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Motorway repairs pull into fast lane


In deepest Kent, maintenance engineers working for road network manager the Highways Agency have been racking up the mileage carrying out repairs to parts of the aging M20. Paul Thompson travels into the Garden of England to find out how a new speedy repair MATERIALS is helping them do it

AS THE main arterial route that helps Britons slake their love affair with all things French, the M20 motorway is a perfect example of a modern highway.

Speedy and ut ilitar ian, the concrete motorway is designed to help whiz traffic towards the UK's umbilical cord with mainland Eu rope, the Channel Tunnel.

Huge vehicle numbers pound its length day in, day out, from its first metre on the outskirts of Folkestone to its last, where it joins London's orbital motorway, the M25 near Swanley.

Upwards of 75,000 vehicles hammer down the road each day and, with almost a fifth of these being fully laden HGVs, it is hardly surprising that the reinforced concrete slabs that the road is built from are starting to feel the pace.

But any repair and maintenance strategy has to be tempered by the impact ser ious t raff ic management on one of the UK's most important roads could have.

When parts of the motorway between junctions 8 and 9 began cracking around the joints, Paul Thompson, highways design manager at the Highways Agency's managing agent contractor InterRoute, a joint venture between highways contractor RCS and consultant Mot t MacDonald , knew speed would be of the essence.

'This is one of the few remaining sections of concrete motorways. It opened to traffic in 1991 and we have to carry out legacy maintenance on it. With the volumes of traffic we can expect to see on the motorway, we are always interested in something that can speed up any repairs we have to carry out, ' says Mr Thompson.

Despite popular belief, the Highways Agency is acutely aware that acres of coned-off motorway are maddening to drivers already stressed by slowed traffic and queues.

There was no chance that Mr Thompson and the team were going to traffic-manage M20 drivers to despair along 22 km of three-laned motorway, if it could be at all avoided.

But how could conventional methods of a joint sealant and concrete patch repair be used overnight?

The simple answer, according to Mr Thompson, is that they can't.

There are two main issues. The first being that the joint sealant can only be used when moisture content is below 3 per cent before the coat can be bonded properly. This loses valuable time while the moisture content is measured and lowered. The second problem is getting the concrete to cure at all.

'It is possible to carry out overnight repairs using conventional MATERIALSs but then they have to sit there - untrafficked - curing for a week, ' says Mr Thompson.

'We knew that the work could cause massive disruption, so we wanted a repair MATERIALS that would allow us to get our traffic management on and off each night.'

The maintenance team were also worried that there was a chance that if they used a cementitious MATERIALS it could shrink back after a few months, debond and the repair would need to be f ixed again.

Enter Norwich-based specialist MATERIALSs company Roadtechs. Its Techcrete TBR synthetic polymer compound contains cement and aggregates and produces an impermeable mass with no voids - ideal for just such a repair project, says Mr Thompson.

'There is no doubt about it, this system is more expensive than standard repair methods, but the advantage of being able to work at such speed and use the Techcrete at such a scale meant that the up-front costs were far outweighed.'

He estimates that Techcrete is seven or eight times more expensive than standard repair methods, a figure not challenged by Roadtechs' David Adams.

'It is more expensive but then it is probably about eight times quicker than using standard repair methods. Add together the construction cycle savings and the repair longevity and I think it works out well, ' he says.

A preconstruction survey revealed that 290 sq m of carriageway, dotted around the three lanes in both directions needed repairing, a nightmare patchwork of small repairs along the 22 km stretch of motorway.

'The first survey revealed that there were just over 290 sq m needing treating but these visual surveys can be a little rough and ready. It is difficult to inspect the fast lane when HGVs are bear ing down on you , ' says Mr Thompson.

In the end, the Roadtechs gang managed to cut the repairs down to 255 sq m, carried out over an 18-n ight per iod du r ing September. That does not sound much, but when you consider at each repair the road surface has to be planned down to fill level, treated, then on to the next pothole where the whole process is car r ied out once again, it is no mean feat.

'The first cones were put down at 10 pm and the planer would start at about 10.45 pm. The last cones were off no later than 4.45 am, so it's a pretty tight working window we are talking about, ' says Mr Adams.

In fact, the actual output of the two repair crews working on the project totalled just under 7 sq m per night with some shifts where the crews could concentrate on the larger repair patches yielding 20 sq m.

All things considered, it was a more robust repair and quicker return rate than standard MATERIALSs would have offered , according to Mr Thompson. He has seen the mater ial used before on a much smaller project.

'If we had used convent ional mater ials, we would have been looking at two to three months of work.

Here, we have a MATERIALS with a high compressive resistance that has already been trialled on an early stage of the M20. We don't know how long it will last because we are still waiting for it to fail and those trials are eight years old.'

No more repair work for at least eight years. That has to be music to the ears of M20 motorists.

Wonders of Techcrete

MOST of the repairs needed on the M20 were at concrete slab joints, where the MATERIALS is prone to cracking, thanks to the constant impact of vehicle tyres.

The crack star ts to f ret back away f rom the joint and usually the affected MATERIALS has to be cut out at least 50 mm away from any visible problem.

A planer is then used to gouge out the failed concrete to 40 mm on the M20, although single-layer repairs can be carried out to as deep as 50 mm. Any th ing above that and the Techcrete has to be laid in separate layers.

Once the section has been cut out, a thermal lance is used to warm up the concrete and open up its pores. This helps to draw the MATERIALS into the concrete and improve its bond. Prebagged MATERIALS is thrown into two preheaters on the Roadtechs' laying machines, where it is melted down at 220 deg C.

'Once it reaches this temperature, all the polymers in the resin have broken down, ' explains Mr Adams, 'and the mixing paddles keep the MATERIALS f lowing.

'All our staff are trained on its use because it is a difficult MATERIALS to lay. You need to get the air out of it as it is laid and then apply the skid-resistant bauxite chips.'