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Highways Agency responds to concerns over SMA - 21 March 2005
In response to media interest on the safety of road users and the adequacy of skid resistance on newly laid thin surfacing materials including stone mastic asphalt (SMA), the Highways Agency has provided the following article for readers of Motorcycle News to convey a fuller understanding of the subject area.
Stone mastic asphalt, also known as SMA, originated from Germany. In the early 1990s, the Highways Agency evaluated this material for its potential use on the trunk road network in England.
However, following carefully monitored trials in the UK, the Highways Agency decided not to pursue its use as the trials revealed the potential loss of surface texture (required for adequate skidding resistance) due to the rich binder content and minor variation in the aggregate grading.
Following these trials, the Highways Agency asked the UK Asphalt Industry to develop bespoke materials, which would ensure the required in-service skid resistance over the long term.
This led to the development of the many proprietary thin surfacings, based both on SMA and French thin asphaltic concrete mixtures. It is these thin surfacings, which are now used on the trunk road roads and indeed on majority of the local road network in the UK.
These proprietary materials provide a number of benefits, principally:
- Good long-term skid resistance performance as their surface texture and resistance to polishing over the longer period are very is satisfactory
- It is quicker to lay, often at night - hence less disruption for the road users and all the benefits to road users that follow from this
- It is durable (in terms of the general wear and tear) and is resistant to rutting and lasts as long as the traditional surfacings such as the hot rolled asphalt
- Many tend to emit less spray in wet - unless very wet.
- Emits low noise, benefiting people living along busy roads and helping to reduce driver stress.
There has been general confusion about these materials and the use of SMA recipe-based materials on road in the UK.
The Highways Agency adds that it is worth noting that the skid resistance of most asphalt surfaces will exhibit lower skid resistance in the first few months when compared with the long-term in-service skid resistance. This includes the traditional surfaces such as hot rolled asphalt.
Lower skid resistance on newly laid surfaces does not mean that the trunk roads are unsafe.
The Highways Agency's initial assessment of early life skid resistance properties of the thin surfaces, using the Pavement Friction Tester, has already indicated that the dry skid resistance is significantly higher than that of wet surfaces - which the Highways Agency routinely monitors.
As part of its management of the network, the Highways Agency decided to carry out extensive research in this subject area. This research has two specific tasks, namely:
a) To determine if there are significant issues of early life skid resistance on newly laid surfaces on the trunk road network and, if necessary, provide new advice, in addition to that already provided at present, to its managing agents.
b) To examine any effect of early life skid resistance on road safety - aiming to quantify the risks on newly laid surfaces and if warranted, develop long term strategies such as improved use of warning signs etc.
Part (a) report will be published in Spring 2005 and part (b) in late 2006.
Safety of the road users is a top priority for the Highways Agency. Research work plus maintaining regular liaison with the Asphalt Industry helps the Agency to provide a lead on safety issues.
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