Fatigue Performance Of Strain-hardening Fiber-reinforced Cementitious Composite And Its Functionanlly-graded Structures

Strain-hardening fiber-reinforced cementitious composite is a cement-based material exhibiting significant pseudo-strain-hardening and multi-cracking behaviors with tensile strain capability of several percent.This novel material is designed with micromechanical principles,and it is also referred to as engineered cementitious composite(ECC),strain-hardening cementitious composite(SHCC),or ultra-high toughness cementitious composite(UHTCC).As a new class of cement-based material with high ductility and durability,it is promising to apply UHTCC in structures that are subjected to repeated loads or fatigue loads.Additionally,the rapid development of modern infrastructures places a greater demand on the service life of concrete structures under severe mechanical or environmental loadings.A solid knowledge of fatigue performance of UHTCC is hence important and urgent for the application of this material.The main objective of this research is to explore the fatigue performance of UHTCC material and its functionally-graded structures.1.The compressive fatigue behaviors of UHTCC at various stress levels are investigated.A probabilistic model of fatigue failure deformation considering the effect of stress level is proposed for a reliable application of this material.During the fatigue failure process,it is found that micro cracks initiate in the fatigue source region,propagate in the transition region and form main cracks in the crack extension region.Besides,three fatigue-induced failure modes of fibers are found.2.The frequency effect on the fatigue behavior of UHTCC in compression is studied.It is found that the fatigue life and deformation pattern are influenced by loading frequency.The relationship between the secondary strain rate,the secondary strain rate per cycle,and the fatigue life is illustrated and equations applied to predict the fatigue life of specimen are developed.A probabilistic model of the fatigue failure strain is proposed to describe the frequency effect.3.A novel model based on the three-parameter Weibull function is developed to describe thefatigue deformation behavior of plain and fiber-reinforced concrete(e.g.,UHTCC).All the model parameters have clear physical meanings.Additionally,a deformation-based method for prediction of the fatigue life of concrete is presented,and the prediction results demonstrate that the proposed model can be successfully applied to the estimation of the fatigue life of concrete materials.4.The tensile fatigue behaviors of UHTCC at various stress levels are investigated.Four stages are observed in the evolution curve of fatigue deformation.“Smooth" and“rough”areas are distinguished on the fatigue failure surfaces with different fiber failure modes.On the basis of the initial distribution of static strength,P-S-N(probability of failure-stress level-fatigue life)models are proposed for a reliable application of this material.5.The static and fatigue behaviors of UHTCC functionally-graded structures are investigated.For the rapid construction of the functionally-graded structures,a series assembled participating permanent formwork using UHTCC are designed and studied.The fatigue behavior of functionally-graded beams with different thicknesses of the UHTCC layers is investigated.The mechanism of the fatigue enhancement of the functionally-graded beam compared to a conventional reinforced concrete beam is revealed.These investigations extend the knowledge of the fatigue behavior of fiber-reinforced cement-based material and its functionally-graded structures.The proposed models and methods can be applied to predict the fatigue life and deformation of concrete materials in the areas of structural design,health monitoring,and maintenance of modern infrastructures.Strategies are also suggested to achieve a greater fatigue performance of UHTCC materials and structures.The findings of this doctoral research are important for the future application of UHTCC in practice,which may contribute to the safety and durability of our future infrastructure systems.