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Best Practices for Placing Concrete Overlays on Prestressed Slab Bridges
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2019-01-01
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[PDF-1.15 MB]
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Abstract:The objective of this research was to identify alternatives to the Maryland Department of Transportation State Highway Administration’s (MDOT SHA's) procedure of placing cast-in-place concrete overlays on prestressed bridge decks. The current practice involves the application of a cement-sand slurry mix over the bridge deck prior to placement of overlay concrete. The reinforcement cage for the overlay is removed from its position to facilitate uniform, consistent application of slurry mix over the bridge deck and then subsequently placed back in position. Overlay concrete is then poured to finish the procedure. The current process is time sensitive as it requires that the slurry mix at the deck-overlay interface is in a non-set condition and is still consistent when the overlay concrete is being poured. To evaluate alternatives to this process, this research tested the shear bond and tensile bond strengths by two different test setups: Double-L test and Split-Prism test, respectively. Seven different cases including the reference test case that uses slurry mix were designed for both test setups and the results compared to suggest recommendations for laying concrete overlays in future projects. Of the six cases other than the reference case with slurry mix, four cases simply had different groove configurations running on the underlying substrate. The other two cases involved use of a commercial bonding agent and anchors used as shear studs. Use of materials, mix design specifications, and curing regimes, were in accordance with current MDOT SHA specifications and industry practices. The results of the research on the prestressed slab panels with pre-existing grooves over which the overlay concrete could be directly poured without the need for a slurry or other bonding agents, were very positive. The average bond shear strength values for all the four test cases that have different square-shaped groove configurations (¼ in., 3/8 in. and ½ in. deep grooves at spacing of ½ in. and 1 in.) along the interface were recorded to be higher than the reference test case that uses slurry mix. Based on consultation with the prestressed precast slab manufacturing plants, precast slabs with standard groove configurations could, theoretically, be manufactured at plants for direct use during bridge construction. The overlay concrete could then be directly poured over these panels replacing the current practice. The interlocking effect obtained through the grooves contributed significantly to the bond shear strength and provided good bond characteristics as indicated in the research findings.
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