ESTABLISHMENT OF DURABILITY DESIGNTECHNIQUES TO PREVENT SPALLING OF
FACADE CLADDING IN RC BUILDINGSRC 建築物の外装仕上げ材の剥離防止に関する耐久設計技術の確立2023 年 3 月
広島大学大学院先進理工系科学研究科 建築学専攻
張 沛怡

SUMMARY
The largest non-structural element in buildings, facade cladding, expresses design aesthetics while
protecting structural components from weather. The most common cladding for RC buildings in Japan isrendered and ceramic tiles; however, it faces severe durability issues due to poor design, material selection,and inadequate maintenance, causing a high degree of deterioration. Typically, this cladding is composedof numerous layers of cementitious materials, with delamination being the predominant anomaly, especiallybetween the rendering mortar and the concrete substrate. Cladding delamination can cause public safetyincidents and shorten the building's lifespan. The lack of reliable methodologies and indications forassessing the durability of facade cladding makes it difficult to raise the standard of such material.
In this sense, the purpose of this study is to develop durability design techniques that can be
implemented to prevent the facade cladding from peeling and spalling over time. Pre-construction
performance design and in-operation preventative maintenance are crucial facade's life cycle stages.
For the performance design phase, proposed a lab-scale durability assessment method for rendering
mortar/concrete adhesion that enables stakeholders to choose specifications based on predetermined targets.
In this technique, a cyclic thermal load is applied to a part of the rendering mortar surface to accelerate thedegradation; this part represents an external wall subjected to solar radiation. Numerical simulation andexperimental results support the effectiveness of the proposed accelerated degradation method. The rate ofbond strength loss can be used to rank the parameters that affect durability. This technique deviates fromthe norm in that it does not employ a combination of hot-cold and dry-wet cycles and only applies "partial"loads. Both are necessary for the accurate reproduction of the degradation mechanism; the former prevents
erroneous reproduction of the collaborative action of the degradation agents and guarantees deterioration,
while the latter offers sufficient constraints.
For the preventative maintenance phase, developed a delamination detection technique with Fibre
Bragg grating sensors. Based on the relationship between the strain behavior of the materials, the technique
identifies the delamination. The adhesive interface of any two cladding composition materials (concrete,rendering mortar, tiles, and tile adhesives) was examined. The moment of damage at any adhesive interface
can be observed together with delamination. This technique's advantage is that continuous detection
prevents early or late repairs, accidental cladding peeling, and high maintenance expenses.
While establishing the above techniques, the debonding evolution of rendering mortar wasinvestigated. Debonding progresses on an "S-shaped" pattern, stalling at a "plateau" period. During thistime, the bond strength does not diminish with the aging cycle. The bond strength test has hysteresis,
requiring debonding to cumulatively approach a threshold before it can be characterized. This finding
supports the need for continuous damage detection.
This dissertation summarizes a series of studies on durability design techniques. The goal is to givepractitioners tools to improve the execution efficiency of a building's life cycle, extending its service life.

i

ACKNOWLEDGEMENTS
Since October 2017, I have spent five and a half unforgettable years at Hiroshima University. I
would like to express my sincere appreciation to everyone who has and continue to offer me support.

To my supervisors, Prof. Ohkubo Takaaki and Prof. Teramoto Atsushi. I cannot adequately
express how much they have helped me. In addition to my academic success, I have also received
strong emotional and spiritual support from their confirmation. Thank them for their time, expertise,and comments regarding this dissertation. Being their student has been a beneficial experience thatwill continue to pay off for the rest of my life. I am grateful for the opportunity.
To Prof. Kawai Kenji and Prof. Tagawa Hiroshi, for the assistance and insightful remarks they
provided during the process of completing my dissertation.
To the Yahata Memorial Scholarship Foundation, for providing financial support during my study
for the period of six months.
To the Next-Generation of Innovative Researchers Scholarship Foundation, for awarding me astudy grant for a year and a half.
To all the personnel of the Building Materials & Components Laboratory, for their assistance andmoral support during the experiments, which considerably facilitated the research, and especially to
my seniors, Qu Fei-Fei and Zhang Yu-Lu, who assisted me in every way upon my arrival in Japan.
To the entire Department of Architecture faculty and staff for their tremendous assistance.
Special gratitude to my close friend, Assist. Prof. Chen Xing-Chen, for her support andencouragement throughout my academic and life.

Finally, I'd like to express my gratitude to my family and friends. My father, Zhang Bin, has
always had a positive outlook on education, and he instilled in me a desire for academics from an earlyage. Huang Ji-Hong, my mother, is the most supportive person I know. Her love and encouragement
have carried me through thick and thin.

ii

CONTENTS

CONTENTSSUMMARY ...................................................................................................................................... i
ACKNOWLEDGEMENTS ............................................................................................................. ii
CONTENTS.................................................................................................................................... iii
LIST OF FIGURES .........................................................................................................................vi
LIST OF TABLES ............................................................................................................................x
NOMENCLATURE LIST ...............................................................................................................xi

CHAPTER I. INTRODUCTION
1.1 Background ...........................................................................................................................1
1.1.1 Facade claddings anomalies ...............................................................................................4
1.1.2 Design interventions for facade durability .........................................................................6
1.1.3 Objective-based durability design ...................................................................................... 8
1.2 Objectives .............................................................................................................................91.2.1 Establishing a lab-scale method to assess the durability of mortar and concrete adhesion
systems ......................................................................................................................................10
1.2.2 Developing non-destructive testing (NDT) techniques for detachment ...........................12
1.3 Scope of Work .....................................................................................................................12
1.4 Dissertation outline .............................................................................................................13

CHAPTER II. LITERATURE REVIEW
2.1 Adhesion failure of facade cladding ...................................................................................16
2.1.1 Adhesion of the rendering mortar to the substrate ........................................................... 16
2.1.2 Assessment and measurement of adhesion ......................................................................21
2.1.3 Environmental effects on facades ....................................................................................24
2.2 Approach to constructing durability assessment systems ...................................................27
2.2.1 Degradation mechanisms of rendering mortars ...............................................................27
2.2.2 AT methods for studying mortar adhesion durability .......................................................30
2.2.3 Degradation patterns of mortars .......................................................................................33
2.2.4 Assessment of rendering mortar/substrate adhesion ........................................................36
2.3 NDT delamination detection techniques .............................................................................37
2.3.1 Field-applicable NDT methods ........................................................................................37
2.3.2 Laboratory-applicable NDT methods ..............................................................................42
2.4 Summary .............................................................................................................................44iii

CONTENTS

CHAPTER III. ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION
3.1 Proposed lab-scale durability assessment method ..............................................................46
3.1.1 Proposed AT method ........................................................................................................47
3.1.2 Dimensional design of specimens and adhesion testing ..................................................48
3.2 Numerical simulation ..........................................................................................................50
3.2.1 Modeling the mortar/concrete composites .......................................................................49
3.2.2 Application of numerical model .......................................................................................52
3.3 Experimental test of the PTL method .................................................................................54
3.3.1 Specimen preparation.......................................................................................................54
3.3.2 Accelerated degradation effect tests .................................................................................56
3.3.3 Results and discussion .....................................................................................................58
3.4 Rendering mortar degradation pattern under PTL cycles....................................................60
3.4.1 Methods ...........................................................................................................................61
3.4.2 Results and discussion .....................................................................................................62
3.5 Summary .............................................................................................................................67

CHAPTER IV. APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD
4.1 Durability comparison of mortar/concrete composites .......................................................69
4.1.1 Specimen preparation.......................................................................................................69
4.1.2 Durability assessment ......................................................................................................70
4.2 A continuity approach for assessing mortar adhesion .........................................................74
4.2.1 Monitoring the strain behavior of specimens with FBG sensors .....................................74
4.2.2 Experimental results and discussion ................................................................................76
4.2.3 Analysis of debonding evolution based on strain monitoring results ...............................83
4.3 Summary .............................................................................................................................85

CHAPTER V. DELAMINATION DETECTION WITH FBG SENSORS UNDERPTL AGING CYCLE
5.1 Experiment on small-scale specimen with tiles ..................................................................87
5.1.1 Tile specimen preparation ................................................................................................88
5.1.2 Temperature monitoring of tile specimens .......................................................................90
5.1.3 Strain monitoring of tile specimens .................................................................................90
5.1.4 Analysis of strain amplitude.............................................................................................94
5.1.5 Response of strain behavior to adhesion state..................................................................97
5.2 Experiment on large-scale wall specimen ...........................................................................98iv

CONTENTS

5.2.1 Specimen preparation.......................................................................................................99
5.2.2 Aging cycles and delamination detection ....................................................................... 100
5.2.3 Results of the bonding mortar ........................................................................................ 102
5.2.4 Results of the organic adhesives .................................................................................... 106
5.2.5 Delamination detection of organic adhesives under freezing-thawing cycles ............... 108
5.3 Summary ........................................................................................................................... 110

CHAPTER VI. CONCLUSION AND FUTURE STRATEGIES
6.1 Conclusion ........................................................................................................................ 112
6.1.1 A laboratory-scale durability assessment method .......................................................... 112
6.1.2 The key parameters affecting the durability of rendering mortar................................... 114
6.1.3 Debonding evolution of rendering mortars under the thermal load ............................... 115
6.1.4 Detecting adhesion damage in tilings with FBG sensors ............................................... 116
6.2 Future strategies ................................................................................................................ 117

Appendix A. Cohesive Zone Material (CZM) Mode Mixed-Mode Debonding law in
AYSYS .............................................................................................................. 119
Appendix B. Effect of Curling Behavior of Mortar Due to Drying Shrinkage on MortarConcrete Adhesion System ........................................................................... 121
B.1 Free curling of rendering mortar ....................................................................................... 122
B.1.1 Experimental procedure ................................................................................................ 123
B.1.2 Experimental result ....................................................................................................... 123
B.1.3 Numerical analyses ....................................................................................................... 125
B.2 Study of restrained shrinkage by numerical analyses ....................................................... 127
B.2.1 FE Model ....................................................................................................................... 127
B.2.2 Interfacial stress estimation under restraint conditions ................................................. 128
B.3 Study of restrained shrinkage by DIC ............................................................................... 128
B.3.1 Experimental procedure ................................................................................................ 129
B.3.2 Post-processing result .................................................................................................... 131
B.4 Effect of curling deformation on the proposed durability assessment method ................. 132
B.5 Summary ........................................................................................................................... 133

REFERENCES. ...................................................................................................................... 135v

LIST OF FIGURESLIST OF FIGURES
Figure 1-1 Average estimated service life obtained for façades claddings, considering the climaticconditions of the region in which the study was carried out, according to [11] ..............3
Figure 1-2 Contribution of anomalies to the evolution of facade degradation, according to [15] ...5
Figure 1-3 Conceptual framework for extending the service life of facades ...................................7
Figure 1-4 Conceptual framework for objective-based durability design ........................................8
Figure 1-5 Dissertation outline....................................................................................................... 13
Figure 2-1 Factors which exert influence on bond, according to [41] ...........................................19
Figure 2-2 The schematic illustration of the locations of the rupture surfaces during the pull-off test,
according to [64] ........................................................................................................... 21
Figure 2-3 Schematic description of different experimental tests to assess the bond strength between
two materials, according to [63] ...................................................................................22
Figure 2-4 Schematic of differences in pull-off test vs. slant shear test with respect to directions ofgravity and loading, according to [50] .......................................................................... 23
Figure 2-5 Transition zone models of bonding interfaces in the literature.....................................24
Figure 2-6 Deformation of rendering mortar .................................................................................27
Figure 2-7 Failure of adhesive unity under daily solar radiation ...................................................28
Figure 2-8 Peeling and cracking due to drying shrinkage in the mortar/concrete composite specimen,
adapted from [81] ..........................................................................................................29
Figure 2-9 Debonding due to curling deformation of the overlayer, corner lift (a) and example ofdebonded edges (b), according to [82] ..........................................................................30
Figure 2-10 Hygrothermal aging cycle, the two most used AT methods, according to [84] ..........31
Figure 2-11 Evolution of the adhesive strength of the specimens during the hygrothermal aging
cycle, according to [85]............................................................................................... 31
Figure 2-12 Hygrothermal aging cycle according from Japanese Society of Finishing Standards
M101 ......................................................................................................................... 32
Figure 2-13 Four typical patterns of deterioration paths, according to [87]................................... 34
Figure 2-14 Evolutionary pattern of debonding of one tile, according to [89] .............................. 35
Figure 2-15 Evolution of adhesion under cyclic mechanical loading (shear stress amplitude fromleft to right: 1.21 MPa, 0.97 MPa, 0.81 MPa.), according to [91] ..............................36
Figure 2-16 Monitoring of interlayer strains in composite tile-rendering-concrete specimens underartificial aging cycles, according to [13] .....................................................................41
Figure 3-1 Application of partial heating to reproduce the actual degradation condition ..............47
Figure 3-2 Rubber heaters for applying PTL .................................................................................48vi

LIST OF FIGURES

Figure 3-3 Specimen dimensions ...................................................................................................49
Figure 3-4 Devices for bond strength tests.....................................................................................49
Figure 3-5 Mortar/concrete composites geometry .........................................................................51
Figure 3-6 Modelling the mortar/concrete adhesive interface, according to [143] ........................51
Figure 3-7 Thermal load conditions ............................................................................................... 52
Figure 3-8 Equivalent (von-Mises) stress (MPa) at the FEM model interface at 120 min ............53
Figure 3-9 Maximum deformation (mm) and maximum principal stress direction of the mortar in
the FEM model ...........................................................................................................53
Figure 3-10 Thermal load cycle and application ............................................................................56
Figure 3-11 Temperature history of the mortar surface during 50 cycles ......................................56
Figure 3-12 Contrast zones for the bond strength ..........................................................................57
Figure 3-13 Change of bond strength in both directions ................................................................58
Figure 3-14 Shear strength of specimens under different thermal load lengths .............................60
Figure 3-15 Aging cycles for studying degradation patterns .........................................................62
Figure 3-16 Evolution of the bond strength in the specimens submitted to the aging cycles – Peaktemperature of 80 °C ...................................................................................................63
Figure 3-17 Cracks on the mortar surface after thermal loading....................................................64
Figure 3-18 Evolution of the bond strength in the specimens submitted to the aging cycles – Peaktemperature of 70 °C ...................................................................................................65
Figure 3-19 Rate of bond strength change with 70 °C aging cycles (vs. 0 cycles) ........................66
Figure 3-20 Average bond strength variation for each cycle between two shear tests ...................66
Figure 4-1 Shear strength of the four types of specimens ..............................................................71
Figure 4-2 Change rate of shear strength of specimens after degradation .....................................72
Figure 4-3 Comparison of dimensions for FBG sensors and strain gauges ...................................75
Figure 4-4 Arrangement of FBG sensors in the specimen .............................................................75
Figure 4-5 Temperature history of the ST specimens ....................................................................77
Figure 4-6 Strain history of the SS specimens ...............................................................................78
Figure 4-7 Residual strain of mortar and concrete at the end of each cycle...................................79
Figure 4-8 Definition of the strain amplitude per cycle .................................................................81
Figure 4-9 Evolution of strain amplitude and strain amplitude ratio of mortar and concrete withaging cycle ..................................................................................................................81
Figure 4-10 Change rate of shear strength of SD specimens after aging cycle ..............................83
Figure 5-1 Outline of tile specimen and ID of detection points .....................................................88
Figure 5-2 Tile specimens in operation and PTL ageing cycle manipulations ...............................88
Figure 5-3 Temperature history of the tile specimens ....................................................................89
Figure 5-4 Strain history of the tile specimens...............................................................................91vii

LIST OF FIGURES

Figure 5-5 Residual strain of each detection point at the end of each cycle ..................................92
Figure 5-6 Comparison of strain behavior before and after accelerated degradation.....................94
Figure 5-7 Evolution of strain amplitude of each detection point with aging cycle ......................95
Figure 5-8 Evolution of strain amplitude ratio with aging cycle, (a) rendering mortar to concrete,(b) tile to rendering mortar ............................................................................................96
Figure 5-9 Outline of wall specimen ..............................................................................................99
Figure 5-10 Schematic of the operation of the PTL aging cycle experiment ............................... 100
Figure 5-11 Temperature history of bonding mortar side............................................................. 102
Figure 5-12 Value (left), change rate (right) of maximum SPL of impact sound ......................... 103
Figure 5-13 Fourier amplitude spectrum of impact sound ........................................................... 103
Figure 5-14 Strain history of concrete and tile surface ................................................................ 104
Figure 5-15 Strain difference between tile and concrete, (a) in the heated zone, (b) in the non-heatedzone, (c) details of 33rd to 37th cycle in the heated zone ......................................... 105
Figure 5-16 Ratio of the strain amplitude of the tile to concrete during each aging cycles ......... 106
Figure 5-17 Comparison of the strain behaviour of the tiles and substrate before and after
delamination ............................................................................................................ 106
Figure 5-18 Temperature history of organic adhesives side ......................................................... 107
Figure 5-19 Acoustic diagnostic results on the organic adhesive side ......................................... 107
Figure 5-20 Strain history of organic adhesives side ................................................................... 107
Figure 5-21 Strain relationship between the tiles and substrate on the organic adhesives side ... 107
Figure 5-22 Sensor configuration (left) and temperature history of concrete and tile surface (right).................................................................................................................................. 108
Figure 5-23 Strain history of concrete and tile surface under freeze-thaw cycles........................ 109
Figure A-1 Mixed-Mode Debonding law in ANSYS, according to [143].................................... 119
Figure B-1 Displacement measurement system ........................................................................... 123
Figure B-2 The displacement of each point changes with time ................................................... 124
Figure B-3 Curling deformation over 72 hours ............................................................................ 125
Figure B-4 Mass loss over 72 hours ............................................................................................. 125
Figure B-5 Free shrinkage model ................................................................................................. 126
Figure B-6 Displacement results on the axial path, comparison between the experimental andnumerical response.................................................................................................... 127
Figure B-7 Restrained shrinkage model ....................................................................................... 127
Figure B-8 Maximum deformation of mortar .............................................................................. 127
Figure B-9 Equivalent (Von-Mises) Stress (MPa) in the interface............................................... 128
Figure B-10 DIC Specimen dimensions....................................................................................... 129
Figure B-11 Measurements of the displacement field at the interface between the mortar layer andviii

LIST OF FIGURES

the concrete in the vertical direction, DIC procedure ............................................... 129
Figure B-12 AOI and subset size (40 × 40 pixels) ....................................................................... 130
Figure B-13 Strain distributions [μ] (in the out-of-plane direction) exhibited by specimens M-1 andT-1 at each moment ................................................................................................. 130
Figure B-14 Comparison of partial thermal loading and drying shrinkage effect on the stress stateat the mortar/concrete interface ............................................................................... 132

ix

LIST OF TABLES

LIST OF TABLES
Table 3-1 Model material parameters .............................................................................................52
Table 3-2 Mix proportions of base concrete ...................................................................................55
Table 3-3 Mix proportions of rendering mortar..............................................................................55
Table 4-1 Impact factors of the adhesion strength between mortar to concrete [145] ....................70
Table 4-2 Specification of rendering mortar specimens .................................................................70
Table 4-3 Initial shear strength (MPa) result for each group of specimens ....................................71
Table 4-4 Rate of decrease of the adhesion at the heated parts and the ends .................................73
Table 4-5 Specimen for different measurements ............................................................................73
Table B-1 Specifications of mortar specimens ............................................................................. 122

x

NOMENCLATURE LISTAT

Acceleration test

ACU

Air-coupled ultrasonicAE

Acoustic emission

CPFC

Component performance in failure conditions

CZM

Cohesive Zone Model

DIC

Digital image correlation

ET

Electromagnetic testing

EVA

Ethylene-vinyl acetate

FBG

Fiber Bragg grating sensors

FEM

Finite element model

GPR

Ground penetrating radar

IE

Impact-echo

IRT

Infrared thermographyLT

Laser testing

NDT

Non-destructive testing

NMF

Non-negative matrix factorization

NSC

Non-structural components

PCA

Principal component analysis

PSD

Power spectral density

PTL

Partial thermal load

RH

Relative humidity

SHM

Structural Health Monitoring

UAV

Unmanned aerial vehicle

UT

Ultrasonic TestingxiChapter 1. INTRODUCTION

Chapter I
INTRODUCTION

1.1 BACKGROUND
The construction materials sector is dependent on concrete and cement products. It is widely
utilized in house building, bridges, airports, highways, railroads, industrial mining, and military
defense projects. It is one of the most vital materials and components of infrastructure and engineering
construction. Significant amounts of energy are consumed at each stage of a building's life cycle, fromdesign and construction through maintenance and decommissioning. As the primary cementitious
material used, approximately 90% of the carbon footprint of concrete comes from Portland cement [1].
Total cement production in 2021 is estimated at 4,400,000 tons [2]. Each ton of cement produced
generates 0.8-1.0 tons of CO2 (depending on kiln operation), consumes 1,700 kWh/ton of energy, andrequires 1.5 tons of raw materials [3]. World construction demand is expected to remain high for along time, and cement and other cementitious materials will remain the primary material for meeting
global housing and modern infrastructure needs. Therefore, extending the service life of reinforced
concrete (RC) structures and maximizing cementitious materials' use will benefit material and energy
savings and reduce CO2 emissions.
Durability is the most fundamental performance requirement of buildings. Durability is definedas "The ability of a building or its components to perform their necessary functions during a specified
service life under the influence of the agents expected" [4]. It is worth pointing out that the term
"service life" here refers to the physical (technical) service life, which is related to the deterioration of
the building elements due to natural aging and deterioration factors. The physical service life ends
when the probability of failure of a building element is more significant than a predetermined
acceptable level of risk [5]. Some researchers have also introduced the concepts of functional andeconomical service life. The former relates to the expectations and requirements of building users. The
motivation to replace materials is related to aesthetics and trends rather than a decline in technicalperformance. The latter is motivated by cost-efficiency considerations, as it is better to introduce newbuilding solutions than to maintain the existing state. These two types of service life are outside thescope of this work [5].
1Chapter 1. INTRODUCTION

In addition to the need to optimize the use of scarce resources, high durability guarantees to build
quality and safety. These combined factors have encouraged the development of durability design
techniques for buildings and components and the study of service life prediction models. Most
countries in the world also have regulations that require the service life of RC structures, such as 50
years for general civil buildings in Japan. Theoretically, each component should have the same service
life as the building. However, this is not the case. Structural elements, such as beams and columns, do
not have the same frequency of deterioration as non-structural elements (NSEs), such as equipment,
suspended ceilings, and interior and exterior finishes.
Many of the RC buildings in Japan were constructed during the period of rapid economic growthin the 1970s, and some of them have reached their required service life but are still maintained to gooduse. In those buildings that did not experience severe disasters (e.g., earthquakes and floods) during
their service life, the durability of structural elements performed beyond expectations. In contrast,
NSEs often exhibit significant degradation behavior early in their service life, even requiring several
renovations and repairs throughout their life cycle, resulting in substantial public property
expenditures. In addition to economic losses, the safety risks associated with low-durability NSEs areof concern, with damage to NSEs far more likely to cause injury or death in safety incidents causedby damage to building’s components than structural elements [6] [7]. During the Tohoku earthquake
in March 2011, injuries caused by falling ceilings in seismic shelters across Japan increased publicawareness regarding the safety of NSCs. Multiple spalling accidents due to the deterioration ofbuilding facade elements are reported annually worldwide. Frequent safety accidents have increased
public awareness of the importance of NSEs, and considerable attention has been paid to theperformance design and safety monitoring of nonstructural components in recent years [6] [8] [9].
However, research on testing and assessment methods, design and construction, and management andmaintenance techniques for the performance of NSEs are still open, and most countries have yet toestablish perfect rules and indicators.
The building envelope (exterior wall cladding, sloped roof cladding, and window frames) is the
NSEs that receive the most attention. Because of their role in safety, thermal and acoustic insulation,
air and water infiltration/mitigation, solar energy, daylight and glare control, and aesthetics [10].
Moreover, the building envelopes are considered the first durable layer of the building, they are highly
susceptible to defects that have a direct impact on the performance of the building, the comfort andquality of life of the users, and the cost of repair and maintenance [11]. The facade claddings are theNCEs with the largest application area. Facade claddings provide aesthetic expression and protect2Chapter 1. INTRODUCTION

