Research On The Early Shrinkage-age Model Of Reactive Powder Concrete In Different Curing Conditions

Reactive powder concrete(RPC)has extreme low water to binder ratio,large amount of active admixture,high dosage of superplasticizer and elimination of which lead to higher early-age shrinkage compared with ordinary concrete.Accordingly,the shrinkage cracking risk is significant.In order to study the early shrinkage development regularity and the shrinkage mechanisms of RPC,and the shringkage of RPC must be predicted and calculated accurately.In this paper,the early-age shrinkage of materials was measured in three curing conditions,normal temperature soaking,sealing and drying.Combining the internal relative moisture,degree of hydration,pore structure distribution and mechanical performance,the shrinkage mechanisms were analyzed.The shrinkage prediction model was built.Based on the model,the influence of different curing temperatures on the early-stage shrinkage of reactive powder concrete(RPC)was examined.The shrinkage development rules and changing mechanisms under the three curing conditions were discussed,respectively.The main work and conclusion are as follows:(1)The shrinkage of RPC under the above stated three curing conditions was measured at the ambient temperature.The shrinkage mechanism was investigated in view of changes of the internal relative humidity,degree of hydration and pore structure.Results suggested that the early-stage shrinkage of RPC was greatly influenced by the curing conditions,and that the increase of shrinkage value was rapid in the early period but t gradually slowed down in the late stage.Besides,the shrinkage increase rate,time and shrinkage value were all different.(2)The shrinkage increase rule under the sealing condition and the drying condition differed from each other greatly.Under the sealing condition,the shrinkage kept on increasing within 28d;while under the drying condition,the shrinkage increase was fast in the first 7d but slowed down from the 7the day to the 28th day.From the perspective of hydration,the hydration reaction under the sealing condition went on without interruption within 28d,so did its autogenous shrinkage.However,under the drying condition,water desorption pulled down the hydration reaction rate after the 7th day,and the autogenous shrinkage was also thus limited.Numerically speaking,the shrinkage value under the sealing condition was higher than that under the sealing condition by 90.5?? on the 28th day.Contrary to the laws of the common concrete,the microscopic mechanisms of the early-age shrinkage law was further discussed based on the results of hydration,pore structure and moisture migration.(3)Based on the degree of hydration,the internal relative humidity,the elasticity modulus and the pore structure distribution of binding materials,the chemical shrinkage,self-shrinkage and drying shrinkage prediction model was built,respectively.Based on the shrinkage strain of the soaking group actually measured,the chemical shrinkage value under the sealing and drying condition was demarcated.Based on the shrinkage strain of the sealing group actually measured,the autogenous shrinkage value under the drying condition was demarcated.In this way,separation of three shrinkages was realized.(4)The shrinkage of RPC placed at three different curing temperatures,namely drying,sealing and soaking,for a long time was measured.From the perspective of the extent of hydration,the influence of the curing temperature on chemical shrinkage,autogenous shrinkage and drying shrinkage on RPC was analyzed.Different curing temperatures had a different influence on chemical shrinkage,self-shrinkage and drying shrinkage.Along with increase of temperature,chemical shrinkage gradually increased,but the increase range was not huge.Under the soaking condition,a higher temperature would cause a dramatic self-shrinkage in the early period.The high temperature could inhibit autogenous shrinkage.The higher the temperature was,the more obvious the inhibiting effect was.In the early stage,the drying shrinkage increased along with the curing temperature.However,the high temperature could lead to a more compact structure and inhibited the shrinkage in the late stage.(5)The shrinkage rules under the high-temperature steam curing condition based on the influence of the curing temperature on three kinds of shrinkage was studied.Along with increase of the curing temperature,the shrinkage also increased in the early period.Curing in a highly-humid environment could result in dramatic shrinkage within a short period of time after the end of curing.The higher the temperature was,the higher the shrinkage value was in the period.This was also a major cause of the crack risk.In the late stage,the shrinkage increase rule of RPC suggested that,the higher the curing temperature was,the more stable the volume in the late stage was,and the smaller the shrinkage variation was.