Win win situation for the clients and bidders

Whether a glass fibre or synthetic fibre liner is used in a sewer rehabilitation with Cured-In-Place-Pipes (CIPP) primarily depends on the cost effectiveness of the systems. Local boundary conditions such as nominal size, geometry, length and location and the number and angle of bends in the sewer to be rehabilitated, or even the groundwater level, can justify a technical necessity of one or another system.

The relevant standards for CIPP systems, the relevant standards and data steets do not make any difference in the suitability of the products. Only the end product is decisive, which must meet the operating, stability and durability requirements. It makes sense to support tenders that do not specify a certain liner material in advance.

The purpose of the tender is to find the technically best and at the same time the most cost-effective option for the pending project. Both CIPP systems, provided they have a national technical approval, are to be considered equivalent when cured. This enables an optimal solution to be proposed to the client for the specific application case. Apart from the purely economic aspects, from the client’s point of view it is equally important to examine the suitability of the bidder and to include appropriate proofs of suitability such as quality markings, certificates, or references.

This procedure generally helps to secure a successful rehabilitation project and thus a win-win situation for both the client and the contractor. A good example of how “unspecified material” tender specifications lead to the required result is a project in Hannover. Here, Aarsleff Rohrsanierung GmbH rehabilitated around 5.9 km of wastewater and stormwater sewers including various combinations of different liner materials as well as installation and curing methods.

Glass fibre or synthetic fibre liner? – That is not the question!

CIPP is a well-known and tested method of rehabilitating wastewater pipes and sewers. This method was used for the first time in England in 1971. Since then, continuous further developments have led to an improvement in the materials and equipment used. What all variants have in common is that a carrier materiel impregnated with reactive resin is introduced into the pipe, inflated, and then cured with the help of a medium (water, steam, UV ligth or, in the area of connection pipes, also with the help of LED light). The differences between the methods available in the market lie in the materials used (carrier material and resin), in the wall construction of the liner, in the installation method and the type of curing.

The materials used for the felt liner system is synthetic polyester fibres and for the glass fibre liner system glass fibres are used. Both types of liner are in the “lining with cured-in-place pipes” group of methods according to EN 15885 [[1]] and are suitable for the rehabilitation of wastewater pipes and sewers according to EN ISO 11296, Part 4[[2]] and DWA-A 143-3[[3]] with the corresponding national technical approvals. In Germany, this national technical approval is exclusively issued by the Deutsche Institut für Bautechnik (DIBt) on request and is generally limited to five years.

Thus, extensive proofs of suitability exist for the standard CIPP systems, which ensure continuous product monitoring. Pipes renovated in this way meet the functional requirements for drainage systems in accordance with EN 752 [[4]] and also verifiably satisfy the legally required factors such as leak-tightness, stability, operational reliability as well as chemical and operational resistance.

Wall construction and installation differences

The needle felt liner basically has a three-layer construction. From the inside a coating, a synthetic needle felt and an outer film called a preliner. The synthetic needle felt (nonwoven) is impregnated with a resin matrix made of unsaturated polyester resin and fillers. The inner coating is generally an integrated part of the liner system and consists of?thermoplastics Polypropylene (PP) or Polyethylene (PE).

To protect the environment from contamination by the resin and the liner from damage during installation the preliner is inverted into the pipe in advance. In the case of the synthetic fibre liners, the laminate serves as a carrier material for the resin matrix – a combination, which forms a structurally load-bearing pipe in the cured end state. The synthetic fibres do not have any strength-increasing or stiffening function.

The glass liner system consists basically of five layers. A temporary inner film, a resin-rich layer, a glass-fibre layer, a pure resin layer or nonwoven layer, and finally an outer protective film. The inner film serves as an installation aid and is removed again after curing of the liner The resin-rich inner layer made of UP resin is made from glass fibre rovings of nonwoven layers and is intended to act as a wear layer The next layer is the glass fibre laminate impregnated with unsaturated polyester resin. On the outside the laminate is a pure resin or nonwoven layer. An outer film is in turn laid around this, which protects the liner from mechanical damage during installation in the sewer, contact with groundwater and UV light. The latter would trigger premature curing of the liner during storage or transport. In this type of liner, the glass fibres in conjunction with the resin matrix act like reinforcement and have a strength-increasing effect.

