Joints in slabs

Concrete Construction, Feb, 2005 by Bill Palmer

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

Contraction 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.

Contraction joints are created with groovers or saws. There are also plastic joint-forming strips, but they are not recommended. A grooving tool can be effective at making a contraction joint, but the groove has to be deep enough to function. The finisher can use the point of a mason's trowel to make sure that the cut is deep enough and then finish it with a groover. The groove should be at least 1/4 the thickness of the slab.

Sawing is the best way to make contraction joints, especially on larger slabs. Timing is critical because if it's too late, the cracks will already have formed. Sawing should begin as soon as the concrete begins to cool so that you are beating the buildup of tensile stresses from shrinkage. For conventional saws, either wet-cut or dry-cut, the joints are cut 4 to 12 hours after the conclusion of finishing--4 hours in hot weather, 12 hours in cold. The depth of the cut needs to be at least 1/4 the slab thickness.

Another way to cut contraction joints is with an early-entry, dry-cut saw. This saw allows the joints to be cut much sooner without damaging the surface and, since the saw is used before the concrete has gained much strength, the cut need not be as deep for the crack to follow it. Early entry saws can be used within 1 to 4 hours after finishing, depending on the ambient temperature. The depth of the cut is usually 1 inch.

Isolation joints

A slab will settle and move (from temperature changes) independent of other parts of the structure, such as walls, columns, or stairways. If the slab moves only slightly more or less than the adjacent element, either horizontally or vertically, then we will get a crack. To prevent that crack, we isolate the slab from the other parts of the structure. For example, say the slab butts up against a wall inside a building. After the slab is poured, it will shrink away from the wall, unless it's tied to the wall. If tied, it will crack.

To create an isolation joint, use preformed joint filler that is the full depth of the slab. Along walls, the isolation joint is placed directly between the wall and the slab. At columns, the isolation joint can be round or diamond shaped (with a contraction joint coming off each point). Fill in the open area around the column with concrete after the slab is placed and after the column is carrying its full load.