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Posted by & filed under Pipes & Manholes.

One thing often misunderstood in drainage pipeline bedding design, is the wide range of bedding options available for specifiers. Without proper guidance and assessment of available options, the likelihood of missing out on the most cost-effective ones is always high.

Structural design of pipeline installations involves dealing with three main types of load: The weight of the trench-fill, loads transmitting to the pipe from the surface such as traffic, and the supporting reaction from the ground under the pipe. Loads imposed by wastewater/ stormwater in the pipe may also be considered for larger diameter pipes. Pipe beddings play a major part in supporting the pipeline in dealing with these loads and are an integral part of a pipeline’s structure. A number of pipe bedding solutions are available. These are described in detail in BS 9295, and other industry standards, and are usually identified as “bedding classes”.

Class S bedding is the most extreme option, structurally, as the pipe is surrounded fully by the granular material. In such option, a significant proportion of the structural strength is derived from the embedment either side of the pipeline, protecting the pipe from the superimposed loads. Other pipeline Bedding Classes, such as Classes B, F and N require significantly less material and would rely more on the pipe’s inherent strength.

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Figure 1. Bedding classes used in pipeline structural design.

Due to their lack of inherent strength, flexible pipe installations depend heavily on a high-quality installation where the surrounding embedment takes most of the loading. The options for bedding are usually limited and the installation will more likely need a Class S full surround bedding with a high degree of compaction and sometimes some level of site supervision to ensure that the installation is carried out precisely to the engineer’s specification.

The use of Class S full granular bedding will lead to more material cost and more vehicle movements to deliver the granular to site. More soil will need to be removed from site and sent to landfill or off-site use. A full surround will also need more time laying and compacting the granular around and on top of the pipeline. This can add to the overall cost significantly. For many pipe bedding installations where stronger pipe material is used, less bedding support may be needed, and the pipeline bedding may need significantly less granular material.

Specifying Class S as a default, without knowing, understanding or considering the other options, is a very common problem in pipeline design. In many cases we have seen in the past, Class S was specified for concrete pipeline installations that would have worked perfectly well with a Bedding Class B or even Classes N & F. To help make these options more transparent and easier to detect, the British Precast Drainage Association (BPDA) developed a web-based Structural Design Calculator tool to help designers and specifiers identify the range of Bedding Classes available for their projects. BPDA also developed another tool to help specifiers understand the costs associated with bedding choices. The Material Cost calculator offers alternative costs for both a concrete and flexible pipe option based on pipe type and diameter, imported bedding costs and the cost of dug soil disposal to landfill.

The BPDA Structural Design Calculator can be accessed here: https://www.precastdrainage.co.uk/page/structural-design

The Material Cost Calculator is available at this link: https://www.precastdrainage.co.uk/calculators/material-cost

Both calculators are available via a mobile phone app through Apple, Android and Amazon platforms: https://www.precastdrainage.co.uk/calculator-app

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The new British standard for concrete pipes, BS 5911-1, offers further type testing requirements and cement mix options to improve pipe performance in a world shaped by fatbergs, Net Zero Carbon, and infrastructures durability challenges.

The British Standards Institute (BSI) published a new revision of BS 5911-1:2021, the concrete pipes specification standard, earlier this month. BS 5911 has been revised and updated numerous times since it was first introduced in the early 1980s. However, this revision is different as it comes at a challenging for society and its infrastructure. In this blog we highlight three issues which have shaped changes and additions to the new version of BS 5911: These are Net Zero Carbon, Durability, and resilience. We explore how the standard in its new format can help sewerage infrastructure address these issues.

Net Zero Carbon

We are in a race against time to reduce carbon emissions and limit global warming to no more than 1.5° C. It is a major challenge in which every industry needs to play its part. National Highways (previously known as Highways England), for example, recently launched a “Net Zero Highways” plan in which the agency targets the use of Net Zero construction products and materials by 2040. Such ambitious targets would require significant change in how products such as concrete pipes are specified. The new BS 5911-1:2021 will allow the concrete drainage sector to reduce embodied carbon emissions significantly as the “Cementitious Content” section of the standard has been revised and can now enable the use of a significantly wider variety of low carbon cements, including pozzolanic cements and a range of ternary cements with limestone fines content.