Figure 1-1 Average estimated service life obtained for façades claddings, considering
the climatic conditions of the region in which the study was carried out, according to [11].
architectural elements from climatic and environmental conditions. As the "skin" of a building, it canbe said that its durability directly affects the durability of the building. However, the durability of
facade cladding in practice is far below what is expected for the building. Silva and Brito (2021)
analyzed 107 publications and built a database on the service life of building envelope elements. They
classified the average estimated service life of the facade coverings according to the climaticconditions of the study area, as in Fig. 1-1 (Fig. 6 in [11]).
The average estimated service life of seven facade claddings (renderings, painted surfaces,
ceramic and stone claddings, External Thermal Insulation Composite Systems (ETICS), architecturalconcrete surfaces, and timber claddings) are listed in the references, which are derived from literature
reviews and expert opinions, field sampling surveys, and standards and guidelines. The facade
claddings in the humid subtropical climate ("Cfa") in which Japan is located obtain the lowest average
estimated service life. It is widely accepted that even the most durable ceramic and stone claddings
require intensive maintenance and partial renovation/replacement after 20 to 30 years of construction

3Chapter 1. INTRODUCTION[6], making it difficult to match the 50-year service life demand for RC buildings. Therefore, the design
technology for high durability of facade claddings has become a challenging and demanding task.
Mortar is essential in any facade cladding. Mortar is used in many applications and can beclassified according to its use as plaster, render, tile adhesive, grout, waterproofing slurry, restorationmaterials, insulation and finishing systems, and others. These are usually dry-mixed mortars from
specialist production plants prepared according to the application. Rendering mortars are the basis for
applying all finishing materials, providing a solid, level surface for the external walls of RC buildings,
and ensuring good adhesion of finishing coatings. The cementitious adhesives for ceramic tiles are
known as adhesive mortar. Renderings and ceramic claddings are the two common types of facade
claddings in Japan [12] and many countries worldwide, such as China [13], Portugal [14], Brazil [15],Poland [16], etc. Consequently, rendering mortars and adhesive mortars are the most produced andconsumed prefabricated dry-mix mortars. World production of ceramic tiles reached 16.093 billion
sqm in 2020. Consumption is 16.035 billion sqm, of which Asia accounts for around 70% [17].
Assuming an average use of 3 kg/m2 for both rendering and adhesive mortars for tile coverings (aconservative estimate), this implies a world production of over 96 million tons of both types of mortar.
Generally, these mortars have a service life of about 20 years and fail even after five years of use.
Overcoming the non-sustainability of mortars is an important research topic and an academic aim of
the authors.

1.1.1 Facade claddings anomalies
Degradation starts when the building is built, and the various anomalies that arise during thedegradation process can be understood simply as defects in the facade claddings. There are many typesof defects, the frequency, and severity of which depend on the geographical characteristics of thebuilding and its use and maintenance. Researchers have generally classified facade claddings damage
into the following four types [15] [18]:- Detachment/Debonding, "Delamination" occurs when the claddings lose adhesion. (Highthermomechanical stress due to sunlight exposure, joint failure, water penetration, moisture
expansion, and excessive building deformation).
- Cracks, single line or interwoven, involving only the finish layer or through the entire thicknessof the exterior wall. (Deformation caused by the movement of the building structure,
thermomechanical stresses by the direct action of the sun).

4

Chapter 1. INTRODUCTION- Joint failure, occurs in voids in the grout between the ceramic claddings and excessive joints
between floors. (Inadequate grout specification, high thermomechanical stress, and interlayer
shift).
- Stains, color changes on the surface of the covering, deposition of salts, microbial adhesion.
(High humidity, high content of soluble salts in bricks and mortar, mold, rusting of metalelements).

The extent of damage to the facade claddings varies from defect to defect. Among them, stains
and minor degrees of cracking can be classified as visual degradation [19], as they have not developed
to the extent that they would lead to actual physical failure and loss of durability of the facade but stillrequire appropriate maintenance strategies to avoid further development. Stains are usually located inareas where more rainwater is exposed, and the accumulation of rainwater in the texture of thecladdings and the recesses of cracks helps microorganisms to multiply, allowing dirt to be depositedon the surface [20].
Cracking in both the cladding material and the joints can evolve toward debonding and areanomalies requiring immediate intervention. The leading cause of cracking is the movement of thesubstrate, which in turn is subjected to thermal solid or mechanical deformation. A high incidence of
cracks is found in external walls or where there are significant dimensional changes or pressures, suchas at the borders of buildings, between floors, and openings. Joint grout failures in ceramic cladding,such as cracks, and material voids, can be precursors to serious mechanical failures such as debonding.
Cracks through the thickness of the facade are the most dangerous situation, which can lead to direct
exposure of the structure to the environment, where water intrusion may trigger reinforcement

Figure 1-2 Contribution of anomalies to the evolution of facade degradation, according to[15].
5Chapter 1. INTRODUCTION

corrosion and threaten the structure's safety.
These failures may eventually evolve into detachment, which depends on the progressive increasein the action of degradation agents (radiation, stormwater) and is directly related to more specificdegradation mechanisms, such as thermal stress, fatigue, and deformation of the facade. Detachment
represents the most severe damage to facade claddings, "peeling" leads to the failure of the protective
layer and impairs the performance of the structural elements. In the worst-case scenario, a deboned
cladding loses its restraint and falls from high altitudes causing accidents to people and property.
Fig. 1-2 shows the contribution of various anomalies in the degradation process of the facade,according to Bauer and Souza's investigation of a group of buildings in Brasilia with ceramic claddings.
It is important to note that Brasilia does not have the same climate as Japan [15]. There is a Tropical
Savannah Climate, with distinct dry and rainy seasons, where the expected service life of ceramic
cladding is longer. Detachment explodes near the limit of service life. In other words, adhesion failures
accelerate the overall degradation process.
Therefore, the prevention of debonding is an important research topic. A cement-rendered facadeis essentially a multi-layer structure composed of different materials and ensuring the bonding integrity
between the layers is essential to the durability performance. The first consideration in the durability
design of a building facade is the prevention of peeling and flaking of the claddings from the concrete
substrate during its life cycle.

1.1.2 Design interventions for facade durability
The facade of a building is affected by changes in the environment and the physical properties ofbuilding materials at every stage of the building's life cycle. Despite significant developments inmortar manufacture and bonding technology in recent years, facade anomalies persist, related to thepoor design of construction details, inappropriate selection or/and application of materials, andinadequate maintenance [21]. Facade design decisions can interfere with service life. In the absenceof a systematic approach to developing a facade design process that considers performance anddurability [22], designers are often reluctant to implement novel solutions with improved durability
and low maintenance requirements because the performance is difficult to prove. At the same time,high costs and technical barriers do not guarantee the efficient implementation of regular maintenance.
The current design methodology for the durability of facade cladding systems is mainly empirical.
It is based on an initial value that meets the minimum requirements of the project and the codes andstandards ((e.g., bond strength after construction) and empirical assumptions. If these rules are met,

6

Chapter 1. INTRODUCTION

the system is considered to achieve an acceptably long but unspecified life. Further consideration ofdurability may lead to an increase in building durability. However, this is a short-sighted andcontroversial approach. Because a design approach that only meets the initial performance requiredby standards without sufficient consideration and subsequent issues (such as environmental impact
and sustainability of performance, inspection needs, and maintainability) does not allow for thedefinition of whole life costs, i.e., there is no way to prove that future maintenance and repair costs
will be reduced. In particular, defects require immediate intervention and can bring repair costs evenhigher than the initial costs (because of the inclusion of a removal step). Properly designed and applied
durability is entirely worthwhile if maintenance costs are considered [22].
Fig. 1-3 shows a conceptual diagram for extending the service life of a facade, where durability
design is essential to achieve the desired tile performance. At this stage, full consideration should begiven to the details of material degradation, including degradants-atmospheric, biological, loading,incompatibility and use, and maintenance actions [23]. According to Chew et al. [24], identifying
materials with improved durability gradients is the solution to resist natural aging. Applying highly
durable materials and construction techniques can slow down the degradation rate. Secondly, the lifeof a facade depends on environmental factors or extreme events beyond the designer's control. Regularinspection, maintenance, repair, or timely replacement of facades can significantly improve building
performance and owner satisfaction. Durability design is about establishing methodologicalprocedures that balance intended function, investment costs, and maintenance efforts to maintain thebuilding at a high performance [5].

1.1.3 Objective-based durability design

Figure 1-3 Conceptual framework for extending the service life of facades.
7

Chapter 1. INTRODUCTION

Concept “Objective-based durability design” is built on the Concept “Performance-based
durability design” [25] [26]. Performance approaches are based on the measurement of material
properties that can be linked to deterioration mechanisms under prevalent exposure conditions. The
measurement of actual concrete material properties of the as-built structure allows accounting for the
combined influences of material composition, construction procedures, and environmental influences
and therefore forms a rational basis for durability prediction and service life design [26]. Applying thisconcept to the durability design of facades, we wanted to establish clear service life objectives.
Durability design techniques are developed around the purpose required by the user, and suchprocedures allow specifications to be selected according to the purpose set. Fig. 1-4 presents an
overview of the proposed conceptual framework.
The designer first needs to define the service life objectives.

Designing durability depends onthe erosion of the environment in the area where it is applied and should be carefully identified andclassified. Once the degradation agents have been identified, the degradation mechanisms and patterns
need to be clarified, and a deterioration model developed. It is important to note that different
degradation models should be developed for other building parts. According to the “layering principle”
proposed by Brand [27], different parts of the same building degrade at different rates and patterns.

Figure 1-4 Conceptual framework for objective-based durability design.
8Chapter 1. INTRODUCTION

High exposure to degradation agents does not necessarily mean higher levels of degradation, and otherconditions are needed to allow degradation mechanisms to develop. This mechanism action is aprocess that involves chemical reactions, deformations, and mechanical efforts, among others [15].
Next, a lab-scale durability test method needs to be established. The method should be able toreproduce the impact and degradation mechanisms of the degradation agent. Further, a predictivemodel for service life is based. Durability metrics are related to the predicted service life and need tointegrate the results of laboratory tests with long-term practical applications. Controllable durability
influencing factors are specified as normative parameters and are optional specifications. The
proposed technology can be applied in practice once it has passed the lab-level durability assessment.
In addition, the control of construction quality, preventive management, and the expected scope of
repairs are all mandatory considerations of the durability design. After all the necessary interventions
have been completed, the techniques/specifications used in the design can be standardized if the
product can meet the service life objectives.
Although it may take decades to establish a complete program, it has long-term implications. The
objective-based design provides stakeholders with a reliable alternative because it uses realistic
environmental and material models and can predict the future behavior of concrete structures, thereby
quantifying the design life.
-Design based on total life cycle costs will be possible - Reduced material and energy
consumption by optimizing the use of materials.
-Provides an objective basis for user-tailored design, primarily when the design is based on
industry-specific material properties.
-The proposed durability testing procedures can be extended to new materials and applications.
The framework for design procedures can provide an objective basis for predicting the behavior of
new materials in use.
-Predictions of service life will be more reliable, thereby reducing premature dismantling andrepair of components.

1.2 OBJECTIVES
The rendering mortar in the facade cladding of RC building is the chosen object of this work. As
the first cementitious material comes into direct contact with the concrete substrate, the rendering
mortar is responsible for connecting the concrete substrate and the finishes, but detachment frequently
9Chapter 1. INTRODUCTION

occurs. Therefore, a series of studies have been carried out to maintain the adhesion integrity between
the rendering mortar and the concrete, to improve the durability of facade claddings.
As shown in Fig. 1-4, the efficiency of the implementation of three phases determines the service
life of a building: 1) pre-construction durability design, 2) preventive maintenance, and 3) appropriate
and timely repair. The authors' research during my Ph.D. was distributed in two phases, 1) and 2), withseparate research objectives established. The research objective for the durability design phase was toestablish a laboratory-scale durability assessment system. The preventive management phase aimed todevelop efficient non-destructive damage (detachment) detection techniques.

1.2.1 Establishing a lab-scale method to assess the durability of mortar and concreteadhesion systems
Reliable lab-scale durability evaluation methods are a prerequisite for selecting durable design
techniques and material specifications based on objective service life. Research on high-adhesion
rendering mortars has been significant gains, but some obstacles still make it challenging to apply
them in practice. First, for commercial dry-mix mortars, current Japanese regulations only provide
requirements for initial performance after construction and acceptance and do not have complete
durability evaluation indicators like concrete. Therefore, manufacturers generally cannot offer
accurate product durability and operational details required about durability. Second, the durability
assessment methods for rendering mortars recommended by national codes are less convincing, as
poor consideration of degradation mechanisms. This makes improving the durability of rendering
mortars in actual projects challenging. Therefore, the establishment of durability assessment methods
is an urgent research task.
Durability assessment methods generally consist of two tests, 1. Accelerated testing canreproduce the impact and degradation mechanisms of degradants in the laboratory. 2. Performance
tests. Therefore, this study can be further divided into the following two objectives.

(1) Elucidate the degradation mechanisms and degradation patterns of adhesion integrity
between rendering mortar and concrete
Determining a performance degradation model is the first step in establishing a durability
assessment methodology. The characteristics of the degradation model depend on identifying current
degradation mechanisms, anomalies, and external mechanical, electromechanical, thermal, chemical,
and biological agents [21] and contain three key parameters, degradation agents, deterioration

10

Chapter 1. INTRODUCTION

mechanisms, and deterioration patterns.
Degradation agents are necessary factors (mainly climatic factors) that trigger the deterioration
process. The material's response to the degradation agent is defined as the degradation mechanism,
which depends on the agent as well as on the materials itself and its location (interference of otherelements).

In the case of rendered mortar-concrete bonding systems, the loss of adhesion is aphenomenon caused by the failure or rupture of the interface between mortar and substrate due to thegeneration of tensions that exceed the resistance of the connection. The leading cause of interface
damage is the hygrothermal effect, and radiation and rain are key degradation agents. Several studies
using both agents to reproduce degradation mechanisms in the laboratory have been conducted, but
no satisfactory results were obtained, proving the complexity of the synergistic mechanisms between
agents. Therefore, elucidating the relationship between degradants and degradation mechanisms on acase-by-case basis is an introductory approach.
A deterioration pattern is the pattern of performance evolution over time under a specificdeterioration mechanism, which a qualitative curve can generally represent. The deterioration patterndetermines the scheduling of performance tests and durability evaluation results and is essential forestablishing durability evaluation methods. Therefore, it is a sub-objective to plot the degradation
curves of the adhesive properties of rendered mortars based on specific degradation agents anddegradation mechanisms.

(2) Determine the Accelerated testing (AT) and bonding performance test methods.
AT can be expressed as "compressed time within a reasonable test time to accelerate the failure
mechanism so that product reliability can be evaluated." Increasing the frequency of use and the load
level of the test equipment is a common model for AT. Since there is usually more than one degradation
mechanism, combined models are recommended. But it is difficult to simulate all degradation
mechanisms and the way they interact with each other. Therefore, selecting the most representative
degradation mechanisms for the specific application conditions of the material to develop a particular
AT is a solution. As mentioned previously, temperature and humidity are essential parameters for
mortar degradation. Therefore, this study investigated the two parameters separately to establish themost suitable AT for rendered mortars.
Directing the durability assessment to the desired performance will ensure a more durable
solution. This study considers the bond between the rendering mortar and the concrete substrate as themost critical parameter, but the bond strength is highly variable depending on the test method. Several

11

Chapter 1. INTRODUCTION

representative bonding performance test methods were investigated to obtain a bonding performancereflecting durability.

1.2.2 Developing non-destructive testing (NDT) techniques for detachment.
Effective damage detection techniques are critical to developing maintenance strategies, andtimeliness and accuracy are crucial. Timeliness requirement prevents missed inspections and reduces
and avoids the presence and evolution of defects. Accuracy requires reducing premature repairs due
to incorrect judgments and reducing maintenance and repair costs. As mentioned earlier, detachment
is the most dangerous form of failure; therefore, delamination detection is a crucial technology. Thisis particularly the case where delamination cannot be directly observed, as it can lead to lowered
vigilance by managers and further deterioration in the form of crack formation and tile spalling. NDTis the ideal method for detecting subsurface damage and is desired by building owners. The authors
investigated several common NDT techniques and concluded they have difficulty balancing timelinessand accuracy. Therefore, the development of efficient layered inspection techniques became a research
objective.

1.3 SCOPE OF WORKThis study is part of establishing the durability design procedure, and the work scope is marked
in red in Fig. 1-4. The degradation mechanisms of the rendered mortar were investigated, and theeffects of temperature and humidity on the adhesion properties were discussed separately. A lab-scale
durability assessment method is proposed, consisting of two tests, the AT and the adhesion strength
test. Numerical simulations and experiments verified the validity of the AT. Based on the AT,
investigated the deterioration pattern of the mortar. The decrease rate of the bond strength was used asa critical durability parameter to determine the best performance evaluation pattern.
Several influencing factors were investigated using the proposed durability evaluation method,
including the mortar's water-cement ratio, admixtures, thickness, workmanship, and curing conditions.
Previous studies investigated the effects of these parameters on the initial bonding performance, andthis study further evaluated their impact on durability.
Several non-destructive methods of delamination detection are investigated, and a technique forstrain monitoring using fiber Bragg grating (FBG) sensors is proposed. The technique was piloted in
two overlays: rendered mortar and ceramic tile. The scope of the study includes the feasibility,
12Chapter 1. INTRODUCTION

sensitivity, limitations, and development potential of the technique.
This study aims to ensure that the proposed durability assessment method enables a comparisonof the durability of several different product specifications. Predicting the actual service life is notconsidered, as this would involve another complex area of research. Furthermore, although renderedmortar is the subject of this study, the proposed method can be extended and adapted to include
different cladding and coating solutions in RC building facades.

1.4 DISSERTATION OUTLINE
The outline of this dissertation is shown in Fig. 1-5 and consists of six chapters. The following isa summary of each chapter's content.

Figure 1-5 Dissertation outline.

13

Chapter 1. INTRODUCTION

Chapter 1
This chapter outlines the study's background, subjects, and goals. Describes the durability
problems and societal requirements of facade claddings for RC buildings, along with the significanceof establishing objective-based durability design methodologies. This work is focused on performance
design and preventive maintenance technologies.

Chapter 2
This chapter reviews exceptional previous studies in the relevant field. Factors impacting
adhesion, adhesion failure, degradation agents and degradation mechanisms, and damage detection
techniques for rendered and tiling facades are the main topics covered. The fundamental procedure forconstructing a durability evaluation method is described, along with the rationale for solar radiation
as the principal environmental degradation agent.

Chapter 3This chapter describes the development phase of the durability assessment method for rendering
mortar/concrete adhesion. An artificial aging model is defined. The degradation mechanism undersolar radiation is reproduced by applying partial thermal load (PTL aging cycle) to the rendered mortar
surface. Experimental and numerical results prove this method works. PTL aging cycles were used to
examine the debonding evolution of rendering mortars, confirming the unsteady nature of this process.
It is concluded that continuous detection technology is necessary to improve execution efficiency as
there is a risk of missed detection with the periodic damage detection methods. Furthermore, theinfluence of curling deformation caused by drying shrinkage on mortar adhesion was investigated(Appendix B). It was determined that the detrimental effects of curling deformation should not beignored, and several procedures (wet curing and application of SRA) to ensure uniform drying of themortar during curing are useful countermeasures.

Chapter 4This chapter describes the implementation phase of the durability assessment method proposed
in Chapter 3. It is possible to rank the durability of four types of specimens created utilizing distinct
processes. The proposed durability assessment method benefits product development, especially foritems lacking application history. It shows the material's response to solar radiation. It can be rapidly
tested for durability by comparing it to a reference material with a long application history. Moreover,14

Chapter 1. INTRODUCTION

discussed the limits of the bond strength tests for measuring adhesion performance; hence a strainmonitoring approach is developed to analyze adhesion loss continually. The experimental results
demonstrate the method’s growth potential because of its ability to capture the instant damage that
occurs precisely.

Chapter 5
Based on the findings presented in Chapter 4, this chapter investigates further the application of
the strain monitoring method to the damage detection of facade tilings. On tile specimens of two
distinct sizes and compositional materials, FBG strain sensors were used to measure the layer strains.
Observed the instant of damage at any interface. Information was provided regarding the viability ofthis procedure, its applicability, and the interpretation of the test results. Strain monitoring methods
can minimize unnecessary or delayed repairs and, as a result, significantly reduce the danger of
inadvertent tile spalling.

Chapter 6
This chapter includes the major conclusions from Chapters 1 through 5 of the dissertations, aswell as recommendations for further research.

15

Chapter II LITERATURE REVIEW

Chapter II
LITERATURE REVIEW
2.1 ADHESION FAILURE OF FACADE CLADDING
Ceramic cladding facades consist of a substrate, a render, an adhesive layer, tiles, and grout
between the tiles. The properties of each layer result in different responses to environmental activities
and aggressive factors. As each layer in the system adheres to the other, the response of each layer also
affects the behavior of adjacent layers. Such differences in behavior are known as “differential
movement” between layers. If differential movement is restricted, internal stresses accumulate,leading to adhesion failure in debonding and cracking. Adhesion failure is a natural degradation
process, and several factors influence its progression, for example, the bond quality of the renderedmortar and concrete, environmental exposure, service conditions, and maintenance frequency [37][39]. Without interference from maintenance strategies, the adhesion quality represents the initialresistance to degradation. The degradants provided by environmental exposure trigger the operationof the degradation mechanism (differential movement) and determine the degradation pattern. Initialbond quality can be improved in various ways, and much effort has been made to do so. Althoughenvironmental conditions are uncontrollable, understanding the effects of various environmental
factors on facades can help decision-makers suit durability design to local needs and are essential forestablishing lab-scale durability assessment methods. The following is a survey of the relevantpublished studies.

