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Inner strength Conventional flexible carriageways are finding it hard to cope with modern traffic loads.

Dave Parker reports on the recent trials of a concrete alternative.

London-bound drivers inching through a contraflow on the M23 near Gatwick Airport recently found it hard to keep their eyes on the road. More than a kilometre of the heavily rutted asphalt on the inside lane was being replaced - apparently by bright green concrete! Main contractor Fitzpatrick's paving division manager Tim Gibbs explains.

'This is the first trial of our Fitzper whisper concrete as an inlay. The green colour won't last, it comes from the special compound we spray onto the freshly-laid concrete to retard the surface and prevent rain damage. Once the surface has been properly textured it'll be the normal whisper concrete colour - dark grey.'

Those who would actually prefer a more colourful surfacing can take comfort from the fact that the noise levels generated by the textured surface will be considerably lower than before. Recent tests by the Transport Research Laboratory on the first trial sections laid on the A13 last year have shown that Fitzper, Fitzpatrick's specially-developed one pass version of whisper concrete, is significantly quieter than the hot rolled asphalt it is replacing (see box).

It should also be much more resistant to the rutting which is plaguing the inside lanes on much of the motorway network. And, as Fitzpatrick demonstrated between the thunderstorms of late September, a Fitzper inlay is a very practical resurfacing alternative for busy motorways.

'This is the first time we've laid Fitzper this late in the year,' Gibbs admits. 'The concrete is maturing much slower than in the summer - and the succession of heavy downpours haven't helped.'

Resurfacing the 1.3km of inside lane close to Junction 10 was scheduled to take just 14 days. Apart from the usual traffic management preparations, the first operation was to plane off 200mm of the existing, 15 year old HRA over the length of the trial.

'This had ruts in it 25mm deep or more,' reports Fitzpatrick engineer Rab McLeod. 'We also took off 50mm over a 10m long transition zone each end, to smooth the passage of the paving train.'

European-style whisper concrete is made up of two layers of two distinctively different concrete mixes. Fitzper's main advantage, the developer claims, is that it is one mix laid in one pass by a conventional slipform paver.

True, the mix used, a blend of Ordinary Portland Cement, fine sand and a virtually single size 10mm to 6mm crushed stone with an air entraining agent and a water reducing admixture, has to be site batched using high PSV aggregates specially imported from South Wales. Setting up the sort of advanced batching plant Fitzpatrick deployed for this trial does not come cheap. But, as Gibbs points out: 'This gives us total control and an assured supply, vital on a lane rental contract.'

On the M23 the concrete was placed in two 12 hour shifts, despite the downpours. A grid of high yield bars - 16mm diameter at 150mm centres longitudinally, 12 mm at 600 centres transversely - provides continuous reinforcement. In summer temperatures the two stage texturing process normally follows within seven to eight hours, once the concrete has gained enough strength to support the wire-brush-equipped tractor. 'Here the maturity meters TRL has placed in the slab every 100m are indicating an insitu strength of about 15N/mm2 at 25 hours,' Gibbs reports.

'Successful texturing depends on concrete maturity. Too early and too much laitence is removed, too late and we have to resort to grit-blasting.'

Fitzpatrick technicians monitor surface maturity by hand brushing test areas. Then the surface is hosed down before the tractor moves forward and its wire brush begins work.

Texture is checked every 50m or so by the sand patch test, although the TRL is also carrying out checks with its laser profile meter which averages out texture over 10m runs. Rain at this stage is as much or even more of a problem as during the concrete placing, Gibbs says.

'The Whispertard spray is taking up to two hours to skin over at this time of year as opposed to 30 minutes or so during the summer, so the concrete is vulnerable to rain damage until then. But if we get rain during the texturing process all the dust turns into a slurry - and the sand patch test is very hard to carry out.'

No transverse joints are required, but, sandwiched as it is between hard shoulder and outer lanes, the Fitzper inlay needs longitudinal joints each side. These are sawn once the texturing is finished and filled with a bituminous compound.

For this particular trial the Highways Agency insisted on a maximum aggregate size of 10mm rather than the 14mm used in the Fitzper A13 trials. Gibbs says that experience on other forms of whisper concrete suggests this change should reduce noise levels even further.

'However, this makes the aggregate more expensive, as we need a higher PSV than with 14mm, and we've seen a greater tendency for it to pluck during the texturing process. But there's been no need to increase cement content beyond the standard 375kg/m3.'

Long term durability, skid resistance and noise levels will be constantly monitored by the Agency. With 40t HGVs fitted with supersingle tyres on the roads the rutting problem on inside lanes is a constant headache. Fitzpatrick confidently expect Fitzper to have an effective life of between 20 and 40 years - and if it lives up to expectations it will be good news for motorists as well as the concrete industry.

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