Asphalt Repair And Maintenance
Maintenance of Flexible Pavements
Flexible pavements generally are referred to as asphaltic concrete pavement (ACP) or bituminous surface treatment (BST). Flexible pavements develop strength from the tight interlocking of crushed rocks with an asphalt material binding them together. This mixture deflects when loaded by traffic and exerts pressure on the subgrade. Consequently, both the pavement and the subgrade must be in good condition to avoid maintenance problems.
Pavement Maintenance Techniques
All flexible pavements require patching at some time during their service life. Surface patching should be performed to a standard commensurate with resource availability and the objective of retaining a smooth ride as long as possible. Since patching materials are one of the larger material costs a high quality patch is one of the most cost effective means of utilizing available resources.
There are two principal methods of repairing asphalt pavements:
- Remove and replace the defective pavement and surfacing or base material.
- Cover the defective area with an overlay of a suitable material to renew the surface, seal the defective area, and stabilize the affected pavement. These repairs can be called ‘dig-outs’ or ‘overlays’ according to the method used.
Damage and deterioration of pavements will become apparent in a variety of ways. A number of factors can contribute to the appearance of pavement deficiencies. For example, an overlay with excess asphalt or poorly graded or inadequately fractured paving material may not have adequate particle interlock; thus pushing, rutting, and humps may develop. Poor subgrade drainage, heavy tonnage, and accelerating or decelerating traffic are all potential sources of surface irregularities. Pavement deficiencies are explained in more detail as follows:
Rutting is a surface depression within the wheel path and is a result of permanent deformation of the pavement or subgrade. This condition is normally caused by heavy loads on roads lacking sufficient strength to support the loading. In some cases, rutting can also be caused by studded tire use. Wheel ruts, if not repaired, can trap water and cause hydroplaning.
The condition known as alligator cracking is attained when discontinuous longitudinal cracks begin to interconnect to form a series of small polygons that resemble an alligator’s skin. This distress is usually caused by poor drainage, poor mix design, or subgrade failure.Ideally the surface should be treated with a seal coat or overlaid with suitable material before water has an opportunity to penetrate the surface and lead to alligator cracking. If it is neglected and alligator cracks appear, heavy traffic can push the surfacing rock into the wet soil beneath it. This forces mud up through the asphalt surface (pumping) causing permanent damage that can not be repaired by a seal or overlay. Spots where severe pumping has occurred will often need to be dug out, and the base rock, surfacing rock, and asphalt replaced.
A longitudinal crack follows a course approximately parallel to the centerline. These are typically resultant of natural causes or traffic loading.
Transverse cracks run roughly perpendicular to the roadway centerline. They may be due to surface shrinkage caused by low temperatures, hardening of the asphalt, or cracks in underlying pavement layers such as PCC slabs. They may extend partially or fully across the roadway.
Potholes are voids in the roadway surface where pieces of the pavement have become dislodged. Areas in which many potholes occur become suspect for fundamental problems such as inadequate drainage, pavement strength, or base/subgrade problems. Single or infrequent potholes may be the only pavement distress to occur in an area, and beyond the treatment of the individual pothole no other pavement repair work may be required.
Raveling and Pitting
Raveling and pitting distresses are characterized by the loss or dislodgment of surface aggregate particles. Oxidized asphalt binder is often the cause of raveling and pitting. It could also be caused by poor compaction, letting the mix get cold when paving, dirty aggregate, not enough asphalt in the mix, overheating the mix during manufacture, or aging. Routine maintenance repairs are made to raveled or pitted surfaces is made as soon as conditions permit and/or materials are available. The most important consideration in scheduling repair of raveled or pitted areas is to perform the repairs before a more serious condition develops, and prior to the onset of inclement weather. Open grade pavements that allow water to drain through and out the side don’t need to be sealed if they are properly constructed. But, pavement that is raveling must be sealed. Unsealed pavements will continue to ravel and will also age and harden at a much faster rate than normal. This condition may also encourage the loss or stripping of the asphalt within the pavement.
Flushing (or bleeding) is free asphalt on the surface of the pavement caused by, too many fines in the mix, too few voids, too much asphalt in patches, or a chip seal that has lost its rock. This type of distress often shows as a shiny, glass-like reflective surface. It is inherent to unstable mixes and often results in other roadway surface distresses if not corrected. Removal and replacement of flushed or bleeding pavement areas is an expensive, but sometimes cost-effective method of repair. Thin overlays of flushed or bleeding areas will frequently have the “fat spot” show through in hot weather and exhibit the same characteristics as in the “before” condition.
Sags and Humps
Sags and humps are localized depressions or elevated areas of the pavement that result from settlement, pavement shoving, displacement due to subgrade swelling, or displacement due to tree roots. The deficiency usually occurs in isolated areas of the roadway surface.
Edge raveling occurs when the pavement edge breaks and is most commonly found on those roadways that were constructed without curbs or paved shoulders.