Multi-scale Design And Static And Dynamic Mechanical Properties Of Ultra-high Performance Concrete

Ultra-high performance concrete（UHPC）,with excellent mechanical properties and durability,has a very broad application prospect in super high-rise buildings,long-span bridges and major infrastructure.UHPC has multi-scale characteristics and complex raw materials components,including large amount of cementitious materials,steel fibers and high efficiency superplasticizer,and each component can significantly affect the mechanical properties of concrete.The current mix proportion design of UHPC is mainly based on the“dense packing”theory,which means that if the granular material has the highest bulk density,the concrete achieves the optimal mechanical properties.However,the method does not take into account the hydration of the multiple cementitious materials,although the hydration degree is very low,but it still has an important impact on the mechanical properties of UHPC.In this paper,based on the multi-scale transition theory,the composition design method based on the micro-meso-macro level was established to provide guidance for the mix design and mechanical performance regulation of UHPC.The main work is as follows:（1）The hydration microstructure model of UHPC was established:Through the experimental techniques,the characteristic parameters of hydration microstructure of UHPC,such as the content of cement clinker and Ca（OH）₂,the reaction degree of fly ash and silica fume,and the porosity of the matrix,were determined.The results showed that the cement hydration mainly occurred in the first seven days,and the reaction amount was small and the speed was slow in the late stage.After 3 days of hydration,the fly ash and silica fume basically did not react.When the hydration age reaches 28 days,the reaction degree of silica fume is higher while that of fly ash is still low.Based on the CEMHYD3D model,the hydration microstructure evolution model for the ternary cementitious system of cement,fly ash and silica fume was established under the condition of extremely low water to binder ratio.The simulation results of phase composition and porosity corresponded well with the test results,indicating that the established model can better predict the evolution behavior of UHPC microstructure.（2）Quantitative analysed the mechanical properties of UHPC microstructures:Based on the hydration microstructure model,the three-dimensional lattice model for the UHPC microstructure was established,and the stress-strain curve of microstructure under tensile stress was calculated,combined with the mechanical properties of each phase in paste.Then,the interface between hydration microstructure and the finite element software of ANSYS/Ls-prepost was developed,and the finite element solid element model was established.The mechanical calculation results show that the the lattice model and the solid cell model are similar and both can be used to analyze the mechanical properties of UHPC at the microscopic scale.（3）A three-dimensional random packing model of aggregate particles was developed and the mechanical properties of meso-scale mortars were calculated:Based on the random packing algorithm,the generation and overlapping detection method of arbitrary convex polyhedron particles was proposed,and a three-dimensional packing model of aggregate particles was established.The identification algorithm of mortar matrix,aggregate and interface transition zone（ITZ）components was developed,then a three-dimensional meso model of mortar was constructed.Furthermore,combined with the mechanical parameters of microstructure,the mechanical properties of mortar at the meso-scale were analyzed.（4）A three-dimensional random packing model of the fiber-aggregate hybrid structure was developed and the mechanical properties of UHPC at macroscopic scale were calculated:The generation and overlapping detection algorithm of steel fibers was proposed,and the packing model of straight and hooked fibers was developed.The overlapping detection algorithm of steel fiber and aggregate were proposed,and a three-dimensional packing model containing steel fibers and aggregates two-phase materials was constructed.In addition,a meso-scale model containing mortar matrix,coarse aggregates,ITZ and fiberes was established.Combined with the mechanical properties of mortar,the mechanical properties of UHPC at macro-scale were calculated.（5）Based on the Mori-Tanaka approach and Upscaling transition theory,the elastic modulus of UHPC was quantitatively calculated as follows:Based on the evolution of hydration microstructure model of UHPC,the information of each phase content in the cement microstructure was determined.The elastic modulus of the UHPC microstructure was also calculated by combining the Mori-Tanaka theoretical method.Based on the Upscaling transition theory method,according to the volume fractions of fine aggregates,fibers and coarse aggregates,the elastic modulus of mortar and UHPC were determined respectively.（6）Based on the multi-scale composition design method,the influence of raw materials on the mechanical properties of UHPC was studied,and the optimal ratio range of material components was determined:The compressive strength,flexural strength and elastic modulus of UHPC with different mix proportions were calculated.The results showed that,1)The fine aggregates have little effect on the compressive and flexural properties of UHPC,but can help to enhance the elastic modulus of UHPC.In addition,coarse aggregates can significantly improve the elastic modulus of UHPC.2)The compressive strength,flexural strength and elastic modulus increase with the increase of fiber content,while the fiber type has little effect on the mechanical properties of UHPC.3)Changing the amount of three kinds of cementitious material results in the differences of microstructure,which would affect the strength and elastic modulus of UHPC.For the raw materials used in this paper,the compressive strength,flexural strength,and elastic modulus could reach the highest when the mass ratio of cement,fly ash and silica fume is 13:4:3,13:5:2,and 12:5:3,respectively.（7）The dynamic mechanical properties of UHPC was predicted:A three-dimensional meso-scale model of UHPC impact compression was established,and then the dynamic compression mechanical properties of UHPC with different mix ratios were predicted based on the multi-scale composition design method.The numerical simulation calculation results corresponded well with the experimental results.The dynamic mechanical properties of UHPC were excellent,showing typical strain-rate effects,and its dynamic increase factor showed a bilinear trend with the increase of strain rate.The multi-scale design method can be used to predict the dynamic mechanical properties of UHPC.