2.1.1 Adhesion of the rendering mortar to the substrate
Adhesion failure can occur at the bonding interface of the layers of material, and someresearchers have suggested that the interface between the bonding mortar and the tiles may be theweakest in the entire tiling system [30] [31]. The authors offer a different view, suggesting thatadhesion failure mainly occurs at the bonding interface between the two cementitious materials, i.e.,
between the rendering mortar and the concrete substrate or between the rendering mortar and the tileadhesive. Firstly, special tile adhesive mortars add more polymers to meet the bonding requirements
and lead to a higher modulus of elasticity. The thickness of the bonding mortar is usually 1/5 to 1/10of that of the rendering mortar, and the more excellent deformability and smaller thickness reduce the16Chapter II LITERATURE REVIEW

difference in movement between the layers. Secondly, to increase the contact area with the adhesive,
manufacturers create textures on the underside of the tiles, and these grooves form a solid mechanicalfit. In addition, organic adhesives with strong deformability have been used extensively in recent years.
Conversely, rendered mortars are more prone to bond failure due to their lower performancerequirements (mainly to provide a smooth surface for the facade).
Rendering mortars made with Portland cement have some disadvantages. These disadvantages
include 1) low tensile, flexural, shear, impact, and adhesive strengths, 2) large drying shrinkage, 3)tendency to crack with changes in temperature and moisture, 4) relatively high moisture absorption
and permeation, 5) low resistance to corrosive agents, and 6) rapid loss of gauging water in thinsurfacing by evaporation and substrate absorption [32]. The dominance of rendering is mainly due to
its low cost and the relative lack of expertise in execution compared to other claddings and coatings[33]. Low investment often means an unacceptable level of degradation due to restricted performanceand execution defects.
Weak adhesion can significantly accelerate the degradation process. Hence the adhesion behaviorof mortars has been extensively studied. This section investigates the adhesion mechanisms of mortars
and discusses the factors influencing the adhesion between rendering mortars and concrete substrates.
However, the understanding of adhesion mechanisms is not limited to concrete substrates. It can beextended to the adhesion of mortars to all porous materials such as stone, ceramics, masonry, and evenwood.

(1) Adhesion mechanism of mortars“Adhesion” describes joining two materials with the same interface in a boundary layer. Kinloch[34] proposed five theories to explain the mechanism of the formation of intrinsic adhesion, including
mechanical interlocking, diffusion theory, chemical bonding, electronic and adsorption theories. In
any adhesion system, several adhesion mechanisms often act in concert, and it is difficult to identify
the individual contribution of each mechanism. The interpretation of the adhesion mechanisms
between two contacting materials is complex, with the structure and properties of the object and theadhesive determining how these mechanisms occur [35]. The mechanical interlocking and chemicalbonding theories are the commonly proposed adhesion mechanisms on mortar to concrete.
- The theory of mechanical interlocking, where fresh mortar penetrates defects (porosity androughness) in the surface of the concrete substrate, occurs after hydration.
- In chemical bond theory, where the degree of adhesion of a mortar is defined by the primary17

Chapter II LITERATURE REVIEW

and secondary chemical bonds formed at the interface. The bond strength depends on the contact
between the phases. The greater the contact, the greater the bond strength.
Mechanical interlocking is considered to be the main contribution to bonding forces [36] [37][36]. Based on the magnitude of concrete surface defects in mechanical interlocking theory, theresearchers divided the adhesion of mortar into microscopic adhesion and macroscopic adhesion.
Microscopic adhesion means that the mortar penetrates the pores on the surface of the substrate andforms a tiny "anchor" after hydration. Macroscopic adhesion is where the mortar fills the texture on
the surface of the substrate, and the substrate holds the mortar projections in place, creating amacroscopic "bite." [36] The adhesion effect depends on the ability of the mortar to penetrate thesubstrate and the contact area of the interface.
Chemical bond also contributes to adhesion and, in particular, can explain the enhanced bondstrength effect of adding re-dispersible powders polymers. Adhesion is understood to be the result ofmolecular attraction between the phases. Van der Waals forces are mainly formed between material
molecules and are responsible for the bonding of multiphase materials [38]. Sakai and Sugita [39]investigated polymer-modified cement and mortar's microstructure and compounding mechanism. It
was concluded that the improvement of mortar adhesion ability was related to the formation ofpolymer films at the interface. Mansur et al. [40] investigated the effect of EVA (poly (ethylene-covinyl acetate)) re-dispersible powders content on the mortar/tile interface by concluding that theenhanced adhesion was attributed to the formation of a hybrid ceramic-polymer interface based onhydrogen bonding between the silanol groups on the tile surface and the hydroxyl groups of thehydrolyzed EVA. The researchers concluded that the chemical interaction between the cement and thepolymer compound, the formation of polymer films, and the formation of hydroxyl groups duringhydrolysis in the polymer are beneficial for adhesion.
(2) Factors affecting the bond between mortar and concrete
Adhesion results from the interaction between two materials and are subject to many material
parameters and environmental conditions. Carasek et al. [41] divided the proven factors into five maincategories, as shown in Fig. 2-1. These are a) the characteristics and properties of the substrate, b) the
characteristics of mortars and their constituent materials, c) the mortar’s application technique, d) the
climatic conditions at the time of execution and the lifetime of the render, and e) the time span after
mortar application.

18

Chapter II LITERATURE REVIEW

Figure 2-1 Factors which exert influence on bond, according to [41].
These factors act in two directions, one affecting the strength of the mortar and the substratethemselves and the other involving the contact between the mortar and the substrate. The strength of
the substrate and the mortar itself also influences the bonding properties, but it is inefficient. Based on
the mechanical interlocking theory, the contact form bonding interface is the decisive factor.

i) Short-term bond strength
The formation of microscopic adhesion depends on the water absorption of the substrate, theporosity, and the water retention capacity of the mortar. The balance between the water absorption of
the substrate and the water retention of the mortar has a strong influence on the bond strength. In
combinations of highly absorbent substrates and low retention mortars, excessive water release fromthe mortar leads to insufficient hydration of the mortar near the interface and a reduction in strength[42] [37]. In the combination of a low absorbent substrate and a high retention mortar, the low intrusion
rate of the mortar into the substrate hinders the formation of microscopic adhesion [37]. In addition, a
low absorbent substrate means that the bonding interface will have a water-secreting layer and a highwater-to-cement ratio at the interface, resulting in a meager mortar strength close to the interface area[43]. The water retention capacity of mortars is mainly influenced by the volume of fines in the mixture.
Paes et al. [44] concluded that mortars produced with larger aggregate particles retain water poorly
and favor water transport from the mortar to the substrate.
The macroscopic adhesion depends on the ratio of the effective adhesion area to the potential
contact area [36]. It is directly related to the interface roughness and the rheological properties of themortar. The interface texture determines the possible contact area, while the rheological properties ofthe mortar determine its ability to penetrate the interface texture, i.e., the effective contact area. A

19Chapter II LITERATURE REVIEW

mortar with low workability will have difficulty filling irregular parts of the substrate, and voids andmicrocracks at the interface will significantly reduce the bond strength [45]. Furthermore, insufficient
contact area accelerates degradation and adversely affects long-term bonding performance. Melo et al.
[46] state that a 30% reduction in contact area may reduce the system's ability to withstand deformation
by more than 70%.
The application technique of the mortar is also a decisive factor. The vast majority of short-termbond failures are the result of poor construction. These aspects include the amount of mortar per unit
area, plastering pressure [47], open time [45] [47], curing method of the mortar [48] [49], pre-treatmentof the interface (degree of wetting and cleaning) [50] [51], and bond agent use [37]. The five main
factors that Silfwerbrand [52] identifies as affecting bonding performance are almost all related to theconstruction technique; the microcracking, laitance, and cleanliness of the concrete, and thecompaction and curing procedures of the overlayer. The first three parameters are related to the pretreatment of the interface. The treatment of concrete substrates is usually used for cleaning, removing
scaling layers, and roughening the interface. However, improper handling can induce microcracking[53], [54], [55], such as hammering and pickling [56]. The use of the correct tools and quality control
during execution is critical.

ii) Long-term bond strength.
Most studies mentioned above relate only to short-term bond strengths, which are generally tested
within six months of the mortar curing. This may be because long-term bond performance is influenced
by the factors mentioned above, but also by environmental factors that must be considered. The
inherent stochastic nature of natural phenomena makes studies relating to assessing and predicting
long-term bonding performance involve much uncertainty. At the same time, environmental exposure
experiments are limited by geographical characteristics and are time-consuming. However, fromstudies relating to repair mortars and bond degradation, it is possible to summarize the two properties
that are most influential in long-term bond performance, the ability of the substrate and mortar to resist“differential movement” and the resistance of the mortar to crack. [7] [57] [58] [59] [60].
Wetzel et al. [59] observed the development of bond failure at the glass-bonded mortar interface
through glass blocks. They concluded that interface damage was caused by initial shrinkage andthermal cycling and was exacerbated by wet thermal cycling and water ingress. Tran et al. [61]simulate debonding propagation at the interface between a matrix of old concrete and a thincementitious bond. Granju [60] also concluded that adding fibers inhibited the thin overlayer's

20Chapter II LITERATURE REVIEW

cracking, thereby increasing the adhesion durability to the substrate. Jenni et al. [45] suggest that using
a low modulus of elasticity polymer-modified mortar between the finishing material and the substrate
can significantly reduce the stress concentration at the bonding interface. Al-Ostaz et al. [62]investigated the durability of the bond between the repair material/concrete using hot-cold cycle aging
tests. It was concluded that there was a strong correlation between the incompatibility of the thermal
expansion and the modulus of elasticity and the reduction in bond strength. In particular, the modulus
of elasticity and the coefficient of thermal expansion of the overlay are critical properties affecting thelong-term bond performance under temperature fluctuations compared to the base concrete.
It is considered that mortar cracking also results from the fact that its deformation is constrainedby the substrate, whether by drying and shrinking or by thermal expansion and contraction. It cantherefore be argued that the "followability" of the mortar to the substrate's deformation determines theadhesion durability.

2.1.2 Assessment and measurement of adhesion
Descriptions of the factors influencing adhesion are often contradictory in the literature, which
the authors consider dependent on the adhesion assessment method. Generally, testing the bondstrength, i.e., the stress required to separate the substrate from the overlayer. There are various methodsof testing the bond strength, e.g., whether to assess the shear strength or the tensile strength of theinterface. It also depends on the location of the fracture, i.e., inside the mortar (mortar cohesion failure),at the interface (adhesion failure), or inside the concrete (concrete cohesion failure), corresponding to(a), (b), and (c) in Fig. 2-2 [64], respectively.

(a) within the substrate(b) at the adhesive interface(c) within the overlayer(d) at the epoxy/equipment
interface

Figure 2-2 The schematic illustration of the locations of the rupture surfaces during thepull-off test, according to [64].
21Chapter II LITERATURE REVIEW

In-situ tests“Pure” shear and shear andcompression tests

Direct and indirect tension
tests
Figure 2-3 Schematic description of different experimental tests to assess the bondstrength between two materials, according to [63].
(1) Bond strength test method
Common bond strength test methods include interfacial shear, torsion, and tensile testing. Bondstrength values obtained by different test methods can vary considerably, as the results depend on
specimen size, test set-up, loading rate, etc. In the literature, these methods are often used in
combination to study the response of a bond interface subjected to different stress states. Espeche andLeón [63] summarize the commonly used method. There are three main groups: tension, ‘pure’ shear,and shear and compression-based tests (Fig. 2-3). The first group includes the pull-off test (a), thedirect tension test (l), and the splitting tension test (m, n). The second group is broader and includes
the torsion bond test (b), the direct shear test (c), the modified vertical shear bond test or compact
shear test (f), the push-out test or push-through cube (j), the bi-surface shear test (k), and the guillotine
test (g, h, i). The third group consists of the slant shear test (e) and the shear-compression test (d) [63].
The direct pull-off test (a) is the most popular because it can be performed in situ. However, the test
results often show a large dispersion because the coring process and fracture location influence them,as shown in Fig. 2-2 [64]. Shear tests are usually performed in the laboratory and the results are
relatively stable. The disadvantage of most common shear test methods is the presence of interfacialbending moments due to the eccentricity of the force [65].
Different views have been presented in the literature regarding the correlation between the results
22Chapter II LITERATURE REVIEW

obtained by different test methods. Júlio [55] et al. reported a linear convergence between the results
obtained by the slant shear test and those obtained by the pull-off test. The torsional bond strength testdeveloped by Silfwerbrand [66] (Fig. 2-3 (b)) is considered a situ-applicable shear strength test
method. And reported that the ratio between torsional shear bond strength and tensile pullout strength
was between 2 and 3. Bentz et al. [50] suggested that the differences in results between the different
testing methods were related to the microstructure at the bonding interface of the samples tested. As
shown in Fig. 2-4, when the substrate surface was in a relatively dry state, the friction provided by the
cement particles that were not fully hydrated in the overlay was able to increase the slant shear strength
but contributed less to the tensile strength. In contrast, a wet substrate surface contributes more toenhanced bonding under tensile loading.

(2) Fracture location - transition zone at the bonding interface
Dispersion of fracture locations is mainly seen in drawing tests, where the fracture is thought tooccur at the most vulnerable location of the specimen. However, in the case of “interface failure,” theactual fracture location is often not examined [65]. Considering the mechanical interlocking theory, it
is almost impossible for the separation of the overlayer and the substrate material to occur without
damage. Bond failure is also not really to occur at the interface because the mortar can't be pulled out
of its original anchoring position after it has cured and formed a mechanical bond. Therefore,
researchers have investigated the bonding interface of composites at a microscopic level to get
information on the weakest points.

Figure 2-4 Schematic of differences in pull-off test vs. slant shear test with respect todirections of gravity and loading, according to [50].
23Chapter II LITERATURE REVIEW(a) the model of interfacial zone [68](b) The three zones-two layers model of new-to-old
concrete bonding [69]Figure 2-5 Transition zone models of bonding interfaces in the literature.
Pigeon and Saucier [67] point out that the interface between old and new concrete is very similar
to the bond between aggregate and cement paste. Due to the wall effect, a transition zone similar to aninterfacial transition zone should also exist between the overlay and the substrate. Xie et al. [68] further
explained the microstructure of the transition zone and proposed a model, as shown in Fig. 2-5(a). The
hardened paste at the bonding interface was divided into a penetrating layer, a strongly affected layer,
and a weakly-affected layer at the microscopic level. The first and third layers are part of the substrate
and overlayer, respectively, and have a small influence on the bond strength properties. The second
strongly influenced layer is located at the interface and is considered the weakest link. The local
increase in the w/c ratio induced by the wall effect results in the porous qualities of this layer, with
Ca(OH)2 and needle-like AFt crystals being the main product phases. The thickness of this layer isinfluenced by the age and porosity of the matrix and the w/c ratio of the overlayer. Yan et al. [69]distinguished the transition zone into a reaction layer and a permeable layer on the overlay andsubstrate sides, respectively, as shown in Fig. 2-5(b). The enhancement of the bond strength byinterfacial roughness and interfacial adhesives was explained accordingly. An increase in interfacialroughness thickens the permeable layer. The bond agent applied to the interface forms a polymer film,
making the reaction layer denser and stopping the cracks' extension. Beushausen et al. [70]
investigated the effect of the wetting of the substrate surface on the microstructure of the transitionzone. An overlayer of approximately 100 µm in thickness at the interface is defined as the transition
zone, and this part of the pore structure is distinct from the cover layer. It is considered to be theweakest part. Beushausen et al. pointed out that the conventional practice of pre-wetting the substrateleads to an increase in the porosity of the transition zone. This reduces the bond strength.
The observation of the microstructure of the transition zone can explain the mechanism of action

24Chapter II LITERATURE REVIEWof the factors influencing bond strength. The location of the fracture in shear bond strength tests almost
always occurs in the transition zone, except in the case of microcracks in the substrate caused by
improper handling. The present authors believe that the shear bond strength is more representative of
the actual situation in the structure.

2.1.3 Environmental effects on facades
The environmental factors with the most significant impact on the facade correspond to the local
climate. In this thesis, we focus on those studies in areas with an environment similar to Japan's, withclear seasonal distinctions. Except for some particular domains (e.g., extreme proximity to pollutants
and the sea), most studies support sunlight and rainfall as the main degradation agents. Still, theweighting of these two degradation agents is controversial.
Silva and Brito [11] analyzed 107 reports summarizing the influence of climate on the estimatedservice life of facades. The subtropical monsoon climate (where Japan is located) was the lowest. They
attributed this to the high frequency of rainfall (no significant dry season), with higher humidity being
the leading cause of defects in rendered facades. Gaspar and de Brito [19] classified the impact ofdefects in rendered facades into five classes to determine their condition level and overall degradation.
The area of the facade affected by rainwater infiltration determines the severity level, and the area
involved in the worst condition should exceed 30% of the cladding area. Rainwater erosion largely
influences the level of building degradation.
Pereira [71] argues that the mere moisture absorption of the material does not cause severe
defects to the facade. Problems related to humidity do not occur independently. Weathering is due tosalts carried by rainwater absorbed by the mortar, which attacks the composition of the cementitious
material (through carbonation, chloride, or sulfate attack), destroying its physical properties. Wetzel[59] et al. conducted a long-term field study on the evolution of the damage mechanism of bonded
tiles under naturally occurring combinations of moisture and thermal loads. They proposed three stepsof bond failure: 1) initial cracking, 2) crack extension, and 3) crack enlargement. The initial drying
shrinkage and thermal cycling caused cracking, followed by water entry, allowing rapid crack
extension and enlargement under wet and thermal cyclic loading. Maranhão [72] et al., however,argued that even in the absence of defects, the increase in mortar water content due to condensation is
sufficient to cause a substantial reduction in deformability and modulus of rupture. They analyzed
polymeric mortars used for tiling, where a moisture content of more than 6% was sufficient to reducethe deformability of the mortar by 50%, which would significantly reduce the resistance of the mortar
to other degradation agents.
25

Chapter II LITERATURE REVIEWThe influence of humidity and temperature is difficult to discern with an extensive range ofstatistical results. But the frequent observation that different parts of the same building deteriorate atdifferent rates provides some information. A statistical analysis based on tile failure incidents acrossJapan concluded that wall orientation and the application of dark tiles were the main risk factors forbond failure. North-facing tiles are less likely to fail than other orientations, which may depend on the
maximum temperature difference between the tile surface and the bonding interface. The north wall istypically 1 °C, while the other orientations can reach a maximum of 6 °C [73]. Zurbriggen andHerwegh [31] studied the failure evolution of a typical tile system under daily and seasonal thermalloads. It was clearly shown that tile bond integrity depends on exposure to solar radiation and diurnal
temperature differences. The fastest crack propagation rates were observed for southern walls withintensive solar radiation and early spring and winter when temperature variations were most significant.
Bauer and Souza [15] et al. reported a clear orientation dominance of facade degradation for buildingsin Brazil. Degradation rates were more blocky for north-facing walls exposed to a large amount ofsolar radiation. Consoli [23] et al. noted similar behavioral phenomena between facade failures ofbuildings, where the highest frequencies and strengths were located at the top and southern facades ofthe buildings. Rashid [74] et al. investigated the effect of moisture and temperature on the adhesion
between polymer cement mortar/concrete in the laboratory. The interfacial tensile strength of thecomposites decreased significantly with increasing temperature, while the effect of moisture wasminimal.
The literature shows that there is an interpenetrating relationship between the effects oftemperature and humidity. One defect is a consequence of the former phenomenon and, simultaneously,
a cause of the latter. Macroscopically solar radiation seems to be the more critical degrader, and thethermal expansion mismatch between materials significantly influences adhesion failure. However, adistinction must be made between the main degradation agents in different building parts. According
to investigations by Gaspar and de Brito [19], continuous facades (without opening sections) do nothave the same degradation pattern as corners and edges. Corners and edges are susceptible to thedifferential movement of the whole building and are more likely to lead to cracking than to loadconcentration problems. On the other hand, different parts of the same building react differently to thesame degradant. Zurbriggen and Herwegh [31] pointed out that for the case of slow long-termcooling/heating (not exposed to direct sunlight), since no thermal gradient in the cross-section isformed, the expansion/contraction between the different parts depends directly on the difference in thethermal expansion coefficient. In contrast, when only the tiles are allowed to cool rapidly,

26

Chapter II LITERATURE REVIEW

cooling/heating, the addition of temperature difference brings about a more intense differential
movement.

2.2 APPROACHSYSTEMSTOCONSTRUCTING

DURABILITYASSESSMENT

According to Lewry and Crewdson [75], the critical steps in durability assessment include: 1)
Factors affecting durability; 2) Defining key degradation factors for accelerated testing, and themechanism of that degradation; 3) evaluation of the significant causes of degradation in the proposedend use environment; 4) construction of models and damage functions for the material's response to
the environment based on the accelerated test results and the environmental measurements. Lab-scale
durability assessments are done by simulating degradation mechanisms and accelerated aging cycles.
Under normal use conditions, building components usually do not fail or considerably degrade withina reasonable testing time. Therefore, accelerated testing methods (ATs) are used in the laboratory topromptly assess or demonstrate the component or subsystem reliability. Specifically, AT experiments
are performed to accelerate the failure mechanisms to ensure that the product can reach the aging state
within a reasonable test period [76].

2.2.1 Degradation mechanisms of rendering mortars
Environmental degradation agents have already been discussed in Section 2.1.3. Rendering
mortars are more susceptible to environmental influences than concrete substrates due to their thinner

In-plane direction

Out-of-plane direction

Figure 2-6 Deformation of rendering mortar.
27

Chapter II LITERATURE REVIEW

cover thickness and greater modulus of elasticity. The differential movement is, therefore, mainly due
to the deformation of the rendering mortar itself. In the case of adhesion failures, drying shrinkage andthermal expansion under solar radiation are considered to be the causes of initial interface damage[59]. Fig. 2-6 shows the four modes of rendering mortar deformation. In practice, these types ofdeformation often occur simultaneously. Deformation in the in-plane direction will lead to cracks andreduce the adhesion of the mortar. Deformation in the out-of-plane direction may lead directly todebonding.

(1) Effects of solar radiation and temperature variations
Facades in actual structures are constrained by structural elements such as beams and columns.
It isn't easy to have deformations that occur purely in the in-plane direction, as in Fig. 2-6 (a) and (b).
In the case of rendered mortars exposed to solar radiation, for example, when the rendering mortar
expands but is restricted in the plane direction, the deformation tends to develop in a less restrictive
sequence. It protrudes from the substrate due to the local temperature increase caused by sunlight, asshown in Fig. 2-7. As the temperature rises during the day, the mortar and substrate begin to movedifferently due to a mismatch in thermal expansion caused by two factors, the temperature difference
between the layers and the difference in material properties. Although this deformation recovers atnight as the temperature decreases, the reduction in bond strength between mortar and concrete causedby the deformation is irreversible, and the cumulative damage will lead to fatigue failure. In the worstcase scenario, due to the low tensile strength of Portland cement, cracks from expansion lead to theintrusion of rain and condensation into the interior, and the mortar can quickly spall.

