An Everyday Model for Low-Carbon Concrete Success
Author: Peyrouz Modarres MEng CEng FIStructE FICE ACGI, Director, Walsh, London, UK
This article was first published in the July 2023 issue of The Structural Engineer magazine.
Multi-storey concrete frame residential buildings, typically ranging between 6-15 storeys, seem to be a preferred choice for developers in the UK and abroad. This is because of their strength, durability, inherent fire resistance, design flexibility, and cost-effectiveness.
While there are many alternative materials and off-site construction approaches available, in the UK, horizontal and vertical ties are required once buildings exceed four storeys due to disproportionate collapse requirements. This often leads developers to move away from traditional forms of construction like load-bearing brick and block walls with timber or pre-cast floors and back towards concrete frames.
There are many reasons for and against the use of concrete frames but it is expected that concrete frame buildings will continue to dominate the market for the near future. Despite some opposing views, there is a growing consensus that multi-storey living offers significant environmental benefits, especially in dense urban areas where it conserves land, enhances energy efficiency, and provides transportation benefits. This is of course after it has been established that a new structure is required in the first place and that re-using existing stock is not viable.
Recent data from the Office for National Statistics (ONS) revealed that 33% of all new construction orders in 2021 were for residential housing. 22% of these new dwellings were created in buildings of three storeys or more, a figure which rises to 53% in urban areas like London. Therefore, the need to lower the carbon impact of multi-storey residential buildings has never been more important, given the shortage of housing stock.
According to Peyrouz Modarres, a director of Walsh, there has been an excessive emphasis on the sustainability credentials of high-profile projects that are beyond the reach of most design teams in terms of budgets and resources. Instead, he suggests that the concrete framed multi-storey residential building, which is essential and abundant but often overlooked, deserves greater attention.
The Lark is a project where the engineering approach reduced the carbon footprint of a concrete-framed projects. As engineers, we tend to gravitate towards logical and quantifiable approaches but as you will see, we also need to bring people and communication to the heart of the process if we are to effect real results.
Project Overview
The residential development known as The Lark, previously referred to as Nine Elms Parkside Plot C1, is situated in the Nine Elms Parkside masterplan area of Wandsworth, London. It consists of three blocks providing 262 residential units spanning across 11 to 13 storeys. The development has an L-shaped layout, with Buildings B and C connected to Building A through a communal courtyard located on a podium covered by landscaped communal gardens. Car parking is provided on the ground floor and a small basement houses plant.
The site, a former Royal Mail sorting office in Battersea, posed several challenges typical of brownfield sites, with the most significant being the development’s location on top of a large Thames Water storm sewer.
Galliard Homes secured outline planning permission and commissioned Walsh in 2018 to facilitate design from detailed planning application through to delivery. The completion deadline was tight with projected completion in 2022.
Walsh prioritised developing a trusted relationship with the client
To work with the proposed scheme, an efficient structural frame needed to be developed and early engagement with Thames Water regarding the sewer was required as this had been identified as the main risk to the development. The design had to fit with the architectural intent, keep the programme tight and wherever possible reduce cost. In many ways, Galliard can be seen as an ideal client in that they run a lean team, set the objectives and then allow the appointed consultants to get on with delivery while providing support along the way.
Galliard had the confidence to listen to engineering proposals that the project could be designed to be very lean because of a trusted relationship that had been built up with the engineers over more than 20 years. A solution was proposed that could not only improve the sustainability credentials of the development (in addition to those already required by Wandsworth Borough Council) but also deliver upon the client’s objectives more effectively.
While we don’t always have the opportunity to work with a long-established client, investing time in the relationship which eventually leads to trust is the key. In this instance, it was fortunate that this previous relationship existed as most of the project was delivered through the peak of Covid which severely hampered the opportunities for face-to-face meetings. This illustrates the need for engineers at the start of their careers to not underestimate the longer-term benefits of in-person meetings and developing relationships with clients.
Function and sustainability were balanced
When it comes to designing multi-storey buildings, the choice of framing material is a critical decision. While steel and timber framing have made significant advances in recent years, concrete remains the most viable option for many projects, including The Lark. This is due to its strength and robustness, flat soffits that facilitate MEP coordination, the ability to achieve high floor-to-floor heights with minimal building height, the inherent fire protection properties of concrete, and design flexibility. This is further reinforced by the current procurement and supply chains of most developers.
