Pavement Distress Recognition
As time passes, asphalt pavements may encounter problems that need to be addressed. Three types of distress that an asphalt pavement may develop are cracking, distortion, and disintegration.
Types of Cracks
Cracking appears in many forms. The treatment of the crack may be as simple as filling the crack with an asphalt emulsion slurry or cutback asphalt mixed with sand, or as complex as complete removal of the affected area and installation of drains prior to the necessary repairs.
In order to adequately repair a pavement, the root cause of the cracking must be found. Often the type of cracking seen on the surface of the pavement will give an indication of the type of problem that is causing the crack. In the main, the types of cracks that are encountered fall into the following groups:
- Alligator cracks
- Edge cracks
- Joint cracks
- Reflection cracks
- Shrinkage cracks
- Slippage cracks
Alligator cracks are interconnected cracks forming a series of small blocks resembling an alligator’s skin or chicken wire. They are usually associated with a granular untreated base that has failed or with soft subgrade.
Since this type of cracking is often caused by a saturated base or subgrade, removal of the wet material and possibly installing some form of drainage will rectify the problem. Once the problem materials are removed, a full depth hot mix asphalt (HMA) patch will provide a strong remedy. If this is not a convenient solution, then placing new granular material compacted in layers, primed and covered with appropriate thickness of HMA may be utilized.
In extreme cases, a skin patch may be used for temporary repairs. It is emphasized that this situation should be done promptly so that further damage is not caused to the surrounding pavement.
Edge cracks are longitudinal cracks near the edge of a pavement, with or without transverse cracks branching toward the shoulder. Edge cracks are usually caused by lack of lateral or shoulder support for the asphalt pavement. These cracks may also be caused by settlement or yielding of the base materials underlying the cracked area. This in turn may be the result of poor drainage, frost heave, or shrinkage from drying out of the surrounding earth.
To repair this type of condition, the cracks may be filled with an asphalt emulsion slurry or cutback asphalt mixed with sand. If the cracks are caused by the edge of the pavement settling, then the surface may be brought up to grade with an HMA patching material.
Joint cracks come in two types: Edge-joint crack and lane-joint crack. An edge-joint crack occurs between the pavement and the shoulder, as shown above. The lane-joint crack happens between two adjoin paving lanes.
Alternate wetting and drying or freezing and thawing beneath the shoulder surface is a common cause of edge-joint cracking. It usually results from poor drainage where water is trapped or ponded in depressions over the pavement-shoulder joint.
Lane-joint cracks, on the other hand, are usually caused by a weak seam or poor bond between adjoining spreads in the pavement.
Where water is a contributing factor to cracking, drainage should first be corrected. The cracks are filled with an asphalt-emulsion slurry. To fill larger cracks special asphalt compounds or heavier-bodied asphalts may be used.
Everyone has noticed and felt reflection cracks. Even if you know nothing about roads, these cracks are recognizable. These cracks cause most of the characteristic thump-thump-thump rides felt and heard on an asphalt road. Reflection cracks occur on asphalt overlays. The cracks heard and felt reflect the crack pattern found in the pavement structure underneath. They are most often found in asphalt overlays over Portland cement concrete and cement-treated bases.
Reflection cracks are caused by vertical and horizontal movements in the pavement beneath the overlay. These movements result from traffic loads, temperature, and earth movements.
If the cracks in the pavement are less than an 1/8 inch wide, it may be prudent to ignore them unless water can enter and cause further harm. In that case they may be filled by a squeegee technique, using emulsified or cutback asphalt covered with sand. Cracks wider than ¼ inch are filled with an asphalt emulsion slurry or a light grade of cutback asphalt and fine sand.
Shrinkage cracks are interconnected cracks forming a series of large blocks, usually with well defined corners and angles. It is often difficult to determine whether this type of cracking is caused by volume change within the asphalt mix or in the base or subgrade.
Another form of shrinkage cracks is caused by aging of the pavement. After years of exposure to sunlight and the elements, the asphalt in the surface of the pavement loses some of its elasticity or resilience. At the same time the pavement is subjected to constant expansion and contractions caused by temperature changes. Unlike alligator cracking, shrinkage cracks are not a sign of structural deficiency.
Shrinkage cracks should be sealed with and emulsion asphalt slurry followed by a surface treatment, a slurry seal, or an overlay on the entire surface.
Slippage cracks are crescent-shaped cracks resulting from traffic induced horizontal forces. These cracks are caused by a lack of adhesion between the surface layer and the layer beneath. This lack of bonding may be caused by dust, dirt, oil, or the absence of a tack coat.
The only effective means of repair to this type of cracking is the removal of the poorly bonded layer. The poorly bonded layer is removed until a region of good bonding is found. At which time the area may be patched with HMA.
