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Filling Concrete Expansion Joints: What to Use and What to Avoid

April 27th, 2023 | 3 min. read

By Sarah Etler

Preventing water from running through concrete expansion joints is a crucial maintenance practice, but how exactly should you go about it?

While it’s essential to have an appropriate amount of expansion joints between concrete slabs, having large openings exposed to the elements can cause many different problems over the lifetime of the concrete. For this reason, it’s important to fill or seal your expansion joints.

But with what, exactly?

After over three decades of working on all things concrete repair and maintenance, here at A-1 Concrete Leveling, we’ve gotten to know what works and what doesn’t when it comes to taking care of expansion joints.

In order to help you learn as much as possible about how to keep your concrete in good shape, we’ve put together this resource that will walk you through what an expansion joint really is, why it’s important to seal them off, and what to use vs. what to avoid when filling them. 

What Is a Concrete Expansion Joint?

In short, an expansion joint is an intentional break or gap between two pieces of concrete that allows the individual sections to move independently from one another. 

As temperatures fluctuate or the concrete absorbs moisture, the slabs expand and contract. Without expansion joints, this expansion and contraction could cause excess stress on the concrete, causing it to break or crack.



Why Should You Fill Concrete Expansion Joints?

Adequately-installed and well-placed expansion joints are a crucial component of long-lasting concrete. However, when these joints are left open and exposed, the concrete can face some of the following problems:

  • Sinking, Settling, or Heaving

When it rains, the rain itself, runoff from surrounding areas, or drainage from downspouts can run into the expansion joint gaps and erode away the soil under the concrete slabs. This causes the concrete to settle, sink, or heave upwards over time.

Related Resource: How Drainage, Gutters, and Runoff Affects Your Concrete

  • Unwanted Weeds and Plants

The moist environment under a concrete slab is a perfect place for weeds and plants to take root and shoot upwards. Not only are weeds annoying to see and constantly remove, but their roots can slowly but surely break down the concrete slab itself.

  • Cracking Due to Freeze-Thaw Cycles

When water enters through expansion joints and is absorbed by the porous concrete, it expands when frozen, causing the concrete to crack both internally and on the surface. After the water thaws, the concrete is left with even more space for water to be reabsorbed and expand during the next freeze-thaw cycle.

What to Use to Fill Concrete Expansion Joints

In order to help prevent problems like cracking, weeds, and settling from affecting your concrete, it’s important to fill the expansion joints with a flexible non-sag and/or self-leveling caulk.

This will provide protection against water and weeds while still allowing the concrete to move as it expands and contracts.



Self-Leveling Caulk

Self-leveling concrete caulk has a thin and fluid consistency, enabling it to smoothly and uniformly fill gaps in joints without the need for finishing. 

However, because it's so thin, the caulk can seep through crevices or gaps, so you have to first make sure there are no large gaps or spaces where the caulk can escape to prevent leaks.

For expansion joints over 1 inch wide, it’s best not to go with self-leveling caulk. This is because it’s difficult to properly prep the joint and the caulk may drop too low after curing, which can cause it to collapse when driven over.

Non-Sag Caulk

Compared to self-leveling caulk, non-sag caulk has a thicker consistency that makes it easier to control. 

It won’t leak through cracks or holes in the concrete, and you can even use it in conjunction with self-leveling caulk to dam up any places where you don’t want the thinner material to leak out.

Because non-sag caulk is thicker and does not level itself out, it will need to be smoothed manually after application to leave your expansion joints with a nice, clean finish.

What Not to Use to Fill Concrete Expansion Joints

While flexible caulk is the best material to use as a concrete gap filler, you may come across these alternatives when researching your options that we recommend avoiding:


Using wood to fill concrete expansion joints was originally thought to be a good idea because, like concrete, wood expands and contracts with moisture and temperature fluctuations. 

However, wood does not create a good seal with the concrete, so water can still penetrate the joint. Also, wood rots over time.

Grout or Mortar

As a general rule of thumb, anything that doesn’t expand and contract with temperature changes will likely not be a long-term repair method for concrete. 

This includes applying grouts, mortars, or concrete overlays to concrete expansion joints, which can break off, crack, and crumble as the temperature fluctuates and causes the slabs to move.

Now What? Filling Your Concrete Expansion Joints

After learning more about what you should use to fill the concrete expansion joints around your property and why it’s so important, you’re ready to get started with this crucial maintenance practice. 

Here at A-1 Concrete Leveling, we’ve been helping homeowners and property managers across the country protect and maintain their concrete for over 30 years, including caulking and sealing up expansion joints.

If you’d like to see how A-1 can help get your concrete expansion joints in good shape, click the link below to request a free onsite consultation and cost estimate.

We also have a DIY guide on caulking concrete cracks and joints if you’re up for a hands-on project instead.

Want to know more about concrete repair and maintenance? A-1’s Concrete Academy has an extensive online library of resources about all things concrete, including expansion joints. Check out some of these related topics:

Sarah Etler

Sarah Etler joined A-1 Concrete Leveling after receiving her Bachelor of Arts degree in English from Northern Kentucky University. As A-1's Content Marketing Manager, she works closely with industry experts to produce content that will best answer questions related to concrete repair and maintenance practices. Sarah loves living a life full of discovery and is excited every day to see what new things she can learn and share with those around her.