Why Are There Joints in Concrete?

January 10, 2023

Perhaps you have often wondered why joints are placed in your concrete driveway and sidewalk.  They detract from the smooth look of the driveway and sidewalk. And they often allow weeds to grow in the cracks.  Ugly? You bet!

But these joints serve a very important purpose.  Properly installed, they help control random cracks in concrete.  You see, concrete expands and contracts.  Hard to believe but it does.  The joints create a weak location in the concrete so as the concrete expands and contracts, it breaks at the weak location.  Once the concrete cracks at the joint, caulk should be installed to stop water entry into the joint.  The caulk will also help manage the ugly weeds that grow in the joints.

So, why so many joints.  Well, there is a method to the madness!  A simple rule of thumb for coplacing joints in concrete is the depth of the concrete in inches times 2 equals the joint spacing in feet on the surface of the driveway and sidewalk.  Here is an example:

Another benefit to installing joints in concrete is the ability to keep water from going through the joints and cracks when caulk is installed in the joints.  If water gets to the dirt below the driveway, the dirt becomes muddy and loses its ability to support the concrete driveway.  If that happens, the concrete may randomly crack when your car is driven on the driveway because there is minimal support for the concrete driveway beginning a cycle of cracking throughout the driveway.

Concrete cracks are managed by the joints and steel reinforcement installed in the concrete.  The steel is commonly provided as a flat mesh placed in the concrete before it gets hard.  The mesh has large squares (maybe 6”x6” square) and made of small, welded wires.   

Properly installed and maintained, these concrete joints are important to maintain your driveway and sidewalk.

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