Embodied carbon gets raised on most projects eventually. The problem is timing. By the time it reaches the estimating desk, the structural design is often already fixed, and the only thing left to do with the number is report it, not improve it.
When this actually belongs in the process
Embodied carbon for a concrete substructure is a design-stage question, not a post-tender reporting exercise. Once the mix, the reinforcement schedule and the structural form are locked, what's left is measurement, not reduction. Raising it during pre-construction review or concept design, while the structural engineer can still change the specification without a redesign, is the only point where a bid can genuinely compete on the number rather than just disclose it.
By the time embodied carbon reaches the estimating desk as a line item, the decision that would have moved it has usually already been made.
The lever that actually moves the number
Cement replacement is the single biggest lever available without a structural redesign. Ground granulated blast-furnace slag, GGBS, or pulverised fuel ash, PFA, substituted for a portion of the Portland cement in a mix reduces the embodied carbon of that concrete meaningfully, since cement production is the dominant carbon source in a standard mix, not the aggregate or the water. A 30 percent GGBS replacement is a commonly specified starting point for a substructure mix, higher replacement rates are achievable where the structural engineer confirms the strength and curing time requirements still hold, since higher GGBS content typically slows early strength gain, which can matter on a tight programme.
This is worth raising as a genuine design conversation with the structural engineer, not a box to tick after the fact, since the achievable replacement rate depends on the specific mix design, exposure class and programme, not a single number that applies everywhere.
What to actually ask your concrete supplier for
- An Environmental Product Declaration, EPD, for the specific mix being quoted, not a generic industry average figure
- The cement replacement percentage already built into their standard mixes, and what a higher replacement rate would cost and what it would do to strength gain timescales
- Confirmation of which modules, A1 to A3 typically, their published figure actually covers, since a number without a stated boundary isn't comparable to anything