A concrete slab on grade or floor is going to crack. There are some exceptions to that rule when heroic measures are taken, like post tensioning or shrinkage-compensating concrete, but most slabs are going to crack. It may appear that concrete doesn't move very much, but it moves enough to crack itself, especially as it dries out after placement. Even after it hardens, a concrete slab moves because of temperature changes or settling of the base course.
That's why we need joints in slabs. We put joints where we think the concrete is going to crack anyway, and use the joint to create a nice straight crack with sharp well-defined edges that can be sealed or filled and more easily maintained.
Cracks are a result of tension (pulling) in the concrete. Any time the slab is “restrained,” kept from moving freely, it will crack, because tension is created. This restraint can be from something next to the slab, from the ground underneath the slab, or within the concrete itself. And when a crack does form, you know that the restraint created tension that was directly perpendicular to the crack.
There are three kinds of joints used in slabs on grade to prevent “uncontrolled” cracks:
- Contraction joints (sometimes called control joints)
- Isolation joints
- Construction joints
When concrete is placed, there is almost always more water in the mix than will be consumed by the hydration reaction of the cement. This “water of convenience” keeps the guys placing the floor from breaking their backs because it makes the concrete somewhat fluid (workable). That water also makes it possible to get a nice smooth surface on the concrete (finishable). But this extra water doesn't just sit there as the concrete hardens; it moves through the concrete to an exposed surface and evaporates. As the concrete dries out, both as it is setting and over the longer term, it shrinks. If it isn't allowed to shrink freely, if the shrinkage is restrained, it cracks.
Theoretically, if the slab could shrink without any restraint from the subgrade, if that surface was frictionless, we wouldn't need joints. But that never happens. So knowing that the slab will crack, we compromise and give it places to crack, weakened lines that the crack is sure to follow. It's like cutting glass—make a line of weakness and the crack will follow.
The designer should specify the location of the contraction joints. The typical spacing in feet is 2 to 3 times the slab thickness in inches. So, for example, contraction joints in a 6-inch-thick slab would be 12 to 18 feet apart. These joints should be continuous across the slab and should ideally create square panels, although the panels can be rectangular as long as the ratio of the long side to the short side is not greater than 1.5. (So a 10x15-foot panel is OK, but a 10x20 panel will probably develop a crack across the middle.) Contraction joints should also be placed wherever there is an abrupt change in the thickness of the slab.