Study On Strength Model Of Ultra-High Performance Concrete Considering The Effect Of Cement Hydration

In recent years,as the number of tunnels increases,the geological conditions traversed become more and more complex,and the differences in material supply,process level and construction equipment are obvious,making tunnel construction more and more difficult.At the same time,the traditional tunnel lining construction process relies heavily on the technical level of the construction personnel and is difficult to quantify and control.The lining defects and diseases of the completed tunnels are serious and the concrete material performance should be improved to cope with the extreme environmental and load-bearing conditions faced by the new application scenarios.The effective way to solve the problems related to concrete precipitation in water in hydraulic tunnels is to develop tunnel lining structures using Ultra-high performance concrete(UHPC)with high compressive strength,good toughness and durability.Therefore,a systematic study on the strength characteristics of UHPC is carried out in this paper.In this thesis,the mechanical properties of different raw materials were tested from the physical mechanism of the mix design in the initial stacking state,and the results of the influence of different raw materials on the strength characteristics of UHPC were investigated.The test results show that the use of silica fume can significantly improve the compressive strength of concrete,while the maximum strength of cement-silica fume-fly ash ternary cementitious material system can reach 120 MPa;combined with the close packing model to quantify the initial packing state of different raw materials,the strength trend of different mix ratios can be accurately predicted.In this thesis,a hydration model of the effect of binary/ternary cementitious material composition on the hydration process of cement is proposed,and the applicability of this hydration model is verified by different scholars’ studies on this basis.The simulation results show that fly ash and silica fume are involved in the exothermic process of hydration mainly including the secondary hydration by volcanic ash activity effect.The participation of fly ash,silica fume and limestone powder in the exothermic process of hydration also includes the microaggregate effect,because their particle size is smaller than that of cement,thus further filling the pores between cement particles and making them dense,thus increasing the strength of concrete.It also allows the release of water between the cement particles to become free water,thus improving the fluidity of the concrete.Based on the multi-component hydration model,this thesis investigates the dynamic change of hydration product volume with time during the hydration reaction,quantifies the magnitude of the volume change of hydration products,and obtains a specific characterization of the effect of hydration as a chemical process on the microstructure and stacking state of concrete,starting from the influence of hydration products on both microstructure and stacking density.Finally,an empirical formula for the strength of ultra-high performance concrete considering the hydration process is proposed,and a parameter system is established to better predict the strength of concrete composed of different cementitious materials.This model is validated based on previous mechanical test results,and a simplified design process for the mix ratio based on this model is proposed.To further validate the applicability of the model,23 sets of mix designs from different literature were used for verification.The results show that the proposed method can be used to achieve strength prediction by conducting a small number of experiments.