Resilience

According to Water UK, water companies deal with up to 300,000 sewer blockages annually, costing £100 million every year to clear them. Fatberg and blockage clearance operations usually involve water jetting with high-pressures that might range from 3,000 to 4,000 psi. With the rise of the fatberg problem, it was necessary to introduce a mandatory jetting resistance test into BS 5911-1. Concrete drainage products will need to be tested using a stationary water jet at 28 Mpa (around 4,060 psi) pressure for a period of 3 minutes.

Durability

Another key addition to the new BS 5911-1 is the addition of clear text which demonstrates how an exposure class DC-4 concrete mix, the main chemical design exposure class used by all concrete pipe manufacturers in the UK, and surface carbonation offers the necessary concrete durability to meet the requirements for a 100 years “Intended Working Life”.

Specification for a “100 years” life has always been possible as BS 5911-1 concrete pipes are already specified to requirements within Special Digest 1 “Concrete in Aggressive Grounds” and BS 8500. However, an explicit reference to such Intended Working Life should offer further assurance to specifiers and asset managers.

The new BS 5911-1: 2021 is now available at the BSI website. For any further information on the new standard please contact email@precastdrainage.co.uk

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In late 2019, BPDA carried out a number of high-pressure water jetting tests for a wide range of gravity pipe brands available in the UK market, including both concrete and lightweight plastic pipes. In this blog, we explore how significant are the findings from these tests and what do the results tell us about both concrete pipes and their lightweight alternatives.

In November 2019, BPDA carried out a number of jetting resistance tests on eight pipe samples using a 280 BAR pressure stationary water jet (equivalent to 4,000 psi). The two concrete pipe samples we tested both passed successfully. However, all lightweight pipe samples, made of different types of plastic, failed the testing criteria. Most of the failed pipe samples experienced internal layer piercing within 3 to 5 seconds only.

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The test results come as no surprise. Four of the lightweight flexible pipes tested are manufactured to plastic pipe specification WIS 0-35-01 with a maximum water jet pressure requirement of 180 bar (2,600 psi) only. It is the same water jetting pressure limit specified in the Manual of Drain & Sewer Cleaning for plastic pipes (see Table). The problem was mainly associated with the time it took such pipes to fail:

Every year, Water Companies deal with 300,000 sewer blockages across Britain. High-pressure water jetting is the usual method used by cleaning contractors to deal with sewer blockages. Cleaning contractors can sometimes exceed 2,600 psi to deal with such blockages. However, the general advice currently is that cleaning contractors should not leave the water jet in one position for more than 60 seconds. It is unclear if such advice was offered with knowledge that some plastic pipes can experience damage after only 3 to 5 seconds of high-pressure jetting! The speed in which some of these samples were fully penetrated should make cleaning contractors and asset owners significantly worried.

After introduction of the new sewers’ adoption code, known as the Design & Construction Guidance (DCG), water companies can no longer introduce additional restrictions and requirements on lightweight pipes such as resistance to 4,000 psi water jetting. This may mean that the only way to protect our sewerage network is by the introduction of new higher limits for pipes’ jetting resistance. Even if a lightweight pipe cannot withstand high-pressure jetting for the entire testing period (which is 3 minutes), it should at least be resilient enough to withstand 60 or 90 seconds under such level of water jetting pressure.

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In September 2020, the BPDA published a new report which demonstrated why the carbon footprint of concrete pipeline products is significantly lower than HDPE alternatives. However, that study did not look at HDPE pipes made of recycled HDPE. In this very lengthy blog, we raise questions about the likely carbon footprint of pipes made of recycled HDPE and explain why 100% recycled content would not necessarily mean a carbon footprint of ZERO kg CO2e.

Understanding recycled HDPE

Recycled plastic resin has been in use in plastic pipes production since the 2000s. In 2018, the main standard for plastic gravity sewer pipes, EN 13476, was revised to allow for more use of some non-virgin plastics, including polyethylene. However, EN 13476 still have restrictions on the use of recycled polyethylene sourced from outside the plastic pipeline industry. Exemptions only include processed plastic from rotational moulding processes with an agreed specification. Even for such exempt products, there is a 5% limit to recycled content. This explains why some HDPE pipeline products with 100% recycled content in the UK do not refer to EN 13476 in their specification.

