tensile strain monitoring in reinforced concrete using non ...1231565/fulltext01.pdftransformers and...

Tensile Strain Monitoring in Reinforced Concrete Using Non-Contact Full-Field

Optical Deformation Measurement Systems

Jenny Lindmark

Civil Engineering, master's level 2018

Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering

Tensile Strain Monitoring in Reinforced Concrete

Using Non-Contact Full-Field Optical Deformation

Measurement Systems

Author: Jenny Lindmark

Supervisor: Cristian Sabau, PhD

Structural and Fire Engineering at Luleå University of Technology

Cosmin Popescu, Associate Senior Lecturer


Examiner: Björn Täljsten, Professor


Program: Master Programme in Civil Engineering – Structural Engineering

Extent: 30 hp

Publication: 2018, Luleå

Department of Civil, Environmental and Natural Resources Engineering

Luleå University of Technology

971 87 Luleå

Sweden

i

Preface This thesis was the last and final task of my Master of Science in Civil Engineering at Luleå

University of Technology. I started my studies nearly six years ago and it is with fond memories

of my studies I am taking this next step in my professional life.

The thesis was conducted with the support of In2Track, a project financed by H2020 – A

European Union Research and Innovation programme.

I would like to express my gratitude to my supervisor Cristian Sabau for all the support I have

received during this thesis and for always having time for all my questions. I would also like to

thank my supervisor Cosmin Popescu for initially introducing me to this topic.

Finally, I would like to thank my family and friends for their constant support and pushing me

to finish this thesis.

Luleå, July 2018

Jenny Lindmark

ii

Abstract As traffic loads increase and bridges age the need for structural health monitoring is growing.

With the digitalization of our society, new non-contact full-field measurement techniques have

been developed. These techniques have the potential to be used in monitoring of existing

bridges. Today visual inspections are carried out every sixth year. These only give a rough

estimate of the structure’s health and only provide information about the surface of the

structure. In addition to these inspections, traditional sensors like linear variable differential

transformers and strain gauges are used to measure parameters such as displacement and strain.

For existing bridges in reinforced concrete it is especially important to monitor reinforcement

strains, as high strains could be indicative of overloading of the structure or even that a failure

is about to occur. The methods available to measure reinforcement strain in existing bridges

today are not very effective and have some limitations.

The aim of this thesis is thus to evaluate the possibility to predict reinforcement strain based on

surface strain measurements obtained by a non-contact full-field optical measurement system.

In this study the software ARAMIS was used to measure surface strains, and traditional strain

gauges were used to measure reinforcement strain. Strain distribution were evaluated at the

initiation of cracks, during sections of cyclic loading and at a load close to the yielding point of

the reinforcement. A correlation factor between the strain registered in the software and the

strain obtained from the strain gauges was introduced.

Based on the results in this study it is not possible to predict exact reinforcement strain based

on surface measurements. Digital image correlation does however show potential to be used as

a non-contact full-field measurement technique for in-situ measurements. Before this is reality

there is still a need for further research in this area.

iii

Sammanfattning Ökning av trafiklaster samt broars åldrande skapar ett behov för tillståndsbedömning av

strukturer. Digitaliseringen erbjuder nya digitala och optiska mättekniker som inte kräver fysisk

kontakt med objektet som observeras (non-contact full-field optical measurement techniques).

Teknikerna visar potential för att kunna användas för tillståndsbedömning av befintliga broar.

Idag inspekteras broar visuellt var sjätte år. Inspektionerna ger endast en grov uppskattning av

brons tillstånd och bidrar således endast med information om dess yta. Förutom dessa

inspektioner används även traditionella sensorer som deformationsmätare och töjningsgivare

för att mäta parametrar som förskjutning och töjning.

I befintliga betongbroar är det av särskild vikt att övervaka töjningen i armeringsjärnen,

eftersom höga töjningar kan indikera att strukturen överbelastas eller att ett brott är på väg att

ske. Metoder för att mäta töjningar i armeringsjärn i befintliga broar är inte särskilt effektiva

och har en del begränsningar.

Målet med denna studie har varit att undersöka möjligheten att förutspå töjningen i armeringen

utifrån töjningar uppmätta på ytan med hjälp av ett digitalt och optiskt mätsystem.

I denna studie har programmet ARAMIS och traditionella töjningsgivare använts för att mäta

töjningar på ytan respektive armeringsjärnen. Töjningsfördelningen utvärderades vid uppkomst

av sprickor, under perioder av cyklisk belastning samt vid en last nära armeringens flytgräns.

En korrelationsfaktor som beskriver skillnaden mellan töjningarna på ytan och armeringsjärnen

har introducerats.

Utifrån resultaten i denna studie är det inte möjligt att förutspå den exakta töjningen i

armeringen utifrån mätningar på ytan. Mätmetoden som användes visar dock potential på att

kunna användas som en ” non-contact full-field” mätteknik för existerande strukturer i fält.

Innan detta är verklighet krävs dock vidare forskning inom detta område.

iv

Contents Preface ......................................................................................................................................... i

Abstract ...................................................................................................................................... ii

Sammanfattning ........................................................................................................................ iii

List of Figures ........................................................................................................................... vi

List of Tables ........................................................................................................................... viii

Notations ................................................................................................................................... xi

Abbreviations ........................................................................................................................... xii

1 Introduction ........................................................................................................................ 1

1.1 Background .................................................................................................................. 1

1.2 Aims and objectives ..................................................................................................... 2

1.3 Methodology ................................................................................................................ 2

1.3.1 Literature review .................................................................................................. 2

1.3.2 Laboratory procedure ........................................................................................... 2

1.4 Limitations ................................................................................................................... 2

1.5 Disposition ................................................................................................................... 3

2 Literature review ................................................................................................................ 4

2.1 Structural health monitoring of bridges ....................................................................... 4

2.2 Strain monitoring in RC bridges .................................................................................. 5

2.3 Digital Image Correlation ............................................................................................ 6

2.3.1 DIC applications – State of the art ....................................................................... 7

2.4 Cracks in concrete ....................................................................................................... 8

3 Laboratory and testing procedure ..................................................................................... 10

3.1 Installation of strain gauges ....................................................................................... 10

3.2 Casting ....................................................................................................................... 12

3.3 Surface preparation .................................................................................................... 15

3.4 Pattern evaluation ...................................................................................................... 15

3.4.1 Speckle size and coverage .................................................................................. 16

3.4.2 Displacement and strain evaluation .................................................................... 17

3.5 Test set-up .................................................................................................................. 24

3.5.1 Specimens ........................................................................................................... 25

3.5.2 ARAMIS ............................................................................................................ 26

3.5.3 Image processing ................................................................................................ 27

4 Result ................................................................................................................................ 29

4.1 Crack initiation .......................................................................................................... 29

v

4.1.1 Normal concrete – small specimens ................................................................... 30

4.1.2 Normal concrete – Large specimens .................................................................. 36

4.1.3 UHPC without fibers .......................................................................................... 39

4.1.4 UHPC with fibers ............................................................................................... 42

4.2 Cyclic loading ............................................................................................................ 43


4.2.2 Normal concrete – large specimens ................................................................... 54


4.2.4 UHPC with fibers ............................................................................................... 65

4.3 Maximum load ........................................................................................................... 68


4.3.2 Normal concrete – large specimens ................................................................... 72


4.3.4 UHPC with fibers ............................................................................................... 77

5 Analysis and discussion ................................................................................................... 80

5.1 Initial observations .................................................................................................... 80

5.2 Crack initiation .......................................................................................................... 81

5.3 Cycles ........................................................................................................................ 85

5.4 Maximum load ........................................................................................................... 90

5.5 General observations ................................................................................................. 92

6 Conclusions ...................................................................................................................... 94

6.1 Suggestions for further research ................................................................................ 94

7 References ........................................................................................................................ 95

Appendix A – Curvature measurements .................................................................................. 98

Appendix B – Test cubes ......................................................................................................... 99

Appendix C – Calibration and scale deviation ....................................................................... 100

