Continuity development between precast beams using prestressed slabs, and its effect on flexure and shear

Development of continuity between precast prestressed bridge girders by post-tensioning the insitu top slab In the regions of hogging moment is a relatively new technique which forms the basis for this research study. Compared to the more conventional method of using reinforcing steel in the slab over the interior supports, prestressed slabs will ensure a crack free more durable bridge deck, and will therefore reduce the maintenance costs. The effect that such a slab has on flexural and shear behaviour of the bridge deck has been studied both analytically and experimentally by considering composite beams based on M-8 standard precast beam section. Comparison of the design of bridge decks with a prestressed slab and a reinforced concrete slab indicated that a partially prestressed slab with a prestress considering up to 50% of the live load will ensure the slab remains crack free under total service load. Although secondary effects and the two stage construction of such a slab tend to increase the prestress requirement for the slab, the same two effects considerably reduce the positive midspan moments, resulting in a decrease in the prestress required in the precast beams (and thus a possible increase in the span range) for given standard precast beam sections. The experimental investigation consisted of testing eleven 1/3-scale M-8 continuous composite beams in two series, Series-A and Series-B. Series A, in which three beams were tested as double cantilevers was planned to study the effects of prestressed slab on overall flexural behaviour. A considerable improvement in crack control under service loads and a higher ratio of measured to calculated ultimate moment capacity was obtained in beams with a prestressed slab. The continuity developed using insitu prestressed slabs was very effective at all levels of loading. Recommendations have been made for the flexural design of continuous bridge decks with this type of prestressed slabs. In Series B, effect of prestressed slabs on shear strength at the continuity connection has been studied. A considerable increase in web shear cracking load was obtained for beams with prestressed slabs, resulting in a decrease In the amount of shear reinforcement required for such beams. The different methods of predicting web shear cracking strength and web crushing strength according to current design codes were compared with experimental values, and based on the results, recommendations for the design for vertical shear of composite beams subjected to hogging moments have been made.