Study On The Influence Of Shale Ceramsite On Type Ⅰ Fracture Properties Of Concrete

When aggregate prevents the propagation of crack in the concrete structure,the expansion path can be divided into two types.One of the expansion path is that if the cement mortar strength is relatively low,the crack may bypass the aggregate and expand along the cement mortar-aggregate interface.The other type of expansion path is that if the strength of the cement mortar increases,the bond strength between the coarse aggregate and the cement mortar increases relatively,or the aggregate strength decreases,the aggregate becomes a weak link of the concrete,and the crack may propagate through the aggregate at this time.Therefore,the type of aggregate and the strength of cement mortar have a great influence on the crack propagation in concrete.Regarding the problems mentioned above,in this text,the crack propagation process and fracture characteristics of concrete three-point bending beam specimens with different water binder ratio,different aggregate types and different shale ceramsite lightweight aggregate volume substitution ratios were compared and analyzed.The main work and conclusions of the paper are as follows:1.Ordinary gravel aggregate concrete,shale ceramsite lightweight aggregate concrete and cement mortar specimens with prefabricated cracks,of which the water binder ratio was 0.48,0.41 and 0.35 had been conducted using the three-point bending beam method.The test results show that when the aggregate type is the same,initial fracture toughness KICini,the unstable fracture toughness KICun,and the fracture energy GF increase with the decrease of the water binder ratio.Under the different conditions of the same type of aggregate,the double K fracture parameters and fracture energy are different and the numerical values show that gravel aggregate concrete is the largest,followed by cement mortar,and shale ceramsite lightweight aggregate concrete is the smallest.2.After changing the composition of coarse aggregate of gravel aggregate concrete,three-point bending beams with different volume fractions(0%,20%,40%,60%,80%,100%)of shale ceramsite lightweight aggregate mixed with gravel aggregate concrete were tested.The results show that KICini and KICun decrease with the increase of volume substitution rate of lightweight aggregate,and both of them have great linear correlation with volume substitution rate.After the analytical solution of the cohesive toughness was calculated according to the bilinear softening constitutive relation,then it was compared with the experimentally obtained cohesive toughness KIC,TC.It is found that the experimental values are in good agreement with the analytical values,which indicates that the bilinear concrete softening constitutive curve can generally reflect the fracture characteristics of concrete with different volume substitution ratios of shale ceramsite.3.The influence mechanism of coarse aggregate,matrix material of cement mortar,microstructure of interface transition zone between aggregate and cement mortar on macroscopic mechanical properties and fracture properties of concrete was analyzed by scanning electron microscopy(SEM).It is found that the gravel aggregate is closely arranged by irregular polygon crystals,and the shale ceramsite is porous network structure which determines the characteristics of water absorption and returning.When the water binder ratio is same,the lightweight aggregate-cement interface area is greater than gravel aggregate-cement stone interface area,and the hydration reaction of cementitious material in the interface area of the former is more sufficient.