Guidance that balances the desire to specify concrete with low environmental impact, whilst ensuring other performance parameters are optimised, can be summarised as follows:
Do not over-specify strength.
Consider the possibility of strength conformity at 56 days rather than the conventional 28 days.
Specify responsibly-sourced concrete and reinforcement.
Do not specify aggregate sizes below 10mm unless necessary.
Permit the use of recycled or secondary aggregates but do not over specify.
Specify that concrete should contain CEM II/CEM III or an addition.
Embodied CO2 (ECO2) of concrete should not be considered or specified in isolation of other factors such as strength gain.
Permit the use of admixtures.
Specify BES 6001 responsibly-sourced concrete and reinforcement to gain maximum credits under BREEAM and the Code for Sustainable Homes.
The specification of recycled and secondary aggregates is often not the most sustainable option, although it may gain most points. BS 8500 allows producers to use up to 20% of recycled aggregates in many concretes, they do this when it is available.
The BRE Green Guide does not recognise the availability or otherwise of recycled product when incentivising the use of recycled content. Recycled aggregates should only be specified when they are locally available, otherwise transportation impacts exceed the intended benefits. Within the current assessment method, this should be discussed with the client or project code assessor to prevent unfair penalisation.
Use of cementitious additions can reduce the embodied CO2 (ECO2) of concrete and influence its visual appearance. When aesthetics are critical, specify the cement/combination to ensure colour consistency.
Admixtures can be used to enhance sustainability credentials and reduce the ECO2 of concrete, as well as modifying its physical properties.