Experimental Study On Mode Ⅰ And Ⅱ Fracture Behaviors Of Concrete With Different Coarse Aggregate Volume Fractions

Due to the high proportion and skeleton function of coarse aggregates,the properties of coarse aggregates,especially the volume fraction,significantly affect the mechanical properties of concrete.Understanding the fracture mechanism of concrete with different volume fractions of coarse aggregate is beneficial to optimal design of concrete structures.This thesis focuses on the mode I and mode II fracture behaviors of concrete with different coarse aggregate volume fractions.The wedge splitting tests and non-notched single-edge compressing test were carried out simultaneously for concrete specimens which were prepared with coarse aggregates in different volume fractions of 19%,25%,31%,37%,43%and 50%.The technique of digital image correlation（DIC）was employed during the tests to analyze the deformation field on the surface of specimen.Based on the maximum paste thickness（MPT）theory and meso-mechanics of concrete,the fracture mechanisms of concrete under different coarse aggregate volume fractions were analyzed,and the meso-mechanical model for toughening and degradation was proposed.Additionally,the meso-scale simulations of corresponding mode II fracture tests were also performed by the cohesive zone model combined with the dot matrix method.The conclusions are summarized as follows:（1）When the coarse aggregate volume fraction is smaller,cracks mainly propagate along ITZ and pass through the cement paste,and thus coarse aggregate debonding occurs on the fracture surface which makes the fracture surface rougher.However,at the larger volume fraction of coarse aggregate,more cracks pass through the coarse aggregates,and the fracture surface is relatively smooth.（2）For mode Ⅰ fracture of concrete,with the increase of coarse aggregate volume fraction from 19%to 50%,the initiation fracture toughness K_ICⁱⁿⁱ,unstable fracture toughness K_IC^un and fracture energy G_IF first increase and then decrease,and the maximum values of these fracture parameters are obtained when the coarse aggregate volume fraction is 37%.The effect of coarse aggregate volume fraction on K_ICⁱⁿⁱ is more significant than that of K_IC^un.It was further found that the relationships between the values of K_ICⁱⁿⁱ,K_IC^un,and G_IF and compressive strength f_t can be described by exponential functions.As the coarse aggregate volume fraction increases,the ductility of concrete first increases and then decreases.When the coarse aggregate volume fraction is 37%,the best ductility is obtained.Moreover,the safety warning parameters for unstable fracture of concrete structures were proposed,which provides a reference for the design of concrete structure.（3）Both the mode II fracture toughness K_IIC and the critical energy release rate G_IIC increase as the coarse aggregate volume fraction increases from 19%to 37%,then decrease as the coarse aggregate volume fraction increases from 37%to 50%.On the other hand,the K_IIC(or G_IIC)vs.coarse aggregate volume fraction is almost symmetrical on both sides of the peak value when the coarse aggregate volume fraction increases from 25%to 50%.With increasing the coarse aggregate volume fraction,the peak load P_max of concrete specimen is enhanced almost linearly.The change of the critical compressive displacementδ_c against the volume fractions of coarse aggregate is almost identical to that of K_IIC.The typical fracture process zone（FPZ）of non-notched single-edge compressing specimen is a curved band.When the coarse aggregate volume fraction increases from 25%to 37%,there are more branches in FPZ and its shape is more irregular,so that the area of FPZ increases.If the coarse aggregate volume fraction increases further from 37%,the shape of FPZ becomes more regular and the area of FPZ decreases relatively.（4）The values of K_IIC/K_IC are not a constant,but strongly depend on the coarse aggregate volume fractions.With the increase of coarse aggregate volume fraction,the value of K_IIC/K_IC^un first increases and then decreases,whereas that of K_IIC/K_ICⁱⁿⁱ first decreases and then increases.Both the K_IIC/K_IC^un and K_IIC/K_ICⁱⁿⁱ obtain the extreme value at the coarse aggregate volume fraction of 37%.The effect of coarse aggregate volume fraction on K_IIC/K_ICⁱⁿⁱ is more significant than that of K_IIC/K_IC^un.In addition,K_IIC/K_IC^un is positively correlated with f_t,while K_IIC/K_ICⁱⁿⁱ is negatively correlated with f_t.And there is a logarithmic function relationship between K_IIC/K_IC(K_IIC/K_ICⁱⁿⁱ or K_IIC/K_IC^un)and f_t.（5）The coarse aggregate volume fraction has a significant influence on the FPZ characteristics of wedge splitting test specimens.With the increase of coarse aggregate volume fraction from 19%to 50%,both the developed length of the FPZ at peak load（D_f）and the fully-developed FPZ length（L_f）first increase and then decrease,and obtain the maximum values at the coarse aggregate volume fraction of 37%.The FPZ of concrete is a curved band around the fracture ligament.Furthermore,the FPZ is more irregular and tortuous as the coarse aggregate volume fraction increases from 19%to 37%,and then becomes straighter as the coarse aggregate volume fraction increases from 37%to 50%.The FPZ length can be predicted effectively and accurately by Saucedo’s analytical model combined with bilinear softening rule of Petersson.（6）Based on the MPT theory,the MPT toughening models were developed,which characterizes the relationship between the toughening effect of concrete and the coarse aggregate volume fraction.By modifying Huang-Li toughening model,the meso-mechanical model for toughening and degradation of concrete was proposed.The trapping-bridging of coarse aggregate plays a leading role in the toughening of concrete,and its toughening effect is much greater than that caused by crack deflection or crack shielding.（7）The mechanical response and fracture morphology from meso-scale simulation of mode II fracture of concrete are in good agreement with those of experimental tests.Furthermore,the scalar stiffness degradation development of the cohesive element can be utilized to characterize the damage evolution of concrete at meso-level,which is valuable to the prediction of macro performance.The cohesive zone model can better characterize the fracture behavior of concrete under complex stress state and predict the damage evolution of concrete,which will help to understand and reveal the physical and mechanical mechanisms of mortar,coarse aggregate and ITZ in the process of concrete deterioration.