Experimental Mechanics Experimental Mechanics

Experimental Mechanics 1

Experimental Mechanics

Prof. Dr.-Ing. Volker Slowik

Prof. Dr.-Ing. Lutz Nietner

HTWK Leipzig, Faculty of Civil Engineering and Architecture

with contributions by Dr. rer. nat. Gerd Kapphahn, Dr.-Ing. Thomas Klinkand Dr.-Ing. Nick Bretschneider


1. IntroductionTerms and definitions, observational errors and their treatment

2. Generation of test loadsGeneration and distribution of forces, application of forces to the test objects, testing machines and test fields, mobile loading devices

3. Measurement methodsMeasurement of displacements, inclinations and curvatures; experimental analysis of stresses and strains

4. Model testsProcedures and materials, photoelastic effects, similarity mechanics

5. Non-destructive testing in civil engineering

Measurement principles, procedures, applications

6. Experimental safety evaluation of existing structuresConcepts and technical implementation, guidelines of the DAfStb (German committee for reinforced concrete), practical examples

7. Monitoring of structuresProblems, measuring concepts, practical examples

(Some illustrations and examples in the actual presentation are taken from: J. Quade, M. Tschötschel,

Experimentelle Baumechanik, Werner-Verlag 1993. )

Table of contents


1. Introduction

1.1 Terms and definitions

Measurand: physical quantity to be measured (e.g. length, strain, force, temperature)

Measurement: quantitative determination of the measurand

Object of measurement: item the physical properties of which are to be determined

Measured value: output from the measuring device

Result of measurement: individual measured value or result derived from several

measured values

Measuring device: technical tool for the measurement, gives the measured value

Measuring method: procedure of the measurement

Measurement principle: physical effect used for the measurement


1.2 Observational errors and their treatment

1.2.1 Observational errors

Observational error: Difference between the measured and the "true" value

Causes: Inhomogeneity of the object of measurement

Imperfection of the measuring device or of the measuring method

Environmental influences

Operating errors

Blunders: result from defective measuring devices or incorrect operation; must be avoided; may be

discovered by plausibility checks

Systematic errors: result from the imperfection of the measuring device or of the measuring method;

have a diagnosable value

Random errors: result from indeterminable influences on the measuring device and on the object of

measurement; varying in direction and magnitude; may be reduced, but not

completely avoided

wa xxx −=∆ ... observational error

... measured value

... "true" value

x∆

ax

wx


random errors random and systematic errors

Determination of systematic errors: - using a defined input quantity (reference standard, etalon)

- measuring of the measurand by a more accurate measuring method

- definition of the measured value as "true“ value

- adjustment of the measuring device (referred to as calibration)

Estimation of random errors: - repeated measurements and statistical analysis


( ) ( )GxxGx rar +≤≤−

1.2.2 Error limit

Error limit: maximum deviation from the ideal characteristic

error range surrounding the ideal characteristic

xa ... measured value

xr ... correct value; close to

the "true" value xw

xe ... known input quantity

G ... error limits


1.2.3 Determination and correction of observational errors

Estimation of random errors:

mean value of a series of n measured values:

sn

t⋅±=υ

∑=

=n

i

ixn

x1

1

∑=

−−

=n

i

i xxn

s1

2)(

1

1

ssz usn

tuuu +=+=

sample standard deviation (mean deviation of the individual valuesfrom the mean value):

confidence interval: range which contains with a certain probability P the "true" value(for technical problems usually P = 95%):

measurement uncertainty:

random deviation systematic deviation


Probability Number of

measured values P = 68.3 % P = 99.7 % P = 95 %

3 1.32 19.2 4.30

5 1.15 6.6 2.78

10 1.06 4.1 2.26

20 1.03 3.4 2.09

∞ 1.00 3.0 1.96

Values of t :


measured value (input parameter) Xi ∆Xi

X1 max F g·max F

X2 a sa

X3 b sb

The tensile strength of a steel sample is to be determined:

... maximum load [N]

... cross-sectional dimensions [mm]

of the unloaded sample

All three input parameters are measured. For the force measurement, a testing machine of accuracy class 1 is used,

i.e., the relative error limit amounts to g = 1 %.

Example:

Propagation of error

maximum deviation:

mean deviation:

∑=

∆⋅=∆m

i

i

i

xdx

dYY

1

∑=

∆⋅=∆

m

i

i

i

xdx

dYY

1

2

ba

Fz ⋅

=max

β

Fmax

ba ⋅


deviations:

absolute maximum deviation:

relative maximum deviation:

maximum deviation: mean deviation:

relative relative

absolute absolute

Solution:

bba

Fa

ba

F

ba

FgY ∆

⋅+∆

⋅+

⋅

⋅=∆

²

max

²

maxmax

b

b

a

ag

Y

Y

Y

z

∆+

∆+=

∆=

∆

β

FmaxgX ⋅=∆ 1

asaX =∆=∆ 2

bsbX =∆=∆ 3

NNFkNF 3853385300max%13.385max ±=±=

mmmmsaa a 01.092.15 ±=±=

mmmmsbb b 11.092.44 ±=±=

²/8.53892.4492,15

385300mmmNz =

⋅=β 0024.0

0006.0

01.0

=∆

=∆

=

b

b

a

a

g

%03.1=∆

Y

Y%3.1013.00024.00006.001.0 ==++=

∆

Y

Y

²/0.78.538013.0 mmNY =⋅=∆ ²/5.58.5380103.0 mmNY =⋅=∆


2. Generation of test loads

2.1 General requirements

All mechanical test loads must be defined in terms of value, direction and time-dependence. Load test must be repeatable.Safety of staff, equipment, and test object must be ensured.

Three major tasks: force generation (hydraulically / mechanically / pneumatically)

measurement of the force (required for the repeatability of the test)

application of the force (load distribution and design of the supports)

Further requirements:

test loads (action forces, in case of statically indeterminate structures also reaction forces) must be measurable

test loads should be applied continuously or stepwise

displacements of the loading points on the test object as well as changing load directions should be avoided (might happen in the case of large deformations)

application of test loads should not hinder deformations of the test object


2.2 Generation of forces

2.2.1 Mechanical force generation

a) Spring effect

mechanical load generation(utilization of the flexibility of bending members)

The compliance of the spring or beam, respectively, should be adjusted to the compliance of the test object.

Application: creep tests, simple field experiments

mechanical load generation by using springs


b) Gravitational forces

Ballast materials: steel, concrete, water in tanks, sand or gravel on trucks

Application: load tests of bridges

alternative solution

actuator


load increase by a lever generation of horizontal forces

loading by water pressuregeneration of a trapezoidal pressure distribution

l

a

F

la

F

sealing

backfill

loads


Dimensions: length: 11,78 m

width: 2,66 m

height: 3,68 m

Weight: 36 t

Test loading by vehicles

source: www.faun.de


A closed loop of forces is built up.

When the compliance of the structure increases, the test load is automatically decreasing. In this way, sudden failure of the structure

can be avoided.

Self-securing loading systems

test object


The test load results from

the weight of the ballast at a neutral base underneath the concrete slab to be tested.


The test load results from the weight of the

movable ballast at a neutral base above the concrete slab to be tested.

ballast


Example: Bridge in Gustav-Esche-Straße in Leipzig

CUT B-B M: 1:50


Loading vehicle BELFA (BELastungsFAhrzeug) in the transportation mode


bridge sensor base line

Loading vehicle BELFA (BELastungsFAhrzeug ) in the operation mode

Counter forces of the test load:

• self-weight of the loading vehicle

• additional ballast (steel elements, water bag)

• anchoring of the vehicle outside the span of the bridge


2.2.2 Hydraulic force generation

a) Conventional actuators

... piston surface area

... hydraulic pressure

... piston stroke

... spring constant of the reset spring

Principle:

generation of oil pressure by using small, adjustable

high-pressure pumps

transport of the pressurized oil to the actuator

by pipes or hoses

pressurized oil causes stroke of the piston

in the cylinder

maximum force depends on surface area of the piston

fatigue tests: using of a “pulsator” in the hydraulic system

ckpAFkeff

⋅−⋅=

kAp

kc

1 cylinder foot

2 cylinder

3 piston

4 reset spring

5 pipe joint to oil supply

6 displacement sensor

7 threaded spindle

8 connecting plate

Hydraulic actuator (compression)

Hydraulic actuator (compression-tension)


Application: quasi-static and low-cyclic fatigue tests

Pulsator in comparison to servo-hydraulic test systems:

advantage: low energy consumption (a part of the applied energy can be recovered

by using a fly wheel)

disadvantage: no closed-loop control

unidirectional actuator (example: product of WPM Leipzig)

Differential test cylinder

a) operating principle

b) direct connection

c) with manual valve

d) with servo valve


b) Servo-hydraulic testing systems

Concept: closed-loop electro-hydraulic control;

accurate control on the basis of the measured strain, force, or piston stroke

Closed-loop controlof a servo-hydraulic testing machine

load cellforce

Amp

strainstrain gagesample

pressure

controller

fluid

Hydraulic power unit

servo valve

actuator

funktion generator 1

funktion generator 2

Amp

Amp

Amp ... measurement amplifierstroke

control signal

Amp


Servo-hydraulic actuator


Principle:

hydraulic power unit: generation of a constant flow of pressurized oil by high-pressure pumps

servo valve: link between electronic control and hydraulic system; directs the oil flow to one side of

the piston in the actuator (according to the command signal generated by the controller,

see below); difference pressure causes axial displacement of the piston rod; oil

from the other side of the piston is flowing back to the oil tank

load cell, strain gage, displacement sensor, pressure transducer: measurement of the respective feed-back

parameter (force, strain, stroke, pressure)

measurement amplifier: amplification of the feed-back signal

function generator: generates the reference signal (desired value of the feed-back parameter)

controller: compares the feed-back value to the reference value; generates the command signal for the servo valve in order to reduce this deviation

Application: complex static and dynamic tests; displacement or strain controlled tests; investigation of the post-peak behavior of materials and structures

Drawbacks: costly; high energy consumption; noise pollution (placement of the hydraulic power unit

usually away from the test set-up); space requirements


2.2.3 Pneumatic force generation

Principle: generation of distributed loads (area loads) by using the air pressure in an air cushion

Application: simulation of internal and external compressive loads; loading of extremely thin-walled

structures

test loading of roof panels with

trapezoidal sections by a

distributed pneumatically generated load

test loading of a spherical shell by

a distributed pneumatically

generated load


2.3 Testing machines

Types and principles:

tension-compression testing machine withelectro-mechanical force generation

tension-compression testing machine withhydraulic force generation


2.4 Mounting plates and load application equipment

set-up for load tests in the laboratory, testing of structural members

anchoring in the grill in the floor anchoring in the mounting plate


mm


The test loading should resemble the loading of the real member. Distributed or line loads are simulated by multiple concentrated loads.