Sources of recycled HDPE pellets/ resin

There is very little information about the current source of recycled HDPE pellets for pipes manufactured in the UK. It is understood that sources are likely to include local waste handlers/ recyclers such as Biffa and Veolia. However, as the UK is a net importer of recycled HDPE resin, it is reasonable to assume that some (if not most) recycled HDPE is sourced from international suppliers in Europe and Asia.

A third viable source is manufacturers’ own processing waste and off-cuts or waste and off- cuts sourced from manufacturers of similar products.

Would the carbon footprint of recycled HDPE pipes be significantly lower than concrete pipes or virgin HDPE equivalents?

There is every reason for a 100% recycled content pipe to have a significantly low carbon footprint on a “kg of CO 2 e per tonne” basis. A study by the European Commission’s JRC found significant reductions in using Recycled HDPE, reaching 28% for some products. But environmental impact is assessed based on a functional unit, not tonnes of product. Reductions in the embodied carbon of recycled content HDPE pipe seem to come at the expense of other characteristics such as the pipe’s likely stiffness class, weight per linear metre, bedding requirements and expected ‘Design Life’:

  • The Design Life is unlikely to exceed 50 years due to lack of any standard or specification support. This will directly affect the number of replacements throughout an asset service life if the rules in EN 15978, clause 9.3.3, are followed.
  • The amount of bedding granular is unlikely to be anything less than a Class S (full surround). This will add significant impact as the embodied carbon of granular, and transport to site for that granular aggregate, will need to be added.
  • A higher stiffness class, such as SN6 or SN8 means that the weight of product per linear metre is likely to be significantly higher than a lighter SN2 pipe, adding 20-30% to the embodied carbon of the recycled HDPE pipe.
  • Collection of waste HDPE for recycling and sourcing of recycled HDPE from multiple locations is likely to have a significant hidden impact which can lead to substantial increase in the carbon footprint of such pipes and undermine their case.
  • End of Life prospects are likely to change too. The PE100+ Association notes that PE reclaimed after use in a pipe application is unlikely to have the required characteristics for reuse in the same application. However, for E-o-L impacts calculated in accordance to EN 15804, there will only be a different value for Module D. The product is likely to have the same Module C impacts as for one made of virgin HDPE (e.g. abandonment, recycling, incineration, landfill).

The carbon footprint of the pipe material itself (kg CO 2 e/t) makes less than 50% of the whole- life carbon footprint when other elements such as Design Life, bedding and likely E-o-L come into play.

Last year, we carried out an assessment for a HDPE pipe with 100% recycled content, using as much information as possible from the manufacturers’ websites and videos. We used the assumptions above to build a list of ingredients for the product (recycled HDPE sourced from own process, from local suppliers and from abroad). We employed a model identical to the one developed by Circular Ecology with modifications to allow for recycled HDPE material, higher weights and stiffness classes (such as SN6 and SN8). We also followed all EN 15978 rules on Design Life and overall assets service life and followed PAS 2080 rules on Whole Life carbon assessment and its recommended assessment period of 120 years. The final result reveals very little difference between a 100% recycled content pipe and some equivalent sizes of virgin HDPE pipes. Indeed, a lighter SN2 HDPE pipe of equivalent size has almost an identical performance. British concrete pipes again performed significantly better in terms of Whole-life carbon emissions. On a cradle-to-construction assessment, concrete pipe installations had a slightly lower carbon footprint to 100% recycled HDPE pipes. Figure 1 (below) shows the main result of that assessment.

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Figure 1. BPDA assessment of recycled HDPE pipes Whole Life carbon compared to equivalent concrete pipes.

The main lesson learnt from this blog is that developers should focus on the carbon footprint of the overall pipeline solution/ system instead of the embodied carbon of products that only make part of the system. The embodied carbon values for a 100% recycled content HDPE pipe looked very impressive for our researchers first. However, as soon as the above functionality elements and R-HDPE waste management and transport factors were added, it became apparent that much of the product’s embodied carbon advantage was being negated by its shortcomings.

