Five facts that make the carbon footprint of concrete attenuation tanks lower than Geocellular alternatives
In this blog, we highlight five elements that make all the difference in such underground tank comparisons:
1. Getting the numbers right
Getting the right calculation methodology is crucial. In Whole-life Carbon Assessments there is need for a robust methodology with sound scenarios for each option. Sourcing of right data is also important: The ICE Database is recognised by many in the construction industry as one of the most credible sources of construction-related carbon footprint data. But we need to bear in mind that carbon footprints change constantly and should be subject to continuous checking and scrutiny. This study uses the ICE Database (v.3) numbers as that version is still very recent. Using very simple math:
The concrete pipe tank's mass is around 22 times heavier than an equivalent Geocellular tank. But the carbon footprint of a tonne of mould-injected polypropylene, the material used in that specific type of crate tank, is almost 31 times higher than the carbon footprint of a tonne of concrete pipe. We also ensured that realistic assumptions were developed for products' transport to site, construction on site and End-of-Life. In a few years' time there will be a need for more up-to-date data, and this can only be achieved using recent and valid construction products' Environmental Product Declarations (EPDs) such as this one from the BPDA.
2. Functional Unit & Durability
Bear in mind that the comparison is not simply between two tanks. It is mainly between the services offered by these two tanks: which is the storage and attenuation of a certain amount of stormwater (300 m3) over a specific period of time (+100 years).
If the storage capacity deteriorates over time due to the accumulation of silt, then this needs to reflect on numbers. If a tank has a design life of 50 or 60 years only, then at least one replacement will need to be accounted for (as described in EN 15978) to fulfil the requirement of a +100 years' service life.
3. Use realistic "Transport to Site" and "site installation" scenarios
Development of scenarios for different lifecycle stages is one of the most challenging aspects in Whole Life carbon assessments as any inaccuracies in assumptions can lead to significant over or under-estimation of carbon emissions.
In the transport to site scenarios, instead of using transport factors (such as 0.1065 kg CO2 e/ tonne.km) designed for GHG Protocol calculations, we opted for a different system based on what we know about deliveries and the maximum capacity for trucks used in delivery. Our calculations show that the plastic tank will likely need two deliveries only (at half laden), while the DN2100 concrete pipes will need 12 deliveries at "full laden" truck impact status.
We also avoided the use of any factors for construction activities and carbon emissions. We instead assumed that all plastic tank assembly is manual (carbon free) and calculated the impacts of concrete pipe installation based on the time consumed by a JCB JZ 141 excavator (around 11½ hours excluding idle-time and break-time). The time for excavation and backfilling was assumed to be similar for both systems.
4. Concrete which is unlike any other concrete
A fact often overlooked is that concrete pipes is made of a low carbon type of cement. The cement blend used with concrete pipes, CEM II B-V, uses a 30-35% fly ash content. Over the last few years, we also saw a rise in the use of CEM III blends (which includes GGBS). According to the ICE Database, the carbon footprint of one tonne concrete pipe is around 146 kg CO2 e/t. This number is dropping every year and is likely to reach around 133.4 kg CO2 e/t later this year. Moreover, the carbon footprint of a concrete pipe reaches its peak at the factory gate and then continues to drop every year due to carbonation. Concrete absorbs CO2 through a chemical process associated with the CaO content within its mix. Based on our 3rd party verified EPD, up to 7% of the carbon footprint is likely to be negated by carbonation and the absorption of atmospheric CO2.
In the next 10 years we will see a significant drop in that carbon footprint as more cement manufacturers invest in Carbon Capture & Storage (CCS) and fuel switching technologies. The significant drop in the electricity grid carbon intensity is also likely to have a significant impact on UK concrete pipe factories in the next 10 years which is likely to become very evident.
5. Don't forget the End-of-Life stage
We tend to focus a lot on upfront carbon and forget about the fate of a pipeline at the End-of-Life. At the End-of-Life, a sewerage or drainage pipeline will need to be decommissioned and either exhumed or filled with foam concrete with a very lean mix. In urban areas where the pipes are partially or fully removed, removed concrete pipes can be reused again or crushed and reused as Recycled Concrete Aggregates (RCA). Crushed concrete can absorb vast amounts of CO2, significantly reducing the overall Cradle-to-Grave carbon footprint. The removed plastic crates are either incinerated, releasing anything between 1,000 to 1,500 kg of CO2 per tonne of polypropylene, or recycled and reused in a lower applications.
Published on 29th November 2021