Occasionally it may be necessary to fill a concrete expansion joint to prevent dirt and moisture from seeping into the joint and under the concrete slap itself, or to smooth out the slab before laying flooring. Each situation that mandates filling a concrete expansion joint requires a slightly different approach. You’ll want to consider the final application before deciding how to fill your concrete expansion joints.
The Purpose of Concrete Expansion Joints
The first thing to consider is the function of a concrete expansion joint. A concrete expansion joint is a cut in a large slab of concrete that goes all the way through the slab to the base material. Its purpose is to allow the concrete slab to expand and contract with temperature without cracking the slab.
What Not to Do
Considering the purpose of a concrete expansion joint, a big no-no that many people try is to fill in the joints with concrete or concrete patching material. If you fill a concrete expansion joint with a material that cannot compress or expand as the slab moves, then the slab itself is more likely to crack. Also, the concrete you filled the expansion joint with will likely be cracked by the movement and can become dislodged from the joint.
Types of Filler
There are two types of filler that are appropriate for concrete expansion joints. One is a sealer which should be used to prevent water and dirt from seeping under the slab. The other is a flexible filler that is used to level the slab before laying flooring. Both the sealer and the filler are flexible so they allow the slab to move. This preserves the functionality of your concrete expansion joint.
How to Seal Concrete Expansion Joints
Whether you plan to fill or seal your concrete expansion joints, the first step should be to clean the joint thoroughly. First, sweep out the joint to ensure that any pebbles, dirt or loose debris are removed from the expansion joint. Then, use a detergent to remove any oily substances that might prevent the filler or sealer from sticking to the concrete. Finally, ensure that the joint is completely dry before moving on to the filling step of the process.
If you’re using a sealer to fill your concrete expansion joints, you want the sealer to be thin. This allows you to use less of the sealer which saves money, but it also prevents the sealer from seeping under the slab where it can be stretched from the top and the bottom by the slab’s motion, causing the seal to wear out more quickly. To ensure that you get a thin seal in your concrete expansion joint, use a backer rod first. A backer rod is made of flexible material, typically a water resistant fiberboard, that you insert into the concrete expansion joint and then pour the sealing material on top.
There are many reasons that you might need to fill a concrete expansion joint, which will determine whether you should use a filler or sealer compound. Always consider the purpose of a concrete expansion joint when deciding on how to fill it, which will help make sure that you do not damage your concrete slab. Regardless of how you decide to fill your concrete expansion joint, be sure to thoroughly clean it first before applying the filler or sealer.