Pavement - Minor Repair Activities

Crack Sealing?– This is simply a narrow band of bituminous sealing material which is painted onto the surface of a road as a temporary measure to prevent water from penetrating a crack in the pavement and causing further damage. Particularly in the case of an active crack, it would be expected that the seal would itself re-crack in a relatively short period of time.

Patching?– a more extensive cracked or rutted area, but still of limited extent, would be patched. This again is a rarely a permanent repair through it may last several years if it is carried out well and depending on the type of failure which has occurred. If, for example the cause of the failure was localised defect in the wearing course material itself, perhaps due to construction joint, then a small patch may effectively remove the problem. However, it is almost impossible to achieve the same standard of homogeneity and density in a small patch area as when laying a full pavement course and this is in itself often limits the life of a patch repair.

Haunching?– Haunch repairs are carried out where the edge of the road has collapsed under the action of traffic often combined with the effects of poor drainage. This tends to happen on narrow roads which take a certain amount of heavy goods traffic. The solution is not simply to repair the damage area but to strengthen it in order to ensure that the problem does not recur. This may also necessitate the introduction of a drain where previously there was none.

Surface Treatments

Whether or not the structural life of a pavement is deteriorating significantly, the surface always tends to wear out over a period not usually exceeding 10 years. Two problems may be occurring, namely loss of skid resistance or loss of ride quality. The second one is commonly associated with a more general structural deterioration. This section outlines the treatment options, which are available to restore the surface properties.

Retexturing?– Retexturing is carried out mechanically hammering at the surface stones, forming new unpolished surface, or alternatively by sand blasting the surface, which has the same effect. Thus, the micro-texturing is improved but not the macro-texture and the process will only be effective if polishing is the dominant problem rather than wearing away (abrasion) of the stone itself. The life of such a treatment may be anything from 1 year to many.

Surface Dressing?– by far the most universal surface treatment, surface dressing is a convenient method of combining the benefits of improved surface texture with a sealed surface. In its simplest form, it consists of a spray of bituminous binder, usually an emulsion or a cut back, applied to the surface, followed by spreading of aggregate chippings. The chippings are held down by the bitumen and under the influence of passing traffic, embed themselves into the existing surface. Surface dressing can be successfully applied to all types of pavement, including concrete. However, the resulting surface, though of high skid resistance, tends to be noisy and not to give such a comfortable ride as most bituminous surface layers. The secondary benefit of sealing the surface can be very important and, by excluding water from the sensitive lower layers of a pavement, the structural life can sometime be extended by a number of years.

The life of a surface dressing is controlled by the degree to which the chippings are bonded to the surface. A tougher surfacing is provided if more than one application of chippings is made. Typically, a second application of smaller stone size is applied. These stones tend to fill up the gaps between the larger particles, forming a more densely packed material. This is often termed ‘racked in’ surface dressing. It is quite possible to go even further and carry out a triple dressing and this type of surface would often be used in countries where the surface dressing forms the only bound component of the pavement.

Though widely used an extremely useful maintenance technique, surface dressing is very sensitive to construction practice. When not carried out correctly, the consequences can be a very poor and even dangerous surface. The following points are important.

• The existing surface must be dry and reasonably free from dust and dirt
• The weather conditions must be suitable. In general, this means that the air temperature has to be sufficiently high to cause the bitumen to ‘set’ from its emulsion or cut-back form
• Traffic has to be controlled during early life of the surface. If vehicles travel too fast, the stones will tend to be loosened rather than embedded, resulting in a less durable surface.

Slurry Sealing?– In contrast to the retexturing and surface dressing described above, slurry seal is aimed solely at sealing the surface against water and does not provide a sufficiently skid resistant surface for high-speed roads. The material is a mixture of fine aggregate, filler and bitumen emulsion. It is mixed and laid by a purpose-built machine using a paving screed. Typical layer thicknesses range from about 3-8 mm depending on the size of the aggregate used.

Micro-Surfacing?– This category comprises layers of bituminous material between 15 and 25 mm in thickness, only slightly greater than a surface dressing. In recent years there has been considerable interest in these materials, and they are being actively considered as alternatives to the standard Hot Rolled Asphalt wearing course. Two of the most widely used are ‘Safepave’ and UL-M. Development has taken place mainly in France where their use is now widespread.

Essentially these materials comprise fairly single sized aggregate held together by a bitumen and fine aggregate mortar which still leaves room for a fairly high void content. The void content introduces what is known as ‘negative texture’, an alternative to traditional macro-texture but which still gives good wet skidding resistance. It has the additional benefit of low noise when compared to most other forms of pavement surfacing. The binder used can either be pure bitumen or an emulsion.

Although not much thicker than a surfacing dressing, a micro-surfacing is a machine laid course of material and can be used to correct minor undulations or ruts in a surface. This is in addition to providing a restored skid resistance and a sealed surface. However, the structural contribution from such a layer remains small.