The structural behaviour of concrete beams subjected to air blast loading was investigated. Beams of both highstrength concrete (HSC) and normal-strength concrete (NSC) were subjected to air blasts from explosives in a shock tube and for reference were also loaded statically. Concrete with nominal compressive strengths of 40, 100, 140, 150 and 200 MPa were used and a few beams also contained steel fibres. Furthermore, beams with two concrete layers of different strength were tested. All beams subjected to static loading failed in flexure. For some beam types, the failure mode in the dynamic tests differed from the failure mode in the corresponding static tests. In these cases, the failure mode changed from a ductile flexural failure in the static tests to a brittle shear failure in the dynamic tests. Beams without fibres and with high ratio of reinforcement exhibited shear failures in the dynamic tests. It was observed that the inclusion of steel fibres increased the shear strength and the ductility of the beams. The investigation indicates that beams subjected to air blast loading obtain an increased load capacity when compared with the corresponding beams subjected to static loading.

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The compressive strength development of sealed high-performance concrete and paste specimens, with and without silica fume, have been studied from 1 day and up to 4 years. The paste and concrete specimens were prepared in such a way that segregation was avoided and the silica fume became well dispersed. Under these conditions silica fume increased the strength of paste just as much as it increased the strength of concrete. It appears that the enhancing effect of silica fume on concrete strength is due to an improved strength of the paste phase as a whole, and not due to an improved bond strength between the paste phase and the aggregate particles, as has been suggested earlier. The concretes and the pastes with 10% silica fume appeared to loose strength over a period of time before the strength increased again. R SUME

IntroductionDynamic loads such as explosions can cause severe damage to concrete structures. An explosion in air is the result of detonating explosive charges, the rapid combustion of a fuel-air mixture or bursting pressure vessels. The explosion generates a blast wave that propagates through the air at supersonic velocity in all directions. As the blast wave strikes an object such as the wall of a building, the pressures are reinforced due to reflections. In a case where the reflected pressures are sufficiently high, local failures of structural elements such as loadbearing walls or columns can occur. In the design of concrete structures to resist the effects of blast loads, impacts or other severe dynamic loads, it is not practical to consider a structural response in the elastic range only. The structural elements should therefore be allowed to deform plastically, which better utilizes their energy-absorbing capabilities. A certain amount of damage, i.e. concrete cracking and yielding of the reinforcement due to flexure, is therefore usually accepted in the design of structures to resist blast loads. Structural elements should generally be designed for a flexural response. However, real events [1] have shown that highly intense loads from blasts at close range can cause local shear failures in concrete structures, which is a brittle mode of failure. In the Oklahoma City bombing, two concrete columns were reported to have failed in shear. Apart from real events, shear failures in concrete elements have also been observed experimentally in several investigations involving blast and impact loads [2][3][4][5][6][7]. In several cases these tests confirm that elements that fail in flexure under a slowly applied (quasi-static) load may fail in shear under dynamic loads.The purpose of the present paper is to conduct a review of the literature on the shear problem of reinforced concrete structures subjected to intense dynamic loads and to identify areas for further research. In this context, dynamic loads refer to intense events due to explosions and impacts. The review focuses on behavioural aspects of dynamic shear and on the parameters that control the mode of failure. Highlighting the initial response of concrete elements under dynamic loads represents another focus. The modes discussed here are flexural shear and direct shear. A third mode of shear failure is punching shear. This type of failure can occur as an object impacts on a concrete surface with inclined shear cracks and the formation of a conical shear plug through the thickness of the concrete element [8]. However, dynamic punching shear is outside the scope of this paper. 2Quasi-static shear Shear failure in reinforced concrete elements can generally be related to flexural shear and direct shear. In principle, flexural shear and direct shear exhibit similar fundamental behaviour since they share many features such as the mechanisms that transfer shear across a crack. These mechanisms are friction, aggregate interlock and the dowel action of the longitudi...