Preparing an Existing Slab for a Mortar Bed
Materials: Muriatic acid, foam rods, silicone rubber caulk, mortar mix, bonding agent
Tools: Bucket, putty knife, caulk gun, paintbrush, steel trowel, safety goggles, rubber gloves, work gloves
Anew patio doesn’t necessarily mean you need to pour a new slab. In many cases you can apply brick, stone, or tile over an existing slab. You’ll need to patch and clean it, but it’s considerably less work than starting from scratch. Control joints in the concrete can cause cracking in the surface above them, so tile and brick work best on an old surface.
Lay out the brick or tile so a mortar joint is directly above the control joint, and fill the joint with caulk instead of mortar or grout. First make sure the existing slab is sound. Cracks and irregularities can be repaired, as can a few small areas that have broken away. Make sure the area is flat— if a crack has created a step in the surface, it will crack again after you patch it, and the crack will eventually work its way through the brick, stone, or tile.
Crumbling areas spell big trouble: Either the concrete was poorly mixed or multiple freeze/thaw cycles have broken the surface. If the slab surface is crumbling, talk to a mason about whether the pad can be repaired. Check that the surface of the slab slopes away from the house at a rate of at least 1/8 inch per foot. If it doesn’t you can correct the problem by sloping the forms when you apply the mortar.
Etch the concrete surface with a mixture of muriatic acid and water to clean it and create a surface to which mortar will stick. Make sure you add the acid to the water—not the water to the acid—to avoid dangerous splashes. Ensure a good bond for the mortar and brick, stone, or tile by applying a bonding agent.
Cracks and shifting occur naturally in concrete slabs. Even the best laid slabs will eventually be affected by years of seasonal changes and freeze/thaw cycles. Repairing cracks and covering an existing slab with mortared bricks or pavers adds a design element to your yard and saves you the labor of removing damaged concrete.
1 Clean the surface. Scrape off gum, wax, and dirt. Carefully mix five parts wafer and one part muriatic acid, adding the acid to the water to avoid dangerous splashes. Carefully pour the solution on the slab. Rinse the slab thoroughly with fresh water. Wear rubber gloves and safety goggles when working with muriatic acid.
2 Fill in the control joints. Patch broken or wide control joints by pushing foam backing rods below the surface with a putty knife. Seal the crack with a silicone rubber caulk. If you are laying brick or tile on top of the concrete, put the mortar joint directly above the control joint to avoid cracking.
3 Fill in cracks. Brush a concrete bonding agent on an area 6 inches wider and longer than the crack. Then fill the crack with mortar and level it with a steel trowel.
4 Apply a bonding agent. Just before you apply mortar, brush on a coat of concrete bonding adhesive to the entire surface, following the directions on the package.
5 Regrade the area around the pad. Once you install the brick, stone, or tile, the surface of the patio will be above the adjacent ground surface. Regrade the area, adding soil and raking it smooth until it is flush with the patio surface.
Shifty surfaces - Steps are cracks that have shifted so much that one surface is higher than the other. Steps can cause problems because they are likely to appear again and may mean that the pad has to be removed and repoured. Consult a professional landscape or concrete contractor to make sure the steps won't reappear, this time through your new patio.
Work Smart - Correcting The Slope - You can add a bed of mortar up to 4 inches thick to a patio without compromising the strength. Take advantage of this method to correct any problems with the existing patio's slope. First build forms around the patio the way you would if you were pouring a new patio, keeping the surface as low as possible while making sure the forms have the correct slope. Fill the forms with mortar and pull a screed across it to flatten it. Let the mortar cure, then lay the stone, brick, or tile.