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 and provides a table to determine joint spacing for plain and reinforced concrete containing up to 0.5% reinforcement (reproduced here as Table 1). Most residential slabs contain reinforcement of less than 0.1%. (For example, 0.1% reinforcement in a 4-inch thick slab is #3 bars in both directions at 24-inches.)
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. The institute doesn’t require contraction joints, but notes that “control joints which are weakened planes formed by tooling, sawcuts, or mechanical devices, can be used to attract and conceal restraint-to-shortening cracks.” Their location, which the engineer-of-record should specify, should avoid areas of high bending moment and shear.
Finally, contraction joints may be necessary even if they’re not shown in construction plans. Most likely, the contract includes language stating that relevant codes be followed. That puts the onus on the contractor to submit a request-for-information, which will alert the licensed design professional that joints are required. Doing this will help deflect blame for random cracks and protect against possible construction litigation.
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.
The first consideration is slab connection to the perimeter footing.
For plain and reinforced concrete, the slab-on-ground may be isolated or monolithic with the footing. When isolated, the first joint next to the footing can have a spacing up to that given in Table 1. For example, if slab thickness is 4 inches, the first joint can be 11.5 feet away when using concrete with ¾-inch maximum size aggregate.
| Slab thickness (inches) | Maximum aggregate size less than 3/4 inch | Maximum aggregate size 3/4 inch and larger |
| 3.5 | 8 feet | 10 feet |
| 4.5 | 10 feet | 13 feet |
| 5.5 | 12 feet | 15 feet |
When the slab is cast monolithically with the perimeter footing, more contraction joints will be required. In this case, ACI’s Guide to Design Detailing to Mitigate Cracking requires that the first joint from the footing have a spacing of half the spacing permitted in Table 1.
Interior footings are located beneath load-supporting walls. Hiding contraction joints under the wall might seem logical but a joint located over an interior footing will not activate—that is, a crack will not develop the full depth of the slab and open during thermal contraction and drying shrinkage. The joint needs to be located where the slab is not over the footing. Joint spacing is the same as for when the slab is cast monolithically with the perimeter footing measured from the footing’s outer edge.
Embedded elements in the slab, such as anchor bolts and plumbing pipes need to be avoided entirely by the joints, another reason for variable spacing and curved or angled joints.
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. ACI requires that the aspect ratio of long-side to short-side for each panel bordered by joints be less than or equal to 1.5. This criterion supersedes the intersecting joint criterion and often requires additional joints. For some residential slabs, the result will be joints that are closely spaced.
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.
Contraction Joint Layout in Post-Tensioned Concrete
Residential post-tensioned slabs typically have maximum moments within 10 feet from the slab edge. In the slab’s central portion, which is called the dormant zone, only small moments, shears, and differential deflections exist. For this reason, contraction joints placed more than 10 feet from and basically perpendicular to the edge won’t interfere with structural capacity.
Contraction joint layout must be specified by the engineer-of-record.
Types of Contraction Joints
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.
Grooving is the predominant installation method for sidewalks, driveways, and garage slabs. Mechanical inserts, such as zip strips, can be used on small (residential) slabs. Early-entry sawcuts are preferable for larger slabs because the distance across slabs can be too great for grooving or mechanical insert joints.
A new mechanical-insert method has recently been introduced that produces a joint that’s not as neat as early-entry sawcutting but that is just as effective and costs about 75% less. A folded strip of plastic tape is embedded into fresh concrete using a tool that’s manually pushed forward across the slab. The operation occurs before or after bullfloating, while the concrete is highly workable and vertical embedment and joint depth can be controlled. The tape is installed slightly below the surface, so finishing operations aren’t impeded. Final joint appearance, after thermal contraction and drying shrinkage, is a relatively straight crack.
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.
