The steel-free deck slab developed in Canada used straps as a confinement system and relied on the stiffness of this confinement system rather than its strength. In this paper, a new approach, using the strength of a confinement system comprising unbonded prestressed bars, was investigated. In addition, the effect of the concrete strength and the steel ratio of the confinement system was examined. Seven large-scale one-way steel-free deck slabs were built using normal- and high-strength concrete, prestressed to different levels, and statically tested up to failure. The overall behaviour of the slabs was similar to that of slabs confined by straps. Cracking strength was not considerably enhanced. The slabs failed in punching shear mode with high loads. The new approach was not able to overcome the problem of the longitudinal crack, which developed in the tested slabs and reportedly appeared in all the steel-free slabs built or tested previously in Canada. Prestressing had a definite effect, however, on delaying the occurrence of this longitudinal crack, thereby enhancing serviceability. The paper recommends that the serviceability limit state of the steel-free slabs would be better assessed by the occurrence of the longitudinal crack, even though the ultimate capacity of the slabs is high.Key words: arch action, cracking, external prestressing, high-strength concrete, punching shear, steel-free deck slab.
The authors would like to thank the original designers of the steel-free slab for their interest in the paper and for presenting a summary of their latest developments on the system. The issues they have raised are discussed in the following sections. The longitudinal crack and ultimate strengthThe authors are now in complete agreement with the discussers that the longitudinal crack of the steel-free concrete slabs is not a danger to the ultimate strength of these slabs. After the original experiment, we have since turned our focus to the fundamental behaviour of the steel-free concrete members, which is thought to be essential before further developing the system. One part of this research dealt with steel-free slab strips, which act in bending and is yet unpublished. Another part tested the change of failure mode of steel-free beam-like members and will be published soon (Hassan et al. 2004). The authors came to believe that the behaviour of steel-free concrete members after cracking is better simulated by using models of masonry structures. Arching action would completely dominate after a steel-free concrete member had substantially cracked (in terms of crack width not crack numbers) and therefore turned into separate blocks. The discussers stated this fact when they mentioned that all steel-free slabs had developed full length longitudinal cracks. Consequently, the new system of concrete blocks restrained by the straps and the girders could be considered as any well-restrained masonry arch. This approach would help allay the concerns and fears of many engineers that the longitudinal crack is a menace to the strength of the system. Heyman (1969) in his fundamental and pioneering studies of masonry arches and vaults showed, for example, that cracking state in a masonry arch, similar to the one present in the steel-free slab after appearance of a longitudinal crack, is completely safe and might transfer the structure to a better shape to resist applied loading. Heyman (1969) also proved that a perfectly flat arch (a slab) has an infinite geometrical factor of safety, meaning that the structure failure would be initiated only by material crushing (punching in the case of slabs) or by failure of the restraining system.In brief, the fear of the longitudinal crack in the steel-free slab is due mainly to the belief that this system is merely a variation on the conventional slab. When a consideration such as the one mentioned above is introduced to explain the behaviour of the steel-free concrete slab after cracking, better understanding of the longitudinal crack would be expected. The longitudinal crack and serviceability limitAs the discussers mentioned, the fatigue tests they referred to were not available to the authors at the time of writing the paper in discussion. All these tests proved that the longitudinal crack would always occur and it would always have no effect on the ultimate strength of the structure. All slabs stabilized and showed superior resistance after the occurrence of this crack, a behaviour tha...
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