Road Surfacing: Why Does It Matter?

Not all roads are the same. Although the driver rarely gets to set foot on the road surface as they move by in their vehicle there is actually huge variety in what their tyres are making contact with.

Underneath that car or lorry there are many different types of road surfacing material each of which is chosen for a particular site depending on countless variables. The ingredients that make up the road surface can be influenced by the type of road, the volume and speed of traffic, and its location among others.

Getting these variables right and applying the correct surface option can literally be a matter of life and death, especially on fast roads.

Surfacing Materials: So What Are the Options?

One of the most common materials used for road surfacing today is Stone Mastic Asphalt (SMA) sometimes also referred to as Thin Surface Course. SMA consists of variably graded aggregate mixed with a bitumen binder. The production of SMA is predominantly carried out in batch mixing plants to provide as close to uniform consistency as possible in order to comply with specification guidelines. The SMA is laid by a ‘Paving Machine' which is able to evenly spread the material to the required thickness. This is a highly skilled process which must be performed properly to ensure the quality of the driving surface.

An alternative material is Hot Rolled Asphalt (HRA). HRA consists of two components, the surface course mixture and pre-coated chippings. The surface course mixture consists of a bitumen-based binder mixed with intermediate quality small aggregate. This course is laid to the required thickness using a paving machine. Once a section of surface course is laid a chipping spreader passes over it distributing high quality aggregate chippings onto its surface to produce texture and a skid resistant surface. After the chippings have been spread they must be compacted into the surface course using a heavy roller. However care must be taken when compacting. If the surface course is too hot the chippings may be pressed too deep into the material effectively losing their purpose as a high friction surface layer. If compacted whilst the surface course is too cold the chippings may not stick to the course adequately and may become detached by passing traffic again compromising their purpose as a high friction surface layer.

High Friction Road Surfacing

Certain areas of road, like the approach to traffic lights, may require surfaces with higher friction values than can de achieved by the methods outlined above. In such situations a ‘high friction surfacing' material may be used. There are two main types of this material.  One which is laid hot consists of a resin binder with thermoplastic properties and a coarse aggregate. These constituents, initially solid, are reduced to their fluid state in a temperature controlled hot-pot. Once in the fluid state the material is poured from the outlet-shoe of the pot onto the road where it is manually spread to the desired thickness by the construction team. A material which is laid cold consists of a resin which is mixed with a hardener in a large container using a paddle before being poured and spread onto the road manually. High quality aggregate is then spread over the resin and any excess swept away when the product is dry.

Safety On Our Roads: Anti-Skidding

For the road user skidding is an undesirable effect of heavy braking or harsh sudden changes in direction. During a skid the driver may lose control of their vehicle which can end with dire consequences.

An important factor in road design is therefore how to reduce the possibility of skidding. Skidding is closely related to friction. This means it is important a road surface can create enough friction between itself and passing tyres to reduce the possibility of skidding. This however must be achieved without causing too much damage to the tyre, erosion to the surface and whilst maintaining a suitable ride quality for the road user.

This is achieved using bituminous material combined with aggregate chippings. The spacing between chippings and the height to which the chippings protrude make up the surface's macro-texture whilst the surface's of the chippings themselves comprise the micro-texture. The closer the surface chippings are situated to one another and the higher they protrude from the surface the greater the friction they create between the road surface and the passing tyres and hence the greater the ability to reduce skidding. Over time, passing traffic causes the surface aggregate to become polished reducing its micro-texture and consequently reducing its friction/skid resistance ability.

Chipping aggregate is therefore assigned a Polished Stone Value (PSV) which is a measure of how well it can resist the action of polishing (loss of surface roughness). A high PSV applies to those with good resistance to the polishing action. 

Aggregate of high PSV value however is relatively scarce and expensive and for this reason it is common practice to use such aggregate in braking hot spots such as the approach to traffic lights, roundabouts or zebra crossings, whilst other sections on which braking is scarce are generally assigned aggregate of lower PSV value. The expense of high PSV aggregate also makes it desirable for the recycling process.

Testing Techniques

Surface texture is most commonly measured using the sand patch method. Here, a set quantity of sand is placed on the road surface and then spread into a circle so that the sand levels with the upper edges of the surface aggregate - the greater the texture, the smaller the patch.  The texture is then calculated from the diameter of the circle.

There are numerous methods used to test the skid resistance of a road surface including the portable pendulum tester, the grip-tester, the pavement friction tester and the Sideways Force Coefficient Routine Investigation Machine (SCRIM). 

Patching And Cyclic Maintenance

Patching and cyclic maintenance operations are planned treatments of relatively small defects in the road surface such as potholes or cracks. This technique can be used to prolong the life of a road avoiding the need for more expensive resurfacing treatments. Repairs to cracks and small holes in the surface are vital to maintain the integrity of the overall structure. These are important to ensure the surface forms a seal against water penetration that may damage the road foundation.

Typical defects that may be treated in this way are:

• Fretting, where the surface looses aggregate and texture

• Cracking of the asphalt surfacing

• Rutting or surface deformation

• Small potholes in the surface

The typical treatment would be to mark out around the defective area well back into sound material, remove the surfacing layer and patch with asphalt material up to about 50mm thick.  The replacement asphalt material would generally be laid and compacted by hand, although larger patches may allow for small paving machines to be used. Often temporary repairs are made to keep the road surface safe with a permanent repair following at a later date, usually as part of a programme of similar work. This makes the process more cost effective.