It should be noted that there are limitations to our assessment as we do not know the true composition of recycled HDPE pipe brands in the UK or the nature of their supply chains. We had to make assumptions. But we should also warn that our small-scale study is not the first to find very little carbon benefit in products made of recycled HDPE. The European Commission’s recent report on plastics LCA also found an unexpectedly small carbon advantage in manufacturing chairs using mould-injected recycled-HDPE instead of virgin- HDPE, around 28% only. Recycled-HDPE was even found to have significant emissions associated with Ozone Depletion compared to virgin HDPE (Figure 2).

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Figure 2. Ozone depletion related emissions associated with chairs made of HDPE, recycled HDPE (R-HDPE) and other types of plastic.

It is important for all manufacturers, specifiers, developers, and asset operators to think about how their products are used in a “Functional Unit”. It is also important for manufacturers who want to invest in innovative products/ solutions to ensure that their plans are backed with 3 rd party verified and credible carbon footprints and EPDs (certified to EN 15804) prior to committing funds to such investments.

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In September 2020, the BPDA published a new report which demonstrated why the carbon footprint of concrete pipeline products is significantly lower than HDPE alternatives. In this blog, we explain the importance of functional equivalence and why a carbon assessment of drainage pipelines should include the pipe and its surrounding bedding, not only the pipe.

 

What is ‘functional equivalence’ and ‘functional unit’?

The concept of ‘functional unit’ in Life Cycle Assessment studies is not new. It was introduced well over 15 years ago in the ISO 14040s series of standards on LCA. It mainly looks at the need to account all the different elements that would enable a specific function to be performed. For example, a carbon comparison between turf lawns vs. grass seed lawns would not be restricted to the carbon footprint of turf rolls vs. grass seed bag(s). The comparison will need to look at every ingredient that would contribute to the growth of a lawn. Including land preparation, fertiliser, transport distances, water consumption, material wastage, performance, life expectancy of the lawn, etc.

The same applies to pipes. Rigid pipeline systems are installed differently to flexible pipelines (such as HDPE). Rigid pipelines are often installed with beddings which would require significantly less granular material as such pipes utilise their inherent strength and would function OK with granular only utilised around the lower half of the pipe. Flexible pipes are different as the structure will depend to a large extent on the surrounding bedding, leading to a full surround of granular bedding. More information can be found at our Factsheet on the influence of bedding on cost and carbon (see Figure 1). Such bedding plays the main role in a flexible pipe’s structural function and when considered as part of the functional equivalence debate in an LCA or carbon footprint comparative study, it makes a huge difference.

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Figure 1. The main bedding classes used in pipeline construction.

Figure 1 shows the difference between the two most common bedding installations for flexible (such as HDPE) and rigid (such as concrete) pipes. Bedding Class B, the most common class of bedding for concrete pipes, uses roughly half the amount of bedding used in Class S, which is the most common type of bedding used with HDPE pipes. The additional bedding with a plastic pipe ‘Class S’ installation will lead to more carbon emissions as more aggregate granular is used and transported to site. Figure 2 shows the carbon emissions associated with ‘Class S’ and ‘Class B’ bedding granular.

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Figure 2. Carbon emissions associated with extracting, sourcing and transporting granular for ‘Class S’ and ‘Class B’ beddings.

 

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In September 2020, the BPDA published a new report which demonstrated why the carbon footprint of concrete pipeline products is significantly lower than HDPE alternatives. In this blog, we explore how the End-of-Life and decommissioning stage (also known as Module C) can make a significant impact and why any infrastructure asset assessment will need to look at that stage with scrutiny.

