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Highways special: Greener courses

The first impact of Transport Scotland’s surfacing trials along the M8 is being felt on a small but perfectly sustainable road project in the Scottish Highlands.

The latest stage in Transport Scotland’s search for the ultimate in long lasting and sustainable carriageways can be witnessed in the reconstruction of a 1.8km section of the A9 close to Inverness.

There, the road’s original lean mix concrete base is being cracked and seated to provide a “flexible” course. This is being topped with an EME2 asphalt concrete base/binder of high durability. Above that is a stone mastic asphalt thin surfacing of unusual density for a British road.

Green credentials and sustainability

The project’s green credentials are substantial. Crack and seat means that no concrete is being removed from site: it is merely being re-used, in situ, in a different way. Planings from the old asphalt carriageway are going into forestry roads nearby. But it is the materials forming the new carriageway that should provide the best overall benefits in terms of sustainability.

They are of high quality and are intended to provide long life with minimum interventions. Design of the thin surfacing is the first to have been influenced by Transport Scotland’s asphalt trials which are currently underway on the M8 near Edinburgh (see box below). The A9 project at Carrbridge involves reconstructing the two lane, single carriageway and creating a third lane to ease overtaking.

The A9 as a whole passes through countryside of outstanding beauty and any increase in capacity has to be handled with great sensitivity by client Transport Scotland and its area network manager TransServe.

“Transport Scotland is promoting employment of appropriate solutions, durability, best value and environmental sustainability,” says Transport Scotland’s materials and quality advisor Dougie Millar. His comment reflects the fact that Scotland is at the forefront of the development and use of improved road building materials.

Trials & tribulations

M8 surface laying

Laying the thin surface of the M8’s trial SMA panels

Eight trial panels of stone mastic asphalt (SMA) have been incorporated into the surface of the M8 motorway in Scotland, to test their effectiveness and to see if they perform better thanconventional UK SMAs.

The asphalt in the panels is of German design: the panels are all different to each other in terms of mix design, four are gritted and four are not. If they do perform better than UK asphalts, Transport Scotland − the trial’s sponsor − proposes to draft a new performance specification based on the trial results. It wants surfacings that last longer, provide better value and are thereby more sustainable.

In general, German specifications require SMA mixes to meet a minimum SCRIM-type requirement which tends to result in a surface texture depth of around 0.9mm. The aggregates used are of 11mm or 8mm nominal size. The result is a dense mix of comparative strength and durability, obtained using stone (by UK standards) of medium PSV. Also the Germans grit their new surfacing to enhance early life skid resistance.

Scottish mixes are produced and laid to achieve a required texture depth of 1.3mm; with aggregate of nominal 14mm size. This results in a comparatively open mix and where required by the specification, high PSV is needed to ensure surface skid resistance.

“The indications are that the Germans produce SMAs that achieve better durability, longer lives and better skidding resistance in early life than we manage here in Scotland,” says Transport Scotland’s materials and quality assurance advisor Dougie Millar.

“Which begs the question, should our specification be changed?”


The M8 trials are a case in point: some thin surfacing stone mastic asphalts (SMAs) have not performed well and the eight trial panels contain variants of German rather than British technology.

Early results from the M8 indicate that a nominal size 10mm aggregate in an SMA mix produces a more robust surface course than the traditional 14mm.

“So this is what has been specified for Carrbridge,” says TransServe pavement engineer Stuart Guthrie. “We’re expecting a much more durable, long lasting surfacing.” The Carrbridge mix is a transitional development in that it still adheres to the standard Specification for Highway Works requirement for stone of high polished stone value (PSV), he says.

“We’re expecting a much more durable, long lasting surfacing.”

Stuart Guthrie, TransServe

The higher the PSV, the greater the aggregate’s resistance to polishing. Aggregate with a PSV of more than 60 is considered to have high skid resistance properties.

The Germans in general use stone of a lower PSV (which is more easily obtained) and ensure early life skid resistance with grit. “Using lower PSV stone at Carrbridge would have been a more sustainable way of proceeding.

Here in Scotland we have to import such stone from Wales and Cumbria. We’re still awaiting the relevant trial test data from the M8 and at this stage feel unable to go down the path of gritting. Even so, the Carrbridge mix is a firm step in the right direction.”

