What are the types of cracking in roads?
by Michal Golos, on 22-Apr-2021 04:20:10
The types of cracking in road surfaces: what are the types of cracking, what causes cracking and how can cracking be avoided?
What are the types of cracking?
The FHWA Distress Identification Manual identifies seven different types of cracking.
Fatigue (alligator) cracking starts as a series of interconnected cracks that develop to break the road surface into multiple, irregular shaped pieces, usually less than 500mm in size. Repeated loading causes incremental damage that manifests a micro crack initiating predominantly from the bottom of the asphalt layer. The crack grows, until it extends through the full asphalt layer thickness. These cracks are often referred to as alligator cracking.
Moderate alligator cracking in wheelpath
Severe alligator cracking
Block cracking is a pattern that occurs as a series of roughly rectangular blocks, ranging from 0.3m to 3m in size.
Severe block cracking
Edge cracking: long cracking in the outer 0.3 to 0.6m of a pavement that curves to the pavement edge.
Wheelpath Longitudinal cracking runs parallel to the pavement's centreline, caused by fatigue.
Non-Wheelpath Longitudinal cracking is randomly located but still parallel to the centreline. They are predominately created between the existing pavement and new widening, but may also be a result of expansive clay subgrade soils.
Transverse Cracking describes cracks that are aligned across the carriageway perpendicular to the centreline. Can be thermally induced or due to underlying joints or cracks.
Reflection cracking describes cracking in asphalt overlay located in concrete pavements, or over shrinkage cracks in cement treated base layers. It occurs over an existing crack in the underlying surface. This causes stress concentration in the asphalt layer that eventually leads to the initiation of a crack that extends up to the surface.
Severe transverse reflection cracking over joints in concrete substrate
(Photo courtesy of Pavement Interactive)
What are the causes of cracking?
Load induced cracking is caused by repeated loading from traffic: either fatigue cracking or reflection cracking, as detailed above.
Load induced cracking is caused by repeated loading from traffic: either fatigue cracking or reflection cracking as detailed above.
Non-load induced cracking is generally thermally induced and is referred to as low temperature cracking. This describes the pavement shrinking at lower temperatures, generating tensile stresses in the asphalt, which lead to transverse (across the road width) cracking. Another form of non-load induced cracking is associated with the presence of expansive clay subgrades soils. If these soils dry out, they shrink, causing longitundinal (along the road) cracking in the asphalt surface.
How can cracking be avoided?
Fatigue cracking results from repeated flexing of the asphalt pavement. The degree of flexure is dependent upon the structural strength of the pavement. Tensar stabilisation geogrids incorporated into the subbase or unbound base layer of a pavement increase the strength of the pavement, reducing deformations and fatigue in the surfacing. The fatigue resistance of an asphalt overlay can be improved by the inclusion of Tensar AX5-GN structural interlayer at the base of the overlay.
Reflective cracking in overlays can delayed by inclusion of an appropriate asphalt interlayer. Tensar has a range of interlayer products that have been proven to reduce reflective crack propagation.
Longitudinal cracking due to the presence of expansive clay soil can be prevented by a Tensar stabilisation geogrid in the unbound granular layers, to control lateral movement of the unbound layers in the event of clay shrinkage.
For more information about Tensar solutions, visit www.tensar.co.uk
Want to tune into our next Tensar Academy webinar?
Tensar will be holding an Academy Webinar on “Pavement Design Incorporating Geogrids” on Thursday 29th April at 11.00BST.
The session will be presented by Piotr Mazurowski – Application Technology Manager, who will be looking at the benefits of incorporating geogrids into pavement design – stabilised vs non stabilised, tried and tested.
To find out more information and to register your place, please visit the link here.