Expansion joints are critical for allowing concrete to move without cracking. But when these joints develop gaps, separation, or water ingress, you can’t just fill them with any rigid repair mortar or epoxy. You need a flexible joint sealant that stretches, compresses, and bonds without cracking. In this guide, you’ll learn exactly how to prepare and seal expansion joint cracks with long-lasting, movement-friendly sealants.
Not all sealants are made for expansion joints. Look for polyurethane, hybrid polyurea, or silicone-based joint sealants rated for concrete use. These sealants remain elastic and weather-resistant, even under heavy traffic or exposure. Choose self-levelling sealant for horizontal slabs (driveways, floors) and non-sag sealant for vertical joints (walls, columns). Make sure it's rated for outdoor/indoor and traffic conditions as needed.
Remove all old joint filler, debris, dirt, and loose material from inside the joint. Use a grinder with a crack-chasing blade, or a utility knife for light work. Vacuum out the joint thoroughly — sealant won’t bond to dust or loose material. If the joint is wet, allow it to dry fully or use a moisture-tolerant primer if the product requires it.
Insert foam backer rod (round closed-cell foam) into the joint to control sealant depth and shape. The ideal depth-to-width ratio is around 1:2 — for example, a 20 mm wide joint should have about 10 mm of sealant depth. The backer rod also prevents three-point bonding, allowing the sealant to stretch and compress properly between the joint edges.
Load your sealant into a high-quality caulk gun or bulk applicator. Fill the joint slowly and evenly, making sure the sealant adheres to both sides of the joint wall. Avoid overfilling — surface should be flush or slightly concave, not domed. Use a sealant tool or plastic spatula to smooth the surface and ensure consistent coverage and bonding.
Follow manufacturer guidance for curing time — typically 12 to 48 hours depending on sealant type and temperature. Don’t allow foot or vehicle traffic across the joint during this time. Protect it from rain, dust, or cleaners until fully cured. Once cured, the joint will stretch, compress, and handle thermal movement without cracking or pulling away.
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