Many UK roads use a composite form of construction. A high performance bituminous wearing course - often polymer modified and designed for low tyre noise - is laid on to an 'upper roadbase', also bituminous, which in turn is supported by a 'lower roadbase' of cement bound material. (CBM).
Contractors generally like CBMs. As environmental regulations begin to really bite, the fact that CBMs can use lower grade locally-won or recycled aggregates is as important as their lower cost.
But the inevitable drying shrinkage cracks that occur as the CBMs harden have always prevented their use for the entire roadbase depth.
'Without a substantial thickness of asphalt on top of the CBM, the transverse shrinkage cracks reflect right through the wearing course, ' explains Scott Wilson Pavement Engineering pavement engineer Ian Thompson. 'Such cracks aren't dangerous in themselves, but they allow water to penetrate right through the pavement structure, reducing its life.'
The answer, according to Thompson, is to add steel fibres. These will transform CBM from a naturally brittle material, with no tensile strain capacity after cracking occurs, into a much more ductile material. Cracking will be minimised and controlled, making it possible for CBM to be used as a full depth high performance roadbase topped only by a thin high performance wearing course.
Before joining SWPE in January, Thompson put this theory to the test during a three-year £180,000 research programme at the University of Nottingham. Funding came from the Engineering and Physical Sciences Research Council and the Highways Agency. The highlight of the programme was a full-scale trial carried out at Breedon Aggregate's Cloud Hill quarry near East Midlands Airport.
A section 120m long by 8m wide was laid using an asphalt paving machine and roller compaction. One third was standard CBM 180mm thick with a cement content of 7%.
The remaining 80m varied from 180mm to 130mm thick, and the otherwise identical CBM mix contained up to 40kg/m 3of steel fibres - in this case Bekaert 45/50FL 500mm long hook ended fibres supplied by research collaborator N V Bekaert SA from Belgium.
'You can't just use rusty nails, ' Thompson stresses.
'What you need are engineered fibres with a high enough aspect ratio - length to diameter - but not so high as to promote balling.
'For the trials the fibres were dispensed on to the dry aggregates as they went up the belt into the continuous batcher, which gave a good, even distribution throughout the mix.'
Cores taken from the base showed that the fibre reinforced CBM (FRCBM) achieved a flexural strength of around 4MPa at seven days. To accelerate the deterioration of the trial sections no bituminous wearing course was added, and the quarry's massive 90t dumptrucks were encouraged to use it as much as possible.
Apart from continued core testing, performance was also monitored by a falling weight deflectometer (FWD) and visual inspection. After 20 months of hard usage the difference in appearance between the two sections was obvious, Thompson claims.
'There were obvious really wide cracks visible in the unreinforced CBM section. The FRCBM had cracked as well, but the cracks were held together by the fibres and were much finer, ' he says.
FWD tests also showed the FRCBM to be still stiff as well. A full-scale road to take relatively light traffic would probably be made up of up to 200mm of FRCBM topped by 30-50mm of high performance wearing course, Thompson says.
Transverse cracks would be induced at about 3m centres.
Precise cost savings have yet to be calculated. 'CBM is about half the price of the asphalt it would replace, but the fibres aren't cheap, ' Thompson points out. 'Overall, the cost savings will depend on how much asphalt is saved, which rather depends on the level of traffic.
'Heavily trafficked roads would need a thicker wearing course. Total thickness, however, would still be less than fully flexible The Highways Agency is looking for trial sites throughout the UK road network. 'We need to test it out on both lightly trafficked county roads and on trunk roads, ' confirmed Agency technical advisor Robert Dudgeon. 'We estimate a composite construction for trunk roads would be 70-80mm of wearing course on top of the 200mm of FRCBM - which would still be at least 50mm thinner than the fully flexible alternative.'