformation of 297?m/m and 61?m/m occurs in the 0% and 30% LWA content groups respectively.Futhermore,about 41% saturated LWA can eliminate autogenous shrinkage 5 days,which indicates that the amount of water less than the internal curing water amount calculated by Bentz formula can completely eliminate the early autogenous shrinkage.(4)The mechanical properties and water absorption properties of concrete with different LWA contents are tested.Compared with the groups with 0% content,the compressive strength of standard cubic concrete specimens with 30%,70% and 100% LWA content decreases by 7%,16% and 30% respectively,and the elastic modulus at 5 days decreases by 2.2%,4.8% and 11% respectively,and the elastic modulus 28 days decreases by 2.3%,3.6% and 5.5% respectively,and water loss after drying at 40 days increase by 4.8%,16.2% and 24.2% respectively,and the water absorption after drying-cooling-immersion increases by 5.2%,11.5% and 19.8% respectively.The results show that The larger the content of saturated LWA is,the worse the mechanical properties of concrete are,the better the water absorption performance is.(5)The " hard core soft shell" model is adopted to simulate the distribution of LWA in concrete of different groups and the proportion of the volume of protected paste in all cement paste.The results show that the larger the content of LWA is,the smaller the "average distance" between cement paste and the surface of LWA is,which means most cement paste can be protected without long distance transmission of internal curing water.Assuming that the cement paste protected by the internal curing water does not shrink,the water migration distance can be calculated by the volume of the protected paste.Based on the above assumption,it can be concluded that if the concrete in which the content of LWA is 41% does not shrink,the migration distance of internal curing water should be within 1-2mm.(6)In order to obtain the time-varying "relative expansion" of concrete caused by the different saturated LWA content,the autogenous deformation with the content of 0% is subtracted from the autogenous deformation of concrete with the content of 30%,70% and 100%.The results show that the "relative expansion" rate of early-age concrete with different LWA content is roughly same,and the "relative expansion" duration increases with the increase of LWA content.In addition,in order to get the minimum LWA content on the premise of eliminating early age autogenous shrinkage,the chemical shrinkage value in Bentz formula is redefined,and the maximum hydration value is replaced by early age hydration degree,so as to reduce the negative impact of excessive LWA.