Patching Cracks in Concrete Blocks

by Brian Knight—GBI Technical Advisor

Someone I know patched the cracks in the concrete block walls of her house using a different technique than the one I described in Repairing Cracks in Concrete. When it rained hard, her basement took on a lot of water through cracks in the walls. The cracks ranged in size from 1/16″ wide to some so large she could see daylight through them. Since concrete blocks are hollow, injecting WEST SYSTEM® Epoxy into the cracks would be unacceptable. So she decided to fill the cracks using epoxy thickened to a mortar-like consistency.

First she used a stiff bristle brush and water to wash the areas to be repaired. She worked the bristles into the cracks and cleaned them as best she could. When everything was dry, she mixed a small batch of 105 Resin and 206 Slow Hardener. To this mixture, she added Portland Cement powder to increase the volume of material, and also a little 406 Colloidal Silica to thicken it to a non-sag mixture. Portland Cement is one of the materials used to make concrete. It is very inexpensive and mixes to a gray color similar to mortar. When mixed with epoxy, it has good compressive strength. She also could have used 403 Microfibers, dry sand, pulverized limestone, ground glass, or just about any dry, inert powder.

With a tongue depressor, she forced the thickened epoxy mixture into the cracks. When she felt that there was enough material in a particular crack, she tooled the surface smooth with the end of the mixing stick and moved on to the next area. Working in this manner she filled all the visible cracks on the inside of the wall. Then she moved the operation outside the house and repeated the procedure. Outside, she did not fill any cracks below ground level because she didn’t want to dig up her yard.

Before she repainted the walls, she decided to wait to see how effective her repairs were. It has rained hard several times since she completed the project, and so far, she is happy with the results. Her mop and pail are gathering dust in the corner of her basement.