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Investigation of fracture toughness on flattened Brazilian disc type molded shotcrete specimens

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2019
Yoncacı, Selin
Monitoring behavior of cracks generated by various reasons in shotcrete/concrete is of crucial importance for the stability of structures in various industries. Despite the need of understanding the crack initiation and propagation mechanisms profoundly, fracture mechanics related studies for shotcrete are limited so far. This topic has not attracted the attention it deserves in structural applications such as tunneling. Beam type specimen geometries are commonly used in fracture mechanics testing of shotcrete/concrete mixtures, since building columns, beams, and tunnel linings are under bending loads in general. However, compressive loads in structures can indirectly induce tensile splitting, that’s why, Brazilian type splitting tests are common in checking the structural state of columns, beams, and the linings. A different geometry, Flattened Brazilian Disc geometry (FBD), is used here for the first time for fracture testing of the shotcrete samples. Targeted FBD sample diameters were up to 200 mm. Preparing samples with regular coring and grinding process were not practical due to irregularities formed during machining of samples. Innovative technologies were used to prepare FBD samples with different dimensions 3D printer technology was used to shape the sample molds to the desired geometries. As the molds were printed with a 3D printer, the sample preparation process was improved. The diameter and the loading angle controlling the length of the flattened end was accurately adjusted for each size group. Shotcrete mixture poured to the molds was prepared carefully to maintain the granular and binding characteristic as the sample size increased. Aggregate size was around 0- 5 mm in the mixture. Water/cement ratio was selected as 0.40. A total of 16 molds were used to obtain the samples with diameters range from 75 mm to 200 mm. Loading angles varied between 20-30 degrees, corresponding to flattened end lengths of 2L= 15.6-43.5 mm. A total of 80 valid mode I fracture tests were conducted. Some test results were discarded, since a clear load drop or a theoretically desired central splitting of samples were not observed. It was assured that there were five valid tests for each diameter group and each loading angle group regarding a specific diameter. With the help of a proposed equation from a previous numerical modeling work, the initial crack length (acn) at the onset of stable fracturing was calculated. This was compared to the experimentally observed ace during the slow pace loading of FBD specimens. The previous theoretical formula was validated and proved to be applicable for the shotcrete FBD testing. It was found that mode I fracture toughness, KIc, increased with increasing specimen diameter. It was found 0.96 MPa√m for the lowest diameter group of 75 mm with 30° loading angle, and it was 1.50 MPa√m for the largest diameter group of 200 mm with a loading angle of 20°. In general, KIc is found to decrease with increasing loading angle. This situation is attributed to the boundary influence issue, since the flattened end length L increases compared to the specimen size. Compressively loaded ends and adjacent stress free boundary get too close to the crack front, which is supposed to be under pure mode I loading state. High stress gradients cause a complex loading state instead. Increase of KIc with size and decreasing loading angle showed clues of a second degree polynomial tendency to a size- and geometry-independent ideal specimen for pure mode I fracture toughness testing of shotcrete with FBD geometry. FBD testing work here provided important results and proved its high potential to develop an ideal testing geometry to measure pure mode I fracture toughness of shotcrete/concrete materials.