What is Crack Width Measurement?
What is crack width measurement?
Crack width is the distance across a crack between its two edges. Crack width measurement is the method that gives this distance in millimeters, with a ruler on site or from an image of the pavement.
Why pavement manuals use crack width
A manual rates each crack as low, medium or high severity. Width is one of the inputs, and the manual states which width counts. The FHWA Long-Term Pavement Performance (LTPP) manual uses the mean width of the crack. The PAVER manuals, which airport PCI surveys use, use the mean width of a non-filled crack.
| Pavement | Manual | Low | Medium or moderate | High |
|---|---|---|---|---|
| Asphalt | FHWA LTPP | 6 mm or less | above 6 mm, up to 19 mm | above 19 mm |
| Asphalt airfield | PAVER | 6 mm (1/4 inch) or less | above 6 mm | by spalling, at any width |
| Concrete slab, corner break | PAVER | below 3 mm (1/8 inch) | 3 mm to 25 mm (1 inch) | above about 25 mm, or severe spalling |
Width is not the only input. Spalling, faulting and filler condition also set the severity. A filled crack can have any width. It is low severity when the filler is in satisfactory condition. In the PAVER asphalt manual, high severity depends on spalling that gives a definite foreign object debris (FOD) potential, not on width.
How to measure on the pavement
Measure across the crack, from one edge to the other. On concrete, PAVER says to measure between vertical walls and not in spalled areas of the crack. If the width changes along the crack, the manuals use the mean width, and PAVER records each portion of different severity separately.
How to measure from an image
An image gives a width in pixels. The ground sample distance (GSD) converts it to millimeters, because the GSD is the ground distance that one pixel represents. For a drone image, GSD = (S x A) / (I x F). S is the sensor width, A is the altitude above the ground, F is the focal length and I is the image width in pixels.
The width in millimeters is the width in pixels multiplied by the GSD. A crack that is narrower than one pixel cannot be measured reliably. The manuals use thresholds of 3 mm and 6 mm. The GSD must be small enough that a width near a threshold covers several pixels.
How it matters for airfield pavement
Width tells the maintenance team how to treat a crack. AC 150/5380-6C gives one repair procedure for cracks less than 1 inch (2.5 cm) wide in flexible pavements and a different one for wider cracks. A width above about 25 mm on a concrete slab also creates a tire damage potential, according to PAVER.
The manuals above describe a person who measures on site. No source that we opened sets an accuracy requirement for a width that an image or a drone gives.
Frequently Asked Questions
- Which crack width makes a crack low, medium or high severity?
- It depends on the manual and on the pavement type. For asphalt, the FHWA LTPP manual gives low at a mean width of 6 mm or less. It gives moderate above 6 mm up to 19 mm, and high above 19 mm. The PAVER airfield manual uses 6 mm (1/4 inch) to separate low from medium for non-filled cracks. For concrete slabs, PAVER uses about 3 mm (1/8 inch) and 25 mm (1 inch).
- Does a wider crack always mean a higher severity?
- No. The severity also depends on spalling, faulting and filler condition. In the PAVER asphalt manual, severe spalling with definite FOD potential makes a crack high severity. Width and filler do not matter then. A filled crack can have any width. It stays low severity if the filler is in satisfactory condition.
- How does an image give a crack width in millimeters?
- Multiply the width in pixels by the ground sample distance (GSD), the ground distance that one pixel represents. The GSD is the sensor width times the altitude, divided by the image width in pixels times the focal length. A crack that is narrower than one pixel cannot be measured reliably from that image.
References
- fhwa-ltpp-dim
- Distress Identification Manual for the Long-Term Pavement Performance Program (FHWA-HRT-13-092, revised May 2014) FHWA
- faa-paver-asphalt
- PAVER Distress Identification Manual, Asphalt Surfaced Airfields FAA / US Army Corps of Engineers
- faa-paver-concrete
- PAVER Distress Identification Manual, Concrete Surfaced Airfields FAA / US Army Corps of Engineers
- faa-ac-150-5380-6c
- AC 150/5380-6C, Guidelines and Procedures for Maintenance of Airport Pavements FAA
- omni-gsd
- Ground Sample Distance (GSD) Calculator Omni Calculator
This is a reference entry. TarmacView provides drone inspection services and doesn't sell the equipment or materials described in this glossary.
