Long-term performance of CIPP
The following paper was presented as part of the conference program at No-Dig Beijing in 2016. Its discusses the short- and long-term design, testing and mechanical properties of cured-in-place pipe (CIPP), which is one of the most popular methods for the trenchless rehabilitation of pipelines.
by William Wong, General Manager, Insitu Envirotech (S.E Asia) Pte Ltd, Singapore
CIPP was invented and first installed in 1971 in the UK. Since then CIPP has grown to be one of the most widely used trenchless methods for the rehabilitation of pipelines.
Materials for this technology are now manufactured globally. Often, the materials are combined without consideration of the system as a whole which can result in poor liner construction. Quality assurance is critical for both short-term and long-term performance and ensures that products are installed to extend the life of pipeline assets.
This paper briefly touches on the design, testing and determination of mechanical properties – both short-term and the 50-year creep – to substantiate these designs. It then looks at three studies conducted on installed Insituform® CIPP liners put into service 20, 25 and 30 years ago in the UK and Japan.
Finally, the long-term creep generally adopted in designs will be compared with the results that are obtained from the studies of the Insituform® CIPP liners that have been in service for 20, 25 and 30 years. This comparison allows us to evaluate the performance of this technology over time.
CIPP was invented in 1971 to rehabilitate an ageing sewer pipe at Riverside Close in Hackney, London. Since then, it has grown to be one of the most widely used trenchless rehabilitation methods for the rehabilitation of sewers, water mains, storm water drains, culverts, industrial pipes and many other pipelines.
As today’s global infrastructure ages, the importance of being able to extend the lifetime of these assets using sustainable methods is ever increasing as societies try to balance civilisation with nature.
These infrastructure assets are central to modern civilisation, in particular are those that cannot be seen underground. Water mains and sewers beneath our streets provide humanity with clean water and sanitation.
An increase in human life expectancy can be associated with providing clean water and sanitation to control water pathogens and waterborne diseases. Clearly the importance of these assets to human civilisation cannot be overstated – not only must we maintain these assets but we must use proven materials that will extend the life of these assets.
CIPP material and resins are now manufactured globally and the combination of tube and resin are often mixed and matched without any thought of the system as a whole. The potential problems associated with incompatible systems are not only associated with installation, which are revealed immediately, but also over the longer term during the extended life of the asset.
In evaluating which CIPP systems to adopt, one must consider the economic cost over the expected life extension of the product. Hence the less expensive solution at the time of installation may not be the less expensive solution over the extended life, especially if remedial works are required after rehabilitation.
The best way to avoid additional costs in later years is the use of international design standards and the adoption of products that have proven themselves over time. One of the most commonly adopted design standards for CIPP is ASTM F1216. It sets out the recommended design considerations for determining the required thickness of the CIPP.
Embedded in these equations are the mechanical properties that can determined by short-term testing and the long-term creep of the material, which is generally assumed to be 50 per cent of short-term mechanical properties over a 50-year period.
In 2016, it was the 45th anniversary of that first installation and it is quickly coming up on the 50 years typically adopted for design life.
Previous studies have been conducted on Insituform® CIPP liners after they have been in service for 20, 25 and 30 years. This paper will review these studies and compare them to current industry practices.
ASTM and WIS
ASTM F1216, or the standard practice for the rehabilitation of existing pipelines and conduits by the inversion and curing of a resin-impregnated tube, is one of the most commonly adopted standards for CIPP.
Another commonly adopted standard for CIPP is the UK water industry WIS 4-34-04, which is the specification for the renovation of gravity sewers by lining with cured in place pipes. Both standards provide guidance on materials, installation, design, inspection and testing.
Under ASTM F1216, the design of CIPP will determine the required thickness of the CIPP for each pipe installation. Hence the performance of an installed CIPP under loading over its life expectancy and subsequently the extended life of the rehabilitated pipe is dependent on the thickness of the installed CIPP. This is determined by calculating the thickness under various design considerations and taking the greatest calculated thickness as the minimum required thickness. Under ASTM F1216 the following design considerations are taken into account for gravity sewer pipes:
- Maximum compressive hoop stress
- External pressure buckling
- Minimum stiffness limitation, deteriorated conduit
- External pressure, deteriorated conduit.
For sewer gravity pipes, the minimum flexural modulus and the theoretical long-term creep assuming a 50 per cent reduction factor are shown in Table 1 and Figure 1 respectively.
In determining the design considerations in Table 1 and Figure 1, the mechanical properties of the CIPP, including the flexural modulus and the flexural modulus reduction to account for long-term effects, are required.
These figures can be found using ASTM D790, the standard test methods for flexural properties of unreinforced and reinforced plastics and electrical insulating materials for short-term flexural mechanical properties, and ASTM D2990, the standard test methods for tensile, compressive, and flexural creep and creep rupture of plastics for long-term flexural mechanical properties.
