What is Longitudinal Cracking in a Pavement?

Longitudinal Cracking

Definition

A longitudinal crack is a crack that runs parallel to the pavement centreline or the laydown direction. A transverse crack extends across the pavement at about a right angle to the centreline.

The FHWA Long-Term Pavement Performance (LTPP) manual adds that the place of the crack in the lane is significant. It separates cracks in the wheel path from cracks outside it. TxDOT says that, depending on its place in the lane, a longitudinal crack is either load associated or environmentally associated.

How to tell it from other cracks

• Transverse cracks. The direction is the only difference. The PAVER manual lists both under one distress, 48, and gives them the same severity levels.
• Block cracking. Block cracking divides the pavement into rectangular pieces. In asphalt pavements, other than asphalt over PCC, if the inspector records block cracking, the inspector records no longitudinal cracking in the same area. The LTPP manual does not rate the longitudinal boundary cracks in a block cracking area separately.
• Alligator (fatigue) cracking. A longitudinal crack in the wheel path can be the first stage. 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.
• Joint reflection cracking. In PAVER, a crack over a joint in a PCC slab under an asphalt surface is a separate distress, 47. Reflection from other bases is listed as a longitudinal or transverse crack. The LTPP manual records a reflection crack at a joint as a longitudinal or a transverse crack. It needs the slab dimensions to identify it.
• Edge cracking. The LTPP manual applies edge cracking only to pavements with unpaved shoulders. It includes longitudinal cracks outside the wheel path and within 0.6 m of the pavement edge.
• Shrinkage cracks in concrete. The PAVER concrete manual rates a hairline crack as a shrinkage crack. It is only a few feet long and does not extend across the slab.
• A spall beside a joint in concrete. The LTPP manual records the part of a crack within 0.3 m of a joint as a spall. It records the rest of the crack as a longitudinal crack.

Causes in asphalt

The PAVER manual for asphalt airfields gives three causes:

• A poorly constructed paving lane joint.
• Shrinkage of the asphalt surface from low temperatures or from hardening of the asphalt.
• A reflective crack from cracks beneath the surface course, including cracks in PCC slabs but not at PCC joints.

AC 150/5380-6C says that oxidation and age hardening shrink the surface, and that thermal fluctuations make it contract. It adds that poorly constructed lane joints speed up joint cracking. These cracks are not load associated.

TxDOT adds more causes. Differential movement beneath the surface is the primary cause of longitudinal cracking outside the wheel path. The origins include:

• Desiccated subgrade soils (edge drying).
• Embankment consolidation or slope failures.
• Widening interfaces.
• Active PCC pavement joints that reflect to the surface.

Load can also cause parallel cracks in the wheel path. Pavement Interactive says that the first signs of fatigue cracking are often longitudinal, in or near the wheel path. It also says that cracks can start at the surface in a pavement that is thick for its loading. Tension from wheel loads and an aged, brittle surface layer cause these top-down cracks.

The longitudinal joint

Pavement Interactive calls the joint the most difficult part of a pavement to compact. The roller either rests part of its weight on the pavement next to the joint, or it misses the edge of the new joint. It advises that longitudinal joints go outside the wheel path, so that traffic loads them only rarely.

FAA Item P-401 sets these rules for airfield asphalt:

• A longitudinal joint offsets the joint in the course below by at least 1 ft (30 cm). The joint in the surface course is at the centreline of a crowned pavement.

• Cut back a longitudinal joint by up to 3 in (75 mm) in any of these cases:

  • It was exposed for more than 4 hours.
  • It has cooled below 175 °F (80 °C).
  • It is irregular, damaged or uncompacted.

  The cut exposes a clean, vertical face for the full depth of the course. A tack coat goes on the joint before the next lane goes down.

• Acceptance tests one core centred over the longitudinal joint in each sublot that has one. The lower specification limit for joint density is 90.5 percent. It is 92.8 percent for the density of a surface course mat.

Causes in concrete

A longitudinal crack in a concrete slab divides the slab into two or three pieces. The PAVER concrete manual says that a combination of load repetition, curling stresses and shrinkage stresses usually causes it. AC 150/5380-6C says that it may indicate poor construction, underlying layers that cannot carry the load, or overload.

In the PAVER concrete manual, a low-severity crack is usually related to warping or friction. It is not a major structural distress. Medium and high-severity cracks are usually working cracks and are major structural distresses. A slab that is divided into four or more pieces falls under a different distress, shattered slab.

