ISO 16611:2026

International
Standard
ISO 16611
Second edition
Plastics piping systems for
2026-01
drainage and sewerage without
pressure — Non-circular pipes and
joints made of glass-reinforced
thermosetting plastics (GRP) based
on unsaturated polyester resins
(UP) — Dimensions, requirements
and tests
Systèmes de canalisation en plastiques pour réseaux
d'assainissement sans pression — Tuyaux non-circulaires,
assemblages de tuyaux et raccords en plastiques
thermodurcissables renforcés de verre (PRV) à base de résines de
polyester non saturé (UP) — Dimensions, exigences et essais
Reference number
© ISO 2026
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ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 General requirements . 8
4.1 Classification .8
4.1.1 Categories .8
4.1.2 Nominal size .8
4.2 Materials .8
4.2.1 General .8
4.2.2 Reinforcement .9
4.2.3 Resin .9
4.2.4 Aggregates and fillers .9
4.2.5 Elastomers .9
4.2.6 Metals .9
4.3 Wall construction .9
4.3.1 Inner layer .9
4.3.2 Structural layer .10
4.3.3 Outer layer .10
4.3.4 Appearance .10
4.4 Reference conditions for testing .10
4.4.1 Temperature .10
4.4.2 Properties of water for testing .10
4.4.3 Loading conditions .10
4.4.4 Conditioning .10
4.4.5 Measurement of dimensions .10
4.4.6 Elapsed time for determination of long-term properties, (x) .11
4.5 Joints .11
4.5.1 General .11
4.5.2 Types of joint .11
4.5.3 Flexibility of the jointing system .11
4.5.4 Sealing ring . . .11
4.5.5 Adhesives .11
4.6 Assessment of conformity .11
5 Pipes .12
5.1 Geometrical characteristics — Dimensions . 12
5.2 Mechanical characteristics . 13
5.2.1 General . 13
5.2.2 Short-term bending modulus . 13
5.2.3 Long-term creep bending modulus.14
5.2.4 Short-term bending resistance .14
5.2.5 Long-term bending strength . 15
5.2.6 Resistance to strain corrosion .17
5.2.7 Minimum initial longitudinal tensile strength .17
5.3 Marking .18
6 Fittings . 19
6.1 All types .19
6.1.1 General .19
6.1.2 Diameter series .19
6.1.3 Mechanical characteristics of fittings .19
6.1.4 Installed leak tightness of fittings .19
6.1.5 Dimensions .19

iii
6.2 Marking .19
7 Joint performance .20
7.1 General . 20
7.1.1 Requirements . 20
7.1.2 Test temperature . 20
7.1.3 Dimensions . 20
7.2 Flexible joints . 20
7.2.1 General . 20
7.2.2 Maximum allowable draw. 20
7.2.3 Maximum allowable angular deflection . 20
7.2.4 Maximum allowable misalignment. 20
7.2.5 Flexible non-end-load-bearing joints with elastomeric sealing rings .21
7.3 Rigid joints .21
7.3.1 General .21
7.3.2 Number of test pieces for type testing . 22
7.3.3 Test pieces . 22
Annex A (informative) Static calculation methods .23
Bibliography .24

iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 138, Plastics pipes, fittings and valves for the
transport of fluids, Subcommittee SC 6, Reinforced plastics pipes and fittings for all applications.
This second edition cancels and replaces the first edition (ISO 16611:2017), which has been technically
revised.
The main changes are as follows:
— the normative references have been updated;
— the definition of the initial ring stiffness has been aligned with that used in other GRP pipe standards,
see 3.3;
— the definition of the wet creep factor has been aligned with that used in other GRP pipe standards, see
3.5;
— the short-term flexural capacity was introduced as alternative parameter to measure the short-term
bending resistance, see 5.2.4;
— a new subclause on “Assessment of conformity” has been introduced (see 4.6);
— inconsistencies in Clause 7 "Joint performance” have been removed.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
Introduction
The purpose of this document is to standardize the requirements for non-circular glass-reinforced
thermosetting plastics pipes (GRP).
This document specifies the requirements for non-circular GRP pipe systems, designed to be used for the
renovation of existing channels or open cut installation.
This document specifies dimensions, requirements and tests of non-circular pipes, including short and long-
term properties. To establish long-term properties, it is recommended to use the circular pipes samples.
Annex A provides commonly used calculation methods for informational purposes. The calculation method
is selected by the designer.
This document does not include the recommended practice for installation.

