Inclusion of carbonation during the life cycle of built and recycled concrete: influence on their carbon footprint

When the service life (or primary life) of built concrete infrastructure has elapsed, a common practice is that the demolished concrete is crushed and recycled, then incorporated into new construction. LCA studies of CO₂ emissions focus on the manufacturing and construction and occupancy/utilization phases, without consideration of the demolition and application of recycled concrete into a secondary construction application. Concrete has a documented ability to chemically react with airborne carbon dioxide (CO₂); however, carbon capture (or carbonation) by concrete during the primary and secondary life, is not considered in LCA models. This paper incorporates CO₂ capture during both primary and secondary life into an LCA model for built concrete.

CO₂ equivalent (CO₂-e) emissions were estimated by calculation of the quantity of CO₂-e emitted per unit of activity at the point of emission release (i.e., fuel use, energy use, manufacturing activity, construction activity, on-site demolition, etc.). Carbonation was estimated for built concrete during the primary life and also during the secondary life when the demolished concrete structure is crushed and recycled for a new application within a new built structure. Life cycle calculations for a built bridge structure are provided which contrasts the net effects of CO₂ emission and capture. The study has analyzed a concrete bridge with primary life of 100 years. Following completion of the primary life, we have considered that the demolished concrete from the bridge will be crushed, recycled, and used in the construction of a replacement bridge. Due to damage caused by demolition and crushing, the quality of the recycled concrete is unlikely to be as high as quarried natural rock, and the recycled concrete is most likely to be used in a more temporary construction application with an assumed 30-year secondary life. Following the expiry of the secondary life, if recycling of recycled concrete aggregate (RCA) was to be undertaken, it is unlikely the quality of RCA will be suitable to enable a third construction application: therefore, our LCA includes the primary life of 100 years plus the secondary life of 30 years.

Carbonation of the built concrete during the primary life is almost negligible compared with the emissions arising from manufacture of raw materials, concrete production, and construction. However, CO₂ capture by recycled concrete during the secondary life is considerably greater: a factor that is not included in LCA estimates of the carbon footprint of built concrete. Crushed concrete has considerably greater exposed surface area, relative to volume, than a built concrete structure: therefore, a greater surface area of RCA, compared with a built structure, is exposed to CO₂ and carbonates. This key factor leads to such high amounts of carbonation during the secondary life when compared with the built structure.

While reducing the amount of solid landfill, recycled concrete provides significant capture of airborne carbon dioxide. The effects of carbon capture of recycled concrete aggregate within the secondary life is significant, imbibing up to 41% of the CO₂ emitted during manufacture of a 100% Portland cement binder. Emission estimates can be overestimated by as much as 13–48%, depending on the type of cementitious binder in the built concrete and the application of recycled concrete during the secondary life. The effects of carbon capture of recycled concrete aggregate within the secondary life are significant and should be included in LCA calculations for reinforced concrete structures.

Carbonation during the secondary life can be affected by the type of application and exposure of the RCA: gravel with fine particle size will carbonate more comprehensively than larger boulders (due to greater exposed surface area), while air-exposed RCA will carbonate more comprehensively than RCA located in a buried and moist environment. An approach is provided in this paper that incorporates carbon capture by concrete infrastructure into LCA carbon emission calculations. If carbonation is ignored, emission estimates can be overestimated by 13–48%, depending on the type of cement binder and the application of RCA during the secondary life.

Due to the considerable number and types of built concrete structures, it is recommended that the LCA model be improved by incorporation of carbonation data obtained from recycled concretes that have been applied to a variety of construction applications and exposed to a range of exposure environments.