Reinforcing Steel Corrosion Repair

The Problem

The parking deck for a large suburban shopping mall was exhibiting severe corrosion of the reinforcing steel. The structural deck was a mildly reinforced concrete deck with expansion joints at 128 ft on center, each way. The deck was supported by columns spaced at 32 ft on center, each way. The concrete deck was 1 ft thick with mild steel top and bottom, each way. The reinforcing steel at the top and at the bottom was corroding at a rapid rate due to the application of deicing salts in the winter. The concrete deck had received an asphalt overlay several years earlier.

The Solution

A clear definition of the extent of the deterioration could not be made with the asphalt overlay on the structural concrete deck. Although the structural design engineer had produced a repair solution, the owner elected to negotiate with Restruction Corporation. Upon review and analysis of the project and with the quantities unknown a Time & Material contract was selected as having the lowest economic cost to the owner while providing the greatest economic benefit.

Restruction Corporation working with the owner, architect and engineer developed a construction engineered solution that met the owners schedule and the construction requirements. Repair to the underside of the structural concrete deck was performed first. This repair required the removal of all the delaminated areas from the bottom of the deck. Based on the amount of salt in the concrete at the lower reinforcing bars, the durability of the concrete and the projected life cycle of the structure, the engineer elected not to remove concrete from around the exposed reinforcing tee. Sand blasting then cleaned the exposed areas and the cavities filled by shotcrete. Applying a curing compound cured the shotcrete areas. The structural deck was then shored and work proceeded on the top of the deck. Upon removal of the asphalt overlay, it was discovered that prior to the placement of the overlay, extensive spall repairs had been made. The spall repairs had been made using a gypsum enriched repair mortar. Without exception, every previous spall repair area had failed. Additional delaminations had occurred as a result of the on going corrosion of the top layer of reinforcing steel. A total of approximately sixty-percent of the deck was delaminated.

To prepare the surface to receive an overlay, concrete had been removed to a depth of approximately 3/4" below the rebar. The delaminated concrete was removed with 30 lb. jackhammers and the concrete around the rebars was removed with 15 lb. jackhammers to minimize the micro cracking at the concrete surface. The exposed rebar as well as the entire concrete surface was then sand blasted to clean the surface and expose aggregate particles in the concrete surface. The loose concrete rubble and dust were removed with 3000psi pressure washers.

Once prepared, a silica fume modified concrete overlay was placed over the entire deck surface. To assure adequate drainage, the overlay thickness varied from 2" - 6" to accommodate several new drains. The paste phase of the concrete was scrubbed into the surface to provide a positive bond that would produce a composite structural system. To assure adequate curing, a curing compound was applied at twice the manufacturers recommendations and then the surface was covered with polyethylene backed burlap. The concrete was allowed to cure for seven days. The shores were then removed and traffic permitted on the deck.

Although there were no cracks in the overlay at the time the polyethylene burlap was removed, cracks did occur when the shoring was removed due to deflections as a result of the increased deadload and the fact that the original concrete was a 3000psi mix and the new concrete overlay was a 4500psi mix. Additional cracking occurred in the overlay as vehicle traffic loading was applied to the deck. These conditions were expected, and the engineer had specified that all cracking was to be sealed with a high molecular weight methacrylate (HMWM). The entire deck surface was then sealed with the HMWM.

Project Images

Poor drainage and previous application of asphalt wearing surface without a membrane created a corrosive environment

Removal of unsound concrete using 30# hammers and sound concrete below oxidized rebar using 15# hammers improves bond strength of the overlay.

Application of curing compound to finished overlay

Macro-cell corrosion of botton rebar mat caused corrosion and concrete delamination in the soffit

Areas of sound concrete remained as new overlay created better deck drainage. Epoxy coated rebar was added to supplement existing reinforcing.

Placement of the silica fume modified overlay using a scrub coat as a bonding agent.