Settlement cracks form when the ground beneath a concrete slab shifts, sinks, or compresses unevenly. These cracks often appear long after the concrete has cured and may widen or offset over time. Unlike shrinkage or hairline cracks, settlement cracks may point to subgrade failure — and fixing them means more than just cosmetic patching. This guide walks you through how to assess, repair, and prevent settlement cracks in concrete floors properly.
Begin by measuring the crack width and checking for vertical movement. Use a straightedge across the crack — if one side sits higher, it’s likely a true settlement issue. Document length, direction, and nearby damage (wall separation, floor sagging, etc.). If the crack is still moving or growing, you may need a structural engineer before repair.
Use a diamond crack chaser blade or angle grinder to widen the crack slightly — this helps remove weak edges and provides a better bond. Vacuum all dust and debris thoroughly. If you find loose fill or signs of water entry, dig deeper and check for voids beneath the slab.
If the slab has settled due to sub-base failure or voids, consider foam injection (slab jacking) or pressure grouting to re-support it. This is essential for cracks showing vertical movement. Without stabilisation, your repair may fail again quickly. For minor voids, compacted flowable fill or microcements can also be used.
For cracks with no movement, use epoxy crack fillers for structural strength. If minor movement is still expected, a flexible polyurethane or hybrid sealant is better. Avoid basic cement fillers — they shrink and fail. Match your product to the environment: industrial floors often need epoxy, while domestic or semi-structural cracks may benefit from a more flexible approach.
Use a caulking gun or syringe to fill the crack from bottom to top. For epoxy, apply in stages and allow it to flow deep into the slab. For larger voids, backer rod may be needed to control depth. Work slowly and remove excess before it hardens. Tool the surface flat while the material is still workable to match the surrounding floor.
Once cured, grind or sand the repaired area flush with the slab. This is essential if the crack is in a traffic area, near walls, or beneath coatings. Avoid overgrinding flexible sealants — some need to remain slightly proud to stretch. If the slab will be polished or recoated, ensure proper surface profile is maintained after repair.
If the floor is exposed to water, chemicals, or heavy wear, consider sealing or recoating the entire slab. Use epoxy, densifier, or silane-based sealers for internal floors. For overlays or polishing, ensure your repair is compatible with the top layer material. This also helps blend the repair visually and extends lifespan.
Settlement usually starts beneath the slab. If the root cause isn’t fixed, more cracks will appear. Improve drainage around the slab, redirect downpipes, and ensure the ground isn’t eroding or washing away. If future slabs are planned, use compacted MOT Type 1 or engineered fill and install a proper damp-proof membrane.
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