Stone Mastic Asphalt Review document

02nd March 2004

This note is intended to give additional advice on the use of stone mastic asphalt (SMA) for the reinstatement of openings in the highway. S6.4.2 of The Specification for the Reinstatement of Openings in the Highway requires that SMA and other hot mix proprietary thin surfaces are reinstated with SMA. Guidance on the use of SMA is contained in Volume 7 Section 5 Pavement Design and Maintenance Design Manual for Roads and Bridges and interim advice note 49103.

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.

It is important that appropriate tack coats or bond coats are applied to the top of the binder course, particularly in maintenance situations when replacing the surface course only and where the binder course layer may be of variable quality. Detailed guidance on the application levels of tack/bond coats is given in BS 594: Part 2 and BS 4987: Part 2.

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 Structure
SMA has a stone skeleton which is filled with mortar of binder, filler and fine aggregate. A major benefit of the use of SMA is that the material has a good resistance to deformation. This resistance relies on the "stone to stone" aggregate skeleton and it is most important that good quality aggregate is used. However, fatting up of the binder when the material is compacted is a potential problem with SMA if the interstices have been over-filled with mortar. If sufficient care is taken in its design and laying, the fatting up can be reduced or even eliminated. It should be noted that fatting up is not a new phenomenon unique to SMA.

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
It is necessary for Authorities and Statutory Undertakers to look at the risks associated with the use of SMA and its application in various classes of road, if not on individual sites. Any risk assessment should take account of the traffic flow and speed, the site conditions, the size and position of the works and the mixture design in terms of their influence on the probability of increasing the risk of accidents over that normally designed for, the length of time before the risk reduces to the normal design value and the number of road users put at extra risk during that period. It is not the purpose of this advice note to undertake a risk assessment on the use of SMA but to provide a framework to facilitate the process.

Traffic Flow and Speed
A risk assessment needs to consider the traffic flow and speed. The higher the traffic flow, the sooner the binder film will be worn away but, in that time, vehicles may have been at increased risk. However, the type of traffic flow will influence the extent of wear with commercial traffic being more aggressive.

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.

Site Conditions
A risk assessment needs to consider the site conditions. The more stress applied by the conditions, whether due to the need to turn, stop or accelerate, the more each wheel will wear the binder film from the aggregate. However, such site conditions generally require higher skid resistance as a design parameter in order to allow the vehicles to turn, stop or accelerate safely. Furthermore, any change in skid resistance resulting from the transition between a newly-laid material and adjacent in-service materials can give rise to added accident risk. Transitions must be avoided at sensitive locations such as on bends and where vehicles are braking for junctions or crossings.

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
A risk assessment needs to consider the size of the reinstatement being resurfaced. For small excavations, the risk of any skidding accidents is low because they are likely to be under only one of the wheels at any one time so that any loss of skid resistance can be averaged over the four (or more) wheels. The reduction in risk will be further reduced if the reinstatements are away from the wheel tracks. However, for these reinstatements considerable time will elapse before the binder film is worn from the aggregate surfaces.

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.

Mixture Design
A risk assessment needs to consider the mixture design. The simplest aspect of mixture design is whether it has been carried out rigorously or a recipe has been arbitrarily selected. It is known that small variations in aggregate grading, blend of fine aggregate, binder content and changes of aggregate source may have significant effects on the performance of the material, particularly with respect to texture depth and permeability. Poor design will increase the risk of excess mortar leading to fatting up and a loss of texture depth. The choice of polished stone value (PSV) for the coarse aggregate will provide the skid resistance once the binder film has been worn off. Finally, the choice of binder has an effect; polymer-modified bitumen tends to have greater tenacity that will slow down the wearing process.

Response to Low Early Life Skid Resistance
There are several ways of reducing the risk:

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.

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