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Pipe Liners: Combined Wall Thickness and Unrealistic Moduli of Elasticity Adieu


The so-called resin-rich layers of pipe liners have been subjected to very specific treatment on the German market since the introduction of DIN EN ISO 11296-4 in 2011. The scope for interpretation resulted in a competition developing to achieve the highest modulus of elasticity, which then became synonymous with high quality. However, the resulting characteristic values are dubious in terms of their technical basis. This practice continues to be upheld, although the update to the product standard has now rendered the basis unfounded.  The introduction of the AoC document for pipe liners is another reason to deviate from the unique German path.


In August 2021, the so-called AoC document for hose liners was published, representing the ISO/ TS 23818-2 [1] pre-standard. It regulates the product requirements for liners used in the gravity segment pursuant to EN ISO 11296-4 [2] and for the pressure segment pursuant to EN ISO 11297-4 [4]. AoC stands for Assessment of Conformity.

Chapter 6.1.2 of the ISO/TS 23818-2 pre-standard makes a further contribution to ending the unique German path for describing the mechanical properties of pipe liners. In contrast to usual international practice, the modulus of elasticity of liners in Germany is still related to the combined thickness. The DWA rules set forth in the current M 144-3 [5] Code of Practice specify the modulus of elasticity groups in relation to the combined thickness, even after publication of the new version of DIN EN ISO 11296-4. This is a geometric reference that no longer exists in the currently valid product standard.  The previous version of the standard, from which this definition originated, was retracted over three years ago [3]. The current version only deals with the composite thickness, which differs significantly from the combined thickness.

Unlike the now amended, internationally accepted definition, which originally had a different objective, the mathematical subtraction of so-called resin-rich layers has been handled very specifically on the German market since the introduction of DIN EN ISO 11296-4 in 2011. This German approach was – and still is – supported by the specifications contained in the National Technical Approvals (abZ)/General Construction Technique Permit (aBG) for pipe liners issued by the German Institute for Building Technology (DIBt). Here too, the characteristic values are still related to the combined thickness of the now superseded standard.

ISO/TS 23818-2 clearly stipulates that the mechanical properties of pipe liners must be related to the composite thickness. Moreover, pipe liners are grouped on the basis of diameters and ring stiffnesses in the course of conformity testing. The classification of pipe liners on the basis of ring stiffnesses represents further alignment with accepted international practice applicable to pipes in general. Pipes, and in this case pipe liners, are typically classified fundamentally as construction products using precisely the two characteristic values of diameter and ring stiffness as the basis.

Function and Significance of ISO/TS 23818

AoC documents are of key significance when it comes to deciding whether construction products meet the requirements of the respective product standard. AoC documents can be divided into two groups in European product standards. Harmonized product standards exist in which the AoC document forms part of the standard as a so-called ZA Annex. Such harmonized product standards are developed on behalf of the European Commission. They are generally introduced in accordance with building regulations, and the ZA Annex is therefore binding. If the construction products already conform, they receive the CE mark. There is currently only one harmonized product standard applicable to pipes, DIN EN 295-1 [6], which sets forth the requirements for vitrified clay pipes.

The second group comprises the large number of non-harmonized product standards, including those applicable to pipe liners, for example. These standards do not contain a ZA annex. Supplementary regulations exist for the Assessment of Conformity in the form of independent documents, which often have the character of a TS (Technical Specification). These are somewhat misleadingly called ‘Vornorms’ (pre-standards) in German. Frequently, Technical Specifications are not converted into Technical Standards. CE marking on the basis of these AoC documents is not possible.

Technical Specifications are one level below the international EN or ISO standards. Irrespective of this, they naturally define the state of the art. As soon as they are published they are used by manufacturers, certification bodies and users to evaluate construction products.

Under item 1 “Scope”, ISO/TS 23818-2 states that it governs the conformity assessment of pipe liners pursuant to the relevant parts 4 of the EN ISO 11296, 11297 and 11298 families of standards. It is intended for inclusion in manufacturers’ quality plans as part of their quality management system and as the basis for certification procedures. This means that type testing, factory production control and external monitoring of pipe liners will in future be carried out pursuant to ISO/TS 23818.

On the basis of the TS, liners must first be classified on the basis of their diameters and ring stiffnesses. When grouping liners, the modulus of elasticity is not included as a characteristic value. All mechanical properties are, of course, based on the definitions set forth in the current standard. In future, the modulus of elasticity will no longer be the focus of mechanical evaluation. The ring stiffness of liners will become increasingly important. Development is therefore following the direction set by pipes in general and GFRP pipes in particular. Manufacturers of GFRP pipes do not specify a modulus of elasticity for their products. Technical evaluation takes place on the basis of ring stiffness, because that is the factor used in the calculations.

Because their businesses are internationally oriented, manufacturers of pipe liners will prepare technical data sheets with standardized characteristic values in order to avoid confusion. This will mean parting with using the modulus of elasticity in relation to the combined thickness.

