River Dane repairs

David Playfor, Saint-Gobain Weber, with Adrian Pike, Highways Manager, Concrete Repairs Limited

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.

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. 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, with Saint-Gobain Weber assisting with this work alongside Concrete Repairs Limited (CRL) as the principal contractor via the construction works framework.

Prior to being selected, in 2021, CRL was appointed to the early contractor involvement stage and spent more than 12 months working closely with 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

It had been explained that the issues rectified at the River Dane typified those of a 60-year-old concrete structure, but the more detailed challenge with this project was apparent in the delivery of the work. This stretch of the M6 carries thousands of road users every day, so it was crucial that the bridge remained open to avoid complicated and circuitous traffic diversions into the surrounding towns and villages. Weight restrictions were put in place while the works were being carried out, with HGVs diverted, but enabling the majority of road users to maintain their normal route, to have a less significant impact.

This stretch of the M6 carries thousands of road users every day, so it was crucial that the bridge remained open to avoid complicated and circuitous traffic diversions into the surrounding towns and villages. 

The project 

The project was broken into three phases.

Phase one – comprising the entirety of the south side of the bridge supports – began in January 2022 and is now complete.

Phases two and three, with the former covering the north side of the bridge and the latter addressing the worst of the corrosion on the south side as well as issues on the soffit, were completed in April 2024.

It was important that disruption was minimal, not only to the M6 road users but also to the surrounding areas while the repair works were undertaken.

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 – the team had to seek permission to put up temporary buildings and construct access roads prior to any works commencing. It meant liaising with the landowner, the Environment Agency, as well as the Warrington Anglers Association, to ensure all concerns from stakeholders had been heard and addressed.

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 – the team had to seek permission to put up temporary buildings and construct access roads prior to any works commencing.

As part of the phase three works, a suspended scaffold was erected to allow the team to access the underside of the bridge. It was entirely separate from the structure and was fully enclosed within a debris net to avoid river pollution and keep the public safe as the footpath running alongside the river remained open throughout.

Silt-busters were also introduced to filter and screen the contaminated water used within the concrete repair procedure. The slightly alkaline-contaminated water resulting from this process was then neutralised and recycled and could 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 had been an increase in damage to the structure of between 50% and 60%.

Some of this damage was caused by compromised waterproofing on the bridge which allowed water to seep into the columns and soffit. This resulted 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, the extent of the repairs required was assessed 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, a condition report was produced which meant the team, subject to client approval, could begin to physically start removing the concrete. Each repair began with hydro-demolition and concrete breakout to ensure there was no corrosion in those areas and that the steel was completely clean.

The bridge before repair.

Column and beam prepared for repairwith rebar exposed.

Work being carried out on the bridge.

Commencement

In February 2022, the Saint-Gobain Weber team was called in to assist in the repairs. To address the corrosion issues, a cathodic protection (CP) system was installed. 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.

Before commencing work, the extent of the repairs required was assessed 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.

In an impressed current cathodic protection 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 team then applied a 50mm depth spray overlay concrete application to cover the mesh and bond to the parent concrete. For this webercem spray CP mortar was used to comply 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 did not require CP, and where it was possible to apply products using a spray method, webercem spray DS was used – which conforms with BS EN 1504-3 as a class R4 repair product – to maximise the application thickness. However, this method couldn’t be employed for all of the repair works as some areas didn’t allow access for a spray machine.

For patch repair, webercem R4 duo was used and in any large areas that were too big for hand placement, webercem advanced repair concrete was used – a flowable repair mortar which offers rapid strength development.

Social values

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. Phases two and three were completed in April 2024. 

David Playfor, Saint-Gobain Weber, with Adrian Pike, Highways Manager

Concrete Repairs Limited