What is Alligator (Fatigue) Cracking?

Definition

Alligator cracking, also called fatigue cracking, is a series of interconnecting cracks in an asphalt surface. Repeated traffic loading causes fatigue failure of the surface, and the cracks connect into a pattern that looks like chicken wire or alligator skin. People also call it crocodile cracking. It is the same distress. Pavement Interactive describes the pattern as resembling the back of an alligator or crocodile.

The pieces between the cracks are small, and the agencies give different limits:

• The FAA PAVER manual for asphalt airfields: less than 2 feet (0.6 m) on the longest side.
• The FHWA Long-Term Pavement Performance (LTPP) manual: usually less than 0.3 m on the longest side.
• TxDOT: less than 1 foot on edge.

Alligator cracking occurs only where repeated loading acts, such as in wheel paths. It does not occur over a whole area unless traffic loads the whole area.

How it forms

The PAVER manual and AC 150/5380-6C describe the usual sequence. The cracking starts at the bottom of the asphalt surface or stabilized base, where the tensile stress and strain under a wheel load are highest. The cracks reach the surface first as a series of parallel cracks. After more loading, the cracks connect and form multi-sided, sharp-angled pieces. Excessive deflection of the surface over a weakened or under-designed foundation has the same effect.

The severity levels follow the same path. Fine parallel cracks come first. A network of interconnected cracks follows, with light spalling at the edges. In the last stage the pieces are spalled, and some rock under traffic.

Bottom-up and top-down

Pavement Interactive separates two cases.

• Bottom-up cracking. In thin pavements the cracks start at the bottom of the asphalt layer, where the tensile stress is highest. They travel up to the surface as one or more longitudinal cracks. This is the classical case.
• Top-down cracking. In thick pavements the cracks most likely start at the top. High local tensile stress at the tire contact and aging of the asphalt binder cause them.

A study of three Indiana highway sites (FHWA/IN/JTRP-2004/06) found top-down cracking in pavements with excellent structural capacity. The authors named non-uniform material from construction, high in-situ air voids, low fines and binder aging as the causes in the surface layer. They found no structural cause. A core through the crack shows where it starts.

How to tell it from other cracks

Alligator cracking and block cracking

Both are patterns of interconnected cracks. The PAVER manual separates them by cause, piece shape and location:

The PAVER manual says that a pattern over a whole area that traffic does not load is block cracking. Block cracking usually shows that the asphalt has hardened significantly. The LTPP manual gives block sizes of about 0.1 to 10 square meters. It also says to rate fatigue cracking inside a block cracking area as fatigue cracking. The block cracking area is then smaller by that amount.

Alligator cracking and longitudinal cracks

A longitudinal crack in a wheel path can be the first stage of alligator cracking. AC 150/5380-6C says that alligator cracking reaches the surface first as a series of parallel cracks. The LTPP manual rates a wheel path longitudinal crack as fatigue cracking if it has associated random cracking or meanders and has a quantifiable area. It also rates an area of short, closely spaced transverse cracks (less than 0.3 m) in the wheel path as fatigue cracking. TxDOT says that a longitudinal crack in the wheel path can be a precursor of alligator cracking.

Alligator cracking and map cracking

The LTPP manual lists fatigue cracking among the asphalt concrete distresses. It lists map cracking among the jointed concrete distresses. Map cracking is a series of cracks that extend only into the upper surface of a slab. The manual does not list it for asphalt surfaces.

Alligator cracking and rutting

The two often occur in the same wheel path. The PAVER manual says that if alligator cracking and rutting occur in the same area, the inspector records each at its own severity. See rutting .

Causes

AC 150/5380-6C lists these probable causes in its repair guide:

• Failure of the base or the subgrade.
• Overload.
• A surface course that is too thin.

Pavement Interactive describes the cause as inadequate structural support, and lists these common sources:

• Loss of base, subbase or subgrade support, for example from poor drainage or spring thaw.
• Stripping at the bottom of the asphalt layer. The stripped part adds little strength, so the effective thickness falls.
• More or heavier loads than the design assumed.
• Inadequate structural design.
• Poor construction, for example insufficient compaction.

TxDOT adds that alligator cracking can appear early in the life of a pavement. It can then point to one of these causes:

• A structural thickness that is too small for the traffic, including a thin asphalt surfacing.
• Delamination of the surface layer.
• Poor construction.
• Weak materials.