With the introduction of ISO/TS 23818-2, there is no need for a different testing methodology, only a different method of calculation for determining the characteristic values. This needs to be emphasized once again for the sake of completeness.


Relationships Between Moduli of Elasticity With Different Geometric References and Ring Stiffnesses

The most important mechanical and geometrical characteristic values for pipe liners, modulus of elasticity, wall thickness and ring stiffness are known to be inextricably linked. There are various possible combinations for describing the mechanical properties of the product. The stability of the liner is ultimately determined on the basis of its ring stiffness, which is calculated from the modulus of elasticity and wall thickness. The wall thickness flows into this calculation by taking its geometrical moment of inertia into consideration. When pipe liners undergo materials testing, stiffness is always determined in any event, i.e. how resistant they are to deformation. The modulus of elasticity is then derived mathematically from the stiffness on the basis of the wall thickness.

Taking into consideration the current product standards for pipe liners, the accompanying DWA (German Association for Water Economy, Waste Water and Waste) Set of Rules and the German National Technical Approval (abZ)/General Construction Technique Permit (aBG), there are four different moduli of elasticity for a specific pipe liner alone. These are short-term values which must, in turn, be converted into long-term values by applying a reduction factor. This means that pipe liners play a special role within the pipe product family. Normally – and sensibly – only one modulus of elasticity exists for a material.

Kompositdicke 3

Fig. 1: Preparation for installing the pipe liner: Insertion of the gliding foil

Kompositdicke 4

Fig. 2: Insertion of the SAERTEX-LINER MULTI, TYPE S+

Kompositdicke 2

Fig. 3: 3-point bending test on the SAERTEX-LINER MULTI Type S+.

Table 1: Characteristic values for the SAERTEX-LINER MULTI and INDUSTRY Type S+ as requirements in the 3-point bending tests


Modulus of elasticity

(combined wall thickness)

Modulus of elasticity

(composite thickness)

3-ply 14,500 MPa 9,100 MPa
4 - 5-ply 16,800 MPa (IST) 12,600 MPa
6 - 15-ply 16,800 MPa (IST) 14,200 MPa


In the following, the four numerical values for the SAERTEX-LINER MULTI and INDUSTRY Type S+ (see Fig. 1 and Fig. 2) are given as examples for clarification purposes. These values are typical for glass-fiber-reinforced pipe liners in the high-performance segment.

  1. Modulus of elasticity from the test on the circular ring, referenced to the combined thickness, see abZ/aBG: 20,500 MPa. This characteristic value is used in the structural analysis.
  2. Modulus of elasticity from the test on the circular ring, referenced to the composite thickness, to-date only internal characteristic values, see illustration in the following point.
  3. The modulus of elasticity from the 3-point test on the curved liner section, referenced to the combined thickness, see abZ/aBG: 16,800 MPa. This characteristic value is used to test the quality of the liner on the construction site.
  4. Modulus of elasticity from the 3-point test on the curved liner section, referenced to the composite thickness, to-date only internal characteristic values, see illustration in the following point.

In recent years, a competition has developed among pipe liner manufacturers in Germany to achieve the highest modulus of elasticity. A high modulus of elasticity has become synonymous with high quality pipe liners. This is reflected in the regularly published supplementary versions of DWA Data Sheet M 144-3 with ever higher moduli of elasticity, as shown in Table 2 for the so-called material characteristic groups. With five supplements already issued over the past nearly ten years, the maximum modulus of elasticity value has almost doubled.

Alongside technical development, the increase in the modulus of elasticity is also an expression of the design possibilities based on the 2011 version of DIN EN 11296-4. By making the largest possible geometric deductions in terms of the so-called resin-rich layers, very high moduli of elasticity can be calculated. For a given stiffness of a specific liner, the modulus of elasticity will increase accordingly as more layer thickness is subtracted from the total wall thickness by re-classifying it as a resin-rich layer. The highest characteristic values for the long-term modulus of elasticity of pipe liners in the current version of the DWA bulletin lie within the range of the maximum values of GFRP pipes with ideal circumferential reinforcement, as described elsewhere in the DWA Set of Rules. This appears to lack credibility, because GFRP pipes have not only undergone a longer period of development, but are also manufactured using optimized industrial processes and are not produced on the construction site in the same manner as liners. Geometric subtractions are sometimes used to determine characteristic values for pipe liners that lie above the technical limits of GFRP. At maximum fiber content and ideal orientation, this limit results in a modulus of elasticity of around 35,000 MPa.

The competition surrounding this one characteristic value is essentially absurd, because the modulus of elasticity is only one building block on the way to achieving the required ring stiffness. Strictly speaking, the modulus of elasticity is even far less effective than the wall thickness. The ring stiffness is calculated using the cube of the wall thickness. Increasing the modulus of elasticity by 20 % increases the ring stiffness by exactly the same percentage. Increasing the wall thickness by 20 % is accompanied by an increase in the ring stiffness to 173 %.

