Seeking a cure for poor relining work

George Woods of Atkins Water explores renovation techniques and identifies the measures that can be taken to prevent below-standard cured in place pipe linings being applied

Cured in place pipe (CIPP) lining is frequently specified as a sewer renovation technique and is offered by many contractors. To achieve a durable product it must be designed, manufactured, installed and tested properly. Many clients and some contractors seem to be unaware of the features of this material and as a result some sub-standard linings are being installed. While this article is based on CIPP lining of rigid pipes, experience indicates CIPP lining of brick sewers is equally prone to quality issues. CIPP lining has been around for more than 30 years and is a popular technique for the rehabilitation of sewers. Hundreds of kilometres have been installed globally and the process accounts for approximately 70% of all pipeline renovation currently carried out.

As a sewer renovation technique used in accordance with WRc's Sewerage Rehabilitation Manual, purchasers reasonably expect a pipeline renovated by CIPP will last for a further 50 years. Figure 1 demonstrates not all linings installed recently have been successful. In the UK, CIPP is covered by Water Industry Specification (WIS) 4-34-04 Specification for Renovation of Gravity Sewers by Lining with Cured-in-Place Pipes. In addition, BS EN 13566-4:2002 Plastics Piping Systems for Renovation of Underground Non-Pressure Drainage and Sewerage Networks - Part 4: Lining With Cured-in-place Pipes was published on February 4, 2003. Interestingly, not all contractors offering the process seem to know this. CIPP is often used to reline sewers constructed from traditional rigid pipe materials, provided any deformation or displacement of the host pipe is within tolerable limits. These sewers have been designed to withstand loads resulting from construction in open trench and, in spite of damage, may still be capable of doing so if relined. The structural design of rigid pipelines is often based on look-up tables, which is a reasonably simple process for the design engineer.

The CIPP lining, however, is a flexible material and it is not installed in open trench. Its design is much more complex and will depend partly on an assessment of the future behaviour of the host pipe. Standards exist covering its design - these typically require complex calculations to investigate pipe deflection and buckling stability.

In the UK, the appropriate standard is BS EN 1295-1:1998, Structural Design of Buried Pipelines Under Various Conditions of Loading - Part 1: General Requirements. Frequently, however, the only force on CIPP lining is due to hydrostatic pressure. To complete the calculations, a value for the flexural modulus of the lining must be used. This is a measure of the bending stiffness of the material.

The value of this modulus will, for CIPP materials, reduce with time as they creep under load. If a 50-year life is to be expected, then the long-term or 50-year modulus must be used. Experts suggest the procedures contained in design standards are conservative, typically because few procedures acknowledge the residual strength of the host pipe. The design engineer's task is now much more complex and purchasers frequently rely on the design capability of the contractor. In reality, if they are in any doubt, the purchasers should seek independent professional expertise to check if the design is robust.

Knowledge of the individual components and processes that are used to manufacture a CIPP lining is beneficial if the material's selection, lining manufacture and installation processes are to be understood. CIPP linings comprise a carrier and a resin system that must be compatible. The carrier is a porous material such as polyester felt or glass fibre. The resin system includes the resin and catalyst and any fillers or additives. Typical resins are polyesters, epoxies and vinyl esters. Manufacturers will often suggest epoxy resins have a higher short-term flexural modulus than polyesters and thus allow a thinner lining to be used.

However, there is some evidence the long-term modulus is not significantly higher than that of polyester and the use of thinner linings must be treated with caution. Most other pipeline renovation techniques involve the installation into the host pipe, using a no-dig technique, of a lining consisting of a pre-manufactured pipe. The lining will have been manufactured under factory conditions where acceptable standards of quality control are achievable. CIPP differs from such techniques in that the final stages of manufacture do not occur until the lining is within the hostile environment of the host pipe.

For the lining to be successful, the resin components must be measured accurately and mixed thoroughly, and the carrier material must be thoroughly impregnated. Some contractors carry out these processes under factory conditions, but many prefer to mix and impregnate on-site. Preparing the lining on-site avoids the need for specialist refrigerated transport to avoid premature curing of the resin, but other aspects of the lining manufacture are likely to suffer.

The manufacturing process

The impregnated lining, which is still flat and flexible, is either pulled or 'everted' into the sewer. One end of the lining is closed and the other end is attached to a compressor or a source of water. The penultimate stage of manufacture of the lining, forming it to the required shape, is achieved by inflation with compressed air or water under pressure. The lining is normally fractionally smaller than the diameter of the host pipe and is expanded into place. If the lining diameter is too great longitudinal wrinkles will occur.

