Q: Our company placed a poured concrete wall with a board finish. There are a few honeycombed areas that have to be patched. How can we make the patches as unnoticeable as possible?

A.: It's nearly impossible to make a patch that will blend perfectly with the rest of the concrete. If the honeycombed area isn't too large, it will probably be less noticeable left as is. A poorly matched patch may draw more attention than the original defect.

If the owner insists on patching the honeycomb, first work on matching colors of the repair material and existing concrete. Patching mixes for shallow surface voids are usually mortars. As a starting point in designing the mortar, use the cement-sand ratio from the actual project design mix. For example, if the design mix calls for 564 pounds of cement and 1,190 pounds of sand per cubic yard, the cement proportion is 32% of the dry ingredient weight.

A batch made with 5 pounds of dry ingredients is about the right size to permit placing before the mix stiffens too much. For a batch of this size containing 32% cement you'll need 1.6 pounds of cement and 3.4 pounds of sand.

Starting with a known amount of water in a container, add water to the mix until it reaches a stiff ball consistency. Sometimes it helps to add an acrylic or latex bonding agent. Use about 10 to 20 milliliters per 5-pound batch.

After mixing in the bonding agent, use the trowel-slump test to measure batch wetness. Pick up mortar on the back side of the trowel and tap the trowel on the side of the mixing bowl twice. Then turn the trowel so the blade is vertical. If the mortar slips off it is too wet and the batch should be discarded. If the mortar falls off it is too dry. Add water as needed to get the right consistency.Compute the final amount of water used by subtracting the amount left from the amount you started with. Record all batch weights so you can duplicate the batch.

To test the color match, try placing some of the mortar in a part of the structure that won't be seen. Let it cure for several days by first spraying the patch with water, then covering it with polyethylene held by duct tape. Color of the patch should be slightly on the dark side since it will bleach out with age.

If necessary, vary the patching mix color by blending white cement with cement originally used for the job concrete. Try three different cement blends, using 10%, 30%, and 50% white cement. Batch quantities for the three trial mixes would be as follows:

Material Batch Weight, Pounds
Batch 1 Batch 2 Batch 3
Original cement 1.44 1.12 0.08
White cement 0.16 0.48 0.80
Dry sand 3.40 3.40 3.40

When you've got a mix that's the right color, make sure all batch quantities are recorded so you can reproduce the mix.

Before the patching begins, use a hand-held hammer or small pneumatic chipping tool to dress the hole and square the edges. Brush out any dust, prewet the concrete surface, and apply a bonding agent. Apply the patch mix and allow it to take an initial set. Finally, take a small section of board like the ones used for forming the original surface and press it into the patch to duplicate the as-cast surface as closely as possible. In some cases it might be easier to try to tool the surface with a trowel or other tool to match the initial markings. Then cure the patch by spraying with water and covering with polyethylene.

The patching process, especially color and texture matching, requires patience and skill. For big jobs, you might consider hiring a concrete restoration specialist. One such specialist is Al Perez, P.O. Box 4045, Irving, Texas 75061. Much of the information given here is based on an article Perez wrote for Concrete Construction magazine (July 1986, page 623).