New Characteristic

Values for SmC Pipe Liners The future adaptation of the characteristic values to the geometric reference set forth in the current version of DIN EN ISO 11296-4, i.e. to the design, means a significant reduction in the modulus of elasticity, since the composite thickness is usually greater than the combined thickness. The composite thickness of a liner is the total thickness minus the thickness of the thermoplastic inner or outer films, if present. The film thicknesses must be specified in the technical data sheets.

This eliminates the common practice of measuring the liners, which employed either the very time-consuming and not very objective measurement of the resin-rich layers, or the equally unsuitable flat-rate deduction in accordance with the manufacturer's specifications. Usually, a fixed value was subtracted from the total wall thickness to account for the so-called resin-rich layer, in addition to the films, in order to define the liner thickness as a geometric reference for the modulus of elasticity.

The changed geometric reference means that the modulus of elasticity undergoes a greater change in percentage terms for smaller wall thicknesses than for larger wall thicknesses. The reason for this is the aforementioned high leverage of the wall thickness, the cube of which is used to calculate the moment of inertia. For a given stiffness, changing the wall thickness from 4.0 mm (composite thickness with a flat-rate deduction of 0.5 mm for the resin-rich layers) to 4.5 mm (composite thickness) equates to a reduction of the modulus of elasticity to 70 %. Changing the wall thickness from 10.0 mm to 10.5 mm causes the modulus of elasticity to drop by only 86 %.

SmC has therefore decided to create wall thickness groups for the new characteristic values for the modulus of elasticity. Deviating from the previous practice of having just one modulus of elasticity for each liner type, different moduli of elasticity now exist within these groups. The dependence of the modulus of elasticity on the wall thickness is known for certain manufacturing processes and liner designs. However, this has not generally been an issue in the past. On the basis of the modulus of elasticity, the ring stiffnesses are determined for the pipe liners as a function of composite thickness and outer diameter.

The characteristic values for the SAERTEX-LINER MULTI and INDUSTRY Type S+ as requirements for the 3-point bending tests (Fig. 3) are reproduced as examples in Table 1. The table also shows the changed geometric references and future liner designations in relation to their wall thicknesses.

In the course of re-evaluation and conversion, new characteristic values were determined for the liner with a composite thickness of 3.5 mm by means of an extensive test program, taking into account statistical scatter. The new characteristic values are also shown in Table 1.

The numerical values in the table were derived by converting them from the characteristic values in the abZ. In future, they will be the subject of material testing performed on the basis of the applicable regulations. The changed characteristic values are only the result of a new evaluation of the proven measurement methods. No changes have been made to the liner design or to the raw materials. The existing approval remains fully valid. SmC will gradually convert its characteristic values and, for a certain period of time, use the characteristic values pursuant to the old and new versions of the product standard in parallel. Of course, this changeover will not affect the required wall thicknesses resulting from the verification of stability certification, i.e. it will not affect the actual selection of the suitable liner.  Within the required ring stiffness, the weights are only mathematically shifted from the modulus of elasticity towards the wall thickness. The conversion of bending stiffness to ring stiffness, as the basis of the structural analysis, will take place using a fixed factor. This is also derived from the characteristic values of the abZ/aBG.


[1] ISO/TS 23818-2 „Konformitätsbewertung von Kunststoffrohrleitungssystemen zur Sanierung von bestehenden Rohrleitungen – Teil 2: Harz-Faser Verbundwerkstoff (RCF)“ (2021-08)

[2] DIN EN ISO 11296-4 „Kunststoff-Rohrleitungssysteme für die Renovierung von erdverlegten drucklosen Entwässerungsnetzen (Freispiegelleitungen) – Teil 4: Vor Ort härtendes Schlauch-Lining“ (2018-09)

[3] DIN EN ISO 11296-4 „Kunststoff-Rohrleitungssysteme für die Renovierung von erdverlegten drucklosen Entwässerungsnetzen (Freispiegelleitungen) – Teil 4: Vor Ort härtendes Schlauch-Lining“ (2011-07)

[4] DIN EN ISO 11297-4 „Kunststoff-Rohrleitungssysteme für die Renovierung von erdverlegten Abwasserdruckleitungen – Teil 4: Vor Ort härtendes Schlauch-Lining“ (2018-09)

[5] DWA M 144-3 „Zusätzliche Technische Vertragsbedingungen (ZTV) für die Sanierung von Entwässerungssystemen außerhalb von Gebäuden – Teil 3: Renovierung mit Schlauchliningverfahren (vor Ort härtendes Schlauchlining) für Abwasserkanäle“ (2018-12)

[6] DIN EN 295-1 „Steinzeugrohrsysteme für Abwasserleitungen und -kanäle – Teil 1: Anforderungen an Rohre, Formstücke und Verbindungen“ (2013)


Ricky Selle, Dr. Eng.

Managing Director

Selle Consult GmbH, Leipzig

Tel: +49 341 30 82 410

Dr. Nils Füchtjohann

Global Director Products

SAERTEX multiCom, Saerbeck

Tel: +49 2574 902 502

Thanks to 3R for providing the article.