ISO 18674-7:2025
(Main)Geotechnical investigation and testing — Geotechnical monitoring by field instrumentation — Part 7: Measurement of strains: Strain gauges
Geotechnical investigation and testing — Geotechnical monitoring by field instrumentation — Part 7: Measurement of strains: Strain gauges
This document specifies the measurement of strain by means of strain gauges and strainmeters carried out for geotechnical monitoring. General rules of performance monitoring of the ground, of structures interacting with the ground, of geotechnical fills and of geotechnical works are presented in ISO 18674-1. This document is applicable to: — performance monitoring of — 1-D structural members such as piles, struts, props and anchor tendons; — 2-D structural members such as foundation plates, sheet piles, diaphragm walls, retaining walls and shotcrete/concrete tunnel linings; — 3-D structural members such as gravity dams, earth- and rock-fill dams, embankments and reinforced soil structures; — checking geotechnical designs and adjustment of construction in connection with the observational design procedure; — evaluating stability during or after construction. With the aid of a stress-strain relationship of the material, strain data can be converted into stress and/or forces (for 1-D members; see ISO 18674-8) or stresses (for 2-D and 3-D members, see ISO 18674-5). NOTE This document fulfils the requirements for the performance monitoring of the ground, of structures interacting with the ground and of geotechnical works by the means of strain measuring instruments as part of the geotechnical investigation and testing in accordance with References [1] and [2].
Reconnaissance et essais géotechniques — Surveillance géotechnique par instrumentation in situ — Partie 7: Mesure des déformations : jauges de déformation
General Information
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Standards Content (Sample)
International
Standard
ISO 18674-7
First edition
Geotechnical investigation and
2025-11
testing — Geotechnical monitoring
by field instrumentation —
Part 7:
Measurement of strains: Strain gauges
Reconnaissance et essais géotechniques — Surveillance
géotechnique par instrumentation in situ —
Partie 7: Mesure des déformations : jauges de déformation
Reference number
© ISO 2025
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Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 3
5 Instruments. 4
5.1 General .4
5.2 Strain gauges .5
5.2.1 Surface-mounted strain gauges .5
5.2.2 Embedded strain gauges .9
5.3 Strainmeters .10
5.4 Instruments for specific applications .10
5.4.1 Monitoring of 1-D structural members .10
5.4.2 Monitoring of 2-D structural members . 12
5.4.3 Monitoring of 3-D structural members .14
6 Installation and measuring procedure . .15
6.1 Installation . 15
6.1.1 Installation of strain gauges . 15
6.1.2 Installation of strainmeters .18
6.2 Measuring procedure .19
6.2.1 Instrumentation check and calibration .19
6.2.2 Measurement .19
7 Data processing and evaluation . 19
8 Reporting . 19
8.1 Installation report .19
8.2 Monitoring report .19
Annex A (normative) Data processing and evaluation.20
Annex B (informative) Distributed fibre optic strain sensing .25
Annex C (informative) Temperature effects on strain measurements .31
Annex D (informative) Geotechnical applications .33
Annex E (informative) Measuring examples .35
Bibliography . 47
iii
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
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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 document 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).
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This document was prepared by Technical Committee ISO/TC 182, Geotechnics, in collaboration with
the European Committee for Standardization (CEN) Technical Committee CEN/TC 341, Geotechnical
Investigation and Testing, in accordance with the Agreement on technical cooperation between ISO and CEN
(Vienna Agreement).
A list of all parts in the ISO 18674 series can be found on the ISO website.
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.
iv
International Standard ISO 18674-7:2025(en)
Geotechnical investigation and testing — Geotechnical
monitoring by field instrumentation —
Part 7:
Measurement of strains: Strain gauges
1 Scope
This document specifies the measurement of strain by means of strain gauges and strainmeters carried
out for geotechnical monitoring. General rules of performance monitoring of the ground, of structures
interacting with the ground, of geotechnical fills and of geotechnical works are presented in ISO 18674-1.
