River Dane Bridge repairs

As part of its £128m commitment to complete maintenance improvements to motorways and major A-roads in the North East of England, National Highways commissioned repair work to the bridge which carries the M6 over the River Dane. We explain how Weber supported the work while causing only minimal disruption to traffic.

River Dane Bridge before repair

Located just north of junction 18 of the M6 motorway, near Holmes Chapel in Cheshire, the 85-metre-long River Dane bridge was constructed in 1962 to transport traffic across the river at high level. Over the years, the migration of salts through the bridge joints to the piers and columns below created heavy corrosion damage. This ultimately led to the de-bonding of the covering concrete. The bridge was therefore earmarked for repair, to improve its integrity and lifespan, and was subsequently included within a National Highways improvement plan.

Concrete Repairs Limited (CRL) was selected as the principal contractor via the Construction Works Framework (CWF) and is now working directly for National Highways. The company has been involved with frameworks of this nature for more than 22 years. Prior to being selected, in 2021, CRL was appointed to the early contractor involvement (ECI) stage and spent more than 12 months working closely with the project design consultant Amey, and National Highways. Together they determined the methodology necessary to overcome the risks of working over water, in a floodplain, on a heavily used ‘live’ structure, in conjunction with a number of other stakeholder considerations to the contract.

WIDER PROJECT CONSIDERATIONS

“The issues being rectified at River Dane typify those of a 60-year-old concrete structure, but the more detailed challenge with this project is apparent in the delivery of the work,” explains Adrian Pike, highways manager at CRL. “This stretch of the M6 carries thousands of road users every day, so it is crucial that the bridge remains open to avoid complicated and circuitous traffic diversions into the surrounding towns and villages. Of course, we have had to implement weight restrictions while the works are being carried out, so HGVs are being diverted, but if we can enable the majority of road users to maintain their normal route, it should have a less significant impact.”

Owing to the challenges of working on a major network motorway, with a full-flowing river and attendant wildlife to consider – as well as occupying a greenfield site while undertaking the works – Adrian and his team had to seek permission to put up temporary buildings and construct access roads prior to any works commencing. They liaised with the landowner, the Environment Agency, and the Warrington Anglers Association to ensure all concerns from stakeholders had been heard and addressed.

“It’s really important to us that we do our best to minimise disruption not only to the M6 road users but also to the surrounding areas while we undertake the repair works,” adds Adrian. “As part of the phase three works, a suspended scaffold has been erected to allow the CRL team to access the underside of the bridge. It is entirely separate from the structure and is fully enclosed within a debris net to avoid river pollution and keep the public safe. The footpath running alongside the river has remained open throughout. We have also introduced Silt-Busters to filter and screen the contaminated water used within the concrete repair procedure. The slightly alkaline-contaminated water resulting from this process is then neutralised and recycled, which can then ultimately be emptied back into the river.”

PREPARING FOR THE BRIDGE REPAIRS

The River Dane bridge has been subject to various surveys throughout its life, including one completed five years prior by the project design consultant Amey. Since then, there has been an increase in damage to the structure of between 50% and 60%.  Some of this damage has been caused by compromised waterproofing on the bridge which has allowed water to seep into the columns and soffit. This results in corrosion of the steel, causing it to expand and form cracks, creating de-bonding. Extreme weather conditions can exacerbate this process which, once it’s begun, cannot be mitigated until it is repaired. Before commencing work, CRL had to assess the extent of the repairs required by conducting a delamination survey to check whether the concrete was structurally sound. This was followed by an assessment of the level of chloride ions and a half-cell check, used to determine the probability of corrosion within the rebar in reinforced concrete structures, to review the condition of the existing concrete.

The team also checked the thickness of the rebar cover. Reinforcement should be a 50mm cover of concrete to the rebar; if it is exposed to the air it will corrode. In this case, there was a lot of unprotected rebar, which placed the structure at risk of further corrosion. Once these checks were completed, CRL produced a condition report which meant the team, subject to client approval, could begin to physically start removing the concrete. Each repair begins with hydro-demolition and concrete breakout to ensure there is no corrosion in those areas and that the steel is completely clean.

CARRYING OUT THE REPAIR WORK

In February 2022, CRL called on the expertise of the team at Weber – with whom they have worked on many similar projects – to review their suggestions for the optimum products to use for the repairs. To address the corrosion issues, CRL installed a cathodic protection (CP) system. This is typically used in chloride-contaminated concrete and concrete that has experienced carbonation to protect the reinforcement in the structure and future-proof protection in the longer term.

In an impressed current cathodic protection (ICCP) installation, the reinforcement (or cathode) is checked for continuity (linkage). A sacrificial anode is then introduced into the structure to allow for the anode to corrode preferentially and protect the cathode. A power supply is ultimately used to power up or energise the installation. Through this, a very low-level current can be used to regulate and maintain the reactive ability of the anode.  The anode – in this case a titanium overlay mesh – required encapsulation and protection once it had been fixed intimately to the parent concrete. The CRL team then applied a 50mm depth spray overlay concrete application to cover the mesh and bond to the parent concrete. For this they used webercem spray CP mortar, which complies with Highways England specifications for repairs to highway structures. It has low resistivity which makes it suitable for application to structures which receive CP treatment. The formulation has been designed specifically for dry process spray application to provide reduced rebound of material and maximise application thickness.

For other large areas which do not require cathodic protection, and where it is possible to apply products using a spray method, CRL is using webercem spray DS – which conforms with BS EN 1504-3 as a Class R4 repair product – making repairs easier and quicker, giving high strength, low rebound and wastage, and to maximise the application thickness.

However, this method can’t be employed for all of the repair works as some areas don’t allow access for a spray machine. For patch repair, webercem R4 duo is being used and in any large areas that are too big for hand placement, CRL is using webercem advanced repair concrete – a flowable repair mortar which offers rapid strength development.

SOCIAL VALUE AND SUSTAINABILITY CONSIDERATIONS

As part of the original tender, CRL had to outline its commitments to supporting the local economy and reducing its carbon footprint. The company hosted a site tour for 30 engineering students from Liverpool John Moores University, which led to a six-week placement opportunity. Furthermore, effective waste management diverted 98.99% of waste from landfill during phases one and two.

AWARD-WINNING REPAIRS

The project has been so successful so far that phases one and two were awarded Project of the Year and Project of the Year Over £1m at the 2023 Concrete Society Awards and Concrete Repair Association Awards respectively.

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