Preventing Random Cracks In Concrete

Because concrete will crack randomly, we attempt to control where it will crack by incorporating contraction joints. Joints are easy to maintain and much less objectionable than a random crack.

Contraction joints may be formed with plastic or hardboard preformed strips, saw cuts or tooled in the plastic concrete.

The placement and depth of these joints is critical for effective performance in preventing random cracking. Use the following chart to help determine where the joints should be placed based on the slab thickness. The maximum joint spacing is 18 feet. Also, keep in mind that the length to width ratio of the joint patterns should not exceed 1.5. It is best to keep the “panels” as close to square as possible.

Depth of Joints

Construction joints need to be 1/4 the thickness of the concrete slab. By jointing the concrete, you are trying to create a plane of weakness. This plane of weakness should dictate where the concrete will crack. If using a hand tool to joint the concrete, make sure that the edge is not worn and will provide the full depth necessary.

Timing of Jointing

Concrete should be jointed as soon as possible. The advantage of using a plastic or hardboard preformed strip is that it is placed in the concrete while the concrete is still plastic. It is important to make sure these strips are aligned properly and not skewed. If not placed properly, a random crack can occur.

Hand tooling joints is another effective method of jointing but is not always practical. When using a hand tool, you should tool the joints with each successive pass of finishing.

If using a saw to cut the joints, the cuts should be made as soon as the concrete can withstand the sawing without raveling the edges of the saw cut. Understand that the longer you wait to cut the joints, the greater the potential for cracking. If a crack develops before the concrete is sawed, the sawed joint will be ineffective. Typically, concrete should be saw cut within the first 6 to 18 hours. Do not wait longer than 24 hours.

If using an early entry saw, cutting may be possible within a few hours of placement. Research has shown that the timing of the saw cut is more important than the actual depth of the cut. Although shallower depth may be utilized, industry standards still recommend cutting to a depth of 1/4 the thickness of the slab.

Don’t Forget to Cure

As mentioned, proper curing is essential for durability. Contractors typically begin the curing process by applying a curing compound following completion of the concrete placement. You can sustain the curing process by maintaining the moist condition of newly placed concrete by moist curing. The usual duration for moist curing is seven days, but the longer the concrete is cured, the better

There are several methods for moist curing, but the most common are continuous spraying/fogging or a wet covering. Spraying/fogging can be achieved with an ordinary lawn sprinkler if good coverage is provided and water run-off is not a concern. Soil-soaker hoses also provide excellent curing. If you choose a moisture-retaining fabric, burlap or cotton mats are recommended. They should be placed on the concrete as soon as it has sufficiently hardened in order to prevent surface damage. The coverings should be kept continuously moist and a film of water should remain on the concrete surface throughout the curing period.

Sealing Joints

While it is not always necessary to seal joints, this can help protect the joints from the infiltration of surface water and incompressible materials into the joint. Surface water infiltration can lead to sub-grade failure and incompressible materials in the joint may contribute to spalling. Any sealant application should be delayed to ensure the shrinkage of the concrete is complete. Sealant materials should bond to the concrete, be impermeable, and be able to handle expansion and contraction.

Disclaimer: Pennsylvania Aggregates and Concrete Association is a trade association organized under the Pennsylvania Nonprofit Corporation Law. The information provided here is intended solely for the continuing education of Qualified Professionals who are competent to evaluate the significance and limitations of the information provided and who accept total responsibility and liability for the accuracy of any application of the information contained in this publication. Others should obtain assistance from a Qualified Professional before proceeding. The Association and its members make no express or implied warranty with respect to the information contained herein or the accuracy thereof. They disclaim any product liability in connection with the publication or any information contained herein. The information is garnered from various publications reflecting current industry recommendations.