Study On The Autogenous Shrinkage And Cracking Sensitivity Of High Performance Concrete At Early Ages

Autogenous shrinkage and its induced restrained stress are paramountcauses of early-age cracking of high performance concrete (HPC). It isprerequisite to study the mechanism of volume change undermulti-parameters for predicting and controlling early-age cracking of HPC.In this thesis, autogenous shrinkage strain and cracking resistance of youngHPC mixtures were investigated and discussed considering testingparameters including water-cement ratio, curing temperature and restraintrate.The self-designed embedded strain gauge with an extremely lowelastic modulus was used to exactly monitor volume change strain of HPCafter initial setting. For the purpose of measuring thermal expansioncoefficient of young HPC and grasping its time-dependent behavior, aspecial testing method was presented. In the testing method, a low temperature range (-1～4℃) and rapid temperature change in the rangewere adopted for alleviating hydration and eliminating influences ofautogenous shrinkage, respectively. And then, based on the measuredthermal expansion coefficients and equivalent age principles, componentsof volume changes of young concrete under sealed condition were dividedinto autogenous shrinkage and thermal strain. Thereafter, time dependenceof autogenous shrinkage of early-age HPC was obtained to evaluate itscracking sensitivity indirectly. Also, temperature dependence of autogenousshrinkage and cracking resistance were preliminarily explored.In order to much more straightforwardly evaluate cracking sensitivityof HPC at early ages, restrained volume change test was executed on thetemperature stress testing machine (TSTM), in which different restraintrates (60%and100%) were take into account. As a comprehensive index ofindicating crack sensitivity in TSTM method, cracking temperature, wasapplied to evaluate cracking resistance directly.The conclusions could be drawn in the range of this dissertation asfollows:1.Time-dependent changes of thermal expansion coefficient of HPCexperienced three phases sequentially, i.e., sharply decreasing (between theinitial setting and final setting time), slowly increasing and continuouslystable phase (after the age of24hrs).2.Higher curing temperature resulted in more autogenous shrinkage strain and lower water-cement ratio led to more autogenous shrinkage strain.Differences of the shrinkage rate with different mixtures and various curingtemperature were remarkable until the age of1and2days, after that, therates no obvious differences.3.For the same mixture, the lower the restraint rate, the lower the crackingtemperature in TSTM method. The lower the water-cement ratio (0.3versus0.4), the lower the cracking sensitivity, but furthermore lowerwater-cement ratio (0.2) could not improve cracking resistance much more.