Study On The Stress Relaxation Properties Of Self-Compacting Concrete Under Autogenous Shrinkage Being Restrained

Self-compact concrete(SCC)carries the characteristics of low water-binder ratio and high dosage of binder materials,its shrinkage mainly consists of autogenous shrinkage,therefore it is subject to cracking risk when constrained condition is applied.Stress relaxation performance is directly related to the amount of residue stress under constrained shrinkage,making it critical to cracking performance of concrete.Since the occurring mechanism and developing rule for basic creep and drying creep are different,and the stress relaxation performance of concrete under autogenous constrained shrinkage differs from that of drying shrinkage or mix shrinkage,it is therefore necessary to investigate the stress relaxation performance of SCC under autogenous constrained shrinkage.Water-binder ratio,amount of cementitious material,constraint level were taken as parameters,and the stress relaxation performance of SCC under autogenous constrained shrinkage condition was investigated by free antogenous shrinkage and ring constrained shrinkage.The detailed work is given as follows:1.Basic mechanical properties of SCC such as cubic splitting tensile strength,elastic modulus were carried out to provide basis for theoretical stress calculation and anti-cracking risk analysis of concrete.2.On the basis of optimization and improvement of testing method,the development relationship between autogenous shrinkage and time and the variation rule caused by different parameters were studied.Autogenous shrinkage test result which was gathered since the completion of molding and was of good repeatability was obtained,and the second exothermal peak of exothermic curve the peak of hydration heat rate second heat emission of slurry was chosen as "starting time" by hydration heat experiment of slurry sieved from SCC with duplicate mixing proportion.Based on the result of autogenous shrinkage and elastic modulus of critical ages,theoretical analysis was conducted and theoretical elastic stress was thereby obtained.3.Sealed specimens were used to carry out ring constrained autogenous shrinkage experiment,where different constraint levels were simulated by adopting different steel ring thickness.Based on strain occurred in the inner side of steel ring,residue stress of concrete with autogenous shrinkage constrained was calculated,and by comparison between residue stress and splitting tensile strength,cracking risk caused by each influencing factor was analyzed and obtained.4.The results showed that the problem of big data noise,susceptibility to external thermal influence existing in eddy current external test method recommended by GB/T 50082-2009 could be solved and test result with good stability and repeatability could be obtained when computer was employed to collect data from the beginning of completion of molding and vibrational chord strain gauges were used to monitor autogeous shrinkage of concrete.The study shows that the problem of the eddy current testing method,which is recommended by GB/T 50082-2009,and has the characteristics of large data noise,easy to be influenced by the external temperature,and it can obtain autogenous shrinkage of stability and repeatability can be solved by taking the embedded type vibrational chord strain gauge collected continuously since moulded.The research result lasting for 100 days shows the free autogenous shrinkage doesn’t mainly occur in early ages,it generates slowly and grows steadily after a slight expansion within one day.With the increase of the water-binder ratio,the antogenous shrinkage of all ages significantly lowered.With the increase of the amount of cementitious material,the antogenous shrinkage has an increasing trend.The stress relaxation of restrained autogenous shrinkage mainly occurs in the early age,and the stress relaxation rate decreases gradually with the increase of age.The stress relaxation rate increased with the water-binder ratio and the amount of cementitious material increasing while the influence of constraint level is not significant.Concrete will be influenced by autogenous shrinkage in the long-term,autogenous shrinkage stress fails to be alleviated by stress relaxation in long age.The effect of autogenous shrinkage with low water cement ratio,high amount of cementitious material of the concrete should be captured to the long-term.Test result of 100 days long indicated that antogenous shrinkage,instead of occurring at early age,took place and developed slowly and continually after small extent of expansion within 1 day;with the increase of water-binder ratio,autogenous shrinkage of each age decreased noticeably;with the increase of addition of cementitious material,autogenous shrinkage of each age exhibited increasing trend;stress relaxation constraining autogenous shrinkage stress mainly occurred at early age,stress relaxation rate gradually decreased and reach flatness with the increase of age;stress relaxation rate increased with the increase of water-binder ratio and cementitious content,while the impact of constraint level was not remarkable;the impact autogenous shrinkage on concrete is long-term,long-term autogenous shrinkage stress was difficult to be alleviated by stress relaxation,and the long-term effect of autogenous shrinkage on low water-binder ratio,high cementitious material content is noteworthy.