Contractor for the Carrbridge improvement is RJ McLeod with asphalt supplied by Bardon Scotgreener land. Normally, Bardon offers Aggregate Industries’ proprietary thin surfacing Hitex which performs well in Scottish conditions.

“In mixture terms, the Carrbridge thin surfacing is different to Hitex. OK, it’s a stone mastic asphalt, but it has a substantially finer graded aggregate,” says Bardon Scotland’s technical manager Scott Buchanan. “There is less surface texture to the mix and the resulting asphalt is more dense than conventional thin surfacings.”

Open to technological advance

Transport Scotland’s openness to technological advance is reflected in the fact that the choice of French developed “Enrobe a Module Eleve” (EME2) surfacing for Carrbridge’s base/binder comes as no surprise.

While take up of the French developed surfacing was slow in the rest of the UK, Scotland was at the forefront in expressing interest in the material a couple of years ago, advocating its use under certain circumstances.

Described simply, EME2 technology relies on smaller nominal size aggregates and high volumes of very stiff bitumen to produce a homogeneous mix that is less likely to segregate, is easily compacted and of low void content.

“We have confidence in the technology resulting in very long life pavements, provided it is laid on a good stable base .”

Jukka Laitinen, Nynas

EME2 has been used with some success in Scotland. An important requirement of EME2 asphalt for a particular job is that its mix design has to relate to the aggregate being used. The design process is an onerous one involving exacting performance parameters but the upside is that all the effort results in a dense and durable material.

As with Carrbridge’s SMA thin surfacing, supply of the binder for the EME2 is by Nynas UK which also provided technical assistance with mix design. “The Carrbridge base/binder contains our NypaveFX20 which is purpose-engineered for EME2 applications,” says Nynas asphalt engineering support manager Jukka Laitinen.

“We have good capacity to assist in the mix design of EME2 and confidence in the technology resulting in very long life pavements, provided it is laid on a good stable base.”

Facts and figures

Carrbridge’s facts and figures are these. Of the 1.8km of two lane single carriageway being rebuilt, 1.01km is being accompanied by a new third lane. The contract is programmed to take 12 weeks and is valued at £2.7M. The road’s new thin surfacing is 30mm thick and its EME2 base/binder is 140mm thick.

The asphalt is laid on 200mm of original lean mix concrete which has been cracked and seated to make it a structural part of the pavement.

Site location

Carrbridge map

Carrbridge is within the Cairngorms National Park so environmental issues are taken especially seriously.

Verges to the road being rebuilt and widened contained 40 wood ant nests − and each of these had to be moved to a safe location, without disturbing the colony within.

“We shifted the nests to either end of the site, to south facing locations,” says TransServe Engineer’s Representative Keith Pratt. “There are three types of ants, all protected. Each nest was carefully lifted and transported to a pre-arranged site.” No casualties were reported…


The original A9 in the Carrbridge area was built in the mid 1970s. More recently it has become clear that traffic flows would benefit from an overtaking lane and also that, while serviceable, the road could do with some fundamental maintenance.

One issue was the unexpected strength of the lean mix for the concrete. The concrete was acting as a series of slabs, fracturing at stress points with reflected cracks appearing in the asphalt above. A decision to opt for crack and seat was taken early on.

“Compared to conventional methods, the crack and seat technique means we effectively buy one kilometre, get the next for free − and provide a neat structural solution.”

Dougie Millar, Transport Scotland

Effectively leaving the concrete in situ rather than breaking it up and disposing of it offsite saved around 300t of CO2 emissions, it is claimed. The process can also be highly cost effective. ”Compared to conventional methods, the technique means we effectively buy one kilometre, get the next for free − and provide a neat structural solution,” says Millar.

Crack and seat involves the careful use of a wheeled guillotine breaker which travels across the existing concrete road slab, dropping a 5.5t weight at (say) 300mm intervals to induce micro cracking.

The micro cracks subsequently act to take up thermal and traffic movement, allowing the concrete to become structurally part of a flexible composite pavement. The concrete is left, to become a useful contributor to the structure, instead of being wasted.

Crack and seat has been carried out at Carrbridge by Antigo Breakers. “The art is not to shatter the concrete but to create cracks while maintaining the interlocking of the concrete matrix,” says Antigo operator/supervisor Brian Marley.

Highways special: Greener courses

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