Over a length of just under 5.5km, through Chiswick, the roads are stacked one above the other – the two-lane M4 runs on a massive reinforced concrete viaduct, which spreads wing-like cantilevers over the dual three-lane A4 below. Every day, a total of more than 138,000 cars, coaches, vans and lorries travel the narrow corridor, 87,000 along the M4 and 51,200 along the A4.
And many of those using the A4 will no doubt have noticed that the M4 viaduct, built in 1967, is revealing its age. “The structure began showing signs of deterioration in the 1990s and we responded with additional inspections,” says Highways Agency project sponsor Andrew Brunning.
Cast in-situ piers are topped with transverse crosshead beams, supporting the precast concrete longitudinal planks that form the deck soffit. Crosshead beams have an inverted T-section. Ends of the longitudinal planks sit on the nibs or bearing shelves either side of the upstand. Inspections revealed that de-icing salts used on the M4 in winter had seeped past the road deck’s asphalt plugs at these joints and penetrated the concrete of the beams, causing near-surface reinforcement to corrode.
“There wasn’t any immediate structural concern because there’s a significant amount of redundancy in the crosshead beams,” Brunning reassures.
A programme of regular monitoring was introduced and concrete was removed from the crosshead beams over badly corroded rebar. In parallel, the Highways Agency began trialling different protection methods to arrest corrosion.
“If deterioration were unchecked, the elevated section would be subject to weight restrictions and in the longer term crosshead beams would have to be strengthened or replaced,” Brunning explains.
Closing the M4 elevated section for replacement or repair was unthinkable. Replacement of a single crosshead beam was trialled by former Highways Agency managing agent, WSP. Wide-scale replacement would require a 9.1km detour of all M4 traffic onto the A4, which would itself have to be subjected to major traffic management to enable the works. West London would grind to a standstill.
Based on WSP’s trial, it was also calculated that a replacement would cost £5M per beam.
While the monitoring regime continued, the Highways Agency sought an alternative solution. “We wanted to start on the viaduct swiftly so that we could carry works out in a carefully planned manner and minimise disruption to road users and local residents,” Brunning says. It commissioned current Area 5 managing agent Mouchel Parkman to look at ways of rehabilitating the structure.
Mouchel Parkman initially drew up a maintenance strategy that focused on reducing seepage from the deck. It also reviewed previous research on surface cathodic protection. “Eight types of surface-mounted cathodic protection had been trialled by WSP,” says Mouchel Parkman structures team leader Scott Munro. “We assessed the crossheads and determined that we needed to protect the beams’ critical internal bending and shear reinforcement, as well as the outer reinforcement, which is what was visually exhibiting corrosion.
“This led us to draw up a performance-based specification and indicative design for titanium mesh with cementitious overlay offering near-surface protection, as well as discrete anodes threaded through the crossbeam reinforcement and deck drainage systems to protect the critical reinforcement for a minimum of 30 years,” says Munro.
Mouchel Parkman also warned against the potential for damaging the reinforcement and drainage systems during the installation of the protection system. Area framework contractor Birse Civils and its specialist concrete repair and protection subcontractor Laser Special Projects were brought in on an early-contractor-involvement basis.
Birse set out to halt the ingress of chloride-saturated water by replacing asphaltic joints on the M4 deck. “Originally, there were two joints at each crosshead beam, one either side of the beam upstand,” explains Birse Civils contracts manager Andy Gibbons. Over time they had deformed under loading from heavy, slow moving traffic. “Restrictive working windows had made maintenance difficult and they were leaking, allowing water onto the beams.”
“The new joints are harder wearing and less liable to deformation, and they are continuous across the top of the beam between the deck ends on either side,” Gibbons says. Work was carried out during night-time lane possessions when traffic disruption would be minimal.
More difficult was breaking out delaminated and spalling concrete, replacing it, and installing cathodic protection.
Access to the underside of the viaduct presented a ticklish challenge. Birse, Mouchel Parkman, the Highways Agency, owner of the A4 Transport for London, borough council noise officers, the emergency services and bus operators worked out a traffic management system whereby a single lane of the A4 in each direction – lane one or three – would be cordoned off 24/7 while work was taking place. A second lane would be handed over to Birse during night-time possessions.
