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Mode II fracture toughness (K IIc ) of fiber reinforced concrete (FRC) has been widely investigated under various patterns of test specimen geometries. Most of these studies were focused on single type fiber reinforced concrete. There is a lack in such studies for hybrid fiber reinforced concrete. In the current study, an experimental investigation of evaluating mode II fracture toughness (K IIc ) of hybrid fiber embedded in high strength concrete matrix has been reported. Three different types of fibers; namely steel (S), glass (G), and polypropylene (PP) fibers were mixed together in four hybridization patterns (S/G), (S/PP), (G/PP), (S/G/PP) with constant cumulative volume fraction (V f ) of 1.5 %. The concrete matrix properties were kept the same for all hybrid FRC patterns. 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. Three different ratios of notch depth to specimen width (a/w) 0.3, 0.4, and 0.5 were implemented in this study. Mode II fracture toughness of concrete K IIc was found to decrease with the increment of a/w ratio for all concretes and test geometries. Mode II fracture toughness K IIc was sensitive to the hybridization patterns of fiber. The (S/PP) hybridization pattern showed higher values than all other patterns, while the (S/G/PP) showed insignificant enhancement on mode II fracture toughness (K IIc ). The four point shear test set up reflected the lowest values of mode II fracture toughness K IIc of concrete. The non damage defect concept proved that, double edge notch prism test setup is the most reliable test to measure pure mode II of concrete.

In the present paper, an experimental work was carried out to study the effect of ground waste tire rubber addition on the mechanical properties and impact resistance of normal strength concrete (NSC) with and without silica fume. Three different volume ratios of cnunb rubber (lo%, 20%, and 30%) were used as a partial replacement of sand. Two different sizes of cylinders were tested , under indirect tension test to study the size effect. Impact compression according to ACI Committee 544's repeated drop-weight impact test was carried out on discs of 150 mm diameter ,. ' ana 63 mm height. Test results showed that, the replacement of fine aggregate by 10% crumb ,. rubber caused a small decrease in concrete compressive strength. The presence of crumb rubber of :,. small size in concrete increased its resistance to crack initiation under impact load. Tbe rubber of small sue bas no varticle brid.eiua effect, hence, the mode of failure of rubberized concrete under static and impact ~ompression&a~ the s&e as that of plain concrete. The ACI impact test failed to differentiate between the impact resistance of rubberized concretes with different rubber contents.

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The most favorable site of diagonal tension crack initiation has been attempted to be located. Due to the numerous interacted parameters affecting both site and angle of diagonal tension crack initiation, twelve possible sites were investigated, at midheight of the shear span and at the bottom surface near the support of the beam with vertical and diagonal orientations. The first diagonal tension crack initiated from the bottom tip of the diagonal pre-crack at midheight of the beam as a result of constraint release. To verify the previous finding, a single diagonal pre-crack has been created at midheight of only one side of the shear spans in both normal and fiber reinforced concrete (FRC) beams. FRC beam showed different behavior, where couple of diagonal tension cracks initiated at both sides from the tip of flexural cracks regardless of the existence of pre-crack at one side of the beam. Ó 2014 Faculty of Engineering, Ain Shams University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

This study focuses the light on the shear behavior of pre-cracked beams, and examines the feasibility of applying fracture mechanics concepts to widen the understanding of shear behavior and mechanisms. The experimental program contains ten beam specimens of high strength concrete (HSC) and steel fiber reinforced concrete (SFRC). Pre-cracks were embedded with different sizes and locations along the favorable path and orientation to shear failure. Three main scenarios of shear failure were reported with minute effect of pre-cracks. The geometrical effect is dominant with marginal effect of the material's nonlinearity in case of severe pre-diagonal crack in HSC while the nonlinearity of the material is supreme to that of the geometrical effect for SFRC and shorter cracks. For verification, numerical simulation was conducted to examine the geometrical effect of the pre-diagonal tension crack in shear span on the structural behavior of RC beams. It is found numerically that, when the crack tip of the tensile crack is away from the tensile reinforcement, the closuring moment of tensile reinforcement increases, and as a result reduces the strain energy release rate. Therefore, the tensile cracks stop and the shear cracks keep propagating leading the failure mechanism to the end failure point.

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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.