Bridge Deck Removal Depth and Method Before Overlay
A 1974 reinforced concrete bridge deck is being rehabilitated. The 60 mm asphalt overlay has been milled off. The deck has about 35 mm average cover over epoxy-coated rebar, widespread chloride contamination (0.35% by mass at 15 mm), and delaminations over 12% of the deck. The specification requires removal of 15 mm of concrete and a bonded concrete overlay. The contractor proposes 15 lb pneumatic chipping hammers followed by wire brushing. One lane must remain open to traffic, and the adjacent lane is being milled at the same time. The engineer is concerned about microcracking and damage to the epoxy coating. The deck is 220 mm thick, and no post-tensioning is present. A waterproofing membrane will be installed over the new overlay.
1. Select a removal method and justify it against the constraints. Include what you would reject and why.
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Reject 15 lb chipping hammers for 15 mm removal over epoxy-coated rebar. They deliver high-impact energy, can microcrack the remaining deck, and can chip the epoxy coating; wire brushing then polishes the surface and does not remove laitance or chloride. Use hydrodemolition with controlled pressure and small nozzles, or a ride-on milling or scarifying machine with depth control, whichever is validated by a trial area. Hydrodemolition is preferred near delicate epoxy rebar because it removes concrete without impact and leaves a rough, sound surface; it also avoids microcracks. The trial area should be tested for profile, chloride at depth, and bar damage. Traffic vibration and concurrent milling are not reasons to accept a damaging method; they require sequencing and monitoring.
2. The specified 15 mm removal depth would leave 0.35% chloride at that depth. How do you resolve this conflict?
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The 15 mm is a nominal removal depth, not a guarantee of chloride removal. With 0.35% at 15 mm, removing 15 mm still leaves concrete well above the corrosion threshold at the new surface. For a bonded overlay, the remaining chloride can drive corrosion and delamination under the overlay. Require chloride profiles at 5 mm increments to at least 50 mm on a grid, and remove until the new surface is below the specified threshold, often 0.05–0.10% acid-soluble, or to the top of the rebar, whichever is deeper. If that means 25–40 mm removal, issue a field change before overlay. Do not rely on the overlay or membrane to stop corrosion.
3. What acceptance checks would you perform immediately before placing the overlay?
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Check removal depth and uniformity; chloride at the final surface; surface profile by comparator chips; no microcracking or bruising by visual inspection and hammer tapping; all delaminated concrete removed; epoxy coating intact except where the engineer approves repair; no exposed, damaged rebar without treatment; a clean, dust-free surface; saturated-surface-dry substrate; and correct edge details. Sound the deck after removal, not only before. Record test locations and photographs. Reject areas with laitance, oil, or standing water, and verify the overlay manufacturer's required surface moisture condition.