Over the last few years, there have been lots of talk about the embodied carbon of assets used in wastewater and drainage infrastructures. However, for products such as concrete pipes, a cradle-to-site carbon footprint should not be treated as a fixed value, as that footprint gets lower over time and can drop by as much as 7.2% at its end-of-life due to carbonation.
The last few years saw a significant rise in the development and use of calculators to estimate Greenhouse Gas emissions (GHG) associated with the production of infrastructure assets and facilities. There has been an increased scrutiny over the role of ‘Embodied Carbon’ (also known as Capital Carbon) and how much it may contribute to the overall footprint of a civil engineering project. A number of asset operators have incorporated generic embodied carbon values within their own calculators using a wide range of sources such as the ICE Database or One Click LCA. There might be a good understanding about the Cradle-to-Gate impacts of concrete. However, very few calculators capture the effects of carbonation and how it may reduce the carbon footprint of concrete at later stages of the lifecycle.
What is carbonation?
What many in the industry do not realise is the fact that a significant proportion of the cradle-to-site carbon footprint of concrete drainage products is associated with temporary CO2 emissions which are absorbed back by the products at later stages of the lifecycle. This process is known as ‘carbonation’. Concrete carbonation is basically a chemical reaction between atmospheric CO2 and the alkaline components of hardened concrete (CaO) to form Calcium Carbonates (CaCO3). This process can occur in virtually all concrete elements made of Portland cement. It basically reverses another chemical reaction in cement production, known as ‘calcination’, which results in the release of CO2 into the atmosphere. Calcination contributes 60-70% of the carbon footprint of Portland cement.
It should be noted that carbonation depends on a number of factors, such as the length and nature of concrete exposure to the atmosphere (fully exposed, submerged in water, buried, etc.), the type of concrete, its total surface area and its cement/ clinker content, and a number other factors. Carbonation can have a negative impact on reinforcement, this is why concrete products are designed with sufficient cover for the reinforcement to allow for surface carbonation without any negative impacts on the structural integrity of the products. The process of total carbonation takes a significantly long time which may extend to tens, hundreds or even thousands of years.
Is it an accepted scientific fact?
There is already substantial evidence that carbonation contributes to the reduction of the carbon footprint of concrete products. In 2019, European standard CEN TR 17310 was published to help the construction industry understand carbonation and the range of factors that may affect it. The current Product Category Rules (PCR) for the carbon footprint of concrete, EN 16757, includes a detailed methodology to calculate CO2 removed via carbonation at different stages of the lifecycle. Carbonation also occurs in concrete structures buried underground or submerged under water.
Is it significant?
The contribution of overall carbonation is significant. It is estimated that between 1930 and 2013, a cumulative amount of 4.5 Gigatonnes of carbon was sequestered worldwide through carbonation, offsetting 43% of carbon emissions from the production of cement over that period (Xi, Davis, Ciais, et al, 2016).
How does it affect the final carbon footprint of a concrete drainage product?
For every 1 metre of DN600 concrete pipe, a reduction of 3% to the carbon footprint is expected due to carbonation during the pipe’s 100 years lifetime. If that concrete pipe is crushed at the end of life, the likely scenario (developed in collaboration with NFDC and The Concrete Centre) is for the crushed concrete to be recycled and then stored on site for a period of 5 weeks prior to removal for another application. During that period a further 5-6% reduction to the carbon footprint is expected. But due to the impacts associated with demolition, it is expected that a benefit of 4.2% reduction only to the footprint will arise, offering a total of 7.2% reduction to the carbon footprint.
Note: Carbonation will differ significantly based on the cement mix. Cement mixes with increased volumes of low carbon cementitious materials (e.g. exceeding 30% of cement content) can experience higher levels of carbonation.
Figure 1. Graph demonstrating reductions to the carbon footprint of a metre of DN600 concrete pipe over the lifecycle (2016 data).