04 Mar, 2022 By Sotiris Kanaris
Carbon emissions and costs for construction of ground bearing slabs for a project in London have been cut with the use of Earth Friendly Concrete.
Finding projects to put new materials through their paces is always a challenge. But one contractor is using its own facility to do just that. Keltbray is using Earth Friendly Concrete (EFC) in its new treatment facility for hazardous and non-hazardous construction, demolition and excavation waste in West Silvertown in London.
The new facility is for its Environmental Solutions division. It will assess, treat and dispose upwards of 350,000t of waste a year and will replace the firm’s existing one at nearby Thames Wharf which is being taken over for the Silvertown Tunnel project.
Reducing operational carbon emissions has been a key part of Keltbray’s vision for the new facility. The site’s proximity to the River Thames, will make it possible for the company to accept material into the facility and dispose of it via barges, rather than solely relying on lorries. This will reduce the number of lorry journeys and therefore carbon emissions.
As well as reducing operational carbon, Keltbray – the project’s client, designer and contractor – looked into reducing carbon embodied during the construction phase. The original plan was to use Ordinary Portland Cement (OPC) concrete for the 10,000m2 of ground bearing slabs, divided into approximately 300m2 bays. Analysis carried out in-house indicated that a switch to EFC for this application would reduce the project’s carbon footprint and cut costs.
On 10,000m2 [of slab] we have 1,800m3 of concrete compared with 2,000m3 and no mesh compared with 90t of mesh
EFC is a cement-free concrete developed by construction materials specialist Wagners. It uses a geopolymer binding system made from the chemical activation of blast furnace slag and fly ash instead of OPC. Blast furnace slag is waste from iron production and fly ash is waste from coal fired power generation.
Keltbray has been using OPC concrete for temporary works and piles since 2019 but its own project has created an opportunity to use it for ground bearing slab construction. Through the previous projects, the contractor came to appreciate the advantages of EFC compared to OPC concrete. These include lower shrinkage, higher flexural strength and lower heat of hydration.
Keltbray strategic engineering director Tim Lohmann says that the lower shrinkage and higher flexural strength of EFC-based concrete allowed for the use of thinner slabs and larger base sizes than if OPC was used. In addition, these material properties removed the need for reinforcement.
Under the original scheme the slab thickness was 200mm with one layer of A393 steel reinforcing mesh if OPC concrete was used. In the amended scheme the slab thickness has dropped to 180mm, with no mesh required. “So on 10,000m2 [of slab] we have 1,800m3 of concrete compared with 2,000m3 and no mesh compared with 90t of mesh,” Lohmann explains.
EFC is more expensive than OPC concrete – Wagners estimates the price to be between 20% to 40% higher. But Lohmann says for this project it was a more cost-effective solution because the thinner slabs and lack of reinforcement led to savings of £4/m2 of product.
Choosing this material also led to a significant reduction in carbon emissions. The switch from C32/40 concrete to EFC resulted in carbon saving of 0.038kgCO2/kg and around 0.835kgCO2/kg for the mesh.
The only way we will stimulate growth of supply and availability of these concretes is by people asking hard questions and gaining confidence
The carbon footprint of the slabs under the original scheme was 555t, the figure dropped by 71% to 162t for the revised scheme.
Using this material does have some limitations in terms of curing techniques. The contractor would usually use wet curing – where the concrete is frequently hosed down – for slab construction to prevent cracking from fast evaporation. But EFC does not react well to wet curing because of the high lime alkaline content in the paste. For this reason, Keltbray is placing specialist curing membranes over the concrete to prevent moisture loss.
With the preparation of the ground bearing slabs to be completed in the coming months, the contractor is already looking for other projects where it can use EFC in permanent construction.
Although it is a strong advocate of the material, Keltbray is taking a cautious approach. “We’ve looked at a risk based hierarchy. Where can we get the biggest win with the minimum risk?” says Lohmann.
“One of the challenges with the new concrete is that long term durability is hard to assert.” For this reason, Keltbray is investing in research at the University of Dundee to assess chemical and environmental durability of EFC.
The company has prepared a report which includes data on the material’s different properties and is sharing it with clients, as many are reluctant about using EFC.
But some clients have been convinced of the benefits of this new concrete type. One is British Land, which gave the green light for 73, 900mm diameter EFC piles to be installed at a petrol station in Canada Water in London.
“This was a collaborative effort between an engaged and committed client, its structural engineers AKT and ourselves. We set high ambitions for the scheme and pushed hard to achieve this with an agreed programme of testing for the EFC through the preliminary phases and during the works,” says Lohmann.
Through testing at those stages, Keltbray has been able to demonstrate that the material it is proposing complies with specification. According to Lohmann, current concrete specifications do not allow for concrete containing no Portland cement, creating a challenge for the use of EFC.
“And so, when we’re trying to get the material approved [by a client], we push for performance specification rather than material specs. We say, the concrete shall achieve [a specific] strength at certain time, shall achieve a certain durability criteria, and we then validate those by testing.”
The testing and research allows the company to gain knowledge about the material so it can start using it in other more complex applications.
Lohman adds: “The only way we will stimulate growth of supply and availability of these concretes is by people asking hard questions and gaining confidence. And we should all be prepared to share the information we’ve got.”
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Tagged with: Concrete Low carbon Net Zero
Just a query about one of the paragraphs,
“Keltbray has been using OPC concrete for temporary works and piles since 2019 but its own project has created an opportunity to use it for ground bearing slab construction…”
Is this supposed to read “Keltbray has been using EFC for temporary works…”
I am just wondering how long Keltbray has been using EFC in the UK?
or a new account to join the discussion.
Brighton and Hove City Council
Brighton and Hove City Council
04 Mar, 2022 By Sotiris Kanaris