Q&A / 

Control Joint Video

Hello, I'm Tim Carter from AsktheBuilder.com. It's a cold winter day and I want you show you something that is pretty neat. If you recall a few weeks ago, we poured this concrete slab. Now look at this.

There are cut lines in the slab that were added later. These are concrete control joints. The contractor came back and used a gas-powered saw with a dry diamond blade to cut these lines in the concrete slab. They were put here for a reason. We are looking at an inside corner of the foundation. That is a point where the slab would want to crack. It would want to crack right at the corner, if the control joints weren't cut into the slab.

Understand this about concrete. When you pour concrete, as it cures and hardens, it shrinks. Some of the water leaves the concrete because it is used in the chemical reaction of the curing. It will shrink one-sixteenth of an inch for every 10 linear feet that is poured. That shrinkage creates tension that will literally rip the concrete apart. So if you saw cut in these control joints into slabs (some times they are cut into sideways with a special tool as the concrete is still in its plastic state), it will help relief some of the tension.

The depth of the control joint needs to be a minimum of 1/4 the thickness of the slab. So if the slab is 4" thick, the saw cut has to be 1" deep. Many times, the contractor doesn't cut the control joint that deep.

Control joints control where the slab should crack. They don't always work that way, but they should.


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