Figure 2-7 Failure of adhesive unity under daily solar radiation.
28

Chapter II LITERATURE REVIEW(2) Effects of drying shrinkage
The change in the volume of cementitious composites due to water loss is defined as drying
shrinkage [77]. The alternation of wet and dry environments has little effect on mortars without surface
defects. However, long-term drying shrinkage can have a detrimental impact on mortars. Drying
shrinkage does not usually directly affect bond integrity, abnormally manifested by cracking, but can
reduce bond durability. To provide good workability, rendered mortars often have a sizeable water-tocement ratio and contain much more water than is required for the hydration of the cement, which also
leads to more severe drying shrinkage. Both external and internal constraints hinder the drying
shrinkage of rendered mortar. External restraint is due to its bond with the concrete. Internal restraint
is caused by the moisture gradient present in the mortar (as drying usually occurs on only one face)
until equilibrium with the humidity of the surrounding environment is reached. Local shrinkage isdirectly related to pore moisture; therefore, a shrinkage deformation gradient exists throughout thedrying process [78] [79], causing the rendering mortar to deform in an out-of-plane direction, with
deformation often manifesting itself as upward curling (Fig. 2-6(c)). This anomaly was frequently
observed in the corners and edges of external walls and floor slabs.
As shown in Fig. 2-8(a), the top surface of the mortar dries and shrinks, while the bottom surface
remains wet with a much smaller amount of dimensional change. As the shrinkage gradient rises, thecurling moment of the mortar increases, inducing tensile stresses in the mortar and substrate and may
eventually overcome the material's tensile strength, leading to cracking and debonding of the mortar[80]. To quantify the effect of curling deformation, Mauroux et al. [81] have proposed an evolutionary
test device for monitoring shrinkage during drying, quantifying the extension of cracks in mortars(location, opening, depth). They investigated the "free" drying shrinkage of mortars and the cracks inmortar/substrate composite specimens due to the limitation of drying shrinkage using the Digital image(a) Curl deformation schematic(b) Map of equivalent tensile strain (in m/m)

Figure 2-8 Peeling and cracking due to drying shrinkage in the mortar/concrete composite
specimen, adapted from [81].
29Chapter II LITERATURE REVIEW

Figure 2-9 Debonding due to curling deformation of the overlayer, corner lift (a) andexample of debonded edges (b), according to [82].
correlation (DIC) technique. As shown in Fig. 2-8(b), cracks were observed in the mortar constrainedby the substrate at a temperature of 25°C and a relative humidity of 30 ± 5%. Microcracks wereobserved on the surface of the mortar and at the adhesive interface (arrow). The shear stresses inducedby the shrinkage of the mortar result in debonding.
Chilwesa et al. [82] investigated four cementitious overlayer materials, applying them to a
concrete slab with dimensions of 1150 × 1050 × 200 mm, providing information on edge curling anddebonding. As shown in Fig. 2-9, significant debonding was observed along with curling of the cornersof the overlay (displacement in the out-of-plane direction). The most severely debonded specimen,
CMR1, showed debonding on the 6th day after the end of casting and debonded 82% of the overlayarea after 28 days (examined by hammering.) Chilwesa et al. noted that the incorporation of fibers and
SRA significantly inhibited the deformation of the overlayer and alleviated cracking and debonding.

2.2.2 AT methods for studying mortar adhesion durability
Humidity and temperature are considered relevant factors in the degradation of facades.
Therefore, many studies have established artificially accelerated aging cycles to assess the effect oftemperature and humidity on the durability of mortar bonds. The main ones are dry-humidity, freezethaw, cold-heat, and hygrothermal cycles combining both factors. Some representative work ispresented below.
Pigeon and Saucier [80] investigated the effect of different interfacial binders on the durabilityof new and old concrete bonds under three other aging conditions. The AT methods included thefreeze-thaw cycle, dry-wet cycle, and drying conditions. They suggested that specimens subjected tothe combined aging cycle showed durability problems not found when exposed to a single aging cycle.

30

Chapter II LITERATURE REVIEW(a) Heat-cold cycle(b) Heat-rain cycle

Figure 2-10 Hygrothermal aging cycle, the two most used AT methods, according to [84].
Yiu et al. [13] conducted a laboratory study on the weathering effects of external tile systems. For the
mortar-tile interface, heating-cooling plus wetting-drying cycles may reduce the shear strength bymore than 50% in the first 100 cycles. However, there was no significant difference in the samples
before and after weathering for the render-concrete interface. Curci et al. [83] assessed the behaviorof a tile system when subjected to temperature changes by applying a heating-cooling cycle to a largescale wall to accelerate aging. The heating was performed by emitting 450 W/m2 of thermal radiationfrom 15 halogen lamps onto the outer surface of the tiles, and the cooling was achieved by projecting
cold water at approximately 20°C from the specimen's top surface. Maia et al. [84] proposed adurability assessment method for thermal mortars applicable to multi-layer systems. Two ATs were
established based on specific European climatic conditions, including heat-cold (HC) and heat-rain(HR) cycles, as shown in Fig. 2-10. According to the results reported, the amount of water penetration
was more significant for the HC-only mortar than after HC+HR. This indicates that comparing the
hot-wet combination, the temperature-only load resulted in more material rupture.

(a) the aging cycles(b) Evolution of the adhesive strength

Figure 2-11 Evolution of the adhesive strength of the specimens during the hygrothermal
aging cycle, according to [85].
31Chapter II LITERATURE REVIEWThe study by Freitas et al. [85] also suggests that the moist heat cycle may not act as thedegradation agent. After the specimens were exposed to the aging cycle depicted in Fig. 2-11(a), an
increase in bond strength behavior was observed, as shown in Fig. 2-11(b). This may be due to thedispersion of the pull-out test results and may also indicate that the provided hygrothermal cycle does
not act as a degradation agent. Al-Ostaz et al. [62] proposed a thermal cycle-only AT method to exclude
the effect of humidity. The average bond strength between the repair mortar/concrete decreased by
17%, 25%, and 33% after 60, 120, and 180 cycles, respectively.
Most of the studies mentioned above have used a combination of aging cycles. The standardized
AT method specified in the Japanese Society of Finishing Standard M-101 [86] also combines
temperature and humidity degradation agents, as shown in Fig. 2-12.
1) Irradiate the infrared lamp for 105 minutes until the surface temperature of the specimens
reaches 70°C.
2) Sprinkle for 15 minutes (water temperature 15±5℃).
3) 1) and 2) as one cycle and lasting 300 cycles
This experiment was originally performed to evaluate the quality of the adhesion promoters usedin plastering mortars by testing the adhesive strength between the mortar and concrete. However,
several problems have been identified in practice. Specifically, the entire specimen is subjected to ahigh-temperature history and water supply, which may favor the hydration of the mortar. In otherwords, instead of acting as a degradation agent, the hygrothermal cycle may increase the bond strength.
Similar AT methods have the same type of limitations. Even if the original intention of the researcher
was to assess the aging behavior under temperature changes, it is difficult to strip out the moisturefactor by heating through thermal radiation and cooling through dispersed water. Even though they

Figure 2-12 Hygrothermal aging cycle according from Japanese Society of FinishingStandards M-101.
32

Chapter II LITERATURE REVIEW

can simulate the various degradation agents, artificial aging cycles have difficulty reproducing theirsynergistic mechanisms of action. At the level of the degradation mechanisms of rendered mortars, thesynergistic mode of action of temperature and humidity still needs to be clarified.
Furthermore, existing AT methods are unsuitable for studying solar radiation's effects on facades,
as no actual degradation mechanisms are reproduced. As shown in Fig. 2-7, the distribution of solar
radiation on a facade must be uneven, with temperature differences arising in both out-of-plane andin-plane directions. This means that the rendering mortar subjected to solar radiation is bounded by
the substrate, columns, and beams and by the mortar in the surrounding unheated areas. In this case,the thermally loaded mortar tries to deform in the out-of-plane direction, causing cracking anddebonding. In the literature, composite specimens of overlayer/substrate or surfaces of the overlayer
are often exposed to cyclic loading in their entirety, in which case the overlayer is only constrained
from the substrate. This results in the stresses generated at the interface being mainly in the in-plane
direction, with the edges of the specimen being the most affected by differential movement.

2.2.3 Degradation patterns of mortars
The general model for the durability assessment is that all specimens are tested for the extent towhich their performance degrades after a certain number of aging cycles. However, it is challenging
to elucidate the correlation between the rate of degradation at the laboratory level and the actual rateof structural aging. Although the different physical, chemical, and microstructural processes associatedwith the formation and evolution of the debonding mechanism of mortars have been studied in detail,the change in bonding properties with aging time has not yet been elucidated. Ideally, bond strength
would decrease linearly with the aging cycle, but this pattern is almost impossible in naturaldegradation. In addition, the degradation pattern may be inconsistent from specimen to specimen, andthe choice of which stage of performance testing is critical, which may alter the outcome of durability
evaluations. Current research into degradation patterns has focused on two levels, the degradation
pattern of the whole facade and the evolution of mortar adhesion failure, with some of the prominent
work described below.

(1) Degradation pattern of the facade
The study of facade degradation patterns is mainly based on visual and empirical judgment,
considering the action of various degradation agents and anomalies. Shohet et al. [87] systematicallyassessed the pathological cases of existing building facades (including render cladding, ceramic

33

Chapter II LITERATURE REVIEW

Linear pattern“Convex-Shaped”
pattern“Concave-shaped”
pattern

S-shaped patternFigure 2-13 Four typical patterns of deterioration paths, according to [87].
cladding, and stone cladding). They proposed four typical patterns of facade deterioration, linear,convex, concave, and S-shaped patterns, as shown in Fig. 2-13. The independent variable is time, thedependent variable is the component performance in failure conditions (CPFC), and 100% represents
a perfect condition.
Linear pattern – This pattern is typical in situations where a permanent deterioration agent exerts
a continuous and consistent impact on the cladding. Shohet et al. suggested that this pattern isrepresentative of weathering effects, such as damage from solar radiation.
“Convex-Shaped” pattern – Indicates physical failure of the entire facade system due to physicalor chemical phenomena. For example, degradation due to differences in shrinkage between mortar andsubstrate.
“Concave-shaped” pattern – represents the action of chemical and physical agents on claddings.
Such impact appears in cases of microorganisms or cyanobacteria development on render and on
natural stone.
S-shaped pattern – represents a deterioration mechanism that changes its intensity over time. Thispattern represents the possibility of some design flaws in the facade, with visually observabledegradation occurring shortly after construction is completed. The primary mechanism then becomesa physical reaction that takes longer to affect the strength of the material, and the degradation enters aperiod of peaceful development. When this process matures, the rate of deterioration suddenly
accelerates.
Other researchers prefer an 'S-shaped' pattern to describe the degradation process of the facade[19] [88]. Three distinct phases of cladding performance over time are considered.1) the first phase ispremature degradation, which occurs rapidly due to defects caused by the quality of materials andor/and construction; 2) the second phase is a slow development phase, which occurs due to the natural
aging process associated with exposure to environmental degradation agents, influenced by theconditions of use and frequency of maintenance actions; 3) the third phase is a rapid development

34

Chapter II LITERATURE REVIEW

phase, which occurs rapidly due to synergies between different degradation agents and thesimultaneous presence of defects; and 4) the third phase is a rapid development phase. Agents and thesimultaneous presence of defects, degradation progresses rapidly until complete failure.

(2) Degradation pattern of the adhesion
Considering debonding as a severe failure requiring immediate intervention, several studies havefocused on understanding, quantifying, and describing debonding damage mechanisms. Feldfogel andRabinovitch [89] [90] investigated the evolution of debonding mechanisms in the square, rectangular,
and round tiles in porcelain cladding under uniform heating. In all cases, two threshold loads played aturning role in the evolution of the debonding mechanism. After the first threshold load is reached, thedebonding mechanism starts abruptly and is accompanied by a slow and steady growth phase. Until
the second threshold load is reached, debonding is rapid but steady. After the second threshold load,
the debonding propagation becomes unstable, and the system will quickly break down and lose static
equilibrium.
Fig. 2-14 depicts the debonding area ratio Ad (debonding area / initial bonding area) about theuniform thermal difference Δθ. Until about 40 °C, no debonding seems to occur. Between 40 and 55 °C(point A1), a steady and slow evolution of debonding can be seen. From points, A1 to A2, a stable but"fast" debonding process is observed. A fracture occurs at the critical load Δθ= 75.53 °C. As theenlarged view in Fig. 2-14, between points A2 and A3, there is an increase in the debonding area ratio
but a decrease in temperature, indicating the unstable nature of the reaction at this stage. From aphysical point of view, the break at point A2 means that the structure is no longer in static equilibrium
for the same Δθ. The response becomes intrinsically unstable, and the structural system fails. After
point A3, the debonding process seems to stabilize and become slow again, and after the debonded

Figure 2-14 Evolutionary pattern of debonding of one tile, according to [89].
35Chapter II LITERATURE REVIEW

area reaches 80% (point A4), the authors do not follow it again.
This finding is consistent with the “S-shaped” degradation pattern described above. The
degradation rate is unlikely to be uniformly linear even when the specimen is subjected to a single
degradation agent. In another study, Cao and Chung [91] monitored the degradation of the bond
between old and new mortar by contact resistance measurements under cyclic shear loading. Fig. 215 depicts the fractional change in contact resistance with the number of cycles for a cyclic shear loadwith a shear stress amplitude of 1.21 MPa, 0.97 MPa, and 0.81 MPa with the adhesion becoming
smaller with larger values on the left vertical axis. The degradation of the bond under mechanicalloading also shows a three-stage “S-shaped” characteristic. It is therefore assumed that the renderedmortar will show a similar degradation pattern during the artificial aging cycle.

Figure 2-15 Evolution of adhesion under cyclic mechanical loading (shear stress amplitude
from left to right: 1.21 MPa, 0.97 MPa, 0.81 MPa.), according to [91].

2.2.4 Assessment of rendering mortar/substrate adhesion
For the bonding of rendered mortar to concrete, the rate of reduction in bond strength before andafter ATs is the basis for evaluating durability. The various bond strength test methods have been
described in detail in Section 2.1.2. It can be summarized from the literature that mechanical adhesion
under tension is very different from adhesion under shear. For example, high interfacial roughness
increases the shear strength, while mechanical bond strength in tension depends mainly on thetransverse anchoring in pores and textures [65]. When establishing a durability evaluation method, it
is essential to correctly select the bond strength test method for a given test parameter based on the AT
being designed.
In addition, almost all of the studies presented in Section 2.1.2 used overlayer and substrate ofcomparable thickness to produce the specimens. It is not a suitable sample pattern for evaluating longterm bonding performance. As described in Section 2. 1.1, long-term adhesion is strongly influenced

36

Chapter II LITERATURE REVIEW

by the differential movement, in which case the same thickness would result in an impractically small
degree of substrate restraint on the deformation of the overlayer. In addition to the issue of thickness,the researcher also pointed out the influence of the geometry of the samples on the results of theadhesion strength tests. For a pull-off test, the concentration of tensile stresses at the edges of square
samples compared to the more uniform distribution in round samples resulted in higher adhesion
values for the latter [92]. This shows that the dimensional design of the specimens has a significant
influence on the results of the performance assessment.
While laboratory-scale studies often use destructive strength tests to assess adhesion, destructive
testing is not appropriate in many situations in the field. And, because destructive tests cannot be
performed on the same specimen in succession, the results often exhibit a high degree of dispersion.
To overcome these drawbacks, researchers have developed several non-destructive testing (NDT)
techniques capable of assessing adhesion. This methodology is described in detail in Section 2.3.

2.3 NDT DELAMINATION DETECTION TECHNIQUESThe essential technique for maintaining and managing facade cladding is "delamination
detection." Claddings usually begin deteriorating with the delamination of tiny areas between themand the substrate. The potential for the delaminated areas to spall under sustained fatigue loading
conditions increases due to reduced adhesion; however, this minute degree of delamination is difficult
to detect by visual inspection alone. NDT is an ideal approach to detect damage beneath the surface.
Several researchers have applied NDT techniques to detect hidden delamination beneath facade
cladding. This detection method is essential for assessing the continued availability of facade cladding
after an earthquake, especially in areas that are difficult to reach. The following section describes someof the methods that have been proven to be applied in the field and those that currently remain at thelaboratory scale.

2.3.1 Field-applicable NDT methods(1) Impact acoustic inspection
Among the various NDT methods, the acoustic method offers the advantages of convenience andcost-effectiveness [93]. The method has been widely adopted owing to its high equipment andenvironmental compatibility and its relative reliability. In general, acoustic methods can be divided
into two main categories: acoustic emission (AE) and impact echo (IE). The difference between these37

Chapter II LITERATURE REVIEW

two categories is the source of the sound waves, with the AE method measuring elastic waves
generated by changes within the building. In contrast, the impact-echo method is based on monitoringechoes generated by external impacts, which do not cause any internal changes.
The impact sound detection method is based on the impact sound signal generated by a small
hard object striking the surface of a concrete structure [94]. In facade inspection, the conventional way
uses the information conveyed by the acoustic signal produced by a hammer or similar metal object
striking the surface of a tile, also known as “percussion inspection.” The inspector detects
abnormalities through the ear based on acoustic characteristics, such as sound pressure and frequency.
This method is prevalent because of its cost-effectiveness and operability. However, diagnosis ishighly dependent on the experience of the practitioner, and individual variation leads to a risk of
accuracy and difficulty in quantitative assessment. In addition, this inspection method has a limited
scope of practice, as the scaffolding and gondola required to survey large areas result in prohibitive
costs.
In recent years, the rapid development of climbing robots [93] [94] [95] [96] has broadened therange of applications for impact acoustic inspection. Several attempts toward automation have beenreported [93] [94] [97] [98] [99] [100] [101] [102]. The research has been focused on the extractionof effective parameters for characterizing defects from acoustic signals and their automatedidentification. The bonding integrity of the object under test can in principle be evaluated withinformation from both the time-domain and frequency-domain [99]. The former is based on theamplitude–time waveform of the impact sounds, while the latter is a power spectral density (PSD)
distribution pattern obtained by performing a fast Fourier transform calculation on the time–amplitude
waveform. Furthermore, the frequency-domain features based on PSD distribution for theidentification of defects is a general method, however is sensitive to environmental noise and theroughness of the measured object surface. Tong et al. [99] proposed a method for defectcharacterization by time-domain features extracted from the initial acceleration peak of the impact
sound, which can not only identify defects and is not easily affected by the texture of the test object
surface. Luk et al. [93] proposed an evaluation method based on wavelet packet decomposition that
overcomes the interference of environmental noise by identifying the outline features that constitutethe PSD pattern. The researchers inspected the facades of high-risk buildings with a cable-driven
climbing robot that examined 400 specimens. When the signal-to-noise ratio was greater than 2 dB, a
successful detection rate of more than 80% could be achieved. In these previous studies, machinelearning techniques such as artificial neural networks [93] [99] and support vector machines [94] [100]38Chapter II LITERATURE REVIEW

have been applied to extract features from the collected signals to achieve automatic recognition.
Although percussion inspection in Japan relies primarily on manual work, the research andpractice of automatic percussion detection systems are receiving attention. Non-contact acoustic
inspection techniques using unmanned aerial vehicles (UAV) have been developed [103] [104].
Although they have not yet been practically applied, it is foreseeable that acoustic inspection has apromising future.

(2) Infrared thermography
The intensity of infrared radiation emitted by an object is related to its absolute temperature. In
infrared thermography (IRT), the infrared specific wavelength signal of thermal radiation is detectedfrom the surface of the facade. Then the temperature distribution of the wall is converted into a visually
distinguishable image for human eyes. The voids created by debonding are filled with air or water,
resulting in the varied overall thermal properties of the wall [105]. Thereby, healthy and delaminatedareas are detected.
Unlike percussion inspection, IRT is non-contact. This method is widely employed for thequantitative evaluation of building diagnostics because of its suitability for recording and reporting,enabling non-contact detection of large areas, and providing visual images and numerical results [105][106] [107] [108]. Most importantly, the ability to simultaneously detect large areas makes IRT
appropriate for timely post-disaster assessment. The IRT approaches qualitative and quantitativeevaluations for building diagnostics to vary from passive to active [105] [108], the difference being
whether manual heating excitation is required. For multi-layered structures, active IRT provides morereliable results. However, for post-earthquake or high-rise buildings, conventional passive IRT using
optical excitation is generally preferred for both safety and cost considerations. This makes thedetection results inevitably subject to conditions, such as obstruction of the camera's view by adjacentbuildings or difficulties in creating a significant temperature gradient on the observed wall due toobjective circumstances (e.g., variations in natural heat sources, such as orientation and weather). In
addition, measurement accuracy is reduced by: variations in emissivity, color, and stain of the finishes;
air conditioning in the room; and contact elements between different materials, such as window frames
and balustrades [109]. Another restriction of IRT is that interpreting post-processing results requiresconsiderable experience, intuition, and judgment [110].
Multiple studies have reported factors that influence detection accuracy and estimated the extent
of influence for these factors, such as wind speed [108], indoor and outdoor temperature differences

39

Chapter II LITERATURE REVIEW[111] [112], tile type [113] [114], and surface color [115]. Meanwhile, the development of thermal
image analysis techniques has improved the detection accuracy of passive IRT. Lourenço et al. [105]reviewed several studies addressing the latest IRT as a diagnostic technique for building facades.
Effective diagnostic methods, both quantitative and qualitative, were revealed, affirming the feasibilityof detecting facade defects through passive IRT techniques. Sato et al. [116] analyzed thermal images
obtained by passive IRT using self-reference lock-in thermography (lock-in processing of random
temperature fluctuations). They verified the improvement in the detection accuracy of tile
delamination. Edis et al. [117] analyzed the time-dependent thermal behavior of defective areas bysimple image subtraction, principal component analysis (PCA), and non-negative matrix factorization(NMF) as different quantitative methods and compared the potential of these approaches in detecting
delamination. They concluded that PCA and NMF have great potential for the time-dependent analysis
of IRT data, as almost all stratifications were checked. Moreover, techniques have been proposed thatverifiably reduce reflected noise and help interpret results [114] [118].
In recent years, with the maturity and usability of UAV technology, the difficulty in inspecting
high-rise buildings is expected to be overcome. Pan et al. [119] applied forward-looking infrared
technology and high-resolution photographic equipment to a UAV to develop a system suitable for the
initial visual assessment of exterior walls. Such advancements in UAV technology have increased therange of IRT applications, specifically those in unsafe and inaccessible locations. However, thefundamental problem is that sufficient temperature variation in the target wall is a prerequisite for
implementing IRT.

(3) Strain monitoring method
As described in Section 2.1, strain differences between each layer induce delamination of facade
cladding; moreover, the occurrence of delamination can theoretically be detected by monitoring thestrains in the components.
Several studies have been conducted to measure strain in ceramic claddings. Yiu et al. [13]measured strain at the tile-rendering-concrete interface using strain gauges in a laboratory AT.
Although the aim of this experiment was not to investigate adhesive degradation by monitoring strains,
the response of the layers of material to weathering could be observed from the strain histories, as
shown in Fig. 2-16. Murai et al. [120] measured the ultimate strain in concrete using strain gauges andconcluded no risk of tile delamination below this limit. Tanigaki et al. [121] assessed the delaminationof the tiles from the concrete substrate during loading using strain differences. They reported that the40

Chapter II LITERATURE REVIEW(a) Location of strain gauges(b) Cyclical strains of the three surfaces

Figure 2-16 Monitoring of interlayer strains in composite tile-rendering-concrete specimens
under artificial aging cycles, according to [13].
deformation consistency of the tiles to the substrate decreased as the specimens underwent wet-dry
cycles. Nachi et al. [122] proposed the deformation-compatibility capability of tiles as a designindicator, which was obtained from the strain transmissibility between the concrete substrate and thetile surface. Okihashi et al. [123] monitored the strain behavior of concrete substrates and tile surfaces
in exterior-exposed environments on real-scale wall specimens. They showed that the repeated strain
differences between the two were caused by wet–dry and heat–cool cycles. Moreover, a method toassess the future exfoliation risk of existing wall tiles has been proposed by assessing the strain on
tiles in existing buildings, as measured by strain release tests [124]. Chang et al. [125] [126]demonstrated the applicability of strain measurement techniques to measure the deterioration of tiled
facades. They combined strain measurement results with acoustic diagnosis to map the deterioration
symptoms diagram of square tiles, affirming the feasibility of this technique for future smart building
development.