2.5 Load distribution


simulation of a line load by 8 single loads


Distribution and application of generated forces to the test object

generation of a

line load

generation of an

area load


Supports: transfer of reaction forces from the test object to the mounting plate orto the frame of the testing machine

supports used for load tests

spherical hinge hinged support with a single axis of rotation

roller supporthinged support for compression-tension tests


2.6. Loading functions

... describe the load-time behavior

... have a major influence on the test results

(especially in the case of dynamic loading)

- Force-time functions:

used for load-controlled experiments

- Displacement-time functions:

used for displacement-controlled experiments

investigation of the so-called post-peak behavior


Typical loading functions for quasi-static tests:

stepwise load increase withrepeated unloading cycles

main field of application:

experimental safety evaluation

technical requirements:

conventional hydraulic actuators or

servo-hydraulic testing systems


Typical loading functions for quasi-static tests:

constant force(creep test)

constant displacement(relaxation test)


endurance testing of materials and structures


servo-hydraulic testing systems or loading

by gravitational forces (creep test)


Typical loading functions for dynamic tests:


investigation of the fatigue behavior of materials and structures; durability tests


electro-mechanical vibration generators,pulsator machines or servo-hydraulic testing systems

pulsating load

alternating load



investigation of the fatigue behavior of materials and structures; durability tests

impact tests

determination of natural frequencies

random load

impact load

excitation and natural oscillation


servo-hydraulic testing systems

servo-hydraulic testing systems; falling masses

electro-mechanical excitation; release of spring forces;falling masses

Typical loading functions for dynamic tests:


3. Measurement methods

3.1 Introduction

deformation and

displacement of a

structure

Actions: mechanical loads, temperature, moisture

Reactions: displacements: settlements, rotations

deformation: (relative) displacements, distortion, curvature, torsion

structural changes: cracks, plasticization


3.2 Measurement of displacements

3.2.1 Mechanical measurement principles

Principle: magnification of the displacements by using levers and gearings

Application: for minor measurement problems and for plausibility checks of results acquired by electrical sensors

Advantages: robust (in view of on-site measurements)

simple operation and maintenance

insensitive to electro-magnetic influences

Drawbacks: limited resolution of measured values

reading of the measured values directly at the object of measurement

comparatively large observational errors

sensitive to variations in temperature

measurement may hardly be automated


a) Dial gages

1 probe tip

2 toothed rack3 gear drive

4 pointer

5 scale

6 tension spring7 lever

8 shaft

9 sensor fixationtarget


measurement of deflectionby using a dial gauge

measurement of strainby using a dial gauge

measurement of displacementby using a dial gauge

target

dial gage

sensor fixation


b) Mechanical strain meters

Basic principle: edges or tips are attached to the object of measurement, one of them is movable

a) Huggenberg's tensometer

b) strain meter with dial gauge (type Albrecht)

c) strain meter (type MK 3, Fa. Holle)

a)

b)

c)

22211 Hh

a

h

v

H

v

h

l

+==

∆and

AB Lever

C yoke

DF pointer

S1 fixed edge

S2 movable edge


c) Stress-probing extensometer

Basic principle: extensometer is shortly attached to two markers at the surface of the object of measurement, movable tip is fixed, extensometer is removed and dial gage allows to read the measured value

type Pfender

1 lever

2 fixed tip

3 movable tip4 marker

5 dial gage

6 locking bracket

7 trigger


3.2.2 Optical measurement principles

a) Measurement of displacements by using leveling instruments or theodolites

Basic principle: measurement of the displacement of markers attached to the object of measurement

measurement of vertical displacements or deflections by using a leveling instrument

scale

reference scale


trigonometric measurement of

vertical displacements by using a

theodolite

top view

side view


b) Optical strain measurement

mirror instrument according to Martens

scale


c) Optical measurement of inclinations (autocollimation method)

Autocollimation telescope: inclination of a mirror allows to read a corresponding value on a scale

optical measurement of an inclination

scale


3.2.3 Electrical measurement methods

Basic principle: transformation of the displacement to be measured into an electric signal

a) Inductive sensors

Measurement principle: variation of the inductive resistance RL

due to the displacement of the

ferrite core within a coil, alternating current (AC) is applied

measurement of the apparent resistance (impedance) RS

(sum of inductive

resistance RL

and ohmic resistance R)

mL

R

wLR

²⋅=⋅= ωω

A

lR

ρ⋅=

RRR LS +=ω circular frequency of the

applied AC

L inductivity of the coil having a ferrite core

w winding number of the coil

Rm magnetic resistance of the ferrite core


Sensor types

a ferrite core

b coil

c movable target (metallic)

s gap

non-contacting distance sensor


Linear Variable Differential Transducer (LVDT)

suitable for comparatively large displacements (>= 100mm) due to the large linear range of measurement

Measurement principle: variation of the degree of coupling between the primary and the secondary coil by the displacement of the ferrite core


b) Vibrating wire gage

Physical concept:

(natural frequency of a vibrating wire)

using Hooke's law:

with

Measuring concept: excitation of the wire by an electric impulse

wire oscillates with natural frequency

the wire's frequency is transformed into an electric oscillation of the same frequency by using an electromagnetic sensor system

linearization and digitalization of the wire’s natural frequency

l

lEE

∆⋅=⋅= εσ

ρσ

l

nfn

2=

ll

Enfn ∆⋅

⋅

⋅=

ρ³4

²²

ρ⋅

⋅=

³4

²

l

EnK

K

fl n

²=∆

fn natural frequency

n number of natural frequency

l length of the wire

σ tensile stress in the wire

ρ density of the wire's material

K constant describing the

physical properties of the wire


vibrating wire gage for measurements within concrete members

vibrating wire gage for surface measurements

Application: long-term measurements under “rough” conditions

long-distance transmission of measured values (up to 5 km)

Advantages: no influence of resistances of the transmission (cable length) on the measured value

long-term constancy of the zero-point

high reliability of the sensors under “rough” conditions

Drawbacks: comparatively high costs of the individual sensor

base length of displacement measurement should be larger than 20 mm

a, b connectors

c vibrating wire

d tube

e sealing

f spring

g welding spots

h flanges

a vibrating wire

b clamps

c end pieces

d tube

e magnetic body

f coil


3.2.4 Special methods of displacement measurements

a) measurement of crack opening and sliding displacements by using gypsum markers

application of the gypsum ribbon perpendicular to the crack

avoid “filling” of the crack by the gypsum

crack opening in the substrate results in easy to detect cracking of the gypsum

reference line allows also to measure crack sliding displacements

b) capacitive measurement of displacementsc) hydrostatic measurement of displacements (water level gage)

reference line


Determination of the elastic curve: measurement of the

inclination at several points along the beam's axis;

determination of a regression curve and integration;

consideration of the displacement boundary conditions

+-

w´

w+

Neigung

Biegelinie

3.3 Measurement of inclinations

Applications: determination of the tilting of supports

determination of the elastic curve of beams

determination of torsional deformations

Typical structures requiring inclination measurements: dams, high-rise buildings, bridges

Measurement principles: liquid systems: observation of a gas bubble on a liquid's surface

pendulum systems: deflection method: pendulum remains in its vertical orientation; change of position with respect to the housing of the transducer is measured, for instance by using inductive or capacitive sensors

servo method: pendulum is kept in a constant position with respect to the housing of the transducer;

required force is measured

inclination

deflection


The calculation of the curvature from the measured displacement f is performed under the assumption of a

quadratic parabola as elastic curve. In case of a short base length lb

this assumption is justified.

ρ1

²

8

²

8

²

8

²

²41

=−=

−=′′

−=′

−=

bb

b

b

l

f

lfw

l

xfw

l

xfw

yMwEI −=′′

3.4 Measurement of curvatures

Measurement principle: measurement of the deflection within a certain base length

f

lb

x

w

inclination

deflection

curvature


Determination of the curvature by strain measurement: influences of normal force and bending

moment need to be separated

Possible solutions: direct measurement of the curvature or measurement of the longitudinal strain at two different distances from the axis of the beam

Example: bending with axial force, two strain sensors required (for instance at the top face and at the

bottom face of the beam)

↓

zEI

MM ⋅=ε

EA

NN =ε

Nε

M

NL

N

( ) ( )h

wh

dxdx uouo εε

ρ

εε

ρ

+=′′=→

⋅+=

1

Special case: bending without axial force and centroid in the middle of the beam

( )h

w uερ

21=′′=


Curvature-strain sensor by Quade: simultaneous measurement of the strain due to normal

force and of the curvature due to bending

object of measurement

object of

measurement

neutral axis


Curvature-strain sensor at the lower flange of a steel girder embedded in concrete


3.5. Measurement of strains

State of the art: electrical resistive strain gage

Measurement principle: measurement of the change in electric resistance of a conductor due to mechanical

strain; measurement of the resistance usually by a Wheatstone bridge circuit

Derivation:

²r

l

A

lR

⋅

⋅=

⋅=

πρρ

R ohmic resistance

l length of the conductor

ρ specific electric resistance

A cross-sectional area

rlR

rA

AlR

ln2lnlnlnln

ln2lnln

lnlnlnln

−−+=

+=

−+=

πρ

π

ρ

differentiated with

respect to R:

( )

r

dr

l

dl

R

dR

dR

dr

rdR

dl

l

dRdr

drrd

dRdl

dlld

dR

rd

dR

ld

RdR

Rd

2

12

1

ln2ln

ln2ln1ln

−=

−=

⋅

⋅−

⋅

⋅=

−==

A. C. Ruge with a specimen which was

instrumented with the first electrical strain gage


The elongation (strain) of the wire is accompanied by a change in diameter (Poisson's law).

l

dlµ

r

dr

l

lµµ

l

llql −=

∆−=−=

∆= εεε

µk

)µ(R

dR

)µ(l

dl

l

dlµ

l

dl

R

dR

21where

21

212

+=

+⋅=

+⋅=+=

ε

ε⋅=kR

dR ε strain

k sensitivity

The sensitivity k (also referred to as k-value) amounts to approximately 2 in case of metallic strain

gages; in case of semiconductor strain gages it amounts to approximately 150.

