Concrete Patching, Additives & Epoxies
Expensive Stuff
Concrete is made from basic, inexpensive raw materials. However, transportation and placement of these materials is costly and labor intensive. Because of its high cost and durability, people expect concrete to last for a long time. This is not an unreasonable expectation, as I have seen concrete 50 plus years old that is in very good shape.
But sometimes things go wrong. Your concrete drive, sidewalk or patio may experience a popout (depression caused by a piece of stone that explodes for numerous reasons). Those living in cold climates may experience spalling, a condition where the top surface of the concrete peels off. A corner of a concrete step may crack and fall off due to a handrail post expanding (rust and/or freezing expansion). In any event, a patch can be employed rather than replacing an entire segment of the concrete.
Challenging Task
I have seen numerous attempts at patching concrete. The majority of them have failed miserably. There are many reasons for patch failure. I'll try to cover some of the most persistent causes.
Concrete, in its original form (when first mixed) sticks together because of the cement. When the cement is mixed with water, a chemical reaction begins that is really pretty neat. Very small crystals begin to grow. These crystals interlock with one another and lock onto the pieces of clean sand and gravel. Also, when concrete is mixed, there is usually lots of extra water around to help keep the crystals growing. And grow they do, as the crystal growth slows down but can continue for years.
However, when patching, these two conditions are not always present. The area to be patched may be dirty and bone dry! If this is the case, there is a great chance that the patch will fail.
Water, water everywhere...
Remember when I spoke above of the growing crystals? The same thing happens, to a large degree, with many concrete patching compounds. They also need water for their crystals to grow.
Many people forget that concrete can and does absorb water. You can demonstrate this easily by getting on your hands and knees. Do this and place a drop of water on a piece of concrete. Observe what happens. The water, in almost all cases, readily soaks into the concrete surface.
When you place water based patching compounds on dry concrete, the concrete robs the water from the patching compound. No water, no crystals. No crystals, no bond. It is that simple.
Dirt also gets in the way as well. If dirt blocks the crystals from bonding to the concrete to be patched, all that happens is that the crystals bond to the dirt, not the old concrete.
Additives / Bonding Agents
To improve the chances of a patch sticking to old concrete, there are two categories of materials that you should consider. One category is additives and the other is bonding agents. They work towards the same goal, but they do it very differently.
Additives are chemicals or compounds such as rubber, vinyls, acrylics and different polymers that are sometimes added to portland cement mixtures. All they do is simply make the concrete mixture stickier. These compounds can be in a dry form premixed with a patching compound, or a wet milky liquid that you add to your patching compound. In either case, they work quite well when applied to a clean surface.
When using concrete patching compounds that contain additives, be sure to follow the instructions carefully. Some of these compounds must be applied to dry concrete surfaces, while others instruct you to wet the area to be patched. You must pay attention!
Bonding agents are glues that are applied to the old or damaged concrete surface that is going to receive the patch. These are almost always liquid compounds that you simply brush onto the clean, dust free area to be patched.
Once again, you must follow directions to the letter to achieve the best results. It is possible to use both compounds to maximize your chances of success. In other words, you can purchase a patching compound that contains additives - these are usually labeled as containing vinyl or acrylic,etc. - and use it in conjunction with a bonding agent. Always check the labeling to make sure the materials are compatible.
Epoxies
If you really want to go all out on your patching project, consider epoxies. These are no different than regular epoxy. You simply mix equal parts of a resin and a hardener together, often with a sand aggregate. You apply it and BINGO, super patch! The problem with some epoxies however, is coloration. You may not be really excited about the finished color. Some are white, beige, and very light grey. In contrast, regular Portland cement patching compounds dry to a medium grey.
The Old Stand By
Guess what? You can use just plain cement, sand and sometimes pea gravel to patch that hole. If done properly (50 percent cement - 50 percent sand and/or gravel), your patch will perform just fine.