(4) Vibration
In this approach, the target structure is impacted, and the movement of claddings is integrated if
adhesion is adequate. Conversely, if delamination exists, the vibration response of claddings likely
differs from that of the substrate. Therefore, the delamination of facade claddings can be determinedby detecting differences in the vibration response.
Ohgishi et al. [127] showed that the degree of delamination for the finishing material can be
identified by the vibration acceleration index. Iwase [128] showed that the peak frequency of vibration
for delaminated tiles is approximately 20 to 30 dB higher than that for healthy tiles. That partiallydelaminated tiles showcased such vibration characteristics only in debonded areas. Takano et al. [129]41

Chapter II LITERATURE REVIEW

visualized the vibrational state of a tile surface using wavelet analysis and proposed a threshold forthe delamination state. Soeda et al. [130] derived the relative maximum amplitude from the impact
sound waveform. They noted that the relative maximum amplitude in a healthy region is greater fortiles adhered with organic adhesives than for tiles with mortar, which may alter the threshold.
This method of detecting delamination based on vibration characteristics is well-proven, but it
has the same disadvantages as the percussion method in terms of its limited detection range. Wheninspecting ceramic coverings, for example, current practice requires that all individual tiles arevibrated.

2.3.2 Laboratory-applicable NDT methods
The NDT methods described in Section 2.3.1 are, of course, also applicable in the laboratory.
Still, a part of the measurement accuracy is often sacrificed to meet the operability in the field. For
this reason, researchers have also developed other NDT methods for internal damage detection.
Although these methods have some drawbacks that make them temporarily unsuitable for field
diagnosis, their potential cannot be ignored as they perform well in laboratory tests, characterizing
internal concrete damage and detecting reinforcement corrosion and adhesion failure in composite
materials.

(1) AE method
AE is another commonly used method of acoustics. Mechanical energy is generated wheneverany object or building is subjected to pressure; this energy is emitted from elastic waves. Usually,
cracks in a structure emit some characteristic frequency of "unhealthy sound" as they propagate, which
can be tracked to determine the location and extent of crack development. In some cases, AE in a
structure needs to be detected using an ultrasonic detector.
AE is often used to follow the transition from a solid to a damaged state and to provide a damage
assessment of concrete and cementitious composites [131]. Grazzini et al. [132] conducted static
compressive tests on composite specimens of stone-repair mortar. They analyzed the evolution of themortar shedding process in a coupled stone-block-mortar system by triangulation of the AE signal,
assessing the adhesion surface energy released by fracture propagation. They report that the AE signal
is related to the energy emitted by the specimen during the delamination phase between mortar andstone. The variation of the AE parameters depends entirely on the specimen of the fracture pattern.
The AE signatures are effective for detecting early damage propagation but are also limited by cost42

Chapter II LITERATURE REVIEW

and detection area, with few examples of application to facade cladding.

(1) Ultrasonic Testing (UT) method
UT is one of the most widely used NDT methods for damage assessment. Although both require
the use of an ultrasonic detector, what distinguishes UT from AE is that it is an active process in which
high-frequency sound is transmitted into the object by the instrument, interacting with the attenuation
or reflection characteristics present in the material. Tan et al. [133] evaluated the bonding state of
cement and tiles using the reflected signal amplitude of pulse echoes, which was able to correspond to
the results of strength tests. Meola et al. [134] compared the ability of IRT and ultrasonic techniques
to inspect internal defects using simulated debonded plaster-marble specimens. In addition, ultrasonictomography was widely applied to locate the thickness and defect areas of concrete and to assess theeffectiveness of the masonry structures repair [135]. Wave velocity variations in tomograms were
found to be able to reflect damage or changes in brick and mortar, specifically voids and cracks.
Conventional ultrasonic testing should use a coupling fluid or gel to ensure good contact between theprobe and the structure's surface, making it more suitable for the laboratory. Recently, the air-coupled
ultrasonic (ACU) technique has attracted much attention due to its being more feasible for bothmaterial characterization and defect detection [136]. It removes the inherent limitation of contact
ultrasonics by using air as the coupling medium and provides an efficient, noncontact inspection
resolution. However, probably due to the complexity of the thickness and composition of facades, theACU field has not explicitly focused on the application of damage detection to facade claddings.

(2) Other methods
In addition to these methods, electromagnetic testing (ET), ground penetrating radar (GPR), laser
testing methods (LT), and electrical resistance measurement have also been attempted to assess theadhesion of cementitious composites. Osaki et al. [137] examined the adhesion of tiles to concrete
walls using radar reflectometry. The basic principle of the measurement is that electromagnetic waves
incident on the surface of a tile obtains different reflectivity due to differences in the physicalproperties of the layers of material. The reflectivity between tiles and the air is much greater thanbetween tiles and adhesive and between adhesive and concrete; GPR is mainly used to detect
underfloor damage in historic buildings; The amount of deformation of the finish material in the outof-plane direction (bumps represent delamination) can be obtained using laser scanners to profile thesurface of the cladding, a technique often combined with UAV for virtual reconstruction and43

Chapter II LITERATURE REVIEW

restoration of large and complex cultural sites [138] [139] [140]; Cao and Chung [91] measured thecontact resistance between old and new mortar to estimate the bond strength under cyclic loading,where the contact resistance increases abruptly upon bond failure, as shown in Fig. 2-15.

2.4 SUMMARY
This chapter reviews the literature and research relating to adhesion between mortar/concrete,
focusing on three main areas covered by the objectives of this thesis, namely 1) degradation factors of
rendering mortars and adhesion failure mechanisms; 2) durability assessment models for
mortar/concrete composites; and 3) non-destructive debonding detection techniques. The adhesion
mechanism of mortars, the factors affecting adhesion, and the adhesion failure mechanism are
explained, and the basis for solar radiation as the leading environmental degradation agent isdetermined. In addition, the general process of establishing a durability assessment method is
described, and the components of the durability assessment method are detailed. The AT methods
currently employed to simulate weathering, as well as their effectiveness and potential misconceptions,are outlined. The advantages and limitations of generic bond strength testing methods are alsopresented. The application of NDT methods for delamination detection, particularly for building
envelopes, is also reviewed, with several examples of published works and an overview of future
trends in the application of NDT in facade maintenance. The following conclusions can be drawn:(1) The initial interface damage is caused by differential movement between the rendering mortar
and the adjacent material, resulting from a mismatch in the amount of expansion/shrinkage caused bysolar radiation and the drying and shrinking of the material. Subsequently, rainwater can intrude along
defects in the material surface, further exacerbating the interface damage. Therefore, while solar
radiation and rainwater are important environmental degraders, this work focuses on the initialdegradation mechanisms, examining both solar radiation and drying shrinkage (ignoring the externalmoisture factor).
(2) The factors influencing short-term bond strength and long-term bond strength are different.
The former depends on forming a strong mechanical bond between mortar and concrete; the latter
relies more on the mortar's ability to follow the concrete's deformation. The dry-mix mortar industry
is now mature enough to achieve adequate short-term bond strengths, provided that the quality of theprocess is up to standard. Manufacturers should consider long-term bond strength as part of their44

Chapter II LITERATURE REVIEW

product design, but there currently needs to be a standard method for assessing long-term bondstrength.
(3) In the part of the study presented, the AT method for simulating weathering did not achieve
the expected accelerated degradation. However, the two variables of ambient temperature andhumidity were adequately considered. This is mainly related to two factors, 1) needing to adequately
consider the constraining conditions in the actual structure and 2) failing to reproduce the collaborative
mechanism of temperature and humidity. A new AT method needs to be proposed to improve these
deficiencies.
(4) When selecting a bond strength test method, it's essential to consider the prevailing interfacial
stress conditions caused by each method, which may or may not be representative of the general stressconditions encountered in the actual structure. Also, consideration must be given to whether the
specimen is dimensionally designed to produce an appropriate stress state. Ideally, a test methodshould be developed to measure important bond strength parameters based on a relatively simple test
set-up and commonly used molds.
(5) Regardless of the loading conditions, the bond failure process of mortars does not proceedsteadily and goes through three distinct phases. In the last stage, the rapid development phase,debonding may occur suddenly. Therefore, the maintenance management model of periodic
assessment of the adhesion of claddings may lead to missed inspections. Technology needs to be
developed to continuously monitor the adhesive integrity of facade claddings.
(6) Only the strain monitoring method can satisfy continuous monitoring and field applicability
in the NDT methods presented. During the research phase, it is possible to study the degradation
process of mortar/concrete composites by monitoring the strain differences between the materials. The
study cases in the literature use resistive strain gauges to monitor strain. However, the physical size ofthe strain gauges can be the starting point for bond failure, which is rarely discussed. A more suitablesensor to monitor interlaminar strain should be selected.

45

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

Chapter III
ESTABLISHING A LAB-SCALE DURABILITYASSESSMENT METHOD FOR
MORTAR/CONCRETE ADHESION
3.1 PROPOSED LAB-SCALE DURABILITY ASSESSMENT METHOD
Durability assessment methods may consist of one or more of the following four tests [75]:-Benchmark testing; The criteria for passing or failing durability of a material under test after AT
is determined using historical data on the material's performance in a given environment.
-Reference material/comparative testing; When AT of unknown new materials is required, adurability ranking can be quickly obtained by comparing reference materials with known
properties.
-Environmental/stress testing; a) attempts to simulate and accelerate the entire service
environment; b) attempts to limit the material by enhancing its susceptibility to environmental
factors.
-Site testing; a) exposure to the field for testing; b) in situ monitoring of buildings.

This work has been designed for accelerated aging cycles based on principle (b) inenvironmental/stress testing, i.e., to enhance the most susceptible environmental factors. The
assessment of durability is done utilizing reference material/comparative tests. The methodologicalprocess for developing a durability assessment method applicable to rendering mortar/concrete
composites went through two phases, the development and implementation phases. This chapter
describes the development phase, where the validity of the proposed AT method is verified throughnumerical simulations and experimental testing. The implementation phase is described in detail in
Chapter 4.

46

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

3.1.1 Proposed AT method
The investigation of facade anomalies reported in the literature is reviewed in Chapter 2, andsolar radiation is considered the most critical environmental factor. Therefore, the proposed AT method
aims to simulate the degradation mechanisms triggered by solar radiation. At the same time, thesummary in Chapter 2 describes the shortcoming of the AT method used in the literature to simulate
solar weathering; that is, the method of applying thermal loads does not take into account the actual
conditions of the facade, resulting in a degradation mechanism that cannot be reproduced.
Considering the facade situation, the rendered generates temperature gradients in both planar andcross-sectional directions after being subjected to thermal loads. The temperature gradient in the plane
direction arises from the uneven distribution of sunlight (from shading and the limitation of the solar
height angle). The temperature gradient in the cross-sectional direction results from the difference
between indoor and outdoor temperatures and the difference in thermal properties between materials.
The methods used in previous studies to apply thermal loads to mortar/substrate composites wereunable to create temperature gradients in both directions simultaneously, either by heating an entire
specimen [13] [62] or by heating the whole surface of the mortar [80]. Therefore, it is necessary toprovide an appropriate restraint for the rendering mortar subjected to thermal load. As shown in Fig.
3-1, the uneven heating due to solarization will cause a temperature difference in the wall. The
temperature difference not only exists between the different layers but also exists in the in-plane
direction of each layer. The temperature difference combined with the physical difference of various
materials to occur the differential movement between layers. The rendering mortar in a real structure
is restrained not only by the substrate, columns, and beams but also by the mortar in the surrounding
unheated area. Therefore, it is necessary to mimic this limited state in an artificial aging model, which
has been overlooked in previous research. To reproduce this state, in the proposed approach, a thermal

Figure 3-1 Application of partial heating to reproduce the actual degradation condition.
47

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

load is applied only in the central part of the mortar, and consequently, the unheated surrounding
mortar provides restraint. The author has named this method the "partial thermal load (PTL) method."
Applying a thermal load to the specimen in a non-contact manner often results in an
uncontrollable temperature, such as halogen lamp irradiation [80], as the distance between thespecimen and the heat source is thus a key parameter. Ideally, the concrete substrate should be heatedpurely by heat transfer from the mortar, creating a temperature gradient across the thickness of thespecimen, which is similar to the condition of an actual building facing exposure to solarization. To
realize this, silicone rubber heaters with wire-wound elements can be placed directly in contact withthe mortar to provide thermal loading. Such heaters exhibit excellent physical strength and canwithstand repeated flexing without considerable deterioration in the service life and performance.
Consequently, such heaters can be placed in close contact with the specimen and cut into any shape.
In this work, high temperature-resistant double-sided tape was used to attach the rubber heater to themortar’s surface to avoid gaps. The setup was connected to the control equipment to control the

Figure 3-2 Rubber heaters for applying PTL.
temperature, heating time, and the number of cycles, as shown in Fig. 3-2.

3.1.2 Dimensional design of specimens and adhesion testing
As described in Section 2.2.4, the dimensional design of the specimens had a significant impact
on the results of the adhesion assessment. The difference in material thickness between themortar/substrate composite specimens in the literature surveyed is too small, resulting in an
unrealistically small degree of substrate constraint on the deformation of the cover layer during theartificial aging cycle. The dimensional proportions of the specimens should be close to the actual48

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

Mortar10 100Concrete
400

100
30 40 30MortarConcrete
Figure 3-3 Specimen dimensions.

Figure 3-4 Devices for bond strength tests.
structure and based on commonly used molds. Therefore, specimens like those shown in Fig. 3-3 areproposed. The concrete substrate was cast in the most used rectangular cast iron mold with dimensionsof 100x100x400 mm, the standard size for bend strength testing of concrete in Japan. The rendering
mortar had dimensions of 40×10×400 mm. The thickness of the overlayer was designed to be 1/10thof the substrate, which corresponds to the actual structure and may allow for a more reasonablemagnitude of stresses generated at the adhesive interface under load. The width of the rendering mortar
was designed to be 40 mm to facilitate the bond strength test. It should be noted that the findings
obtained from small-sized specimens are difficult to use directly as an engineering design reference
but are valuable for fast comparison and screening of samples during the product development phase.
When selecting a bond strength test method, the main interface stress conditions caused by the
proposed AT need to be considered. The peeling of the mortar is caused by the restraint stresses thatact both in-plane and out-of-plane; consequently, strength tests in both directions should be performed.
Although tensile stress perpendicular to the interface is rare in structural design. Under solar radiation,
49

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESIONthe interface in the structure undergoes a mixed mode involving tensile and shear stress, but the shear
stresses were more dominant than the tensile stresses. Rumbayan et al. [141], who examined a wall
tile structure, reported that for a tile surface temperature of 50 ℃, the shear stresses generated at the
tile-mortar interface were 2.6 MPa. In contrast, the tensile stresses were only 0.008 MPa. During thedevelopment phase of the durability assessment method, pull-out tests and direct shear tests werecarried out to assess the degrading effect of the PTL method on the mortar/concrete adhesion. The
purpose was to clarify the effect of the bond strength test method on the results of the durability
assessment. The test machine was in accordance with the Japan Society for Finishing Technology
standard requirements, as shown in Fig. 3-4.
For both tests, the rendering mortar had to be cut to 40x40 mm before the operation, which is
why the mortar was 40 mm wide. In the pull-off test, the attachment is glued (e.g., epoxy resin) to themortar surface, and care is taken to prevent the bond from bridging the cut area. The testing machineis connected to the attachment, and a perpendicular tensile load is applied; slowly rotate the handle
until the mortar and concrete are separated, and the failure load is recorded. The direct shear test
measures the in-plane adhesion strength by separating the plastering mortar from the mixed clay
substrate by applying a force from the horizontal direction. After the specimen is fixed, the handle isshaken slowly and vertically, and the hydraulic oil pump transmits force to the side of the mortar until
the mortar slides against the substrate.

3.2 NUMERICAL SIMULATIONAs discussed in Chapter 2, the adhesion failure between the rendering mortar and concrete is a
result of interlayer stress accumulation. The numerical model can be used to help investigate the stateof interfacial stresses for a given load condition and the contribution of some factors to thedevelopment of adhesion of the mortar on the substrate. The variables can be quickly filtered bychanging the properties of the material. This section validates the rationality of the proposed PTL
method by performing numerical simulations by comparing it with the method of applying loads tothe specimen as a whole in the literature. Comparison of the expected deterioration effect of heating
the whole and part of the mortar by the stress state at the adhesive interface. The analysis was
performed using ANSYS Workbench, which is a 3D finite element method (FEM) program.

3.2.1 Modeling the mortar/concrete composites
50

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

Figure 3-5 Mortar/concrete composites geometry.
As shown in Fig. 3-5, the 3D-FEM model consists of three components, concrete with dimensionsof 100×100×400 mm, mortar with dimensions of 40×10×400 mm, and the adhesion interface. The
primary purpose of this section is to validate the soundness of the proposed AT approach. Therefore,
the material behavior of these elements has been simplified. In this case, the rendering mortar andconcrete behavior are assumed to be linearly elastic. The ANSYS Workbench element library includes
interfacial finite elements representing the cohesion zone between two elements, considering theseparation across the interface. The 3-D 8-Node contact element CONTA174, schematically
represented in Fig. 3-6(a), is selected. The CONTA174 is a surface-to-surface contact element. The
contact detection point uses a Gaussian integration point by default, which usually provides moreaccurate results than when using the node itself as an integration point [142]. Cohesion zone models(CZMs) have been chosen to describe interface problems. CZMs are widely used to model discrete
fracture processes in some homogeneous and inhomogeneous material systems. The CMZ model isdefined by introducing a critical fracture energy, which is the energy required to break the adhesiveinterface between the adherent and the adhered. The traction acting on the interface and thecorresponding interfacial separation (displacement jump across the interface) depends on the element
and material model [143]. The form of the constitutive relationships and parameters varies from model(a) CONTA174 contact element(b) Mixed-Mode Debonding law in ANSYS

Figure 3-6 Modelling the mortar/concrete adhesive interface, according to [143].
51

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

to model. In this work, a mixed fracture model (Mode I and Mode II) of CZM is used, as shown in
Fig. 3-6(b), considering that the debonding of the mortar is caused by stresses in both the normal andtangential directions.
σmax, τmax, ucn, and uct are the inputs of the interface elements. Where, σmax is the maximum normal
contact stress; τmax is the maximum equivalent tangential contact stress; ucn is the contact gap at thecompletion of debonding; uct is the tangential slip distance at the completion of debonding. The
parameters were set based on the results of bond strength tests obtained under the same conditions in
the laboratory, as detailed in Section 3.3. In addition, a theoretical reference to the CZM used is
supplemented in Appendix A of the dissertation. The material parameters of the concrete and mortar
components were set according to the JSCE recommendations [144], as summarized in Table 3-1.
Table 3-1 Model material parameters
Parameter
Concrete
MortarDensity (kg/m³)
2300
2100
CTE (1/℃)
1.10E-05
1.50E-05
Young's Modulus (GPa)28
20
Poisson's Ratio
0.2
0.2
Tensile Yield Strength (Pa)
2.78E+06
3.50E+06
Compressive Yield Strength (Pa)3.00E+07
3.96E+07
Isotropic Thermal Conductivity
1.6
1.5(W/mꞏ℃)
Specific Heat (kJ/kgꞏ℃)
0.7
0.6
Maximum Normal Stress (Pa)Maximum Tangential Stress (Pa)Normal separation distance (μm)
Tangential slip distance (μm)
Artificial Damping Coefficient (s)-3.2.2 Application of numerical model

Figure 3-7 Thermal load conditions.
52

Interface
1.05E+06
9.73E+05
400
400
0.001

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

Partial thermal load length160mm

Whole thermal load length 400mm

Figure 3-8 Equivalent (von-Mises) stress (MPa) at the FEM model interface at 120 min.

Figure 3-9 Maximum deformation (mm) and maximum principal stress direction of the mortar in
the FEM model.

The model was evaluated by performing transient thermal and structural analyses. The concrete
bottom surface in the model was set as a fixed constraint, similar to that in the actual conditions. One
thermal load cycle was applied to the entire and central part of the mortar surface with a load lengthof 400 mm and 160 mm, respectively. Fig. 3-7 shows the temperature history of the mortar surface
and the adhesion interface below the thermal load zone. It can be noted that after the peak temperatureload of 70 °C was maintained for 30 min., the temperature increased only around the loaded portion
owing to the low thermal conductivity of the concrete. The purpose is to obtain the stress distributionof the adhesion interface at high temperatures to verify the rationality of the proposed partial thermalload method. The dry shrinkage effect, cracks, and decrease in the adhesive strength owing to thecyclic thermal loads were not considered.
Fig. 3-8 shows the equivalent stress at the interface of the FEM model under thermal load lengthsof 160 mm and 400 mm, at the 120th minute of a cycle (after heating at 70 °C for 30 min). It can benoted that in the case of the model subjected to a partial thermal load, the stress is concentrated at the53

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

center of the mortar; in contrast, in the model subjected to the whole load, the stress is concentrated at
the ends, in accordance with the movement trend of the mortar. Fig. 3-9 shows the maximum
deformation of the mortar under one thermal loading cycle, the red arrow represents the direction of
the maximum principal stress of the adhesion interface at that moment. The maximum principal stressof the heated part of the interface is directed in the out-of-plane direction, and the mortar and concrete
have a tendency to delaminate. However, the maximum principal stress of the interface subjected to
the whole thermal load is located at the end, and the direction is almost parallel with the interface.
From the deformation results, the expansion caused by the heating of the mortar constrained by thesurrounding area tends to deform the material in the vertical direction. Therefore, the stress isconcentrated on the loaded part. In contrast, the mortar not constrained by the surroundings tends todeform toward both ends, and the stress is concentrated at the ends.
The FEM analysis results indicate that the deformation of the mortar subjected to partial thermalload is consistent with that of the actual rendering mortar for external walls. The complete heating
method cannot reproduce the degradation mechanism of the actual walls owing to the thermal
expansion of the mortar not being sufficiently constrained. Therefore, it is considered that providing
thermal load at the center, which can degrade the target location by realizing an appropriate constraint
for the heated part, is a superior accelerated deterioration approach compared to the complete heating
approach.

3.3 EXPERIMENTAL TEST OF THE PTL METHOD
It has been verified by numerical analysis that the damage pattern caused by the PTL method ismore representative of what happens when the facade is exposed to solar radiation. In this section, theactual degradation effect of the PTL method is verified experimentally, i.e., by comparing the change
in mortar adhesion before and after loading.