This document is applicable to:
— performance monitoring of
— 1-D structural members such as piles, struts, props and anchor tendons;
— 2-D structural members such as foundation plates, sheet piles, diaphragm walls, retaining walls and
shotcrete/concrete tunnel linings;
— 3-D structural members such as gravity dams, earth- and rock-fill dams, embankments and
reinforced soil structures;
— checking geotechnical designs and adjustment of construction in connection with the observational
design procedure;
— evaluating stability during or after construction.
With the aid of a stress-strain relationship of the material, strain data can be converted into stress and/or
forces (for 1-D members; see ISO 18674-8) or stresses (for 2-D and 3-D members, see ISO 18674-5).
NOTE This document fulfils the requirements for the performance monitoring of the ground, of structures
interacting with the ground and of geotechnical works by the means of strain measuring instruments as part of the
geotechnical investigation and testing in accordance with References [1] and [2].
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 18674-1:2015, Geotechnical investigation and testing — Geotechnical monitoring by field instrumentation
— Part 1: General rules
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18674-1 and the following 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
strain gauge
field instrument for measuring strain
Note 1 to entry: The strain is sensed over the full length of the gauge, commonly by a vibrating wire sensor (VW), an
electrical resistance strain gauge sensor or an Fibre Bragg Grating sensor (FBG) with optical sensing).
Note 2 to entry: Typical configurations are strain gauges mounted to a surface of a steel member [see 3.3 and
Figures 1 a), 1 b), 3 a) and 3 b)], strain gauges embedded in concrete [see 3.4 and Figure 1 c)], or FBG integrated into
the structure or fixed onto the surface [see Figure 3 b)].
Note 3 to entry: A series of FBG sensors with a single lead cable is called an FBG array (see Figure 4).
Note 4 to entry: For mechanical strain gauges, see Reference [3].
Note 5 to entry: Distributed fibre optic strain measurements (DFOS) are not subject to this document, as this new
technology is still under intensive development and change. Usage principles and examples for DFOS are given in
Annex B.
3.2
strainmeter
strain gauge for measuring strain by means of a displacement measurement
Note 1 to entry: The strain is sensed over the defined gauge length of the strainmeter (see Figure 2)
Note 2 to entry: An extensometer with a defined gauge length, for example a probe extensometer (see ISO 18674-2)
has the function of a strainmeter.
Note 3 to entry: A typical configuration is a continuous chain of strainmeters embedded in fill, soil or concrete.
Note 4 to entry: Alternative terms for a strainmeter used in practice are “fill extensometer”, “soil strainmeter”, “soil
extensometer”, “embankment extensometer” or “linear continuous extensometer“
Note 5 to entry: The term strainmeter is sometimes (incorrectly) used for specific strain gauge sensors, e.g. rebar
strainmeter
3.3
surface-mounted strain gauge
strain gauge designed for attachment at the surface of a structural member
Note 1 to entry: There are different types of instruments for surface-mounting: spot-weldable, arc welded and
adhesive bonded strain gauges.
3.4
embedment strain gauge
strain gauge for the embedment in a medium
Note 1 to entry: Typically, the medium is mortar, grout, reinforced concrete, shotcrete or mass concrete.
Note 2 to entry: See Figure 1 c).
3.5
instrumented reinforcement bar
piece of reinforcement bar into which a strain gauge is integrated
Note 1 to entry: When installed alongside the structural reinforcement, this is commonly known as a "sister bar".
Note 2 to entry: When installed as part of the structural reinforcement this is commonly known as a "rebar strain meter".
Note 3 to entry: Sister bars and rebar strain meters measure the same parameter: overall strain in a reinforced
concrete element. The difference in their use is that when using sister bars the overall steel area in the structural
element increases slightly leading to a reduction of the actual strain at the measuring section.
Note 4 to entry: See Figure 5.
3.6
gauge length
L
initial length over which the strain is measured by the strain gauge or initial length over which the
displacement is measured by a strainmeter
Note 1 to entry: For vibrating wire strain gauges and strainmeters the length is defined by the mounting blocks, end
plates or anchors.
Note 2 to entry: For a Fiber Bragg Grating (FBG) strain gauge, depending on the mounting, the gauge length is either
the length of the grating or the distance between the mounting blocks / end plates, anchors or spots of adhesive (see
Figures 3 and 4)
4 Symbols
Table 1 lists all symbols and subscripts used in this document.