To be noted: The change of the electric resistance does not necessarily result from mechanical strain.

Temperature changes also cause variations of the resistance.


Carrier: made of non-conductive material, e.g. paper or plastics

Measuring grid: electric conductor, printed circuit or made of wire

Cover: insulating, protection against moisture and mechanical impact, preferably

made of silicone, sometimes additional protection against solar radiation required

(reflective cover)

Connections: used for the connection of wires by solder joints

Adhesive joint: connects the strain gage to the object of measurement, should be as thin as

possible, should not be hygroscopic, preferably epoxy resin

Strain gage applied to a structural member


c)


Wheatstone bridge circuit:

special case: bridge is balanced

unbalanced bridge due to the change

of resistances:

43

4

21

1

RR

R

RR

R

U

U

E

A

+−

+=

03

4

2

1 =→=E

A

U

U

R

R

R

R

4433

44

2211

11

RRRR

RR

RRRR

RR

U

U

E

A

∆++∆+

∆+−

∆++∆+

∆+=

:For 1<<∆

R

R

∆−

∆+

∆−

∆≈

4

4

3

3

2

2

1

1

4

1

R

R

R

R

R

R

R

R

U

U

E

A

( )4321

4εεεε −+−=

k

U

U

E

A

UA output voltage

UE excitation voltage

R1 ... R4 resistances

ε1 ... ε4 strains

k sensitivity

The “diagonal” resistance changes are

added, the “neighboring” resistance

changes are subtracted from each other.


a) half-bridge circuit: resistances of the cables are aligned with those of the strain gages

-> errors do to influences on cables and connectors

compensation of temperature:

application: in-situ measurement

b) full-bridge circuit: cable connections are outside the bridge circuit

-> influences of the cables may be neglected

compensation of temperature :

-> application: long-term measurements

Application types of the Wheatstone bridge circuit:

ε⋅= kU

U

E

A

0and 4321 ==−= εεεε

εεεεε =−==−= 4321

1, 3

2, 4

1

2ε⋅=

2

k

U

U

E

A


c) quarter-bridge circuit:

compensation of temperature is not possible

application: multi-channel measurement

excitation voltage must be constant

amplification of the measured

voltage required -> application of

alternating current (AC) useful

Because of the application of

alternating current (AC), the errors

due to the variation of resistances

at the contact points and due to

thermoelectric effects are smaller.

Carrier frequency amplifier:

14

εk

U

U

E

A =

1

excitation measured phase-dependent outputvoltage voltage demodulated signal signal

balancing


It must be considered:

required length of strain gages depends on the inhomogeneity of the material:

steel: comparatively short gages concrete: longer gages (up to 10 cm), recommended: three times maximum

aggregate size

accuracy of the measurement is higher if angles in a rosette are as different as possible:

isotropy: the directions of principal normal strains and principal normal stresses are identical

aaxyayaxa ϕϕτϕσϕσσ cossin2²sin²cos ⋅⋅+⋅+⋅=

aaxyayaxa ϕϕεϕεϕεε cossin2²sin²cos ⋅⋅+⋅+⋅=

aaxyayaxa ϕϕγϕεϕεε cossin²sin²cos ⋅⋅+⋅+⋅=

⋅

⋅

⋅

=

xy

y

x

cccc

bbbb

aaaa

c

b

a

γ

ε

ε

ϕϕϕϕ

ϕϕϕϕ

ϕϕϕϕ

ε

ε

ε

cossin²sin²cos

cossin²sin²cos

cossin²sin²cos

°⋅°⋅°⋅ 1202/602/452

precondition: angles φa, φb and φc are not identical

aσ

yσ

xσ

xyτyxτ

aτ aϕ


stresses are independent of the material (equilibrium of external and internal forces)

Principal normal stresses:

Principal shear stresses:

Plane state of stress:

yx

xy

σσ

τϕ

−=

22tan

2/1

²4)²(2

12/1 xyyx τσστ +−±=

²4)²(2

1

22/1 xyyx

yx τσσσσ

σ +−±+

=

−−

=

xy

y

x

xy

y

xE

γ

ε

ε

µ

µ

µ

µτ

σ

σ

)1(2

100

01

01

²1

0)90()(

)90(

)(

11

12

11

=°+=

→°+=

→=

ϕτϕτ

ϕσσ

ϕσσ

Minimum

Maximum

Extremum→°± )( 451ϕτ


Advantages of the electrical strain gages:

applicable for multi-channel measurements (multiplexing)

high sensitivity

low space requirements for application

low mass

direct contact to the object of measurement

applicable for static and dynamic measurements

easy compensation of temperature variations

measurements at high (up to 800°C) and low temperatures possible

Drawbacks of the electrical strain gages:

in case of outdoor measurements or when embedding the gages in concrete: protection against moisture is required

strain gage may be damaged by cracking of the object of measurement

Application of electrical strain gages in load cells:

( )43214

1εεεε −+−⋅⋅⋅= kUU EA lql εµεεεεεε ⋅−===== 4231 andwith

( ) ( )µεεµεεµε 224

1

4

1+⋅⋅⋅⋅=⋅++⋅+⋅⋅⋅= lEllllEA kUkUU

( ) ElA UkU ⋅⋅⋅+⋅= εµ12

1


3.6 Measurement of forces

Forces may only be determined by measuring their effects.

Effects of forces:

mechanical effects: deformations

accelerations

hydrostatic pressure

electrical effects: electric charge

Requirements:

- load cells should be incorporated in the mechanical system without influencing it, i.e., they

should be comparatively stiff

- forces should be measurable with: - small hysteresis

- small amount of mechanical work

- small creep effects

- sufficient long-term stability


Mechanical force measurement

a) Spring force meter (spring dynamometer)

Principle: - utilization of the elastic deformation of steel springs having a spring constant K

- measurement of the displacement f caused by the force F

K

Ff =

Tension spring force meter

a) with coil spring small forces up to 1 kN

b) with coil springs in a light metal casing

c) with flat springin a metal casing forforces up to 250 kN

light metal casing

fixed beam

spring

tension spring

guiding rod

toothed rack

movable beam


b) Force measuring ring

Force measuring rings

a) tension and compression force measuring device with dial gauge (ring-shaped)

b) tension and compression measuring device (yoke-shaped)

c) 2,5 kN compression measuring ring (type Wazau)

Measurement principle: utilization of the elastic deformation of a ring- or yoke-shaped spring element

made of steel

measurement of the deflection by using a displacement transducer

(e.g. dial gauge or LVDT)

yoke


Force measurement by using electrical load cells

Principle: deformation of a solid body is measured by using electrical strain gages; load cell must be as rigid as possible; sensitive strain gages required (semiconductors); full-bridge circuit for compensating temperature influence

Electrical load cells

a) compression force load cell

b) tensile force sensor

c) bending membrane sensor

casing

end plate

hollow cylinder


Force measurement by using vibrating wires

Vibrating wire gage for the measurement of forces in reinforcing bars

Hydraulic stress sensor

Principle: increase of the pressure in the outer circuit by using a pump until the pressure is equal to the

pressure in the embedded cushion

opening of the membrane valve in the sensor box, liquid starts to flow from the pump to the tank without further pressure increase

pressure at the manometer is equal to the vertical compressive stress in the object of

measurement



Pendulum manometer

used in mechanical testing machines

Principle: the tangent of the inclination angle

corresponds to the hydraulic pressure

back-pressure valve prevents sudden

swing back

Application: limited to the case of low testing velocities

due to the low natural frequency of the

pendulum

oil pressure from the

actuator of the testing

machine

piston


Bourdon gage

Principle: tubular curved spring bends under the

action of increasing internal pressure

and moves a pointer

casing


3.7 Measurement of vibrations

Piezoelectric acceleration sensors

Types of acceleration sensors

prestressing jacket


Piezoelectric acceleration sensors

Since the seismic weight is constant, a force which is proportional to the

acceleration (F = m·a) is acting on the piezoelectric measuring element.

Piezoelectric acceleration sensors consist of a casing, the piezoelectric

measuring element, and seismic weights.


Capacitive acceleration sensors

The principle of a capacitive acceleration sensor is

the measurement of the varying capacity of a

capacitor. The latter is affected by the

displacement of an accelerated mass.

This displacement changes the gap width at both

sides of the mass with opposite signs.

The resulting capacity differences unbalance a

electrical bridge circuit.


Basic configuration of an electrodynamicvibration velocity sensor

The measurement principle is that

a potential is induced if an electric

conductor is moved within an

electric field.

The induced potential is

proportional to the velocity.

Electrodynamic vibration velocity sensor


4. Model tests

4.1 Introduction

Model: scaled copy of the original; in mechanics used for gaining information on the mechanical behavior of the original structure

Applications of models:

• for the conception of structural systems in case reliable analytical modeling can not be guaranteed

• for studying details of structures, especially where static of geometric discontinuities occur (not easy to simulate with analytical models)

• for investigating the origins of structural faults

• for demonstrations in engineering education

Principle: linking the model test and the theoretical analysis (calculation); comparison of the results obtained in the calculation to those of the test


Advantages:

laboratory experiments require less effort than load tests at the original structure and may be performed under suitable conditions (no environmental influences)

some effects concerning the mechanical behavior of structures will occur in a more pronounced way

idealizing assumptions like in the analytical model are not required

Fields of application:

model tests in case of elastic material behavior: application to complex geometrical structures; for stability problems, for dynamic processes, and for teaching

model tests with real materials characterized non-linear deformations and damage processes including cracking, plasticization, bond failure

Hybrid technique:

experimental and analytical techniques are combined


4.2 Introduction to similarity mechanics

4.2.1 The principle of physical similarity

Physical processes will proceed similarly under similar influences and in similar geometric systems.