3.3.1 Specimen preparation
The dimensional design of the specimens is described in Section 3.1.2. The mixture proportionsof the concrete substrate and rendering mortar are listed in Tables 3-2 and 3-3, respectively.
The concrete substrate for all the specimens was prepared by demolding the concrete the day
after casting and subsequently curing it in 20 ℃ water for more than two months. The concrete wassufficiently dried to reduce the errors caused by drying shrinkage. Specifically, before plastering, thesubstrates were dried for more than one week at 40 ℃.
54

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESIONThe specimens were prepared with reference to the Japanese Architectural Standard
Specification—JASS15 Plastering Work [145]. Generally, to achieve higher adhesive strength, a seriesof pretreatments, such as polishing, cleaning, and adhesive application, are performed on the plasteredsurface. There are different technique classifications according to the concrete removal depth [56].

Strength(N/mm³)

Slump
(cm)

36

18

MortarPre-mixed
mortarTable 3-2 Mix proportions of base concrete.
WaterWeight (kg/m³)
cement ratio, Water
CementFine
CoarseW/C (%)
44
180
410
798
887

Table 3-3 Mix proportions of rendering mortar.
Cement: Fine
CementFine aggregate
aggregate (byWeight)Ordinary Portland cement
S: Crushed
1: 2.580%
sandBlast furnace slag 20%

Admixture
3.32

W/C (%)
50/60

Sand blasting, shot blasting, grinding, low-pressure/high-pressure water demolition andmanual/mechanical milling are the common methods used to create rough concrete surfaces. However,some of the more destructive methods increase the roughness while also increasing the risk of microcracking on the substrate surface, thereby reducing the bond [54], [56], [146]. This study aimed tovalidate the proposed AT method; therefore, the initial bond strength was not deliberately optimized.
In this work, 60-grit sandpaper was used to clean up the substances generated during curing and theresidual release agent on the substrate surface until the surface of the substrate exposed the fineaggregate, and no visible texture can be observed visually. The roughness was between 1 and 2
according to the International Concrete Repair Institute (ICRI) standard [147]. Acetone was used toremove dust and grease, which improved the adhesion of the mortar. Finally, ethylene-vinyl acetate(EVA), which is an adhesion promoter, was applied to the surface two hours prior to plastering. This
step is necessary to prevent the water in the mortar from being absorbed excessively by the concrete.
The effect of the degree of dryness of the interface on adhesion has been subject to debate. Traditional
treatments recommend pre-wetting the surface of the substrate, but some researchers consider thispractice detrimental to adhesion. In this case, it peels off the concrete very easily. The specific reasonsfor this have been discussed in Section 2.1.1. Lukovic and Ye [148] also state that dry substrates resultin more voids at the interface as the water loss from the substrate reduces the effective W/C and thehydration of the applied material. Therefore, adequate drying of the substrate before the mortar is cast,
55

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

then using EVA to control water transport may be a compromise; meanwhile, the formation of polymer
films and the formation of hydrogen bonds during hydrolysis in the polymer favor adhesion [38] [40].
The rendering mortar used in this work is a commercial ready-mixed mortar developed to plasterand repair fracture surfaces. This type of mortar is usually described in terms of the strength grade andis reasonably stable. According to the manufacturer's data, this rendered mortar has a compressive
strength of 30 N/mm² after 28 days of curing. Because rendering mortar is fragile in the early stage ofcuring, it was maintained in a stationary state for two days after pouring and then moved to an
environment with a temperature of 20 °C and relative humidity of 60% for 28 days until the
degradation experiment was performed.

3.3.2 Accelerated degradation effect tests(1) Accelerated degradation
The same thermal load as that in the FEM analysis described in Section 3.2.2 was used in theexperiment. One cycle was divided into four temperature steps for a total of three hours: 25 °C (30min) → 25 °C to 70 °C (60 min) → 70 °C (30 min) → 70 °C to 25 °C (60 min), as shown in
Fig.3-10(a). Notably, the cycling pattern is not unique, as alternative cycles with varied peaktemperatures and temperature gradients have been utilized in subsequent research. The influence ofthe cyclic mode of thermal loading is outside the scope of this dissertation and will be studied in the(b) thermal loading lengths(a) the aging cycles

Figure 3-10 Thermal load cycle and application.

Figure 3-11 Temperature history of the mortar surface during 50 cycles.

56

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

future. During the heating experiment, the surface temperature of the rubber heater and mortar wasmonitored in real-time by using thermocouples. The laboratory temperature was maintained at 20 °C
because the rubber heater could only be operated when the set temperature was higher than the room
temperature. The peak temperature of 70 ℃ was determined based on the most severe load conditions
that an exterior wall may be subjected to in summer. The number of cycles was set to 50 based on thetolerance of the weakest part. In particular, it was necessary to ensure that the specimen with theweakest adhesion strength in the degradation experiment did not peel. Fig. 3-11 shows the temperature
history of the mortar surface of a specimen under 50 thermal loading cycles. It was confirmed that thedesired temperature could be applied to the mortar surface by the rubber heater.
To validate that the partial heating method could degrade the adhesion, the entire specimen washeated using the same rubber heater, and the temperature history was recorded. As shown in Fig.310(b), the two thermal load lengths (160 mm and 400 mm), as in the FEM analysis, were applied tothe specimens by controlling the length of the rubber heater.

(2) Bond strength test
After the specimen was cured (after 50 load cycles) and cooled to room temperature, the mortar
was evenly cut into 10 parts with dimensions of 40 mm×40 mm×10 mm using a grinder and tested
sequentially before testing. The adhesive strength was defined as the ratio between the failure load andthe test area:
𝑆

𝑁/𝐴

(3-1)

where S is the adhesion strength, N is the maximum shear/tensile failure force, and Ao is the testarea (16 mm²).
Owing to the symmetry of the specimen, the average value of the strength at a location equidistantFigure 3-12 Contrast zones for the adhesive strength.
57

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

from the heated part was used. For example, the average of a and a’ was considered the adhesivestrength of the middle part A. The adhesive strength of the specimen was divided into five parts fromthe center to the end as A–E for the evaluation, as shown in Fig. 3-12. Because the adhesive strength
test was destructive, at least two specimens were prepared (not heated after curing) to obtain the initialvalues for each group. The average value was considered the experimental result.

3.3.3 Results and discussion(1) Degradation effect of PTL aging cycle on adhesion
Fig. 3-13 shows the adhesion of the specimens in the tension and shear tests after degradation
over 50 cycles of thermal loading compared to the initial value. It can be noted that the adhesivestrength of the central part (A, B) of the specimens subjected to the thermal load is significantly
reduced, whereas that of the unloaded part (C, D, E) remains nearly constant. The decrease in theadhesive strength of the loaded boundary area B is slightly larger than that for the load center A inboth the shear and tensile tests. Mahaboonpachai et al. [12] reported that under a temperature gradientand thermal expansion mismatch, shear stress occurs along the interface between the concrete andadhesive mortar. In particular, high shear stress occurs at the two edges of the specimen along theinterface between the concrete and adhesive mortar. In addition, the stress state of the adhesiveinterface subjected to high temperatures, as shown in Fig. 3-8, can also explain the large decrease in
the adhesive strength of region B. The stress concentration occurs not only in the central part of theload but also in the edge region. The high correlation between the experimental and numerical results
supports the reliability of the numerical model. Moreover, it can be speculated that as the adhesivestrength of the central part gradually decreases with the increase of the number in thermal cycles, thestress concentration part moves from the load center to the load edge.

(b) direct shear test (out-of-plane)(a) the pull-off test (in-plane)

Figure 3-13 Change of bond strength in both directions.

58

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

It can be considered that regardless of the employed strength test method, it is possible to evaluate
whether the adhesive strength of the specimen decreases after the specimen is subjected to accelerated
deterioration through the thermal cycle loading method. However, during the pull-off test, it wasobserved that the attachment attached to the mortar (used to connect the tester) peeled off from thespecimen before the adhesion failure. In contrast, the damage in the shear tests essentially occurred in
the interface transition zone, where a thin layer of mortar was observed to cover the damaged interface,which is more comparable to the situation in the actual structure. Therefore, in the proposed durability
evaluation system, the direct shear test was selected to evaluate the adhesion.
In both the tests and especially in the tensile test, the adhesive strength of the end of specimen E
was lower than that of other parts. According to Georgin et al. [149], mortar can undergo curling
deformation (deformation in the out-of-plane direction) owing to drying shrinkage. Thisphenomenon's causes are detailed in full in Section 2.2.1, the moisture gradient present in the rendering
mortar thickness during the curing process thus likely caused a differential shrinkage in the thickness.
Appendix B of the dissertation documents a series of studies to assess the effect of curling deformation
on mortar adhesion. Using laser sensors and the digital image correlation (DIC) technique, respectively,
the curling of mortar in the free drying shrinkage and constrained shrinkage states were studied. The
stresses at the adhesive interface between mortar and substrate were anticipated by numericalsimulation. The research confirmed that curling had a negative effect on adhesion. DIC revealed outof-plane deformation of the mortar/concrete specimens' ends. Nevertheless, according to numericalmodels, the PTL offers a significantly more potent force for debonding than drying shrinkage evenunder extreme drying circumstances. Furthermore, effective countermeasures, such as the use ofshrinkage reducing agents (SRA) and the placement of an overlayer on the mortar surface, which bothpromote uniform drying of mortar, can be used. Although curling distortion of mortars is a nonnegligible factor affecting adhesion, it does not interfere with the evaluation of durability, it is found.
Effective countermeasures can be implemented, such as using shrinkage reducing agents (SRA) andapplying an overlayer on the mortar surface, both of which promote uniform drying of mortar. It is
concluded that although curling deformation of mortars is a non-negligible factor impacting adhesion,
it does not interfere with the durability assessment.

(2) Degradation effect of the thermal load lengths
To verify the degradation effect of the partial thermal loading experiment on the adhesion, thecomplete thermal loading experiment was performed as a control group. Fig. 3-14 shows the shear

59

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION(a) Direct shear test result(b) Rate of change of shear strength after degradation
Figure 3-14 Shear strength of specimens under different thermal load lengths.

test result and change rate of the specimens before and after degradation, induced by the change in thethermal loading lengths (160 mm and 400 mm). The results indicate that the partial heating method
can significantly degrade the adhesion of the target location. The adhesion of the middle part (A, B)
of the specimens subjected to a partial thermal load significantly decreased, whereas that of the otherparts only decreased slightly. In contrast, the complete load specimens exhibited more obvious damage
only at the end.
The experimental and FEM analysis results indicate that the proposed partial thermal load method
can simulate the actual degradation mechanism and is a reasonable and effective means to realize
accelerated degradation. The direct shear test gives more consistent bond strength results than the pullout test. A high correlation exists between stress generation and the mechanism of reduction inadhesive strength. It is necessary to monitor the interfacial stress in future experiments.

60

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

3.4 RENDERING MORTAR DEGRADATION PATTERN UNDER PTL CYCLESThe health status of a structure can be determined by analyzing the features extracted fromcontinuous or periodically spaced measurements. Where continuous monitoring is not possible, it is aquestion of how to set the periodicity of the measurements. In section 2.2.3 of Chapter 2, two studies
on the degradation pattern of the adhesive properties of mortars are presented. The debonding processof the mortar is accompanied by two crucial inflection points, both under thermal loading in theuniform warming mode and under cyclic mechanical loading. Before the first inflection point,debonding proceeds slowly and is even difficult to observe; after the first inflection point, debonding
develops rapidly but steadily; after the second inflection point is encountered, stability is lost and soon
progresses to destruction. However, the pattern of change in debonding behavior before and after the
inflection point is influenced by the loading pattern. Furthermore, according to Cao and Chung [91](Fig. 2-15), the amplitude of the cyclic mechanical load (maximum load) also affects the location of
the inflection point and the pattern of development of debonding. The degradation pattern of adhesion
may be correlated with the load's form and magnitude.
The decision on the measurement period is, therefore, crucial for the results of the durability
evaluation. In section 3.3, the testing of the adhesion state was carried out after 50 thermal load cycles.
On the one hand, there is no basis to justify this as good timing for the test, as it is difficult to determine
at which stage of debonding evolution after 50 cycles. On the other hand, the durability comparison
was only carried out after a given load cycle, whether this mode of assessment is justified. Whether it
is necessary to output combined results after parallel testing in multiple dimensions. All thesequestions need to be judged based on the degradation pattern of mortar adhesion under the proposed
AT method.
This section, therefore, investigates how the bond strength of the mortar decreases along with the
PTL cycle.

3.4.1 Methods
The specimens were prepared according to the method described in Section 3.3.1. The only
difference is that the curing method of the rendering mortar was adjusted to reduce curling deformation
due to drying shrinkage; according to previous studies [150], curling deformation occurs mainly in theweek after mortar casting. Therefore, the surface of the mortar was covered with a wet sheet for the
first 7 days of curing:61

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

The mortar adhesion degradation pattern was obtained through bond strength tests every six PTL
cycles. The thermal load was applied in the same way described in Section 3.3.2. The difference isthat the bond strength of one set of specimens is tested every six cycles until the bond strength in theheated area falls below 0.4 MPa, which is the minimum permissible value in the specification for thebond strength of exterior rendering mortars. Direct shear tests obtained the bond strength. Each groupcontains three specimens, and given the symmetry, six data can be obtained from the same position.
The thermal cycle is shown in Fig. 3-15. As mentioned earlier, the degradation pattern is related
to the loading magnitude and the loading form. By adjusting the peak temperature, experiments werecarried out in two cyclic modes with different loading magnitudes and loading rates (warming andcooling rates). In this study, one cycle was 4 hours: 25 °C (60 min) → 25 °C to 70/80 °C (60 min)→ 70/80 °C (60 min) → 70/80 °C to 25 °C (60 min). It is worth mentioning that the experimental
environment was slightly different between the 80 °C cycles and the 70 °C cycles. The 70 °C cycles
were carried out at a temperature of 20 °C and 60% humidity, whereas the 80 °C cycles only controlled
the room temperature (20 °C) and not the humidity.

3.4.2 Results and discussion
As the plastering was done by hand, the results of the bond strength tests were inevitably highly
scattered, although the variables were controlled as far as possible. In discussing the results, the bondstrength for each group of specimens was calculated as follows; the average of the six results from the

Figure 3-15 Aging cycles for studying degradation patterns.
62Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

exact location after the maximum and minimum values had been removed.

(1) Accelerated degradation at 80 °C cycles
In the first study, the specimens were subjected to a maximum of 48 load cycles. Evolution of thebond strength in the specimens submitted to the aging cycles is shown in Fig. 3-16, (a) (b) shows thebond strength value and the rate of bond strength change, respectively. The following equation obtains
the rate of bond strength change:
𝑅

𝑆

𝑆 /𝑆

(3-2)

where RD is the rate-of-change of the bond strength, S0 is the bond strength before load, and SD
is the bond strength at the same position after degradation.

(a) Variation of bond strength values with aging cycles(b) Rate of bond strength change with aging cycles (vs. 0 cycles)

Figure 3-16 Evolution of the bond strength in the specimens submitted to the aging cycles– Peak temperature of 80 °C.
63Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESIONFirst, for the heated regions (A and B) in the left column of Fig. 3-16, the bond strength of theconcrete and mortar gradually decreases with PTL cycles. However, the rate of decrease was not
uniform and was significant between 6 and 12 cycles at a maximum temperature of 80°C for the PTL
cycle. In other words, the first inflection point of the degradation process occurs between 6 and 12
cycles when debonding suddenly starts to develop. Subsequently, debonding gradually evolved
between the 12th and 24th cycles but showed a steady trend. No decrease in bond strength wasobserved at this stage, which could be due to two reasons; 1) debonding proceeds so slowly at thisstage that the bond strength test cannot observe it. 2) the variability of the initial bond quality of thespecimens. Fig. 3-17 (left) represents the results of the bond strength test in area A. The results of the
12-30 cycles show a large dispersion. There was another significant drop in adhesion between cycles
24-30. Although the test results fluctuated in cycle 36, combining the results from cycles 42 and 48,
the debonding process appeared to stabilize and slow down again after cycle 30 until cycle 48, whenhalf of the specimens already had bond strengths below 0.4 MPa, and loading was terminated.
In addition, after the 30th cycle, cracks were observed on the surface of the heated mortar in somespecimens, and the morphology of the cracks is marked in Fig. 3-17. Complete debonding of themortar in part with cracks (area A) occurred at the time of cutting, which was because the tensile force
generated by the expansion of the mortar in the out-of-plane direction under thermal load exceeded its
tensile strength, resulting in fracture damage. This may indicate that the cracks on the surface of therendered facade represent a small area where the rendering mortar has debonded.
A decrease in bond strength was also observed for the non-heated areas. After 48 cycles, the rateof decrease in bond strength in the non-heated areas was approximately half that in the heated areas,
with an average reduction of 39% in parts C, D, and E and 65% in parts A and B. The most significant
decrease in bond strength was also observed between cycles 6 and 12. The most significant reduction
in bond strength throughout the process also occurred between cycles 6 and 12, with the difference
that no second inflection point was observed afterward. In fact, the decrease in the strength of the C,

Figure 3-17 Cracks on the mortar surface after thermal loading.
64Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

D, and E parts was around 35% in both the 12th and 48th cycles. Therefore, it can be assumed thatdebonding hardly evolves in the non-heated region after cycle 12. This 35% loss of adhesion is most
likely due to the drying shrinkage of the mortar.

(2) Accelerated degradation at 70 °C cycles
In the second study, the specimens suffered a maximum of 78 cycles due to a decrease in peaktemperature. Evolution of the bond strength in the specimens submitted to the aging cycles is shownin Fig. 3-18, which shows the trend lines for each variable. The degradation pattern of mortar adhesionchanges as the peak temperature of the thermal load decreases. For the heated zone, unlike the"stepwise" decrease observed at the 80 °C cycles, the delamination between mortar and concrete at
the 70 °C aging cycles proceeded in a relatively steady and gradual manner. The overall decrease inadhesion for B, representing the edge of the heated area, was more significant than for the center ofthe heated area (part A), with complete debonding observed at the earliest, consistent with thedescription in section 3.3. Part A's shear strengths were 1.65 MPa and 0.36 MPa after 0 and 78
accelerated aging cycles, respectively, with a shear strength loss of about 78%. While for part B, theshear strengths were 1.76 MPa and 0.23 MPa, respectively, with a shear strength loss of about 87%.
In both thermal loading modes, the non-heated and heated zones had similar degradation patterns,

Figure 3-18 Evolution of the bond strength in the specimens submitted to the aging cycles– Peak temperature of 70 °C.
65

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

Figure 3-19 Rate of bond strength change Figure 3-20 Average bond strength variationfor each cycle between two shear tests.

with 70 °C aging cycles (vs. 0 cycles)

except that the overall loss of shear strength was less in the unheated area. From the center of thespecimen to both ends (C→E), 36%, 23%, and -5% were lost, respectively. The rate of shear strength
loss shows that the non-heated region is also affected by thermal loading to some extent, and the closer
to the thermal loading zone, the greater the trend of shear strength loss. In addition, there is almost no
strength loss in the end E of the specimens, which may be related to the humidity control in thisexperiment. The increase in ambient humidity and the change in the curing method reduced the curling
deformation of the mortar.
Fig. 3-19 depicts the rate of change in bond strength with 70 °C cycles of heating the region.
Since there are no results for cycles 42 and 54, Fig. 3-20 shows the average shear strength change per
cycle between the two shear tests to observe the adhesion evolution more clearly. Calculated from thefollowing equation,
𝑆̅

𝑆

𝑆 / 𝑖

𝑗

(3-3)

where 𝑆̅ is the average shear strength change per cycle, Si is the shear strength after the i-th cycle,
and Sj is the shear strength after the j-th cycle.

From Fig. 3-19 and Fig. 3-20, it is observed that cycles 0-6, 12-24, and 60-78 are the phases withthe fastest shear strength reduction. The overall degradation pattern is similar to that observed in the
80 °C cycles, which an "S" curve can represent. Three phases characterize the evolution of debonding,and cycles 24 and 60 can be considered crucial inflection points. After a certain percentage of shear
strength is lost, the debonding process tends to slow down and develops rapidly only when the next66Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION

critical point is reached. Although the fluctuation of shear strength between cycles 24 and 60 is
significant, the evolution of debonding in this phase can be considered slow when viewed as a whole.
This "S-shaped" degradation path is consistent with that reported in the two studies presented in theliterature review [89] [91] (Fig. 2-14 and Fig. 2-15). Therefore, it is considered that this is not a chance
result but a general pattern of mortar debonding evolution.
Interestingly, similar shear strength losses were observed in both loading modes before the slow
progression of debonding. In the 80 °C cycles, the inflection point occurs at cycle 12 when the shear
strength loss is A (40%) and B (42%). In the 80-degree cycle, the inflection point occurs at cycle 24
when the shear strength loss is A (43%) and B (40%). It can be concluded that after a loss of about 40%
of shear strength, the evolution of debonding enters a slow development phase until the next criticalpoint, which may be due to fatigue accumulation. This slow development phase is referred to as the"plateau" in this paper. The plateau is prolonged as the load strength (peak temperature) decreases.
This means that more aging cycles need to be run to pass the plateau of debonding evolution before
the shear strength continues to decrease. Conversely, it might be inferred that the lower the durabilityof the mortar/concrete adhesion, the shorter the plateau.
Based on these results, the author concludes that an adequate number of aging cycles should be
set according to the cycling pattern when comparing the durability of the two specifications ofmortar/concrete composites. At the very least, it should be guaranteed that the debonding evolution of
the less adhesive specimen has passed the plateau after accelerated degradation. If the debonding ofboth specimens happens to have progressed to the plateau when the loading is stopped, each has likely
lost 40% of its initial strength, making it difficult to compare durability. A better approach might be toperform a multi-dimensional durability test, that is, to compare the loss of bond strength after different
cycles to obtain a comprehensive durability evaluation result.

3.5 SUMMARY
This chapter describes the development process of the durability assessment method. In this study,
an AT method was proposed to evaluate the durability of the rendering mortar in terms of adhesion to
concrete substrates. Experimental results and numerical analysis demonstrated the validity of the AT
method. Based on the proposed AT method, the debonding evolution of the rendering mortar wasinvestigated. The main conclusions of this work are:67

Chapter III ESTABLISHING A LAB-SCALE DURABILITY ASSESSMENTMETHOD FOR MORTAR/CONCRETE ADHESION(1) The effectiveness of the partial thermal load conditions was demonstrated, and it was notedthat the restraints provided by the surrounding parts of the mortar must be considered to reproduce thereal degradation environment in the laboratory.
(2) Loss of bond strength between the heated mortar and the concrete was observed in both thepull-off tests and direct shear tests. It is considered that the thermal load will cause stress in both thein-plane and out-of-plane directions at the adhesion interface between the mortar and concrete.
(3) Results of the numerical analysis performed using a 3D-FEM model confirmed that the inplane and out-of-plane stresses were concentrated in the area in which the adhesive strength was
reduced by the partial heating method, and the stress generation and mechanism of the adhesion failure
were reasonably correlated.
(4) The stress concentration occurred at the edge of the load portion at which the joint strength
decreased most during the experiment. Presumably, the weakest areas for plastering the mortar
correspond to the junctions subjected to both sunlight and darkness, instead of those subjected only to
direct sunlight.
(5) Under cyclic thermal loading, the mortar adhesion deteriorates in an "S-shaped" path. A"plateau" period can be observed in the debonding evolution, during which the bond strength does notdecrease as the aging cycle runs. The debonding evolution hit a plateau when the loss of bond strength
reaches about 40%. The duration of the plateau is related to the load strength. After the plateau, thedebonding develops rapidly until the adhesion failure.
(6) Adhesion at the specimen ends is lost due to curling deformation caused by drying shrinkage,
which can be improved by wet curing. For durability assessment, the drying shrinkage resistance ofmortar can be determined by the loss of adhesion at the specimen ends.