Table 1 — symbols
Symbol Definition Unit
A area m
C thermo optic coefficient MHz/°C
T
C strain transfer coefficient MHz/%
ε
D diameter m
d distance m
E Young’s modulus Pa
F normal force N
H height m
L gauge length m
s spacing between strain gauge sensing element and steel surface mm
T temperature °C
t web thickness mm
web
W width m
-1
α coefficient of linear thermal expansion K
T
ΔT change in temperature at location i °C
i
a
Δε change in strain at location i -
i
Δν change in Brillouin peak frequency at location i MHz
bi
a
ε strain -
a
ε axial strain -
ax
ε micro strain µm/m
µ
ν Brillouin peak frequency MHz
b
σ stress Pa
a
Strain and strain changes are expressed as an elongation per m, so m/m, resulting in a dimensionless parameter.
5 Instruments
5.1 General
5.1.1 When using strain gauges for geotechnical monitoring, distinctions should be made between:
a) the type of the strain measuring instrument (strain gauge type versus strainmeter);
b) the location of the measuring point (at the surface of a structural member versus embedded in a
medium);
c) the measurement principle of the instrument (e.g. vibrating wire (VW), Fiber Bragg Grating (FBG) or
electrical resistance)
d) the material in or on which the instrument is placed (e.g. steel, concrete, soil…)
Table 2 and Figures 1 to 5 show different types of instruments, the location of the measuring point and
measurement principles .
NOTE 1 Annex D provides an overview of the various types of strain gauge and strainmeters in some common
geotechnical applications.
NOTE 2 Examples of typical application for various types of strain gauges and strainmeters are presented in
Annex E.
Table 2 — Types of strain gauges and strainmeters in geotechnical monitoring
Common gauge
Strain measuring instrument Location of meas- Member / Common
length, L
Subclause
and common configuration uring point medium sensor
(mm)
a
steel 5 to 150
at the surface of a
surface-mounted strain gauge 5.2.1
vibrating wire
structural member b
concrete 50 to 350
(VW), FBG
c
embedment strain gauge concrete 50 to 500 5.2.2
inside medium
displacement
d
strainmeter fill 1 000 to 3 000 5.3
transducer
a
For example, reinforcing bar, steel pile, sheet pile, strut.
b
For example, pillars, girder, beam.
c
For example, concrete pile, diaphragm wall, gravity dam, shotcrete tunnel lining.
d
For example, earth dam, rock-fill dam, embankment.
5.1.2 In line with the sign convention for most geotechnical engineering applications, compressive strain
should be taken as positive.
NOTE 1 Within the ISO 18674 series, this sign convention is adopted in ISO 18674-4, ISO 18674-5 and ISO 18674-8,
however not ISO 18674-2. The latter (extensometers) adopts the sign convention established in geodesy.
NOTE 2 In ISO 18674-1, no unified rules were set on the sign convention for geotechnical monitoring
instrumentations, as the sign convention is often context dependent. It is emphasized that a clear sign convention is to
be made for each application (see ISO 18674-1:2015).
5.1.3 The instrument shall be selected such that it has negligible influence on the elastic stiffness of the
monitored structure.
5.1.4 The measuring range of the instrument shall be selected according to the expected strain range
within the project. The measuring range of the instrument shall be 25 % higher than the expected strain in
the member to measure.
NOTE Many types of vibrating wire strain gauges and strainmeters allow the adjusting of the range starting point
towards tension or compression.
5.1.5 If the stiffness of the medium is changing in time (e.g. in course of curing or consolidation of the
medium), an instrument should be selected which is designed for a low stiffness in the strain measuring
direction.
5.1.6 For an instrument installed on matrix or composite material, sturdiness and the gauge length of the
strain instrument shall be selected considering the size of the aggregates in the medium. The ratio of the
gauge length to the largest aggregate should be more than 5.
5.1.7 In order to measure homogenous strain, strain instruments shall not be located at or near the ends
of a structural member or connection points with other structural members. A distance of at least three
times the width or the diameter, of the member can be considered as sufficient.
NOTE Recommended distance in dynamic load testing (see Reference [10]) is a distance at least 1,5 times the
width or diameter of pile.
