Potential and practicality: Low‑carbon concrete in large‑scale rail projects

Concrete remains fundamental to large‑scale rail infrastructure. Tunnels, stations and ancillary structures rely on it for strength, durability and long‑term performance. At the same time, growing pressure to reduce embodied carbon is prompting us to rethink established approaches to materials selection.

Low‑carbon concrete has emerged as a premier potential solution. It offers a pathway to reduce emissions while maintaining performance, provided it is applied with a clear understanding of standards, construction methods and local conditions. For major rail projects, the conversation has shifted from whether low‑carbon mixes can be used to how they can be specified responsibly and delivered consistently.

Low‑carbon concrete is not a new concept, but the context shaping how it is used has changed. What was once driven by project-specific sustainability targets is increasingly shaped by regulation. In New South Wales, regulatory requirements have sharpened expectations and reduced variability between projects, with the Decarbonising Infrastructure Delivery Policy reinforcing the expectation that carbon reduction is embedded in material choices across publicly funded infrastructure. Similar trends are emerging elsewhere in Australia, bringing greater consistency alongside greater scrutiny.

Regulation alone does not define the challenge. Supply chains are now central to the sustainability conversation. Cement substitutes, aggregates and supplementary materials vary widely by location, availability and performance characteristics. For rail projects that span across regions, this variability matters. A mix that performs well in one location may be impractical or non‑compliant in another.

Ethical considerations are also becoming harder to ignore. Sand, a core component of concrete, is under increasing global pressure. Environmental impacts from extraction, supply constraints and illegal mining have reframed low‑carbon concrete as more than a carbon issue. It is also a question of responsible sourcing and long‑term resilience. For transport owners and contractors, these issues influence procurement strategies and risk profiles across the life of a project.

Our work across complex transport projects has shown that early engagement with these realities supports clearer decisions later. For the Sydney Metro West – Western Tunnelling Package (WTP), material choices intersect directly with regulatory compliance, constructability and long‑term asset performance. Addressing them in isolation rarely delivers the intended outcomes.

The tension between ambition and achievability complicates how we deliver low‑carbon concrete. Sustainability targets are often set early, sometimes before methodologies are confirmed and without full consideration of construction realities. As designs progress, the practical implications become clearer, making low-carbon outcomes harder to achieve.

Lower‑carbon mixes can introduce challenges related to durability, fire resistance, curing times and structural performance. To address these potential challenges in lower-carbon mixes, high levels of supplementary cementitious material (SCM) such as fly ash are specified, particularly in marine or water-adjacent infrastructure projects. In these contexts, SCM content can exceed 60 per cent, delivering performance benefits alongside carbon reduction when supported by appropriate testing and specification. Concrete mixes with high SCM content have also been used in Australia for decades and are embedded in standards and specifications, so lower-carbon mixes have a precedent to follow that provides long-term durability improvements and asset management opportunities.

Precast concrete adds further complexity. Its application often depends on rapid strength gain and early demoulding, which can conflict with some lower‑carbon formulations. However new advances in concrete technology, including modern admixtures, and highly reactive SCMs such as calcined clay, are shifting this dialogue. While reducing embodied carbon in precast elements remains a challenge, the solution is not far away.

Projects such as Central Station Metro illustrate how these tensions emerge in practice and how we resolve them through design and delivery. The station upgrades combine heritage constraints, operational rail interfaces and modern performance expectations. In this context, low‑carbon outcomes rely on careful specification and realistic alignment with construction sequencing.

Our role on major rail programs often involves helping teams navigate these constraints. We interpret standards, assess what is achievable under current conditions and support informed departures where they are technically justified. We engage with the supply chain early, working with manufacturers and suppliers to test options, understand constraints and build confidence before decisions are locked in. We are also clear about limitations. Over‑promising on carbon reductions without considering constructability and long-term performance introduces risk rather than value.

Standards play a critical role in setting minimum performance expectations, but they also shape how innovation is adopted. For rail projects, compliance with structural performance standards and embodied carbon requirements cannot be treated as an afterthought; it must be embedded in specification and design from the outset.

Understanding how standards apply in different contexts supports better decisions. It helps project teams to distinguish between requirements that are fixed and those that allow flexibility based on evidence. This distinction is particularly important where sustainability aspirations push against established testing regimes or prescriptive limits.

Research and innovation have a role to play, but how they are framed matters. Low‑carbon concrete should not be positioned as a finished solution, and the ultimate goal of fully decarbonising concrete is still a long way to go. Applied research, testing and learning over time are more credible and more useful. Collaboration with academic institutions and industry partners is helping to explore alternative materials, including recycled aggregates and manufactured sand substitutes, while building confidence through data rather than claims.

Large transport programs create space for this approach. Their scale allows incremental improvement, feedback loops and refinement over time. Perth Airport’s Terminal 2 apron expansion demonstrates how sustainability outcomes improve when materials strategies evolve alongside delivery rather than being fixed at the outset. 

Across rail and road interfaces, including work on the Tonkin Gap project, we have seen the value of positioning materials advice as an enabler of decision‑making. The focus remains on balancing carbon reduction with safety, durability and compliance, supported by evidence rather than rhetoric.

There is no single approach to low‑carbon concrete in large‑scale rail projects. Each asset is shaped by its environment, construction method and operational demands. Applying universal solutions risks overlooking these realities.

Progress comes from bespoke thinking. We need to understand regulatory intent, engage honestly with supply‑chain constraints and treat sustainability as a series of informed engineering choices rather than fixed targets. Low‑carbon concrete is part of a broader system, not a standalone material decision.

For the rail sector, the most resilient outcomes emerge when ambition is matched with achievability and when materials strategies can evolve. At GHD, we have a proven track record of providing practical insight, standards interpretation and long‑term commitment to deliver major infrastructure projects that implement innovative decarbonisation methods for sustainable development.