Appendix D – Strain distribution at cracks ............................................................................ 101

D.1 TT4-8 ........................................................................................................................... 101

D.2 TT5-8 ........................................................................................................................... 102

D.3 TT3-0 ........................................................................................................................... 104

D.4 TT1-15 ......................................................................................................................... 106

D.5 TT6-8 ........................................................................................................................... 107

D.6 TT7-8 ........................................................................................................................... 109

D.7 TT8-8 ........................................................................................................................... 112

vi

List of Figures Figure 3.1 Sketch of tension tie ................................................................................................ 10

Figure 3.2 Procedure of gluing strain gauges to rebars ............................................................ 12

Figure 3.3 Cross-section of tension ties with measured distances marked .............................. 14

Figure 3.4 Mean displacement for pattern evaluation .............................................................. 18

Figure 3.5 Maximum displacement for pattern evaluation ...................................................... 18

Figure 3.6 Mean strain for pattern evaluation .......................................................................... 19

Figure 3.7 Maximum strain for pattern evaluation .................................................................. 20

Figure 3.8 Maximum strain for final three patterns ................................................................. 22

Figure 3.9 Mean strain for final three patterns ......................................................................... 22

Figure 3.10 Mean displacement for final three patterns ........................................................... 23

Figure 3.11 Maximum displacement for final three patterns ................................................... 23

Figure 3.12 Strain noise for pattern 3-80-25 ............................................................................ 24

Figure 3.13 Displacement noise for pattern 3-80-25 ................................................................ 24

Figure 3.14 Load sequence 100x100 specimens - Normal concrete ........................................ 26

Figure 3.15 Load sequence 150x150 specimens - Normal concrete ....................................... 26

Figure 3.16 Load sequence - UHPC ......................................................................................... 26

Figure 3.17 Placement of the cameras and lights and their distances ...................................... 27

Figure 3.18 Components in ARAMIS used for all specimens ................................................. 28

Figure 4.1 Displacement and strain at the initiation of the 1st crack in specimen TT2-15 ...... 31

Figure 4.2 Displacement and strain at the initiation of the 2nd crack in specimen TT2-15 ...... 31

Figure 4.3 Displacement and strain at the initiation of the 3rd crack in specimen TT2-15 ...... 32

Figure 4.4 Displacement and strain at the initiation of the 4th crack in specimen TT2-15 ...... 33

Figure 4.5 Displacement and strain at the initiation of cracks in specimen TT4-8 .................. 34



Figure 4.8 Displacement and strain at the initiation of cracks in specimen TT1-15 ................ 37






Figure 4.14 Strain distribution at all ten cycles for specimen TT2-15 ..................................... 44

Figure 4.15 Zoomed in image of the crack located at 140 mm in specimen TT2-15 .............. 45




Figure 4.19 Strain distribution at the 4th cycle in specimen TT4-8 .......................................... 49

Figure 4.20 Zoomed in image of the crack located at 465 mm in specimen TT4-8 ................ 50





Figure 4.25 Strain distribution at the 4th cycle in specimen TT1-15 ........................................ 55



vii



Figure 4.30 Strain distribution at the 3rd cycle at both 50 and 90 kN in specimen TT6-8 ....... 60







Figure 4.37 Strain distribution at the 3rd cycle at both 50 and 90 kN in specimen TT10-8 ..... 67

Figure 4.38 Strain distribution at the 3rd cycle at both 50 and 90 kN in specimen TT11-8 ..... 68

Figure 4.39 Displacement and strain at maximum load in specimen TT2-15 ......................... 69

Figure 4.40 Displacement and strain at maximum load in specimen TT4-8 ........................... 70












Figure 5.1 Correlation factor of crack 1 in TT2-15 throughout the loading sequence ............ 81

Figure 5.2 Correlation factor evolution of 1st crack in each specimen at the initiation of new

cracks ........................................................................................................................................ 82

Figure 5.3 Evolution of correlation factor between DIC and theoretical reinforcement strain at

1st crack in each specimen ....................................................................................................... 83

Figure 5.4 Correlation factor plotted against SG strain at crack initiation............................... 84

Figure 5.5 Correlation factor plotted against DIC strain at crack initiation ............................. 85

Figure 5.6 Correlation factor for all cracks and cycles in specimen TT2-15 ........................... 86

Figure 5.7 Correlation factor at 4th cycle for specimens in normal concrete and 3rd low and

high load cycle for UHPC specimens ...................................................................................... 88

Figure 5.8 Definition of amplitude, peak and valley ................................................................ 89

Figure 5.9 Correlation factor plotted against SG strain amplitude........................................... 89

Figure 5.10 Correlation factor plotted against DIC strain amplitude ....................................... 90

Figure 5.11 Correlation factor plotted against SG strain at maximum load ............................ 91

Figure 5.12 Correlation factor plotted against DIC strain at maximum load ........................... 91

Figure 5.13 Correlation factor plotted against maximum load ................................................ 92

viii

List of Tables Table 2.1 Sensors used for SHM ................................................................................................ 4

Table 3.1 Location of strain gauges ......................................................................................... 11

Table 3.2 Diameter of reinforcement bars after grinding ......................................................... 11

Table 3.3 The five different recipes for normal concrete ......................................................... 12

Table 3.4 Strength evaluation for different concrete recipes ................................................... 13

Table 3.5 Concrete recipes for batches 1-4 of normal concrete ............................................... 13

Table 3.6 UHPC recipes both with and without fibers ............................................................ 13

Table 3.7 Mean compressive strength after 28 days for all concrete batches .......................... 15

Table 3.8 Variables for speckle pattern evaluation .................................................................. 15

Table 3.9 Evaluation of speckle diameter and pattern coverage .............................................. 16

Table 3.10 Mean and maximum displacement for all stages for usable patterns ..................... 17

Table 3.11 Mean and maximum strain for all stages for usable patterns ................................. 18

Table 3.12 Comparison between strain and displacement for pattern evaluation .................... 20

Table 3.13 Displacement final three patterns ........................................................................... 21

Table 3.14 Strain final three patterns ....................................................................................... 21

Table 4.1 Cracking loads registered in the DAQ system for all specimens ............................. 29

Table 4.2 Peak strain from ARAMIS and interpolated SG strain at the initiation of the 1st

crack in TT2-15 ........................................................................................................................ 31

Table 4.3 Peak strain from ARAMIS and interpolated SG strain at the initiation of the 2nd

crack in TT2-15 ........................................................................................................................ 32

Table 4.4 Peak strain from ARAMIS and interpolated SG strain at the initiation of the 3rd

crack in TT2-15 ........................................................................................................................ 32

Table 4.5 Peak strain from ARAMIS and interpolated SG strain at the initiation of the 4th

crack in TT2-15 ........................................................................................................................ 33

Table 4.6 Peak strain from ARAMIS and interpolated SG strain at the initiation of cracks in

specimen TT4-8 ........................................................................................................................ 34


specimen TT5-8 ........................................................................................................................ 35


specimen TT3-0 ........................................................................................................................ 36


specimen TT1-15 ...................................................................................................................... 37

Table 4.10 Peak strain from ARAMIS and interpolated SG strain at the initiation of the only

crack in TT1-8 .......................................................................................................................... 38

Table 4.11 Peak strain from ARAMIS and interpolated SG strain at the initiation of the only

crack in TT2-8 .......................................................................................................................... 39


specimen TT6-8 ........................................................................................................................ 39


specimen TT7-8 ........................................................................................................................ 40


specimen TT8-8 ........................................................................................................................ 42

Table 4.15 Average peak and valley load for all ten cycles in specimen TT2-15 ................... 44

Table 4.16 Difference in SG and DIC strain for crack at 140 mm for specimen TT2-15 ....... 45


ix


Table 4.19 Difference in SG and DIC strain for crack at 640 mm for specimen 2-15 ............ 48

Table 4.20 Average peak and valley load at the 4th cycle in specimen TT4-8 ......................... 49

Table 4.21 Difference in SG and DIC strain for specimen TT4-8 ........................................... 50




