Patching concrete is a process that begins with evaluation. You don't want to bother with a concrete patch if your concrete is beyond repair.
You'll just end up resurfacing over your concrete patch before too long. Fortunately for most home applications, evaluating concrete is pretty simple, particularly when it's a sidewalk, walkway, or driveway that you will be repairing. On the other hand, if you see a structural element with a crack, you need to call in a structural engineer Once you see a crack in concrete, the next step is to try to determine what caused it. Concrete is one of the most versatile and flexible construction materials available. Its compressive strength is unmatched, and a concrete foundation is the start of structures from the smallest house to the largest skyscraper. However, most concrete is exposed to everything humans and mother nature can dish out: rain, ice, snow, heavy loads, constant stress from car tires ... it is no wonder that cracks in concrete are so common. Surface cracks that show up after a hard rain and a hard freeze are pretty to easy to classify. But sometimes the source of a problem with concrete is a little harder to diagnose. For example, suppose you have a set of concrete stairs that has a stain on it. Upon closer inspection you may see that the stain is made from rust. This probably means that the reinforcing steel in the concrete is deteriorating, and that's a problem that a concrete patch won't fix. Ask yourself how widespread the damage is to a piece of concrete. Can you tell if a crack is superficial, or deep? Are cracks still progressing? Is surface delamination confined to the surface? While a concrete patch can fix superficial damage quite well, it will be useless against deeper structural damage. The nature of the crack or damage will determine the supplies and materials you'll need to make a concrete patch. For many repairs, high quality concrete is the best repair material. Repair materials often are made of polymer modified concrete, which is a basic Portland cement and aggregate mixture with a polymer such as latex added to it. Patching material comes either premixed in bags, or in the form of raw materials. You'll have better control with the raw materials because you'll be able to vary the amount of Portland cement to your preference. In general, for patching concrete you will need the following tools and materials: * patching mix or water, sand, and cement * gravel for aggregate if the repair is deep * container to mix cement and water * paintbrush for applying the cement/water mix * bucket or other container to mix patching concrete * wood float to smooth out the surface of the repair * tarp * spray bottle * concrete stain or paint if necessary If you're patching a hole of 8 cm or deeper, you can use 1 inch gravel as your aggregate. Shallower concrete patches will need pea gravel or sand instead. Your patch mix should be made up in the following proportions: 3 parts gravel2 parts coarse sand1.5 parts cement But, if you're repairing stucco or brick, you simply use 2.5 parts sand with 1 part Portland cement. To prepare the area for repair, clear away all loose stones, grit, and dust. Mix a 1 to 1 solution of water and cement. It should have the consistency of paint. Dampen the repair area slightly with a damp cloth or spray bottle, then paint the water/cement solution onto the area where the concrete patch goes. Then apply the patching compound on top of it. Using a wood float, smooth the surface of the patch. Instead of mixing your own repair cement, you can use an acrylic bonding agent, which is chemically similar to the resins in paint. You can find acrylic bonding materials that you add to the patching compound. There are also acrylic bonding agents that are simply painted onto the old concrete, in the same manner as the water/cement solution described above is painted on. You can use one or the other, or both if you want to take a "belt and suspenders" approach to the repair. These acrylic resins will have to cure slightly before you apply the patching compound. You can tell when this happens because they turn from a thin, milky color to being clear. When it has turned clear, then you add the patching compound. The key to having a long lasting patch is to allow it to cure properly. That means placing a piece of plastic or tarp over the patched area and not letting it dry completely for about a week. You can mist it with a spray bottle once or twice a day. It shouldn't be wet to the point of having standing water, just consistently damp. This allows the chemical reaction between the cement, water, sand, and gravel to proceed optimally to form the small crystals that hold the concrete together. For those who are very ambitious, larger repairs such as re-attaching a corner from a step or a patio are often feasible if you use steel pins for the patching mixture to grab onto. The concrete patch instructions already discussed, which only use a bonding agent, will not be strong enough for a larger repair like this. You'll need some 1/2 inch rebar pieces of 4 to 5 inches long, metal primer to paint it with, and a hammer drill, which you can rent if you don't own one. Once you've painted the rebar with metal primer and it's dry, use the hammer drill to make a half-inch hole at least three or four inches deep into the the center of the old concrete. Then drive in the rebar with a hammer. This won't be easy, because it's a very tight fit. However, the rebar is necessary to hold the repair in place and make the repair as permanent as possible. If you have made a concrete patch on painted or stained concrete, you will need to wait until it has cured before you repaint or re-stain. Be aware, however, that the new concrete will have a different profile and a different porosity than the old concrete, and will therefore take paint or stain differently. You would be wise to paint a small, inconspicuous area on the patched concrete to be sure that the results will match well enough to suit you. If the results aren't good, mitigative options include applying a thin micro-topping or a concrete overlay over the entire concrete slab. It is very important to note that patching concrete is only successful if any underlying cause of the original crack or damage has been taken care of. Otherwise you'll find yourself having to make another concrete patch, and another until you find out what is causing the damage and fix it. When done properly, however, a well executed concrete patch can last 20 to 30 years and can blend in well with the original concrete.
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