In my work as a home inspector, I see plenty of bad concrete. I also hear about disputes people have with their contractors when problems like cracking and surface loss show up. Contractors point out that soil settlement, salt, weather and loading will cause damage, and they can't control these circumstances. That makes sense as far as it goes, but there are many other aspects of a concrete job that they can control. The difference between a reputable cement contractor and a fly-by-night company is in how well they deal with the factors that are within the scope of their work. If you are having some flatwork (driveway, sidewalk, garage slab, basement floor) done, you should be aware of these things, and make sure the contractor knows you're aware!
Cracking:
Concrete shrinks as it cures. The shrinkage causes cracks, and these are hard to avoid on very large surfaces such as basement floors. If the top surface is allowed to dry too fast, it shrinks more rapidly than the bottom surface, causing the slab to warp upward at the edges. Any subsequent loading will push the edges back down, producing cracks on the surface. There are several things that can minimize the cracking. Keeping the surface wet for several days after finishing can help a lot. Traditionally, wet straw was used for this purpose. These days, polyethylene sheeting (Visquine) is often used. Flooding the surface for several days is sometimes possible, and very effective. Even if everything is done right, a basement floor may still develop a pattern of fine cracks, due to the lack of expansion joints in so large a slab.
If the mix was too wet to begin with, severe cracking is inevitable. The water content of the mix is determined at the batch plant, but the transit mix driver can, at the contractor's request, add extra water at the site. The resulting soupy mix is easier to place, but will produce a poor quality product.
When you order concrete work, be sure there is a clear, written description of how thick the concrete will be. Typical residential driveways are 4 inches, and many cities require six inches for the apron and the sidewalk adjacent to it. The thickness should be uniform all the way across the slab. This can be measured by pulling a string line or laying a straight board across the forms, then measuring down to the subgrade.
Expansion Joints:
Lack of proper expansion joints is another cause of problems. The spacing of joints is something of an art in normal concrete work. As a rule, in 4" concrete, any piece larger than 8 feet in any dimension will crack. Narrow, tapering pieces, such as at the ends of curved walkways are weak and will crack. Intelligent joint placement can do a lot to control cracking. If the concrete abuts a basement wall, there should be at least 1/2 inch of expansion paper against the wall.
Scaling (surface flaking off):
This is another one of those problems that can be the contractor's fault, or the owner's. If you put salt on a concrete driveway or walkway within the first year, the surface will begin to peel and may not stop until it's gone. The salt doesn't have to be deliberately applied, either - what drips from your car may be enough. It is possible to mix the concrete to resist salt damage, but this has to be done at the batch plant, and will add to the cost of the work.
Similar damage can be caused by overfinishing. This is another part of cement work that's more art than science. When to trowel the concrete, how hard, with what tool, and most importantly, when to stop troweling are all factors affecting durability. Too much smoothing can bring up excess sand and water, resulting in a surface so fine that it looks like cake frosting and is just about as durable.
Weather:
Exposure to subfreezing temperatures during the first 48 hours will cause scaling, spalling(loss of chunks from the surface), cracking and sometimes total failure of the concrete. Yes, sudden cold snaps may be acts of God, but very few of them sneak by the Weather Channel. Anyone who pours concrete when the temperature is 40 and falling is asking for trouble. Some suppliers add calcium chloride to the mix to allow cold-weather curing. The Michigan Department Of Transport swears by the stuff, and some other engineers swear at it. The best course, if you have a choice, is just to avoid pouring concrete in cold weather.
Loading:
Your new driveway is cracked?! Who drove on it? When? With how much weight? Did they move the substantial barricades that the contractor says he placed, or did they just ignore the flimsy ones he really did place? Or, had he already moved them to his next job? You moved them? Ohh!
Barricading is important! The waiting period for driving on concrete varies with the type of mix, thickness and temperature. The longer you wait, the stronger it gets. The time that the contactor tells you is the minimum needed to prevent cracking. If the job is remote from your location, (an unoccupied new house, for instance,) you would be wise to cruise by once a day to see what things look like. Take a camera.
Ground Settlement:
Yes, the ground under a concrete job can settle, causing cracking. This often happens next to new basements, and it means that the backfill around the basement wasn't packed down properly. Voids were left, or trash in the fill slowly decayed, creating voids. It's up to the builder to ensure that backfilling is done properly, before the cement contractor arrives on the site. If its your house that's being built, ask the builder how the fill under the driveway, garage floor or family room floor will be compacted.
For older sites, settlement is very rare and gets blamed for more problems than it actually causes. If mushy soil is encountered prior to pouring the concrete, it should be removed and the hole filled with well-compacted material such as slag. Old stumps and large roots should get the same treatment. Pouring concrete over a dubious base is an invitation to problems. This aspect of settling is entirely within the contractor's control.
Other things than settling can cause the concrete to move: tree roots can easily lift a sidewalk or driveway; frost does a lot of damage, too. The wetter the soil, the more it will heave during freeze-thaw cycles. If all the rainwater from the roof of your carport drips next to the driveway, the driveway will lift every year. Maybe it will settle back down in the summer. Good design and meticulous maintenance of gutters and downspouts can prevent most frost problems, but this is the homeowner's responsibilty, not the contractor's.
So what can you do to make sure your concrete job holds up? Here are some suggestions:
copyright 1996 Matthew J. Bezanson