Concrete joints are built to absorb slab movement, but over time the sealant can crack, shrink, or come loose. When this happens, water gets in, edges break down, and surface coatings can fail. Raking out and re-sealing a joint is a straightforward task — but doing it properly ensures long-term durability and clean appearance. This guide explains how to rake out and re-seal concrete joints step-by-step for driveways, slabs, and warehouse floors.
Begin by examining the joint for signs of ageing: look for cracking, shrinking, debonding, or brittle sealant. Probe the material with a utility knife — if it crumbles or pulls out easily, it's ready for removal. Also check the joint edges for spalling or contamination. If the slab edges are crumbling, you may need to address structural issues before resealing. Most resealing jobs are straightforward, but identifying signs of deeper damage early can prevent wasted work or future repairs.
Use a joint raker, oscillating multitool, or right-angle grinder with a crack chaser blade to remove the old sealant. For soft sealants, a hooked scraping tool or heavy-duty utility knife might be sufficient. For hard-set or thick sealants, mechanical tools are faster and provide a cleaner edge. Always wear PPE, including dust mask, gloves, and safety glasses. If you're working indoors, use dust extraction. Your goal is to remove all the old filler down to the backer rod or base of the joint, leaving a clean, square-sided slot ready for resealing.
Once the old sealant is removed, use a vacuum to clear all remaining dust, debris, and particles from the joint. Even small fragments of the old filler can stop new sealant from bonding properly. If the joint is greasy or oily, use a degreasing cleaner and let it dry fully. Moisture or contamination in the joint can lead to early failure. For vertical joints or indoor settings, compressed air can be helpful to clear narrow channels. Clean, dry, dust-free joint walls are essential for proper adhesion and sealant longevity.
Backer rod is used to control sealant depth and give it a flexible base. If your old joint had one, replace it with a new piece of closed-cell foam rod. Choose a diameter about 25% larger than the joint width so it fits snugly. Push it to a depth that leaves the joint about half as deep as it is wide. This helps ensure proper sealant performance and avoids excessive usage. Never leave voids below the sealant — without backer rod, it may sag or fail prematurely. Some narrow joints may not require rod but always check specs.
Use a flexible joint sealant designed for concrete expansion or construction joints. Polyurethane and hybrid polymers are the most common for driveways and slabs. Ensure the product is UV-stable and traffic-rated if it's outdoors or subject to vehicles. For heavy-duty industrial floors, consider semi-rigid epoxies or self-levelling polyurethanes. Avoid rigid materials like mortar or caulk — they’ll crack under movement. Always check the product’s joint width and depth limits, cure time, and whether primer is required for bonding to dry concrete.
For clean edges, especially on decorative concrete or visible surfaces, apply masking tape along both sides of the joint. Leave a consistent reveal — typically 2–3 mm wider than the joint opening. This step makes it easier to achieve a professional finish and prevents over-spill on adjacent surfaces. It’s especially useful for coloured or stamped concrete where sealant smears are hard to remove. Remove the tape immediately after tooling the joint before the sealant skins over.
Cut the nozzle at a 45° angle and load the cartridge into your sealant gun. For large joints, use a sausage gun or bulk pour system. Slowly fill the joint in one smooth, continuous pass, ensuring the sealant contacts both sides fully. Slightly overfill to allow for tooling. For self-levelling types, use a funnel and ensure it spreads evenly. Avoid air gaps or breaks in application — these become weak points. Work in manageable sections and always follow the product’s pot life if using two-part systems.
Use a jointing tool, rounded spatula, or even a gloved finger (if recommended) to smooth the sealant. Create a concave or slightly domed finish depending on the slab application. Tooling improves adhesion by pushing sealant into the sidewalls. For best results, tool within 10 minutes of application before skinning begins. Remove masking tape immediately after tooling. If needed, wipe the edges clean with solvent on a cloth — but avoid dragging it through uncured sealant.
Follow manufacturer guidelines for curing time — most sealants take 24–72 hours to fully harden depending on weather and depth. Protect the joint from rain, traffic, and dust during this time. After curing, inspect for shrinkage, bubbles, or gaps. Well-sealed joints protect the slab and reduce the risk of edge spalling. Reinspect every 2–4 years and re-seal as needed — especially in areas exposed to UV, chemicals, or freeze-thaw conditions.
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