The cement finisher shown here is masking an area to be patched.
Jason Sperry The cement finisher shown here is masking an area to be patched.

The first thing a customer needs to know is that patching concrete is a natural part of doing concrete work—the material isn’t perfect. But the art and science of installing durable, invisible patches is performed best by companies able to hire people with good skills in this area. These individuals have a mix of artistic and craftsman skills, patience, and good perception of color and texture. Companies that specialize in architectural cast-in-place work are often the ones who afford to keep these people on staff—these companies look for the most experienced individuals.

Jason Sperry, a superintendent for Morley Construction, Santa Monica, Calif., is currently responsible for a large cast-in-place architectural concrete project using titanium oxide as an integral white color admixture. He has a very skilled, professional cement mason on his team to patch blemishes, such as spalling, bugholes, and corner voids, removing them from view.

The masking follows the exact edge of the patch area.
Jason Sperry The masking follows the exact edge of the patch area.

Preparation

The first step involves the construction of mock-up panels on the jobsite so owners or architects can approve color and finish. They also become the experimental panels where patch formulas are worked out. When patch colors match panel colors, the recipe is defined by volume measurements. Sperry says some environments demand more stringent recipes with weight measurements.

He adds they usually allow a drying period of three to four weeks after forms are removed. The color of the concrete becomes more defined and stable after this interval, including the effects of efflorescence.

The next step involves carefully masking the concrete around the patch; this is critical to the process. Patch experts place painters tape around the void and cut the zig-zag shape of the crack edge so the tape is always right on the edge of the hole. There can’t be any straight lines. If a patch cavity isn’t deep enough, it should be deepened with a cold chisel and hammer, being careful not to create more patch area.

Their basic tools include a 3-inch mason trowel, a razor to cut the painters tape to match the void shape, a small straightedge, and rags and sponges.

The finished patch is undectable.
Jason Sperry The finished patch is undectable.

Guidelines for patching

When a patch is deeper than 1/8 inch, a base “rough” patch is installed first, holding it down 1/8 inch for the finish coat. It’s made with the same cement used in the concrete, a rapid-setting cement with sand aggregate, and an acrylic bonding agent. The acrylic polymer also acts as a waterproofing agent, restricting movement of moisture into the patch.

One or two days later, workers make the finish patch material. It’s a combination of the original cement powder, a commercial polymer-cement product, and color to match the concrete—the mix which was worked out on the mock-up panel. It’s applied with the small masonry trowels and finished to resemble the surrounding area. As it begins to set, the masking is removed so final patch blending can be achieved.

Using a small pointed mason trowel, the cement finisher applies the color matched patching material.
Jason Sperry Using a small pointed mason trowel, the cement finisher applies the color matched patching material.

Sperry and his crew let their patches dry out for the same amount of time the concrete dried out, about three to four weeks. Then the color match is viewed. If it is way off, the top surface can be removed and the patch process repeated. However, normally the patch is close to the mark and can be tweaked with translucent stains lightly applied to the entire area around the patch with rags or sponges.

Decorative concrete contractors sometimes make the mistake of thinking that patching is something anyone can do. But installing good patches is artistry at its best, and the people who can do undetectable patch work are very skilled.