What is Block Cracking?

Block cracking is a pavement distress where interconnected cracks divide an asphalt surface into approximately rectangular pieces. The pieces range from about 0.3 m by 0.3 m to 3 m by 3 m. The FHWA highway manual gives the same range as 0.1 to 10 m2 per block.

Cause

Contraction of the asphalt and daily temperature cycling cause block cracking. The cycling puts the asphalt through a stress and strain cycle every day. Traffic loading does not cause it. Its presence usually shows that the asphalt is now much harder.

Block cracking normally covers a large part of the pavement. It sometimes occurs only in areas without traffic.

Block cracking and alligator cracking

Alligator cracking forms smaller pieces with many sides and sharp angles. The PAVER manual gives pieces of less than 2 ft (0.6 m) on the longest side. Repeated traffic loading causes alligator cracking, so it occurs only in traffic areas such as wheel paths. The manual calls it a major structural distress. Cracking in a pattern over a whole area with no traffic loading is block cracking.

Severity on an airfield

The PAVER manual for airfields rates block cracking at three levels:

• Low: the cracks have no spalling or only light spalling, with no potential for foreign object debris (FOD). An unfilled crack has a mean width of 6 mm (1/4 in) or less. A filled crack has filler in satisfactory condition.
• Medium: the cracks have moderate spalling, or unfilled cracks have a mean width over 6 mm, or the filler is unsatisfactory. All of these have some FOD potential.
• High: the cracks have severe spalling and a definite FOD potential.

FAA AC 150/5380-6C says loose material from pavement distress can strike propellers or enter jet engines, and that all pavement inspections should address it.

The FHWA highway manual uses crack width alone. Low is a mean width of 6 mm or less. Moderate is more than 6 mm up to 19 mm. High is more than 19 mm. Adjacent random cracking can raise the level.

Measurement

PAVER measures block cracking as surface area in square feet or square meters. Record areas with distinctly different severity separately. In the same area of an asphalt pavement, do not also record longitudinal and transverse cracking.

The FHWA manual records the area at each severity level in square meters. It subtracts any fatigue cracking inside the area. An occurrence must be at least 15 m long to rate as block cracking.

Repair

FAA AC 150/5380-6C lists these steps for cracks in flexible pavement: remove old sealer, clean and prepare the cracks, then seal them. Its sealing procedure is for cracks less than 1 inch (2.5 cm) wide. Crack filler material should meet ASTM D5078. Some cracks need a partial depth or full depth repair of the damaged pavement.