Repairing Spalled and Damaged Concrete First
What Spalling Actually Is
Spalling is the loss of a slab's surface layer: the top few millimetres flake, chip or sheet away, leaving shallow craters with exposed aggregate. It is a surface condition, not a structural one, which is why a badly spalled garage floor can still be a sound slab underneath. Three causes cover most of it. Freeze thaw, where water in the surface pores expands as it freezes and pushes the top layer apart, which in Allen happens on the handful of hard freezes each winter. De-icing salt, carried in on tyres after a highway trip. And a weak surface from the pour itself: too much water in the mix, or troweling carried on after bleed water appeared, which pulls fines to the top and leaves a skin with little strength.
Pop Outs and Delaminated Laitance
Pop outs are small conical craters, typically 10mm to 50mm across, where a piece of aggregate near the surface absorbed water and blew the concrete above it off. They are localised and cosmetic. Delaminated laitance is more serious: a weak layer sitting on good concrete, still in place but no longer attached. Tap across the floor and listen for a hollow, papery note. It has to come off, because anything coated onto it is bonded to a layer already detached.
Grind Back to Sound Concrete
Every repair starts by removing what is not sound. Grind the damaged area until the tool runs on dense concrete with visible aggregate rather than paste, and keep going past the visible edge of the damage, because the weak zone extends further than the flaking does. Square up the perimeter of a deeper repair, since a patch that tapers to nothing chips under a tyre. Vacuum, and tap again before mixing anything.
Cementitious Patch or Epoxy Mortar
Two families of repair material cover almost every garage floor.
- Polymer modified cementitious patch. Cement based with a polymer added for adhesion and flexibility. It moves with the slab, tolerates a slightly damp substrate, feathers easily and takes a coating well. The right choice for broad shallow spalling and for resurfacing a whole bay.
- Epoxy mortar. Resin loaded with graded sand. Very high compressive and bond strength, cures fast, does not shrink, unaffected by moisture once mixed. The right choice for deep localised holes and for thresholds and edges that take impact. It is stiffer than the slab, so it is not a material to spread over a whole bay.
Depth usually decides. Localised and deep points at epoxy mortar. Wide and shallow points at a cementitious product.
Filling a Low Spot Versus Resurfacing
A low spot where water pools is a different job from spalling. Small depressions can be filled and feathered with a patching compound. Once you are filling more than a few dips, the honest answer is a resurfacer over the whole area, giving one continuous surface rather than a floor made of patches. A coating over many patches telegraphs every one, because each material takes gloss differently.
A Patch Must Be at Least as Strong as the Slab
This is the rule that governs everything above. If the repair is weaker than the concrete around it, the coating fails at the patch first and takes the patch with it. Check the stated compressive and bond strength against a residential slab, commonly around 3000 psi, and choose a product that exceeds it. Follow the mix ratio as stated, not by eye: over watering a cementitious patch is the most common way a good product ends up weak.
Prove the Repair Bonded Before You Coat
Once the patch has cured for the full time specified, tap across it and its edges with a hammer handle. It should ring the same as the surrounding slab, with no change in tone at the boundary. Check the perimeter for a hairline crack tracing the outline of the repair, which means it shrank away rather than bonded. For a firmer answer, glue a steel dolly to the patch and pull it: a break in the concrete below is a pass, a break at the patch to slab line is not. Coating over an unproven repair hides it until the tyre finds it.