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Without good joints cracks can grow

QuestionOur concrete driveway is cracked and in need of repair. It is as wide as our two-car garage and has two joints, both running parallel to the street. I have heard concrete joints called "expansion" joints and "control joints." What is the difference between the two? What kind of joints and how many of them should a driveway have, and how close together should these joints be placed? Should they run lengthwise or crosswise—or in both directions? Can I cut these joints into my driveway myself? Thanks for your help!

AnswerThanks for your questions about expansion and control joints. It can be confusing, so for accuracy I turned to the Portland Cement Association and spoke with David Shepherd, AIA, director of sustainable development at Portland Cement.

He tells me that an expansion ("isolation") joint is used to separate two building elements. A good example of this is when a concrete driveway, which is subject to frost and heave, butts up to a concrete foundation wall, which should never move. The joint isolates the two concrete structures. Typically, asphaltic-impregnated fiberboard is inserted to create the joint between the two surfaces.

Shepherd also explained that a 100-foot-long driveway will shrink about three-quarters of an inch as the concrete cures. With no joints, this concrete strip will develop random cracks as shrinkage creates tension in the slab. These are the cracks we all dislike.

They do not indicate defective concrete, nor can they be prevented. However, the cracks can be controlled through control joints, also known as shrinkage control joints or contraction joints.

Control joints are predetermined points of possible stress. Part of concrete placement and finishing, these joints can be tooled into a slab, cut with a saw, or made by an insert (typically plastic) pushed into the freshly placed slab. Then, when the concrete cracks, these cracks will occur at the bottom of the control joints where they can't be seen.

If sawcutting is the selected approach, the slab should be cut to a depth of one-fourth of the thickness, for example, a 4-inch slab will need a 1-inch deep cut. Since most of the shrinkage occurs in the first four to 24 hours, when the freshly placed concrete is the weakest, sawing must be coordinated with the setting time of the concrete. It should be started as soon as the concrete hardens sufficiently to prevent aggregates from being dislodged by the saw (usually within 4 to 12 hours after the concrete hardens) and completed before drying shrinkage stresses become large enough to produce cracking. A combination of art and science, timing the cuts depends on factors such as mix proportions, ambient conditions, and type and hardness of aggregates.

Cutting the joints a week or two after placement is far too late.

Correct spacing of contraction joints in concrete driveways depends on slab thickness, shrinkage potential of the concrete, subgrade friction, environment, and the absence or presence of steel reinforcement. The panels created by contraction joints should be approximately square since panels with an excessive length-to-width ratio (more than 1.5 to 1) are likely to crack at intermediate locations. In joint layout design, it is also important to position contraction (control) joints so they terminate at either a free edge or an isolation joint. Also, contraction joints should never terminate at other contraction joints as cracking will be induced from the end of the terminated joint into the adjacent panel (as in a T intersection).

Generally, contraction joints for a 4-inch-thick slab are placed 8 to 10 feet on center maximum in both directions. A 20-foot by 40-foot drive should be divided into eight to 10 equal pieces, one joint lengthwise and three to four joints perpendicular to the length.

Shepherd said he has used this technique for all his projects and never had a random crack develop. As he pointed out, "This is really cheap insurance for a slab with a 30- to 50-year life expectancy."

Adding reinforcing steel does not eliminate cracks. Rather, it keeps the cracks closed (tight) when the concrete does crack.

The Portland Cement Association was instrumental in providing the facts for this column, and they offer a great video titled "Building Quality Concrete Driveways" (VC329) for $6.95 that's worth checking out through its on-line bookstore (or call them at 800-868-6733). Its website (www.cement.org/bookstore) offers a wealth of information and resources from the cement and concrete experts at PCA.

Copyright © 2004, 2006, & 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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