Types of Distortion
Pavement distortion is a problem caused by compaction. This compaction may occur in the subgrade soil where additional movement or compaction has occurred, or where base compaction has happened. This type of problem may or may not be accompanied by cracking, but either way this is not a good situation as it becomes a traffic hazard, permits water to accumulate, and eventually makes matters worse. Distortion comes in a variety of forms:
- Corrugations & shoving
As with any other defect, the type of distortion has a cause, and it must be determined before the proper remedy can be applied. Pavement distortions are more likely to occur at intersections and other stop-start areas where traffic, especially heavy trucks, imposes severe horizontal stress on the pavement surface. Repair techniques range from a relatively simple procedure of leveling the surface by filling with new material, to the complex treatment of complete removal of the affected area and replacing with new material.
In general causes of distortion are often narrowed down to the following causes:
- Insufficient compaction
- Too many fines
- Too much asphalt binder
- Swelling, and
Rutting, or permanent longitudinal deformations are channelized depressions that develop in the wheel tracks of asphalt pavements. Rutting may be caused by consolidation or lateral movement under traffic in one or more of the underlying courses, or by displacement in the asphalt surface layer itself. New asphalt pavements that did not receive sufficient compaction during construction may be susceptible to channeling. Also rutting may happen in a pavement where the asphalt mix does not have the stability to support traffic.
It is necessary to identify the cause of the channeling. In some cases it may be necessary to remove the pavement layer identified as the cause. In less severe situations the channels or ruts may be milled to a level plane or simply filled in with HMA to be followed by an overlay across the entire pavement.
Corrugations and Shoving
Corrugations, or washboarding, is a form of plastic movement typified by ripples across the asphalt pavement surface. Shoving is also a form a plastic movement, but its results are exhibited by localized bulging of the pavement surface. Both corrugating and shoving occur where traffic starts and stops, or on downgrades where vehicles brake when going down a hill.
Corrugations and shoving also usually occur in asphalt pavement mixtures that lack stability. This may be the result of too much asphalt, too much fine aggregate, improper gradation, or round or smooth textured coarse aggregate in the mix. If emulsified or cutback asphalt mixes are used, it may be the result of insufficient aeration.
In cases where an aggregate base with a thin surface treatment develops corrugation, the surface should be scarified, mixed with the base, and recompacted before resurfacing. For the effective repair of an area with shoved pavement, these areas should be removed and patched.
Depressions are localized areas of limited size that may or may not be accompanied by cracking. The danger caused by these depressions is the collection of water. This water may ultimately become detrimental to the pavement surface and accelerate deterioration. Water is also hazardous to the driving public. Depressions are caused by traffic heavier than that for which the pavement was designed and by consolidation or movement within the subgrade. The movement within the subgrade may have been caused by utilities companies that did not perform adequate compaction when closing their trenches.
Depressions should be filled with HMA and compacted to restore the area to the same grade as the surrounding pavement.
Upheavals are caused by an expansion of the material beneath the pavement surface. This may be due to freezing water during the winter, or by an expansive soil in the subbase. The cure for these both require drainage. In the case of the freezing water under the surface of the pavement, the water source can be isolated from freezing temperatures by placing layers of insulation under the pavement structure or by replacing the native subgrade material with free draining granular material to a depth below the frost line. If the water source is localized, an under drain system may prove adequate. Either way, the pavement must be removed and replaced to allow permanent repair.
For upheavals that are caused by expansive soils, the problem may be arrested by installing a drainage system to remove water access to the subgrade. Remedial maintenance to restore ride quality may be done by cold milling the surface and placing a surface treatment or thin overlay. If drainage is not successful, complete pavement replacement may be the only solution.
Disintegration is the breaking up of a pavement into small, loose fragments. This includes the dislodging of aggregate particles. If not stopped in its early phases, disintegration may lead to complete pavement replacement. Two common types of early stage disintegration are pot holes and raveling. Repair ranges from simple seals in early stages to deep patches if allowed to go to an extreme level.
Pot holes are bowl shaped and range in severity. These are localized disintegrations caused by traffic. Usually caused by weakness in the pavement resulting from a weak subgrade or base course, too little asphalt, too thin an asphalt surface, too many fines, too few fines, or poor drainage, they are seen where ever asphalt has been damaged.
Pot holes often appear when the right type of maintenance is not promptly applied once the disintegration begins. Temporary fixes usually involves cleaning out the hole and filling it with a premixed asphalt patching material. A permanent repair is conducted by a deep patch with HMA.
Raveling is the progressive loss of surface material caused by weather and/or traffic abrasion. Exhibited by the wearing away of the fine aggregates which leaves behind the telltale signs of small pock marks in the pavement surface, the erosion continues until larger pieces of aggregate break free. This leaves a rough and jagged appearance to the pavement surface. Raveling may be caused by poor construction methods, inferior aggregates, or poor mix design. Early fog seal application upon first detection of the problem generally slows down this progressive deterioration.