Largest ever concrete repair completed on viaduct

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The UK’s largest ever concrete repair project has been jointly completed by Concrete Repairs Limited (CRL) and VolkerLaser. Valued at over £100m, the M5 Oldbury Viaduct Major Renewal Scheme was recently announced as the Concrete Repair Association (CRA) ‘Repair and Refurbishment Project of the Year’ at the Concrete Society Awards 2019*.

The M5 is one of the busiest motorways in the country, carrying traffic through the Midlands and onwards to routes in both the north and south. The section of the M5 that was repaired – between junctions 1 and 2 in Sandwell – is exceptionally busy with up to 120,000 vehicles using the viaduct each day.

Originally constructed in the late 1960’s and opened in 1970, the viaduct is a 1.8 mile (3.2km) long elevated section to the west of Birmingham. Although the viaduct was considered structurally sound, Highways England realised the need to carry out essential repair work to the concrete decks, deck ends and cross-head beams as the waterproofing system and expansion joints on the motorway deck had degraded over the years.

The decision to repair this section of motorway was based on the current condition of the viaduct and to avoid large-scale projects occurring at the same time around the ‘Birmingham Box’ – the M5/M42/M6. The work formed part of the government’s long-term Road Investment Strategy to build a modern and resilient road network which would last well into the future.

The viaduct is split into 165 separate steel reinforced concrete deck sections (slabs) linked by expansion joints in the gap between the ends of each section. A waterproofing layer then protected the decks, but this had deteriorated over time, resulting in chloride contaminated water permeating into the concrete and causing the reinforcement to corrode and spall.

The concrete repair work was carried out in three stages to allow for continued two-way traffic flow. The first was the southbound carriageway, followed by the northbound section and finally the central reservation. CRL was appointed as the concrete repair contractor alongside VolkerLaser. The principal contractor was a joint venture between BAM Nuttall, Morgan Sindall and VolkerFitzpatrick – known as BMV.

The concrete repair work carried out by CRL involved the northern section of the viaduct closest to Junction 1, comprising a 0.93 mile stretch of deck, covering 50,000m2 – essentially half the viaduct. Even though the nature of the contract was typical of the type of works commonly undertaken by CRL – involving hydro-demolition, continuity testing, installation of sacrificial anodes and then reinstatement with a pre-bagged flowable micro-concrete – the scale of the work was unprecedented.
In total CRL carried out full repairs to their allocated section – namely 125 deck ends, over 4,500 individual mid span areas and replacement of half a deck as part of a single repair covering 200m2. To achieve this, 10 million litres of water was used during the hydro-demolition works, 1.5 million kilograms of concrete repair material was applied, 21,000 sacrificial anodes installed and almost 350 individuals contributed to the project success in CRL’s section alone.

The southbound (first) phase was carried out over a period of 13 months, during which time the four lane carriageway was closed to traffic for the entire duration. Both northbound and southbound traffic used the northbound carriageway via a contra-flow system which enabled operatives to work two full shifts a day, six days per week. Some areas of the motorway had other roads passing directly underneath, which required CRL to complete concrete repairs during full weekend closures of the road underneath from 8pm on Friday until 6am the following Monday.

The work process started with BMV planing off the existing surfacing and removing the failed waterproofing system. A third party – Socotec – then tested the deck to identify where concrete repairs were needed. Hammer testing identified where the concrete had delaminated, and half-cell testing pinpointed areas of potentially corroded steel. Chloride testing was also carried out to identify where chloride salts had penetrated the deck to the depth of the steel, and surface defects identified via visual inspections.

Once the areas requiring repair were identified, operatives were able to saw cut around the patches that needed repairing and proceed with the hydro-demolition. A 3m x 3m enclosure was created and surrounded with two layers of debris netting to contain flying fragments of concrete.

The next stage of the process involved replacing badly corroded reinforcement. CRL used a number of on-site welding devices/systems to connect the new reinforcement to the existing. The welds were subsequently tested using non-destructive testing methods including magnetic particle tests and ultrasonic testing. Continuity tests were also carried out on the reinforcement before installation of the sacrificial anodes which were used to prevent potential future corrosion of the reinforcement, and thereby significantly extend the life of the structure. Over the course of the contract, more than 21,000 sacrificial anodes were installed by CRL.

Once the patches had been prepared, the necessary reinforcement replaced, and the sacrificial anodes installed, the broken out areas were then filled in with a high performance, free flowing concrete repair micro-concrete conforming to the requirements of Highways England Specification for Highways Works and EN1504-3 Structural and Non-Structural Concrete Class 29F.

Due to the scale of the repairs, large 500kg bags of repair material were used instead of the standard 25kg bags. This eliminated manual handling as the bags were lifted using telehandlers and an opening on the bottom allowed for the material to be discharged directly into large pan mixers. The mixed repair material was then transferred to the individual repair areas, placed and finished by hand using trowels and floats.

As repair work was required to be carried out throughout winter months, CRL implemented a winter working procedure whereby a heated enclosure was constructed over the completed repair. In addition, warm water was added to the mix and blow heaters were used to keep the temperature above 5⁰C. until the mortar had cured.
Once all the concrete repairs had been carried out on the southbound carriageway, the area was handed over to the deck waterproofing team before the CRL team moved to the northbound carriageway for a period of 10 months, replicating the processes used for the first phase.

CRL was then appointed to carry out the concrete repairs to the central reservation across the full length of the viaduct. The central reservation (final phase), carried out over a period of two months, was the shortest phase. During this period operatives worked in lanes either side of the central reservation to carry out the required repairs to damaged/spalled concrete.

The works carried out has ensured that the structure has been effectively and efficiently repaired, the concrete protected and the lifetime of the viaduct extended significantly, thus ensuring the ultimate Highways England objective of having a robust and resilient road network.

* The Concrete Repairs Limited and VolkerLaser Alliance was awarded the ‘Repair and Refurbishment Project of the Year’ accolade from the Concrete Repair Association (CRA) at the prestigious Concrete Society Awards on Wednesday 20th November 2019 for the Alliance’s work on the M5 Oldbury viaduct.

Concrete Repairs Limited formed an alliance partnership with VolkerLaser, who worked together to undertake the largest ever concrete repair project carried out in the UK.

The Concrete Society Awards celebrate excellence in concrete across the UK and the judges praised the Alliance for genuine collaboration and innovative methodology, which enabled the sharing of learning, resources, materials and plant allowing the project to be delivered on time despite its size and complexity.

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