The thickness of the liner serves in combination with the mechanical properties to ensure the structural integrity of the installed liner. The glass fibre liner benefits from its higher mechanical properties when it comes to thickness but for the basic properties of the CIPP systems, for example, its physical or chemical resistance, leak tightness or operating reliability the parameters are not depending on the thickness before a recommended minimum of 3 mm. The higher thickness of the needle felt liner could cause lower flow rate in the relined pipe. But calculations shows that this is an insignificant effect compared to the manholes, bends and irregularities in the line. Therefore, it is difficult to argue that the E-modules is the main measure of liner quality.

Regarding the installation options, a differentiation is made between inversion with water or compressed air, pulling in or a combination of both. The liner can be cured either with heat from hot water or steam or light from UV or LED lamps as well as a combination of heat curing and UV light. Table 1 shows an overview of the possible uses and use limits of the two types of liner.

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Table 1: Possible uses and use limits

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“Unspecified material” tender procedure

Within the scope of an invitation to tender, according to VOB/A §7 [[5]], the work and services must be described clearly and exhaustively so that the bidders must understand them in the same sense and, among other things, can calculate their prices reliably. The specified performance must be feasible and must not burden the contractor with any unusual risk. According to §7, Paragraph 2, the technical specifications may only refer to a special method, a product provided by a specific company, or to brands, patents, etc. in justified exceptional cases. For example, if the subject of the contract award cannot be described sufficiently and in a way that is generally understandable, the tender specification must be supplemented with the note “or equivalent”. Conversely, the party inviting the tenders must understand methods that are not practical in the individual case in hand in order to exclude them from the competition.

For the tender specification of pipe lining methods, this means that: As long as the different types of installation and curing are possible for a renovation due to the prevailing local boundary conditions, the material used (synthetic or glass fibre liner) may not be an excluding criterion. On the contrary: According to §2 VOB/A no company may be discriminated against in the award of construction works and competition-limiting and unfair or dishonest behaviour must be combated. The group of potential bidders can be extended by “unspecified material” tender specification for the pipe lining. Implementable and reasonable variant solutions should also be allowed. The fact that the individual companies are able to submit tenders for the technically most sensible and cost-effective option in their view, based on the tender specification documents, can only be an advantage for the client.

However, the suitability of the bidder to meet the work offered for tender and proofs of the quality of the materials to be used should be further important criteria for the contract award. Quality markings, recognised test and quality seals, DIBt approvals, references and independent quality controls provide good orientation aids in this respect.

Good experience in Hanover

The municipal drainage company of?Lower Saxony state capital Hannover opts for method-neutral and thus “unspecified material” tender specifications for renovation measures using CIPP. For example, in the rehabilitation area Wülfel Süd, in which – starting in 2017 – around 5.9 km wastewater and stormwater sewers have been renovated. In the method-neutral tender specification, only the method group with pipe lining to be used was specified. The “sewer rehabilitation” team of Hannover’s municipal drainage company considers proper rehabilitation planning and “unspecified material” tender specification to be prerequisite for sustainable rehabilitation success: “We describe the local situation precisely and in detail.

This procedure offers companies the opportunity to tender the method that in their opinion is optimal for the construction project. Due to this method freedom, we assume that we receive the best and most cost-effective tender for the respective rehabilitation project; in addition, no one is disadvantaged in the invitations to tender. We have acquired good experience with this procedure so far.” The individual tenders must first pass through the complete legal contract award process, which as the municipal drainage company explains, includes all contract award checking steps. Among other things, this also includes verifications such as DIBt approvals of the methods and the presence of the relevant quality sign of the sewer construction quality association (Gütegemeinschaft Kanalbau).

By meeting the quality and test provision requirements of RAL-GZ 961, bidders demonstrate their particular technical qualification (technical knowledge and skills, technical performance and reliability in technical contract performance), which are required by clients, particularly for the award of construction works as defined in §6a Para.3 of VOB/A. If multiple tenders remain at the end, the municipal drainage company of Hannover is also obliged to award the contract to the most cost-effective tender.