5.1.8 The temperature shall be monitored at the measurement point or at a representative location.
NOTE Commercially available vibrating wire strain gauges generally have integrated temperature sensors.
FBG sensors usually have loose sensing (dummy) elements for compensation of temperature effects or integrated
temperature sensors.
5.1.9 Temperature effects on the instrument as well as on the structure itself shall be taken into account
when reporting and analysing the data.
NOTE 1 Temperature effects on the stresses in the structural members are often large and insufficiently known;
see Annex C.
NOTE 2 The use of strain gauges compensated for specific materials (steel, aluminium, concrete) avoids the need to
correct the measured strain to obtain the structural strain and calculate the stress.
5.1.10 When FBG sensors are used in series, FBG shall be manufactured with different Bragg wavelengths.
NOTE The number of FBG that can be in series will depend on the FBG interrogator as well as the amount of strain
to be measured.
5.2 Strain gauges
5.2.1 Surface-mounted strain gauges
5.2.1.1 The fixing and the type of strain gauge should be selected depending on the surface onto which it
is to be mounted.
NOTE 1 Micro spot-welded gauges [see Figure 1 a)] are commonly applied to rebars. Arc-welded gauges [see
Figure 1 b)] are commonly applied to larger steel members such as struts, steel sets or sheet piles.
NOTE 2 The advantages of the micro spot-welded gauge are its small size and the minimization of the errors that
result from bending of the member due to the close proximity of the strain gauge to the surface of the member.
NOTE 3 If adequate space is available on a steel surface, an arc-welded version is commonly preferred, see Reference [4].
NOTE 4 Adhesive bonding is commonly applied on compound material or for FBG sensors on steel. FBG strain
arrays are sometimes bonded into pre-manufactured grooves of steel members for better protection.
NOTE 5 Strain gauges based on the electrical resistance principle (foils sensors) are commonly applied using
adhesive bonding to a surface.
5.2.1.2 When arc-welding is applied, a dummy rod should be used as a spacer whilst the mounting blocks
are welded [see key reference 1 in Figure 1 b)]. After welding, the dummy rod can be replaced by the strain
gauge sensor [see key references 2, 3 and 4 in Figure 1 b)].
NOTE This procedure is carried out to shield the sensitive strain sensor from arc-welding effects.
5.2.1.3 When mounting blocks are used, these can have set screws acting in different directions.
NOTE 1 As example, see Figure 12 for the position and acting direction of the set screws.
NOTE 2 For examples with mounting blocks see Figures 3 c), Figure 3 d) and Figure 4 a).
5.2.1.4 For gauge length installation of FBG strain arrays and FBG strain gauges, the cable / sensor shall
be pre-tensioned. The pretension level shall consider the strain to be measured.
a) Spot-weldable strain gauge
b) Arc-weldable strain gauge
c) Embedment strain gauge
Key
1 anchor/flange, mounting block/tab 5 substrate/medium (e.g. steel/concrete)
2 plucking coil 6 screw nut on setting screw
3 steel vibrating wire 7 spring
4 protection housing
Figure 1 — Features of vibrating wire strain gauges
Key
1 anchor (here illustrated as flanges) 4 telescopic tube
2 displacement transducer with electric cable 5 extension rod
3 guide 6 sleeve
Figure 2 — Features of a strainmeter
a) Spot-weldable FBG strain gauge
b) Adhesive-bonded FBG strain gauge
c) Bolt-on FBG strain gauge
Key
1 medium (steel / concrete) 5 optical fibre
2 mounting block / end plate 6 FBG (strain or temperature)
3 welding (row or points) 7 epoxy bonding
4 protection housing 8 anchor / bolt
Figure 3 — Features of a FBG strain gauge
a) Clamp-mounted FBG strain array (gauge length installation)
b) Adhesive-bonded FBG strain array (continuous or spot)
Key
1 medium (steel / concrete) 5 FBG (strain)
2 mounting clamp 6 adhesive
3 cable jacket 7 anchor / bolt
4 optical fibre
Figure 4 — Features of FBG strain arrays
5.2.2 Embedded strain gauges
5.2.2.1 Embedment strain gauges can be equipped with flanges at both ends of the gauge. The diameter of
the flanges should be about 10 % to 20 % of the gauge length and at least twice the diameter of the gauge.
