Hydration Process And Correlation Of Macro-and Meso-/Micro-Properties Of Early-age Concrete

Early age is the most crucial period in the formation and development of various properties of concrete. With the proceeding of hydration, the microstructure of concrete evolves vehemently and the mechanical properties of concrete develop rapidly. Studying the hydration process, the microstructure development and the correlation between macro-and meso/micro-properties of early-age concrete are of great importance for understanding the microscopic origin of the mechanical properties of concrete. The main research works of this thesis are as follow:The hydration process of early-age concrete was divided into seven stages by non-contact electrical resistivity measurement, and the formation mechanism of each stage was interpreted. The threshold and the duration of the induction period were defined. Based on microstructure observations and theoretical calculation, a new interpretation on the formation of the second deceleration period and the third acceleration period of the electrical resistivity was proposed. The effects of water to binder ratio and mineral admixtures on the hydration process of early-age concrete were studied. The evolution of the pore structure of early-age concrete was studied via MIP measurement. Prediction equation of the capillary porosity of early-age concrete was established with consideration of the bi-phase conductive properties of concrete, and the evolution of solid phase density with age was quantitatively described through parameter analysis. Prediction equation of the tortuosity of early-age concrete was established, and the evolution of the tortuosity of early-age concrete was predicted and analyzed.A new testing method for concrete permeability was developed based on the non-contact electrical resistivity measurement, and the evolution of the solution permeation characteristic parameter of cement paste of various cement to binder ratio and concrete of various mix proportions during the age of12hours and28days were tested and analyzed. The evolution of the permeability of concrete at different stages was interpreted from the perspective of meso-/micro-structure development, and a new definition of the capillary depercolation threshold of concrete was proposed.An improved model for elasticity prediction of early-age concrete was developed. A crucial parameter, the aspect ratio of hydrates, was introduced in the improved model to consider the morphological effect of early-age hydrates. The effect of the aspect ratio of hydrates on the mechanical threshold and the development of elasticity of early-age concrete were analyzed, and the aspect ratio of hydrates was determined by electrical resistivity measurement. The prediction results by the improved model were compared with experimental results. It was found that the improved model significantly improved the prediction accuracy for the mechanical threshold, the elasticity of cement paste and mortar of hydration degree below0.3and the elasticity of concrete of hydration age less than1day. An improved model for elasticity prediction of hardened concrete was established. The most recent discovery on the microstructure composition of hardened cement paste (The Ultra-High-Density C-S-H and nanoCH) was in the first time taken into consideration in the proposed improved model. The real morphology of hydration products and anhydrous clinkers was also taken into consideration in the improved model. The validity of the improved model was verified through comparison with experimental data and some advice on further amelioration of the improved model was proposed.The development of compressive strength of concrete from early to later ages, and the effects of mineral admixture on the development of compressive strength of concrete at different stages were studied. The correlation between the compressive strength and the Formation Factor was established. A new correlation equation between the compressive strength and the meso-/micro-structure characteristics of early-age concrete was established. The tortuosity parameter was introduced in the new correlation equation to consider the effect of the development of the solid phase. Finally, the mechanism of the development of the compressive strength of concrete after the age of3days was interpreted from the perspective of solid phase evolution.