What is D-Cracking (Durability Cracking)?

What D-cracking is

D-cracking is a distress in concrete pavement. Certain coarse aggregates either fracture or swell when they go through many cycles of freezing and thawing while saturated with water. This cracks the mortar around the aggregate and deteriorates the concrete. Pavement engineers call the result D-cracking, from “durability” cracking.

FAA AC 150/5380-6C describes it as a pattern of cracks in the vicinity of and parallel to a joint or linear crack. The cause is the inability of the concrete to withstand freeze-thaw cycles because of variable expansive aggregates.

How it looks and develops

The FAA PAVER manual for concrete surfaced airfields lists durability cracking as distress 64. It appears as a pattern of fine cracks parallel to a joint or linear crack, usually with dark colouring around the cracks. FHWA adds that dark staining often comes first, and that the cracking then appears at joints and cracks.

Over time the concrete can disintegrate within 0.3 to 0.6 m (1 to 2 ft) of the joint or crack. FHWA gives 10 to 25 years as the time for this distress to appear.

PAVER rates severity as follows:

• Low: hairline cracks in a limited area of the slab, such as one or two corners along one joint. Little or no disintegration. No FOD potential.
• Medium: cracking over a large area of the slab with little or no disintegration, or cracking in a limited area with missing pieces. Some FOD potential.
• High: cracking over a large area of the slab with disintegration or FOD potential.

Which concrete is at risk

FHWA states that most susceptible aggregates are sedimentary, commonly limestone, dolomite or chert. They have higher total porosity and more medium-sized pores (0.1 to 5 micrometres), so they hold much freezable water when saturated. FAA AC 150/5320-6G adds that durability problems are most severe along joints and at panel corners, where the moisture level is highest. FAA AC 150/5380-6C notes that poor subsurface drainage can contribute to D-cracking.

Prevention in airport specifications

FHWA states that the most effective prevention is to prohibit susceptible aggregate by requiring coarse aggregate to pass freeze-thaw tests. The most common tests are regional variations of AASHTO T 161 and ASTM C666.

FAA AC 150/5370-10H puts this into Item P-501, Cement Concrete Pavement. It accepts coarse aggregate from sources with a 20-year service history without D-cracking. Other aggregate needs a durability factor of at least 95 to ASTM C666. The guide text also lists crushed granite, quartzite, basalt, diabase, rhyolite and trap rock as meeting the D-cracking test requirement.

Role in airfield inspection

D-cracking is one distress type in a PCI survey to ASTM D5340, which FAA AC 150/5380-6C names as the method for the PCI. PAVER rates each case by severity, and the rating considers the FOD potential of loose pieces.