Pavement - Causes of Deterioration and Treatments

Pavement deterioration if allowed unchecked gives rise to problems in two principal areas:

Safety:?This can be divided into two further categories:

• Skid resistance – low skid resistance is a key contributory factor to a large proportion of accidents
• Unevenness – this gives rise to a poor ride quality and the danger that the motorist underestimates the difficulty in controlling a vehicle; the problem becomes particularly acute in wet weather if unevenness allows standing water in the wheel paths.

Economics:?While there is a direct economic cost if safety levels fall, there is an additional and sometimes very significant cost to the economy of a region or a country if a key highway starts to deteriorate. In broad terms, increased roughness leads to slower travelling speeds and less efficient transportation of goods. It also leads to increased fuel consumption and wear and tear on the vehicles. This economic equation is the basis of many pavement management systems in use in various parts of the world.

The following are the main causes of pavement deterioration:

Deterioration Cause No: 1 – Environment

Whilst the principal distress mechanism acting on a pavement are usually traffic related, there are several which stem directly from the environment.

Binder Ageing:?The phenomenon of hardening of bitumen is well known. It is leads to stiffening of the mixture and reduction in the strains developed in a pavement under traffic load. However, it also makes bitumen become more brittle. This can lead to more rapid deterioration under the action of traffic. Ageing is caused principally by:

• Oxidation:?The rate of oxidation will be greater in a material with high air voids since this will allow the oxygen easy access to the bitumen; it also occurs more quickly at high temperature and under the action of ultra-violet sun rays
• Loss of Volatiles:?In some cases, the aggregate used in a bituminous mixture can, over a period of time, absorbs some of the volatile elements of a bitumen, reducing the quality of the bond between the binder and the stone.
• Flow in Hot Weather:?Bitumen behaves as a liquid under most conditions. In hot weather, the temperature of the surface can reach 50C even in the UK and the material is sufficiently soft to flow. This would mainly be seen as increased rutting under the action of traffic but can also take the form of flown down a slope if the road is on a gradient.
• Frost:?A properly designed and constructed bituminous surface should not normally be susceptible to damage by frost. However, the danger exists that freezing of water trapped in the mixture may cause stress which damage the structure of the material.
• Settlement and Heave:?Ground movement is one of the design considerations for roads. Bituminous mixtures can crack under the action of a slow build-up of strain over a period of days, weeks or months, although the slow the movement the more strain can be accommodated before the material fails.

Deterioration Cause No: 2 – Surface Wear Due to Traffic

The effects of this type of deterioration can be quite serious in terms of safety and ride quality although the damage is restricted to the surface. It is possible to identify several basic types of problems:

• Wear to the stones at the surface damages the microstructure at the surface of the stone. This linked directly to the level of achievable skid resistance and therefore to safety. When the level of microstructure drops too far, it is necessary to apply some form of surface treatment to restore it to acceptable levels.
• Abrasion of the stones leads to a loss of textural depth. This is a problem in wet conditions since it increases the likelihood of water coming directly between the tyre and road surface. The result is aquaplaning.
• Stones can be plucked from a brittle surface giving a very rough texture and a very poor ride quality. This is most likely to happen in hot climates, where the binders harden most severely. The resulting phenomenon is termed as ‘ravelling’ and it extreme conditions, it can result in potholes development.
• ‘Bleeding’ is the appearance of a film of bitumen at the surface, giving a smooth and dangerous condition. This is caused by excess binder in the surface layer which is squeezed out under the action of traffic in hot weather.

Deterioration Cause No: 3 – Structure Damage Due to Traffic

Asphalt Rutting:?Asphalt rutting is very familiar sight on the road all over the world, particularly as the weight and volume of goods transported on the roads has seen sustained and dramatic increase.

Rutting is the longitudinal depression in the wheel path in bituminous pavements, which can be attributed to excessive consolidation, formed by an accumulation of permanent deformations caused by repeated heavy loads. It is due to two modes of deformation:

1. Densification occurs if the material has been under compacted. This can give rise to the relatively early development of ruts to a depth of perhaps 5mm, depending on the thickness of bituminous material affected.
2. Shear flow represents the more damaging form of rutting. It is commonly seen at traffic lights or bus stops, where heavy vehicles are travelling slowly. The shape of the resulting rut gives an indication of the layer or layers which are affected by the problem.

Cracking of Bituminous Material:?Cracking occurs in a bituminous material due to high and repeated tensile strains. Conventional analysis shows the highest tensile strain occurring at the base of the bituminous material, causing a crack to propagate upward from the bottom of layer. This is still the assumption made in most analytical pavement design. However, experience clearly shows that it is much more usual to find cracks originating at the surface of the road if the bituminous materials are greater than a certain thickness, perhaps 200mm. This can be attributed to a number of effects:

• Increased brittleness at the surface due to ageing
• Horizontal stresses caused by the way the tyre deforms when in contact with road surface
• Shear forces at the edge of tyre contact area.

If the cracking is allowed to proceed to proceed unchecked, then it will penetrate the full depth of the bituminous layer. Cracks will then multiply and grows in width to form crazing. In parallel, water will enter the pavement through these cracks, softening the underlying unbound materials and accelerating the failure process.

Bond Problems:?One of the inevitable problems with a bituminous road is that the materials have to be applied in layers and in order to maximise the effectiveness of the pavement as a whole, these layers have to be well bonded. Unfortunately, this is a little understood area and pavement life is sometimes reduced significantly because of inadequate bond. In general, traffic action helps to build up the strength of the bond, but this can only happen where, there is sufficient bituminous bond coat in place.

The results of poor bond are that the pavement is no longer so good at spreading load and strains are therefore higher both in the bituminous material itself and in the underlying materials.

Maintenance Treatments

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

• Routine – includes sweeping, winter maintenance, ditch clearance
• Periodic – includes crack sealing, patching, edge repairs, white line repairs
• Special – surface dressing, overlays, reconstruction etc.
• Emergency – accident damage repairs, unblocking drains.

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

Minor Repairs:

The main minor repair activities 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 limiting 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.

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 remoulded or 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.