End of Life: Different scenarios and approaches

Module C is the lifecycle stage in which a building or infrastructure asset reaches the End of its Life (E-o-L) and is demolished, dismantled, decommissioned, or abandoned. Accounting for this stage in construction-related carbon assessments is compulsory under EN 15804 and a number of other standards such as ISO 21930 and PAS 2080. Although the process of accounting for ‘Module C’ in buildings is widely understood, accounting for the E-o-L of infrastructure projects, such as pipelines, is more complicated. Sewerage and drainage pipes inside cities, towns and suburban areas may be removed and replaced, but some sewerage pipelines and drainage networks in motorways are rarely dismantled and may simply be decommissioned and abandoned as specified in CESWI. Specifications across Water Companies vary for sewerage pipelines installed at shallow depth as total or partial asset demolition/ removal is sometimes necessary.

Even at product level, we have different interpretations of what might happen at the E-o-L. Both concrete and HDPE pipe manufacturers have in the past promoted deconstruction, recycling and reuse at the End-of-Life. With the rise of the ‘Circular Economy’ criteria, we will see more consideration of such principles in the design of concrete and HDPE pipeline systems.

Given this very wide interpretation of the most likely E-o-L scenarios (deconstruction vs abandonment), we decided to take an approach identified in draft standard prEN 17472, Sustainability assessment of civil engineering works – calculation methods:

Taking the prEN 17472 approach

PrEN 17472 (the 2020 consultation version) simply allows for all the E-o-L scenarios above to be recognised as long as, among other requirements, the site is made “ready for future re-use”. In other words, if a decommissioned pipeline would not hinder any future re-use of the site, then a partial-destruction and abandonment scenario (manholes partially removed, pipes capped and filled with foam concrete) may be considered. Otherwise, the assessment will need to account for total deconstruction and removal of the pipeline.

account for such requirement, and in the absence of any reliable data on the likely E-o-L scenario for pipelines, we simply decided to account for all possible scenarios. We modelled the scenarios as follows:

  • Concrete pipes E-o-L: scenarios considered included recycling (including carbonation while in use and at E-o-L), landfilling, re-use and abandonment.
  • Plastic pipes E-o-L: scenarios considered included mechanical recycling, incineration, landfilling and abandonment.

The process of digging out the pipelines was excluded as it was assumed to be similar for both systems as both would be of the same diameter size and within the same depth of cover. The process of crushing the concrete pipes was considered but all the CO 2 e emissions associated with crushing the pipes are negated by the End-of-Life carbonation and absorption of CO 2 (for more on carbonation, check this blog). Impacts from shredding the plastic pipes (ready for incineration or mechanical recycling) were excluded due to lack of reliable data. However, the impacts of recycling and incineration were taken from an academic study published in Nature journal last year. One impact we decided to exclude, due to lack of reliable data, is transport associated with recycling or incineration as information about the likely place where the plastic is recycled/ incinerated is unknown.

The E-o-L stage had a significant impact on the overall whole-life carbon assessment and is colour coded in Figure 1, below, in purple.

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Figure 1. Concrete Pipes vs. HDPE Plastic Pipes Cradle-to-Grave GWP comparison (DN2100 pipe) with additional adjustments, including E-o-L impacts.

Our study clearly shows that, regardless of the carbon assessment methodology used for a pipeline project, it is very important to accurately account for carbon emissions associated with End-of-Life of pipeline systems. This is why a number of standards and specifications, such as the ICE Database and the LETI Carbon Primer, consider E-o-L as part of the definition of construction products’ “embodied carbon”.

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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.

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Figure 1. Concrete Pipes vs HDPE Plastic Pipes Cradle-to-Grave GWP comparison – DN2100 pipe (Assuming 50 year plastic pipe design life).

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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 explore one of the main findings of this report: The impact of HDPE resin origins. We explain why it is important to get the supply chain and raw materials’ sources right. We also explain why the carbon footprint of HDPE products in the UK is significantly higher than originally thought.

Raw materials’ sourcing: Findings from the Circular Ecology pipes carbon model

The current version of the ICE Database, published in November 2019, reports a carbon footprint for concrete pipes of around 146 kg CO 2 e/t. That carbon footprint is based on an independently verified and certified Environmental Product Declaration (EPD) for British concrete pipe manufacturers, published by certification scheme IBU in 2017. However, within that database, the generic carbon footprint for an HDPE pipe alternative (around 2,520 kg CO 2 e per tonne) is based on a study carried out over 15 years ago by European trade association ‘Plastics Europe’. That carbon footprint can also be found in most databases used by Water Companies in the UK. An update to that Plastics Europe report is expected soon.