Table 4.25 Difference in DIC strain for specimen TT3-0 ........................................................ 54

Table 4.26 Average peak and valley load at the 4th cycle in specimen TT1-15 ....................... 55

Table 4.27 Difference in SG and DIC strain for specimen TT1-15 ......................................... 56




Table 4.31 Average peak and valley load at the 3rd 50 and 90 kN load cycle in specimen TT6-

8 ................................................................................................................................................ 60

Table 4.32 Difference in SG and DIC strain for specimen 6-8 ................................................ 61


8 ................................................................................................................................................ 62



8 ................................................................................................................................................ 64



8 ................................................................................................................................................ 66

Table 4.38 Average peak and valley load at the 3rd 50 and 90 kN load cycle in specimen

TT10-8 ...................................................................................................................................... 67

Table 4.39 Average peak and valley load at the 3rd 50 and 90 kN load cycle in specimen

TT11-8 ...................................................................................................................................... 68

Table 4.40 Maximum load registered in the DAQ system for all specimens .......................... 68

Table 4.41 Peak strain from ARAMIS and interpolated SG strain at maximum load in TT2-15

.................................................................................................................................................. 69


.................................................................................................................................................. 70


.................................................................................................................................................. 71

Table 4.44 Peak strain from ARAMIS at maximum load in TT3-0 ........................................ 72


.................................................................................................................................................. 73


.................................................................................................................................................. 74


.................................................................................................................................................. 74


.................................................................................................................................................. 75


.................................................................................................................................................. 76

x


.................................................................................................................................................. 77

Table 5.1 Correlation factor evolution of 1st crack in each specimen at initiation of new

cracks ........................................................................................................................................ 82

Table 5.2 Evolution of correlation factor between DIC and theoretical reinforcement strain at

1st crack in each specimen ....................................................................................................... 83

Table 5.3 Correlation factor at 4th cycle in all cracks in normal concrete specimens .............. 86

Table 5.4 Correlation factor at 3rd 50 and 90 kN load cycle for all cracks in UHPC specimens

.................................................................................................................................................. 87

xi

Notations Roman letters

A – Cross sectional area of reinforcement

Ac – Cube area

b – Width of concrete test cubes

E – Modulus of elasticity

F – Force applied to tension tie

Fc – Force applied to test cubes

fcm – Mean compressive strength at 28 days

fcm(t) – Mean compressive strength at an age of t days

h – Height from bottom of formworks to top of rebar

l – Height of concrete test cubes

s – Coefficient dependent on the cement type

t – Age of concrete in days

wl – Distance from left edge of formworks to the middle of the rebar

wr - Distance from right edge of formworks to the middle of the rebar

Greek letters

βcc(t) – Coefficient dependent on the age of the concrete

βs – Correlation factor between DIC and SG strain

ε – Strain

σ - Stress

xii

Abbreviations DAQ – Data Acquisition

DIC – Digital Image Correlation

FOS – Fiber-optic sensors

RC – Reinforced Concrete

SG – Strain Gauge

SHM – Structural Health Monitoring

STA – Swedish Transport Administration

TT – Tension Tie

UHPC – Ultra High-Performance Concrete

1

1 Introduction

1.1 Background The Swedish Transport Administration (Trafikverket) (STA) manages approximately 20 600

bridges, out of which the majority are concrete bridges. Two important parts of STA’s

operations are planning upcoming maintenance and making sure all bridges fulfil their service

requirements as part of the Swedish transport system. To aid STA in their maintenance

planning, inspections and health assessments are carried out. Manual and visual inspections are

carried out at least every sixth year where the current state of the bridge is inspected. To further

determine the structural health of bridges, health assessments in the form of structural health

monitoring (SHM) can be used. (Trafikverket, 2016)

SHM is mainly used to monitor the behavior of a structure to determine its health, service load

performance and can also be used to locate and prevent damages before they occur (Bakht &

Mufti, 2015) (Choi, et al., 2008). The first step of SHM is mapping of the structure and

determining its needs. There are a number of different parameters that can be of interest to

measure on structures. According to (Hejll & Täljsten, 2005) these include deflection, velocity,

acceleration, strain, force, temperature and moisture.

For reinforced concrete (RC) structures it is especially important to monitor reinforcement

strain. If the reinforcement strain is too high it could indicate an overloading of the structure or

even that a failure is about to occur (Brault, et al., 2015).

Strain gauges (SG) is the most common way of measuring strain (National Instruments, 1998).

Studies by (Bagge, et al., 2014) and (Zhang, et al., 2011) mentions the use of SGs to measure

reinforcement strain on existing bridges during load tests. The only way to attach SGs to the

reinforcement of an existing bridge is by removing the concrete cover and attaching SGs to the

visible reinforcement. Another way of measure strain in existing bridges with the use of SGs is

by installing the SGs during construction of the bridge. However, there are a few problems with

that solution. The strains recorded by SGs are local strains over its gauge length (Bakht, et al.,

2011). The reinforcement strain is at its highest at the location of cracks and it is not feasible to

predict the exact location of cracks in a structure before they occur.

Digital image correlation (DIC) is a non-contact and non-destructive measurement technique

that uses digital images to visualize surface displacement and strain. According to (Hoult, et

al., 2016) it has potential to be a new alternative to more traditional sensors used to assess RC

structures. DIC is a technique that has been around since the 80’s (Reu, 2012) and has been

used in a number of recent studies.

In a study by (Brault, et al., 2015) a model for correlation of crack width to reinforcement stress

is presented. This study also gives examples of studies that have used DIC as a way of

measuring crack widths in reinforced concrete. (Fayyad & Lees, 2014) investigates crack

propagation using DIC. (Gencturk, et al., 2014) presents a study on the use of DIC in full-scale

testing of prestressed concrete structures. (Küntz, et al., 2006) are among the first to use DIC to

assess a bridge under operating conditions. A following study by (McCormick & Lord, 2010)

presents a number of in-situ applications for DIC for large structures, including bridges.

2

1.2 Aims and objectives This thesis is part of a project which focuses on the possibilities of evaluating reinforcement

strain on existing bridges using non-contact measuring methods. This study evaluates the use

of DIC for this purpose. Thus, the research question that acts as a base for this thesis is:

“How well can reinforcement strains be predicted based on strains measured on the concrete

surface using digital image correlation?”

To answer this question a number of steps are taken, these are:

- Literature review

- Experimental tests

- Numerical analysis

1.3 Methodology This section describes the methods used to answer the research question in this master thesis.

1.3.1 Literature review

Initially a literature review was carried out. This, in order to gain knowledge and identify

previous research within this topic. In the literature review both books, research papers and

scientific articles were studied. The literature review in this study focuses on the concept and

use of digital image correlation.

1.3.2 Laboratory procedure

Since the aim of the study was to investigate a way of correlating DIC measured surface strains

with reinforcement strains, the literature review was combined with laboratory tests. These tests

focused on the use of DIC on concrete specimens. In this study a non-contact optical 3D

deformation measuring system named ARAMIS is used.

For these tests to be carried out different procedures were necessary. This section only presents

a short summary of the different procedures, for a full description see chapter 3 Laboratory and

testing procedure.

For this study 15 tension ties were created. Two of the tension ties were created as dummies

and used to test load sequences and methods of applying a suitable speckle pattern. These two

specimens are therefore not included in this study.

Initially SGs were installed onto reinforcement bars. Thereafter the formworks for the casting

was created and different concrete recipes were tested out. The tension ties were thereafter

casted and left to cure for more than 28 days. Different speckle patterns were tested to find the

optimal pattern for these tests. After curing of the tension ties the speckle pattern was sprayed

onto the concrete surface. The specimens were thereafter tested in pure tension while the

ARAMIS system continuously captured images. These images were then processed using the

ARAMIS software.

Strains from SGs are acquired using a data acquisition (DAQ) system.

1.4 Limitations The focus of this study is the evaluation of the correlation between surface and reinforcement

strain using DIC. There are possibilities of using DIC as a way of examining crack patterns and

3

evaluating the tension stiffening effect. However, these uses of DIC falls outside the scope of

this study and are therefore not included.

