The hand application of SMA requires special attention in order to achieve the quality required but can be successfully used provided sufficient care is taken in its design, production, transportation and application.
SMA has a relatively high binder content that makes it "sticky" and difficult to lay by hand because it adheres to the rakes and other implements. Therefore, the material needs to be deposited in place as nearly to the relevant thickness as possible.
Provided the SMA is maintained at a suitably high temperature, adequate compaction of SMA is relatively easy. The degree of compaction possible is a function of the material design as much as applied compactive effort.
SMA also has a relatively thick bitumen film covering the aggregate particles, including those particles on the surface. The binder film on the aggregate particles at the surface can result in relatively low skid resistance for SMA in its early life because the binder hides the micro-texture of the aggregate. This issue has led to some Authorities and Statutory Undertakers questioning the use of the material. In the case of SMA that is trafficked, the surplus binder can be worn away by the abrasive tyre action of passing traffic within months. However, the time taken will depend on the site conditions and the properties of the binder. Nevertheless, the low early life skid resistance of SMA due to the presence of the binder film on the aggregate is an issue that needs to be addressed in a systematic way.
Need for Risk Assessments
Traffic Flow and Speed
The traffic speed may have some effect on the rate that the binder film wears, but the influence is not believed to be very significant. However, the speed does affect whether the micro-texture or macro-texture is more influential on safety. At low speeds, micro-texture (provided by small protuberances on the surface of the aggregate particles) is more influential whereas, at high speeds, macro-texture (provided by the interstices between aggregate particles and measured as the texture depth) is more influential. Both properties have an effect and, in order to stop, vehicles at high speeds need to pass through slower speeds. Nevertheless, maintenance of texture depth, including the absence of binder flushing, is more important for high speed roads.
As an example, straight non-event roads with high traffic levels require relatively limited skid resistance and many wheel-passes will wear the binder film. In such locations, the impact of the aggregate micro-texture in the SMA (hidden in the surface) may not be as significant as elsewhere and should not remain as long. However, these roads are usually trunk roads whereas unclassified low speed roads form the bulk of the highway network in urban locations.
Reinstatement Size and Position
Conversely, larger-scale works and narrow trenches have an increased risk because they can make total contact with the tyres of a vehicle. Narrow trenches that are in line with the direction of travel produce a particular risk for motorcycles until the binder film is effectively removed.
Response to Low Early Life Skid Resistance
Application of grit
Slippery surface signs
Use of alternative materials
The application of grit to the surface on completion of the compaction process is similar to the dusting applied to surface dressing fatting up problems with the exception that it is applied at the time of construction. The grit helps to "grind off" the binder film and provides a clean aggregate surface to tyres. However, the grit can accumulate in the road gutters and/or fill the surface voids, and hence reduce the texture depth. The use of grit on 10mm material is recommended at all times. Any grit applied should be specified to comply with BS 4987: Part 1 Cl 7.9 and have a minimum PSV of 55. The rate of spread of grit should be approximately 0.5 – 0.7 kg/m², but in practice the lowest rate to give an even cover should be applied.
Slippery road signs (ref Diag 557 with a supplementary plate to Diag 570) may be used as a temporary measure to warn road users of the surface conditions, although excess use of these signs tends to invalidate their use in the eyes of the travelling public.
Authorities may consider the use of alternative surfacing materials should there be concerns about the use of SMA but this should only be considered as a last resort. The nature of alternative materials is different to SMA. The size and nature of the reinstatement should be factors in the choice of alternative materials. The range of materials that may be considered are 0/10 mm close graded surface course macadam, 55per cent 0/10 mm hot rolled asphalt and 30 per cent 0/14 mm hot rolled asphalt with pre-coated chippings. However, the surface shape of the last of these alternatives is very different to that of SMA, which can have implications for the surface drainage.