Application in the mechanics:

Mechanical processes in the original (H) and in the model (M) proceed similarly if they may be described by the same physical model.

4.2.2 Scales

Scale GV : ratio of a quantity GM measurable at the model and the equally named quantity GH

at the original

Equally named quantities:

quantities with the same name, the same physical meaning and same dimension in the original and in the model (e.g. lengths, displacements, etc.)

H

MV

G

GG =


a) Reference scales (basic quantities of the SI-System)

reference scales for mechanical model tests:

Length:

Force:

Time:

Temperature:

-> in case the reference scales constant, we have rigorous mechanical similarity

rigorous geometric similarity in all three directions:

rigorous force similarity:

rigorous time similarity:

rigorous kinematic similarity:

rigorous static similarity:

rigorous dynamic similarity:

H

MV

H

MV

H

MV

H

MV

T

TT

t

tt

F

FF

l

ll

=

=

=

=

.lV const=

const.=VF

const.=Vt

const.andconst == VV t.l

.Fl VV constandconst. ==

const.andconst.andconst. === VVV tFl


b) Derived scales

Precondition: rigorous similarity

Velocity:

Moment:

Stress:

Weight density:

Strain:

c) Scales of material properties

Young's modulus:

Poisson's ratio:

Density:

1/

/

²/

/

/

/

=∆

=∆

∆==

⋅=⋅

⋅==

====

⋅=⋅

⋅==

===

V

V

HH

MM

H

MV

VV

HH

MM

H

MV

V

V

V

V

HH

MM

H

MV

VV

HH

MM

H

MV

V

V

HH

MM

H

MV

l

l

ll

ll

gg

g

l

F

A

F

AF

AF

lFlF

lF

M

MM

t

l

tl

tl

v

vv

εε

ε

ρρρ

γγ

γ

σσ

σ

H

MV

H

MV

H

MV

E

EE

ρρ

ρ

µµ

µ

=

=

=


4.2.3 Model laws

• describe the relations between quantities in M and H

• allow to design models with respect to size, material, loads, measurement method and range of expected measurement results

Example 1: Bending of a beam due to a distributed load

Elastic curve (deflections) and resulting internal forces are to be determined.

l

p

b … width

12

3bh

I =


The differential equation of the elastic curve is presented for both M and H. Then, the quotient is

formed in order to obtain a dimensionless equations.

4

4

4

44

4

4

4

4

4

V

v

H

H

HV

M

H

H

M

M

l

w

dx

wd

dxl

wd

dx

wd

dx

wd

=⋅

=

H

M

H

HHH

M

M

MM

p

p

dx

wdIE

dx

wdIE

′

′=

⋅

⋅

4

4

4

4

bppdx

wdIE ⋅=′=⋅

4

4

and in case of rigorous geometric similarity in all three directions (bV=h

V=l

V)

³3

3

VV

HH

MMV

H

MV hb

hb

hbI

E

EE ⋅=

⋅

⋅== andwith

and the relation of the differential quotients

( )

V

H

H

H

HV

H

H

H

HV

H

H

H

M

w

dx

wd

dx

wdw

dx

wd

dx

wwd

dx

wd

dx

wd

=

⋅

=

⋅

=

4

4

4

4

4

4

4

4

4

4

4

4

todue

H

MVVV

x

xllI == andwith 4


yields:

In case of rigorous geometric similarity: wV

= lV

, i.e., the ratio of the deflections in M and H

is equal to the length scale.

Then,

In case a single load is adopted instead of the distributed load p, yields

The internal forces may be obtained by:

Transformation of the results to the original H:

VVVV

HH

MM

H

MV lppb

pb

pb

p

pp ⋅=⋅=

⋅

⋅=

′

′=′

VVVV

V

VVV wElp

l

wlE ⋅=⋅=⋅

4

4

1und1 =

==

=

VV

V

VV

V

p

E

p

E

l

w

l

w

VVVVV FQlFM =⋅=u n d

V

MH

VV

MH

V

MH

F

QQ

lF

MM

l

ww =

⋅==

u n du n d

dxbpF ∫ ⋅=

Hooke's model law

2

VVV lpF ⋅=

and

and

1²

=⋅

VV

V

lE

F

and

and and


The differential equation for the deflection w(x,y) of an isotropic thin plate is:

The flexural rigidity of a plate is rigorously similar only in case:

For the internal moments:

For the shear forces:

Example 2: Bending of a plate

py

w

yx

w

x

wKwK =

∂

∂+

∂∂

∂+

∂

∂=∆∆⋅

4

44

4

4

²²2

²)1(12

³

µ−

⋅=

hEK

1=Vµ Poisson's model law

VxyVVyVVxV FmFmFm ===u n du n d

V

VyV

V

VxV

l

Fq

l

Fq == und

differential equation of the thin plate


4.3 Model materials

4.3.1 Characteristics

The selection of a material for a model test depends on the objective of the investigation:

The model should represent the behavior of the original as realistic as possible.

The model has to be producible with the required accuracy concerning size and shape.

The model material must exhibit sufficient deformability in order to allow for measureable deformations under comparatively small loads.

Precondition: The properties of the model material (e.g. Young's modulus, stress-strain-

curve, Poisson's ratio, creep and temperature behavior) must be known.

Examples for model materials:

metals

mineral materials

plastics


4.3.2 Metals

Characteristics: - distinctive elastic behavior up to the yield limit

- small creep deformations

Advantage of aluminum: for measurable deformations, smaller stresses are required when

compared to steel (Ealu

= 1/3 Esteel

)

4.3.3 Mineral materialsGlass: - ideal-elastic and no creep

- mirror effect: utilization for optical measurement methods

- high risk of brittle fracture

Gypsum: - hygroscopic and brittle with low fracture strain

- moldable (almost arbitrary shapes may be obtained)

- reinforcement of gypsum by using thin wires -> simulation of reinforced concrete

- short hardening period

Fine-grain concrete: - simulation of the load-carrying behavior of (un-)reinforced concrete members

- maximum aggregate size approximately 7 mm

Polymeric concrete: - faster hardening when compared to cement-based concrete

- high tensile strength

- strength and deformations strongly depend on temperature


4.3.4 Plastics

For instance polyvinyl chloride, polycarbonate, polyester resin, epoxy resin

Characteristics: - low Young's module -> even low stresses result into measurable deformations

- Young's module depends on temperature

- higher Poisson's ratio when compared to metals- good workability

- low thermal conductivity (unintentional occurrence of temperature gradients)

- photoelastic effect: optically isotropic when unloaded; optically anisotropic due to

- mechanical stresses

time-dependent deformation

in case of cyclic loading

σ is constant


4.4 Optical measurement methods in model tests

4.4.1 Photoelasticity

Physical effect: some transparent materials such as epoxy resin are birefringent (double refracting) under mechanical load:

- in each direction of refraction different velocities of light propagation - the directions of principal normal stress and refraction are identical

a) Stress-optical bench

Setup: light source, pair of filters (polarizer and analyzer), model located between the filters

Effect: The light emitted from the source „oscillates“ in all directions.

The first filter (polarizer) allows just one oscillation direction to pass.

The polarized light A0 is refracted in the model in a way that the new oscillation directions

correspond to the directions of the principal normal stresses.

The resulting components A1 and A2 pass the model with different velocity. Hence, an

optical path difference s occurs.

The elliptically polarized light reaches the analyzer which is rotated by 90° with respect to the

polarizer. It allows only the corresponding components A‘1 and A‘2 to pass. These two

components have always the same absolute value.

∆


principal normal stresses σ1 and σ2


Generation of isoclines

Isochromates: - lines of the same color or of the same brightness in case of monochromatic light

- correspond to lines of the same difference between the principal normal stresses

- according to the optical path difference, the horizontal components interfere:

● s is equal to zero or equal to an integer multiple of the wave length -> cancellation

● accordingly: s is equal to one half of the wave length -> maximum brightening

- isoclines may be eliminated by additional filters

Isoclines: - lines of the same orientation of the principal normal stresses

- result from the coincidence of the directions of polarization and principal normal stress

- allow the determination of the principal stress trajectories

∆∆

cancellation


( ) dtvvs ⋅−=∆ 21td = runtime of the light through the unloaded model

d = thickness of the model

v1, v2 = velocity of the components A1 and A2 , respectively

v0 = velocity of the light in the unloaded model

( ) ( )

( )

0

2112

2112

121212

2211122112

0

1212

12212

22111

v

d)()kk(s

)()kk(

)(k)(k

kkkkvv

v

dvvtvvs

kkv

kkv

d

⋅−⋅−=∆

−⋅−=

−⋅+−⋅=

⋅−⋅−⋅+⋅=−

⋅−=⋅−=∆

⋅+⋅=

⋅+⋅=

σσ

σσ

σσσσ

σσσσ

σσ

σσ

(Brewster's law)

0v

dtd =


)kk(

vS

dS

)kk(d

v)(

v

d)()kk(

s

12

0

12

0

21

0

2112

−

⋅=⋅=

−⋅

⋅⋅=−

⋅−⋅−=⋅

⋅=∆

λδλδσσ

σσλδ

λδ

with

S = stress-optical constant

= order of the isochromate

cancellation at (δ = 0; 1 ; 2 …) and amplification at (δ = 1/2; 3/2; 5/2 …)

δ


Result of a measurement at the stress-optical bench: Investigation of the plane state of stress, stress

concentrations at the points of geometric discontinuity may be studied.

