Study On Early-Age Shrinkage Performance And Cracking Tendency Of High Performance Concrete

Early-age cracking of High Performance Concrete (HPC) usually occurs in field engineering. The occurrence of early-age cracks reduces the service performance of concrete structures. Early-age cracking also deteriorates the durability of concrete engineering and shortens its service life. In order to optimize structural design of HPC and ensure its service ability and long-term durability, early-age cracking needs to be further studied. The mechanism of early-age cracking should be studied combining the investigation of setting and hardening, shrinkage, shrinkage stress, tensile creep and cracking resistance. Scientific and reasonable cracking criterion should be established to predict early-age cracking relative precisely.Ultrasonic nondestructive detection method was utilized to investigate the development of acoustic parameters of ultrasonic wave passing through the HPC mortar. According to the investigation results, the setting time was determined. The results indicated that initial setting time corresponded to the time at which the steep increase of ultrasonic dominant frequency occurred. Final setting time corresponded to the time at which ultrasonic pulse velocity started to increase slowly after rapid increase stage. Through ultrasonic method, setting and hardening were investigated according to hydration process of cementitious materials and development of internal micro-structures. So the results of ultrasonic method characterized the setting principle. Compared to penetration resistance method, ultrasonic method exhibited operational and technical advantages.The internal Relative Humidity (RH) change of HPC in sealed condition was investigated. The self-desiccation induced RH change was predicted based on the maturity and hydration degree theory. The calculation model of early-age moisture diffusion coefficient was modified. Based on moisture diffusion theory, the internal RH distributio of HPC under unilateral dryng condition was predicted. The results indicated that w/c (water to cement raio) and mineral admixtures have significant influence on internal RH change induced by self-desiccation. A simple linear relationship exsited between self-desiccation induced RH change and hydration degree. Prediction model of self-desiccation induced RH change could be established based on the mentioned linear relationship. Relatively high correlation was obtained between the predicted results and investigated results. Based on Fick's second law, internal RH distribution prediction considering interaction between self-desiccation and moisture diffusion was relatively resonable.Self-developed non-contact shrinkage measuring device was applied to investigate early-age autogenous shrinkage and shrinkage distribution under unilateral drying condition. The influencing factors of autogenous shrinkage and shrinkage distribution were studied. Early-age autogenous shrinkage prediction model was established according to capillary pressure theory. Based on the linear relationship between free shrinkage and RH, prediction model of shrinkage distribution under unilateral drying condition was established. The results indicated that w/c and mineral admixtures had significant influence on autogenous shrinkage and shrinkage distribution. For HPC with water to binder ratio of 0.40, Relative high correlation existed between predicted autogenous shrinkage and investigated data. For HPC with lower w/c, chemical shrinkage of paste was more dramatic and setting process lasted longer time. So contribution of chemical shrinkage to autogenous shrinkage was more significant. This resulted that predicted autogenous shrinkage was lower than investigated data. Through the comparison of predicted shrinkage distribution and experimental data, existence of interlayer restraint was verified. Considering interlayer restraint, the predicted shrinkage distribution corresponded well with the experimental data.Based on investigated development of early-age elastic modulus and splitting tensile strength of HPC with age, cracking sensitivity of restrained HPC was analyzed. Through the modified restrained ring test, shrinkage stress, stress gradient, tensile creep of restrained HPC were studied. The influencing factors of the mechanical properties mention above were also studied. Through the application of micro-technique, surfacial and internal cracking of restrained concrete ring was investigated. Based on the experimental work mentioned above, early-age cracking tendency of restrained HPC was analyzed comprehensively. The results indicated that restrained HPC exhibted early-age cracking sensitivity. Curing conditions, superplaticizer, w/c and mineral admixtures had significant influences on shrinkage stress. Curing conditions, w/c and mineral admixtures had significant influences on early-age creep performance. Reduction of w/c, effect of drying condituon and addition of silica fume resulted in higher early-age cracking tendency of restrained HPC. The influence of addition of grouded blast-furnace slag on early-age cracking tendency was not evident.Considering early-age visco-elastic properties, shrinkage stress in restrained HPC was predicted. Early-age cracking criteria were established based on strength and energy theory. Accroding to the predicted shrinkage stress and established cracking criteria, early-age cracking age of restrained HPC was predicted. The results indicated that before release of residual shrinkage stress, predicted shrinkage stress considering visco-elastic properties approached to experimental data. Predicted cracking according to strength and energy theory occurred earlier than investigated cracking. The difference between predicted cracking age and investigated one was not great. This showed that early-age cracking prediction method mentioned above was relatively effective. The influence of initial crack on early-age cracking was considered during the determination of cracking resistance based on energy theory. So predition method based on energy theory was more suitable for early-age cracking prediction of restrained HPC. Predicted results according to energy theory presented greater security and reliability.