“The problem with night work is that there are homes only 14m from the structure. Hounslow Council has fairly strict noise limits,” Gibbons says. “We carried out noise trials for breaking out concrete, grit-blasting rebar, drilling holes for the cathodic protection, and spraying back new concrete cover at Laser’s yard and onsite. We were planning to do all of the work over lane three during the day and over lanes one and two at night.”
“But when we started work for real, we found the noise levels were exceeding what we’d predicted.”
Birse and Laser went back to the drawing board. Birse devised a platform system built up using I-beams that cantilevered off trestles erected around the piers, providing access to the sides of the beams over all three lanes in both directions. This enabled 95% of work to be carried out during daytime hours, when noise thresholds were higher.
Problems were not entirely resolved, however. “The A4 is a high-load access route into London. You can’t compromise headroom,” Gibbons says. Installing cathodic protection using traditional discrete anodes would normally have required holes to be drilled from the under-side of the crosshead beams. To meet Mouchel Parkman’s performance requirement, 58 anodes per beam had to be installed.
Unable to get at the under-side of the beams during the day, and prohibited from drilling at night by noise limits, “drilling the holes could have been a show stopper”, Gibbons admits.
Laser’s designer Mott MacDonald came up with an unorthodox proposal. Instead of drilling multiple holes, why not core a single hole from end to end of the beam, right through its centre, and avoid working beneath it at all?
“As far as we knew it had never been done before – at least not on the UK highways network,” says Brunning. “It had always been assumed that you couldn’t control the direction of drilling sufficiently over long distances to avoid hitting reinforcement,” explains Mott MacDonaldsenior corrosion and protection engineer Chris Atkins.
“From all angles, though, drilling a single hole looked attractive. Going straight through the centre of the beam would keep the drill away from reinforcing steel, which is concentrated around the outsides, and put your cathodic protection exactly where you wanted it. Once you had the hole, installing it would be simple – you’d simply feed in a string of anodes. Backfilling the hole would be easy: It’s far simpler to inject grout along a horizontal hole than against gravity, up into a vertical hole. And you’d be drilling a far shorter distance. The length of the M4 crosshead beams is 18m. By comparison, the combined length of the 58 holes that we were initially looking at would have been 184m.”
Atkins says that drilling time for the single hole-through-the-middle approach was three days compared to 30 days or more for a conventional approach, and consumed 60% less energy. Gibbons equates that to a 20% cost-saving per pier – had conventional drilling been practicable.
Before deploying the approach onsite, however, it had to be proved. “We needed to demonstrate that the drill string wouldn’t kick out as it went through the beam,” Atkins says. “We decided to core the hole rather than use percussion drilling equipment for accuracy.” Coring is also quieter than percussion drilling.
A row of concrete traffic barriers was laid out in Laser’s yard and a sample core was taken. “There was less than 50mm deviation over 20m – well within tolerance,” Atkins says.
Gibbons notes that the deviation that did occur was due to the drill dipping under gravity. “All the crossheads have a camber, so we simply drilled from the high end.”
Birse designed the cantilevered working platforms with a stage at the end to provide access to the beam ends. Two platforms were fabricated and Birse Civils tackled the first three crosshead beams earlier this year. It is now carrying out repairs to a further six. Repair costs per pier and beam are in the order of £300,000.
Birse is required to remove 25mm of concrete behind all corroded rebar. Despite the structure’s large margin of redundancy, it was only possible to remove concrete over small areas without compromising its performance. “We’ve had to tackle concrete repair in 1m strips,” Gibbons says.
Repairs to the soffits over lane 2 are still required, and Birse has another trick up its sleeve. “We’ve bought a truck of the kind used by airports to load baggage onto and off of planes,” says Gibbons.
It is effectively a giant scissor lift.
“We’ve lined out the platform with noise-suppressing material and will be able to raise that up and enclose the crosshead. Operatives will be able to work in there at night without disturbing local residents.”