On The Design And Preparation,Microstructure And Tensile Behaviors Of Gap-Graded Cement-Based Engineered Cementitious Composite

Cracking of cement-based materials is one of the most important factors that influence the durability of structures.Control of the cracks in cement-based materials is good for service life prolongation,and environmental load reduction of infrastructures.Strain hardening cementitious materials or Engineered cementitious materials?SHCC/ECC?provide a new idea for crack width control problem in cement-based materials.By deliberate design of the amount and the mechanical properties of matrix,fiber,and the interface between them,ECC presents a tensile strain capacity that is hundred times than traditional concrete and excellent crack width control ability?less than 100?m?.Since the design theory sets crucial limits on the parameters of the multiphase in ECC,it is common that ECC cannot present satisfied stable strain hardening property and multiple cracking behaviors with the interference of randomness.As a result,ECC is not widely applied nowadays.Matrix is an important part of ECC,and the present research corresponding to matrix design such as introducing large flaws or applying a larger surplus strain hardening parameter in the design process will be accompanied by raw material limitation or deterioration on the structure and performance of ECC.To fundamentally narrow the gap between the practical matrix and the theoretical matrix caused by the inhomogeneous phenomenon in the matrix,the ECC matrix design theory is specified from the view point of homogenized matrix solid phase in the present study.Guided by gap-graded blended cement theory,the microstructure of ECC matrix solid phase?except flaws with the size of millimeters?is designed to reduce the formation probability of large packing gaps and to standardized the interface between large unhydrated particles and hydration products,then a series of gap-graded matrices and the corresponding ECCs were prepared.In addition,the micro mechanical parameters in the matrix solid phase were corrected with experimental data and theoretical calculation.Furthermore,the finite element model of gap-graded ECC was established to analyze the enhancing mechanism of strain hardening performance and multiple cracking behaviors.The main research works are included afterwards.With the guidance of gap-graded blended cement theory,the gap-graded ECC matrix were designed,prepared,and characterized,then the relationship between the matrix fracture toughness,the matrix homogeneity,and the tensile behavior of ECC were established.The mechanical performance of matrix solid phase is mainly controlled by the packing gaps and the large interface between unhydrated particles and hydration products.By applying the gap-graded particle size distribution theory,the initial packing density of matrix is increased and the distribution homogeneity is improved.The large interface is standardized by limiting the type and size of particles in each size fraction,especially the larger size fractions to avoid random distribution of various interface and the corresponding performance fluctuation caused by inhomogeneous matrix.According to this design principles,a new type of gap-graded ECCs was prepared and its fracture toughness is tailored to approach that of the ref-ECC.The gap-graded ECC presented an initial cracking strength of 2.31 MPa similar to the ref-ECC,and better tensile behaviors?with a strain capacity of 4.83%and an average crack spacing of 1481?m?than ref-ECC.In the second part,the microstructure of ECC matrix solid phase is analyzed,and the micro mechanical parameters of the multiphases are calculated and corrected with experimental data and simulation,then the matrix mechanical property and its discreteness are quantitatively evaluated.Specifically,the complicated matrix solid phase is simplified in micro-structurally designed cement matrix,and the structural model of Portland cement paste and micro matrix are constructed in microscale,then the structural model of micro-structurally designed matrix is constructed in meso scale.Afterwards,considering the effect of water content,the type and the size of the added particles,the micro mechanical parameters in micro-structurally designed cement matrix are used in model construction of gap-graded ECC matrix and ref-ECC matrix.By multiple loadings on these two complicated matrix models,their mechanical properties and the related discrete range were calculated and the distribution patterns of the shrinkage stress in these matrix solid phases were analyzed accordingly.According to the multiscale simulation theory of ECC,a lattice finite element model of gap-graded ECC was constructed considering the concepts of 2-dimensional simulation,elements with the brittle and ductile constitutive model,regular grid,and randomly distributed pores.By applying a homogeneous hypothesis on the matrix solid phase,ref-ECC formed 9 cracks while gap-graded ECC and homo-ECC formed27 cracks and 48 cracks under tensile loading in simulation.The tested crack numbers of gap-graded ECC and homo-ECC are in the predicted range according to the simulation.However,the predicted results of the initial cracking strength and crack number of the ref-ECC in simulation are distorted with the tested results,resulting from the randomly mismatched distribution patterns of the inhomogeneous shrinkage stress and the introduced pores with the sizes of several millimeters.In this research,gap-graded fly ash particles and ultrafine cementitious particles are used and a type of gap-graded ECC is prepared with a tensile strain capacity of4.83%,an initial cracking strength of 2.31 MPa,an averaged crack spacing of 1481?m,and a compressive strength of 32.5 MPa.The micro mechanical parameters of the multiphases in the gap-graded ECC are calculated based on experimental data and theoretical simulation.Accordingly,the finite element model of gap-graded ECC is constructed to analyze the relationship between the mechanical property of matrix,its discreteness,and the tensile behavior of ECC.The results provide theoretical foundation and technical support for the preparation of high performance ECC with larger amount of industrial waste?especially for low activity or inert industrial wastes?.By this method,a huge amount of resources and energy can be saved and the CO₂ emission can be significantly reduced,resulting in numerous economic,social,and environmental benefits.