Prestressed FRP Fabrics for Flexural Strengthening of Concrete

Marco Menegotto, Giorgio Monti, and Marc’Antonio Liotta

Abstract The feasibility and advantages of strengthening reinforced concrete (RC) beams by prestressed Fibre-Reinforced-Polymer (FRP) fabrics are examined. Actually this is possible, as a prestressing machine has been designed, suitable to properly seize and prestress the fabric, without risk of ripping it. The device is illus­trated and the case of RC beams with inadequate steel reinforcement for bending is treated. A method is proposed and the analytical verifications are shown, that make use of closed-form equations and follow the usual steps commonly taken when strengthening, i. e. to assess the resisting moment capacity of the existing beam, to evaluate the demanded strength increase and to design the amount and prestress level of the strengthening. A design example is also given.

17.1 Introduction

The use of prestressed FRP reinforcement in the shape of laminated strips, for flex­ural strengthening of RC members, has broadly increased over the past decade.

In some cases, it may be advantageous to bond the external FRP reinforcement onto the concrete surface in a prestressed state. Both laboratory and analytical research (e. g. Triantafillou and Deskovic 1991; Triantafillou et al. 1992; Deuring 1993) show that prestressing represents a significant contribution to the improve­ment of the FRP strengthening technique, and that procedures have been developed to prestress the FRP composites under real life conditions (Luke et al. 1998).

M. Menegotto (*) • G. Monti • M. Liotta

Department of Structural Engineering and Geotechnics, Sapienza University of Rome, viaA. Gramsci, 53,1-00197 Rome, Italy

e-mail: marco.; giorgio.;

M. N. Fardis (ed.), Innovative Materials and Techniques in Concrete Construction: ACES Workshop, DOI 10.1007/978-94-007-1997-2_17,

© Springer Science+Business Media B. V. 2012

Prestressing the strips prior to bonding has several advantages: it provides stiffer behaviour, as at low loading most concrete is under compression, thus better con­tributing to the inertia. Crack formation is delayed and, when it appears, cracks tend to be thinner and more finely distributed. Prestressing also favours the closure of pre-existing cracks, improving both serviceability and durability, while it also enhances the shear resistance.

Further, the same strengthening can be achieved with smaller areas of FRP, if pre-stressed, and, with adequate anchorage, prestressing may increase the ultimate resisting moment by avoiding peeling-off at crack locations and at the strips ends. Finally, prestressing significantly increases the load level which makes the steel reinforcement yield.

Prestressing also has drawbacks: it is more expensive than normal strip bonding, due to the number of operations and the equipment required, and it requires the pre­stressing force to remain in place longer, until the adhesive has hardened enough.

Were the prestressing force too high, failure of the beam would occur due to the release of the prestressing force at the ends, inducing excessive shear stresses in concrete just above the FRP. Hence, design, detailing and execution of the end zones require special care. Tests and analyses have shown that FRP strips shear off (from the ends) if no special anchorages are provided at the ends and prestress levels are above 5-6% of their tensile strength (for CFRP). But technically and economically, a considerably higher degree of prestressing would be effective – around 50% of the FRP tensile strength – which can be achieved only by use of special conhning anchorages. These have been developed, for research purposes as well as for practi­cal applications.

So far, devices that are effective in seizing and prestressing laminated strips, thanks to their high transverse stiffness, have been produced, and field applications have been already carried out. However, the use of prestressed FRP reinforcement in shape of fabrics has not developed as much. This is mostly due to the difficulty of seizing the fabric when prestressing it without ripping the fibres. In fact, signifi­cant problems have been encountered in developing appropriate devices. This paper tries to contribute in this respect, by presenting a tensioning machine for FRP fab­rics. The concept and a design scheme are illustrated. Some considerations regard­ing the design of a strengthening intervention through prestressed FRP are also discussed, along with an example.

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