Research On The Development Of Restraint Stress And Relaxation Of Concrete At Early Age

Cracking in the concrete of constructions like foundation, bridge, tunnel lining and other medium concrete seems more acute in recent years, which severely impairs the durability of constructions. The study on the restraint stress in concrete at early age due to shrinkage is valuable to the evaluation of the cracking sensitivity. The investigation focuses on the restraint stress and stress relaxation of concrete under uniaxial restraint at early age and the modelling of prediction of restraint stress. The summary of the finding and conclusions from this study are presented below.1. The development of completely restraint stress due to temperature shrinkage and autogeneous shrinkage was studied. The effect of stress relaxation of early age concrete was also considered. The influence of the deformation and mechanical properties, as well as the effect of the stress relaxation were both considered in the restraint stress. The restraint stress could be seen as a compositive index of cracking sensitivity of concrete at early age.2. The study on the restraint stress revealed that the temperature stress is one of the most important factors which induce cracking in medium concrete. The temperature stress of concrete structural members increases due to the accelaration of strength and elastic modulus of concrete with high placing temperature and high temperature rise, while the internal restraint temperature stress increases due to high tmeperature difference between inner and surface concrete. The restaint stress due to autogeneous shrinkage of concrete increases with the decrease of water-cement ratio, which makes concrete more cracking sensitive.3. The experiment data of creep in literature were analysed to estimate the stress relaxation at early age with the use of rheological model. An approach for the modeling of restraint stress for normal concrete was explored on the basis of the disscussion on the mechanical properties and deformability. The prediction was proved to be precise enough for application in engineering and the model was believed to be available to predict the restraint stress and cracking sensitivity. 4. The restraint stress and elastic-visco behavior of High Volume Fly Ash concrete at early age was studied. It showed that the use of fly ash can not only reduce the temperature rise and temperature stress, but also relief part of the restraint stress as the relaxation occurs. However, the concrete was in tension in any case. The combination use of fly ash and expansion agent could eliminate the tensile stress and produce compressive pre-stress, which can improve the cracking sensitivity.