Concrete Joints: Sealer or Filler?

Posted on: 5 December 2016


In concrete construction, sealing a joint and filling a joint are not the same thing—though they often serve a similar purpose. If you're laying concrete slabs and you're not sure how to handle sealing and filling your joints, read on.

What is sealing a joint, and when should you do it?

Concrete sealant is a soft material that allows a concrete slab to naturally expand and contract with the weather and general usage. It helps to prevent damage from water—including ice damage—and keeps dirt, grit and other intrusions from getting into cracks or rising up from underneath a concrete slab. Sealed concrete can also look better than concrete left unsealed, as it has a more uniform appearance and looks more 'finished'.

There are many different brands of concrete sealant. All that is really important when selecting a material to use is that it is flexible, impermeable and highly adherent.

The Concrete Institute of Australia does not have a specific recommendation for depth-to-width ratio when creating plugs of sealant to use in concrete joints. The American Concrete Institute, however—who are also amongst the world leaders in concrete construction research, guidelines and information—recommend that your sealant plugs are created at a ratio of 3:2 to ensure good adherence to the slabs they are designed to hold together.

What is filling a joint, and when should you do it?

Concrete sealant is soft and flexible, but concrete filler is a rigid material designed to support the edges of a joint and secure the join itself. It's best used in high-traffic areas, as its main job is to prevent the gap between two slabs from widening over years of pressure from above or from either side.

It's important to bear in mind that you should only fill concrete joints if they are completely straight and have been saw-cut. A joint that is rounded, tooled or otherwise irregular won't be able to support the filler and you will need to seal the joint instead.

What should you bear in mind whichever you do?

It's important that sealant or filler is only installed on a clean joint; you'll need to vacuum the gap thoroughly before you begin. This prevents grit and other dirt from being trapped inside the sealing or filling substance, and if you skip this step, you'll find that your sealants and fillers are less effective and more prone to failure.

You're also going to need to give your newly laid concrete ample time to shrink before you carry out this final step of the work. Concrete shrinkage is a natural process that will occur throughout the lifetime of any concrete slab; the best way to handle this is to seal or fill as late as you can. You're also going to need to maintain the work as the concrete continues to shrink over the course of its lifetime.