The tests are limited to three types of concrete and reinforcement bars with a diameter of 16

mm. Specimens are only tested in pure tension and can thus only provide some insight to the

behavior in tension.

1.5 Disposition This thesis is divided into six chapters. A brief summary of the content in these are presented

here.

1 – Introduction

This chapter presents the background and importance of this problem. In addition, aims and

limitations of this study are stated.

2 – Literature review

In this chapter the literature review is presented. This chapter mainly focuses on the importance

and concepts of DIC.

3 – Laboratory procedure

The different parts of the laboratory procedure are extensively described in this chapter.

4 – Results

The results from the laboratory tests are presented in this section. Different parts presents strain

distribution at crack initiation, cyclic loading and maximum load.

5 – Analysis and discussion

This chapter presents an analysis and discussion of the results presented for crack initiation,

cycles and maximum load. A section on general observations is also presented.

6 – Conclusions

In this chapter the research question is answered and some suggestions of further research

needed in this area is mentioned.

4

2 Literature review In order to understand the upcoming tests, the importance and concept of digital image

correlation must be explained.

2.1 Structural health monitoring of bridges To comprehend the importance of DIC a brief introduction to SHM is necessary.

Due to increasing traffic volumes and loads it is important to have an understanding of

structures’ behaviors, from construction to demolition. Another reason is the use of road salt.

The salt creates a hostile environment for RC structures and causes corrosion of the

reinforcement. (Hejll & Täljsten, 2005)

In Sweden visual inspections are carried out every sixth year (Trafikverket, 2016). The

advantage of these inspections are their simplicity and low cost. This technique is well-known

and well-established. There are however some limitations to these inspections. The inspections

only give a rough estimate of the structure’s health and only provide information about the

surface of the structure (U.S Department of Transportation, 2014). Since these visual

inspections are carried out by humans there is the possibility that different inspectors assess

damages differently based on their own experience.

SHM is a structured way of monitoring the health of existing bridges by in-situ measurements.

It also provides a way of evaluating new more complicated structures. (Hejll & Täljsten, 2005)

and (ISIS Canada & SAMCO Network of the European Commission, 2005) presents a number

of steps taken in the development of a SHM system. Initially a diagnosis of the bridge is made.

The diagnosis can either be made in a general way or more advanced. The purpose of the

diagnosis is to provide information on the aim of the measurements and what needs to be

measured. Once necessary measurement parameters are decided there are numerous types of

sensors to choose from in the design of the monitoring system. (Hejll & Täljsten, 2005) presents

a table of different sensors categorized by measurement parameter, see Table 2.1.

In addition to sensors, data acquisition, management and data interpretation systems are chosen.

A decision is also made on the type of monitoring that will be conducted. These decisions

include whether the monitoring should be static or dynamic, continuous or periodic and have

controlled or ambient loading. Finally, the system is installed and the data is assessed (ISIS

Canada & SAMCO Network of the European Commission, 2005).

Table 2.1 Sensors used for SHM

Parameter Types of sensors

Displacement LVDT (Linear Variable Differential Transformer)

Interferometry

Accelerometers and time-based numerical integration (transient signals)

Optical laser triangulation

GPS

Velocity Accelerometers and time-based numerical integration (transient signals)

Geophones

Acceleration Piezoelectric accelerometers

Force-balance accelerometers

Capacitive accelerometers

Strain Traditional electric strain gauges

5

Bragg gratings

Interferometry

Force Traditional electric strain gauges

Piezoelectric sensors

Temperature Thermometers

Thermocouple

Thermistor

Humidity MEMS-sensors Note. Adapted from Civil Structural Health Monitoring (CSHM) by Arvid Hejll and Björn Täljsten

2.2 Strain monitoring in RC bridges In RC structures reinforcement strain is one of the most important parameters to monitor.

During design of RC structures, the yield stress of the reinforcement is often used to calculate

the structures’ resistance to load. The yield stress is the stress at which the reinforcement

changes from elastic to plastic behavior. (Isaksson, et al., 2010)

For lower steel grades the stress-strain curve is linear up to the point where the material yields.

This is also valid for reinforcement steel, however since reinforcement steel often is of higher

grade it can behave a bit differently. The change between elastic and plastic behavior might

occur more gradually in higher steel grades. (Moseley, et al., 2012)

Due to cyclic loading structures may experience fatigue. Fatigue is the formation of microscopic

cracks that eventually will lead to a fracture in the structure. According to a study by (Kopas,

et al., 2016) no RC highway bridges in service have noticeable fatigue fractures. But (C.E.B,

1989) states that fatigue cracks are difficult to identify in concrete due to the lack of identifiable

surface topography. In addition, (Kopas, et al., 2016) mentions that the reinforcement can fail

due to fatigue without any visual signs other than local cracking of the concrete. Concrete

structures such as bridges are design to withstand fatigue as the elastic stress in serviceability

limit state should not exceed 80 % of the characteristic stress. There are however some factors

that can generate a shorter fatigue life than designed. These factors include corrosion, bar type

and form of manufacture (Kopas, et al., 2016).

Due to the linearity of the stress-strain curve and a known modulus of elasticity, the

reinforcement stress is easily obtained from the reinforcement strain. (Isaksson, et al., 2010)

(National Instruments, 1998) states that the most common way of measuring strain is by using

strain gauges. To measure reinforcement strain SGs are attached to the reinforcement. During

loading of the structure, the reinforcement experiences strain. As this happens the strain in the

reinforcement is transmitted to the grid arranged metallic foil inside the SG. This change of

strain in the metallic foil corresponds to a change in electrical resistance. The change in

resistance is thereafter transmitted to a readout system.

According to (Bakht, et al., 2011) SGs averages the strain underneath its gauge length (grid

pattern). Dependent on what strain is of interest different gauge length can be used. A smaller

gauge length provides more accurate strain readings at a specific location whereas a longer

gauge length is useful when the aim is to measure average strain over a specific length (Bakht,

et al., 2011).

(Bakht, et al., 2011) states that due to their simple installation good accuracy they have been

used in laboratory tests for decades but are also useful in SHM of bridges. Studies by (Bagge,

6

et al., 2014) and (Zhang, et al., 2011) mentions the use of SGs to measure reinforcement strain

on existing bridges during load tests.

To measure reinforcement strain in existing RC structures the concrete cover is removed and

SGs are attached to the visible reinforcement. Another way to measure reinforcement strain in

existing bridges is to install SGs before the bridge’s construction. However, this solution has

some disadvantages. According to (Brault, et al., 2015) it is not feasible to predict the exact

location of cracks in a structure before they occur. Another problem is the fact that SGs only

provide local strain under its gauge length (Bakht, et al., 2011). Another solution presented by

(Masukawa, 2012) and (Scott & Beeby, 2005) is installing numerous SGs along the rebar.

(Brault, et al., 2015) also reject this idea stating that the bond between the reinforcement and

concrete might be affected due to the amount of SGs and cables needed.

Both (Hejll & Täljsten, 2005) and (Brault, et al., 2015) mentions the use of fiber optic sensors

(FOS) to measure strain in civil structures. FOS can according to (Brault, et al., 2015) be used

to measure strain along the entire length of the fiber optic cable as well as measure strain over

lengths comparable to the gauge length of traditional SGs.

Another way to evaluate strain in structures is DIC. (Hoult, et al., 2016) mentions that DIC

has the potential to be a new alternative to more traditional sensors used to assess RC

structures

2.3 Digital Image Correlation DIC is a full-field technique that visualizes the deformation and strain of an object. This is done

by comparing images taken before and after deformation with the use of computer software, as

stated by (McCormick & Lord, 2010) and (Gencturk, et al., 2014).

