United States Department of Transportation
i
Use of fiber reinforced concrete in bridge approach slabs.
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2008-12-01
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Abstract:Bridge approach slabs are deteriorating at a much faster rate than expected resulting in a massive need for repairs and premature replacement
costing millions of dollars annually. Both environmental and traffic loading causes the concrete to worsen which accelerates the deterioration
process by allowing deleterious agents to enter the concrete. In order to enhance the service life of bridge approach slabs the material needs to
comply with certain performance criteria such as crack resistance, durability enhancement and post crack flexural stiffness. A Hybrid Fiber
Reinforced Concrete (HyFRC) was developed for the use in bridge approach slabs in Area III jurisdiction which are exposed to severe
environmental conditions. Deflection hardening was used as a performance goal in the design of the HyFRC composite.
The research program consisted of four parts. First (1), a performance based materials approach to bridge approach slabs was developed. The
fiber reinforced composite was designed to exhibit deflection hardening through an average measure of ductility exceeding the yield strain of steel
rebar (i.e. > 0.002). Four point flexure tests on 6 in. (152 mm) deep beam specimens were conducted in order to quantify the material flexure
performance. Second (2), the existing California Department of Transportation (Caltrans) bridge approach slab design was used as the basis for
element sizing and reinforcement ratios. Four-point flexure tests (1/2 scale models of an existing bridge approach slabs) were conducted and
HyFRC beams were compared against plain concrete control beams with and without conventional steel reinforcement. Third (3), an extensive
durability study was conducted on how the crack resistance provided by the HyFRC mitigates durability problems associated with frost action and
corrosion. The bridge approach slab is highly susceptible to deterioration from freeze-thaw cycling and reinforcement corrosion due to the
anticipated location (i.e. Area III jurisdiction). Compared to the plain concrete specimens the HyFRC exhibited improved performance in regards to
both freeze/thaw resistance and scaling resistance. To study the effect of cracking/crack resistance on corrosion behavior, cyclic flexure tests were
conducted on reinforced beam elements composed of HyFRC and a comparable plain concrete in order to induce surface cracking. For a specified
load demand in excess of the plain concrete fr and below the yield strength of the rebar, the HyFRC flexural specimens showed a high propensity
for crack resistance (no surface cracks were visible as opposed to the reinforced plain concrete). Corrosion behavior was then monitored by
ponding salt solution on the cracked surface and higher corrosion rates icorr (measured by polarization resistance) by nearly 1 order magnitude were
observed in the reinforced plain concrete specimens. The observed increased corrosion rates were confirmed by direct gravimetric analysis of the
bars after removal. The study concludes (4) with a discussion of how the observed improvements in flexural performance in the presence of the
fibers can be used for a new design detail for bridge approach slabs.
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