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Concrete Crack Prevention

concrete cracks

Concrete Crack Prevention

DEAR TIM: We are having a new concrete driveway installed. What can be done to prevent cracks in the new surface? Is it realistic to expect the surface to last for 25 to 30 years with no maintenance? How thick would you pour the drive? Would you install any steel reinforcing? Does the driveway need to be sealed once it is poured? Maria B., Janesville, WI

DEAR MARIA: Concrete is a wonderful driveway surface. If your driveway is installed by a seasoned professional, then your driveway should easily last 30 years. In fact, I have seen concrete driveways in my town that are nearly 50 years old that are still in good condition. Long lasting, crack free concrete does not happen by accident. It must be ordered correctly, installed with great care, finished properly, and cured carefully.

Cracks are to be expected in new concrete. Professional concrete masons realize this fact and take measures to minimize their appearance. Concrete is very strong if you try to squeeze or compress it. But if you try to stretch it or bend it, it usually only exhibits 1/10th the strength that it possesses when you compress it. If the new concrete in your drive is poured over soft, uncompacted soil, your car or a heavy delivery truck will bend and crack the concrete as it passes over the soft spot. This is why concrete needs to be poured on solid, compacted soils.

Concrete also shrinks as it dries and hardens. It can shrink about 1/16th of an inch for every ten feet of length. This shrinkage creates tension within the concrete that tries to tear it apart. Professionals who install concrete driveways install crisp tooled lines in the slabs. These are called control joints. The lines act as pre-weakened stress points that encourage the concrete to crack at these locations. The joints can also be installed as soon as the concrete hardens by using a diamond saw that cuts the line. The control joints must be deep enough to perform their job. The minimum depth of the joint should be 1/4 the thickness of the slab.

Spacing of control joints is critical. Make sure the joints on your driveway are spaced at intervals of 15 feet or less. It is a common practice to make the spacing equal to the width of the driveway. If your driveway slab will have sharp 90 degree corners, control joints should extend from these corners out into the slab. Concrete cracks often radiate from corners, so make sure you discuss the control joint layout with the contractor before the pour begins.

Thicker concrete is a good idea for driveways. I would like to see you install a minimum 5 inch thickness. Often contractors might try to tell you that a 4 inch thickness is sufficient. The cost to upgrade to a 5 inch thickness on an average driveway is chicken feed when you compare it to the total cost of the job. For example, if your driveway measures 720 square feet, a 4 inch thick pour will require 9 cubic yards of concrete. Increasing the thickness to 5 inches requires 11 cubic yards of concrete. The extra two yards of concrete will only cost about $130.00. (1999 pricing - adjust for inflation)

Reinforcing steel is a must in my opinion. Steel is very inexpensive and it helps to strengthen the concrete significantly. Steel will hold cracked slabs together. Without steel, cracks can grow in size and you can get offsets where one part of the slab is higher or lower than an adjacent piece. Steel needs to be placed no more than 2 inches down from the top of the slab for maximum performance.

Concrete that has been ordered with enough cement so that it is a 6 bag mix (564 lbs of cement) or a 4,000 pounds per square inch design strength, contains just enough mix water to give it a 4 inch slump consistency, is finished without the use water, and is moist cured with plastic or with a liquid curing compound should not have to be sealed.

If your concrete driveway is poured correctly, the surface will withstand cold weather, deicing salts, vehicle traffic, basketball games and tricycle road races for many years. Weak concrete that has been ordered and installed improperly needs to be sealed with high quality silane or siloxane water repellents. Do not use film-forming silicone sealers.

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Author's Notes:

May, 2004

Keep in mind that the above strength rating of 4,000 psi concrete is a minimum standard. There is no reason why you can't order concrete that is 4,500 or even 5,000 psi. It will cost slightly more simply because they are adding more cement per cubic yard. But if you expect the concrete to be subjected to harsh conditions, this extra cement could extend the useful life 25 to even 50 more years!

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7 Responses to Concrete Crack Prevention

  1. Moderation? We feel we have a fine contractor. I needed to know the expectations. Our Village has restrictions and inspects along the way also. Thank you.

  2. Thanks Tim! What about the driveway base. Replacing our 33 year old drive. One contractor says our old landscape river rocks that we are getting rid of will be fine additional base. These are rounded 1 - 1 1/2" stones. This seems a nice environmental solution but not sure that is the best base. Don't jagged edges compact better?

  3. I sealed my concrete driway last summer and it was beautiful; however, a couple of white lines have appeared in the middle of the driveway (like discoloring of the concrete). Why and whay can I do?

  4. Is this correct? "Steel needs to be placed no more than 2 inches down from the top of the slab for maximum performance."

    If the steel is 2" from the top of a 6" slab, it's along the neutral axis - 0 compression, 0 tension. The steel needs to be in the tension zone to do any good. For a 6" slab the steel should be 2" from the bottom. For a 4" slab the steel should be 1" from the bottom.

    • The neutral axis in a 6-inch slab would be 3 inches down, not 2 inches as you say.

      The farther away from the center of the slab, the more benefit you get from the steel, but you need to keep in mind the size of the aggregate in the concrete. If the concrete has 3/4 or 1-inch aggregate and the steel is only 1-inch up from the bottom or top, you're going to have strength issues for sure.

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