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From the call for tenders to the tender submission

Aarsleff Rohrsanierung was signed to carry out the rehabilitation works in Wülfel Süd. Yet before the team backing the Aarsleff site engineer and ZKS consultant Maik R?deker could begin with the execution, the entire project had to be estimated for the submission of the tender. R?deker describes the process from the invitation to tender to the submission of the tender in this way: “After examining the design documents of the municipal drainage company of Hannover, we first defined rehabilitation sections for the different methods: needle felt/steam, needle felt/hot water and glass fibre/UV light based on the local boundary conditions and under the aspect of cost effectiveness.

Apart from the nominal size of the sewer, the local boundary conditions include the presence of bends, sub-bends or backdrop structures, the gradient of the pipe, groundwater levels, etc. For example, a straight pipe length between 180 and 200 m can be rehabilitated in one section with a synthetic fibre liner DN 200 and steam curing. Considerations such as these led to the definition of rehabilitation sections and construction phases, which in turn formed the basis for the construction time calculation and the precise cost estimating.”

After the contract award, preliminary CCTV inspections were undertaken to check whether the local boundary conditions corresponded to the description of works and whether the planned methods could be applied technically. “In one case, this led to the planned needle felt liner with nominal size DN 300 and steam curing not being used, because the rehabilitation section ended in another sewer”, remembered R?deker. Due to the limited workspace, an alternative using a UV light chain was not possible. Therefore, the decision was ultimately made in favour of a synthetic fibre liner with hot water curing. R?deker also attributed the ability to easily change between the different methods to the broad-based Aarsleff product portfolio.

In total, in Hannover, 4,295 m were rehabilitated successfully with synthetic fibre liner/steam curing DN 200 to DN 400, 863 m with glass fibre liners/UV curing in nominal sizes DN 500 to DN 1000 and with synthetic fibre liner/hot water curing in 683 m DN 1000 to DN 1200 and 7.8 m with DN 300. The workflows were also optimised so that different works could be carried out in parallel. The result: Instead of the 43 individual weeks of work that would have been necessary for strict sequential completion, the last connection was integrated after only 32 weeks.

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Conclusion

Renovation work by Cured-In-Place-Pipes has a high standard and has long since established itself as an alternative to renewal. The circumstance that manufacturers and users, with a view to further application options, cost effectiveness and sustainability, continue to work together to improve the technical performance, creates many viable solutions. Therefore, there is not one liner system, which is the best and only one for all rehabilitation measures. Rather, the local boundary conditions or the cost effectiveness decide which liner and which installation or curing method represents the best solution. Clients who leave precisely this choice open in the tenders receive the opportunity of a broader competition and thus the chance of achieving the technically most reasonable and most cost-effective result.

The municipal drainage company of Hannover has acquired very good experiences with this type of tender specifications to date. Furthermore, the success of a rehabilitation project needs a reliable partner, who has the necessary know-how, the technical expertise and technical equipment.?Aarsleff Rohrsanierung GmbH, a subsidiary of Aarsleff Pipe Technologies, is such a reliable partner, which can always tender the suitable method according to the technical, economic and enviromental aspects. With a broad product portfolio, which also extends beyond the different pipe lining systems, as a bidder, the company has no corporate or economically motivated preferences for individual methods. Aarsleff can also offer integrated rehabilitation concepts for connection pipes, manholes, large profiles, main sewers, structures, syphons, etc. from a single source, from which the clients ultimately benefit.

[1] EN 15885: Classification and characteristics of techniques for renovation, repair and replacement of drains and sewers; German version DIN EN 15885:2018; Beuth Verlag (2018).

[2] EN ISO 11296-4: Plastics piping systems for renovation of underground non-pressure drainage and sewerage networks – Part 4: Lining with cured-in-place pipes; German version DIN EN ISO 11296-4:2018 + A1:2021; Beuth Verlag (2021).

[3] DWA-A 143-3: Sanierung von Entw?sserungssystemen au?erhalb von Geb?uden – Teil 3: Vor Ort h?rtende Schlauchliner; Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall e. V. (DWA, May 2014) (Rehabilitation of drainage systems outside buildings with cured in place pipes).

[4] EN 752: Drain and sewer systems outside buildings - Sewer system management; German version DIN EN 752:2017; Beuth Verlag (2017).

[5] Vergabe- und Vertragsordnung für Bauleistungen – Teil A, 2019 version. https://dejure.org/gesetze/VOB-A (retrieved in February 2022) (German Construction Contract Procedures - Part A - VOB-A)