Lining: The objective cure

CIPP is the most popular method of sewer renovation in the UK, but liner quality can vary considerably. Nick Sheehan, engineering director of Perco, explains how the findings of independent tests in Germany are especially pertinent to UK contractors

The method of sewer renovation used in the UK more than any other is cured-in-place pipe (CIPP) lining. Compared with other techniques, CIPP can accommodate the bends, undulations and eccentricities of our ageing sewer and drain systems relatively easily. It also falls reassuringly between the least and most expensive renovation methods.

The basis of all CIPP systems is to insert a flexible tube of material in the host pipe and then cure it to create a rigid lining. With several options available, water industry engineers are keen to evaluate performance, in order to select the right system for a particular project.

In Germany, where the technology has been pioneered, and where manufacturers have developed many systems, a comparative report has recently been published by the independent testing body, the Institute for Underground Infrastructure (IKT). Its 2006 liner report tested 1,084 samples taken from live installations throughout Germany last year.

Independent testing

Cured-in-place liners are composites, made up of a reinforcement or support material impregnated with a resin, which is normally polyester.

IKT tested two distinct types of liners – glass fibre reinforced (GRP) and needle-felt reinforced – for:

  • Short-term flexural modulus
  • Flexural strength
  • Wall thickness
  • Watertightness

Superficially, these may appear to test only the intrinsic characteristics of the materials. In reality, they also give an indication of how well the installer has performed, particularly in carrying out the curing procedure correctly.

Crucially, more than 80% of the samples were tested at the request of municipal clients, rather than liner manufacturers or installers, avoiding any conflict of interest.

Significant results

The short-term flexural modulus demonstrates the elasticity of the sample. This indicates the liner’s ability to withstand changing loads, such as are caused by traffic, ground movement and groundwater pressure.

The results were significant for the UK because the UV-cured Brandenburger GRP liner, installed here exclusively by Perco, passed the test in 57 out of 57 installations – 100% met the design modulus.

In contrast, the needle-felt liner whichis most widely used in the UK achieved the target in only 84% of its samples. This indicates that in Germany, there are approximately 30 liners with below-par elasticity installed.

The flexural strength of the liner samples was determined by recording the stress at which they failed in three-point bending.

Again, the Brandenburger GRP liner samples had a 100% pass rate. The UK’s number one felt liner type only achieved 56%.

Watertightness was tested by applying a negative pressure to the outer liner wall and an aqueous dye solution to the inner liner wall. Any water transmitted right through the liner wall indicated porosity and therefore a failure. Once again, all of the Brandenburger samples passed the test, while more than 31% of the predominant felt liner samples showed porosity.

The measurement of liner wall thickness was taken at six points on each sample to establish a mean value. GRP is more prone to variation from the nominal wall thickness than felt liners, yet the Brandenburger system returned a pass rate 89.5%. The UK’s major felt liner fared worse at 80.8%.

Margin for improvement

A clear message from the tests is that, when it comes to meeting their targets of water tightness, modulus of elasticity and flexural strength, GRP UV liners such as Brandenburger, on average, score nearly 30% better than felt. This is not all down to the inherent properties of the liner system. It suggests that GRP liner installations are performed more efficiently and that is probably due to a better reproducibility and quality with this liner type.

In closing its report the IKT comments: “A number of installation contractors still have adequate margin for improvement of their liner quality and work. This applies particularly to those companies using needle-felt liners. This group of technologies needs to improve their quality to catch up with GRP liners.”

View the report at

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