Concrete is not a ductile material—it doesn’t stretch or bend without breaking. That’s both its greatest strength and greatest weakness. Its hardness and high compressive strength is why we use so much of it in construction. But concrete does move—it shrinks, it expands, and different parts of a building move in different ways. This is where joints come into play. Although many building elements are designed and built with joints, including walls and foundations, we’ll limit this discussion to joints in concrete slabs. Here’s an overview of the types of joints, their function, and tips for locating and installing joints.

As concrete moves, if it is tied to another structure or even to itself, we get what’s called restraint, which causes tensile forces and invariably leads to cracking. Restraint simply means that the concrete element (whether it’s a slab or a wall or a foundation) is not being allowed to freely shrink as it dries or to expand and contract with temperature changes or to settle a bit into the subgrade. Joints allow one concrete element to move independently of other parts of the building or structure. Joints also let concrete shrink as it dries—preventing what’s called internal restraint. Internal restraint is created when one part of a slab shrinks more than another, or shrinks in a different direction. Think how bad you feel when part of you wants to do one thing and another part wants to do something else! Concrete feels the same way.

In slabs, there are three types of joints:

Isolation joints – Isolation joints have one very simple purpose—they completely isolate the slab from something else. That something else can be a wall or a column or a drain pipe. Here are a few things to consider with isolation joints:

• Walls and columns, which are on their own footings that are deeper than the slab subgrade, are not going to move the same way a slab does as it shrinks or expands from drying or temperature changes or as the subgrade compresses a little.

Construction joints – On many jobs there will be starting and stopping points—you won’t pour the entire slab or driveway all at once. That’s where you will place a construction joint.

Contraction joints – The most important type of joints and certainly the most likely to cause problems. Both isolation and construction joints are formed before the concrete is poured; contraction joints (or control joints) are “placed” in the fresh concrete before it has a chance to create its own joints—also known as cracks. What a contraction joint really is in the end is a crack in the slab that we force to follow a line of our own choosing. We create a weakened line across the slab and let nature take its course. When the slab does crack, that’s called “joint activation.” Contraction joints are formed by saw cutting, by tooling a joint with a grooving tool, or by inserting a plastic strip into the concrete during finishing (zip-strip). Proper timing and depth of cut are essential. If you wait too long, the slab will crack where it wants to rather than where you want it to. If the joint is not cut deep enough it will not create the plane of weakness needed to guide the crack.

Different joints in concrete slabs all have the same bottom-line purpose of preventing cracks.

Control joints are planned for cracks which allow for movements caused by temperature changes and drying shrinkage. In other words, if the concrete does crack-you want to have an active role in deciding where it will crack and that it will crack in a straight line instead of randomly.

Space joints properly. Space joints (in feet) no more than 2-3 times the slab thickness (in inches). A 4″ slab should have joints 8-12 feet apart.

Cut joints deep enough. Cut joints 25% of the depth of the slab. A 4″ thick slab should have joints 1″ deep.

How to cut joints. Grooving tools cut joints in fresh concrete. Saw cutting cuts joints as soon as the concrete is hard enough that the edges abutting the cut don’t chip from the saw blade.

Cutting joints soon enough. In hot weather, concrete might crack if joints are not cut within 6-12 hours after finishing concrete. In this condition, if you don’t want to use a grooving tool to cut joints, there are early-entry dry-cut lightweight sawsthat can be used almost immediately after finishing. These saws cut 1″ to 3″ deep, depending on the model.

Place joints under walls or under carpet areas.Under walls they won’t be seen. Under carpet areas the joints won’t have a chance to telegraph through vinyl areas.

Avoid re-entrant corners. Planning the joint pattern can sometimes eliminate re-entrant corners.

Correctly laid out joints. Note: Inside corners, where cracks
would typically occur, have correctly placed joints.

Most plans don’t have joint spacing marked on them. So don’t leave this important part of concrete construction to chance.

Jointing is often not taken seriously enough and the “sawcutter” comes to your job and puts the cuts where he feels they belong or where it is convenient for him.