NOTE See Figure 1 c).
5.2.2.2 Instrumented reinforcement bars should be long enough to ensure load transfer from the
surrounding concrete.
Key
1 re-bar anchors 4 plucking coil
2 strain sensor body 5 vibrating steel wire
3 strain gauge end blocks
Figure 5 — Features of an instrumented reinforcement bar
5.2.2.3 Embedded FBG strain arrays / cables with a protective coating to avoid direct contact between
concrete and the optical fibre and gratings shall ensure that strain from the external coating/jacket is fully
transferred to the optical fibre.
5.3 Strainmeters
Embedment strainmeters can be either fixed-installed or moved between equally spaced measuring marks
embedded in the medium.
NOTE 1 For fixed-installed strainmeters, see Figure 2.
NOTE 2 For moveable strainmeters, see probe extensometer, ISO 18674-2:2016, 5.3.
NOTE 3 In contrast to strain gauges, embedment strainmeters are often aligned in a continuous chain of single
strainmeter elements; see Figure 8.
5.4 Instruments for specific applications
5.4.1 Monitoring of 1-D structural members
5.4.1.1 The strain-sensing direction of the instrument shall be along the axis of the 1-D structural member.
NOTE Typical 1-D structural members are concrete pile, steel pile, strut, prop, anchor tendon
5.4.1.2 The location of the instrument shall be specified in relation to its distance from the axis.
NOTE In the case of Figure 6 a), the measuring location coincides with the axis.
5.4.1.3 When bending moments occur or are to be measured in addition to axial strain the instrumentation
design should consider the dimensions of the structural member to define the number and location of strain
gauges per measuring section.
EXAMPLE 1 Concrete pile, see Figure 8.
EXAMPLE 2 Steel I-Beam, see Figure 9.
NOTE 1 In the layout of Figure 6 b), there are two opposite strain instrument locations at equal distance, d, to the
axis. This configuration allows monitoring of axial strain and monitoring of bending of the member in a pre-specified
direction.
NOTE 2 The configuration of Figure 6 c) allows monitoring of bending in the principal strain directions.
NOTE 3 Pairs of gauges are typically installed as far as possible from the axis of the steel member and opposite
across the axis (see Figure 7 for a cross-section of an I-beam identifying axis) so as to detect the strain due to bending
when present.
5.4.1.4 Along the axis of a 1-D member, the strain measuring locations may be either discrete or continuous.
For discrete locations, strain gauges should be selected; for continuous measuring lines a continuous chain
of strainmeters should be used.
NOTE 1 Figure 8 shows an example with discrete and continuous strain gauges in a pile.
NOTE 2 Discrete measuring locations require interpolation of the strain measuring data, particularly in respect to
bending. They also might serve as a redundancy check, e.g. for inclinometer measurements (see ISO 18674-3)
a) Axial strain in a pre-specified b) Axial strain plus bending in
c) Axial strain plus bending
direction principal directions
Key
1 concrete pile + pile axis
2 strain measuring location d distance of strain instrument to axis
Figure 6 — Possible strain monitoring layouts in a concrete pile (cross-section)
a) Strain in ZZ-axis and b) Strain in ZZ-axis and
bending around YY-axis bending around XX-axis
Key
1, 2 number of strain gauge
d distance of strain gauge to XX-axis
d distance of strain gauge to YY-axis
a
½ height of I-Beam
b
½ width of I-Beam
Figure 7 — Possible layouts for monitoring of axial strain and bending of a steel I-beam
Key
F normal force
1 ground
2 pile head
3 pile
4 strainmeter (e.g. probe extensometer), continuously aligned in pile axis
5 strain gauges, pair equidistant to pile axis at three discrete measuring locations
Figure 8 — Continuous and discrete strain measuring locations in a concrete pile
5.4.2 Monitoring of 2-D structural members
5.4.2.1 The strain sensing direction of the instrument shall be in the plane of the 2-D structural member.
NOTE Typical 2-D structural members are foundation plate, sheet pile, diaphragm wall, shotcrete/concrete
tunnel lining.
