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Mode I fracture toughness and tensile strength investigation on molded shotcrete brazilian specimen

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2019
Tayfuner, Tuğçe
Tensile opening mode I loading state is important for shotcrete-concrete type materials, since these are weak under tension. Brazilian type splitting tests are commonly used for checking the structural effectiveness of concrete in opening mode. Tensile strength is measured indirectly by these tests. In concrete industry, beam tests under three- and four-point bending loads are used to measure tensile strength and mode I fracture toughness of beams and columns under almost pure tensile loading state. However, some structural parts are under compression and indirect tensile loading may result in these. Flattened Brazilian Disc (FBD) is a modified splitting type geometry. It is preferred due to its simplicity of specimen preparation and the compressive load application. Testing this geometry under compression provides means to measure both tensile strength and mode I fracture toughness with a single specimen and test. This work is original in the sense that FBD geometry and the testing method are successfully applied to measure the tensile strength and fracture toughness of molded shotcrete with a water/cement ratio of 0.5. Molds are designed in different sizes and 3D printing technology are used to produce these accurately in desired geometries. vi The aim of this study is to investigate the relations between fracture toughness, specimen size, curing time and the tensile strength of shotcrete for a particular mixture design. Loading angle that is defined as the angle between the paths drawn from the specimen center to the edges of flattened loading ends, are kept constant at 22° in all testing work. This corresponds to a half of the flattened end/radius ratio of L/R=0.19. For investigating the effect of specimen size on the fracture toughness, specimen diameters were varied between 100 mm and 200 mm. Curing time was constant as 7-day for this group of tests. Fracture toughness of shotcrete was found to increase from 0.93 to 1.46 MPa√m while specimen diameters were changing between 100 mm to 200 mm. The effect of curing time on fracture toughness was investigated with constant 160 mm diameter disc specimen geometry. One to seven-day air cured FBD specimens were tested. It was found that mode I fracture toughness of shotcrete increased from 0.66 to 1.18 MPa√m with increasing curing time. The tensile strength calculated from Flattened Brazilian Disc tests were compared to the results of the conventional Brazilian Disc tests. Processing the experimental results, a size independent fracture toughness/tensile strength ratio was proposed for concrete type of materials.