Long Lasting Concrete Repair
Permanent concrete repairs are possible, well, nearly permanent. What I mean to say is that it is possible to expect a 20 - 30 year life expectancy from a properly executed repair. That isn't so bad if you ask me.
The trick to a long lasting concrete repair lies in bonding the new material to the old material. Yes, I know you already probably knew that, however, the point needs to be stressed. Many people think that the new patching material will magically stick to the old concrete. No such luck! You need to understand the process of bonding.
The Magic Crystals
The concrete that makes up your steps, driveway, sidewalk, etc. is usually comprised of four basic ingredients: water, sand, gravel, and cement. The cement is the glue that holds the sand and gravel together. It does this my reacting chemically with the water that you add to the mixture. As soon as you add water to cement powder, a chemical reaction begins to happen. Very tiny crystals begin to form. These crystals interlock with one another and lock into and onto any irregularities of the sand and gravel particles.
The more crystals that form, the stronger the bond will be. Hmmmm, did I say more crystals? Well, you can get more crystals if you don't skimp on the cement! You can also increase your chances of success with concrete patching if you keep wallpaper in the back of your mind.
Have you ever seen those home repair shows on TV? Some show the fairy tale couple who mix a concrete batch up and pour it into a hole. If we were to go back and do an autopsy of the patch, we would see lots of stones from the new mix that are touching the old concrete. There would not necessarily be a uniform amount of cement paste coating the old concrete. This is the primary reason for most concrete patch failures.
As soon as you understand that you must glue the old concrete to the new mix, you will be on your way. There are several ways to accomplish this. You can do it the old fashioned way which, I might add works very well, or you can choose to use some newer additives or bonding agents to your concrete mix.
The old method of securing a patch involves simply mixing up a cement paint. Nothing could be simpler. You take Portland cement, add water until you have a paint consistency, and then brush this onto the area to be patched. Of course, the area to be patched should be ready to go. This means that the old concrete must be free of all loose stones and grit, free of all dust, and finally slightly damp.
Just before you are ready to install the concrete patching compound, you simply paint a thin coat of the cement paint onto the clean, solid, damp old concrete. The patching compound is then added. The result, as long as you finish it, cure it, and protect it, will be a long lasting patch.
If you wish not to use the cement paint method, you can use many of the acrylic bonding agents that are available. These chemicals are not much different than the resins used in paints. Yes, paints are glues too! They stick to walls, don't they?
There are bonding agents that you add to the patching compound, and there are bonding agents that you paint onto the old concrete. You can actually use both, if you wish. These compounds work very well if you follow instructions to the letter.
Often, the agents, that are painted onto the old concrete, must cure slightly before you add the patching compound. They are usually a milky white color when you paint them on. Depending upon the temperature, wind, and humidity, they then become clear. When the agent turns clear you can then add the patching compound.
Pinning the Patch
Large concrete patches such as a step, corner of a driveway, sidewalk, or patio must be attached to the old concrete with a mechanical pin. The cement glue or additives alone will not do the job.
I have successfully employed standard reinforcing steel bars for years. They are inexpensive, easily obtainable, and the new concrete readily grabs onto the bumps and knobs on the reinforcing steel. The rough profile of the bar also is an advantage when you drive it into the old concrete. As long as you drill the right sized hole for the rod it is virtually impossible to remove the rod. In fact, as you drive the rod into the old concrete it will feel tighter than when you drive a nail into wood.
Installing the Pins
I have had the best luck in my repair work when I used 1/2 inch reinforcing steel driven into a 1/2 inch hole. Hammer drills quickly drill holes into old concrete. If you don't own one, they can be rented at any tool rental shop. I prefer to drill at least a 3 to 4 inch deep hole when possible. Try to stay away from the edges of the old concrete. You want to drill directly into the center of the old concrete. Use a 4 pound or heavier hammer to drive the steel pin. The length of the pin should be predetermined. In other words, don't cut off the steel after it has been driven into the old concrete. Always try to maintain a 1 inch or better coverage of patching material over the pin.