5.4.2.2 For monitoring of the general 2-D strain state at a measuring location, an assemblage of three
independently oriented instruments shall be installed. A rosette adapter can be used to facilitate installation
in pre-specified directions.
NOTE 1 A rosette with three directions 120° apart is the optimal layout.
NOTE 2 More than three instrument orientations create a redundancy which can be useful for checking the
consistency of the measurements.
5.4.2.3 For specific engineering questions, the number of differently oriented instruments per measuring
location may be reduced.
EXAMPLE 1 Bending of a diaphragm wall towards an excavation: one pair of vertically oriented strain gauges
per measuring location (see key reference 6 in Figure 9). This layout of strain instruments can be supplemented or
substituted by vertical inclinometers, see ISO 18674-3.
EXAMPLE 2 Estimate of the circumferential stress developing in a shotcrete tunnel lining due to increasing
ground pressures acting onto the lining: pair of strain gauges oriented in circumferential direction of the tunnel (see
Figure 10) and conversion of strain into stress by employing a stress-strain relationship of the shotcrete.
Key
F normal force 4 1-D member: strut 8 adaptor
1 ground 5 strut strain gauges 9 base of excavation
2 excavation 6 pair of strain gauges 10 topographic surface
3 2-D member: diaphragm wall 7 optional load cell (ISO 18674-8:2022)
Figure 9 — Example of strain measuring locations in 1-D and 2-D structural members (strut and
diaphragm wall)
Key
1 excavation surface 3 strain gauge 5 inner lattice girder
2 shotcrete 4 outer lattice girder 6 surface of shotcrete lining
Figure 10 — Example of strain gauge layout in shotcrete lining
5.4.3 Monitoring of 3-D structural members
5.4.3.1 Strain monitoring in 3-D structural members can be carried out by strain gauges and strainmeters.
NOTE 1 Typical 3-D structural members are mass-concrete structures (see Figure 11), earth dams, embankments.
NOTE 2 High-precision strainmeters are instruments for monitoring the behaviour of the dam structure in
response to changing water levels of the reservoir.
NOTE 3 One purpose of combined strain and temperature measurements is monitoring of the concrete’s curing
process. Commonly, commercially available vibrating wire strain gauges have a built-in temperature sensor.
Key
1 concrete embedment strain gauge with integrated temperature sensor
2 high-precision mobile strainmeter, gauge length 1 m
Figure 11 — Strain measuring instruments in a gravity dam
5.4.3.2 For monitoring of the general 3-D strain state at a measuring location, an assemblage of six
independently oriented strain gauges shall be installed. A rosette adapter can be used to facilitate installation
in pre-specified directions.
NOTE Monitoring of the general 3-D state of strain by strain gauges is theoretically possible, however rarely
carried out in practice.
5.4.3.3 Strain measurements in earth dams and embankments should be carried out by means of a
strainmeter. The instrument can be embedded as a single element, as a cluster of differently orientated
elements or as a chain of several elements.
NOTE A horizontal chain of strainmeter elements embedded inside and near the crest of an earth-fill dam or
embankment is commonly used.
6 Installation and measuring procedure
6.1 Installation
6.1.1 Installation of strain gauges
6.1.1.1 General
6.1.1.1.1 The installation work of strain gauges should be shielded from the activities of the construction
site and protected from adverse weather conditions.
6.1.1.1.2 Strain gauges and their cables shall be protected against mechanical damage which can be caused
by construction activities (e.g. pile driving; pouring of concrete; concrete vibrators), traffic or vandalism.
6.1.1.1.3 Strain gauges shall be protected against temperature effects due to direct sunlight.
NOTE When the structural member and the strain gauge do not have the same temperature (eg direct sunlight
exposure of part of the structural element or the strain gauge) this will lead to differential strains in the strain gauges
en the structural member. One way to minimise differential temperature effects is to provide each gauge with a
suitable thermal and protective cover.
6.1.1.1.4 The entire assembly (strain gauge element, mounting blocks and/or welds) should be protected
against corrosion.
EXAMPLE Wrapping by self-vulcanizing tape or protection with resins.
