In September 2020, the BPDA published a new report which demonstrates why the carbon footprint of concrete pipeline products is significantly lower than HDPE alternatives. In this blog, we explain why the service life of construction products is such an important element in carbon footprint assessments and comparison studies. We also demonstrate how it has affected the ‘Number of replacements’ in our carbon study.
To understand why products’ and assets’ lifespans can make a significant difference in LCAs it is important to find out, first, how the main construction environmental assessment standards (such as EN 15978 and EN 15804) look at product lifespans and why plastic pipes are specified to have a 50-60 years Estimated Service Life (ESL) while concrete pipes’ ESL is considered as 100-120 years. We explain this point and then explain how EN 15978 uses ESL to assess the number of product replacement across the lifecycle of a built asset (e.g. pipeline).
Reference Service Life (Design Life) vs. likely maximum lifespan
In technical specifications and literature, there are several definitions that describe different periods within the lifespan of an asset in service. Two of the main lifespan concepts in LCA and carbon footprinting literature are ‘Design Life’ and ‘Service Life’. The main construction environmental assessment standard, EN 15978, defines ‘Design Life’ as the lifespan intended by the designer. It is the lifespan identified in product standards and the lifespan against which construction products are tested and approved. This is different to the likely maximum lifespan, which is usually difficult to verify and may need to be “guestimated”. For example, a laptop may be designed to last for exactly 4 years, but it may typically serve for 4-6 years before it is no longer able to function due to extensive wear & tear. However, there are no guarantees that every single laptop will survive beyond 4 years as the units are only designed and tested to serve for 4 years.
In LCA assessments based on EN 15978, the construction products’ lifespan is defined as ‘Reference Service Life’ (RSL) or ‘Estimated Service Life’ (ESL). Despite its name, RSL/ ESL is mainly determined by the ‘Design Life’ of a construction product as defined in its standard. The standards still allow for a maximum lifespan to be used, but this can only be employed if sufficient proof exists (e.g. proof of multiple identical product units which survived and lasted for X years). RSL/ ESL can also be obtained by the use of a methodology identified in the ISO 15686 “Building and constructed assets — Service life planning” series of standards. But as the use of ISO 15686 requires extensive research and the use of data not usually available, almost all products’ RSL/ ESL is determined by the use of ‘Design Life’ as stated in European product standards and Eurocodes.
What is the RSL/ ESL of concrete and plastic pipes?
The RSL/ ESL of British concrete pipes is as defined in BS 8500 supported by specific provisions within Special Digest 1 (SD-1). This has always been 100+ years. As the Eurocodes National Annex aligns the 100+ intended working life with Highway standards’ 120 years, concrete pipes (as concrete structures) use an RSL/ ESL of 120 years. There is already proof that concrete pipes can survive for decades after 120 years as many 19 th Century concrete pipes in cities such as New York, London and Malmo are still serving in excellent or good condition today. However, the ‘Design Life’ of concrete pipes remains only as 120 years.
The RSL/ ESL of plastic pipes is as defined in a series of standards such as ISO 9080 or EN 13598. In our Whole-Life Carbon study, we looked at different RSL claims by the plastic pipe sector. One report by TEPPFA claims that 100 years is a viable lifetime for SN4 (4kN/m 2 ) and SN8 (8kN/m 2 ) plastic pipes. We were unable to use that report as it did not meet ISO 15686 requirements, has not been verified by any parties outside the plastic pipe industry (including CEN/ ISO), excludes SN2 pipes and interprets excessive deflection levels (8-15%) as acceptable under ‘Design Life’. British Precast had to stick with the ‘Design Life’ employed and used for testing in numerous ISO and CEN standards for HDPE pipeline products, which is 50 years.
How the number of product replacements is specified in EN 15978
In a pipeline assessment, the intended lifespan of the overall asset should be at least 120 years, as specified in the Eurocodes UK National Annex for “Category 5” structures and in PAS 2080. EN 15978 refers to such lifespan as Required Service Life (ReqSL)
If any product Design Life/ RSL/ ESL fails to meet the overall asset lifespan of 120 years, then this product is likely to be replaced at least once. Clause 9.3.3 of EN 15978 states that the number of replacements should be calculated as the rounded-up number from this equation:
= (ReqSL/ ESL) – 1
The use of this equation would suggest 1 to 2 replacements for an HDPE pipe. A single replacement was employed as a ‘50 year’ design life in European/ ISO standards is likely to be aligned with ‘60 years’ in UK Highway Specifications.
How the Whole-life carbon footprint of plastic vs concrete pipes will look like
Based on EN 15978, the embodied carbon of a replaceable HDPE pipe will double at Lifecycle Module B4 as an old 1m of pipe unit within the overall asset is decommissioned and replaced with a new 1m of pipe. The new unit is likely to have a different carbon footprint as it will be replaced 50 years into the future. But EN 15978 requires that only present-day technologies/ practices are used as reading into the future can lead to overestimation/ underestimation of the impacts of future technology.
The final result of a comparison based on EN 15978 will show impacts for plastic pipes more than double of that for concrete pipes: 50% of the impacts for plastic pipes, as shown in the Figure below, are associated with the replacement of a plastic pipe as specified in EN 15978. Readers should also keep in mind that the intended working life of the pipeline asset may not only be 120 years, it could be up to 800 years (as suggested in HM Government’s ‘Water for Life’ report, section 4.25). In such case, both concrete and plastic pipe units will need to be replaced multiple times, making it even more viable to consider products’ ‘design life’ and ESL as a viable element in pipeline assets carbon assessments.
Figure 1. Concrete Pipes vs HDPE Plastic Pipes Cradle-to-Grave GWP comparison – DN2100 pipe (Assuming 50 year plastic pipe design life).