Concrete is one of the oldest and widely used of construction materials and possesses many inherent qualities which can be used to benefit the client, designer and contractor.
Concrete can be manufactured to an inexhaustible range of specifications to suit all applications. This is possible by using different proportions of the constituent materials to create durable concrete while minimising the whole-life carbon content.
Building in concrete provides an opportunity to provide an extraordinary range of surface finishes, whether an impression from the form-face material or is applied after it has hardened and struck. This creates the opportunity for architectural expression to go hand in hand with structural integrity.
Concrete can be moulded into any shape by using appropriate formwork. This capability can be used to provide bespoke design solutions to specific problems and aesthetically pleasing structural forms.
Unlike other materials or coatings to materials, concrete cannot be set on fire and does not emit toxic fumes. Importantly it does not smoke or drip molten particles or add to the fire load. As with all materials, concrete does lose strength in an intense fire but its low thermal conductivity makes concrete is naturally and inherently fire resistant and needs no additional application of fire protection.
Well designed, detailed and correctly concrete offers exceptional durability and long life in any structure. Concrete structures built over 100 years ago, indeed as long ago as the Romans, are still in active service today.
Concrete consists of naturally occurring materials, produces no emissions and needs no toxic preservatives. It is recognised that whole life embodied carbon is a major concern, the main source of concrete's carbon coming from the cement. Although it is important to reduce the contribution from the cement in particular, the whole construction process of a structure, be it steel, concrete, masonry or timber, and its operating and maintenance need to be considered over its lifetime. In this respect concrete bridges are inherently robust with current innovation in design and materials striving towards minimising the whole-life carbon content.
Abridged from How to specify low-carbon concrete now, Concrete, vol 55, issue 2,
Concrete Society, Camberley, March 2021 p31-32
Concrete is made from aggregate, cement and water and typically includes an admixture to modify the rheology. Aggregates and water have very low embodied carbon and where locally sourced primary aggregates this is about 4kgCO2/tonne. It is the cement, forming about 10–15% of the mix, which holds most of the embodied carbon.
The UK Concrete and Cement industry aim to provide net zero concrete by 2050. The industry has already made considerable advances due to investment in fuel switching, changes in product formulation and energy efficiency, including plant rationalisation. Since 1990, its direct and indirect emissions have reduced by 53%.
All concretes to BS 8500 Concrete - Complementary British Standard to BS EN 206 are based on Portland cement, or CEM I, but most contain secondary cementitious materials (SCMs) or additions, such as GGBS, fly ash, silica fume, limestone powder and pozzolana. These SCMs have a much lower embodied carbon than CEM I and can make significant savings to the embodied carbon of concrete (see Table 1).
Table 1 - Embodied CO2 of UK concretes (based on a cement content of 320kg/m3 of concrete)
|Broad designation of
cement type in concrete
|Percentage of addition||Embodied CO2
kgCO2/m3 of concrete
|IVB||36-80% fly ash or pozzolana||130-188|
BS 8500 allows ternary blends of cements which use CEM I with two types of additions, normally
limestone fines with either fly ash or GGBS. All these cements are based on CEM I, but there are
also geopolymers or alkali-activated cementitious materials (AACMs) that can be specified using
PAS 8820 Construction materials. Alkali-activated cementitious material and concrete.
Specification, a publicly available specification produced by British Standards Institution.
These are normally based on GGBS which is activated by a chemical rather than Portland cement.
Higher proportions of additions slow the strength gain of the concrete. Where the concrete does not need to be struck quickly or support load shortly after being cast, the construction programme may not be excessively extended. For example, foundations are frequently cast against the ground and the load is applied only slowly as the project progresses. Although the concrete strength is specified at 28 days, a concrete with a high proportion of SCM (e.g IIIB) will still be gaining strength after 28 days and the strength at 56 days may be 40% greater. The designer could take advantage of this by specifying a 56-day or even 90-day strength.
Elements that need to have a faster strength gain, such as suspended slabs or post-tensioned elements, can still use SCM additions. Several projects have used IIIB for a post-tensioned suspended slabs, the setting time improved using set accelerating admixtures.
By designing structurally efficient sections, such as rib or voided slabs, the volume of concrete is reduced and hence the embodied carbon. Higher concrete strengths can also reduce the embodied carbon of a structure, as less concrete is required despite higher cement contents. Superplasticiser admixtures can also help reduce the embodied carbon by reducing the water: cement ratio and hence providing a stronger concrete without increasing cementitious content.
In summary, the embodied carbon of concrete can be lowered by: