Concrete ·
Concrete Joints Are Planned Cracks
Concrete shrinks as it hardens and dries, and some cracking is expected. Joints give that movement a planned place to go, which matters for the floor finish too.
Why a slab needs lines cut into it
Look closely at a sidewalk, a garage floor or a restaurant back-of-house slab and you will often see straight grooves dividing it into panels. Those grooves are not decoration. They are joints, and they exist because concrete moves.
The National Ready Mixed Concrete Association (NRMCA) explains that concrete expands and shrinks with changes in moisture and temperature, and that the overall tendency is to shrink. Because concrete is weak in tension, shrinkage that is restrained can crack it. NRMCA's short description is a good one to remember: joints are simply pre-planned cracks.
Three kinds of joints
NRMCA describes three main types for slabs on grade:
- Contraction joints create weakened planes so cracks from shrinkage form where they were planned.
- Isolation or expansion joints separate a slab from walls, footings, columns and other points of restraint, so the parts can move independently.
- Construction joints are where two separate placements of concrete meet, such as at the end of a day's work.
Spacing, shape and depth
NRMCA's recommended practice is a maximum contraction joint spacing of 24 to 36 times the slab thickness. For a 4-inch slab, that works out to about 10 feet, and NRMCA further recommends limiting spacing to a maximum of 15 feet. Panels should be square or nearly so, with the length no more than 1.5 times the width, and L-shaped panels should be avoided.
Depth matters as much as spacing. For contraction joints, NRMCA calls for a groove at least one quarter of the slab thickness, and not less than 1 inch. The American Concrete Institute's floor and slab guide, ACI 302.1R-15, adds that contraction joints should be continuous, not staggered or offset.
These are general practices, not a design. ACI 302.1R-15 states that establishing slab joint spacing, thickness and reinforcement requirements is the responsibility of the designer, and it lists many factors that affect the choice, from the concrete mixture to the layout of pits, trenches and equipment pads.
Timing is part of the plan
Joints are not always cut on the day of the pour, but they cannot wait long either. NRMCA notes that early-entry dry-cut joints are generally run 1 to 4 hours after finishing, and conventional saw-cut joints within 4 to 12 hours after the concrete has been finished, depending on the method and the concrete. If sawing is delayed too long, it becomes difficult and uncontrolled cracking may occur. That is why NRMCA's first rule for jointing is to plan the exact location of all joints, including the timing of the sawing, before construction.
Joints and cracks do not disappear under a floor
Even a well-jointed slab may still crack. ACI 302.1R-15 says some random cracking should always be expected, and that random visible cracks in 0 to 3 percent of the slab panels formed by saw cuts and construction joints are reasonable to expect. It also notes that reinforcement will not prevent cracking, although properly sized and located reinforcement helps keep cracks tightly closed.
This matters when tile, vinyl or another finish goes on top. ACI 302.1R-15 points out that the potential for joints to telegraph through the flooring should be addressed by the design, and that the slab design should be coordinated with the floor covering specification. For an owner, the practical step is to make sure the flooring installer can see where the slab's joints and cracks are before the new floor is selected and laid out, and to follow the flooring manufacturer's instructions for treating them.
Questions to ask about a new or existing slab
- Who designed the joint layout, and is it shown on the drawings?
- When and how will the contraction joints be cut or formed?
- Where does the slab meet walls, columns or footings, and how is it isolated there?
- For an existing slab getting a new floor, where are the joints and cracks, and what do the flooring manufacturer's instructions say about them?
Conclusion
Concrete is going to move, and some cracking is normal. Joints are how a project decides ahead of time where that movement goes. Planning their layout, depth and timing before the pour, and accounting for them before a new floor goes down, helps keep a slab in Florida looking and performing the way the owner expects.