The final stage of manufacture, curing the resin, is achieved either by relying on the resin's ambient-cure characteristics, by circulating hot water around the inside of the lining (hot cure) or by the introduction of ultra-violet light. Each curing system has its own advantages and limitations. So, what can go wrong? Provided the lining is selected, designed, manufactured, installed and tested in accordance with the appropriate standards, and provided the contractor is experienced and competent, little or nothing should go wrong. However, it is often the case not all of these are achieved.

Both WIS 4-34-04 and BS 13566-4:2002 require the CIPP lining to comply with specified requirements for both long and short-term flexural moduli. Type tests are included to determine these parameters. For the 50-year modulus a three-point bending test should be carried out for a period of at least 10,000h (approximately 60 weeks) and the modulus extrapolated from the results. Few suppliers provide this information for their products and hence do not comply with these standards. Some will supply the results from a 1,000h test extrapolated to 50 years. In reality, the difference between the 1,000h test result and that for 10,000h is not likely to be significant and suppliers might argue the standards are unreasonable.

BS 13566-4:2002 lets the contractor declare the 10,000h test result as the specified long-term flexural modulus value, provided it exceeds 300MPa. Samples of linings installed on-site should meet the declared value when tested. In practice, type testing is often conducted on samples prepared under factory conditions and tested for 1,000h. High results are achieved and declared to give the contractor a competitive edge - these results are used in the contractor's design.

Samples taken on-site are unlikely to achieve the same results and the design is therefore immediately suspect. BS 13566-4:2002 recommends one sample should be taken per lining installation. Few linings are actually tested in accordance with this BS recommendation - of those that are, many fail to achieve the correct modulus. Testing of the lining immediately following installation identifies weaknesses and allows remediation before the consequences become catastrophic.

Undertaking lining design

Once a long-term modulus has been established, the lining thickness design can be undertaken. This requires an understanding the engineering requirements and considerable computational skills to perform the structural calculations. Many contractors now offer CIPP lining. While some are becoming experienced, few can demonstrate long-term competence. Competitive tendering on a design and construct basis will encourage less scrupulous contractors to cut corners by paring their design to the bone to reduce materials costs. Proposed contractors therefore need to be vetted very carefully.

Most problems occur on-site. Incorrectly mixed resins will not cure properly and accurate measuring equipment is unlikely to be used on-site. Impregnating the carrier, or 'wetting out', is a specialist process and if not done correctly, the impregnation will not be thorough and uniform. Adequate pressure must be maintained to inflate the lining against the wall of the host pipe.

The inflation pressure must be maintained until the resin has cured sufficient to resist external pressures. Some resins are not tolerant of water until they have cured. If these resins are used in areas of high groundwater without a pre-lining, or if flow is re-introduced into the sewer before curing is sufficiently advanced, the resin will be contaminated by water, will not cure properly and the lining may fail catastrophically (see Figure 3).

Curing, especially hot curing, requires careful control and should ideally be monitored via thermocouples placed between the lining and sewer wall. Generally, suppliers identify the lining needs to be cured for a set time at a specified temperature. An inexperienced contractor will heat the water to the required temperature and maintain the temperature for the set time. This ignores the temperature gradient through the lining thickness, with the outer face of the lining losing heat to the host pipe. Unless the outer face is cured properly, the lining will not achieve the flexural modulus specified in the standards.

Once the lining has cured, a CCTV survey will identify poor installation but will not confirm the structural quality of the lining. This can be checked only by obtaining a sample from the installed lining and carrying out bending tests. A short-term bending test will give an almost instantaneous value for the short-term flexural modulus - if the value falls well below the specified figure, the lining quality is suspect. If the value is reasonably close to the specified value, then rather than assume the worst, it would be appropriate to carry out a 1,000H test to determine the long-term flexural modulus. After the test has progressed for a few hundred hours it should be possible to see if the sample is likely to fail. Figure 4 shows a sample which has been on test for 828h. The predicted 50-year modulus is only 51MPa.

To summarise, CIPP lining projects will benefit from experienced engineering expertise throughout the procurement process. If this is not available within the purchasing organisation, it should be sought independently. CIPP lining is popular; nonetheless it is a specialist process. Proper selection of the materials, and checking the design and workmanship, are important to achieve a quality product. Just as mixing and placing in-situ concrete requires stringent supervision and testing, so with CIPP lining, relying on acceptable results to be achieved without ensuring appropriate quality control and testing procedures are applied could result in an end product that does not live up to expectations.

George Woods thanks Dave Harrison of Bodycote PDL Testing Laboratories of Salford, who assisted in the writing of this article.

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