Got a driveway that’s too low to meet the garage floor? Or a patio full of “bird-baths?” How about a cracked concrete step or a scaled sidewalk? There’s no need to live with these problems. You can either hide them under a new layer of concrete or patch them. What you do depends on how extensive the damage is and whether- they’re concrete slabs or concrete walls. You can resurface a slab but not a wall.
A cracked concrete wall can only be patched. Resurfacing is used to renew, cover up or raise an existing concrete slab such as fixing concrete foundation. It also can beautify, as in resurfacing with a colored concrete or rustic terrazzo topping. Concrete is resurfaced in one of two ways: bonded or unbonded. Bonded resurfacing is stuck strongly to the top of the old slab and can be as thin as ⅝ inch, even thinner in some cases.
Building an unbonded resurfacing is much like casting a whole new slab on top of the old one, using the old slab merely as a subbase. The resurfacing must be at least 2 inches thick and should be reinforced with steel mesh weighing at least 30 pounds per hundred square feet. The mesh should be placed in the middle of the resurfacing, a good trick if you can do it. A layer of sand or a polyethylene bond breaker is spread over the base slab before placing the resurfacing concrete. It keeps the two from bonding into one structural unit.
Control joints and isolation joints in the new slab are made according to the rules given in another section. An unbonded resurfacing should be used where a raise in the elevation is desirable, or where the old concrete slab is badly cracked.
To take a bonded resurfacing, the existing concrete slab must be structurally sound. If it’s cracked, the resurfacing will crack, too, within a short time. To get a good bond for resurfacing, first chip the old slab’s surface with a mason’s hammer. Brush off all the loose particles. Thoroughly clean the surface of grease, oil, paint, wax and other materials.
Grease and oil can be removed by scrubbing with trisodium phosphate (TSP) sprinkled onto hot water poured over the concrete surface. Paint must be chipped or ground off. Provide “tooth” to the old surface by etching with muriatic acid, one part acid in five parts water. Flush the surface to remove all traces of acid. Keep the concrete slab wet overnight. Before placing your resurfacing remove all free water on the surface.
You have your choice of two methods of bonding a resurfacing: portland cement slurry bond, and bonding agent. The cement slurry bond is cheaper but requires the chipping and acid-etch previously described. To use it, sprinkle portland cement over the wetted slab and scrub it into a thick slush coat. Brush this out well to avoid a too-heavy coating. Resurfacing concrete is placed right over the slush coat before it dries and whitens.
If it whitens, its bonding properties are lost. When using a bonding agent, such as Weld-Crete, you can omit the chipping and acid-etch. Tightly clinging paint need not be removed. Loose paint should be taken off. Bonding agent comes in 1/2 pints, pints, quarts and gallons and should be available through your building materials dealer. Material cost is about 3 cents a square foot or more, depending on the size of the job. Big jobs lower the per-square-foot cost. The agent may be applied by brush, roller or spray. Resurfacing concrete is placed on top of the bonding agent after an hour of drying.
If a thin resurfacing is wanted, such as one that will trowel out to a feather- edge, bonding agent can be applied both to the surface and as an additive in the resurfacing mix water. Make the mix water of one part Weld-Crete in three parts plain water. Used this way, bonding agent permits feather-edging, according to the manufacturer.
A patch is simply a small, thin feather edged resurfacing. You may use the Weld-Crete system just described for resurfacing. Or you can make your own patching mix using the following formula: Add 2 pounds of concrete plaster to 45 pounds of ready packaged sand mix concrete. Mix dry. To use, add ice water and patch quickly on top of a cement slurry or bonding agent treatment. You don’t need to wait long for the patch to set up. The concrete plaster kicks off the reaction Quickly.
Finish your patches to look as much like the surrounding concrete as possible. Don’t expect a perfect color match. Close is all you can expect. Patching and resurfacing sure beat breaking up an old slab and building a whole new one.
Plywood siding gives interesting textures. Casting concrete in a pegboard form gives it a pimple-dimple effect. Take such a form off before the concrete gets too hard. Other materials can be fastened inside a wood form to give texture. Rubber matting and formed plastic are made for this purpose, but not generally available to us concrete handymen. Rubber stair tread materials will work nicely, however. So will carpet-like rubber mats.