Together with the above conclusions, it is concluded that the proposed durability assessment
method can conduct the next step for implementation testing.

68

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

Chapter IV
APPLICABILITY OF THE DURABILITYASSESSMENT METHODThis chapter describes the implementation phase of the process for establishing a durability
assessment method. After the feasibility of this method is proved, some upgrading attempts are made.
First, the proposed AT method was applied to several mortar/concrete specimens prepared according
to different specifications, and their durability was compared by shear strength tests. Secondly, anapproach to continuously monitor the adhesion is proposed to provide higher accuracy of theadhesion test results in place of the shear strength test.

4.1 DURABILITY COMPARISON OF MORTAR/CONCRETE COMPOSITES
A durability assessment is run by comparing tests with reference material. This means thatspecimens prepared in strict accordance with the methods recommended by the code are used as thereference material. After performing the same AT, the durability ranking of the new material isdetermined by comparison with the reference material. This approach does not correlate with actual
service life but allows for rapid comparison and parameter selection at the beginning of productdevelopment. Meanwhile, the reference material as a standard value, according to the actual service
life of the reference material, can roughly estimate the expected service life of the new material.

4.1.1 Specimen preparation
Specimens prepared according to the method described in Section 3.3.1 were used as referencematerial (control group) and designated SV. Three experimental groups were set up, each varying
only one parameter based on the SV specimen, that is, the water-cement ratio (W/C) of the mortar,
the use of a shrinkage reducing admixtures (SRA), and the thickness of the mortar to prepare
specimens with different adhesion between mortar and concrete. These parameters were selected as
69

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

they influence the bond strength of the mortar and concrete, as specified in the JapaneseArchitectural Standard Specification—JASS15 Plastering Work (2019) [145], as shown in Table 41. Although no recommendations are given in the code, according to the research in Chapter 3,
drying shrinkage is also a factor in the decrease of bond strength. Therefore, the use of SRA is set
as a parameter. SRA was invented by Goto et al. In the 1982s [151] and has been proven bynumerous studies to effectively reduce the shrinkage and cracking of cement-based materials [152][153]. The experimental parameters of the four types of specimens are listed in Table 4-2.
On specimen SV basis, the W/C of the L-1 specimens is 0.6, which is higher than the W/C of
0.5 recommended by the manufacturer. The mortar thickness of the L-2 specimens was set as 15
mm instead of 10 mm (an increase of 50%). In addition, because dry shrinkage influences theadhesion strength, the amount of. SRA was set as a parameter, and the mortar of the L-3 specimens
was mixed with 1% SRA. Theoretically, group L-3 should have the best adhesion, SV second, andgroups L-1 and L-2 are worse because of the selection of unfavorable parameters to the adhesion.

Table 4-1 Impact factors of the adhesion strength between mortar to concrete [3.7].

Factors

Adhesiveinterface
MortarRoughness
Adhesion promoter
W/C
Thickness

Adhesion strength
Lower
Lower
Smooth
<
Rough
Water
<
EVA
Large
<
Small
Thick
<
Thin

Table 4-2 Specification of rendering mortar specimens.
Experimental parameterW/C (%)
Thickness (mm)Admixture

Specimen notation
L-1
L-2
60
50
10
15
-

SV
50
10
-

L-3
50
10
SRA

4.1.2 Durability assessment
The PTL-based aging cycle model is shown in Fig. 3-10(a). The principles for setting the aging
cycles are discussed in the conclusions in Section 3.4. The degradation of AT should allow the worst
adhesion specimens to pass the "plateau" in the evolution of debonding; that is, the specimens shouldlose at least 40% of their bond strength after AT. According to the results in Chapter 3.3, the bondstrength loss of SV specimens after 50 aging cycles has met this requirement, with heating centers
A at 35.5% and B at 56.4%. Therefore, 50 aging cycles with a peak temperature of 70 ℃ is
considered a reasonable choice in this study. The bond strength was obtained by direct shear test.
70

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD(a) Initial shear strength (No heating)(b) Shear strength after 50 thermal load cycles
Figure 4-1 Shear strength of the four types of specimens.
Fig. 4-1 shows the shear strength results for the four groups of specimens before and after
being subjected to 50 partial thermal load cycles. The strength without heating decreases in thefollowing order: L-3 (mortar mixed with SRA) > SV (standard value) > L-1 (mortar W/C is 60%) >L-2 (mortar thickness is 15 mm).
Table 4-3 Initial shear strength (MPa) result for each group of specimens.
CenterEnd

Specimen notation

A

B

C

D

E

SV

1.15 (100%)

1.16 (100%)

1.30 (100%)

1.07 (100%)0.95 (100%)L-1

1.02 (89%)

1.02 (88%)

1.01 (78%)

0.75 (70%)

0.70 (74%)

L-2

0.88 (77%)

0.92 (79%)

0.86 (66%)

0.91 (85%)

0.42 (44%)

L-3

1.38 (120%)

1.41 (122%)

1.18 (90%)

1.35 (126%)

1.05 (110%)

The values obtained in the shear tests are presented in Table 4-3. The influence of each71

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

Figure 4-2 Change rate of shear strength of specimens after degradation.
parameter on the strength of the adhesives is presented as a percentage of the SV group (this latter
is assigned 100%, from which the others follow), making it possible to observe the gain or loss ofresistance as a function of the adopted plastering process, considering the recommended of the code
as the reference value.
As speculated, the L-3 specimens exhibited a higher adhesion strength than that of the SV
specimen. As mentioned in Section 2.2.1, the mortar produces a moisture gradient in the thickness
during the curing. Because the moisture loss at the top of the mortar is higher than that at the bottom,
the shrinkage at the surface is greater, and an upward curl deformation occurs at the edge. Thus,
compared to SV specimens, L-3 specimens with the addition of SRA showed a 10-20% increase in
shear strength, especially at the ends most affected by drying shrinkage. In contrast, specimens withthicker mortar (L-2) had lower shear strength due to a more significant difference in shrinkage
between the upper and bottom surfaces, with only 44% of the shear strength of SV specimens at theends. This result demonstrates the notable effect of dry shrinkage on the mortar–concrete adhesion
system. Thus, dry shrinkage should be considered a key factor influencing the failure mechanism.
Fig. 4-2 shows the rate-of-change of the shear strength of the four specimen groups after 50
aging cycles, as calculated by Equation 3-2. All the specimens exhibit a higher reduction in theshear strength in part B than in part A. This was due to the higher stress levels at the edges of theheated region, as described in Section 3.3. Subsequently, another significant loss of shear strength
occurred at the end of the specimens. This was due to the accelerated moisture loss from the mortar
surface caused by the heating, and drying shrinkage again led to a reduction in bond strength at bothends.
In this work, the criterion to evaluate the durability of the rendering mortar–concrete adhesion
system was as follows: A lower decrease rate of the thermal loaded parts (A and B) was considered
to correspond to a higher durability of the specimen. Table 4-4 presents the decrease rate of the72Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

shear strength at the heated parts (A and, B). The durability was noted to decrease in the following
order: L-3 > L-2 > SV > L-1. The average decrease ratio of the shear strength of the specimens (L1) with a 10% increase in water-cement ratio was 1.13 times that for the SV group. In contrast, thedecrease ratio of the L-3 specimens (mixed SRA) was 0.67 times that of the SV group, indicatingthat the mortar–concrete adhesion system under this treatment method exhibited superior stability.
On the other hand, the resistance of mortar to curling deformation can be obtained by comparing
the shear strength change rate of part E. Here, the strength increase exhibited by specimen L-2 at
the end may be due to the lower initial value (shear strengths of 0.42 MPa and 0.47 MPa before andafter degradation, respectively). Specimens with larger w/c (L-1) are more susceptible to drying
shrinkage. This may also indicate that the low durability of L-1 specimens under thermal loading
may be due to their more significant drying shrinkage rate. The specimen with SRA added (L-3) did
not show excellent resistance to drying shrinkage but obtained high durability evaluation results.
This may indicate that SRA does not improve durability only by reducing dry shrinkage but most
likely by changing other physical properties of mortar, such as elastic modulus and linear expansion
coefficient, a hypothesis that requires more research.
Table 4-4 Rate of decrease of the adhesion at the heated parts and the ends.
Rate of decrease of shear strength (%)
Heated zone
Both ends
Specimen notation
E
A
B
17.9
SV
35.7
56.0
44.2
L-1
41.2
62.7
-11.9
L-2
33.5
51.1
22.9
L-3
23.9
38.3

Moreover, the factors affecting the adhesion and durability of plastering mortar are ranked. A
comparison of the absolute value of the difference between the average initial shear strength of eachexperimental group and the SV group, the influence of the parameters on the initial adhesion in thefollowing order: mortar thickness (10/15 mm) > W/C ratio (0.5/0.6) > addition of admixture (yes/no).
Similarly, by comparing the absolute value of the difference in the decrease ratio of the adhesivestrength between the SV specimens and other specimens, under the conditions set in this experiment,
indicated that the influence of the parameters on the durability decreased in the following order:
addition of admixture (yes/no) > W/C ratio (0.5/0.6) > mortar thickness (10/15 mm).
The initial adhesion strength comparison results and durability evaluation results of thespecimens indicate that although satisfactory short-term adhesion can be obtained with theplastering methods recommended by existing specifications for external walls, the durability of thesystem, i.e., the long-term adhesion, cannot be guaranteed. Consequently, it is necessary to evaluate73Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

the durability of the materials when evaluating or developing new plastering/rendering mortarmaterials or workmanship techniques.

4.2 A CONTINUITY APPROACH FOR ASSESSING MORTAR ADHESION
The validity of the proposed durability assessment method has been verified in the previous
section. The method can be used to compare the durability of mortar/concrete adhesion made indifferent ways and to obtain a general idea of the influence of various factors on durability. However,
the method still has some shortcomings, in particular, the bond strength test used to obtain the lossof adhesion. Regardless of which bond strength test method is used, the results show a high degreeof dispersion and variability. This is because the plastering is carried out by hand, the same as in theactual construction. The low durability of rendered is often caused by poor construction quality [22].
The destructive bond strength tests may influence the judgment of durability due to the variability
in the construction quality of the specimens, as the bond strength before and after degradation was
obtained on different specimens. On the other hand, the degradation pattern of the adhesion wasinvestigated in Section 3.4, and it was determined that debonding does not occur at a uniform rate.
Therefore, the testing period of bond strength may affect the evaluation result of durability. In
summary, developing a method that can continuously monitor adhesion on the same specimen isnecessary. The strain-tracking method described in Section 2.3.1 was considered suitable as a nondestructive test method capable of monitoring the whole process of mortar debonding.

4.2.1 Monitoring the strain behavior of specimens with FBG sensors
As discussed in Section 2.3.1, significant strain differences are expected between the elements
when the adhesion between two materials fails. In other words, the response of the composite to an
external force impacts changes from an integrated behavior to a dispersed behavior. Theoretically,
it is possible to detect debonding by analyzing the characteristics of the strain behavior of therendering mortar and concrete. At the same time, this method allows continuous monitoring of thesame specimen and may help to study the evolution of debonding. Therefore, the work in this section
investigates the strain behavior of mortar/concrete specimens under PTL cycles.

(1) FBG sensors
Strain gauges are typically used to obtain the strain in each component element. However, notonly can the gauge sections reduce adhesion, but also the effect of an approximately 1–2 mm thick74

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

Figure 4-3 Comparison of dimensions for FBG sensors and strain gauges.
wire cannot be ignored. This implies that the application of strain gauges can potentially contributeto delamination. In this study, FBG sensors are attached to the concrete substrate and mortar surface
to measure strain.
FBG sensor is a grating-based point fiber optic sensor. Unlike distribution fiber optic sensors,
the measurement range is limited to the distance where the grating is carved. FBG reflects a portionof the incident light of a specific wavelength, called the Bragg wavelength, which is determined by
the refractive index of the fiber and the grating spacing. When local deformation occurs, the grating
period and reflected wavelength change accordingly, allowing the local strain to be detected [154].
The FBG sensor used in this experiment is a polyimide-coated sensor with a diameter ofapproximately 0.15 mm. As shown in Fig. 4-3, the FBG sensor limitedly affects the bonding
properties of the tiles compared to the strain gauge. By virtue of their small size, signal stability andsuitability for multiplexing, FBG sensors have been widely used for damage monitoring ofcomposite materials [155]. Moreover, there are practical reports that FBG sensors should be suitable
for long-term health monitoring of real structures [156]. FBG sensors can be set with intensive
sampling periods (1 kHz), allowing the tracking of strain behavior during earthquakes. In addition,compared to traditional sensing techniques that require many strain gauges and protective cables,

Figure 4-4 Arrangement of FBG sensors in the specimen.
75

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHODFBG enables the integration of many sensors in a single fiber and the ability to measure from either
end of the fiber. However, the slim dimensions of FBG sensors are similar to those of a doubleedged sword, with a high risk of wire breakage. Therefore, the FBG sensors used in this experimentare coated with glass-fiber-reinforced plastics (approximately 1 mm outside diameter) on the wire(away from detection points) to increase the stiffness and prevent wire breakage encountered during
the experiments.

(2) Specimen preparation
The specimens were prepared according to the procedure in Section 3.4. The difference wasthat FBG sensors were attached to the concrete substrate surface (adhesive interface) before themortar was poured. The three most representative locations for measurement were chosen, thecenter of the heated area, the edge of the heated area, and the center of the unheated area. At eachlocation, two FBG sensors with an effective length of 10 mm are installed parallel to the surface ofthe concrete substrate and mortar to obtain the strain on each element. To refer to the degradation
paths obtained through the bond strength tests, the thermal load cycles applied in this work were
the same as the 70 °C cycles in Section 3.4. The thermal load was applied to the central 160 mm x
40 mm area of the mortar; in one cycle: 25 °C to 70 °C (60 min) → 70 °C (60 min) → 70 °C to
25°C (60 min) → 25 °C (60 min). Fig. 4-4 shows the installation of the FBG sensor.
In addition to the specimens used for strain measurement (SS), other specimens of the same
specification were prepared for different measurements, as shown in Table 4-5. ST specimens wereused to measure the temperature change at the corresponding position of the FBG sensor for
temperature compensation calculations. SI and SD specimens were used for shear strength tests tomeasure the change in bond strength before and after the load, respectively. All specimens were
subjected to the same temperature load, and the experiments were carried out at constanttemperature and humidity (20 °C and 60% relative humidity).
Table 4-5 Specimen for different measurements
Measurement
Specimen notation
Temperature
ST
StrainSS
Initial shear strengthSI
Degradation
shear strengthSD

4.2.2 Experimental results and discussion
As shown in Fig. 4-4, the detection points are named as follows; the center of the heated area,
the boundary of the heated area, and the unheated area are named H, HB, and N, respectively; C76

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

Figure 4-5 Temperature history of the ST specimens.
represents the sensor located on the concrete surface and M represents the sensor located on themortar surface. For example, H-M stands for a mortar surface located in the center of the heatedarea.

(1) Temperature monitoring
Fig. 4-5 depicts the measured temperature evolution at each detection point during aging cycles
for the mortar/concrete composites. Notably, for point H-M, the temperature curve (black solid line)
displayed relevant fluctuation at 70 ± 2 °C (equilibrium level). The automated temperature control
system of the rubber heaters causes this effect. The system switches the heater on/off depending on
the thermocouple detection of the mortar surface temperature, i.e., if the mortar is cooling down or
heating up by over ± 1 °C. During the high-temperature plateau (about 1 hour) of each cycle, there
is a temperature difference of about 15 °C between the concrete and the mortar at the thermal loading
center, which is sufficient to cause the expected differential movement. The temperature at HB-M,
which straddles the boundary between the heated and unheated areas, is approximately 30 °C lower
than the center of the heated area (H-M), which could explain the loss of adhesion in part C in Fig.
4-2. In contrast, the temperature at N-M in the non-heated region is almost the same as room
temperature, which can ensure that this region is not damaged by the thermal load while providing
the expected restraint for the heated part.

(2) Strain monitoring
Fig. 4-6 reveals the cyclical strain induced in different layers by the thermal cycles detected by
the FBG sensors; (a) (b) (c) lists the strain results comparing the mortar surface and the concretesurface in parallel positions at the area subjected to different thermal load levels, respectively. To
observe the strain development trend more clearly, Fig. 4-7 shows the strains of mortar and concreteat the end moment of each cycle, defined as the residual strain per cycle.
77

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD(a) Thermal loading center(b) Thermal loading boundary(c) Non-heated area
Figure 4-6 Strain history of the SS specimens.
i) Thermal loading center (H-C & H-M)
In the center of the heated area, the strain trajectories of H-C and H-M overlap only in the firstcycle and then gradually separate. In the 1st-7th cycles, the strains of H-C developed toward theshrinkage side, and from the 8th cycle onward, it turned to the tensile until the 17th cycle, when it
started to develop slowly toward the shrinkage side again. On the contrary, the strain of H-M keeps
growing toward the contraction side.
According to the strain trajectory, cycles 1-7, the concrete follows the mortar towards the78Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD(a) Thermal loading center(b) Thermal loading boundary

Figure 4-7 Residual strain of mortar and concrete at the end of each cycle.
shrinkage side with the same strain behavior. The bond integrity may still be maintained during thisperiod. However, the gradual increase in strain difference indicates that degradation is ongoing.
Starting from cycle 8, the concrete shows a strain behavior opposite to that of the mortar, and it ispresumed that debonding may have occurred at this moment when the concrete loses the restraint
from the mortar and the prestress applied initially to the FBG sensor starts to relax (H-M installedat the adhesive interface). This speculation is also supported by the residual strains in Fig. 4-7(a),where the residual strains of the concrete increase from cycle eight at a constant temperature,indicating that the pressure on point H-C is gradually lost. The gap between the strain trajectory of
the concrete and the mortar becomes progressively larger in cycles 8-17, and the extent of debonding
is slowly expanding. From cycle 17 onwards, the concrete gradually develops towards shrinkage
again, probably because the debonding within the detection range of the FBG sensor has been
completed (complete release of prestress) and the drying shrinkage becomes the dominant factor in
the strain trajectory. Penetration cracks were observed on the mortar surface in the heating center
after 24 aging cycles, and it can be inferred that debonding has occurred.

ii) Thermal loading boundary (HB-C & HB-M)
The strain trajectories of mortar and concrete located at the thermal load boundary follow apattern similar to that of the center of the heated area. HB-M progresses toward shrinkage with theaging cycle, while HB-C undergoes a three-stage transition from shrinkage-tension-shrinkage. Dueto the lower strength of the thermal load applied and the minor temperature difference between
mortar and concrete, HB-C has been following the strain trajectory of HB-M in cycles 0-11. Startingfrom cycle 12, the strains of HB-C and HB-M gradually separate, indicating that the debonding
evolution begins from this moment. From the residual strains of Fig.4-7(b), the debonding develops
until the 20th cycle, until the residual strains of H-C cease to change. After the 20th cycle, theresidual strains of HB-C show only a weak decreasing trend due to drying shrinkage. It can be79

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

expected that, without further increasing the strength of the thermal load, the debonding will notcontinue.

iii) Non-heated zone (N-C & N-M)
According to the strain history in the non-heated zone, there is no damage to the bond integrityof mortar and concrete here. The strain trajectories of N-C and N-M maintain a good following,
both towards shrinkage. However, there is a slight separation between the strain trajectories of N-C
and N-M with drying shrinkage, which may indicate a partial loss of adhesion. It can be expectedthat the strain trajectories of mortar and concrete will further separate as the drying shrinkage
increases.

iv) Effect of drying shrinkage
Comparing the shrinkage trends of the mortar in the three regions, the drying shrinkage wasstrongly influenced by the temperature. Since the temperature in the room was constant, the mortar
temperature did not ever fall below 20 °C during the experiment, which means that the shrinkagecame only from drying. After 30 cycles, the residual strains in the mortar at the thermal load center,thermal load boundary, and unheated area were -763µ, -287µ, and -118µ, respectively. At thethermal load center, PTL leads not only to an expansion deformation of the mortar in the out-ofplane direction but additionally to a drying shrinkage of about 645µ in the in-plane direction. Thisphenomenon can explain the results of the durability evaluation in section 41. In the heating center(part A), L-1 (larger w/c) specimens obtained the maximum loss in adhesion, while L-3 (with SRA
added) specimens showed the least adhesion loss. This may indicate that the drying shrinkage does
not counteract the thermal expansion but rather exacerbates the differential movement of mortar andconcrete and accelerates the debonding evolution.

(3) Cyclic strain amplitude analysis
The strain amplitudes of mortar and concrete were calculated for each cycle, as shown in Fig.
4-8, where the strain amplitude is defined as the difference between the maximum and minimum
strain values. Fig. 4-9 shows the strain amplitudes at each detection point (left column) and the ratioof the strain amplitudes of the mortar to that of the concrete (right column). The "strain amplitude
ratio" is given as follows:

𝑆𝑡𝑟𝑎𝑖𝑛 𝑎𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝑟𝑎𝑡𝑖𝑜

80

(4-1)

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

Figure 4-8 Definition of the strain amplitude per cycle.

(a) Thermal Load center(b) Thermal load boundary(c) Non-heated area
Figure 4-9 Evolution of strain amplitude and strain amplitude ratio of mortar andconcrete with aging cycle.

Where 𝐴is the strain amplitude of mortar, 𝐴

corresponding position.

81

is the strain amplitude of concrete at theChapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

The strain amplitude reflects the magnitude of the deformation of the element at each cyclewith constant thermal loading cyclic pattern and represents the variation of the constraint condition
to which the element is subjected. At the center of the thermal load, the strain amplitude of H-M
increases cycle by cycle, with a faster growth rate from 0-10 cycles, a slower growth rate from 1020 cycles, and no change after 19 cycles. The growth of the strain amplitude implies that theconfinement imposed on the mortar is gradually decreasing; that is, the adhesion of the mortar tothe concrete is slowly lost. The growth rate of strain amplitude may also represent the rate ofdebonding evolution. After 19 cycles, the strain amplitude of the mortar stops growing as debonding
does not continue to develop. For H-C, the strain amplitude is stable from cycles 0-7 (the fluctuationin the 3rd cycle may be due to the temperature control variation), decreases from cycle 8, and returns
to stability after cycle 19. From the 8th cycle onwards, the restrained equilibrium of the concrete is
broken and gradually returns to "free." Due to the lower temperature of the concrete, the strain
amplitude gradually decreases after losing the traction of the mortar until reaching the next
equilibrium condition (cessation of the debonding evolution). Observing the strain amplitude ratio,
the strain amplitude ratio shows an increase from cycles 0-19 as the deformation of the mortar
increases and the concrete decreases. This phase represents the debonding evolution. After 19 cycles,
the strain amplitude ratio shows a slight decrease as the strain amplitude of H-C slowly increases(H-M remains constant). This may be due to the reduction of elastic modulus due to the decline inmoisture content of concrete with drying shrinkage [157].
At the thermal loading boundary (note that the difference between the range shown in thevertical axis of Fig. 4-6(a)(b)(c), HB-C and HB-M show fluctuations at cycles 2 and 3. Comparing
the strain histories in Fig. 4-6(b), it can be assumed that this phenomenon is not due to debonding
but to some measurement errors. Apart from this, the strain amplitude of the mortar did not show amassive increase. The strain amplitude of HB-C gradually decreased from cycle 9, indicating thatdebonding probably started from this moment. From cycles 19-30, the strain amplitudes of HB-M
and HB-C maintained a relatively stable state, indicating stagnation in the evolution of debonding.
In addition, a slight increase in the strain amplitude ratio was observed in cycle 9, but the overallchange was not significant.
In the non-heated area, the strain amplitudes of the mortar and concrete and the strain amplitude
ratio between the two always remained stable, so it was possible to reconfirm that the adhesion
integrity at this location had not been damaged.