Principal stress trajectories in a notched beam


b) Surface photoelasticity

Application: investigation of non-transparent M or H objects of measurement, e.g. of objects made

of concrete or metal

Principle: application of a double-refracting layer on the surface of the test object, for instance by gluing a

film to the surface (underneath the film will be a reflective layer)

measurement of the principal normal stress difference und of the principal normal stress

directions by using a reflection polariscope

L light source

P polarizer

A analyzer

V quarter-wave plate

F film

SP,K mirror with

adhesive layer

HSP semipermeable

mirror

Different lighting techniques


4.4.2 Moiré technique

Principle: Moiré effects arise from the superposition of a deformed grid with an undeformed reference grid

a) Moiré of rotated patterns b) Moiré of parallel patterns

Applications: determination of strains or in-plane inclinations in plane surfaces

determination of deflections and out-of-plane inclinations of curved surfaces


1 reference grid

2 test object

3 camera

4 object grid

5 semipermeable mirror

Different Moiré techniques

a) superposition of a grid which is applied to the test object

with a reference grid

b) superposition of a reference grid with its own shadow image

c) superposition of two grids which are projected onto the test object before and after the loading, respectively


4.4.3 Holographic interferometry

Principle:

Laser: generation of coherent light

Dividing mirror: division of the primary beam into two coherent beams (object wave

and reference wave)

Object wave: diffuse reflection at the surface of the test object

Photographic plate: receives a part of the reflected light; superposition with the

reference wave -> interference; development of an interference pattern,

the so-called hologram

photographic plate


5. Non-destructive testing in civil engineering

Content:

5.1 Rebar locator (electromagnetic)

5.2 Pulsed radar

5.3 Radiography

5.4 Corrosion monitoring / Potential field measurement

5.5 Ultrasonic testing

5.6 Impact-echo method

5.7 Acoustic emission analysis

5.8 Infrared thermography

Significance of the concrete covering:

A sufficiently dense and thick concrete cover is one of the most important preconditions for the durability of reinforced concrete structures.

The requirements are specified in DIN 1045 under consideration of the environmental conditions.

The analysis of damage patterns at concrete members reveals that inadequate concrete cover along with incorrect curing is the predominant reason for these damages.

Guidelines for the planning and implementation of the reinforcement as well as for the quality assurance are given in the recommendation Betondeckung und

Bewehrung (“Concrete cover and reinforcement”) of the DBV (Deutscher

Beton- und Bautechnik-Verein e.V.), a German association for structural engineering.

Furthermore, the measurement of the concrete cover and the statistical evaluation of the results is regulated.


5.1 Rebar locator (electromagnetic)


Measurement principles of magnetic methods

Unidirectional magnetic field

• The simplest instrument is a permanent magnet, which is

moved over the concrete surface. The detection depth can

be estimated by using a reference specimen with known

concrete cover. This depth is usually below 20 mm.

• It is also possible to measure the attracting force between

the permanent magnet and the reinforcing steel.

• The calibration has to be conducted for the each

reinforcement diameter separately.


Alternating magnetic field

• The measurement concept is based on the principle of a

transformer: an alternating (AC) current in a primary coil

induces a voltage in a secondary coil.

• The magnitude of the induced voltage depends on the

amount and on the proximity of magnetizable material. After

calibration in an nonferrous environment, the change of the

induced voltage may be measured accurately by using a

bridge circuit and the distance to a reinforcement bar with

known diameter may be determined.


Concept of the concrete cover measurement by using an alternating

magnetic field which is influenced by the steel reinforcement

AC


Eddy current

• Techniques based on eddy current differ from methods

based on alternating fields only by the strength of the

generated magnetic field.

• In case of non-magnetic reinforcement (e.g. stainless steel),

only the eddy current technique allows to detect the

reinforcement and to measure the concrete cover. The

measurement effect results from the formation and damping

of an alternating current in an electrically conducting

material.

• In principle, it is possible to determine reinforcement

diameter and concrete cover by detecting the real and the

imaginary part of the complex impedance Z. However,

currently no such instrument is offered on the market.

Commercial rebar locators are based on the application of alternating

magnetic fields or on the eddy current method with pulse induction. Only

the eddy current method allows to detect stainless steel.

Modern devices are coupled with a path measurement and allow linear

or planar presentation of the results similar to radar methods. Their

major field of application is the measurement of the concrete cover.

Statistical evaluation, e.g. according to the aforementioned DBV

recommendation, is usually implemented in the software. The accuracy

strongly depends on the diameter of the reinforcement.

The devices are calibrated for the detection of reinforcement and can not

distinguish between steel bars and other metallic inclusions.


Rebar locators based on inductive techniques


• maximum detection depth ranges from 10 cm to 18 cm

• precise determination of the reinforcement position; localization problems may occur in case of close meshes (spacing < 10 cm)

• concrete cover up to 50 mm ± 1 mm for single rebars in case of known rebar diameter; reduced accuracy for close and multilayered reinforcement as well as for large concrete cover

• identification of individual rebars possible if spacing is larger than diameter

• determination of diameters nearly impossible or only with high uncertainty

• inclined rebars detectable in case of a planar presentation

• interpretation of the results difficult at overlapping reinforcement meshes

Rebar locators based on inductive techniques

Summary of performance parameters


Commercially available instruments

Imaging rebar locator (path measurement)


Measurement range

and accuracy

as specified by the

manufacturer


Measurement range and accuracy

(comparative study)


Representation of results as line scan with preset target

concrete cover


Area scan


Area scan

Ceiling at the support

with bent-up bars


Line scan of a concrete wall with irregular concrete cover



Example: Measurement of the concrete cover at a bridge


Example: Measurement of the concrete cover at a bridge


5.2 Pulsed radar

Applications:

• Localization of inclusions in concrete (e.g. reinforcement,

tendon ducts, anchors, dowels)

• Investigation of layers (thickness, inhomogeneities)

• Detection of imperfections and damages (e.g. detachments,

hollow spaces)

• Detection of objects in soils (foundations, tanks, pipelines)

• Screening of material properties (moisture content, salt

content, homogeneity)

Basics

• Radar is a general term for electromagnetic waves in the

frequency range between 106 Hz and 1010 Hz.

• In the case of pulsed radar, short electromagnetic pulses are

emitted. The mean frequencies are ranging from 20 MHz to

approximately 2 GHz.

• The measurement principle is based on the propagation law for

electromagnetic waves. Relevant material properties are the

electrical conductivity б and the relative dielectric constant εr .

• For the detection of steel reinforcement, antennas with a

frequency range from 1 GHz to 2 GHz are commonly used. With

these antennas, an inspection depth of approximately 0,5 m is

achieved in concrete.


oz

o

r

ezc

tEtz ÊEαε −

−= 0),(


With the radar method, the time-dependent electric field is measured

and evaluated.

The propagation of electromagnetic waves is described by the

Maxwell equation. In the case of structural materials, a number of

simplifications may be made. The relative magnetic permeability

may be set to µ r ≈ 1. The conductivity б is rather small, hence the

loss angle is also small.

In that case, the electric field may be described as plane

wave impulse for a linear polarization.),( tzE

2

0 tcs

r

⋅=ε

with: co velocity of light in the vacuumεr relative dielectric constant of the material

α absorption factor

Then, the propagation velocity v is

and the depth s of a reflector:

When the wave hits an interface between two materials, a reflected and a transmitted wave are formed. The reflection coefficient r depends on the angle and on the polarization of the incoming wave. In case of perpendicular

approach:

ε r

cv 0=

εεεε

21

21

rr

rrr+

−=



The depth d of a reflector may be determined approximately on the basis of the

runtime t and of the mean propagation velocity v.


Formation of

reflection hyperbolas


Transmission method


Reflection characteristics, formation of radar images

Simplifying, real impedances may be assumed for dry non-conducting

materials. The reflection factor depends on the difference between the

relative dielectricity constants and the conductivities.


Reinforcement detection

Positioning of a linearly polarized send-receive dipole (SE) lateral to the direction of movement (arrow)

- objects that are aligned parallel to the dipole (black) are easy to detect

- objects that are perpendicular to the dipole (gray) are hardly detectable

Reinforcement bars are detectable with maximum intensity if their orientation is perpendicular to the direction of the antenna's movement.

Detection of different materials

on the basis of the

phase shift


Detection of reinforcement layers and determination of wall thicknesses


Area scans by combining x- and y-traces


Scan at the side of a reinforced concrete beam

Stirrups

Inclined bar

with hook


Representation of time slices (in-depth scans)



Advantages of pulsed radar:

– larger depth range

– reliable detection of stainless

steel

– non-metallic reflectors are

detectable as well

– quick and comprehensive

scans with large information

content

Advantages of pulsed radar:

– larger depth range

– reliable detection of stainless

steel

– non-metallic reflectors are

detectable as well

– quick and comprehensive

scans with large information

content

Advantages of rebar locators:

– concrete cover measurement

more accurate

– low price

– simple handling

Advantages of rebar locators:

– concrete cover measurement

more accurate

– low price

– simple handling

Disadvantages of both techniques:

• determination of bar diameters inaccurate or impossible

• errors in case of close rebar spacing

Disadvantages of both techniques:Disadvantages of both techniques:

• determination of bar diameters inaccurate or impossible

• errors in case of close rebar spacing

Comparison of pulsed radar and rebar locator


Application

• detection of reinforcement steel• detection of imbedded objects• measurement of layer thicknesses

Detection of the upper reinforcement of a reinforced concrete beam


Beam

Beam

Column

Column


Identification of the structure of a slab

Steel beam

Masonry

Concrete

Detection of stirrups and inclined bars


Column

Column

Detection of stirrups


Radar scan of a floor wall, comparison of different sampling rates


Comparison of radar and electromagnetic rebar locators

at the example of a floor wall


Floor wall, Ferroscan analysis with software


Floor wall, radar line scan, determination of thickness



For comparison: ultrasonic measurement of wall thickness


Example: road construction

Determination of the layer thicknesses by using a horn antenna

Possible results



Screening of railway ballast

mobile measuring device

for the screening of

rail track gravel

Tendencies for the

development

of measurement

instruments



5.3 Radiography using x-ray and gamma radiation

Scope of application:

• Testing of welded joints

• Determination of position, number, and diameter of the

reinforcement, even in case of complex geometry

• Localization of installation parts

• Diagnosis of damages

• Determination of grouting defects

• Detection of voids and compaction defects



Principle of a transport and operation container for isotopic sources

Containers for different isotopic sources


GAMMAMAT TK 30 with Cobalt-60 source in operation


Detection of the reinforcement of a concrete beam


Generation of x-radiation

x-ray tube


Energy spectrum of

x-radiation


X-ray tube


Betatron (accelerator)