vi
International Standard ISO 16611:2026(en)
Plastics piping systems for drainage and sewerage
without pressure — Non-circular pipes and joints made
of glass-reinforced thermosetting plastics (GRP) based
on unsaturated polyester resins (UP) — Dimensions,
requirements and tests
1 Scope
This document specifies the required properties of the piping system and its components made from glass-
reinforced thermosetting plastics (GRP), based on unsaturated polyester resin (UP) intended to be used for
drainage or sewerage without pressure including culverts.
It is the responsibility of the purchaser or specifier to make the appropriate selections taking into account
their particular requirements and any relevant national regulations and installation practices or codes.
This document is applicable to non-circular GRP-UP pipes, with flexible or rigid joints, primarily intended
for use in buried installations but may also be used to reline existing non-circular pipe lines.
NOTE 1 GRP-UP includes pipes with vinyl ester liners or made entirely from vinyl ester.
NOTE 2 Piping systems conforming to this document can be used also for non-buried applications provided the
influence of the environment, e.g. from UV-radiation, and the supports are considered in the design of the pipes,
fittings and joints.
This document is applicable to pipes, fittings and their joints of nominal sizes from DN 150 to DN 4000,
which are intended to be used for the conveyance of surface water or sewage at normal service conditions.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 75-2, Plastics — Determination of temperature of deflection under load — Part 2: Plastics and ebonite
ISO 178, Plastics — Determination of flexural properties
ISO 3126, Plastics piping systems — Plastics components — Determination of dimensions
ISO 4633, Rubber seals — Joint rings for water supply, drainage and sewerage pipelines — Specification for
materials
ISO 8513, Plastics piping systems — Glass-reinforced thermosetting plastics (GRP) pipes — Test methods for the
determination of the initial longitudinal tensile strength
ISO 8533, Glass-reinforced thermosetting plastics (GRP) pipes and fittings — Test methods to prove the design of
cemented or wrapped joints
ISO 8639, Glass-reinforced thermosetting plastics (GRP) pipes and fittings — Test methods for leaktightness and
proof of structural design of flexible joints
ISO 10468, Glass-reinforced thermosetting plastics (GRP) pipes — Determination of the ring creep properties
under wet or dry conditions
ISO 10928, Plastics piping systems — Glass-reinforced thermosetting plastics (GRP) pipes and fittings —
Methods for regression analysis and their use
ISO 10952, Glass-reinforced thermosetting plastics (GRP) pipes and fittings — Determination of the resistance
to chemical attack for the inside of a section in a deflected condition
EN 681-1, Elastomeric seals — Material requirements for pipe joint seals used in water and drainage applications
— Part 1: Vulcanized rubber
CEN/TS 14632, Plastics piping systems for drainage, sewerage and water supply, pressure and non-pressure —
Glass-reinforced thermosetting plastics (GRP) based on unsaturated polyester resin (UP) — Guidance for the
assessment of conformity
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
nominal size
BN × HN
alphanumerical designation of size, which is common to all components in a piping system, it is a convenient
pair of numbers for reference purposes that is related to the maximum internal breadth (B) and the
maximum internal height (H)
Note 1 to entry: The designation for reference or marking purposes consists of the letters BN(B) × HN(H) and the
numerical values for (B) and (H), when expressed in millimetres.
3.2
specific ring stiffness
S
physical characteristic of a circular pipe which is a measure of the resistance to ring deflection per metre
length under external load
Note 1 to entry: Specific ring stiffness is determined using the formula below and is expressed in Newtons per square
metre (N/m ):
EI
S 
d
m
where
E is the apparent modulus of elasticity, which can be derived from the result of the ring stiffness
[1] 2
test, i.e. ISO 7685, expressed in Newtons per square metre (N/m );
d is the mean diameter of the pipe, in metres (m) (see 3.5);
m
I is the second moment of area in the longitudinal direction per metre length, in metres to the
fourth power per metre, (m /m):
e
I=
where e is the wall thickness of the pipe, in metres.

3.3
initial ring stiffness
S
[1]
value of ring stiffness S obtained when a circular pipe is tested in accordance with ISO 7685
Note 1 to entry: Initial ring stiffness is expressed in newtons per square metre (N/m ).
3.4
mean diameter
d
m
diameter of the circle corresponding to the middle of a circular pipe wall cross-section and given by either of
the following formulae:
dde
mi
or
dde
me
where
d is the internal diameter, in millimetres(mm);
i
d is the external diameter, in millimetres (mm);
e
e is the wall thickness of the pipe, in millimetres(mm).
Note 1 to entry: It is expressed in millimetres (mm).
3.5
wet creep factor
α
x,creep,wet
ratio of the long-term ring creep stiffness, S at x years to the ring stiffness S at position 1 at 0,1 hour
x,wet 0,1
Note 1 to entry: See 4.4.6 for value of x.
Note 2 to entry: Determined using circular test-pieces under sustained diametrical loading in wet conditions when
tested in accordance with ISO 10468
.
Note 3 to entry: Wet creep factor is a dimensionless number given by the formula:
S
x,wet
 
x,creep,wet
S
01,
3.6
long-term ring stiffness
S
x,wet
determined value of ring stiffness S at x years for circular pipe
3.7
rerating factor
R
RF
multiplication factor that quantifies the relationship of a product’s mechanical, physical and chemical
properties under service conditions above 35 °C [design service temperature (3.17)] to those applicable at a
standard test temperature of 23 °C

3.8
short-term bending strength
σ
f
maximum flexural stress sustained by the test specimen during a bending test, when tested in accordance
with ISO 178
Note 1 to entry: Short-term bending strength is determined using the following formula and is expressed in
megapascals (MPa):
3FL
 
f
2bh
where
F is the load, in newtons (N);
L is the span, in millimetres (mm);
h is the thickness of the specimen, in millimetres (mm);
b is the width of the specimen, in millimetres (mm).