A short-term flexural test will ensure that the mechanical properties of the CIPP will be above the short-term flexural modulus used in the design and calculation of the minimum required thickness for the CIPP (see Figure 2).
For the long-term flexural test, the samples are put under load for 10,000 hours to determine the reduction factor to account for long-term creep. This is compared to the reduction factor assumed in the design and calculation of the minimum required thickness for the CIPP to ensure that the mechanical properties of the CIPP are sufficient (see Figure 3).
Testing of CIPP after being in service
Insituform has installed over 40,000 km of CIPP worldwide; the first of these was installed in 1971. To better understand the performance of CIPP over time, Insituform has conducted a number of tests on installed CIPP after being in service for 20, 25 and 30 years.
In executing these tests, sample panels were cut out from the existing installed CIPP (see Figure 4). These sample panels were then sent to the respective laboratories and prepared to be tested under the relevant flexural modulus testing methodology.
The results of the flexural tests conducted on installed Insituform® CIPP after being in service for 20, 25 and 30 years are summarised below.
Test on installed CIPP after 20 years in service (see Table 2)
- Year of test: 1991
- Location: UK
- Test method: BS2782 Part 3 Method 335A:1978
Test on installed CIPP after 25 years in service (see Table 3)
- Year of test: 2015
- Location: Japan
- Test method: JIS 7171
Test on installed CIPP after 30 years in service (see Table 4)
- Year of test: 2001
- Location: UK
- Test method: BS EN ISO 178:1997
We’ve briefly covered short- and long-term testing of CIPP and looked at the results of short-term testing of CIPP that had been in service for up to 30 years. The question is: why is the short- and long-term testing of CIPP material of interest and how they are related?
CIPP is classified as plastic material, hence the creep is often defined to be the non-elastic portion of strain. Therefore, it is assumed that the CIPP exhibits a time dependent behaviour whereby the stress and strain are a function of time (see Figure 6).
In theory, when plastic material, in this case CIPP, is subjected to constant load, it deforms continuously over time until it either ruptures or yields, causing failure. When failure occurs in pipelines, many of these are catastrophic failures.
Due to the nature of the CIPP material, it is required that we look beyond the short-term testing results and take into consideration the behaviour of the CIPP material over time. To achieve this, we conduct long-term testing in accordance with ASTM D2990 for 10,000 hours or just over 13 months to estimate the material properties of the CIPP at 50 years.
We emphasise that this is an estimation based on a model of creep and the assumption of superposition principles that allow us to predict creep behaviour in plastic materials. Hence the only way to confirm that these assumptions are correct is to do physical tests of the CIPP after they have been in service.
By combining the long-term testing results, the theoretical design long-term flexural creep for 50 years and the short-term flexural tests of Insituform® CIPP that has been in service for 20, 25 and 30 years, one can compare the actual flexural modulus versus the theoretical flexural modulus. See Figure 7, which captures the results discussed.
While the CIPP technology has not been adopted for over 50 years, it is important to collect long-term data from field samples. The tests conducted on Insituform® CIPP that has been in service illustrates how we can evaluate the correctness of the underlying assumptions.
In 2016, it was the 45th anniversary of the first CIPP installation and it is quickly approaching the 50 years typically adopted for design life. The question of how well CIPP performs over time can be answered in the summary above.
Clearly, the results of the flexural testing done on Insituform® CIPP installed and put into service over 20, 25 and 30 years show that the mechanical performance and flexural modulus of the CIPP are above the required minimum flexural modulus, in accordance with ASTM F1216 and WIS 4-34-04, and also the theoretical long-term creep of the CIPP. From these results, the Insituform® CIPP is a quality product, proven over time.
ASTM D2990-01, Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep Rupture of Plastics for long term flexural mechanical properties.
ASTM D790-03, Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials for short term flexural mechanical properties.
ASTM F1216-09, Standard Practice for Rehabilitation of Existing Pipelines and Conduits by the Inversion and Curing of a Resin-Impregnated Tube.
Bodycote Material Testing Ltd. (2001). Determination of the flexural properties of an Insituform sewer lining from riverside close, Hackney London.
BS EN ISO 178:1997, Plastics. Determination of flexural properties.
BS2782 Part 3 Method 335A:1978, Determination of flexural properties of rigid plastics.
JIS K7171:2008, Plastics. Determination of flexural properties.
MTS Pendar Limited (1991). Tests prove durability of Insituform.
Nippon Steel & Sumikin Pipeline & Engineering Co. Ltd. (2015). INS Technology, Nagoya City DN1350 of rehabilitating pipe, follow up report after
UK Water Industry WIS 4-34-04, Specification for Renovation of Gravity Sewers by Lining with Cured in Place Pipes.
This article was featured in the Summer edition of Trenchless International. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
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