Applications



Applications


Applications


testing device and

radiation energy

maximumconcrete thickness

[cm]

exposure timein case of 30 cm concrete and

1 m distance, D8 film

x-ray tube 200 keV 25-30 ca. 30 min

Cobalt-60 1,2 MeV 45-50 ca. 5 min

MegascanTM 7,5 MeV [1] 150 ca. 0,3 min

Comparison of various radiation sources


Application to reinforced concrete structures

Q Q Q'

radiographic inspection double exposure

concept of the radiography

Q - sourced - reinforcement diameter

b - concrete cover (up to rebar center)d' - rebar width on the filmV - displacement

a - distance between source and film

d

bd

x-ray filmd = (a-b) d' / ab =

a V

q+ V

Principle of the detection of reinforcement using radiographic tests

Application principle of radiography to detect reinforcement


Gammaquelle

image on x-ray film

Drill hole radiography

gamma source

Lateral transmission

image on x-ray film

Significant areas for the reinforcement detection in a beam


Example: Beam


Example: Bridge


Example: Beam


Example: Beam


Radiogram of a tendon duct


Radiogram of a concrete slab


Formation of concrete cracks in a slender prestressed beam


Defective reinforcement in a suspender beam


Defective reinforcement (stirrups)



5.4 Corrosion monitoring / Potential field measurement

For the purpose of concrete maintenance, usually the following inspections are required:

- determination of the carbonation depth

- determination of the chloride content (depth profile)

- determination of the concrete cover

Mechanism of the carbonation

Carbonation is the conversion of calcium hydroxide which is solved in the pore water to calcium carbonate under the presence of CO2. The crystallization is accompanied by a volume increase.



pH-value as indicator for the carbonation depth


a) beginning carbonation without steel corrosion (not detectable by visual

inspection or tapping the concrete surface)

b) advanced carbonation with steel corrosion (visible or detectable by tapping the concrete surface)

Progression of carbonation and subsequent corrosion of reinforcement

• The severest carbonation occurs in rooms which are continually

dry. However, the corrosion protection is given by the lack of

moisture.

• Concrete which is always stored in water is not affected by

carbonation. Corrosion protection is given due to the lack of

oxygen.


K … carbonationR … corrosion

Carbonation rate



Determination of the chloride content

- direct addition of chlorine (hardening accelerator) has been prohibited in Germany for about 30 years

- only free ions induce corrosion; but the total chloride content is determined

• allowable limit values: (chloride content by mass in relation to the cement)

plain concrete 1,0 %

reinforced concrete 0,4 %prestressed concrete 0,2 %

- distinction between the critical chloride content, which causes a depassivation of the steel surface, and

- the chloride content, which leads to corrosion phenomena classified as damage

• best evaluation criterion is the Cl-/OH- ratio, but there is no in-situ

measurement method

• the total chloride content is determined quantitatively by the acid digestation of concrete flour and the subsequent potentiometric titration using a silver nitrate solution (DAfStb Heft 401)


Principle of the chloride corrosion


Corrosion in cracks

rust


Electrochemical corrosion of steel in concrete

In case of a relative humidity between 65 and 90%, the moisture of

concrete is high enough to enable electrochemical corrosion. There are

anodic (steel resolving) and cathodic (steel preserving) areas on the

reinforcement. The following reactions take place:

Generation of a local element in an electrolyte drop on iron


area of anodic effect


Electrochemical corrosion

of steel in concrete


Basics of potential field measurement

• The electrochemical potential field measurement is a method to

evaluate the corrosion state of the reinforcement.

• The potential difference is measured between the reinforcement

steel and a reference electrode on the concrete's surface.

• Various reference electrodes are common, for instance:

- copper electrode in a solution of copper sulphate

- silver / silver chloride electrode in a solution of potassium

chloride 0,5 mol/l

• The corrosion potential depends on the electrode.

• The inspection of the structures is non-destructive and may be

performed comprehensively.


Principle of the potential field measurement

Electrical field and current in a macro element (steel in concrete)

(--- current flow potential field)


Schematic diagram of the potential field measurement

Layout of a measuring electrode

Cu/CuSO4 – half cell with

typical potential range


Local corrosion spots, in particular in case of chloride induced corrosion, may cause the formation of distinctive potential peaks.



Potential field measuring device

Typical applications

• inspection of bridges

• inspection of park decks

Prestressing steel of a

precast girder

detachment of concrete caused by corrosion


intact passivation layer

Corrosion due to cracks and construction defects



Damages of a bridge resulting from construction defects

Potential distribution on

the surface

of a runway


Corrosion detected

at the reinforcement after

removing

the concrete cover

Example: parking deck


Mapping of the potential



5.5 Ultrasonic testing

• Physical background

– Wave types

– Behavior at interfaces

– Generation of ultrasonic waves

• Transmission

• Ultrasonic echo techniques

• Ultrasonic wave propagation

Wave types in solid

bodies



Behavior at boundary layers

Assumption: perpendicular incidence Example: air-filled hollow in concrete

ZB = 9,2 ⋅106 Ns/m3

ZL = 4,3 ⋅102 Ns/m3 (ρL = 1,3 kg/m3, cL = 333 m/s) The reflection coefficient results in: R = -0,999 In case of a water-filled hollow it comes to: R = -0,72

Reflection: 100% at the boundary concrete / air 50% at the boundary concrete / water 70% at the boundary concrete / steel


Influencing factors for the wave velocity in concrete

• size and type of aggregate

• amount of hollow spaces

• temperature

• water/cement ratio

• density

• cement class, concrete additives

• reinforcement

Imaging method

used with the impulse-

echo technique


Fig. 3a: Ultrasonic intensity versus time or depth (A-Scan)

Fig. 3b: Succession of A-scans by shifting the test probe (B-Scan)

Generation of ultrasonic wavesImportance of the coupling for the transmission of energy


Measurement principle

in transmission


Simulation of the wave propagation in concrete

considering different air entrainments


Concrete modelwith crack


air content: 2% air content: 4%

Calculated A-scan with

air content 0%


air content 2%



air content 4%


Wave propagation in concrete without air voids



Sound propagation in concrete having 2% air voids


Sound propagation in concrete having 4% air voids


Applications

• testing of welded joints

• thickness measurement of walls made of steel or concrete

• determination of concrete compressive strength;

evaluation of homogeneity

• evaluation of hardening behavior – prediction of strength development

• flaw detection

• pile integrity test

Thickness measurement of walls made of steel



Transverse wave probes in array arrangement for concrete


Measurement of the wall thickness of a corridor wall


0

100

200

300

400

500

100

200

300

0

10

0

20

0

30

0

40

0

50

0

10 20 30 40 50 60

50

100

150

200

250

300

vL= 3959 m/s

vT= 2415 ...

2533 m/s

Measurement of the wall thickness of a corridor wall


Corridor wall, approximate position of the reinforcement

0

100

200

300

400

500

100

200

300

0

10

0

20

0

30

0

40

0

50

0

10 20 30 40 50 60

50

100

150

200

250

300

vL= 3959 m/s

vT= 2415 ...

2533 m/s


Determination of vL in case of one-sided access

Linear regression for:

Y = A + B * X

parameter value error

--------------------------------------------------------

A 8,28 1,59018

B 2,52629 0,04083

10 20 30 40 50 60

20

40

60

80

100

120

140

160

180

runtim

e in µ

s

distance in cm

Determination of the wave velocity from the slope

vL = s/t

vL = 1 / 2,5263 = 3959 m/s

determination of the US-speed ν of one sideL


Correlation between longitudinal and transverse wave velocity

in case of concrete

)21(

)1(2

)1(2

1

)21)(1(

)1(

2

2

2

2

µ

µ

µρ

µµρ

µ

−

−=

+⋅=

−+

−=

T

L

T

L

v

v

Ev

Ev

LTTL

LTTL

vvvv

vvvv

61,063,120,0

64,0,57,116,0

,==⇒=

==⇒=

µ

µ

The relation vL/vT depends only on µ.

In case of concrete:

vT = 0,61….0,64 vL


Examination of the homogeneity of a pillar using the transmission method

2nd floor; pillar B03 2nd floor; pillar B04

measurement grid 10 x 20cm measurement grid 10 x 20cm

starting from 10 cm over ground starting 10 cm over ground

direction A direction C direction A direction C

75 72 83 65 61 66

73 78 81 68 67 69

77 75 80 64 58 65

69 77 75 65 63 67

71 73 73 65 66 67

73 71 74 67 68 81

75 85 86 73 81 73

81 98 81 69 70 78

77 79 76 67 70 69

72 83 77 65 67 66

2nd floor; pillar B08 2nd floor; pillar B18

measurement grid 10 x 20cm measurement grid 10 x 20cm

starting 10 cm over ground starting 10 cm over ground

direction A direction C direction A direction C

86 68 93 79 85 /

96 78 83 82 79 83

87 81 83 75 103 90

76 82 76 84 79 81

80 73 75 83 71 76

78 82 91 78 79 75

98 99 71 70 86 79

77 72 71 70 67 73

68 65 67 91 70 75

58 63 59 72 69 98

2nd floor; pillar B10

measurement grid 10 x 20cm

starting 10 cm over ground

direction A direction C

73 74 70 color runtime sonic speed

70 71 68 ≥ 90µs ≤ 2222 m/s

71 71 71 ≥ 80µs ≤ 2500 m/s

72 79 74 ≥ 70µs ≤ 2850 m/s

76 71 75 ≥ 60µs ≤ 3333 m/s

73 79 75

85 81 82

83 79 72

73 79 68

88 78 67

Homogeneity check of a pillar

using the transmission method


Correlation of wave velocity and cube compressive strength

for concrete with different compositions



5.6 Impact-echo method

Fields of application:

- thickness measurement of concrete slabs (even

including top layers)

- detection of flaws, detachments, and compaction

deficiencies in slabs

- (detection of cracks normal to surfaces)

- detection of tendons

- (examination of groutings)

- inspection of masonry

Basics of the impact-echo method

Fast Fourier Transformation (FFT)

The evaluation of the measuring signals is conducted at the frequency sprectrum


dd = thickness resp. flaw depth

f = frequency

cP = sonic speed

f

cd

2=

f

cd

2=


Principle of the impact-echo method

Demonstration of the wave propagation

impact by falling steel bullet

Wave propagation during impact-echo examinations

Signals without and with large flaw



Wave propagation during impact-echo examinations

Signals without and with small flaw

Characteristic results as dependent on the

lateral extension of a flaw


Frequency spectrum of the „sending signal“

The energy and the frequency spectrum depend on the diameter of the bullet and on the contact time tc.