Severity

PAVER (airfields), asphalt

The PAVER manual for asphalt airfields rates a longitudinal crack as follows:

• Low: no spalling, or minor spalling with little or no FOD potential. 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 satisfactory condition.
• Medium: one of these four conditions.
  • Moderate spalling with some FOD potential.
  • Filler in unsatisfactory condition.
  • A non-filled crack wider than 6 mm with no more than light spalling.
  • Light random cracking near the crack.
• High: severe spalling with definite FOD potential, filled or not filled, of any width.

PAVER (airfields), concrete

For a non-reinforced concrete slab, PAVER rates a non-filled crack as low below 3 mm (1/8 in) wide with no more than minor spalling. It is medium from 3 mm to 25 mm (1 in) or if moderately spalled. It is high if severely spalled or wider than about 25 mm, which can damage tires. The inspector counts the slab once.

Highway manuals

Highway manuals use other bands. State which scale a report uses.

• LTPP, asphalt. Low is a mean width of 6 mm or less. Moderate is above 6 mm up to 19 mm. High is above 19 mm. A crack of 19 mm or less is moderate if low-severity random cracking is within 0.3 m. It is high if moderate or high-severity random cracking is within 0.3 m.
• LTPP, jointed concrete. Low is a width below 3 mm, with no spalling and no measurable faulting. Moderate is 3 mm up to 13 mm, or spalling below 75 mm, or faulting up to 13 mm. High is 13 mm or more, or spalling of 75 mm or more, or faulting of 13 mm or more.
• TxDOT, flexible pavement. TxDOT rates a crack only if it is at least 1/8 in wide, shows spalling or pumping, or has been sealed. It rates a crack wider than 2.0 in, or faulted more than 2.0 in, as a failure.

Measurement

The PAVER manual measures longitudinal and transverse cracks in linear feet (linear meters). The inspector records the length and the severity of each crack. If the severity changes along the crack, the inspector records each part on its own.

The LTPP manual records the length in meters at each severity level, in the wheel path and outside it. It also records the length of crack with sealant in good condition. It does not count the sealant as good unless at least 1 m of continuous sealant in good condition is present. A crack of less than 1 m needs good sealant over its whole length.

TxDOT measures in linear feet per 100-ft station. TxDOT also says that how active a crack is, and where it comes from, must be part of any rehabilitation plan. To see the activity, measure the crack width at a few cracks during different seasons.

What it means on an airfield

Pavement Interactive lists three problems of a longitudinal crack. It lets moisture in. It adds roughness. It may be the first sign of alligator cracking and structural failure.

A spalled crack matters on an airfield because it can release debris. AC 150/5380-6C says that distress which generates loose material can strike propellers or enter jet engines. PAVER uses the FOD potential of the spalling to set the severity.

AC 150/5380-6C lists filling and sealing of cracks as routine maintenance. It says that timely maintenance keeps the load-carrying capacity of the pavement and reduces the potential for FOD.

A PCI survey to ASTM D5340 records the crack as part of the Pavement Condition Index. TarmacView captures pavement survey photos today. Crack detection and the rating are in development.

Repair

Asphalt

AC 150/5380-6C says that simple crack filling may be the right repair for a longitudinal or transverse crack in a flexible pavement. The steps are to remove old sealer, clean and prepare the crack, and seal or reseal it. Joint heating is an option for longitudinal cracks only under the direction of an engineer.

For a crack narrower than 1 in (2.5 cm), Appendix A1 gives this procedure:

1. Do not repair in bad weather. The pavement temperature must be 50 °F (10 °C) and rising, or meet the sealant maker’s limit.
2. Do not apply sealant if the crack has moisture.
3. Clean the crack with compressed hot air. If needed, saw or rout it to the width and depth that the sealant maker gives.
4. Fit backer rod if the width-to-depth ratio needs it.
5. Apply the sealant from the bottom up, without voids. Hot-applied sealant meets ASTM D6690. Cold-applied sealant meets ASTM D977.
6. Leave the sealant surface 1/4 in to 3/8 in (6 mm to 9 mm) below the pavement surface.
7. Allow no traffic until the sealant cures.

Some cracks need a partial depth or a full depth repair. Partial depth repair can suit a pavement thicker than 5 in (13 cm). Full depth repair is typical for a thinner pavement. Both procedures in Appendix A apply to cracks wider than 1 in.

Pavement Interactive splits the repair by severity. For a low-severity crack (under 1/2 in wide and infrequent), seal it. The seal keeps moisture out of the subgrade and stops the edges from raveling. For a high-severity crack (over 1/2 in wide and numerous), remove and replace the cracked layer, or place an overlay.

Concrete

In a concrete pavement, sealing a crack keeps surface moisture out of the structure. AC 150/5380-6C prefers a concrete saw to a router, because a router can cause micro-cracks in the adjacent concrete.