Pavement Maintenance Techniques

There are various ways of categorising the different maintenance activities, but the usual way is to divide them according to the regularity with which they have to be carried out.

Routine Maintenance: Activities that are likely to be needed continually on all roads independent of the traffic volume or geometric characteristics. Examples are cutting grass, clearing drains, ditches and culvert maintenance, bridge maintenance, road sign maintenance etc.

Recurrent Maintenance: Activities which may be required at intervals throughout the year depending on the traffic, topography and climate. Examples are repairing potholes, surface patching, sealing cracks, edge repairs, road marking repair etc.

Periodic Maintenance: Activities which may be required at intervals of several years, depending on the traffic and rate of deterioration. Examples are surface dressing, thin and thick overlays, mill and replace, fog seal, slurry seal, single surface dressing, re-gravelling unpaved roads, full depth reconstruction, etc. 

Special Maintenance: Emergency work which needs to be carried out immediately to avoid danger to traffic. Examples are accident damage repairs, clearing debris, replacing washout, salting/gritting, snow removal, unblock drains, etc.

The activities described above as ‘periodic’ are those which have to be carefully planned and programmed and which are the most expensive.

Minor Repairs

The principal tasks involved in minor repairs are as follows:

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.

Major Repairs

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.

Adequate skid resistance of road surface is quite important for safe travelling. Surface retexturing provides a quick, efficient and cost-effective way of maintaining skid-resistant road surfaces.

Various techniques available for retexturing are detailed below. It is vitally important that correct treatment is specified to deliver the correct end results.

Bush Hammering: The bush hammering process involves a number of independent fully controlled treatment tips which mechanically restore skid resistance to any existing surface. The technique is applicable to all road surface types including surface dressings, asphalts, e.g. hot rolled asphalt, thin surfacing, and concrete. This treatment improves and restores microtexture by re-profiling aggregate to recreate the original sharp angularity of the aggregate and removing polished particles and fines. Macrotexture will also be improved depending on the surface.

The technique follows the surface profiles and can cope with surface deformation and variations in road width, using instant variable width control, providing a consistent and even skid resistance to the treatment area. The surface levels of the treatment area remain the same as the surrounding surface and treatment is repeatable.

The treatment causes no damage to joints/repairs and traffic loops. Road markings, road studs and ironwork do not need to be removed prior to treatment and can be avoided if necessary.

The process can be used in any weather conditions and equipment is available to treat large and small areas.

Shot Blasting: Abrasive blasting is effective in removing surface polish, giving an improvement in micro and macro texturing.

Shot blasting involves graded steel shot being projected at high speed and at an optimum velocity from a rotating wheel. The technique improves surface texture by abrading and re-profiling aggregate and removal of bituminous matrix, fines, and detritus. Equipment is available which is suitable for treating both large and small areas including different blast head widths.

The treatment causes no damage to joints/repairs and road markings and road studs do not need to be removed prior to treatment. The surface levels of the treatment area remain the same as the surrounding surface.

Longitudinal Scrabbling: Uses hardened tips set into steel washers, loosely mounted side by side hydraulically loaded and drawn/rotated (on several axles around a central axle) along the road surface. The arisings are swept up separately.

Microtexture is improved by cutting through the surface aggregate and exposing new aggregate faces, creating a corduroy effect

Depending on the surface initially surface texture depth can also be improved by creation of longitudinal grooves but the process can also reduce texture depth.

Longitudinal Grooving/Grinding: Grinding with longitudinal grooving (following surface profile) involves the creation of longitudinal grooves using diamond tipped saw blades. This closely spaced saw blades cut grooves at a predetermined width and depth and this process follows the profile of the surface. The surface levels of the treatment area remain the same as the surrounding surface and the treatment is repeatable.

This technique improves microtexture and macrotexture and has the potential to reduce tyre/road noise. The treatment causes no damage to joints/repairs and road markings, road studs, ironwork and traffic loops do not have to be removed prior to treatment.

Transverse Grooving/Grinding: Uses diamond tipped blades and pressure washing to remove slurry. It can be used to provide discreet grooving patterns and can aid surface water drainage (has little effect on skid resistance) and can lead to increased tyre/road noise levels.

Both processes can be used under wet conditions.

Fine Milling: Fine milling (following surface profile) involves the creation of longitudinal grooves using tungsten tipped cutting tools set at 6mm spacing. The machine has accurate level control and removes the top 2-6 mm of the road to achieve a new running surface. This can also be used to provide a key for an overlay treatment. The cutting drum can adjust the number of revolutions per minute to alter the texture depth in different surface materials. In reducing the overall level of the surface, all cat eyes and street furniture need to be removed or reset. The technique is fast and improves microtexture and macrotexture, thereby improving skid resistance and increasing texture depth. It can be used under all weather conditions.

