On a recent job, we needed to restore a section of structural concrete. This particular job was a repair to an overhead section in a lighting niche in the mechanic’s well at a garage that services school buses. Though it’s not the most usual example, it provided a textbook case on concrete restoration.
Repair work began by chipping out the unstable concrete around c…
Surface prep. The first step was to remove the damaged concrete, using a rotary hammer to chip away the bulk of the loose material and a grinder to take the concrete down to solid material. The goal was to work back to a solid base and to fully expose the existing rebar so we could fully encase it in repair mortar. In addition, we used a grinder to remove the old paint around the repair area. This would help blend in the repair so when the final work is painted, you won’t see where the new concrete meets the existing surface.
Some surface corrosion of the rebar had occurred but not enough to decrease the dimension of the steel to any significant extent. We used a wire brush to clean up the rebar and then painted it with Rust-oleum Green Rebar epoxy paint. It’s important not to get any paint on the surrounding concrete; otherwise, it can interrupt the bond of the new concrete with the old material.
Before setting the form, a crew member sprays the form board wit…
Repair mortar. For the mortar, we used Sika’s VOH (which stands for “vertical and overhead”)—a fast-setting repair mortar. We used this to first mix up a loose “slurry mix,” which we painted on with a mason’s brush after spraying the surfaces with water to achieve what is described in the engineer’s spec as “SSD,” or saturated surface dry.
As the slurry mix cured, we set the form. For a small form like this, we simply used WD-40 as a release agent, being careful to spray down the form away from the repair area. We then mixed up a new batch of VOH to a stiffer spec and packed this material in small lifts, working it in to make sure we wouldn’t have any voids when we stripped the form. There was also one small overhead section at the back of the niche, outside of the form area, that we would need to fill by hand after the form was stripped.
All photos by the author.
