Joints spall for one reason: the filler or the concrete edge cannot accommodate the movement the slab actually makes. Repairing the surface without addressing movement produces a patch that fails at the next seasonal cycle.

Failure pattern

Pattern	Cause
Crumbling edges along the joint	Rigid filler, no movement allowance
Filler popped out intact	Bond failure, dirty or damp joint
Wide broken zone, 20 mm+	Slab movement or subbase issue
Hairline crack beside the joint	Load transfer or curling

Repair sequence

1. Cut or grind the joint edges back to sound concrete, typically 3-6 mm each side.
2. Clean dry, then check for moisture. Damp joints need a moisture-tolerant filler.
3. Install backer rod where the joint is deep, so filler depth matches the specified profile.
4. Apply primer if the manufacturer requires it, then fill slightly proud.
5. Cure fully, shave flush, then polish the joint last with a light pass.

Filler stiffness

Semi-rigid epoxy is right for interior joints with modest movement and heavy traffic. Pure rigid epoxy cracks where the slab moves. Soft sealant absorbs movement but collects grit and cannot be polished. On joints that carry both traffic and movement, specify a semi-rigid product rated for the temperature range and document the rating.

Polish without destroying the repair

• Do not cut into the filler with a coarse metal bond.
• Keep the head flat over joints; a tilted head digs the edge.
• Clean pads between joints so filler dust does not streak the floor.
• Finish joints with the same grit as the surrounding floor, not a finer one.

When to stop patching

If a joint reopens within one season, the problem is movement, not the filler. Options are widening and re-profiling the joint, installing a proper expansion assembly, or an engineering review of the slab. Repacking the same joint a third time is a decision to keep paying.