6.1.1.2 Spot-welding onto steel members
6.1.1.2.1 For the application of strain gauges to steel members, flat and clean surfaces shall be secured at
the measuring point, e.g. with the aid of a grinder and sandpaper.
NOTE If the surface has irregularities, an air gap can occur between the surface and the sensor. Spotwelding on
such surface can deform the sensor and affect the measuring results.
6.1.1.2.2 Spot-welding of the gauge carrier to the prepared surface shall be carried out according to the
procedures prescribed by the supplier of the instrument. The procedures shall be documented.
NOTE 1 Commonly, there are two to three rows of welding spots, spaced about 2 mm, which are applied on each
side of the vibrating wire or FBG carrier in a specific order.
NOTE 2 Following the welding procedure supplied by the manufacturer reduces the risks of locking-in welding
stress in the gauge.
6.1.1.2.3 Any contact between welder tip, the sensor and the lead cable shall be avoided. As a precaution,
an insulation sheet may be held between welder tip and the sensor and cables.
6.1.1.2.4 When using vibrating wire sensors, the plucking coil unit [see key reference 2 in Figure 1 b)] shall
be placed on top of the vibrating wire element and fixed in its position, e.g. by cable-ties, straps or tapes.
6.1.1.3 Arc-welding onto steel members
6.1.1.3.1 At the measuring point, a flat and clean surface shall be secured as per 6.1.1.2.1.
6.1.1.3.2 Arc-welding of the mounting blocks shall be carried out in the sequence shown in Figure 12.
6.1.1.3.3 During arc-welding, the strain gauge element between the two mounting blocks shall be
temporarily replaced by an inert setting element of equal external dimension.
NOTE This procedure prevents welding-induced damage to the strain gauge.
Key
1 arc weld with sequence of application 3 mounting block (top view) with two inclined set screws
2 mounting block (top view) with a single set screw 4 setting element
NOTE The sequence of welding is indicated by #1,#2…
Figure 12 — Sequence of arc-welding when installing strain gauges to a steel surface
6.1.1.4 Attaching to concrete surfaces
6.1.1.4.1 The strain gauges should be attached to the concrete surface by anchors or adhesion
NOTE 1 Mounting blocks with groutable or expandable anchors inserted into predrilled holes are commonly used,
see Figure 13.
NOTE 2 Epoxy gluing is a typical type of adhesion.
6.1.1.4.2 When applying adhesive material, its specification should take into consideration the
characteristics, quality and durability during the complete monitoring period.
NOTE 1 Creep and strain transfer are characteristics to consider, as well as possible presence of micro-cracking.
NOTE 2 Calibration tests in regard to strain transfer provide a good understanding of the physical properties of the
adhesive.
Key
1 vibrating wire strain gauge element with integrated temperature sensor
2 mounting blocks
3 concrete
4 anchor
NOTE The anchor key reference 4 can be fixed with grout or epoxy
Figure 13 — Vibrating wire strain gauge attached to a concrete surface (example)
6.1.1.5 Embedding into concrete
6.1.1.5.1 Prior to a concrete pour of a reinforced structural member, the gauges, shall be placed and held
in their pre-determined position and orientation by means of steel wires wrapped around the instruments
and connecting to the reinforcement cage.
6.1.1.5.2 In mass concrete applications, the gauge may be installed either before or immediately after
placement of the concrete. When, at a measuring point, placing several strain gauges in different pre-
determined orientations, use should be made of a rosette adaptor.
6.1.1.5.3 Embedded FBG strain cables shall be fixed to reinforcement with cable ties at intervals no greater
than two metres. The cable should be kept straight and taut all along the section to be monitored.
6.1.1.5.4 Strain gauges may also be installed by providing a reservation tube at the reinforcement cage
and embedding afterwards the gauges in this reservation tube with a grout/mortar.
EXAMPLE Array of FBG's on a reinforcement or a carrier bar.
6.1.1.5.5 To avoid possible damage of the gauge when pouring and vibrating the concrete the gauge may
be precast in a briquette of concrete identical to that used in the member, provided that the briquette is cast
maximally 48 hours prior to the main concrete pour.
