Experimental Results and Validation of the Design Technique Used for FRP Strengthening of RC Bridges


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Title: Experimental Results and Validation of the Design Technique Used for FRP Strengthening of RC Bridges

Author(s): D. I. Kachlakev

Publication: Special Publication

Volume: 209


Appears on pages(s): 187-206

Keywords: bridges; composites; fiber reinforced polymers; repair and retrofit; structures

Date: 9/26/2002

The Horsetail Creek Bridge (HCB), constructed in 1912, is located along the Historic Columbia River Highway in Oregon. The cross beams of this historic structure were found to be 50 percent deficient in flexure and 94 percent deficient in shear, mainly due to the traffic loads increase. Analysis of the alternative designs indicated that glass FRP (GFRP) laminates would be most suitable for shear strengthening, while carbon FRP (CFRP) laminates would be best for flexural capacity enhancement. Concurrently, four full size beams, as similar as possible to the actual bridge beams, were constructed to simulate the retrofit of the bridge. One of the beams served as a control; one beam was strengthened for shear capacity increase only; one beam was reinforced with CFRP for flexure; and one beam was reinforced with CFRP for flexure and GFRP for shear. Results revealed that addition of either GFRP or CFRP composites strengthening provided static capacity increase of 45 percent compared to the control beam. The beam strengthened with CFRP for flexure and GFRP for shear, which simulated the HCB cross beams after the retrofit, exhibited nearly 100 percent of moment capacity increase. Post cracking stiffness of all beams was increased, primarily due to the flexural CFRP laminates. Results suggested that capacity of the experimental beam, retrofitted in the same fashion as the bridge, should exceed the bridge design load of 720 kN-m (after strengthening), sustaining up to 868 kN-m of applied moment. The addition of GFRP for shear alone was sufficient to offset the lack of steel stirrups in the actual bridge, allowing for a conventionally reinforced concrete beam with significant shear deficiency to fail by yielding of the tension steel. The ultimate deflections of the shear GFRP reinforced beam were nearly twice those of the control shear-deficient beam. The experimental beam retrofitted with only CFRP for flexure failed as a result of diagonal tension cracking at a load 45% greater than the control beam. A design method for flexure and shear was proposed before the onset of this experimental study and used on the HCB. The design procedure for flexure was refined and allowed for predicting the response of the beam at any applied moment. The flexural design procedure includes provisions for non-crushing failure modes, and was shown to be slightly conservative using the design material properties.