Concrete bridge structures are typically designed to last 50 to 75 years, but seldom last half that time before needing major rehabilitation, due to degradation caused by corrosion of steel reinforcement similar to that shown in Fig. 1. Corrosion in commonly used epoxy-coated steel bars has raised concern with its use and has raised interest in the use of alternative reinforcement like fiber-reinforced polymer (FRP) bars. Glass FRP (GFRP) bars are a cost-competitive alternative to epoxy-coated steel bars and have been found to not corrode (see UDOT Report No. UT-11.16).
Many transportation costs and user impacts associated with typical corrosion problems could be potentially eliminated with a proactive approach of using non-corrosive reinforcement (e.g. GFRP) in the original construction of concrete elements. Experimental tests were conducted recently at the University of Utah on circular concrete columns reinforced with GFRP and/or steel longitudinal bars and GFRP confining spirals to evaluate their behavior and viability as a potential construction alternative.
One set of columns was reinforced with GFRP spirals and GFRP longitudinal bars, another set of columns was reinforced with GFRP spirals and steel longitudinal bars, and a final set of columns was reinforced with double GFRP spirals and a combination of GFRP and steel longitudinal bars (see Fig. 2). Tests were performed on 12 in. diameter short (3 ft tall) and slender (12 ft tall) columns. These are the only tests known to the authors which have investigated the stability of slender FRP-reinforced concrete columns.
An analytical confinement and buckling model was developed and validated against the tests to provide a means to predict the behavior and capacity of FRP-reinforced concrete columns. This enabled the analysis of additional reinforcement scenarios utilizing FRP (glass or carbon) longitudinal bars and spirals.
In general it was found that FRP spirals and FRP longitudinal bars can be a viable method of reinforcement for concrete columns, particularly in corrosive environments. FRP spirals, however, need to be placed at a closer pitch spacing to provide confinement levels similar to steel spirals due to the lower modulus of elasticity of FRP composites. On the other hand, FRP longitudinal bars can provide increased deflection capacity as compared with steel bars due to the higher tensile capacity of FRP composites.
Additional research is needed to better quantify the confining capacity of FRP spirals and the required pitch spacing needed. Also research investigating the behavior of FRP-reinforced columns under seismic loading will be an important consideration.
This guest post was written by Thomas A. Hales, PhD, SE with the UDOT Research Division and Chris P. Pantelides, PhD, SE with the University of Utah and was originally published in the UDOT Research Newsletter.