Rapid curing concrete has been introduced to reduce the time, costs and traffic disruption associated with replacing worn out concrete carriageway on the M25. Jon Masters reports.
As part of a drive to cut costs and enhance the value of its ‘lifecycle renewals’ works on the M25, Connect Plus, which has a 30-year concession to maintain and operate the orbital motorway, has worked with its contracting and materials suppliers to develop a method of replacing concrete carriageway without needing daytime lane closures.
“Airports have experimented with rapid cure concrete, which is where we took the learning from, but it has not been used on the strategic road network before,” says Connect Plus chief executive officer Tim Jones.
Concrete accounts for around 8% of the M25 carriageway, largely on the south western quarter, and Connect Plus is currently replacing around 300 concrete bays, 150 in each direction, between junctions eight and 11.
“There are no charges for working at night, but daytime lane closures are very expensive, typically costing £500,000 over a whole weekend”
Tim Jones, Connect Plus
Jones says that, using conventional techniques, it takes a minimum of four nights and one day to repair each bay, sometimes spilling over into a second day due to the time it takes for the concrete to cure. “There are no charges for working at night, but daytime lane closures are very expensive, typically costing £500,000 over a whole weekend,” he adds.
Using rapid cure concrete (RPC), the process takes just two nights, according to Jones, while additional costs are marginal.
Connect Plus initially estimated that RPC repairs would cost £45,000 per bay, not including lane closure charges, but refinement of the process has reduced this to about £30,000, and costs are expected to fall below £25,000 as the repair programme continues.
“This is about developing labour and plant efficiencies rather than costs of the RPC,” Jones says. “The RPC bays have the same value in the long term, but we are able to repair surrounding problematic joints and carry out fine milling of the new surface to match adjacent bays without daytime closures. We are generally looking to do 2km of other work within the same 4km long nightly closures.”
Connect Plus initially developed the RPC repair procedure and mix design with contractor Jackson Civil Engineering and supplier Grace Concrete Products. A pilot project on the M20 was followed by trials on the M25 with contractors Balfour Beatty, Skanska, Osborne and Jackson and concrete suppliers Lafarge Tarmac and Aggregate Industries. “All were given targets of achieving efficiencies and sharing their experience with others,” says Jones.
The methodology involves mixing and laying the RPC in situ with one 6m3 volumetric mixer and one team per bay. The first night of the lane closure is reserved for saw cutting the concrete into 1t sections, which are then lifted out and replaced with a single RPC pour the following night.
The RPC material contains a curing accelerator and super-plasticiser, and has a very low water-cement ratio of 0.28.
“The biggest difficulty has been getting the workforce used to working with the RPC and then achieving consistency across all of the contractors,” says Connect Plus roads asset manager James Burdall. “The material is very dry at first, but activation with pokers causes it to become jelly-like. It then cures very quickly. It cannot be worked after 30 minutes to one hour.”
Connect Plus’ contractors have now repaired over 120 bays with RPC during night lane closures, with between one and three bays being repaired at a time, depending on proximity and available resources.