Ductility of Beams Prestressed with FRP Tendons
Miguel Morais
The linear elastic behaviour of FRP materials means that ductility needs to be precisely defined. In order to design a safe structure it should have enough rotation capacity for the use of plasticity theory and in seismic regions should be able to dissipate energy. At failure it should have strain hardening behaviour in order to warn of incipient failure and low stored elastic energy available for release so that failure is not explosive. The current measures of ductility and ways to design ductile structures with FRP are presented; FRP structures can have rotation capacity but not dissipate much energy. The failure mode of the structure is also of relevance; snapping of the FRP tendon or failure of confined concrete can be explosive and hence undesirable.
Tests on nine small-scale prestressed concrete beams are described. Steel and AFRP tendons were used. Three types of concrete were used; normal strength concrete, concrete confined with AFRP spirals and steel fibre reinforced concrete. Some beams were designed to be over-reinforced and other beams were designed to be under-reinforced. The over-reinforced beam prestressed with AFRP, and with the concrete confined, had a large deformation capacity and energy dissipation ratios but failure was explosive by snapping of the AFRP spiral confining the concrete. Steel fibre reinforced concrete is a good way to enhance the concrete plastic behaviour provided care is taken in the fibre alignment. The under-reinforced AFRP prestressed beam failure was by snapping of the tendon without a post-peak behaviour but with a large deformation capacity.
A new technique was used to measure deflections. Using a still digital camera, pictures of the beam were taken as it was being loaded. These pictures were later analysed by a computer program. As a result the displacements of about 1,200 points on the side of the constant moment region of the beam are known. This method was used to study localization in the compression zone of concrete, crack spacing, neutral axis position and to better visualize the post peak behaviour of the beams.
Integration of the moment-curvature relationship was used to determine the load-deflection behaviour. Prediction of the load capacity of over-reinforced beams is dependent on the concrete strength since failure is not governed by the tendon capacity. Thus it is very important to know the concrete properties well.
Key words: Prestressed, concrete, beams, flexure, FRP, ductility, over-reinforced