Carbon optimisation through material selection: where to start

Work to cut a building's embodied carbon usually starts in the wrong place: teams set out to find a greener alternative for every material. The distribution, though, is extremely uneven. In a typical building, ten per cent of material lines account for eighty per cent of embodied carbon.

Hotspot analysis first

Before looking for any alternative, work out how the current design's embodied carbon distributes across elements. In almost any concrete-framed building the answer is similar: structure and foundations are more than half the total, with concrete and reinforcing steel taking most of that share.

That analysis decides where effort goes. Hunting alternatives for lines that are small in mass and low in impact consumes design time without a measurable gain.

The five decisions that move the number

• Retain the existing structure — refurbishing instead of demolishing typically saves more than fifty per cent
• Optimise the structure — a more efficient frame directly reduces material quantity
• Lower the clinker factor in cement — substitution with fly ash, slag or calcined clay
• Use recycled reinforcement — EAF-route steel carries about a third of the integrated route's emissions
• Extend the life of short-lived components — this cuts the B4 replacement burden directly

Concrete: the single largest line

Concrete is the largest component of a building by mass, and although its carbon intensity is low, that mass makes it the dominant line. The good news is that the abatement levers in concrete are mature and accessible: clinker substitution, specifying the strength class to the real requirement, and using later-strength-gaining mixes with extended curing.

The most frequently missed opportunity in practice is over-specification. Writing C40/50 into the specification for margin when C30/37 is structurally sufficient means unnecessary cement. That is a design decision, not something the material supplier can solve.

Care in material substitution

When substituting one material for another, functional equivalence must be preserved. A lower-carbon but shorter-lived product can be worse on a whole-life basis. Equally, a change that lowers GWP can raise another impact category.