Dry-spray concrete enables fast refurbishment of River Neath Bridge
Specification of high performance webercem spray RS, a rapid setting dry-spray concrete by Saint-Gobain Weber, has enabled specialist contractor Freyssinet Limited to carry out the strengthening of the bridge piers that carry the A474 over the River Neath in South Wales.
Located in a tidal zone, this project required a unique application method and an advanced technical material to achieve the required result in the shortest possible time.
Neath Port Talbot County Borough Council (NPTCBC) carried out a full refurbishment of the River Neath Bridge reported to cost more than £1million. Freyssinet were contracted to install a reinforced concrete strengthening jacket around each of the eight structural piers that carry the bridge. These piers are located in the tidal zone of the river where a traditional shuttering and poured concrete method was not possible. A unique project design was conceived by Freyssinet and implemented during September and October 2016.
Freyssinet Limited was formed in 1950 and is now part of the Soletanche Freyssinet Group, a global provider at the forefront of innovative civil and structural engineering solutions. Originally a specialist in bridge components and post-tensioning applications, Freyssinet now has an expanded portfolio of products and services providing the repair, protection and strengthening of structures. This portfolio provides specialist diagnostic surveys, repairs, refurbishment, bridge bearings and cathodic protection to reinforced concrete for the building, civil and marine sectors. With a strong in-house engineering design team the company provides services to major contractors, the Highways Agency, local authorities and developers.
The core solution for the River Neath project was the specification of webercem spray RS, a rapid setting, polymer-modified, dry-spray concrete ideal for permanent repairs where time constraints, such as tidal flow, demand early strength gain. This rapid set capacity prevents wash out from the tidal water flow and also minimises the penetration of chlorides. Research shows that salt penetration into webercem spray RS is limited to 10mm whereas fresh, traditional concrete absorbs chlorides faster than hardened concrete as the crystalline structure has not yet fully formed.
The material contains inert limestone aggregates, dust suppressants and accelerators. This product has passed the stringent freeze/thaw test required for durability and the bonding adhesive standards of BS EN1504. This formula is designed for dry-spray processes where the material is blown by air to the nozzle and mixes with water on exit from the lance. This reduces rebound on contact with the substrate surface and allows high build material application. The dry-spray process fires the material at the substrate at over 80mph giving good compaction with very few voids.
This highly technical product gives an initial set in just 15 minutes and in six hours has a compressive strength of 10-15 n/mm². It has been designed by Weber specifically for use in marine applications, repairs to river bridge abutments or piers, and structural repairs to concrete beams, columns and soffits. This product meets the standards contained in Exposure Class XF4 for long term durability being exposed in flood plain environment. Freyssinet has successfully used webercem spray RS in previous projects and recommended this to NPTCBC. At the River Neath Bridge application some areas of the new jacket had a maximum material depth of 395mm. In total, 78 tonnes of material was required.
Following a detailed survey of each of the piers, removal of any redundant or damaged concrete was carried out using hydro-demolition delivered by Freyssinet subsidiary Aquaforce Limited. All the concrete surfaces were made ready for the webercem spray RS application following hydro-demolition. A reinforced steel jacket was then installed to each pier. Cores taken from test spraying showed that this product builds good density behind the rebar.
A bespoke ‘backing-board’ system was designed by Freyssinet to allow the material to be applied in panels during low tide. This reduced the need for full shuttering to be installed prior to works and enabled the fast setting attributes of the concrete to achieve large build-up of depth of material. Due to the quick setting times of webercem spray RS there are no application thickness restrictions - continuous spray build can be achieved. The accurate control of the dry-spray technique also addressed the environmental issues of working adjacent to water and the riverbank where material loss would have been unacceptable. The application process proved accurate and fast enabling the contract to be delivered to the agreed schedule.
Daniel Harkin, Freyssinet’s site manager, was confident in the specification of webercem spray RS concrete: “We have used this excellent product before, for bridge and pier repairs, so we were confident of its use at Neath. We had first class technical back-up from Weber at the specification stage and also on site. The application process ran to schedule and we delivered the project to meet the client’s plan.”
Neath Port Talbot County Borough Council’s technical team recognised the values of webercem spray RS: “At first we were sceptical that sprayed concrete could be used in such a challenging situation. However, the evidence from Freyssinet’s previous use of the product was convincing enough to allow a trial to go ahead. This proved the precision with which an experienced nozzleman could place the material, and subsequent test cores proved the density of the finished product. The test spraying was also witnessed by representatives from Natural Resources Wales who were initially concerned about potential pollution of the River Neath. Their concerns were allayed and they were happy for us to proceed.
“It was agreed to leave the concrete ‘as-sprayed’ rather than have a trowelled finish. Time will tell how this weathers but initial thoughts are that it gives a visibly pleasing, textured surface similar to rough-cast render.”