What is Delamination in Concrete?

A delamination is a horizontal crack in concrete that lies at or above the top layer of reinforcing steel. The concrete above the crack stays in place until the cracked layer breaks away.

This page covers concrete and asphalt pavement. It explains how a delamination forms and how it differs from a spall. It also covers the tools that find it and the way airfield documents use the word.

Delamination, spalling and debonding

The three words describe different stages or places:

• Delamination. The crack is inside the concrete. The surface can look sound.
• Spall. The cracked concrete breaks away. Spalling is the later stage.
• Debonding. Two layers no longer bond, for example an overlay and the slab below. ASTM D4580 warns that on a concrete overlay, an area that sounds delaminated may be a lack of bond between the overlay and the deck.

How it forms in reinforced concrete

FHWA describes four stages of chloride-induced deterioration:

1. Chloride reaches the steel and corrosion starts.
2. The corrosion stresses exceed the tensile strength of the concrete, and cracks form.
3. Cracks that run parallel to the surface form a delamination, often at rebar level.
4. Inclined cracks reach the surface. Freeze-thaw cycles and traffic then break the cracked portions away. This is a spall.

The corrosion products occupy three to six times the volume of the original steel. This is the stress that cracks the concrete. The cracks also let water and chlorides reach the steel faster.

How inspectors find it

A delamination is often not visible from the surface, so inspectors use tools that sense what is under it. Each method has limits.

Chain drag and hammer sounding

The inspector drags a chain across the surface or taps it with a hammer or steel rod. Sound concrete gives a clear ring. Delaminated concrete gives a dull, hollow sound. The hollow sound comes from flexural oscillations of the cracked layer. FHWA says a chain drag is a fast way to find the approximate location. A hammer is slow but accurate for the boundaries. Both depend on the judgment of the inspector, and an overlay reduces how well they work.

ASTM D4580 covers three procedures: a cart-mounted electro-mechanical device, a chain drag and rotary percussion. The practice is not intended for decks with asphalt overlays. Traffic noise, vibration and moisture affect the result, and the practice should generally not be used on frozen concrete.

Infrared thermography

A delamination acts as an insulator. In sun, the area above it heats faster and gets warmer than sound concrete. When the air cools at sunset, the area cools faster. FHWA describes the same pattern as hot spots by day and cold spots at night. ASTM D4788 describes a vehicle-mounted infrared scanner for exposed and overlaid decks, with overlays up to 4 in. (100 mm) thick.

The limits are clear. The method needs a temperature difference of about 0.5 degrees C. Wet areas, shadows and debris cannot be inspected. It gives the area in the plane but not the depth, so cores confirm the depth. ASTM D4788 has no precision and bias statement, so it is not a basis for accepting or rejecting material.

Ground penetrating radar

Radar finds a delamination or void only if it holds significant air or water. In a Florida DOT study, infrared found the existing delaminations. Radar found a mix of delaminations and the conditions that come before them. These are chloride contamination and corrosion activity. Salts in the concrete and steel mesh in the surface can degrade the signal.

Impact echo

Impact echo finds delamination and debonding in slabs and overlays. A large, shallow delamination lowers the resonant frequency. A deep one raises it. The method needs trained staff, and severity is only roughly indicated.

Which tool is best

No single tool settles every case. ASTM D4580 and ASTM D4788 both say that their results can be used with other methods. WisDOT confirms infrared findings by sounding suspect areas and by taking cores. Indiana tested several methods on bridge decks (JTRP 2022/31). It recommended aerial infrared as a first screening step. Vehicle-mounted infrared then scans the decks with significant delamination, and impact echo was reliable and repeatable at project level.

Drones and delamination

A published study tested a low-cost drone infrared camera on two bridge decks and a test slab. The camera located suspected delamination on the decks, comparable to a research-grade camera. On the test slab with simulated delamination, the maximum detectability was about 70 to 72 percent. A drone can scan a large area without lane closures, but infrared still gives no depth. This page found no such test on airfield concrete. For what a drone scan of a runway resolves, see the article on the low-cost drone survey of a runway surface.

Delamination on airfield pavement

FAA AC 150/5320-6G says that airports use reinforced concrete pavements to a limited extent. The corrosion process above applies only to reinforced slabs. Airfield documents use the word for other defects:

• Scaling of a concrete slab. The FAA PAVER manual says scaling is generally shown by delamination or disintegration of the slab surface. It lists over-finishing, added water, lack of curing and poor air entrainment among the causes.
• Bleed water under the surface of new concrete. An FHWA curing brief says that rising bleed water can become trapped under a prematurely finished surface layer. Bleed water that rises after a curing compound is applied can disrupt the membrane and form a lens that causes spalling or delamination. Bleeding typically stops at about initial set. In slipformed paving, finishing ends before that, so bleeding may continue when the curing compound goes on.
• Debonded asphalt layers. Poor bond between layers causes slippage cracks (AC 150/5380-6C). A thin surface course left after milling can delaminate under construction traffic. AC 150/5370-10H warns that delamination potential exists whenever the milling depth is about equal to the thickness of the layer placed. Ground penetrating radar can investigate delamination between layers. An FAA conference study found debonding at 40 to 70 mm depth with infrared thermography. Rubber on the runway made the debonding harder to identify.

Joint spalling and corner spalling are different defects. They are the breakdown of the slab edge or corner at a joint.

What to do about it

A finding from sounding or infrared is a map of suspect areas. Cores confirm the depth. FHWA guidance for partial-depth repairs of concrete pavement gives the steps for the repair itself:

1. Find the unsound concrete by sounding with a rod, a hammer or a chain.
2. Mark the boundaries. Use a minimum length of 12 in (300 mm) and a minimum width of 4 in (100 mm).
3. Remove the unsound concrete and sound the bottom for weak spots.
4. Switch to a full-depth repair if dowel bars show, or if the crew must keep chipping to find sound concrete.