This non-contact and non-destructive measuring method has been around since the 1980’s when

(Yamaguchi, 1981) and (Peters & Ranson, 1982) were among the first to introduce it. As digital

cameras were developed they were used in experiments using digital speckle pattern

interferometry. Over time the method evolved and eventually three main branches of digital

image correlation have been developed. (Reu, 2012)

- 2D-DIC for in plane measurements

- 3D-DIC for x, y and z data

- V-DIC for measurements within volumes

For 3D measurements a stereo-rig consisting of a minimum of two cameras is necessary. These

cameras must also be calibrated and working as a unit. Calibration is the method used by the

DIC-software to scale and orient images to the real world. In the early days of DIC calibration

was a tedious task. It was highly dependent on camera translation and measurements. Today a

calibration object can be used. (Reu, 2012)

For the ARAMIS system there are two types of calibration objects available, a cross and a panel.

The calibration of the system is carried out by taking a series of images of the calibration object

in different positions. (GOM, 2013)

(Reu, 2012) describes the general function of a DIC software as breaking down images into

independent subsets. The subsets, or facets, are square sections defined in the first stage or

reference image and thereafter identified in every following image using grey level structures

in the images according to (GOM, 2009). During deformation of a specimen these subsets will

7

change shape. A mathematical shape function is thereafter used to conform the change in shape

and motion. (Reu, 2012) also states that any material discontinuities can cause errors in the

linear shape function mostly used.

The ARAMIS software correlates facets by defining 2D coordinates from the subset’s corners

and midpoint in both the left and right image. These 2D coordinates are thereafter used to create

a 3D coordinate using photogrammetric methods (GOM, 2009).

For the system to work as intended it is important with a good surface structure. The surface

must be smooth and have a speckle pattern. The pattern should follow the specimens’

deformation. It should also have a good contrast and have a dull finish as any reflections can

prevent facet computation. (GOM, 2009)

(GOM, 2009) also states that the best patterns are those adapted to the test set-up itself. That is

measuring volume, camera resolution and facet size. According to (Reu, 2012) the four most

important attributes of speckle patterns are: Speckle size, contrast, speckle edge and speckle

density. Furthermore (Reu, 2012) states that speckles should not be smaller than 3 pixels.

Similarly, a study carried out at the Tsinghua University uses a speckle size of 3 pixels in their

speckle patterns (Chen, et al., 2007).

One of the aims for images used in DIC is increased contrast and decreased noise. This can be

done by optimizing the lighting and using non-reflective paint. For subset computation each

subset should contain 2-3 speckles which give a good contrast inside the subset itself. (Reu,

2012)

Speckle density can be described as the ratio of black pixels in the pattern. According to

(Mazzoleni, 2013) the coverage should be between 40-70 % whereas (Reu, 2012) suggests

around 50 %.

It is preferred when speckles have a soft edge instead of hard edges. However, speckle edge is

the least important speckle pattern parameter of four previously mentioned. If necessary speckle

edge can be disregarded in order to optimize the other three parameters. (Reu, 2012)

2.3.1 DIC applications – State of the art

There are many researches whom have been using DIC software in laboratory conditions to

evaluate material properties.

Among these are (Brault, et al., 2015) whom presented a model for correlation between crack

width and reinforcement strain using DIC and FOS. Furthermore, a study by (Fayyad & Lees,

2014) used DIC to investigate crack propagation in reinforced concrete. (Mahal, et al., 2015)

used DIC to evaluate the fatigue behavior of fiber reinforced concrete beams. (Gencturk, et al.,

2014) presents a study where DIC was used in full-scale testing of prestressed concrete

structures.

These studies show that DIC is a feasible way to carry out measurements and monitoring crack

profiles in small-scale RC beams. However, even in laboratory tests some limitations are

presented. (Gencturk, et al., 2014) states that the light sensitivity of the measurements and the

random speckle pattern were two major limitations. Additional limitations presented by

(Gencturk, et al., 2014) include data loss in areas where the concrete experiences spalling or

formation of large cracks.

8

However, the step between using DIC in laboratory and in-situ tests is large and the research in

the latter area is thus far limited.

(Küntz, et al., 2006) were among the first to use DIC to assess a bridge under operating

conditions. A following study by (McCormick & Lord, 2010) presents a number of in-situ

applications for DIC for large structures, including bridges.

According to (Küntz, et al., 2006) the surface of the specimen required little preparation and

little equipment was required for capturing images. In addition, it is stated that the uncertainty

of the final results is heavily dependent on the care taken during image acquisition. (McCormick

& Lord, 2010) presents different techniques for compensating for in-situ effects. It is stated that

weathering of concrete provides enough texture and randomness to the structure’s surface to

enable full-field measurements. Furthermore, it is stated that the National Physical Laboratory

has designed a DIC code which takes lighting differences between images into consideration.

In addition, they state that one major limitation of in-situ measurements is the need to reposition

cameras.

2.4 Cracks in concrete Concrete is brittle and has a very low tensile strength, due to small cracks in the material. The

tensile strength normally varies between 8-15 % of the compressive strength. (McCormack &

Nelson, 2005).

Dependent the concrete’s loading different types of cracks can form. These crack-types include:

Tensile cracks

These cracks occur when its tensile strength is exceeded in axially loaded concrete. According

to (Leonhardt, 1988) the cracking strain of concrete is another way to determine when concrete

cracks. The cracking strain of concrete is 0,010-0,012 % which is equivalent to 100-120 μm/m

and is independent from the concrete’s tensile strength. When a bar is axially loaded with a

tension force these cracks form through the entire cross-section of the bar (McCormack &

Nelson, 2005). According to (Moseley, et al., 2012) the reinforcement bar will take all tension

at the point where cracks occur.

Flexural cracks

Flexural cracks are formed due to flexural loading of a beam and occur in the tensile zone in a

RC beam. These cracks extend vertically from the tension side to the neutral axis of the beam.

The cracks start to form when the tensile strength of the concrete is exceeded. (McCormack &

Nelson, 2005)

Flexure-shear cracks

Flexure-shear cracks are the most common cracks that occur due to shear forces. These cracks

are located in the web of RC beams and can be seen as an extension of existing flexural cracks.

They occur when the shear capacity of the concrete is exceeded. (McCormack & Nelson, 2005)

Web-shear cracks

In conformity with the flexural-shear cracks the web-shear cracks forms when the shear

capacity of the concrete is exceeded. The web-shear cracks are independent and not in

proximity of flexural cracks. (McCormack & Nelson, 2005)

Shrinkage cracks

9

Shrinkage cracks can occur during the curing of the concrete. Fresh concrete contains a large

amount of water, out of which a part evaporates and disappears during the curing of the

concrete. If the concrete is unable to move cracks will most likely appear. The shrinkage causing

these cracks is highly dependent on the composition of the concrete and the cross-section.

(Isaksson, et al., 2010)

10

3 Laboratory and testing procedure This chapter presents the different stages of the laboratory and testing procedure.

For these tests 15 tension ties (TT) were created. A tension tie is a concrete prism with one

reinforcement bar.

3.1 Installation of strain gauges Initially the rebars of the type B500B were cut to a length of 1500 mm. When finished the

tension ties have an 800 mm long prism of concrete in the middle of the bar with 350 mm of

reinforcement on each side, see Figure 3.1. A distance of 350 mm was marked on each side of

the bar.

Figure 3.1 Sketch of tension tie

11

The bars were thereafter marked at the location of the SGs. In the rebars with eight SGs the first

one is located at approximately 50 mm from the edge of the concrete and the distance between

the gauges is approximately 100 mm. In the rebars with 15 SGs the distance between them is

approximately 50 mm. See Table 3.1 for actual location of gauges for all bars. For bars 1-8, 3-

8, 4-8, 5-8, 7-8, 9-8 and 10-8 HBM strain gauges with a gauge length of 6 mm and a gauge

factor of 2,05±1% were used. For bars 2-8, 6-8, 8-8, 11-8 and 1-15 Kyowa strain gauges with

a gauge length of 5 mm and a gauge factor of 2,10±1% were used. For bar 2-15 the Kyowa

gauges had a gauge length of 5 mm and a gauge factor of 2,08±1%.