Measurement of the velocity of the

longitudinal wave using two sensors.



Measuring devises

� test probes

� coupling

� measuring instruments

Commercial measuring devices


Impact-echo measuring probe

with

integrated impactor


Measuring probes with separate

impactor

Coupling with lead foil


new sensor

developments


Commercial

measuring devices


Movable test probe



The problem:

Tunnels are often constructed with shotcrete. In the construction process, an outer shell

is built first as a temporary protection. Then, an inner shell - usually made of reinforced concrete - is cast.

This two-layer design offers the opportunity to install a plastic sealing sheet between the outer and the inner shell in order to ensure the resistance against a hydrostatic water

pressure on the structure.

During the construction of the inner shell, flaws and reduced thicknesses may occur, in particular in the area of the ridge. In some cases, the outer reinforcement is exposed.

Example of application:

Quality inspection of the inner shell of tunnels; non-destructive verification of a sufficient shell thickness




Approach to a solution: thickness measurement using the echo method


Restoration monitoring possible?


The monitoring is only possible under the condition of a bond between concrete substrate and repair mortar.

In case of segregation, a film of cement paste may be formed, which

leads to the formation of an gap.


Guideline for the conduction of non-destructive inspections

of the inner shells of tunnels (RI-ZFP-TU)



5.7 Acoustic emission analysis

• Basics:

– origins of acoustic emission, propagation

– measurement principle

– characteristics of acoustic emission (AE),

terminology, sensors

– localization and accuracy of localization

• Specifics of reinforced concrete

• Application in load tests

• Further technical applications


Origin of acoustic emission

• Acoustic emissions occur when materials are strained beyond

the elastic limit.

• Microcracks develop at points of material flaws or

inhomogeneities. This process is accompanied by a sudden

release of an elastic wave - an acoustic emission event.

• This released impulse is very broadband. It is used for technical

purposes within a frequency range of 20 – 300 kHz, i.e. within

the ultrasonic range.

• Breaking of glass is an example of acoustic emission within the

audible range, but microcracks in steel and other solid bodies

emit also intensive ultrasonic impulses.

• By acoustic emission analysis, such signals are detected and

interpreted. Hence, it offers the opportunity to detect and study

damage processes.

• The acoustic emission analysis is a passive technique, i.e. the

ultrasonic impulses are recorded in the moment of their formation.

It is not possible to detect flaws that are not active during the

mechanical loading.

• Due to the KAISER effect, the material must be stressed beyond

the previously reached maximum stress in order to allow a

detection of the flaws.

• Leakages or delaminations (in composite materials) may also

cause acoustic emissions.

• Discrete and continuous emissions are distinguished.



Discrete and continuous acoustic emissions

Discrete signal of AE

Continuous signal of AE


Acoustic emission measurement

Signal interpretation

Detection of the wave onset when a threshold value is exceeded


Signal interpretation

Analysis of characteristics of classical signal parameters


• onset time (time of the first threshold exceedance)

• maximum amplitude

• rise time (from first threshold exceedance to the time of max. amplitude)

• signal duration

• amount of exceeding oscillations (counts) (of the threshold in one polarity)

• energy

• RMS (effective value) of the continuous background noise (before the respective hit)

Layout of a AE sensor



Influences on the precision of localization

• Signal attenuation

• Anisotropy of the wave velocity

• Different attenuation of different wave types

• Disturbing signals (threshold exceedance)

• material inhomogeneities


0 mm

545 mm

1010 mm

1535 mm

Options for the presentation of results



Mechanism of the formation of micro cracks in normal concrete



Cracking process in concrete under compressive loading

Applications

• laboratory tests

• experimental safety evaluation of existing structures

• further technical applications


Example for the detection of AE signals during a Brasilian test



Formation of cracks during a during a Brasilian test

fracture surface

Applications for the experimental safety evaluation of existing

structures

• mainly used in areas which are inaccessible for other

methods

• in case of a risk of brittle failure, especially shear failure

• for prestressed concrete structures



The acoustic emission analysis as accompanying method in load tests

• integral method � qualitative information about active cracking processes

• qualitative evaluation of active cracks

� improved determination of the cracking load (transition to cracked mode)

• detection of active areas (multichannel system)

� prevention of crack formation (prestressedconcrete)

� monitoring of damaged areas

� prevention of shear failure

• KAISER effect � information on the loading history

Acoustic emission

analysis in the field of

structural engineering

Investigation of crack

formation in beams


Monitoring of cantilevers and

areas vulnerable to brittle

failure during load tests


Acoustic emission

analysis in the field of

architectural engineering

Load tests of pillars


Neues Museum in Berlin

Historic structure


Loading vehicle BELFA

Reinforced concrete Bridge across the Werra having nine spans,

partially destroyed during world war II

AEA applied to shotcreted beams




Niederbrücke damaged by a flood in Döbeln

AEA applied to an unreinforced concrete arch bridge


AEA applied to reinforced concrete beams

Loading vehicle BELFA - Kaiserin-Augusta-bridge in Berlin



Ferry dock Blexen

AEA used for the pre-

vention of crack formation

in prestressed concrete

beams


Prototype BELFA-DB

Viaduct in Rhena

Railroad viaduct in Rhena


AEA applied to the concrete arches and

to the damaged sandstone pillars


Prototype BELFA-DB

AEA applied to a masonry arch


Further technical applications

• Inspection of containers for liquid and compressed gas – crack

detection

• Monitoring of technological processes

• Analysis of corrosion

• Monitoring of transformators (partial discharge, gas formation)

• Tribology (investigations into friction and wearing)

• Geology

• detection of leakages


5.8 Infrared thermography

• Physical background

– Planck's law

– Wien's law of displacements

– Stefan-Boltzman law

– Influence of the atmosphere

• Applications

– Examination of hidden details of structures

– Application in the field of building physics

Physical background


Planck's radiation law

The law explains the correlation between the spectral distribution of

the thermal radiation and the temperature T for the black body (emits

a spectrum which depends only on the temperature).

From this, two phenomena with practical relevance arise:

• Wien's law of displacements

The location of the radiation's maximum λmax is displaced to smaller wave lengths with increasing temperature.

• Stefan-Boltzmann law

The emitted total radiation increases proportionally with T4.


Atmospheric influences

• Radiation attenuation by absorption and dispersion

some components of the atmosphere such as CO2 and H2O

have strong absorption bands in the range to 14 µm

• Characteristic radiation of the atmosphere

interferes with the radiation of the object


Applications

• Examination of hidden construction details

• Detection of thermal bridges and flaws in the field of

building physics

• Combination with Blower-Door method

• Detection of overheating of technical instruments



Heated exit of an underground garage



6. Experimental safety evaluation of existing structures

6.1 General remarks

Normal case: prove of structural safety by calculations

Special case: sufficient safety can not be proved

by calculations

option 1: - completion of the analysis assumptions (inquiry of

documents, survey of the structure, testing of the building

materials, non-destructive testing)

- improvement of the computational model (static system,

mechanical boundary conditions, spatial load-carrying behavior, non-linear material behavior)

- repeated (improved) calculation of the structural safety

- if still no sufficient safety level is provable:

option 2: - experimental evaluation of structural safety

- by load tests


Principle of the experimental safety evaluation by loading tests

Structure

Application of test loads

Measurement of the Reactions (Deformations, Acoustic emission, Reaction forces)



Load test in 1890

On the history of load tests (in German):

G. Bolle, G. Schacht, S. Marx, Geschichtliche Entwicklung und aktuelle Praxis der Probebelastung, Teil 1: GeschichtlicheEntwicklung im 19. und Anfang des 20. Jahrhunderts, Bautechnik 87(2010)11, 700-707, Teil 2: Entwicklung von Normen und heutige Anwendung, Bautechnik 87(2010)12, 784-789.


Guideline of the DAfStb (Deutscher Ausschuss für Stahlbeton,

German Committee for reinforced concrete) for the application of

load tests at reinforced concrete buildings:

„… make provision against sudden (without warning) failure“

This requirement is met by:

> self-securing loading devices

> on-line measurement and evaluation of the structural behaviour


Safety concept

1

G

anal

ysis

exp

eri

men

t

ultimate load

Lo

ad

Reaction

Load-Reaction

(idealized)

Loading test

service load

Gd

,j

+ Q

d

exp. target load

design load (analysis)

∆∆ ∆∆Q

d

experimental limit load

permanent load

ext Ftarg ≤ ext Flim

ext

Fta

rg

ext

Flim


concrete compressive strain 600 µm/m800 µm/m for ≥ B25

crack opening ∆w = 0,3 mm

from that max. 20% remaining

remaining deflection 10% of the maximum value

Criteria for the experimental limit load


Procedure of the planning of load tests

Formulation of the task

• identification of the load test's objective

• examination of the chance of success under consideration of the actual structure and the experimental capabilities

Pre-examinations

• determination of material parameters, of the building's geometry, and of the static system

• review of existing documents

• pre-calculations and determination of critical load positions and combinations

Planning of the load test

• determination of the target load for the different load cases

• planning of the loading technique including reaction forces (anchoring)

Planning of the measurements

• identification of the required measurements and of the sensor positions

• discussion of the test concept with the client


Loading technique: self-securing loading principle

• self-securing loading by hydraulic actuators

• mobile, adaptable loading frame system


Loading technique: anchoring against dead loads


Measurement methods used for the experimental safety evaluation of structures

Essential components of in situ measurements are:

• computer-aided real-time data acquisition system

• load cells

• inductive displacement sensors (LVDTs) and special sensors designed on the basis of them

• strain gauges

• inclination sensors

• acoustic emission analysis system

The immediate availability of all measured values on a display and the possibility to influence the test procedure at any time are required.


