Expanding the Frontiers of Research - Prestressed Concrete Beams Reinforced with ComBAR GFRP

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Expanding the Frontiers of Research - Prestressed Concrete Beams Reinforced with ComBAR GFRP

Dritan Topuzi

Fibre-reinforced polymer (FRP) composites have been successfully used as an alternative to steel in reinforcing concrete structures, in particular because of their corrosion-resistance and electrical and magnetic transparency. In spite of many advantages compared to steel, FRPs have lower moduli of elasticity, which may lead to higher deflections of FRP reinforced structures in certain cases. To overcome this serviceability concern, FRP bars can be prestressed.

To further investigate such applications, Schöck/Fiberline sponsored an NSERC IPS research project at the University of Waterloo [1], designed to assess the long-term performance of concrete beams prestressed with ComBAR GFRP. The GFRP bars were prestressed up to 40% of their ultimate capacity, which is more than the 25% prestressing limit at transfer that is set by CAN/CSA-S6-06 (the Canadian Highway and Bridge Design Code).

Twenty beams with dimensions of 150x255x3,600mm (Fig. 1) were cast and tested under different sustained load levels for 300 days. The long-term deflections and the concrete and GFRP strains were monitored throughout the testing period. The main parameters considered were the prestressing level (0%, 25%, and 40% of the ultimate bar strength), and the sustained load level (35%, 60%, and 80% of the ultimate capacity of the beam). Following the sustained loading phase (Fig. 2), all beams were tested to failure in four-point bending (Fig. 3). The transfer length measurements were taken for the prestressed beams, and data on the early relaxation of the GFRP bars was recorded for 277 hours. 

Fig. 1 - Beam Specimen Details [1]

Fig. 2 - Test Setup for the Sustained Load Test [1]

Fig. 3 - Test Setup for Static Beam Testing [1]

The experimental results showed that subjecting the beams to different levels of sustained loading for 300 days did not affect their ultimate capacity and no measurable relaxation was observed up to 277 hours. 

The code limit of 25% prestressing level, set by the Canadian Highway and Bridge Design Code, appears to be conservative, at least for ComBAR GFRP.

ComBAR GFRP, once again, has proven itself to be a highly-performing reinforcement, exceeding all applicable code requirements.

[1] More details on this research can be found on the PhD thesis of M. Zawam, from the University of Waterloo website: https://libuwspaceprd02.uwaterloo.ca/handle/10012/9976