Table 3.1 Location of strain gauges

SG/TT 1-8 2-8 4-8 5-8 6-8 7-8 8-8 9-8 10-8 11-8 1-15 2-15

1 50 49 43 53 51 48 47 55 48 50 51 46

2 153 149 145 154 151 148 149 155 150 149 100 97

3 253 248 247 253 251 248 249 255 249 249 151 146

4 353 349 348 353 351 349 350 356 350 350 200 195

5 454 449 449 452 450 450 448 456 450 448 253 246

6 551 549 549 552 550 550 550 559 552 549 301 299

7 656 648 648 651 652 650 649 658 653 649 352 349

8 756 746 747 752 752 750 747 758 752 749 402 397

9 452 447

10 502 499

11 550 548

12 605 598

13 655 647

14 704 698

15 754 748

The installation of the SGs was done in accordance with (FSEL, 2016). A belt grinder was used

to grind the area where the SG will be located. To further smoothen the surface sandpaper was

used. Once grinded the new diameter of the bar was measured. These measurements can be

seen in Table 3.2.

Table 3.2 Diameter of reinforcement bars after grinding

TT/

SG 1-8 2-8 4-8 5-8 6-8 7-8 8-8 9-8 10-8 11-8 1-15 2-15

1 15,38 15,81 15,54 15,37 16,05 15,57 15,71 15,34 15,26 15,44 15,84 15,81

2 15,58 16,04 15,60 15,37 16,09 15,60 15,76 15,39 15,69 15,60 15,81 15,66

3 15,75 16,10 15,57 15,36 16,00 15,54 15,66 15,39 15,76 15,39 15,75 15,59

4 15,64 16,04 15,71 15,44 16,05 15,45 15,72 15,38 15,77 15,45 15,78 15,80

5 15,52 15,95 15,66 15,54 16,03 15,28 15,60 15,41 15,02 15,50 15,68 15,72

6 15,57 15,85 15,57 15,35 15,92 15,28 15,65 15,38 15,70 15,68 15,75 15,71

7 15,59 15,79 15,59 15,31 16,00 15,42 15,54 15,36 15,59 15,70 15,75 15,62

8 15,45 15,85 15,45 15,34 15,95 15,43 15,64 15,39 15,60 15,59 15,82 15,47

9 15,80 15,72

10 15,72 15,67

11 15,78 15,64

12 15,80 15,70

13 15,72 15,61

14 15,64 15,72

15 15,49 15,70

The grinded surface was thereafter cleaned with acetone and wiped dry with a cloth.

Strain gauges were removed from the protective plastic and normal tape was applied to the top

of the strain gauge. The tape was used to apply the gauges straight along the rebar. The tape

12

was lifted on one side and the adhesive (Rapid Adhesive Z70 by HBM) was applied to the strain

gauge and connecting cables, see Figure 3.2.

Teflon paper was used to press down on the tape. Pressure was kept for 60-120 seconds and

then the adhesive had to cure for a few minutes in normal humidity.

The tape was removed and a protective coating (Protective Coating SG 250 by HBM) was

applied. A touch-dry skin forms after approximately 2 hours. A 0,5 mm thick layer is cured

after 24 hours.

Figure 3.2 Procedure of gluing strain gauges to rebars

3.2 Casting In this study three different kinds of concrete were mixed: normal concrete, UHPC and UHPC

with fiber-reinforcement. The different types of concrete were used in the following specimens:

Normal concrete

TT2-15, TT4-8, TT5-8, TT3-0, TT1-15, TT1-8 and TT2-8

UHPC

TT6-8, TT7-8 and TT8-8

UHPC with fibers

TT9-8, TT10-8 and TT11-8

In order to obtain an optimal concrete recipe for this study a number of test cubes were casted

using five different concrete recipes. The five recipes for the normal concrete are presented in

Table 3.3.

Table 3.3 The five different recipes for normal concrete

I II III IV V

Cement [kg] 8,4 7,6 7,6 7,6 5,4

Water [kg] 3,2 2,9 2,7 2,1 3,56

Aggregate 0-4 mm [kg] 17,2 17,2 25,8 25,8 17,4

Aggregate 4-8 mm [kg] 17,2 17,2 8,6 8,6 13,4

Filler [kg] 0,8 0,8 0,8 0,8

Superplasticizer [kg] 0,152 0,152 0,1 0,05

13

The cubes from recipe I were tested after 7 days. Recipe II was unusable. The cubes from recipe

III-V were tested after 3 days. The average cube strength for the different cubes are presented

in Table 3.4.

Based on the average strength after 3 and 7 days the 28-day strength was calculated. This in

accordance with equations 3.1 and 3.2 from SS-EN 1992-1-1 (Swedish Standards Institute,

2005), see eq. 3.1 and 3.2 below. In equation 3.1 fcm(t) is the mean compressive strength at an

age of t days and fcm is the mean compressive strength at 28 days. The coefficient βcc(t) is then

given by eq. 3.2 where t is the concrete’s age in days and s is a coefficient depending on the

type of cement used. The concrete used in this study is of class N which gives s=0,25. Results

from these calculations can be found in Table 3.4

𝑓𝑐𝑚(𝑡) = 𝛽𝑐𝑐(𝑡)𝑓𝑐𝑚 ↔ 𝑓𝑐𝑚 =𝑓𝑐𝑚(𝑡)

𝛽𝑐𝑐(𝑡) (3.1)

𝛽𝑐𝑐(𝑡) = 𝑒𝑥𝑝 {𝑠 [1 − (28

𝑡)

1/2

]} (3.2)

Table 3.4 Strength evaluation for different concrete recipes

Recipe no. I II III IV V

Average cube strength, 3 days [MPa] - - 26,23 26,00 9,080

Average cube strength, 7 days [MPa] 67,25 - - - -

Average cube strength, 28 days [MPa] 86,35 - 43,85 43,46 15,18

From these results and the behavior of the concrete, recipe IV was chosen.

Four different batches of this recipe were casted. Batch 1 was 45 liters, batch 2 and 3 were 30

liters each and batch 4 was 40 liters. These recipes can be found in Table 3.5 below.

Table 3.5 Concrete recipes for batches 1-4 of normal concrete

30 liters 40 liters 45 liters

Cement [kg] 11,40 15,20 17,10

Water [kg] 5,817 7,756 8,725

Aggregate 0-4 mm [kg] 38,70 51,60 58,05

Aggregate 4-8 mm [kg] 12,90 17,20 19,35

Filler [kg] 1,200 1,600 1,800

Superplasticizer [kg] 0,075 0,100 0,1125

The concrete recipes for the UHPC was obtained from another study carried out at Luleå

University of Technology and are presented in Table 3.6. ‘Sand Type I’ and ‘Sand Type II’ are

two different sand types where type I is finer. In Table 3.6 the column labeled ‘32 liters’ is the

recipe for the UHPC without fibers, batch 5, and the column labeled ’37 liters’ is the recipe for

the UHPC with fibers, batch 6.

Table 3.6 UHPC recipes both with and without fibers

32 liters 37 liters

Cement [kg] 32 37

Silica fume [kg] 6,4 7,4

Quartz [kg] 9,6 11,1

Water [kg] 7,36 8,51

14

Superplasticizer [kg] 0,48 0,555

Sand Type I [kg] 11,2 12,95

Sand Type II [kg] 11,2 12,95

Fibres [kg] 0,64 0,74

After deciding on the concrete recipe, the formworks were created. In total five molds were

built, four with a volume of 800x100x100 mm3 and one 800x150x150 mm3. On the two edges

an 18 mm hole was drilled.

After spraying the formwork with oil, the rebars were attached to the formworks. To enable

mounting of the rebars one side of the mold was removed. Once the rebar was in place the final

side of the formworks was screwed back on and the molds were measured to obtain the

curvature of the rebar. The measurements taken were the distance to the first strain gauge from

the formworks, distance from the middle of the rebar to the edges of the formworks and the

distance from the bottom of the formworks to the top of the rebar, see Figure 3.3. All

measurements can be found in Appendix A. In addition to the formworks for the tension ties,

molds for test cubes were prepared.

