The corners and edges of a concrete slab are the most vulnerable spots — exposed to impact, weather, and vehicle wear. Over time, they can chip, break away, or crumble, leaving sharp edges, safety hazards, or water entry points. Whether it’s a driveway, path, shed base, or industrial slab, patching a broken edge is a straightforward job — if done right. This guide explains exactly how to repair slab edges and corners for a durable, professional finish.
Begin by inspecting the broken area closely. Is it just the outer edge that’s chipped, or is the corner undermined and crumbling? If more than 100 mm of slab edge has broken off, a structural patch may be required with mesh or pins. Smaller surface-level breaks (under 50 mm) can often be treated with patch repair mortars. Check for vertical cracks extending into the slab — these could indicate structural movement or freeze-thaw damage. Also note if the damage is due to poor compaction beneath the slab or recurring impact. This will inform whether reinforcement or subgrade repair is necessary in addition to patching.
Use an angle grinder or concrete saw to cut clean, square edges around the broken section. Feathered edges or irregular breaks won't allow patch materials to bond properly and are prone to future failure. Cut back until you reach sound concrete — usually 15–30 mm deeper than the break. If patching a corner, cut two lines at 90° to recreate the missing shape. A flat chisel can then be used to knock out the damaged chunk. Be careful not to undercut or chip further into the good slab. Aim for clean, rectangular boundaries that give your new material a solid key to grip onto.
After cutting and shaping, remove all dust, debris, and contaminants from the repair zone. Use a stiff brush, vacuum, or compressed air to get into every edge and crevice. For dirty or oil-contaminated slabs, use a degreasing concrete cleaner followed by a rinse and dry. If moisture is present, use a propane torch or wait until the surface is completely dry — most patch mortars won’t bond to damp edges. This step is crucial: any dust or loose material left behind will act as a bond breaker and shorten the life of your repair. Surface prep makes or breaks the patch.
Apply a liquid bonding agent to the edges of the exposed concrete. These agents improve adhesion between old and new material and reduce the risk of patch delamination. Use a brush or spray applicator to coat the entire interior surface of the repair cavity. Don’t allow puddling, but make sure every surface is covered. Let it go tacky, not dry, before applying the patch mortar. Some high-performance mortars come with their own primer or require a slurry mix instead. Always follow product instructions for the bonding stage — it's what gives your patch structural grip on the slab.
For large or exposed edge repairs, mechanical reinforcement can greatly extend durability. Drill two or more 10–12 mm holes into the slab edge and insert steel pins or rebar dowels using epoxy adhesive. These act as anchors to lock your new patch to the existing slab and help it resist cracking from future impact or movement. Alternatively, for large corners or wide cracks, use stainless mesh within the patching mortar. Reinforcement is especially recommended in freeze-thaw zones, vehicle-accessible slabs, or any edge taking repeated dynamic load. It's a small step that provides long-term insurance.
Use a high-strength, fibre-reinforced concrete repair mortar rated for external edges. Follow the mix ratio exactly — don’t overwater, as it weakens the product. Apply the mortar using a pointing trowel or formwork depending on the size. Pack it tightly into all corners, making sure to encapsulate any pins or mesh. Overfill slightly to allow for finishing. Mortars designed for edges often contain shrinkage control additives and fast-set compounds — read the cure time carefully and work within your open window. Avoid basic cement mixes — they shrink too much and rarely hold on exposed edges.
After placing, use a steel or magnesium trowel to shape the repair to match the slab profile. For corners, recreate the 90° edge with a corner float or edging tool. Try to blend the surface texture — brushed, smooth, or stippled — while the mix is still workable. Be careful not to overwork, which can draw water to the surface and weaken the patch. If you're matching a broom finish, lightly drag a broom across before it sets. The goal is a patch that doesn’t stand out — both functionally and visually. If it looks seamless, you’ve done it right.
Cover the repair with damp hessian, plastic sheeting, or apply a curing agent to prevent rapid moisture loss. Curing is essential — even small edge repairs can shrink or crack without it. Leave for at least 24–48 hours before subjecting to load. Monitor the patch over the next few weeks for signs of separation, colour change, or surface cracking. Reseal if needed, especially if water exposure is likely. Well-executed patches can last for decades, but poor curing is the #1 reason edge repairs fail early. Don’t skip it — it’s what makes the difference between temporary and permanent.
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