BPDA has long argued that the European carbon footprint of HDPE pipes, which is entirely based on a European resin scenario manufactured 100km away from the pipe manufacturer, does not reflect the true emissions of that sector in the UK. The carbon footprint of that European HDPE resin is reported in the ICE Database as 1,930 kg CO2e/t (a more recent PlasticsEurope report suggests 1,800 kg CO 2 e/t). By working with Circular Ecology, and with the help of European market data from 2016/ 2017 and recent European Commission reports offering details on the origins HDPE resin imports, we developed an alternative scenario based on the likely sources of HDPE resin imported from outside Europe.

Assessing the impact of imported HDPE resin

The first step was to estimate the likely carbon footprint of HDPE resins based on countries of origin. A multi-scenario model developed by Circular Ecology helped us come up with the likely embodied carbon values for HDPE resin produced in a number of countries in Asia, North America and the Middle East. For example, it is estimated that resin from the Middle East was likely to have an embodied carbon of 2,180 kg CO 2 e/t. North American HDPE resin footprint was around 2,010 kg CO 2 e/t. The carbon footprint of East Asian HDPE resin was estimated to be around 2,200 kg CO 2 e/t. Based on Plastics-Europe numbers, the embodied carbon of an average European HDPE resin is 1,800 kg CO 2 e/t only.

We then looked for data on the likely sources of such resin in the UK. It is believed that the vast majority is imported from European hubs like Belgium’s Antwerp where a range of international and European plastic resin is handled prior to re-export to the UK and other destinations. We found one European market study from 2016 which offered a breakdown of sources of HDPE resin consumed in Europe, including imports from the Middle East, North America and Asia. That market study was significantly conservative compared to values published in 2020 by the European Commission which suggested that almost two thirds of HDPE resin imported to Europe is from the Middle East. We also added the carbon emissions associated with shipping the resin from central Europe and different ports in the Middle East (Saudi Arabia), Asia and the United States. This enabled us to develop two distinct footprints for pipes with European and International resin.

How the HDPE resin origins affect the carbon footprint

Figure 1 shows the differences between HDPE pipes made of European and International resin based on our study. We decided not to amalgamate the two values as the proportion of International resin used in the UK is unknown. Internationally sourced resin has been used in making plastic pipes in the UK in the past. There is also a suggestion that Middle Eastern HDPE resin makes a significant proportion of resin used and re-exported from the EU (Platts, 2016).bpdagraphic

Figure 1. Carbon footprint of DN2100 HDPE pipe based on a European and International resin mix.

The results clearly show that the carbon footprint of a plastic pipe would increase by 6-7% on average with an internationally sourced HDPE resin. That percentage is likely to be higher if the up-to-date figures from the European Commission were used. It is imperative that databases, such as the ICE Database, seek more representative data from the industry or clearly add information on the limitations of many of the sources used for such footprinting data.

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In 2018, the British Precast Drainage Association (BPDA) sought the services of Circular Ecology, the consultancy behind the ICE carbon footprinting database, to help them quantify the true whole-life carbon emissions associated with concrete and plastic pipeline systems. What Circular Ecology came up with (see report here) did not only help BPDA plans to improve their concrete pipeline products; it also revealed a wide range of hidden factors that can significantly change how the industry perceives embodied carbon and generic products’ carbon footprints.

How generic carbon footprints are used

The last five years saw a significant increase in the number of carbon calculators developed by Water Companies and asset operators to help them assess and manage carbon emissions from their infrastructure projects. Asset operators generally prefer to use generic values to assess the embodied carbon of different construction products. By multiplying the quantities of construction products (tonnage, m 3 , etc) within bills of quantity by the generic carbon footprint values, it is possible to come up with a very quick estimate of the embodied carbon emissions of entire projects. This is why such generic carbon footprints are highly valued by the industry. For the last 15 years, the most used and trusted source for generic carbon footprints in the UK has been the ICE Database (known previously as the Bath University database).

