The Effect Of Age On The Fracture Characteristics Of Fully-graded Concrete And A Practical Analytical Method To Determine K_R-curves Of Concrete

Concrete is a kind of widely used construction material, with high compressive strength, low tensile strength and apt to cracking. The fully-graded concrete with a maximum aggregate size of150mm is widely used in concrete dam. The fully-graded concrete has a large difference with ordinary concrete. The fracture indexes of ordinary concrete measured cannot truly represent and fully reflect the actual fracture properties of fully-graded concrete. Age is the main factor affecting the concrete strength and deformation. Studying the effect of age on the fracture characteristics of concrete has important engineering significance. In order to describe the crack propagation in concrete, the study of KR resistance curve is important. According to the above problems, the main research work is as follows:1、 The fully-graded concrete wedge splitting specimens whose size is800mm×800mm×450mm, the wet-sieved concrete wedge splitting specimens200mm×200mm×120mm and the two-graded concrete wedge splitting specimens200mm×200mm×120mm were cast in the construction site repectively. The fracture parameters were tested in28days,90days and180days age. It was found that the intial fracture toughness KICim, unstable facture toughness KICun and fracture energy GF of fully-graded concrete increase with age; the intial fracture toughness KICini and unstable fracture toughness KICun of wet-sieved concrete and two-graded concrete increase with age and then tends to be stable; but the fracture energy GF of wet-sieved concrete and two-graded concrete increase with age and then decrease;2、 Based on the crack propagation criterion for mode I crack, an analytical method is developed for determining the crack extension resistance curve of concrete. In this method, the crack extension resistance is composed of the initial fracture toughness KICini and the resistance caused by cohesive force, equaling the driving force of crack extension. It is concluded that as long as the initial fracture toughness KICini, the elastic modulus E, the tensile strength ft and the fracture energy GF of concrete are obtained, the KR extension resistance curves can be calculated by using this method without testing the load-crack mouth opening displacement curves. Further, the analytical method is applied to calculate the KR extension resistance curves, the fracture process zone lengths and cohesive stress distribution for concrete with varied strength grades from C20to C120, different initial crack length/depth ratios and different depths under the three-point bending condition. Based on the numerical results, it is found that the KR extension resistance curves grow with the increase of strength grades, but the growth amplitude decrease, when the strength grades is larger than C60, the growth amplitude tend to stable values; the KR extension resistance curve is dependent of the initial crack length and specimen depth, when the ligament is long enough, the crack extension resistance increases and then tends to be stable value; the maximum of the complete fracture process zone lengths decreases with the increase of concrete strength grades; the fracture process zone length is dependent of specimen size.