Determination Of The Double-K Fracture Toughness Of Concrete By Using The Specimen With A Bottom Notched Split-cube

Concrete is a heterogeneous composite material which composed of coarse and fine aggregates and cementitious materials.With the advantages of convenient material acquisition,high compressive strength,low price and simple manufacturing method,it has been widely used in various engineering structures such as hydraulic structures,bridge structures,building structures,etc.However,due to the low tensile strength of concrete materials,microcracks are easy to appear in the construction and normal use stage,which may further expand during the service period,leading to structural damage and engineering accidents.It is a direct and effective method to study the fracture characteristics of concrete by fracture test.So far,the commonly specimen geometries for fracture testing on concrete fracture are three-point bending test and wedge-splitting test.Compared with the traditional test method,the proposed test method in this study has the advantages of simple specimen preparation and convenient test operation,which provides an effective supplement for the existing experimental methods of determining concrete fracture parameters.Based on this,the research work and main conclusions are as follows:?1?Determination of double-K fracture parameters of concrete by using three-point bending beamThree-point bending beam test were conducted on concrete beams with prefabricated cracks at three different specimen sizes?500mm×100mm×100mm?900mm×200mm×100mm and 1300mm×300mm×100mm?and four initial crack ratios?0.2,0.3,0.5 and 0.7?.The crack initiation load,ultimate load,double-K fracture parameters and fracture energy were measured with the change of the initial crack ratios and specimen size.The test results show that the cracking load Pini and the peak load Pmax of the three-point bending beam increase with the increase of the specimen height,and decrease with the increase of the initial crack ratio.The critical equivalent crack length ac and the critical crack opening opening displacement CMODc gradually increase with the specimen height,and decrease with the increase of the initial crack ratio.In the research scope of this paper,the values of double-K fracture toughness measured by three-point bending beam are basically stable,and the calculation results have no obvious size effect.?2?Formula derivation of stress intensity factor and crack opening displacement of bottom notched split-cube specimenFinite element analyses were carried out using ANSYS code to simulate stress development and calculate stress intensity factor and crack mouth opening displacement in bottom notched split-cube specimen under external load.From the horizontal stress distribution diagram,it can be observed that the larger the initial crack length of the specimen,the easier it is to generate cracks from the crack tip,which provides a theoretical basis for the test method of bottom notched split-cube specimen.Through varying the initial crack ratio and the edge length of specimens,the formula of stress intensity factor at the tip of initial crack and the open displacement at the middle position of the crack can be obtained by fitting its geometrical function.?3?Experiment study on the crack propagation under bottom notched split-cube specimen by using DIC.The digital image correlation method was used to observe the split-cube specimens under four different initial crack ratios with the height of 100mm.From the DIC measurement results,it can be known that the fracture process zone length of the split-cube specimens increases with the variation of load before reaching the peak load.The length of the fracture process zone is related to the length of the initial ligament length.The fracture process zone length corresponding to the peak load decreases with the increase of the initial crack ratio.?4?Determination of double-K fracture parameters of concrete by bottom notched split-cubeBottom notched split-cube test were conducted with prefabricated cracks at three different side lengths?100mm?200mm and 300mm?and four initial crack ratios?0.2,0.3,0.5 and 0.7?.The crack initiation load,ultimate load and double-K fracture parameters were measured with the change of the initial crack ratios and specimen size.The test results show that the cracking load Pini and the peak load Pmax of the split-cube increase with the increase of the specimen height,and decrease with the increase of the initial crack ratio.The critical equivalent crack length a_c and the open displacement at the middle position of the crack CMOD_1/2 decrease with the increase of the initial crack ratio.Compared with the TPB specimens,the double-K fracture toughness of the split-cube specimens showed no significant difference,and the calculation results have no obvious size effect,which verifies the effectiveness of the method.