■ Inspection/testing during service life

However, in practice it is not often clearwho is taking responsibility forwhat andwhere the accountability lies. The key opportunities for the end user to ensure QA is applied are:

■ Inclusion of QA requirements for the supplier/contractor at the tender drafting


■ Appropriate evaluation of QA elements within bids.

■ Supervision on-site. ■ Post installation evaluation. ■ Warranty period acceptance.


Material Properties – Qualifying as a Flexible Pipe: In addition to the flexuralmodulus, described above, there are several other importantmaterial properties that are fundamental to a CIPP liner. The underpinning assumption behind the use of CIPP liners is that they performas per a flexible pipe. In order to be considered a flexible pipe there are several criteria outlined inmany international standards thatmust bemet. If one ormore of these criteria is notmet, then it cannot be considered a flexible pipe and therefore it cannot be designed using the flexible pipe equations. In order qualify as a flexible pipe it shouldmeet the following criteria general:

■ Have a ring stiffness of >250 kPa ■ Flexural strain of >0.75% ■ Ultimate elongation of >0.5% ■ Resistance to a specified set of chemicals

Prior to an asset owner selecting a liner and resin systemfor use on its sewer, they should seek fromthe supplier independently certified evidence proving that the systemmeets these criteria.

For an off the shelf product, it should be expected that themanufacturer/supplier is able to provide declared values for the product’s performance andmake clear any limitations on the products use. This is normally achieve using a combination of short and long termtests, the latter normally including a 10,000 hour (14month) long term flexuralmodulusmeasurement of any resin/ linermatrix (to derive declared values) from which expected performance out to an beyond 50 years can be extrapolated.

Where a bespoke design is needed the manufacturers declared values are needed to support the design calculations. Froma practical point of view, it is conventional to


measure the expected short-termflexural modulus of each installed liner as a first indicator of performance. If a lowvalue of flexuralmodulus is found then further testing may need to be undertaken tomeasure a broader range of properties to determine the actual structural performance of the installed product.

Alongside the short and long-termtesting undertaken bymanufacturers to provide their declared values for their product to verify performance and support design calculation, short termtests on installed liners to confirm theymeet expected values aremost commonly performed.

There are four short termtests in common use:

■ Modulus of elasticity (short termflexural modulus) – indicator of load bearing


■ Flexural strength (flexural stress at rupture) – this indicates the point at

which the liner fails due to excessively high stress. If it is too low, the linermay rupture.

■ Wall thickness (mean combined thickness).Minimumvalues are specified

in the structural-analysis calculations for liners.Wall thickness andmodulus of elasticity jointly determine the stiffness of the liners. Excessively lowwall thickness can endanger stability

■ Water tightness – this is a test to check is the liner is fullywetted outwith resin

and/or fully cured. A coloured dye is applied to the inner surface of the liner and a -0.5 bar pressure is applied to the outside. If the vacuumcauses the dye to pass through it focuses attention on checkingwetting out/curing.

Themethod of evaluation is to determine pass/fail of a test against the expected performance values for each individual liner – themanufacturers declared values for an off the shelf product or the design values stipulated for a bespoke installation.

Failing a test does notmean that a linerwill fail as there are safetymargins built into design, sowhen interpreting the results of short-termtests both the contractor and the client need to fully understand theirmeaning. Short termtests, particularly thewater tightness test are very usefulways to detect developing QA issues, identify causes and address themthrough continuous improvement.

These short-termtests alongwith visual inspection and pressure testing can combine to provide re-assurance that an installed liner

| February 2021 |

will performas expected.


Every year since 2004 the IKT Test Centre for Building Products has published a report on the results of its evaluation of samples of installed CIPP liners using the four short-term tests. It compares the performance of contractors and products and aimis to provide rehabilitation project clientswith an overviewof the quality of installation projects actually achieved in thismarket. The LinerReportwas created at the request of German sewer network owners following IKT research on excavated CIPP liners,which found thatmost reflects observed could be traced back to installation issues. Samples are provided by the client or the installer and the source is identified in the report. It is freely available online and is read around theworld.

The results showed a trend for improving pass rates over the first decade and a variable but higher pass rate subsequently. Initially focused on German contractors, it has expanded to include contractors fromthe Netherlands, Czech Republic, Switzerland, France and the UK, and results from>2000 installations each year.

Whilst published performance and testing standards provide a basis for assessment the actual host pipe damage and loading scenarios that clients expect rehabilitation products to address,may be different. Consequently, research projects are undertaken to evaluate actual performance and understand limitations and quality assurance issues.

Since its formation 25 years ago IKT – Institute for Underground Infrastructure has been investigating the issues that sewer network owners experiencewith their deteriorating assets by evaluating rehabilitation technologies in realistic 1:1 scale assessment of performance. These evaluations do not repeat performance testing undertaken against standards/approvals but simulate the actual damage scenarios that the network owners experience andwant to solve. For this, bespoke tests are createdwith the approval of the network owners and the technology suppliers are given the opportunity to comment on themand confirmwhether their product is suitable before it is included. The aimis to understand howavailable technologies actually performunder in these realistic scenarios against groundwater pressure and other loadings, and fromthe results provide recommendations forwhat quality assurance considerations network owners need to be taken into account in selection, design, procurement, installation and acceptance.

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