Water Jetting: The process involves the controlled jetting of water through a series of nozzles at high pressure onto the road surface. It does not restore skidding resistance lost through the polishing action of the traffic. It thoroughly cleans the surface and removes surface contaminants and rubber deposits to expose and improve the existing macrotexture.

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.

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.

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.

In-Situ Recycling

Hot Recycling

As per practice, 30% of recycled planings could be used in a new bituminous mix. However, recycling material in an asphalt plant does not overcome the expense and damage associated with delivery vehicles. This can only be achieved by an in-situ process. Two such processes are in use. Repave is the simpler. As in the Remix process, the pavement surface has to be heated, to a temperature of around 80-90C. At this temperature the bituminous material is workable. The main piece of equipment consists of a scarifier, a device which breaks up the surface to a depth of around 20mm, and an asphalt paver which applies an additional 20mm of new wearing course material on top. The heated scarified material and the fresh mix bond together and, when compacted, form a single new 40mm wearing course. The treatment is suitable when the existing wearing course has started to deteriorate, either by rutting or cracking, but the underlying layers are still in a sound condition.

Limitations are as follow:

• A fairly consistent existing wearing course is necessary;
• A careful mix design is required to establish the proportions needed for the added material;
• Material deeper than 20mm below the original pavement surface is not heated sufficiently to be significantly improved;
• The process can only be considered as partial recycling because of the need for new material;
• An increased surface level has to be accommodated;
• There is a danger of toxic fumes being emitted during the heating process if it is not carefully controlled.

The Remix process differs from Repave in that the top 20mm of the existing surface is actually scraped up and mixed in with the new material before being laid by the paver as a single layer. This means that any inadequacy in the mix used in the original wearing course can be corrected by adding appropriate proportions of bitumen and aggregate in the new material. In the case of Repave, the majority of the original layer remains in the lower 20mm and is only slightly mixed in with the new material.

Cold Recycling

The following three types of binder can be used to form a cold mix:

• Cement (or other hydraulic binder e.g. slag, ash etc.);
• Bitumen emulsion;
• Foamed bitumen.

Bitumen emulsion is a very widely used material, for example as a binder for surface dressing. It consists of droplets of bitumen suspended in water by the action of a chemical, an emulsifying agent. This allows the emulsion to be used as a liquid to be attracted to the aggregate surface. However, this process remains a slow one-unit compaction of the mix greatly accelerates it. The result is a bitumen bound mix with a considerable amount of water, which has to be allowed to evaporate.

Formed bitumen is less widely used but has considerable advantages over a bitumen emulsion. Hot bitumen (> 100C) is fed under pressure down a tube into which water is also introduced (usually along with certain other additives). The water turns to steam, forming bubbles within the bitumen, and the result is foam, some 20 times the volume of the original bitumen which is workable enough to be mixed with cold aggregate. However, the foam is only short lived, having a half-life of typically 20 seconds. Mixing time is therefore limited. The principal advantage over bitumen emulsion in the cold climate is that less water is used and therefore less has to be evaporated before the mix reaches is final strength.

In in-situ recycling, the pavement is milled using a very powerful tool to a depth of up to 350mm. This breaks the existing bound layers into quite small pieces. The broken pavement fragments are then mixed with whichever binder is to be used, compacted and usually at a later date, a new surface is applied. The process mixes all the materials together which have been milled so, in a depth of up to 350mm, it is quite possible that bituminous, granular and even soil may be included. The quantity and type of binder added has to be suited to the materials present.

Cold In-Situ Recycling

In general, cement will be usable with almost any material. Foamed bitumen and emulsion should be satisfactory except with clay soil. However, if emulsion is used the depth may have to be restricted in order to achieve adequate evaporation of the water content.

In any deep recycling process, the danger of material variability exists. Indeed, it is certain that the mix will vary enormously from one spot to the next and, of course, it is the weakest elements which will fail first and therefore determine the life of the road. In addition, it is fairly certain that the depth of recycled material will also vary, or at least the depth to which the binder has penetrated.

If bitumen emulsion or foamed bitumen are used there is also the added difficulty that strength gain takes place over a period of weeks or months. There is even the danger that strength gain never takes place fully if the water is unable to escape from the mix.

For these reasons, the Department of Transport in the UK does not approve these materials on its roads that are trunk roads and motorways. However, the potential economic advantages have meant that other authorities have experimented, with some degree of success and these processes are also used in a number of other countries.

Overlay and Reconstruction

In practice, many of the roads end up requiring a treatment more substantial than any of those mentioned above. Clearly thick overlays and reconstruction can be carried out in much the same way as new pavement construction. Unfortunately, they require a site to be closed to traffic for a significant time which is something that road authorities are increasingly trying to prevent.

The design of overlays and reconstructions presents a number of difficult issues stemming from the way in which the conditions of the existing road are assessed. However, a study of these issues would lead outside the field of pavement maintenance and into pavement design and hence cannot be covered in detail here.