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SKID IN THE DRY -The Tale of Stone Mastic Asphalt.
There is an apparent increase in the use of Stone Mastic Asphalt [SMA] in the repair of road surfaces because of its properties, in that it can be laid thinner than traditional 'Hot Rolled' asphalt, has better lateral drainage properties; reducing the risk of aqua-planing, is more resistant to rutting and produces less noise from tyres going over it.
Advantages
- SMA provides a textured, durable, and rut resistant wearing course.
- The surface texture characteristics of SMA are similar to OGA so that the noise generated by traffic is lower than that on DGA but equal to or slightly higher than OGA.
- SMA can be produced and compacted with the same plant and equipment available for normal hot mix, using the above procedure modifications.
- SMA may be used at intersections and other high traffic stress situations where OGA is unsuitable.
- SMA surfacings may provide reduced reflection cracking from underlying cracked pavements due to the flexible mastic.
- The durability of SMA should be equal, or greater than, DGA and significantly greater than OGA.
Great by the sounds of it.
So, why is it banned in Eire for roads other than 30mph maximum speed restriction?
Why are the British Horse Society [BHS] up in arms about it?
So what exactly is stone mastic asphalt and why is it being used?
According to the powers that be here in Britain it lasts longer, though possibly not so says NRA of Eire.
Why is Derbyshire County Council issuing warnings over its use?
Why is the National Roads Authority [Eire] 'Not yet convinced of its benefits'?
Disadvantages
- Increased material cost associated with higher binder and filler contents, and fibre additive.
- Increased mixing time and time taken to add extra filler, may result in reduced productivity.
- Possible delays in opening to traffic as the SMA mix should be cooled to 40°C to prevent flushing of the binder to the surface.
- Initial skid resistance may be low until the thick binder film is worn off the top of the surface by traffic. In critical situations, a small, clean grit, may need to be applied before opening to traffic.
Gary Fitch, of Kent County Council and member of the County Surveyors Society (CSS) told NFU Countryside: "SMA is a relatively new road surfacing material that has become available to UK highway engineers during the last ten years. It is one of a number of materials available to engineers for repairing road surfaces and its particular benefits include high durability, low noise and reduced spray characteristics. It is most likely to be used on more heavily trafficked roads.
"The durability characteristics are due to the material having a high bitumen binder content which when newly laid can result in lower skidding resistance than would be expected with other materials. However, tests have shown that early life skidding resistance is within acceptable limits.
"In the light of the publicised problems with horses slipping on SMA, the County Surveyors Society is considering the extent of the problems and has set up a working group to try to resolve them. Depending on the outcomes from the working group, CSS will advise local highway authorities on measures to be taken (if any) when applying SMA on roads that experience high equestrian usage" Tests on a newly-laid type of road surface have revealed motorists could skid on it - not in the wet, but in dry conditions. But File On 4; a BBC programme that investigated the problem and that of overbanding found that skid resistance tests were conducted in the wet and on surfaces that were worn down, but that tests in dry conditions were overlooked.
Derbyshire County Council is now alerting local authorities across the country and putting up signs where the surface is laid.
Concerns were first raised about SMA, after two car accidents in different areas of the county.
When Derbyshire police carried out tests in dry conditions they found that newly-laid SMA had "lower than expected" skid resistance.
Weathered-in
John Fern, from the county council, said: "These skid tests are carried out in the worst possible scenarios at high speeds. If motorists are driving carefully there won't be a problem, but we believe warning signs will make drivers aware of the risks."
Experts from the Berkshire-based Transport Research Laboratory to test the road surface under skidding conditions. They discovered that once the surface is "weathered in" the skid resistance does improve.
Derbyshire County Council's member for environment and transport, Councillor Walter Burrows said: "We believe we're the first authority in the country to investigate potential problems with SMA when it's newly laid." The police have already said that skidding played no part in the two accidents but it was only when testing took place in dry conditions that the problem was identified.
After suspending use of SMA, the material is being used again, but with warning signs placed in the area until sufficient cars have passed over it to wear it in.
SMA has only been used in Ireland in the last two years. However, close monitoring has indicated the possibility of a faster deterioration of surface texture than traditional materials.
Local Authorities in Eire have been instructed by the NRA to confine usage of the new process to 30 m.p.h. zones as a positive safety measure.
The NRA's rigorous performance requirements ensure that No Hazard is presented for drivers at any location where SMA has been introduced.
Wear and tear of road surfaces with declining skid resistance is a normal feature of the life cycle of all roads. The NRA's experience with modified Stone Mastic Asphalt indicates the possibility of a faster loss of surface textures than is the case with the traditional road surface material, Hot Rolled Asphalt, which could have implications for the future skid resistance performance of the roads concerned.
The possible shorter operational life of modified Stone Mastic Asphalt, if not rectified in further development and testing of the material, would have road maintenance cost implications. The NRA has thus decided that further use of the material should be restricted until product development establishes that surface texture performance over an acceptable life cycle can be guaranteed.
'Gentle hiss'
Sergeant Jim Allen said he experienced grave difficulties when conducting routine skid tests in optimum conditions on a Derbyshire road newly laid with SMA. "It was a sunny day in August. I jumped on the brakes and the car just kept going and going. Instead of
the scream of tyre on road and a cloud of smoke there was just a gentle hiss as I passed over the road, and I skidded far further than I ever expected to."
Concerns about grip have also been raised in Holland and Germany, where SMA was pioneered in the 1960s, in.
Notably The Department for Transport would not be interviewed for the BBC's File On 4 programme on the subject. But, in a statement it said: "A number of factors would have to come together to make a road unsafe, including volumes of traffic, speeds and the nature of the road itself...Just because roads have reached a level requiring further investigation, it doesn't necessarily mean they are unsafe."
The government would not accept that road repairs are in crisis, saying it was committed to spending more than £31bn over a 10-year
period.
Where have we heard that before?
The following site has an idiot guide to SMA and a notable comment: http://www.highwaysmaintenance.com/SMAlaypics.htm feel it is necessary for me to suggest to you that when the words "SMA" are used to describe a material, especially if "SMA" and "Clause 942" are mentioned in the same breath, to be cautious.
The lack of knowledge as to what actual attributes a material needs to possess to be correctly described as a stone mastic asphalt is causing confusion in marketing some proprietary bituminous mixtures.
This incorrect use of terminology may be due to a lack of knowledge, or sad to say it could be being used by the "marketing boys" to gain an advantage over competitors when advertising the products for which they have been asked to create marketing material.
I know I am not alone amongst engineers and technicians who know a little bit about bituminous materials, that the content and information provided in some modern bituminous material advertising leaves us less than impressed.
And notably he does not argue against SMA when laid properly.
All food for thought.
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