What is Reflective Cracking in Asphalt Overlays?

Reflective Cracking in Asphalt Overlays

Reflective cracking is a crack in an asphalt overlay that copies a crack or joint in the pavement layer under it. FAA AC 150/5380-6C calls it reflection cracking.

How it forms

Vertical or horizontal movement in the pavement under an overlay causes this distress. The movement comes from expansion and contraction with temperature and moisture changes, or from traffic loads. The cracks in the overlay then copy the crack or joint pattern of the old pavement.

It occurs most often in hot mix asphalt (HMA) overlays on Portland cement concrete (PCC) pavement. It also occurs in overlays on HMA pavement. This happens when the old cracks or joints were not repaired correctly. A cement-treated base can cause it too. AC 150/5370-10H says Item P-304 is susceptible to reflective cracking. It also tells designers to avoid higher cement strengths, because they can cause shrinkage and reflective cracks.

How it is recorded in an airfield survey

The PAVER manual for asphalt airfields splits the distress in two.

• Joint reflection cracking (distress 47) is only for asphalt over a PCC slab. The main cause is movement of the slab under the asphalt because of thermal and moisture changes. It is not load related. Traffic can break down the asphalt near the crack. This causes spalling and a risk of foreign object debris (FOD). The slab size under the asphalt helps to identify these cracks.
• Other reflective cracks are recorded as longitudinal and transverse cracking (distress 48). This includes reflective cracks from other bases, such as cement-stabilized base, and from cracks in a PCC slab away from the joints.

On asphalt over concrete, the surveyor records block cracking, joint reflection cracking, and longitudinal and transverse cracking reflected from old concrete separately.

Severity depends on spalling, on crack width, and on the condition of the crack filler. For a joint reflection crack:

• Low: no spalling or light spalling. A non-filled crack has a mean width of 6 mm (1/4 in) or less. A filled crack can have any width if the filler is in good condition.
• Medium: moderate spalling, or a filler in poor condition, or a non-filled crack wider than 6 mm.
• High: severe spalling with a definite FOD risk.

The surveyor measures the crack in linear feet (meters) and records each part of the crack that has a different severity separately. The Pavement Condition Index (PCI) uses these records. AC 150/5380-6C says the PCI is determined to ASTM D5340.

How to reduce it

AC 150/5320-6G gives these points for airport overlays:

• Thicker asphalt overlays generally perform better. Thin overlays of rigid pavement may cause or worsen reflection cracking. The minimum overlay thickness on existing rigid pavement is 3 in (75 mm).
• FAARFIELD does not treat reflection cracking of the overlay as a failure mode. The designer must manage it by other means.
• Rubblization breaks the concrete panels so that they act as a tightly interlocked base. This minimizes the chance of reflective cracks. Crack and seat also breaks the slab into pieces, but rubblization has almost replaced it.
• An interlayer between the old pavement and the overlay can help. Types include aggregate-binder courses, double chip seal, stress absorbing membrane interlayers (SAMI), paving fabrics and grids. It does not remove the need to fill the cracks in the old pavement. Do not use an interlayer where the old pavement shows excessive deflections, substantial thermal stress or poor drainage.