Concrete delamination

Delamination is a separation between a thin top layer and the concrete underneath. The layer is still in place but no longer bonded, which is why it sounds hollow when tapped and why it eventually breaks away in flat plates rather than crumbling.

The hollow sound is genuinely useful, because it tells you the failure exists before the surface visibly fails. A slab that sounds hollow across an area has already delaminated there, whatever it looks like.

What it is

During finishing, the surface can be closed and densified while air or bleed water is still trying to rise out of the concrete beneath. Trapped just below a sealed surface, that creates a weak plane, and the layer above it is only loosely attached.

The result is characteristic: separation at a consistent shallow depth, plates that lift away with fairly clean undersides, and edges that are thin rather than tapering into sound material.

It differs from scaling, which is progressive loss of the very surface, and from spalling, which usually goes deeper and is often associated with something in the concrete rather than the finishing of it.

What seeing this cannot establish

• A hollow sound establishes debonding, not its depth or extent. Tapping maps where; it does not measure how thick or how far.
• Delamination and a failing applied topping or overlay can present almost identically. Whether the separating layer is original concrete or something added later changes the repair entirely.
• The concrete beneath a delaminated layer is often perfectly sound, but that cannot be confirmed until the loose material is off.

What this can be associated with

Candidate explanations, not findings. Nobody can establish which applies to your concrete without seeing it.

• Finishing that closed the surface before bleed water and air had finished rising, which is the most commonly cited association
• Air-entrained concrete that was hard-troweled, which can be associated with trapping air beneath a densified surface
• Placement in conditions where the surface set much faster than the body of the slab
• An applied topping or overlay that never bonded properly to the substrate
• Freeze-thaw cycling working into an already weak plane and extending it

What you can safely check yourself

• Tap across the slab with a hammer handle or a steel rod and listen. Chalk the boundary where the sound changes — that outline is your extent map and is the most useful thing you can produce.
• Photograph the chalked area from above so the pattern is recorded.
• Look at the underside of any plate that has come free. A clean, flat break at consistent depth points at delamination; a rough, tapering break suggests something else.
• Measure the thickness of a loose plate. Consistent thin plates are diagnostic.
• Note whether the area is trafficked, wet, or salted, since those affect how fast it progresses.
• Check whether the slab has ever had a coating or overlay applied. If so, what is separating may be that rather than the concrete.

When this is frequently cosmetic

• Small isolated hollow areas in a low-traffic slab that are not breaking through
• Delamination on a utility slab where appearance and dust are the only consequences

When professional evaluation matters more

• The hollow area is extensive rather than isolated
• Plates are actively breaking out and leaving open edges that catch feet or tyres
• The area is in a walkway, a step, or anywhere a sudden hole is a hazard
• You are planning to coat, tile or overlay the slab, since a coating over a delaminated layer fails with it
• There is any chance the separating layer is a previously applied overlay, which changes the scope

When replacement enters the conversation

• Delamination covers a large proportion of the slab area
• The concrete beneath, once exposed, is also deteriorating
• Repeated patch repairs have already been done and the debonded area keeps extending

Evidence worth having before you ask for estimates

Every item here is something you can produce without tools or risk, and every one of them makes an estimate more accurate.

• A chalked and photographed map of the hollow-sounding area
• A loose plate kept for reference, with its thickness measured
• Photographs of the underside of a broken-out plate
• The slab's age and whether any coating or topping has ever been applied
• A note of traffic, salt exposure and drainage in the affected area

Questions to ask whoever comes out

• How are you determining the extent — sounding, coring, or visual only?
• Is the separating layer original concrete or a previous overlay?
• Once the loose material is off, what happens if the concrete beneath is not sound?
• How will a patch or overlay bond where the original layer did not?
• Will the repair be flush and will it match, or will it be visible?

Where estimates for this commonly differ in scope

These are the places two prices for apparently the same work stop being comparable.

• The extent is discovered rather than known, so estimates commonly carry an allowance. Ask what happens to the price if the sounded area turns out larger.
• Removing loose material and resurfacing the whole slab are different scopes that both get called repair.
• Bonding preparation is the part that determines whether a patch lasts and the part most often described in one word.

Who tends to do this work

• A concrete repair specialist
• A decorative or specialty concrete finisher

ConcreteOwner does not inspect, certify or approve contractors. This describes which trade usually holds this work, not a recommendation of anybody in particular.