MODE II FRACTURE TOUGHNESS ESTIMATES FOR FIBER REINFORCED CONCRETES USING A VARIETY OF TESTING GEOMETRIES

On the shadow of the ongoing debate on attempting to estimate mode II fracture toughness (K_IIc), the role of fibers and, or pozolan addition arises to importance. In this paper an experimental investigation of plain concrete, high strength concrete, and fiber reinforced concretes is reported. Three different types of fibers, namely steel, glass, and polypropylene fibers with a constant volume fraction v_f of 1.5%, are added to high strength concrete specimens. In an attempt to estimate a fairly accepted value of fracture toughness K_IIc, four testing geometries and loading types are employed in this investigation. Four points shear, Brazilian notched disc, double notched cube, and double edge notched specimens are investigated in a trial to avoid the limitations and sensitivity of each test regarding geometry, size effect, constraint condition, and the crack length to specimen width ratio a/w. The addition of pozolan enhanced both the compressive strength and mode II fracture toughness of concrete for almost all test geometries, while the addition of different types of fiber reduced the compressive strength and increased mode II fracture toughness in pure mode II tests. Mode II fracture toughness of concrete K_IIc decreased with the increment of a/w ratio for all concretes and test geometries. An element of mode mixity was traced in four points shear, and notched disc specimen tests. Double edge notched specimen test was found to be inconvenient for the current study due to its sensitivity to brittleness and anisotropity of concrete. Mode II fracture toughness K_IIc reported from double notched cube test showed higher values than all other tests due to the crack propagation miss alignment opposing sliding of crack surfaces.