To repair a concrete stair or anything else made of concrete the first step is to remove all loose pieces and to be sure that what remains is solid. Before making a solid concrete repair you must remove loose concrete, sand, dirt and other debris from the area to be patched. This is the most important part of the repair process. A clean, solid base will ensure a positive connection and a lasting repair.
Next, two oiled wooden form boards are needed, one for each side of the corner. Plywood is all right to use. Wood is porous and can easily stick to freshly poured concrete. Therefore, wood forms must be oiled to prevent them from sticking to the concrete. Although special lightweight oil is available for this purpose (form oil), lightweight motor oil can be used as well.
If the concrete being repaired is surrounded by dirt, the form installation process will be simple. Stakes can be driven into the ground and nailed into the forms to hold them in place. On the other hand, if the repair area is completely surrounded with concrete, heavy weights will be needed to hold the forms in place. Flower pots, bricks, and even garden furniture can be used. Whichever bracing method is used, be sure to nail the form boards together at all joints in this case, at the corner. Curved shapes can be formed using several layers of bender board, thin plywood or thin sheets of Masonite. Lay a board onto the surface to be repaired and let one end protrude over the top of the form. The top of the form can then be aligned exactly to the top of the step.
With the repair area thoroughly cleaned and forms properly in place, all that is left is to wet the repair area, apply the bonding agent (concrete glue) and fill the hole with the patching compound. Be careful here. Each manufacturer has specific recommendations on how their bonding agent should be applied and how long it should cure (dry) before the patch is made. The best repair is made when the bonding agent and patch material are made by the same manufacturer. Although you can make your own patching compound from one part Portland cement mixed with three parts sand and one quarter part hydrated lime, we recommend that a commercial latex patching compound be used. These compounds are strong, flexible, adhere well and are available in smaller quantities, making them easier to handle and more cost effective. Once the patching compound has been placed, it will have to set up for a while before it can be troweled, edged and/or broomed.
Once the patching compound is in place it should be tamped gently with a block of wood or a concrete trowel to ensure that all cracks and crevices are completely filled. After tamping, the exposed areas will need to be "finished" to match the undamaged, adjacent surface area. If the patch is small enough, a putty knife can be used to do the initial surface smoothing. For larger areas a metal concrete finishing trowel should be used. Once the surface is smoothed an edging tool can be used to form a curved edge if one exists. Before going to the store to purchase an edging or nosing tool, cut a pattern of the shape of the stair edge on a piece of paper. This can be used at the store to purchase the proper tool. Trowel the flat surface first and then trowel the rounded portion. When this is done the final phase of finishing can be implemented. The forms are normally removed before the patch can be completely troweled, nosed and finished. The removal should be done very carefully to prevent damage to the patch.
Most exterior concrete surfaces have a slightly rough finish to prevent slipping when wet. The most common finish is the "broom finish." Once the concrete has had a chance to become firm it is brushed with a broom. A similar look can be achieved for a small patch by using a stiff paint brush.
Once the repair is made, you might consider staining the entire area with a concrete stain. Doing so will make the area look fresh and new.