Other planned cyclic treatments could include:

• Crack sealing/overbanding

• Filling ruts and depressions

• Replacing small sections of High Friction Surfacing

Emergency Road Repairs

Sometimes highway authorities must act quickly to repair parts of the road that have been deemed a danger to users. Emergency repairs are usually carried out where defects in the road or pavement have become an issue or danger to the road user or the pedestrian. The safety issue makes quick treatment essential. The quickest and most cost effective way to remedy the problem is to isolate and treat only the defected area leaving the surrounding road in its present condition. Common emergency repairs cover the treatment of potholes and cracks, for instance, and the methods used to effectively treat these defects vary.

Surface Dressing: Prolonging Road Life

Surface dressing is a low cost method of increasing the life of a road and costs less than re-surfacing. However, the process does not add strength to the road and should only be carried out if the underlying road is structurally sound. Surface dressing is used to create a fresh hard wearing surface with increased texture and therefore skid resistance. It also seals the surface of the road preventing the penetration of water, whilst arresting the disintegration of the existing surface.

The process of surface dressing is a relatively fast operation and is achieved by initially spraying a bituminous adhesive coat (usually bitumen emulsion) onto the existing road, followed by the careful spreading of high quality chippings of aggregate onto the adhesive surface. 

The new surface is then compacted. Any loose, excess chippings are swept away from the surface after compaction and again several days after trafficking to minimise the risk of damage to passing traffic. After laying the new surface, it is important traffic speed is kept to a minimum to prevent the removal of adhered chippings and help prevent windscreen damage.

Chippings may be lightly coated in bitumen and/or heated before spreading to aid adhesion. An added advantage of surface dressing is that it can be repeated cost-effectively, for example every five years, thereby extending the life of the pavement even further. However, care must be taken as repeated layering can increase the risk of areas ‘fatting up' and may cause bleeding of excess binder in hot conditions. 

By The Side Of The Road: Footways

Just as great care and planning goes into the surfacing of Britain's roads, the approach to footways for pedestrians is equally considered. There are many types of footway across the country, including the standard grey/black asphalt ones common in residential streets, to the more attractive paved options often seen in city centres. During the design stage, consideration goes not only into visual appearance but also into cost-effectiveness, suitability, availability of material, ease of maintenance and environmental protection. The approach to footway design and construction is greatly dependent upon the type of user the pavement is likely to entice. Footways are categorised into one of three types: pedestrian only, light traffic (bicycles) and heavy traffic (motor vehicles). Clearly heavy traffic footways must be designed to withstand greater loading than the other categories. The ability of a footway to withstand greater loading is achieved by increasing the depth or density of the underlying base course and sub-base of the pavement or in some cases by use of alternate thick rigid surfacing, such as concrete, often seen in cycle paths. Another important aspect of footway design and construction is the use of edge restraints and kerbs. In addition to their use as footway edge constraints, kerbs aid the prevention of water penetration into the footway, providing a vehicle and pedestrian boundary, as well as creating a raised anti-vehicle barrier whilst forming a channel for surface water aiding drainage.

Going Green: Recycling Our Roads

Recycling is becoming more apparent every day including in the roads sector. This is good news for the environment and where cost effective can end up saving companies a great deal of money. Roads surfaces are recyclable due to the plastic properties (fluid when hot and solid when cold) of the bituminous materials used, and due to the fact that most of the aggregate contained within its matrix will be of the same quality as it was when it was originally laid.

The recycling process is extremely valuable when working with scarce aggregate of high quality, typically those in SMA. Although there are variations of the recycling process, dependent on the intended use of the finished product, most follow the same initial procedures.

When recycling a road surface the existing solid surface must first be broken up or planed before being crushed into smaller more manageable, melt-able pieces.  These small pieces are then graded by type and heated to a temperature dependent of the material being recycled in order to reduce the bituminous binder to its fluid state. At this stage a small amount of extra bituminous binder, cut-back oil or flux oil may be added to the mixture to achieve the desired ‘workable' consistency.

Recycling Techniques

Recycling technologies are advancing rapidly. Recent developments include enhanced technology for recycling aggregates and the use of glass and other recycled waste products in asphalt. The exact method of road surface recycling depends on the purpose for which the recycled material is intended.  Some methods include:

Repave - this process involves the heating and breaking up of the road surface to a depth of 20-25mm. A layer of new material (HRA or SMA) is then laid over the top of the broken road surface while it is still hot. Chippings are then added to the new surface and finally all are compacted.

Remix - similar to repave although in this case the existing road surface is broken up, removed and crushed before being graded and eventually heated/melted. The melted material is then thoroughly mixed with an appropriate amount of new material before being re-laid and compacted at the original site.

Hot recycling of road planings - this process is similar to the remix process except that the bituminous road surface material is removed by the method of planing. Once planed, crushed, graded and heated the material is mixed with added bituminous binder, cut-back oil or flux oil to generate the required workable consistency.

Hot recycling of road planings on site - ideal for small-scale operations, this process is the same as that for off-site however every stage is carried out on-site using portable machinery.