How to Fill Expansion Joints in Concrete | A Working Method

Figuring out how to fill expansion joints in concrete without causing cracks comes down to one principle: the joint must stay free to move. Expansion joints exist because concrete expands and contracts with temperature and moisture. Bridge that gap with something rigid and the slab will crack somewhere else instead. The working method is to install a compressible filler at the bottom of the joint and cap it with a flexible sealant that moves with the concrete.

Why Rigid Fillers Ruin Expansion Joints

Expansion joints are designed as a controlled separation point. When concrete heats up and expands, the joint closes. When it cools, it opens. A rigid filler like mortar or concrete has no give, so the pressure transfers to the slab and causes spalling or cracking along the joint edges.

The U.S. Bureau of Reclamation’s guidance on concrete structures describes expansion joints as needing to provide separation, often with a gap around one inch wide, and calls for a compressible filler such as corkboard that goes the full depth of the slab with no bond between adjacent concrete units. That unbonded separation is what lets each slab move independently.

Standard filler materials are covered by industry specifications. ASTM D1751-23 covers preformed expansion joint filler for concrete paving and structural construction, and ASTM D1752 covers preformed sponge rubber, cork, and recycled PVC fillers.

What You Need to Fill the Joint Properly

You need three things: a compressible backer rod, a self-leveling elastomeric sealant, and a clean joint. The backer rod does two jobs. It controls how deep the sealant sits, and it prevents the sealant from bonding to the bottom of the joint, which would lock the two slabs together.

Sealant depth matters. Sika’s technical guidance for its Sikaflex SL 1 sealant says the sealant depth should be half the joint width when possible, with an allowable depth between ¼ inch and ½ inch. A closed-cell backer rod should be about ⅛ inch larger than the joint width so it fits snugly.

For horizontal slabs, you need a self-leveling sealant. Sikaflex SL 1 is a one-component, non-priming, self-leveling elastomeric polyurethane made for expansion joints in concrete floors and decks. It meets ASTM C 920, Type S, Grade P, Class 35, plus Federal Specification TTS-00230C, Type 1, Class A. Class 35 means it handles 35 percent movement, which suits most driveway and garage joints.

Step-by-Step: Filling the Joint

Prep decides whether the sealant lasts. The joint must be clean, dry, and sound before anything goes in it.

1. Clean the joint of debris, dust, laitance, oils, and curing compounds. Remove old sealant or mechanical residues if the joint was previously sealed. Oils require removing enough contaminated concrete to expose clean substrate.
2. Install the backer rod at the proper depth. Press it into the joint so the top of the rod sits at the depth where you want the sealant to end—usually half the joint width down.
3. Pour the sealant from a spouted container, filling from the bottom up. This avoids bridging and air voids. The sealant self-levels to a clean finish.
4. Check the cure time before letting traffic over the joint. The sealant needs to cure into a flexible, traffic-tolerant joint.

For concrete that sits under periodic water immersion, priming is required with Sika Primer-173. The primer dries in about 15–30 minutes, and sealing should happen the same day. In standard dry applications, Sikaflex SL 1 is non-priming.

Common Mistakes That Cause Joint Failure

• Skipping the backer rod. Filling the whole joint with sealant wastes material and can bond the slabs together.
• Sealing a dirty joint. Dust, laitance, and curing compounds block adhesion.
• Bridging the joint. Pouring sealant across the top traps air voids underneath and weakens the seal.
• Using the wrong movement class. A sealant rated for less movement than the joint experiences will tear.
• Sealing before the concrete is cured. Fresh concrete must be fully cured before any sealant goes in.

For garage floors and driveways, the same method applies: clean the joint, set the backer rod, pour a self-leveling sealant. The best concrete expansion joint filler options are covered in our product roundup. For large cracks or spalled edges, repair the concrete surface before sealing the joint.

For heavily trafficked areas, choose a sealant with a higher ASTM C 920 class to handle more movement. For joints subject to water immersion, apply the recommended primer before sealing.