[Univ of Cambridge] [Dept of Engineering]

Flexure of concrete beams pre-tensioned with aramid FRPs


J. M. Lees


The current work investigates the flexural behaviour of concrete prestressed with aramid fibre reinforced plastics (AFRPs). FRPs are linearly elastic materials and, unlike steel, do not yield. Hence a major point of debate is the question of, and even the definition of, ductility. Failure of a beam due to either concrete crushing or tendon rupture will be brittle and hence the applicability of existing design philosophy based on steel reinforcement needs to be addressed.

The aim of the current work was to determine how both large rotations and a high ultimate load could be achieved in prestressed concrete beams with FRP tendons. The particular focus was the influence of the bond between the AFRP tendon and concrete on the flexural response of a pre-tensioned beam.

Preliminary development work was required to determine a means of pre-tensioning the FRP tendons and to elucidate the parameters to be used in the design of the main experimental test series. A prestressing system based on the use of expansive cement couplers proved to be viable and a series of prestressed pull-out tests were carried out to investigate the bond stresses generated between the tendon and the concrete.

In the main test series pre-tensioned concrete beams were cast using either one of two types of AFRP tendons or steel tendons. The influence of bond was investigated by testing beams with bonded tendons, unbonded tendons or partially-bonded tendons. It was found that, although the bonded beams had a high ultimate load capacity, only limited rotation occurred prior to failure. In contrast, large rotations were noted in the unbonded beams but the strength of these members was significantly (25%) lower than that of the bonded beams. The only beams that achieved both a high rotation capacity and a high ultimate load capacity were the beams with partially-bonded tendons.

An analytical study based on a rigid body formulation was carried out to investigate the behaviour of the experimental beams. The analysis, which included modelling aspects of the behaviour using finite elements, provided insight into the effect of the bond on the cracking behaviour of the beam and clarified the nature of the bond stresses generated between the tendon and the concrete. The correlation between the analytical and the experimental results was good.

It is concluded that the bond between the AFRP tendon and concrete has a significant effect on the flexural behaviour of an AFRP-prestressed concrete beam. Furthermore, the performance of these beams can be enhanced by using partially-bonded tendons. It is suggested that the use of such tendons could be incorporated into a new design basis for FRP-prestressed beams.

Key words : Prestressed concrete, flexure, bond, advanced composites, aramid, FRP

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