Study On Shrinkage And Cracking Sensitivity Of Concrete At The Early Age Based On Temperature-stress Testing Machine

Cracking of concrete in the engineering has always been a problem for engineers.Cracking not only affects the appearance of the building,but also makes it easier for aggressive media to enter the interior of the concrete,accelerating the deterioration of the reinforced concrete and affecting its durability.The restraint stress caused by autogenous shrinkage and temperature deformation of concrete in the early age is the main cause of cracking,and the factors affecting the early age deformation of concrete are complicated.At present,most of the testing methods are to study the deformation properties of concrete after hardening,and actually,the concrete properties in the first day after cast change very rapidly.The rise in temperature is often completed within the first day.Researchers at home and abroad have conducted many researches to overcome the shortcomings of traditional testing methods,and have also developed new test equipments.Some new test methods are established,and the most effective one is the uniaxial restraint test.In this paper,a new temperature stress testing machine?TSTM?was used to study the early age autogenous shrinkage and restraint stress of concrete.The main results are as follows:1.Through free deformation curve and temperature curve measured in the free deformation mode by the TSTM,the free deformation can be separated into temperature deformation and autogenous shrinkage,and the influence of concrete strength,curing temperature and PVA fiber on autogenous shrinkage are studied respectively.With the increase of strength,the autogenous shrinkage of C60 and C80 strength grade concrete increased by 27.1%and 53.1%at 96 hours compared with C40.Under three different curing modes with temperature peaks of 32.7°C,48.1°C and 60.1°C,the 96h autogenous shrinkage of C60 concrete was increased by 23.8%and 56.5%,respectively.Based on the C60 strength grade concrete,the effect of volumetric content of PVA fiber on autogenous shrinkage was studied under the natural curing model.Compared to the C60 concrete,when the volumetric content of PVA fiber was 0.25%,0.5%and 0.75%respectively,the autogenous shrinkage was decreased by 7.1%,15.4%,and 20.7%.It can be seen that the higher the fiber content,the more pronounced the suppression of shrinkage.2.Autogenous shrinkage prediction models such as CEB-FIP model,Tazawa model and EN-1992 model,which are commonly used at present,are introduced.Based on the comparison results with the test results,a modified model suitable for this experiment based on the EN-1992 model is selected and revised.Considering the three factors of concrete strength,curing temperature and PVA fiber content,the revised autogenous shrinkage prediction model can well meet the experimental results.3.The early age restraint stress of different w/c concrete was studied under the full-restraint mode of TSTM.The decrease of w/c will increase the temperature rise and increase the restraint stress.From the perspective of cracking time and cracking probability,the increase of strength from C40 to C60 and C80 can improve the cracking resistance of concrete to a certain extent.Lightweight aggregates can reduce the cracking sensitivity of concrete.As the volume replacement rate of lightweight aggregates gradually increases from0%to 10%,30%and 50%,the cracking time of concrete is gradually delayed,and the averaged growth rate of tensile stress at the cooling stage is gradually decreased,and the cracking probability in 3^rd day decreased by 8%,11%,and 15%.With the increase of the volume replacement ratio of lightweight aggregates,the crack resistance of concrete gradually increases.The early stress relaxation effect of concrete has a very important influence on retarding the cracking time of concrete.The experimental result shows that the tensile stress of concrete at cracking is much lower than the calculated elastic stress,and the stress relaxation is more than half of the calculated elastic stress.