Tensile Creep Characteristics And Its Evaluation Of High Strength Concrete Containing Mineral Admixtures At Early Ages

The early-age volume deformation of high strength concrete?HSC?is one of the key factors in terms of concrete cracking.The tensile creep is an important component of the volume deformation of concretes at early ages,so the tensile creep of early age concrete is an important content and basis for the prediction and control early-age cracking of concrete,which is of great significance in theory and practice.Adding mineral admixtures into HSC,such as fly ash?FA?and blast furnace slag?BS?,is an economical and effective method to improve the crack resistance at early ages.Under restrained conditions,the cracking potential of HSC at early ages is closely related to its internal stress development,volume deformations,restraint form and degree of restraint in structure,and the development of properties including elastic modulus,tensile strength and tensile creep.The tensile creep plays an important role in delaying the early cracking potential of HSC,because it changes the internal stress distribution,and alleviates the development of HSC‘s internal tensile stress under the constraints.HSC?C50?containing mineral admixtures were the main research object in this thesis.The experimental parameters were:type of admixtures?FA,BS?and contents of admixtures?Referece group-Ref.,group with 10%FA-FA10,group with 20%fly ash-FA20,group with 30%fly ash-FA30,group with 50%BS-BS50,and group with20%FA and 25%BS-FA20BS2?,loading levels?stress level at 0.2,0.3 and 0.4?,and loading ages?1d,2d,3d,5d and 7d?.The tensile Young‘s modulus of the HSC pastes and the tensile creep characteristics at early ages of a series of HSC mixtures were studied based on test results under the above-mentioned experimental parameters.The applicability of the existing tensile creep models to predict the earl-age tensile creep of HSC with admixtures was anlysized.In order to explore the influence of mineral admixtures on early-age tensile creep of HSC,a mineral admixture parameter was proposed in modified ZC model,and the feasibility the modified model was verified with the independent experimental results.Within the scope of this dissertation,the main conclusions are summarized as follows:?1?The tensile modulus of HSC with admixtures decreased at the early age compared with HSC without admixtures.This reflects that the tensile stiffness of HSC with admixtures decreased,the flexibility increased,and its deformation resistance increased.However,the development of the paste‘tensile elastic modulus with age was not the same as that of concrete.The tensile elastic modulus of concrete was significantly greater than that of the paste at the corresponding age,which was more than twice the tensile modulus of the paste.?2?The hydration temperature rise of HSC with FA was significantly lower than the Ref.concrete.However,the hydration temperature rise of HSC with BS was higher than that of the Ref.concrete,and the age at the temperature peak was earlier than that of the Ref.concrete.The value of autogenous shrinkage in each group with FA at 28 ages decreased by more than 20%,but the value of autogenous shrinkage did not increase monotonously with the increase of FA content.The BS also can reduce the autogenous shrinkage of HSC,but the effect was not as significant as FA.?3?The influence of FA on early-age tensile creep of HSC increased with the increase of FA content.The"micro aggregate effect"of FA was significant when the loading ages were 1 d,2 d and the content of FA was 10%,and this can restrain the early age creep of HSC.The BS can weaken the early-age tensile creep of HSC.The initial loading age has a significant effect on the tensile creep of HSC.At the initial stage of loading,the development of tensile creep was faster.The value of tensile creep of HSC under loading for 3 d was more than half of that under loading for 28 d.In the Ref.concrete,the tensile creep value when the initial loading age was 3 d with load holding to 28 days was 71%of that when the initial loading age was 1 d,and it was 37%when the initial loading age was 7 d.This phenomenon was more obvious in HSC with FA,and this trend was enhanced with the increase of FA content.Similar rules also existed in the HSC with BS.?4?The early-age tensile creep of HSC with admixtures did not follow the linear rule of"Davis Glanville"for all loading ages.The experimental results showed that the"Davis Glanville"linear rule of creep was still applicable to the HSC without admixture.The early-age tensile creep of HSC with admixtures also showed linear creep characteristics after a certain age,but the loading age with linear characteristics was later than that without admixture,and the the linear characteristics were more obvious with the loading age more later.The influence of loading age was more significant with the increase of admixture content.?5?The tensile creep velocity was quickly when the loading age was earlier in the early stage of loading,and the tensile creep velocity decreased as a power function.In the middle stage of loading,the creep velocity gradually became stable and decreased as an exponential function.In the late stage of loading,the creep velocity was stable basically,and the difference of each HSC‘s creep velocity values was small.The influence of the admixture on the tensile creep velocity was more significant while the initial loading age was earlier.The tensile creep velocity was more quickly with higher the content of FA.The influence of admixtures on the tensile creep velocity decreased gradually with the extension of the load-holding period compared with HSC without admixtures.The influence of admixtures was the most significant in the early stage of load holding,and decreased in the middle stage of load holding,and it can be ignored in the later stage of load holding.?6?Comparative analysis between the early-age tensile creep of FA30‘s experimental values and the predicted values of six creep prediction models were performed.The results showed that models as BP-2;B-3;MC2010;ACI209R and GL2000 were not suitable for predicting the early-age tensile creep of HSC when the loading age was earlier than 7d,and they were also not suitable for the predicotn of the early-age tensile creep of HSC with admixtures.The ZC tensile creep model improved the mechanism of model establishment from the physical and chemical significance of model parameters considering the change of physical properties of cement stone components with age.It had a clear direction in determining the model parameters.The ZC model had a great advantage in predicting and evaluating the early-age tensile creep of HSC with admixtures.The predicted values of ZC model were in good agreement with the experimental values when the loading age was 1d,2d,3d,and there was a big deviation between the predicted values of other five models and the experimental values.As the loading age was 5 d and 7d,the ratios deviation of the predicted values of ZC,GL2000 to the experimental values were close to 0.8,and the predicted values were smaller than experiment values;and the ratios of the predicted values of ACI2009R to experimental values were close to 1.0,which was more accurate.In general,ZC model was more suitable for the prediction and evaluation of early-age tensile creep of FA30 concrete.?7?When adding FA,the admixture parameterq'in modified ZC model has a linear correlation with the loading age,and the correlation tends to be closer with the increase of FA content;while adding BS,the linear relationship disappeard and tended to be an exponential function relationship.The_M??,t ₀?in modified ZC model declined with the loading age,well following the law of exponential function.The(?,ta good linear decreasing law with the loading age.The relationship between parameters?and loading age was similar to that of parameters _MC??,t ₀?,showing the decreasing relationship following an exponential function,but the correlation with exponential function was not as obvious as that of_MC??,t ₀?.The influence of admixtures on parameters E_H/_VE was very significant,and the influence of FA was greater than that of BS.There was a good linear relationship between the parameters E_H/_VE of the control HSC without admixture and the loading age,and when the admixtures were added,the relationship between the parameters E_H/_VE and the loading age followed a power function.?8?The comparison between prediction values of the modified ZC model and the experimentals value of others showed that the deveiation of prediction values of the modified ZC model at different FA contents and the experiment values fluctuated,the deviation 93%of the measurement points were within 15%,which meant good applicability of the modified ZC mode.