How to Strengthen an Existing CMU Wall

Clean the CMU wall thoroughly. The surface must be free of dirt, paint, grease and other materials. Use a suitable cleaner, bristle brush, scraper and other materials, if necessary.

Fill any gaps or low spots on the wall face. Prepare a small amount of surface bonding cement according to manufacturer's directions, wet the areas where the cement will be applied and pack the cement into the gaps so that it is flush with the surrounding material. Let the cement dry for at least an hour before applying the wall coat.

Mist or otherwise moisten a small section a few feet wide at one end of the wall prior to applying the cement coat. This will prevent the blocks from absorbing moisture from the applied cement and hampering the curing process.

Prepare a small amount of surface bonding cement. Mix the cement with clean water according to manufacturer's directions, adding color pigments if desired. Prepare the cement in small batches so that it does not begin to dry before it can be applied.

Apply the prepared cement to the wall. Pile cement on a mortar hawk, then use a square-ended trowel to create a coat 1/8 inch to 1/4 inch thick on the wall surface, spreading the material by holding the trowel at a slight angle and using broad, upward strokes.

Smooth the cement after it has been adequately spread by running a wet trowel over the cement surface. Rinse the trowel frequently to keep it clean and wet.

Apply surface bonding cement along the rest of the wall. Moisten a section of the wall, apply the mortar to the damp blocks and smooth it with a wet trowel.

Cut vertical control joints into the cement with a groover every 8 feet or so once the cement begins to harden but before it becomes unworkable. The presence of these control joints will prevent or limit random cracking later.

Protect the cement-coated wall for at least 24 hours. In cold weather or rainy weather, cover the wall to protect it. In warm or dry conditions, mist or fog the wall to allow for proper curing.

Apply a bead of silicone caulk to the control joints once the cement has hardened. For outdoor walls, use a silicone caulk suited for exterior applications.