Figure 3.3 Cross-section of tension ties with measured distances marked

All ingredients for the concrete were weighed and measured. The mixing procedure varied

slightly depending on the type of concrete casted.

Normal concrete

Initially the dry ingredients were mixed using a concrete mixer. After mixing for a few minutes

water and superplasticizer was added and then the concrete was mixed for another couple of

minutes.

UHPC

Initially the two sand types and the silica fume were mixed for 5 minutes using a concrete mixer.

Thereafter the cement and quartz were added and mixed for 5 minutes. Lastly the water and

superplasticizer was poured into the mixer for 1 minute and then everything was mixed for an

additional 12 minutes.

UHPC with fibers

15

The procedure for UHPC with fibers is similar to the procedure for UHPC with the difference

that the fibers were added. After the 12 minutes of mixing that ended the procedure for the

UHPC the fibers were added and then everything was mixed for another 5 minutes.

In total 6 batches of concrete were mixed. Batch 1-4 were normal concrete, batch 5 UHPC

without fibers and batch 6 UHPC with fibers.

After 28 days of curing a compressive test was carried out on all test cubes. All results from

these tests can be found in Appendix B. The mean compressive strength for all concrete batches

can be found in Table 3.7 below.

Table 3.7 Mean compressive strength after 28 days for all concrete batches

Batch 1 2 3 4 5 6

fctm [MPa] 46,70 50,63 50,88 56,35 115,2 146,3

3.3 Surface preparation Before testing the specimens, the surface must be prepared. Holes in the surface can during the

testing be recognized as speckles and may have an impact on the result. To prevent this, any

holes in the surface was filled using wall putty. The surface was then painted white using a

white contrast spray paint and white paint. Once the white spray paint had dried a speckle

pattern was painted on the surface using black spray paint and a template with the chosen

pattern.

3.4 Pattern evaluation To find the most suitable speckle pattern for this study and determine its noise, different patterns

were evaluated. The patterns were created using the software Speckle Generator and evaluated

with the ARAMIS system.

The software Speckle Generator uses three different variables when generating speckle patterns.

‘Diameter’ changes the diameter of the circles of which the speckle pattern is built. ‘Density’

changes the number of speckles and the distance between them. ‘Variation’ changes the

perturbation of the speckle grid.

In this evaluation three different values were used for each of the three variables. See Table 3.8.

Table 3.8 Variables for speckle pattern evaluation

Diameter [mm] Density [%] Variation [%]

1 50 25

2 65 50

3 80 75

All patterns were printed on paper and displayed. The area of the patterns was the same as the

final concrete surface, 100x800 mm. The ARAMIS system was set up and 10 stages were

captured for each pattern display.

The properties evaluated in this study are speckle size, coverage, displacement and strain.

16

3.4.1 Speckle size and coverage

According to (Reu, 2012) each speckle should have a diameter of at least 3 px. The ARAMIS

system gave the following values for image resolution and measuring volume at the current set

up.

Image resolution: 2448x2050 px

Measuring volume: 1005x880 mm

By dividing the image resolution by the measuring volume, the following values were given

for the three different speckle diameters.

Horizontally Vertically Average

1 mm = 2,44 px 1 mm = 2,33 px 1 mm = 2,38 px

2 mm = 4,87 px 2 mm = 4,66 px 2 mm = 4,77 px

3 mm = 7,31 px 3 mm = 6,99 px 3 mm = 7,14 px

The coverage is the ratio of black pixels in the entire pattern and should be between 40-70%

according to (Mazzoleni, 2013) but ideally around 50% (Reu, 2012). To determine the coverage

of the patterns the website by PHP Tools (http://www.coolphptools.com/color_extract#demo)

was used. When using the image color extract, the options on the website was chosen as follows:

Number of colors – 2, delta – 255, Reduce brightness – No, Reduce gradient – No.

In Table 3.9 the speckle diameter and coverage is presented for all 27 patterns. Speckle sizes

below the allowed value and values outside the allowed range of coverage are marked red.

Table 3.9 Evaluation of speckle diameter and pattern coverage

Pattern Speckle diameter [px] Coverage [%] Usable

1-50-25 2,38 25,2 No

1-50-50 2,38 26,8 No

1-50-75 2,38 27,3 No

1-65-25 2,38 53,9 No

1-65-50 2,38 52,5 No

1-65-75 2,38 50,6 No

1-80-25 2,38 97,0 No

1-80-50 2,38 86,6 No

1-80-75 2,38 78,5 No

2-50-25 4,77 23,9 No

2-50-50 4,77 24,1 No

2-50-75 4,77 24,0 No

2-65-25 4,77 40,0 Yes

2-65-50 4,77 41,0 Yes

2-65-75 4,77 40,0 Yes

2-80-25 4,77 67,4 Yes

2-80-50 4,77 63,9 Yes

2-80-75 4,77 60,4 Yes

3-50-25 7,14 22,2 No

3-50-50 7,14 22,3 No

3-50-75 7,14 22,1 No

3-65-25 7,14 37,4 No

3-65-50 7,14 37,4 No

3-65-75 7,14 36,8 No

3-80-25 7,14 59,0 Yes

3-80-50 7,14 57,7 Yes

http://www.coolphptools.com/color_extract#demo

17

3-80-75 7,14 54,4 Yes

3.4.2 Displacement and strain evaluation

The nine usable patterns were then evaluated for displacement and strain.

According to (Mazzoleni, 2013) the average value of every displacement matrix can be

computed for every pattern to determine noise.

In this test a surface component was created on each pattern. The surface components used had

a facet size of 40 px and a point distance of 20 px. This facet size and point distance gave a

good middle ground between accuracy and computation time (GOM, 2009). The tables and

graphs below show the mean and maximum displacement and strain for these surface

components over the different stages. An average value of these are then calculated, see Table

3.10-Table 3.11 and Figure 3.4-Figure 3.7.

