Pultruded GFRP sections as stay-in-place structural open formwork for concrete slabs and girders
Honickman, Hart Noah
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Commercially available glass fiber-reinforced polymer (GFRP) off-the-shelf structural shapes have great potential as stay-in-place open structural forms for concrete structures, including bridge decks and girders. The system simplifies and accelerates construction, and the non-corrosive GFRP forms can fully or partially replace steel rebar. In this study, eight concrete slabs were constructed using flat pultruded GFRP plates, and nine girders were constructed using trapezoidal pultruded GFRP sheet pile sections as stay-in-place structural forms. No tension steel reinforcement was used. All specimens were tested in four-point monotonic uniaxial bending. Four adhesive and mechanical bond mechanisms were explored to accomplish composite action. The most effective mechanism, considering structural performance and ease of fabrication, was wet adhesive bonding of fresh concrete to GFRP. Although failure was by debonding, no slip was observed prior to failure. Other parameters studied were concrete slabs’ thicknesses and their shear span-to-depth ratios. For the girders, three different cross-sectional configurations were examined, namely, totally filled sheet piles, one with a voided concrete fill, and an all-GFRP box girder developed by bonding flat GFRP sheets to the upper flanges of the sheet piles with a cast-in-place concrete flange. Girders were tested in positive and negative bending to simulate continuity. The built-up box girders showed superior performance, with up to 70% higher strength and 65% lower weight than the totally filled sections. It was found that similar size conventional steel-reinforced concrete sections of comparable stiffness have considerably lower strength, while those of comparable strength have considerably higher stiffness than FRP-concrete members. An analytical model was developed to predict the behaviour and failure loads of slabs and girders, using cracked section analysis. A unique feature of the model is a multi-stepped failure criteria check that can detect flexural, shear, or bond failure. The model was successfully validated using the experimental results, and used in a parametric study. It was shown that using the typical value of 1MPa for shear strength of cement mortar predicts debonding failure, which occurs slightly above the interface, quite well. Also, in practical applications of longer spans, flexural failure is likely to occur prior to bond failure.