But questions remain on whether such generic footprint values are always reliable and sufficient. The model developed for us by Circular Ecology, and the subsequent work we did to make up for a few missing pieces within that model, was specifically to help answer this question. More information on that study can be found here.

Circular Ecology’s assessment/ Model

The British concrete pipe sector has long claimed that concrete pipes have a significantly lower carbon footprint compared to HDPE alternatives. We published two studies in 2002 and 2010 to prove this. However, due to lack of any certified carbon footprints for UK- produced HDPE pipes, such comparisons have always been difficult. The only publicly available carbon footprint for HDPE pipes was published by Plastics Europe almost 15 years ago. It was updated only once in the last few years. That carbon footprint is based on European plastic resin and extrusion factors, using a methodology which is not fully compliant yet to the main footprinting standard, EN 15804. We asked authors of the ICE Database, Circular Ecology, to come up with a new HDPE pipe footprint which can enable comparisons in accordance with EN 15804. Circular Ecology was also asked to oversee the comparison to ensure that the main functional equivalence elements are accounted for.

Circular Ecology came up with a multi-scenario model to enable a viable whole-life comparison between concrete and plastic pipeline systems. The model accounted for different options for GWP factors, sources of HDPE resin, steam cracker allocation methods, raw material shipping scenarios, “Design Life” and a number of other elements. Circular Ecology left out some elements where they felt sufficient data may not be available to enable a viable comparison. Extra work not covered by the original contract (such as extra work to account for realistic factory waste or road transport) was also not added.

BPDA made a couple of corrections to account for realistic factory waste and transport (factory to site) impacts for both concrete and plastic pipes. In both cases we used published industry figures. BPDA also sought help from Professor Göran Finnveden, an academic from the KTH Royal Institute of Technology in Sweden, to help build up the End-of-Life impacts of HDPE pipes, which were missing from the Circular Ecology model. Prof. Finnveden worked on GHG emissions associated with the degradation of plastics in the past and he suggested a CO 2 / CH 4 degradation factor which we can use for buried HDPE within its first 100 years of installation, running into post End-of-Life. We used the most conservative degradation factor (within 100 years only) to ensure that such GHG emissions are not over-estimated.

So, what did the study unearth?

Where to start? Based on the most-realistic scenario we developed using the Circular Ecology’s model, and Prof Finnveden’s E-o-L factors, we found the following:

  • The choice of characterisation factors for Global Warming Potential (GWP) can make a noticeable difference. Using a methane GWP which accounts for carbon feedbacks (as recommended by latest standards such as EN 15804 +A2 and ISO 14067) can lead to a 2% increase in the resin’s embodied carbon.
  • Aligning the allocation method within steam cracker production (the process which produces raw material ethylene) to EN 15804 rules roughly adds an extra 7-8% uplift to the carbon footprint of HDPE resin.
  • The source of HDPE resin can make a significant difference. An HDPE pipe manufactured from Middle Eastern HDPE resin can have a carbon footprint 33% higher than one made of resin imported from a European country like Norway.
  • Including pipes’ bedding in the carbon assessment can add up to 33% to the carbon footprint of an installed concrete pipe compared to 72% for the carbon footprint of an HDPE pipe.
  • The End-of-Life carbon emissions of HDPE pipes are so significant, it can increase the final carbon footprint by around 14-17%.
  • Accounting for Whole-life as recommended in EN 15978, where the Design Life of pipes are primarily considered, will lead to a 100% increase in HDPE pipes’ embodied carbon emissions as pipe replacement would be needed to cover a service life of 120 years.
  • There could be further environmental impacts (including GWP emissions) from the use of plastic pipes. Plastic pipes degradation beyond the service life (over 100s or 1000s of years) was not accounted for. This is despite the availability of viable figures from the KTH Institute research. We also decided not to include carbon emissions associated with pre-production HDPE resin pellets finding their way into the ocean, as we couldn’t find a reliable way to estimate such impacts.

Over the next few weeks, we will be publishing a number of detailed blogs demonstrating the significance of each of these findings.

pipe stock (2)

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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.

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Figure 1. Graph demonstrating reductions to the carbon footprint of a metre of DN600 concrete pipe over the lifecycle (2016 data).