Concrete is not without its environmental drawbacks. It accounts for a significant portion of global carbon emissions. As engineers, it is our responsibility to minimise concrete volumes and specify low carbon concrete mixes in order to deliver a sustainable solution. In the case of The Lark, the client’s requirements and supply chain made concrete the most suitable option, but every effort was made to balance function and sustainability in the design choices at every stage.
Designs were developed in partnership with the wider team
By joining the project at an early stage, there was an opportunity to influence the design and reduce the amount and type of concrete in the superstructure. To assess the embodied carbon and costs of different design options, various grids and slab thicknesses were compared, including post-tensioned concrete. Walsh has an in-house Embodied Carbon Assessment Tool (ECAT) which also benchmarks embodied carbon calculations against LETI and IStructE ratings (figure 1).
Figure 1: Embodied Carbon Assessment comparison between Stage 2 and Stage 5
An efficient grid is typically the key to lowering the embodied carbon of a building, and this was discussed and explored fully with the architect. Working closely with Fourfoursixsix, the grid was optimised in their designs which allows the lean design and embodied carbon goals to be achieved.
The building features a central core and corridor with residential units surrounding this which is typical of most residential developments. The column spacing on the perimeter was reduced to under 5 metres, which could be accommodated due to the façade design, and internal spans to under 6 metres wherever possible. Typical residential buildings are approximately 18m wide, which makes a central column line either side of the corridor inefficient as it results in 9m+ span. Placing two central grid lines and positioning bathrooms to the back of corridors creates the approx. 6 metre internal spans (see figure 2). Positioning bathrooms to the back of corridors had the added benefit of moving SVPs away from the building perimeter. Keeping penetrations through the slab away from edge columns greatly reduced the amount of additional reinforcement required for punching shear.
Figure 2: Grid and Scheming tips
The cores were kept compact, and load transfers were minimised through the efficiency and regularity of the grid. This allowed the internal slab depth to be reduced to 225mm with equivalent 50% cement replacement (215 kgCO2e/m3) and the basic reinforced matt to just H10@200. As a result, the average reinforcement density of slabs was only 87kg/m3, significantly lower than the industry standard for this type of building.
Upon running carbon calculations, it was found that taking the slab down further to 200mm actually increased the density and concrete grade such that it also increased the embodied carbon. The 225mm slab was also found to be more efficient due to secondary fixings such as balconies and masonry support. However, on a larger development, economies of scale could have made the 200mm slab more suitable”. This really highlights the importance of optioneering and analysing the embodied carbon profile of different slab depths because relationships are not linear, there are sweetspots that need to be indentified for each project/location/client rather than taking a standardised, one-size fits all approach.
These kind of technical and design decisions are part and parcel of being an engineer but their implementation could only be achieved through the collaborative efforts of the wider design team. On this project, the team really clicked and collaborated to achieve the results. It is impossible to decipher what made this particular team click, but important factors were certainly trust and mutual respect. The ability to develop these relationships can therefore be seen as a vital engineering competency and one that none of us should neglect.
Trade offs had to be made… and battles picked!
The site presented a significant constraint in the form of the Thames Water storm sewer that ran underneath it. To ensure a safe and compliant design, engagement with Thames Water started at an early stage. This ensured that their requirements were fully understood and the initial geotechnical assessment could be used to develop a solution that worked around the sewer.
The optimum solution was found to be the more complex and material intensive one, despite being less environmentally friendly. Combining the surrounding pile caps was necessary to mitigate risk to the sewer and provide a robust foundation solution. A lower material foundation option was reviewed but would have resulted in an above-ground transfer, leading to temporary works and higher risk during and after construction. Transfers were kept below ground so the headroom of each floor could be maximised.
The Lark features a landscaped podium on top of the car park with a 350mm slab to carry the weight of the landscaping and this kind of feature will always add embodied carbon to a building. The most sustainable, lowest carbon option would be to have no landscaped podium at all! However, the wellbeing of residents, the desirability of the development and the ongoing bio-diversity benefits of creating green space, counterbalance the initial carbon and concrete investment.
