Cracked concrete is ugly, but it’s rarely a structural emergency. Most cracks in domestic paths and driveways are caused by ground movement, frost heave, tree roots, or the concrete being too thin for the load it carries. The good news is that many cracks can be repaired effectively without breaking out and relaying the entire slab.
Types of Concrete Cracks
| Crack Type | Width | Cause | Repair Method |
|---|---|---|---|
| Hairline cracks | Under 3mm | Shrinkage during curing, minor thermal movement | Concrete crack filler or sealant |
| Medium cracks | 3–10mm | Ground movement, frost damage, settlement | Polymer-modified repair mortar |
| Wide cracks | Over 10mm | Significant settlement, tree roots, overloading | Epoxy filler or backer rod + sealant |
| Heaving/raised sections | N/A | Tree roots, frost heave | May need lifting and relaying |
| Crumbling/spalling surface | N/A | Freeze-thaw cycles, salt damage, weak concrete | Resurface with bonding compound + screed |
Repairing Hairline Cracks (Under 3mm)
Hairline cracks are cosmetic and don’t usually indicate a structural problem. However, water gets into them, freezes in winter, and widens the crack over time. Sealing them early prevents this progression.
- Clean the crack — use a wire brush to remove any loose material and debris. Blow out dust with compressed air or a stiff brush
- Apply a flexible concrete crack filler — squeeze it into the crack directly from the tube. Flexible fillers accommodate minor ongoing movement without cracking again
- Smooth the surface — use a putty knife or wet finger to smooth the filler flush with the surrounding concrete
- Allow to cure — typically 24 hours. The filler will be slightly visible but the crack is sealed against water
Repairing Medium Cracks (3–10mm)
These cracks need a more substantial repair to prevent further deterioration:
- Undercut the crack — use a cold chisel and hammer or an angle grinder with a diamond blade to widen the crack slightly at the base (creating a V-shape or dovetail profile). This gives the repair material more to grip onto. Wear safety glasses and a dust mask
- Clean thoroughly — remove all loose material, dust, and debris. A pressure washer is ideal for cleaning deep cracks
- Prime the crack — apply a PVA bonding agent or SBR primer to the clean surfaces of the crack. This improves adhesion between the old concrete and the new repair material
- Fill with repair mortar — use a polymer-modified repair mortar (like Sika MonoTop or Fosroc Renderoc). Mix according to instructions and pack it firmly into the crack using a pointing trowel. Press hard to eliminate air voids
- Smooth and finish — strike off flush with the surrounding surface. For a textured finish matching the original concrete, stipple with a stiff brush while still wet
Repairing Wide Cracks (Over 10mm)
Wide cracks often indicate ongoing movement — the crack may continue to widen if the underlying cause isn’t addressed. For the repair to last, it needs to accommodate movement:
- Clean the crack thoroughly — remove all loose material down to solid concrete on both sides
- Insert a backer rod — a closed-cell foam rod (slightly wider than the crack) pushed about 10mm below the surface. This limits the depth of sealant needed and creates a proper sealant profile
- Apply flexible polyurethane sealant — fill the crack above the backer rod with a flexible sealant. The sealant bridges the crack and flexes with seasonal movement. Sikaflex is a reliable choice for concrete joints
Resurfacing Spalled or Crumbling Concrete
If the surface is generally deteriorating — flaking, crumbling, pitting — rather than cracked, a full resurface is more effective than patching individual spots:
- Pressure wash the entire surface to remove loose material and contaminants
- Apply a concrete bonding agent to the clean surface
- Apply a self-levelling concrete resurfacer (like Ardex K301 or Sika Level 30) to a minimum 5mm thickness
- Smooth with a trowel or squeegee
- Allow to cure (typically 24–48 hours before foot traffic, 7 days before vehicle traffic)
For driveways that need resurfacing, our driveway restoration guide covers alternative approaches.
Preventing Future Cracks
- Control joints — concrete paths should have control joints (cut or formed grooves) every 2–3 metres. These create deliberate weak points where the concrete cracks in a controlled line rather than randomly
- Adequate thickness — footpaths need a minimum 75mm of concrete; driveways carrying cars need at least 100mm. If your concrete is thinner than this, cracking is inevitable
- Proper sub-base — concrete laid directly on soil moves with the ground. A 100mm compacted sub-base (MOT Type 1) provides stability
- Drainage — standing water undermines concrete edges and causes frost damage. Ensure water drains away from concrete surfaces
- Tree roots — if cracks are caused by tree roots, you may need to install a root barrier or remove the tree. Consult an arborist before removing mature trees
When to Replace Rather Than Repair
Consider full replacement if:
- Cracks are extensive across the entire surface — more crack than concrete
- The slab has heaved or sunk significantly (more than 25mm difference between sections)
- Tree roots have undermined the sub-base — patching is pointless until the cause is fixed
- The concrete is very thin (under 50mm) — it will keep cracking regardless of repair
- The surface is extensively spalled and crumbling across most of the area
A new concrete path costs about £50–£80 per square metre for materials and is a feasible DIY project for paths and small driveways. The Paving Expert concrete slab guide covers the full laying process.
