Pavement Surfacing Materials

The surface course of a pavement has to provide smooth, skid-resistant running plane with good surface regularity and adequate resistance to the onset of defects, including most commonly deformation and cracking.

The commonly used road surfacing materials fall into following six generic categories.

Hot Rolled Asphalt (HRA)

HRA is characterised by a mix of coarse and fine aggregate bound by a bituminous binder with mineral ‘filler’ added to provide additional stiffness. Pre-coated, single sized ‘chippings’ are rolled into the surface immediately after laying to provide the necessary skid resistance. This offers advantages, compared with alternative systems, in terms of overall reduced aggregate requirements but is relatively noisy compared with most other surfacings. Concerns about noise as well as construction delays and associated costs mean that HRA is no longer used on the trunk road and motorway network, but on many local authority roads, especially where noise is not an issue, HRA may provide the optimum solution.

Coated Macadam (‘Asphalt Concrete’) Mixtures

Coated Macadam (Asphalt Concrete) Mixtures are characterised by an interlocking ‘skeleton’ of course aggregate, usually bound by a bitumen or modified bitumen binder, with relatively little fine aggregate. They provide limited texture depth compared with other options, but High Stone Content Macadams are better in this respect and are often used as surface courses, primarily on low speed and/or lightly trafficked roads;

Stone Mastic Asphalt (SMA) Mixtures

SMA is gap graded asphalt that has a coarse aggregate skeleton. These also comprise an interlocking coarse aggregate skeleton but with a ‘mastic’ mortar comprising bitumen and filler with little or no fine aggregate. They provide a negatively textured surface which combines noise reduction with good texture depth and skidding resistance.

Thin Surfacing Systems

These modern, negatively textured surfaces generally use smaller sized coarse aggregate and enhanced performance binders which allow them to be laid at reduced thicknesses. They primarily include Thin Asphalt Concrete and Thin Stone Mastic Asphalt systems. Some concerns regarding their durability compared with HRA surfacings have been expressed by some quarters but this is likely to improve as technology advances and as experience in the use of these materials grows. They will always have advantages in terms of reducing noise and traffic delays.

Surface Dressing Systems

Surface Dressing Systems are intended primarily as a maintenance treatment to prolong the life of a structurally sound pavement that has fallen below the ‘investigatory level’ of skid resistance. Surface dressing unavoidably produces a relatively noisy running surface, compared with thin surfacing, although the latest technological developments offer significant improvements in this area. Taking this into account, the best surface dressing systems could actually provide a serious alternative to thin surfacings in terms of overall sustainability.

Surface dressing is very useful and cost effective process for restoring skid resistance to a road surface that is structurally sound. In a new road, adequate surface is designed into the running surface by specifying requirements for both aggregate properties and texture depth. However during its service life, the surface becomes polished under the action of traffic and the skid resistance will eventually fall below the minimum specified value. A major benefit of surface dressing is that the process restores skid resistance to a surface that has become smooth under traffic.

With care, surface dressing can be used on roads of all types from the country lane carrying only an occasional vehicle to Trunk Roads carrying tens of thousands of vehicles per day. However in some location, particularly those where vehicles undertake sudden or sharp turning manoeuvres, surface dressing may not be appropriate treatment. This is due to relatively poor ability of the process to resist tangential forces. Surface dressing as a maintenance process has three major aims:

• To provide both texture
• To stop the disintegration and loss of aggregate from the road surface.
• To seal the surface of the road against the ingress of water and thus protect its structure from damage resulting there from.

In addition, it can be used as a treatment to provide:

• A distinctive colour to the road surface
• A more uniform appearance to a patched road.

High Friction Surfacings

High Friction Surfacings using imported calcined bauxite are applied as overlays to other types of surfacing. In the design of road surfacing materials, attention has, understandably, been focused on skid resistance and noise reduction. Durability has clearly also been considered but mainly from the perspective of ensuring that materials are fit for purpose and provide acceptable value for money.

Some of the advantages of high friction surfacing (in addition to providing enhanced skid resistance) are:

• Unlike bitumen-based surfacing, resins tend to be colourless and can be coloured, and can thus be coloured much more easily to make a visual impact.
• The high friction surfacing are relatively expensive surface treatments but the skid resistance is durable and they have proved very cost effective in accident reduction.
• The products with reflective aggregates are proving effective at segregating traffic on unlit roads and improving the visibility in urban areas where street lighting is being removed or reduced to lower the consumption of electricity with the aim of reducing the carbon footprints.