Whilst this is a Project Focus about low carbon concrete, these kind of trade offs are significant because they are the kind of decision that engineers are faced with on a daily basis. Counterintuitively, sometimes the extra carbon and extra concrete will achieve the required balance. It is crucial to take a holistic view of the project to achieve the optimal outcome both in terms of sustainability, safety, aesthetic and wellbeing benefits.
Covid restrictions at the time made working together as a team and communicating effectively even more important and challenging.
Close collaboration with the contractor was key
To mitigate the impact of the additional concrete in the sewer buildover and landscaped podium, a cement replacement was specified to minimise the carbon intensity. However, contractors are often concerned about the curing time and program extension associated with specifying cement replacements. This was absolutely the case with the contractor on The Lark, OHOB, who expressed their apprehension about using such replacements. Rather than insisting on our choice of materials, the time was invested in developing a relationship with them.
The client had already set the condition that they were content to use the materials recommended to them, as long as they did not increase costs or extend the program. Therefore, meetings were arranged with the contractor. During these intense sessions, the contractor was encouraged to express their concerns frankly and was then presented with detailed information and case studies of cement replacements being used on similar sites and projects.
This wasn’t the end of the conversation though. Instead of just demanding a wholesale commitment, they were persuaded to try ‘just one slab’ and assess the results. If any issues arose, these could then be discussed and a solution found. After a month, a follow-up call revealed that the first slab had been successful, and the contractor had continued using it on all other slabs.
A note on cement replacements
The cure time of cement replacement is affected by weather conditions. A higher cement replacement ratio can be achieved during the late spring to early autumn months. As a result, a strategy that specifies an “annual average” of replacement will allow contractors to adjust the mix seasonally. For example, during winter, only 20-30% cement replacement may be achievable, which could be increased to 60% during summer, depending on the program and volumes poured. Please refer to Figure 3 for more information.
It is also worth noting that although the use of cement replacement materials like GGBS in the UK can reduce the carbon footprint of concrete, which is a positive development., if the GGBS is being imported from a faraway location, such as China, the environmental benefits may be offset by the carbon emissions associated with transportation. It can even harm the local markets where the cement replacement materials are made. When all of the cement replacement materials are shipped overseas for profit, local contractors are left with no other choice but to use traditional cement. It is also worth noting that although the use of cement replacement materials like GGBS in the UK can reduce the carbon footprint of the concrete being used for an individual project, the impact on global emissions may not be the same, as outlined in
When all of another country’s cement replacement materials are shipped overseas for profit, local contractors are left with no other choice but to use traditional cement – it’s another worrying example of carbon being off-shored, reducing our domestic carbon emissions at the expense of those abroad.
Results
An embodied carbon calculation was conducted at both RIBA Stage 2 and Stage 5 of the project’s development – see Fig 1. This revealed that the estimated embodied carbon was reduced to 182kgCO2e/m2 for A1-A5 for the substructure and superstructure only. This was achieved by implementing a lean approach and value engineering the design, resulting in an 30% decrease in the embodied carbon at Stage 5 from the initial Stage 2 designs. Of the total 30% reduction, 20% was due less material, and 10% due to cement replacement, so without enhanced cement replacement we would have had 20% reduction to 211 kgCO2/m2
What The Lark project really demonstrates though is that designing a low carbon concrete solution and making decisions about how best to reduce carbon is simpler than actually delivering it. In addition to innovative engineering abilities and knowledge of low-carbon concrete, engineers need soft skills like communication and collaboration. They need to be able to make balanced decisions based on the needs of all stakeholders and not just lower carbon arbitrarily by any means possible. By fostering mutual respect among all members of the design and construction team on The Lark, the project was moved in a more sustainable direction. In this way, engineers can play a significant role in reducing the embodied carbon every development, regardless of size, and make a real difference to the health of our planet.
Project Credits
Structural & Civil Engineer: Walsh
Architect: Fourfoursixsix
MEP: Ridge
Landscape Architect: Turkington Martin
Client: GDL (Nine Elms) Ltd
Frame Contractor: O’Halloran and O’Brien (OHOB)
