Control Joints in Concrete Slabs

Properly creating and locating joints keeps concrete looking its best

By Bill Palmer, ConcreteNetwork.com Columnist

Find Concrete Saws & Blades

Joint Types:

Load transfer - Transferring loads across joints

Sealing joints - Tips for sealing and filling joints

Jointless slabs - Reducing or eliminating joints

Decorative joints - Joint tips for decorative concrete

Bill Palmer

Joints in concrete can serve both to prevent cracking and as a decorative element.

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.

As it 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 (see Subgrades and Subbases).

This is where joints come in. 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.

Although many building elements are designed and built with joints, including walls and foundations, we'll limit this discussion to joints in concrete slabs. In slabs, there are three types of joints:

  • Isolation joints (also sometimes functioning as expansion joints)
  • Construction joints (which can also function as contraction joints)
  • Contraction joints (also sometimes called control joints)

Bill Palmer

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

Let's look at each type of joint, then we'll cover how to improve joint function or appearance and how to make concrete without (or at least with many fewer) joints.

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