Contraction Joints

Not installing joints opens the door to potential litigation in addition to customer complaints. Here’s how to protect your company.

Creating a contraction joint

Creating a contraction joint, image from cement.org

Assuming they’re not required or that they’ll compromise structural integrity, many concrete contractors don’t install contraction joints in residential slabs-on-ground. If you forego joints to avoid being blamed for cracks, beware.

First, the American Concrete Institute (ACI) Residential Code Requirements for Structural Concrete (ACI 332-14) requires contraction joints . Most residential slabs contain reinforcement of less than 0.1%. (For example, 0.1% reinforcement in a 4-inch thick slab iinches.)

Second, contraction joints are rarely used in post-tensioned (PT) slabs although the Post-Tensioning Institute found that neither random cracks, which are common, nor contraction joints (also called control joints) affect structurae performance.

Contraction Joint Layout in Non-Post-Tensioned Concrete

Interferences by interior footings, re-entrant corners, and embedded elements usually make uniformly spaced, parallel, straight-line joints impossible. Spacing often varies and joints may be angled or even curved. The appearance seldom matters, though, because the slab will be covered with flooring materials. The important thing is to minimize random cracks.

Residential slabs often have numerous re-entrant corners, which often don’t line up on opposite sides of the slab. Corners can be connected by angled and even curved joints to avoid an interior footing and should be intersected by at least one contraction joint. However, that’s not always possible.

In some cases, re-entrant corners may have to remain without a joint. A crack may form at those corners, but it will be short and narrow.

Types of Contraction Joints

Contraction joint, diagram, image from cement.org

Contraction joint diagram, image from cement.org

Contraction joints, or control joints, are installed by using grooving tools or mechanical-inserts in fresh concrete or by sawcutting after the concrete has set.

The most functional joints are those installed in fresh concrete. They form a weakened plane before any shrinkage occurs, which can arise from chemical reactions during setting, moisture loss during the first hours and days after placement, and decrease in concrete temperature from the time of concrete hardening.

Early-entry sawcuts are the next best joints because they’re installed within hours of placement.

Conventional, wet or dry, diamond blade sawcuts are the least desirable because they’re installed after the concrete has gained enough strength to resist raveling. To gain sufficient strength, sometimes the concrete is allowed to cure overnight, which can be too long before installing joints. Cracks may have already developed due to thermal contraction and secondarily due to drying shrinkage.

Contraction Joint Depth

The common requirement is one-fourth the slab thickness or a minimum of 1 inch, whichever is greater. The one-fourth depth criterion applies to conventional wet or dry, diamond blade saws. When early-entry saws are used, according to ACI 360 and 332, the depth criterion of 1 inch for slabs up to 9 inches thick is permitted. Logically, this criterion also applies to tooled and mechanical-insert joints because these joints are also early-entry contraction joints.

Responsibility

Concrete contractors often take the blame for random cracks. This will change when the building plans include a requirement for contraction joints. If the plans do not call for contraction joints, the residential concrete contractor should follow the recommendations of ACI 332 and install the joints or inform the engineer that joints are required.

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