Water plays a part. AC 150/5380-6C says that water weakens a saturated base, subbase or subgrade. It calls this one of the main causes of failure in flexible pavements. It also says that no other factor matters more for a pavement than its drainage system.

Severity and measurement

PAVER (airfields)

The PAVER manual for asphalt airfields rates alligator cracking as follows:

• Low: fine, longitudinal hairline cracks, parallel to each other, with no or only a few interconnecting cracks. The cracks are not spalled.
• Medium: a well-defined pattern of interconnecting cracks, with all pieces securely held in place. The cracks may be lightly spalled.
• High: a network in which the pieces are well defined and spalled at the edges. Some pieces rock under traffic and may cause FOD potential.

The manual measures the distress in square feet (square meters) of surface area. Two or three severity levels often exist in one area. If they are easy to tell apart, the inspector measures and records them separately. If they are not, the inspector rates the entire area at the highest severity present.

LTPP (highways)

The LTPP manual also has three levels:

• Low: an area of cracks with no or only a few connecting cracks. The cracks are not spalled or sealed, and pumping is not evident.
• Moderate: an area of interconnected cracks that form a complete pattern. The cracks may be slightly spalled or sealed, and pumping is not evident.
• High: an area of moderately or severely spalled interconnected cracks that form a complete pattern. The pieces may move under traffic, and pumping may be evident.

The LTPP manual defines pumping as the seeping or ejection of water from beneath the pavement through cracks. It can leave deposits of fine material on the surface. The inspector records the area at each severity level in square meters. If the levels cannot be told apart, the inspector rates the whole area at the highest severity present. The manual sets no crack width limits for alligator cracking.

TxDOT

TxDOT does not define severity levels for alligator cracking in the section on flexible pavement visual surveys. It converts the amount of alligator cracking into a percentage of the wheel path area of the rated lane.

Standards

PAVER distress code 41 is alligator cracking. In the LTPP manual it is distress type ACP 1. ASTM D5340 describes each distress type, its severity levels and its measurement for the Pavement Condition Index (PCI) survey of airport pavement. The PCI runs from 0 for a failed pavement to 100 for a new one. Agencies do not use one scale, so state which manual a report follows.

What it means on an airfield

The PAVER manual calls alligator cracking a major structural distress. Pavement Interactive lists three problems that it causes: it shows structural failure, it lets moisture in, and it can become a pothole.

Fatigue cracks that interlock can work loose and form a pothole. AC 150/5380-6C says that sections of cracked pavement that work loose can be picked out by wheel loads. It also notes that potholes are not common on airfields.

Loose pieces are FOD. AC 150/5380-6C says that distress which generates loose material can strike propellers or enter jet engines. It says that all pavement inspections should address FOD. In the PAVER rating, the high severity level names pieces that rock under traffic and may cause FOD potential.

ICAO Annex 14, paragraph 10.2.2, says that the surface of a runway shall be maintained so that harmful irregularities do not form. It does not name alligator cracking.

Federally obligated airports in the US must inspect airfield pavements in detail at least once a year. AC 150/5380-7B says that a PCI survey to ASTM D5340 may extend the interval of those detailed inspections to three years.

Repair

Start with the cause. Pavement Interactive says to dig a pit or take cores, to learn the structure of the pavement and to check whether moisture is a factor.

• Small areas. The cracking points to a loss of subgrade support. Remove the cracked pavement, replace the weak subgrade, improve the drainage if needed, and patch.
• Large areas. The cracking points to a general structural failure. Place an asphalt overlay that is strong enough to carry the load, because the cracked pavement adds little or no strength.

AC 150/5380-6C lists removal and replacement of the damaged pavement, including the base or subbase course if required. Appendix A3 gives a full depth repair procedure for flexible pavement:

1. Saw cut or mill the full depth of the asphalt. Extend the cut at least 12 in (30 cm) beyond the distressed area.
2. Repair and re-compact the base as necessary.
3. Apply a tack coat to the bottom and sides of the repair area.
4. Fill with P-401, P-403 or an equivalent State DOT mix, and compact it to the minimum density.
5. Check with a straightedge that the patch is flush with the pavement.
6. Allow no traffic until the asphalt has cured.

Pavement Interactive says that once the alligator pattern is clear, crack sealing is generally ineffective. TxDOT says that a minimal rehabilitation removes the affected material and patches it before a new surface goes on. TxDOT adds that sealing or an overlay without patching leads to a rapid return of the distress.