Table 3.10 Mean and maximum displacement for all stages for usable patterns

Stage

Pattern 1 2 3 4 5 6 7 8 9 10 Average

2-65-25

Mean [mm] 0,024 0,029 0,013 0,016 0,02 0,022 0,01 0,012 0,021 0,017 0,0184

Maximum [mm] 0,05 0,064 0,04 0,049 0,046 0,05 0,033 0,04 0,044 0,039 0,0455

2-65-50

Mean [mm] 0,031 0,01 0,014 0,012 0,013 0,026 0,017 0,012 0,013 0,012 0,016

Maximum [mm] 0,06 0,029 0,031 0,032 0,045 0,063 0,049 0,041 0,045 0,034 0,0429

2-65-75

Mean [mm] 0,016 0,016 0,018 0,019 0,026 0,03 0,013 0,013 0,016 0,019 0,0186

Maximum [mm] 0,043 0,037 0,042 0,048 0,06 0,057 0,029 0,036 0,047 0,052 0,0451

2-80-25

Mean [mm] 0,016 0,021 0,019 0,012 0,013 0,023 0,039 0,016 0,03 0,012 0,0201

Maximum [mm] 0,038 0,046 0,04 0,029 0,039 0,045 0,077 0,036 0,053 0,036 0,0439

2-80-50

Mean [mm] 0,021 0,027 0,022 0,026 0,066 0,042 0,015 0,013 0,021 0,027 0,028

Maximum [mm] 0,056 0,077 0,066 0,057 0,12 0,08 0,062 0,036 0,054 0,07 0,0678

2-80-75

Mean [mm] 0,012 0,012 0,019 0,009 0,009 0,009 0,016 0,023 0,011 0,009 0,0129

Maximum [mm] 0,037 0,035 0,061 0,034 0,025 0,031 0,046 0,054 0,037 0,031 0,0391

3-80-25

Mean [mm] 0,014 0,013 0,01 0,016 0,019 0,02 0,016 0,014 0,013 0,018 0,0153

Maximum [mm] 0,043 0,042 0,057 0,057 0,051 0,056 0,046 0,047 0,064 0,051 0,0514

3-80-50

Mean [mm] 0,212 0,206 0,222 0,227 0,204 0,212 0,203 0,203 0,217 0,21 0,2116

Maximum [mm] 1,036 1,021 1,053 1,028 0,959 1,016 1,049 1,027 1,027 1,012 1,0228

3-80-75

Mean [mm] 0,017 0,012 0,012 0,011 0,024 0,014 0,018 0,014 0,016 0,014 0,0152

Maximum [mm] 0,047 0,036 0,044 0,035 0,063 0,036 0,046 0,04 0,04 0,047 0,0434

18

Figure 3.4 Mean displacement for pattern evaluation

Figure 3.5 Maximum displacement for pattern evaluation

Table 3.11 Mean and maximum strain for all stages for usable patterns

Stage

Pattern 1 2 3 4 5 6 7 8 9 10 Average

2-65-25

Mean [μm/m] -14 14 14 -56 -14 -17 -37 -28 -36 -25 -19,9

Maximum [μm/m] 377 390 320 280 339 433 415 619 308 334 381,5

2-65-50

Mean [μm/m] -18 -20 -28 -20 -6 63 -20 3 -18 -27 -9,1

Maximum [μm/m] 389 457 459 391 479 518 229 725 383 455 448,5

2-65-75

Mean [μm/m] -2 -24 19 -17 -15 4 -3 1 -16 8 -4,5

Maximum [μm/m] 542 512 497 536 443 360 875 702 320 735 552,2

2-80-25

Mean [μm/m] -34 16 -30 1 10 13 -9 16 24 -19 -1,2

Maximum [μm/m] 471 596 274 526 537 489 425 551 536 538 494,3

0

0,05

0,1

0,15

0,2

0,25

1 2 3 4 5 6 7 8 9 10

Mea

n d

isp

lace

men

t [m

m]

Stage

Mean displacement

2-65-25

2-65-50

2-65-75

2-80-25

2-80-50

2-80-75

3-80-25

3-80-50

3-80-75

0

0,2

0,4

0,6

0,8

1

1,2

1 2 3 4 5 6 7 8 9 10

Max

imu

m s

trai

n [μ

m/m

]

Stage

Maximum displacement

2-65-25

2-65-50

2-65-75

2-80-25

2-80-50

2-80-75

3-80-25

3-80-50

3-80-75

19

2-80-50

Mean [μm/m] -30 -26 -88 4 7 27 -5 -10 -21 -3 -14,5

Maximum [μm/m] 335 314 352 471 586 457 633 424 418 366 435,6

2-80-75

Mean [μm/m] 2 7 59 33 35 0 17 3 22 21 19,9

Maximum [μm/m] 405 344 402 482 592 458 454 587 373 513 461

3-80-25

Mean [μm/m] 0 32 -56 8 25 3 -15 13 7 -9 0,8

Maximum [μm/m] 319 323 447 474 347 343 456 344 472 300 382,5

3-80-50

Mean [μm/m] 5 11 -9 8 37 -11 -10 -59 13 24 0,9

Maximum [μm/m] 332 600 579 338 372 487 364 405 615 430 452,2

3-80-75

Mean [μm/m] -36 38 9 27 3 15 0 -1 18 19 9,2

Maximum [μm/m] 647 674 672 746 835 448 787 488 618 690 660,5

Figure 3.6 Mean strain for pattern evaluation

-100

-80

-60

-40

-20

0

20

40

60

80

1 2 3 4 5 6 7 8 9 10

Mea

n s

trai

n [μ

m/m

]

Stage

Mean strain

2-65-25

2-65-50

2-65-75

2-80-25

2-80-50

2-80-75

3-80-25

3-80-50

3-80-75

20

Figure 3.7 Maximum strain for pattern evaluation

When sorted by falling average mean displacement this are the results, see Table 3.12.

Table 3.12 Comparison between strain and displacement for pattern evaluation

Strain [μm/m] Displacement [mm]

Pattern Mean Maximum Mean Maximum

2-80-75 19,9 461 0,0129 0,0391

3-80-75 9,2 660,5 0,0152 0,0434

3-80-25 0,8 382,5 0,0153 0,0514

2-65-50 -9,1 448,5 0,016 0,0429

2-65-25 -19,9 381,5 0,0184 0,0455

2-65-75 -4,5 552,2 0,0186 0,0451

2-80-25 -1,2 494,3 0,0201 0,0439

2-80-50 -14,5 435,6 0,028 0,0678

3-80-50 0,9 452,2 0,2116 1,0228

Pattern 2-80-75 is the one with least mean displacement, it is however also the one with one of

the highest strains and is therefore not suitable to use in this study.

Pattern 3-80-25 however has a low value on both mean strain and displacement. In the graphs

showing mean and maximum strain, this pattern is the dark blue line and seems to be one of the

most even lines. For displacement however, it has higher values in some stages. Overall it is

even over the different stages.

Pattern 2-80-25 have low values of both mean strain and displacement and would also be a

suitable option. In Figure 3.4 and Figure 3.6 this pattern is represented by the yellow line and

is even over the different stages.

0

100

200

300

400

500

600

700

800

900

1000

1 2 3 4 5 6 7 8 9 10

Max

imu

m s

trai

n [μ

m/m

]

Stage

Maximum strain

2-65-25

2-65-50

2-65-75

2-80-25

2-80-50

2-80-75

3-80-25

3-80-50

3-80-75

21

Pattern 3-80-50 is one of the patterns with lowest mean strain but Figure 3.4 shows that its mean

displacement is a lot larger than for all other patterns. This indicates that there might have been

some disturbance of that pattern during testing and is thus interesting to examine further.

Since all patterns were not evaluated in the same images it is of interest to examine a smaller

number of patterns that can fit in one set up. This to minimize the impact of noise in the image

itself.

Patterns 2-80-25, 3-80-25 and 3-80-50 are then displayed and evaluated again in the same way,

see Table 3.13-Table 3.14 and Figure 3.8-Figure 3.11

Table 3.13 Displacement final three patterns

Stage

Pattern 1 2 3 4 5 6 7 8 9 10 Average

2-80-25

Mean 0,014 0,014 0,014 0,014 0,014 0,014 0,014 0,014 0,016 0,015 0,0143

Maximum 0,018 0,017 0,019 0,018 0,018 0,018 0,017 0,017 0,019 0,018 0,0179

3-80-25

Mean 0,009 0,008 0,009 0,008 0,008 0,008 0,008 0,009 0,008 0,01 0,0085

Maximum 0,012 0,011 0,011 0,011 0,011 0,012 0,011 0,011 0,012 0,013 0,0115

3-80-50

Mean [mm] 0,005 0,006 0,006 0,004 0,004 0,005 0,005 0,006 0,001 0,007 0,0049

Maximum [mm] 0,009 0,009 0,011 0,008 0,009 0,009 0,008 0,01 0,005 0,01 0,0088

Table 3.14 Strain final three patterns

Stage

Pattern 1 2 3 4 5 6 7 8 9 10 Average

2-80-25

Mean -34 -27 -31 -30 -44 -34 -37 -26 -29 -33 -32,5

Maximum 147 172 226 308 172 213 205 203 173 183 200,2

3-80-25

Mean -37 -32 -29 -23 -34 -32 -34 -27 -61 -28 -33,7

Maximum 212 195 171 290 260 187 241 146 143 189 203,4

3-80-50

Mean [μm/m] -19 -10 -21 -27 -35 -13 -30 -8 -28 5 -18,6

Maximum [μm/m] 416 575 405 342 309 400 698 485 470 395 449,5

22

Figure 3.8 Maximum strain for final three patterns

Figure 3.9 Mean strain for final three patterns

0

100

200

300

400

500

600

700

800

0 2 4 6 8 10 12

Maximum strain

2-80-25

3-80-25

3-80-50

-70

-60

-50

-40

-30

-20

-10

0

10

0 2 4 6 8 10 12

Mean strain

2-80-25

3-80-25

3-80-50

23

Figure 3.10 Mean displacement for final three patterns

Figure 3.11 Maximum displacement for final three pat

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