If you have the time, I would also recommend that you prepaint the pin with a good metal primer. Rebars can and will rust within your concrete patch. If it gets bad enough, the rusting pin will expand and actually crack the patch! The extra time spent painting the steel pins is worth it.
Concrete Repairs in General
Loose paving bricks, flagstone, marble, sandstone, etc. can be successfully repaired. Keep in mind the earlier points concerning concrete paint, dust, and a rich patching compound. Weather is also important. How many people think that hot or warm, dry, breezy weather is the best time to repair concrete? Raise your hands now.
Wow! Have we ever got a lot of work to do! This is the worst time to do these repairs! Cool damp, overcast weather is the best. Now mind you, I didn't say rain or drizzle, I said damp.
Remember earlier, when I said the cement needs water to start the chemical reaction that starts the crystals growing? Well, the cement also needs water to maintain the chemical reaction. How long? Well, let's say for 2 - 3 months or so. In actuality, it is really longer. No kidding! So, if you mix up a patching compound batch, install it in hot breezy weather, the water may leave the mix before enough crystal have grown. The patch will be super weak. It will crumble. What?, This sounds like what has happened to you? Well, we now know it won't happen again!
Take the time to dampen the area to be patched. Take the time to cover your work with plastic after you are finished. Take the time to douse the patch with water after it has become stiff. Do this for 4 - 7 days and you will be amazed at how strong your patch will be! If the patch is out in the open, try to shade it from the hot sun as well.
Mixing - Easy as 3 - 2 - 1 (Well maybe 1.5)!
Concrete patching material can be purchased two ways: premixed in bags or raw materials that you have to mix. The raw material route is almost always cheaper by a long shot. Plus, you will have the necessary pure Portland cement on hand.
One thing you always need to keep in mind is the size of the gravel you need. If you are patching a deep hole, say 3 inches or deeper, you can use 1 inch gravel. Common sense would tell you that a shallow hole just can't be patched effectively with a mix containing large gravel. Shallow patches will require pea gravel or sometimes even no gravel!
When you do use gravel in your mix, use these proportions: 3 parts gravel - 2 parts coarse sand - 1.5 parts cement. The old rule of thumb was always 3 - 2 - 1. However, since the cement is the glue and you only want to do the patch once, why not add a little extra???
If you are doing a stucco patch or replacing a flagstone, brick etc., you will not use gravel. Then mix 2.5 parts sand to 1 part cement. This will be a great mix. The patching compound should be at least 3/8 inch thick.
Finishing concrete is easy. It takes two things - a little time and some patience. Oh yes, some decent tools wouldn't hurt, but are not always necessary. Believe it or not, a simple block of wood will produce excellent results. I've done it on many occasions.
To achieve a nice sand finish (the easiest), you need to drive the gravel pieces at least 1/4 inch below the surface. This can be done by rubbing a board across the top of the concrete patch back and forth in a seesaw fashion. Do this several times after you initially pour the concrete. This board removes excess concrete and brings sand and cement to the surface.
Now you must be patient. Remember the crystals? Well, every minute that passes, more and more grow. To achieve really nice finish results, enough crystals must form so that the surface of the concrete is firm but plastic. It is hard to describe. When you initially pour the concrete, the mix is fluid. This is no good, as your tool makes marks. However, let the concrete get too hard and you will have problems. You simply need to watch over and experiment. Temperature and humidity play a big part in the set time. Watch yourself in hot weather. The concrete can get away from you!
NEVER trowel back in bleed water which will in many cases appear at the surface. Bleed water is what makes the concrete look as if it is sweating. Leave this water alone! It will evaporate quickly in most cases. Troweling it (or any other water for that matter!) into the surface will weaken the surface of the concrete. You are diluting the cement!! This is the primary cause of concrete scaling!
If you desire a smooth, steel trowel finish, you must first finish the surface and achieve a sand finish. This is done by gently swirling a wood float, block, or magnesium trowel over the stiffening concrete. Once a few more crystals grow, the concrete can be finished with a steel trowel. This trowel and skilled workmanship bring the cement paste to the surface. As this hardens, you can get a surface as slick as glass. It is truly and art form. Don't expect perfect results your first try.