6.1.2 Installation of strainmeters
6.1.2.1 In stiff media (e.g. concrete, rock, stiff soil), mobile strainmeters should be employed. For
installation, refer to ISO 18674-2:2016, Clause 6.1.4.
6.1.2.2 In soft media (e.g. fill, unconsolidated soil), fixed-installed embedded strainmeters should be
employed. In embankments, strainmeters shall be installed in trenches which should be formed and back-
filled concurrently with the construction work.
NOTE A common configuration is a horizontal continuous chain of strainmeters for monitoring lateral movements
inside an embankment.
6.2 Measuring procedure
6.2.1 Instrumentation check and calibration
6.2.1.1 Before installation, the functionality of the instrument shall be checked by performing tests
suggested by the manufacturer. For general function checks and calibrations, reference shall be made to
ISO 18674-1:2015, 5.6.
6.2.1.2 A calibration certificate or batch factor shall be supplied by the manufacturer for each instrument
delivered.
6.2.2 Measurement
The measurement shall be carried out according to ISO 18674-1:2015, Clause 7.
7 Data processing and evaluation
7.1 Data processing of the strain measurements and their evaluation shall be carried out in accordance
with Annex A.
8 Reporting
8.1 Installation report
The installation report shall be in accordance with ISO 18674-1:2015, 9.1.
8.2 Monitoring report
The monitoring report shall be in accordance with ISO 18674-1:2015, 9.2.
Annex A
(normative)
Data processing and evaluation
A.1 Data processing in terms of strain
This annex can be applied to calculate strains and stresses which are present in structures due to forces and
temperature variations, acting on a structural member.
A.1.1
To calculate the change of strain Δε in the period between reference and follow-up measurements at the
i
measuring point iFormula (A.1) is used:
Δε = ε – ε (A.1)
i F, i R, i
where
ε is the strain reading of the follow-up measurement at location i;
F, i
ε is the strain reading of the reference measurement at location i.
R, i
A.1.2
The change of the axial strain Δε due to loading/unloading of a 1-D member can be calculated using
ax
Formula (A.2):
Δε = (Δε + Δε + . Δε )/n
ax G1 G2 Gn
(A.2)
where
Δε is strain changes monitored by instruments G1 . n (strain gauges or strainmeters);
Gi
n is number of instruments in a given section of the member (aligned in direction of the neutral
axis) of the member.
A.1.3
The change of the axial strain Δε ’ due to bending of a 1-D structural member in a predetermined direction
ax
can be calculated using Formula (A.3) (see Figure A.1):
Δε ’ = [(Δε - Δε )/2] * D/2d (A.3)
ax G1 G2
where
Δε strain changes monitored by two strain instruments G1 and G2 located on opposite sides of
G1,G2
the neutral axis
d distance of the measuring points from the neutral axis
D diameter of the member
NOTE 1 For the determination of the principal strain directions due to bending, three independently oriented
strain instruments are required per measuring section. For the optimal configuration, see Figure 6 c). Additional
instruments lead to a redundancy in the strain measurements.
NOTE 2 For the determination of the principal strain directions due to bending, see Reference [5].
NOTE 3 Identical processing routines are applicable for 2-D structural members.
a) Side view
b) Section A-A’
Key
1 surface of member (here: concrete pile) G1,2 strain gauge (here: embedded)
2 medium d distance of gauge from neutral axis
3 neutral axis of member D diameter of member
Figure A.1 — Possible strain gauge layout in a 1-D structural member
A.1.4 The change of the axial strain Δε ’ due to bending of an I-Beam in any direction can be calculated
ax
using Formula (A.4) (see Figure A.2):
Around the XX-Axis:
Δε ’ = [(Δε + Δε - Δε - Δε )/2]*a/c (A.4)
xx G1 G2 G3 G4
Around the YY-Axis:
Δε ’ = [(Δε - Δε + Δε - Δε )/2]*b/d (A.5)
yy G1 G2 G3 G4
where Δε strain changes in measuring section, monitored by instruments G1 to G4
G1,G2,G3,G4
NOTE 1 For dimensions a, b, c and d, see key of Figure A.2.
NOTE 2 The distance d of the strain gauges to the YY-Axis is
d = t /2 + s
web
where
t web thickness
web
s spacing between strain g
...
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