3.9
long-term bending strength
σ
fx
calculated value of σ at x years for circular pipe
f
Note 1 to entry: Long-term bending strength is determined using methodology given in 5.2.5.3, using linear regression
analysis according to ISO 10928 on data of bending stress in hoop direction, and is expressed in megapascals (MPa):
d 1
m
6F 
max
2 
 
fx ki
le
with
35de
i
 
ki
33de
i
where
F is the maximum force, in newtons (N);
max
d is the mean diameter, in millimetres (mm);
m
d is the inside diameter, in (mm);
i
e is the wall thickness, in millimetres (mm);
l is the length of specimen, in (mm);
α is the correction factor inner surface.
ki
3.10
short-term bending modulus
E
f
value of flexural modulus obtained when tested in accordance with ISO 178
Note 1 to entry: Short-term bending modulus is determined using the following formula and is expressed in
megapascals (MPa):
F
L
max
E  
f
s
4bh max
where
F is the maximum force, which corresponds to maximum deflection, in newtons (N);
max
s is the maximum deflection of specimen, in millimetres (mm);
max
b is the width of the specimen, in millimetres (mm);
h is the wall thickness of the specimen, in millimetres (mm);
L is the span of the specimen, in millimetres (mm).

3.11
long-term creep bending modulus
E
fx
calculated value of E at x years for circular pipe when tested in accordance with ISO 10468
f
Note 1 to entry: Long-term bending modulus is determined using the following formula and is expressed in
megapascals (MPa):
EE 
f0xx ,creep,wet
where
E is the short-term bending modulus as a result of the creep test on circular profiles according to
ISO 10468, in megapascals (MPa);
α is the wet creep factor, see 3.6.
x,creep,wet
3.12
declared flexural capacity
M
B
maximum unitary bending moment declared by the manufacturer
Note 1 to entry: Declared flexural capacity is determined using the following formula and is expressed in N·mm/mm:
 e
fd
M 
B
where
σ is the declared short-term bending strength, in megapascals (MPa);
fd
e is the wall thickness of the pipe, in millimetres (mm).

3.13
short-term flexural capacity
M
B,u
maximum unitary bending moment sustained by the test specimen during a bending test, when tested in
accordance with ISO 178
Note 1 to entry: flexural capacity is determined using the following formula and is expressed in N·mm/mm:

FL
M 
B,u
4b
where
F is the load corresponding to the maximum flexural stress σ as defined in ISO 178, in Newtons
fM
(N);
L is the span of the specimen, in millimetres (mm);
b is the width of the specimen, in millimetres (mm).

3.14
type test
test carried out to prove that a material, component, joint or assembly is capable of conforming to the
relevant requirement
3.15
laying length
total length of a pipe minus, where applicable, the manufacturer's recommended insertion depth of the
spigot(s) in the socket
Note 1 to entry: Laying length is expressed in metres (m).
3.16
normal service conditions
conveyance of surface water or sewage, in the temperature range 2 °C to 50 °C, with maximum water table
of 10 m, for 50 years
3.17
design service temperature
maximum sustained temperature at which the system is expected to operate
Note 1 to entry: Expressed in degrees Celsius (°C).
3.18
flexible joint
joint which allows relative movement between the pipes being joined
Note 1 to entry: Examples of this type of joint are: socket-and-spigot joint with an elastomeric sealing element
(including double socket designs)
3.19
rigid joint
joint which does not allow relative movement between the pipes being joined
Note 1 to entry: Examples of this type of joint are: flanged joint, including integral and loose flanges, wrapped or
cemented joint.
3.20
angular deflection
δ
angle between the axes of two adjacent pipes
Note 1 to entry: See Figure 1.
Note 2 to entry: Angular deflection is expressed in degrees (°).

3.21
draw
D
longitudinal movement of a joint
Note 1 to entry: See Figure 1.
Note 2 to entry: Draw is expressed in millimetres (mm).
3.22
total draw
T
sum of the draw, D, and the additional longitudinal movement, J, due to the presence of angular deflection
Note 1 to entry: See Figure 1.
Note 2 to entry: Total draw is expressed in millimetres (mm).
3.23
misalignment
M
distance by which the centrelines of adjacent pipes fail to coincide
Note 1 to entry: See Figure 1.
Note 2 to entry: Misalignment is expressed in millimetres (mm).
a)
b)
c)
d)
Key
D draw
J longitudinal movement arising from angular deflection of the joint
δ angular deflection of the joint
T total draw
M misalignment
Figure 1 — Joint movement
4 General requirements
4.1 Classification
4.1.1 Categories
Pipes conforming to this document shall be classified according to nominal size (BN × HN) (see 3.1), and
joint type, i.e. flexible or rigid (
...