Acoustic emission sensor


Central market hall in Leipzig

Loading with dead loads (left) and

with mobile loading frame (right)

6.3 Case studies


Loading frame on a reinforced concrete ceiling


Loading frame in a historic department store


Loading frame in the ground floor

Load distribution „moves“upward from floor to floor

Loading frame in a historic department store


Load distribution on three spans of the ceiling


Measurement of deflections in the historic department store


Example of anchored reaction frame

309309





Example of the examination of balconies


Load test of a prestressedconcrete girder which wasdamaged by fire

Anchoring against dead loads

Deflection measurement using a suspension technique


Load test of a prestressed

concrete girder which wasdamaged by fire


Loading frame on a concrete bridge



Development of the principle anchoring against dead loads

BELFA specifications:

• street-legal heavy-duty vehicle which moves to the examination objects autonomously (i.e. driving weight < 80 t, axle weights < 10,5 t and turning circle 22 m)

• loading capability according to DIN 1072 for bridge classes 12 to 60 in the main lane (up to bridge classes 30 without anchoring)

• bridge spans up to 18m (therefore applicable for more than 70% of all existing road bridges in Germany)



319319

BELFA in operation


BELFA - options for ballast


BELFA on a reinforced concrete bridge


Measuring base underneath the bridge


BELFA on an arch bridge


BELFA on a bridge with brickwork vaults


Profile of the bridge with masonry vaults


BELFA on a bridge with masonry vaults


BELFA on a bridge with masonry vaults


Measurement base underneath the bridge


Sensors underneath the bridge


BELFA on a reinforced concrete bridge


Inductive displacement sensor for deflection measurementSensor positions

Inductive displacement sensor for strain measurement

Acoustic emission sensor

12

3

40 50 645

1.38

60 645 50 40

1530


Measurement base


Displacement sensor in the shear area


Loading vehicle BELFA-DB (prototype)


Load test of a railway bridge


Measurement base and sensors


Hydraulic loading system


7. Monitoring of structures

7.1 Problem

Commonly, the so-called structural health monitoring means the long-term

measurement of deformations or displacements of structures. The term long-

term monitoring of structures is also used.

The objectives of these long-term measurements are:

• early detection of changes of the structural condition (formation of damages)

• detection of changes to the structural system

• detection of changing load levels

On the basis of the results, preserving or retrofitting measures may be planned.

From the long duration of the measurement arise special requirements for the

measurement methods. The measured values should be acquired stably and

without the need of calibration over the period of the measurement. Furthermore, the installed sensors are expected to be cost-efficient.


Short-term measurement long-term measurement

inductive displ. sensors

electrical strain gauges

vibrating wire gages

fiber Bragg grating sensors

Strain measurement for the characterization

of the structural behavior

Usually, measurement methods which base on the measurement of the frequency or of the wave length are more long-term stable than those basing on the measurement of a resistance or amplitude. For this reason, especially vibrating wire gages and Fiber Bragg grating sensors have proved to be suitable sensors for the monitoring of structures.


7.2 Measuring concepts

7.2.1 Vibrating wire gages

wiretheofdensity

wiretheinstress

wireoflength

wireoffrequency

21

=

=

=

=

=

ρ

σ

L

f

/)L/(f ρσ

The characteristic frequency of a vibrating wire changes with the stress.

image: Geokon company


22

22

)4(

)4/(

fE

L

E

Lf

⋅⋅

=

⋅=

⋅=

ρε

εσ

ρσ

:lawsHooke'using

strain = constant · frequency²

For long-term measurements: constancy of - length- density

- Young's modulus

• Length: influence of temperature is compensated by calculation

• Density: wire within a protective glass

• Young's modulus: elastic loading of the wire


Sensor for the external application on steel or concrete

• measuring range 3000 µm/m

• reusable

• temperature sensor included



Sensor for the embedded application during the placement of concrete

• measuring range 3000 µm/m

• not reusable




Sensor for the measurement of displacements

e.g. measurement of crack widths or displacement of bearings

• measuring range is independent of wire length (commenly 25 - 50 mm)

• reusable


• anchor embedded in concrete or welded to steel



Load cells

• measurement of bearing or prestressing forces

• various force ranges



Components of a monitoringsystem

in case of discontinuous readings:handheld measuring device for all sensor types image: Geokon company


reflection and transmission at the grid

segments

constructive interference:

• intensification of light intensity upon reflection

• reduction of light intensity upon transmission

• only in case of a certain wave length which satisfies Bragg's conditions of

reflection→ Bragg's wave length

When changing the distance between the grid segments (strain, temperature), Bragg's wave length

changes proportionally.length wavesBragg'

sensor theoflength period

grid theofindex refractivemean

2

2

0

0

0

=

=Λ

=

Λ⋅=

=

=Λ⋅

λ

λ

λλ

λ

m

m

m

n

n

n

7.2.2 Fiber Bragg grating sensors


• very good long-term stability; no need for calibration

• very good resistance to corrosion

• exposition to temperatures > 700°C possible (custom-built)

• very suitable for the measurement of strains in composite materials; immediate

integration possible (airplanes, wind power stations)

• measurement of large strains, >1% possible

• sensors are small and lightweight

• insensitive to electromagnetic fields

• sensors operate passively (no electric power source needed); application in explosion-prone areas possible

• transmission of signals over large distances (> 50 km) possible

• up to 100 Fiber Bragg gratings may be written into a single fiber

Disadvantages of fiber Bragg grating sensors:

Advantages of fiber Bragg grating sensors:

• considerable temperature dependence; 1°C corresponds to 8 µm/m mechanical strain


opticalspectrum analyzer

(OSA)

Measurement using an optical spectrum analyzer in transmission

light source fiber Bragg sensor

opticalspectrum analyzer

(OSA)

coupler

Measurement using an optical spectrum analyzer in reflection

fiber Bragg sensorlight source


Analysis of the light spectrum

Bragg's wave length

local minimum

(reflection: maximum)

limiting value


Comparison of fiber Bragg grating sensors (FBG) and vibrating wire gages

FBG Vibrating wire

long-term stability very good very good

price in € 200 € for grating only 150 € including temperature sensor

application

• on steel

• in hardened concrete

• in fresh concrete

• for displacements

• on steel

• in hardened concrete

• in fresh concrete

• for displacements

smallest dimension 100 µm 40 mm

installation still expensive simple

electromagnetic compatibility very good usually without any problems

measurement instruments

and data loggerunder development well-engineered


Monitoring of a bridge made of high performance concrete (HPC) in the Weißeritztal

• single-span bridge with a span of 32 m

• first application of HPC for a bridge in Saxony

• built without sealing and without pavement in order to allow the monitoring directly at the structural member

7.3 Practical examples


• 4 displacement sensors Geokon4420-1-1: measurement of the

relative movement between bearing and superstructure; temperature measurement

• 2 strain sensors Geokon VSM 4000: measurement of the strain at midspan on the upper side of the bridge; temperature measurement

• 1 strain sensor Geokon VSM 4000: measurement of the strain at midspan on the lower side of the bridge; temperature measurement

• In parallel to the application of the

vibrating wire gages, fiber Bragg grating sensors were installed.

The following sensors were applied:


downside of the bridge data logger

strain sensors (FBG + SSA)

displacement sensor at the bearing


-10

-5

0

5

10

15

20

19

. J

an

24

. J

an

29

. J

an

03

. F

eb

08

. F

eb

13

. F

eb

18

. F

eb

23

. F

eb

28

. F

eb

05

. M

rz

10

. M

rz

Datum/Zeit

Te

mp

era

tur

in °

C

-0,35-0,3

-0,25

-0,2-0,15

-0,1-0,05

00,05

19

. J

an

24

. J

an

29

. J

an

03

. F

eb

08

. F

eb

13

. F

eb

18

. F

eb

23

. F

eb

28

. F

eb

05

. M

rz

10

. M

rz

Datum/Zeit

Ve

rsc

hie

bu

ng

in

cm Auflager Mst. 4

Auflager Mst. 5

Auflager Mst. 6

Auflager Mst. 7

Measurement results

date / time

date / time


-400

-350

-300

-250

-200

-150

-100

-50

0

19. Jan 24. Jan 29. Jan 03. Feb 08. Feb 13. Feb 18. Feb 23. Feb 28. Feb 05. Mrz 10. Mrz

Datum/Zeit

De

hn

un

g in

µm

/m

Dresden-oben Mitte-unten

Freital-oben

-10

-5

0

5

10

15

20

19

. J

an

24

. J

an

29

. J

an

03

. F

eb

08

. F

eb

13

. F

eb

18

. F

eb

23

. F

eb

28

. F

eb

05

. M

rz

10

. M

rz

Datum/Zeit

Te

mp

era

tur

in °

C

date / time

date / time

Measurement results


-0,1

-0,05

0

0,05

0,1

0

100

200

300

400

500

600

700

800

Datum/Zeit

Vers

ch

ieb

un

g i

n m

m

Auflager Mst. 4

Auflager Mst. 5

Auflager Mst. 6

Auflager Mst. 7

-140

-120-100

-80-60

-40

-200

20

0 100 200 300 400 500 600 700 800

Deh

nu

ng

in

µm

/m

Dresden-oben

Freital-oben

Mitte-unten

Measurement results for the crossing of a 26.6 t vehicle

date / time

displacement at the bearings


Monitoring of a bridge made of high performance concrete (HPC)

across the Mulde near Glauchau


Measurement program

1. Temperature measurement

• hydration heat during construction

• under service conditions

2. Strain measurement within cross-sections

• during load tests

• long-term measurements

3. Deflection measurements



Measuring program

4. Displacements at the bearings

5. Dynamic measurements

• during construction due to hydration heat

• under service conditions



Vibrating wire gage


Cable connections to the sensors


Data logger

Experimental Mechanics Experimental Mechanics

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