(4) Shear strength results82Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

Figure 4-10 Change rate of shear strength of SD specimens after aging cycle.
Fig.4-10 shows the average rate-of-change of the shear strength of the SD specimens after 30
cycles (versus 0 cycles), as calculated by Equation 3-2. Sensors H-C and H-M are located at part
A, HB-M and HB-C are located at the junction of part B and part C, and N-M and N-C are locatedat the intersection of parts D and E. A significant decrease in bond strength was observed for part A.
Although the average shear strength loss of SD specimens did not reach 100%, complete debonding
was observed on one of the specimens, and penetration cracks were also observed on the mortar
surface of this specimen (the same as those observed on the SS surface). Thus, the shear strength
results support that complete debonding of H-C from H-M occurred.
Based on the strain measurement results, HB-M and HB-C are believed to have delaminated.
However, parts B and C retain some shear strength. Two reasons could cause this, 1) variation in
the initial bonding of SD to SS; 2) Delamination at the thermal loading boundary occurred only ina small area near the FBG sensor. Although the FBG sensor is small, the adhesive used may haveaffected the initial bond strength. However, the delamination of the small area was insufficient tocause adhesion failure in the 40 mm × 40 mm area. This illustrates the limitations of the bondstrength test.

4.2.3 Analysis of debonding evolution based on strain monitoring results
Section 3.4 studies the debonding evolution based on the shear strength test. In the 40 mm ×
40 mm range, the debonding evolution follows an "S-shaped" path, characterized by three stages
with two critical inflection points. As mentioned before, the strength tests are periodic, the results
show dispersion and variability, and the "S-shaped" degradation path is based on statistical findings.
The strain monitoring approach focuses on the evolution of debonding over a tiny area (effective
length of the FBG sensors). The strain histories of H-M and H-C provide insight into the wholemortar/concrete adhesion process, from health to damage. The strain trajectory of the 1st cycle
represents a healthy state of adhesion, where the strains of H-C and H-M almost overlap, and the83Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

gap is shown only at the peak strain. In cycles 2-5, the strain trajectories gradually separate, butobserving the residual strains, the tilt direction of H-C and H-M is the same (toward shrinkage), andthe strain behavior of the two materials maintains mutual follow-through during this period; hence
that adhesion integrity is not damaged. However, the gradual separation of strain trajectories may
represent that debonding is developing. As mentioned earlier, cementitious materials are bonded toeach other primarily by mechanical interlocking, which means that the separation of strain
trajectories may represent the breakage of some mechanical bonds, and the bond integrity can be
maintained before these breaks are joined into a small debonding area. Thus, although the materials
move in the same direction, their connection becomes weaker. In cycles 5-7, the residual strain in
H-M continued to become larger in the negative value, but H-C did not follow this trend. In thisperiod, debonding has developed to the extent that concrete is not allowed to follow the deformationof mortar, more mechanical bonds break, and small areas of debonding gradually form. In cycles 719, H-C moves in the opposite direction of H-M, and the growth rate of residual strain goes througha fast-to-slow process.
The 7th cycle can be considered an inflection point in the debonding evolution, where thebroken mechanical bonds within the sensor's detection range form a small debonding area. The
mortar and concrete in this area lose their restraining effect on each other, and the strain behavior ofeach element tends to a "free" state. Subsequently, the debonded area gradually increases. Fig. 49(a) shows that the cycle-by-cycle increase in the deformation of mortar and concrete leads to anincrease in the strain amplitude ratio, which indicates that the constraint between the two becomes
weak. After the delamination of point H-C, the evolution of debonding in the surrounding area
continues, and the growth of the residual strain slows down. Nineteen cycles later, a complete
adhesion failure is reached within the sensing range of the FBG sensor (as evidenced by thepenetration cracks observed on the mortar surface), and the stagnation of the strain amplitude ratio
indicates a dynamic equilibrium between the movement of the mortar and that of the concrete.
The initial shear strength of the specimens in this work is low, and the evolution of debonding
in the thermal loading center is accompanied by the beginning of the aging cycle. However, moredetails can be observed at the thermal loading boundary where the temperature applied is lower.
According to the HB-C and HB-M residual strains (Fig.4-7(b)), the debonding evolution starts
abruptly in the 11th cycle and develops rapidly in cycles 11-17. After 20 cycles, the strain amplitudes
of both HB-C and HB-M (Fig. 4-9(b)) no longer change, and it is considered that, at this moment,
the two have regained equilibrium. According to Feldfogel and Rabinovitch [89] [90], the debonding
mechanism initiates abruptly after encountering a threshold load. After rapid but stable development
until the next threshold load, debonding propagation becomes unstable, and the system breaks
84

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD

rapidly. Only similar first two phases (initiation phase and stable propagation phase) could be
distinguished in this study. This may be because this study focused on a tiny area, while their study
was conducted on a tile. The final unstable propagation phase may represent the accumulation ofsmall areas of debonding. On the other hand, adhesion failure may not have occurred where HB-C
and HB-M are located (refer to the shear strength after 30 cycles). If the aging cycle continues,
fatigue-induced adhesion failure may occur.
In contrast to Section 3.4, no plateau period for the evolution of debonding (bond strength does
not decrease with the aging cycle) was observed in this work. The initiation of the debonding
mechanism on a tiny area maintains a steady propagation until the bonding failure. The assumption
can be made that a plateau period during the degradation of adhesion over a large area may represent
that some tiny delamination is accumulating in the area and that the shear strength continues todecrease only after a certain threshold is reached, e.g., if the debonded area gets to a certain
percentage.
The application of FBG sensors to monitor the debonding evolution of mortars has provenfeasible. Moreover, the embedding of the FBG sensor had almost no effect on the adhesive
properties of the tested material compared to previous studies using strain gauges. By monitoring
the strain of each element, it is possible not only to capture the moment when delamination occurs
but also to estimate the progress of degradation based on the change in strain behavior. In durability
assessment tests, a combination of strength tests and strain monitoring can be considered to assess
the loss of adhesion of specimens, which may improve the accuracy of durability assessment results.

4.3 SUMMARY
This chapter describes the implementation process of durability assessment methods andproposed improvements. In the first study, the durability assessment was run by comparing the loss
rate of bond strength with four types of specimens prepared using different techniques. The
feasibility of the durability evaluation method is verified, which consists of the PTL aging cycles
and a direct shear test. The second study proposed a method for continuous assessment of adhesion
loss. The purpose is to improve the accuracy of adhesion measurement, thus, the reliability of
durability evaluation. The strain behavior of mortar and concrete under the PTL aging period wasmonitored with FBG sensors, and the debonding evolution process was analyzed. Several strain
characteristics representing the adhesion loss were put forward. The following conclusions were
derived:85

Chapter IV APPLICABILITY OF THE DURABILITY ASSESSMENT METHOD(1) The durability assessment method can be used to compare the durability of mortar–
concrete adhesion systems made using different methods and to approximately understand theinfluence of various factors on the durability.
(2) Assessing adhesion loss by shear strength tests can yield statistically significant durability
assessment results, but there is a risk of accuracy. This is especially true when comparing two
specimens with small differences in durability. More valid parameters need to be defined fordurability assessment.
(3) Essential parameters affecting the long-term adhesion of mortar to the concrete are thedrying shrinkage, modulus of elasticity, and thermal expansion coefficient of the materials. These
three physical properties combine to determine the difference in deformation between mortar andconcrete.
(4) The strain monitoring method enables observation of adhesiveness from health to damageon the same specimen. The degradation progress of the adhesion can be estimated from thecharacteristics of the strain behavior. When the adhesion is intact, overlapping mortar and concrete
strain trajectories can be observed; during the cumulative stage of adhesion loss, strain trajectories
gradually separate but progress in one direction; strain trajectories lose their followability to eachother, representing the occurrence of delamination.
(5) According to the strain monitoring results, the degradation effect of the PTL aging cycle
comes not only from the thermal expansion mismatch between materials but also from the drastic
drying shrinkage under the influence of high temperature. Therefore, the durability evaluation
results represent the combined ability of the specimens to resist solar radiation and drying shrinkage.

At present, the prototype of the durability evaluation method has been formed. The method isvaluable for achieving evaluation in the early stages of product development. Still, it is only suitablefor comparing several specimens with widely varying specifications at the laboratory level. Some
improvements are needed to make it universally usable. The definition of valid parameters is an
issue that deserves to be studied. After an experimental study, it was concluded that a combinationof strength testing and strain monitoring could be considered to assess the loss of adhesion of
specimens. The combination of continuous monitoring and periodic testing can improve thereliability of the durability assessment results. However, the small sample size is insufficient to havean explicit knowledge of the strain behavior of the composites, and further studies are needed. On
the other hand, the volume of specimens is small. The reference value of the results of durability
evaluation of mortar/concrete composites on a small scale for practical engineering design is one of
the issues that must be considered in the future.
86

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

Chapter V
DELAMINATION DETECTION WITH FBGSENSORS UNDER PTL AGING CYCLE
In Section 1.2 of Chapter 1, the three crucial phases that determine the service life of a structure
are outlined, namely pre-construction durability design, preventive maintenance, and appropriate
and timely repair. For the first stage, Chapters 3 and 4 establish a laboratory-scale durability
assessment method. This chapter discusses a study to achieve preventive maintenance with highimplementation efficiency, including the development of effective NDT techniques for detecting
facade cladding damage.
Preventive maintenance refers to periodic damage detection and maintenance work on
buildings. As a building element directly exposed to environmental erosion, the efficiency of thepreventive maintenance of facade cladding materials is also a matter of public safety. The BuildingStandard Law in Japan, revised in 2008, emphasizes the maintenance and management of exteriortiles on specific buildings (designated by government decree or by specific administrative bodies),
requiring a report on diagnostic results every ten years. Similar to Japan, in Singapore [158], Hong
Kong [97], the United States, and Canada [159], periodic inspection laws for buildings in service
have been enacted to prevent the spalling of building finishes. In this context, researchers have madeefforts to develop efficient damage detection techniques.
The basic technique for maintaining and managing facade cladding is "delamination
detection." Section 2.3 describes several field-applicable methods and laboratory-applicable
methods. Among them, the strain monitoring method can meet high accuracy and field applicability.
Moreover, it is a pre-embedded sensor method, meaning the inspection can be carried out
continuously. According to previous studies on the evolution of mortar debonding, the debonding
mechanism is abruptly initiated. It develops rapidly, and the continuity detection technique can beused to minimize missed detections. On the other hand, this technique can determine the safety,
extent of damage, and continued availability of building facades as soon as possible after a majordisaster such as an earthquake. The strain monitoring method is therefore considered to have thepotential for development.
In Chapter 4, the strain monitoring method was practiced on mortar/concrete specimens using
FBG sensors. The debonding evolution was reflected in the strain behavior of the mortar andconcrete. The possibility of monitoring strains by means of FBG sensors to detect damage to facade
87Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

cladding materials was initially validated. However, it must be acknowledged that the insufficient
sample size and the simple composition of the specimens of the study are mentioned above. Further
research is needed in this area if strain behavior is to be fully understood and appreciated. Therefore,
in this chapter, the application of FBG sensors for damage detection on facade cladding materials isfurther investigated using tiles as the object of study and the proposed PTL aging cycle to simulate
natural degradation. The aim is to clarify the feasibility of the strain monitoring method, its
applicability, and the interpretation of the detection results.

5.1 EXPERIMENT ON SMALL-SCALE SPECIMEN WITH TILES
The first study was carried out on a small scale with tiled specimens. The aim was to investigate
the applicability of the strain monitoring method in simulating multilayer composite structures withtile finishes.

5.1.1 Tile specimen preparation
Detection point ID
Concrete

Renderingmortar

Tile

H-C

H-M

H-T

Thermalload
HB-C
boundary

HB-M

HB-T

Nonheated
area

N-M

N-T

Thermalload
centerN-C

Figure 5-1 Outline of tile specimen and ID of detection points.
Thermal Load cycles
Peaktemperature

Cycle

40℃

1-6

50℃

7-12

60℃

13-18

80℃

19-76

40℃

77-83

Figure 5-2 Tile specimens in operation and PTL ageing cycle manipulations.
88

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE(a) Thermal loading center(b) Thermal loading boundary(c) Non-heated area(d) Enlarged view of the 18th cycle

Figure 5-3 Temperature history of the tile specimens.
The dimensions of the specimens are shown in Fig. 5-1. The materials and preparation
procedures for the specimens (concrete and rendering mortar) were identical to those in previous
studies, except for the parts other than adhesive mortar and tiles, referring to standard specifications(SV specimens). The concrete substrate has dimensions of 100 mm × 100 mm × 400 mm, and therendering mortar has dimensions of 10 mm × 50 mm × 400 mm. After the rendering mortar had
cured for 7 days, mosaic tiles (45 mm x 45 mm x 6 mm) were installed on the upper surface withadhesive mortar. Adhesive mortar is a pre-mixed commercial product with high water retention andviscosity. The adhesive mortar was applied according to the manufacturer’s specifications and JASS
19 [160], which establishes the execution procedures and ceramic tile installation control withorganic and cementitious adhesives on facades.
Fig 5-1 shows the configuration and ID of the FBG sensors for the specimen. FBG sensors
89Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

were attached to substrate surfaces (between concrete and rendering mortar), rendering mortar
surfaces (between rendering mortar and adhesive mortar), and tile surfaces. As in Section 4.2, thestrain and temperature evolution were monitored in the thermal loading center, thermal loadingboundary, and non-heated areas. Fig. 5-2 shows the programmed settings of the rubber heater. The
aging cycle pattern is essentially the same as in the previous study, with the difference that thesetting temperature of the rubber heater is gradually increased from 40°C to 80°C to maintain arelatively long healthy period. After the last cycle at 80 °C, the peak temperature was dropped again
to 40 °C, and the strain response of specimens before and after accelerated degradation wascompared.

5.1.2 Temperature monitoring of tile specimens
The temperature history of the detection points is shown in Figure 5-3. At the thermal loading
center, the maximum temperature difference between H-T and H-M is about 20 °C, and that between
H-M and H-C is about 10 °C. This is because the thermal conductivity of tiles is less than that of
cementitious materials. At the thermal loading boundary, the temperature difference between HBM and HB-C is still 10 °C, although the maximum temperature of HB-T is lower than that of H-T
by about 20 °C. It can be speculated that the thermal expansion differences between rendering
mortar and concrete are similar at the center and boundary of the thermal loading. In the non-heatedarea, the temperatures of the three materials remain almost the same, which can ensure that thethermal load does not damage the area.
Fig. 5-3(d) zooms in on the temperature history at the thermal loading center for cycle 18. The
temperature evolution in the tile system is different from that in the mortar/concrete system. One
effect observed is the reversal of the temperature pattern after the onset of cooling, where thetemperature of the tile surface decreases faster than the temperature of the other layers. This is thesame as the temperature evolution of the prototype wall tilings noted in reference [31]. The rapid
warming and cooling of the tiles induce more stress accumulation at the adhesive interface.

5.1.3 Strain monitoring of tile specimens
Fig. 5-4 shows the strain history of the tile specimens; (a) (b) (c) lists the strain evolution of
the tile surface, the rendering mortar surface, and the concrete surface in parallel positions in areas
subjected to different levels of thermal loading, respectively. Fig. 5-5 shows the evolution of theresidual strains of each layer (definition of residual strains in Section 4.2).

(1) Thermal loading center (H-C, H-M and H-T)
90

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE(a) Thermal loading center(b) Thermal loading boundary(c) Non-heated area
Figure 5-4 Strain history of the tile specimens.
The strain history of the heating center shows that in the early stages of heating, there is nodifference in the strains of the tiles, rendering mortar and substrate, which move as a whole.
However, when the heating temperature was increased to 80 °C, they each began to behave
differently. For the tile/rendering mortar interface, the direction of motion of H-T and H-M diverged
from cycle 18, with H-T moving toward the tensile side and H-M moving toward the compressive
side. This indicates that the adhesion between the tiles and the rendering mortar starts to lose after
18 cycles. As for the rendering mortar/concrete interface, the debonding evolution starts from cycle
33. the motion directions of H-C and H-M diverge from cycle 33, with H-C turning around andmoving toward the tensile direction.
The variation of residual strain shows more clearly (Fig. 5-5(a)) the point of divergence in thestrain behavior of the layers. Starting from cycle 18, the adhesion between the tiles and the rendering
mortar becomes less and less able to restrain the drying shrinkage. The residual strain of the tiles
91

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE(a) Thermal loading center(b) Thermal loading boundary(c) Non-heated area
Figure 5-5 Residual strain of each detection point at the end of each cycle.
was observed to show an abrupt gain at cycle 29 and then started to increase gradually (with constantresidual temperature). This may be due to the voids at the interface causing the tiles to protrude in
the out-of-plane direction. The debonding evolution of the concrete/mortar interface is the same as
in Section 4.2. During the damage accumulation (cycles 18-33), the concrete follows the mortar
towards the shrinkage side, but the gradual increase of the strain difference indicates that theadhesion is degrading. Starting from cycle 33, the concrete shows a strain behavior opposite to thatof the mortar. It is presumed that debonding may have occurred at this moment, rendering the mortar
loses its ability to confine the concrete.

(2) Thermal loading boundary (HB-C, HB-M and HB-T)
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Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

The strain trajectories of the tiles and rendering mortar located at the heating boundary follow
a pattern similar to that of the heating center. From cycle 18 onward, HB-M progressed towardshrinkage with the aging cycle and separated from the strain trajectory of HB-T. However, observing
the residual strain of HB-T (Fig. 5-5(b)), HB-T has maintained a steady motion and showed a slighttendency to shrink. It is likely that the tile/rendered mortar interface at the boundary of the heatedregion did not delaminate and only partially lost its adhesion.
Interestingly, concrete (HB-C) exhibited anomalous strain behavior. The residual strain of HBC underwent a three-phase transition from tensile-shrinkage-tensile. It gradually increased fromcycle 18, showed a brief decrease in cycles 47-55, and maintained at a stable high-level position
after a massive outbreak of increase in cycle 56. That is, the concrete is released from the mortar
restraint from cycle 18, and cycles 18-56 represent the period of development of debonding.
However, interpreting the strain anomaly behavior for cycle 56 is challenging. One reasonablespeculation is that with the accumulation of debonded areas, a sudden outbreak of large debonding
occurred in cycle 56, i.e., complete adhesion failure within the detection range of the FBG sensor.
The loss of pressure from the overlayer kept the HB-C sensor at a tensile state. Another possibility
is that the FBG sensor has been damaged along with the debonding.

(3) Non-heated part (N-C, N-M and N-T)
Based on the strain in the non-heated zone (Fig. 5-4(c)) and the residual strain (Fig. 5-5(c))
history, no debonding occurred at either interface. N-C and N-M strain trajectories maintained goodfollow-through, both towards shrinkage. N-T did not show a shrinkage tendency, which may be dueto the buffering effect of the adhesive mortar layer. The adhesive mortar layer absorbed most of the
traction force from the rendering mortar, and the joints allowed for more deformation of a single
tile. The residual strain results indicate that even with healthy adhesion, to some extent, the tiles do
not move in the same direction as the rendering mortar (note the small range of representation of
the vertical axis in Fig. 5-5(c)).

(4) Comparison of strain behavior before and after accelerated degradation
Next, the strain responses of the specimens were compared before and after accelerated
degradation at the same aging cycle (peak temperature of 40 °C). As shown in Fig. 5-6, there wasno significant difference in the strain trajectories of the layers of material before degradation. In thethermal loading center, the peak strain of the mortar showed a difference of about 15 μ. The
maximum temperature difference between mortar and tile at this moment is about 7 °C, but thecoefficient of thermal expansion of mortar is much larger than that of tile. After subjecting to 76
93

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE(a) Thermal loading center(b) Thermal loading boundary
Figure 5-6 Comparison of strain behavior before and after accelerated degradation.
aging cycles, the strain trajectories of the layers of the material showed dispersion. The dispersion
increased to 30 μ between mortar and tile and more than 40 μ with concrete. This indicates that themortar is bound by the tiles and concrete before deterioration but has the highest thermal expansion
coefficient and can expand freely after the adhesion loss.
At the thermal load boundary, the strain trajectories of the layers before degradation almost
overlap. After degradation, the mortar and tiles maintain a healthy adhesion, but the concrete
exhibits a different strain behavior. Combined with the previous speculation, it is believed that anadhesion failure occurred at HB-C and that a construction defect may exist at this location.

5.1.4 Analysis of strain amplitude
The evolution of the strain amplitude for each layer after 18 cycles was calculated (for thedefinition of strain amplitude, refer to Fig. 4-8) as shown in Fig. 5-7. Notably, the difference in therange of vertical coordinates in Fig. 5-7 (a) (b) (c). In the non-heated area, the strain amplitudes of
the three materials remain stable so that it can be reconfirmed that the bond integrity is notcompromised at this location.
At the thermal loading center, the strain amplitude of H-M increases cycle by cycle, with afaster growth rate from cycles 18-30, a slow growth rate from cycles 30-42, and no further change
after cycle 42. As mentioned earlier, the growth of strain amplitude implies that the mortar is losing
constraint, and the adhesion is slowly lost. The strain amplitude of H-T shows a slight decrease once
94

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE(a) Thermal loading center(b) Thermal loading boundary(c) Non-heated area
Figure 5-7 Evolution of strain amplitude of each detection point with aging cycle
at cycle 29, decreasing by about 50 μ. The reduction in deformation of the tiles is accompanied by
the loss of adhesion due to the smaller thermal expansion coefficient of the tiles. The decrease instrain amplitude of H-C occurs after cycle 33 for reasons similar to those of H-T. According to theabove observations, the debonding evolution of the tile/rendering mortar interface starts at cycle 29,
and the debonding evolution of the rendering mortar/concrete interface starts at cycle 33.
Here, "debonding evolution" represents a significant loss of adhesion, not an adhesion failure.
Over the 10 mm effective length of the FBG sensor, the debonding area is gradually accumulated
until bond failure is reached within the detection range. In the strain monitoring method, the damage
is determined by comparing the strain response of the healthy part. This is a qualitative rather thana quantitative diagnosis. It is possible to determine the generation of debonding by the anomaly ofstrain response, but it is challenging to decide on the size of debonding area. The authors concluded95Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE(a) Rendering mortar/concrete interface(b) Tile/rendering mortar interface
Figure 5-8 Evolution of strain amplitude ratio with aging cycle, (a) rendering mortar toconcrete, (b) tile to rendering mortarthat the anomalous change in strain amplitude implies the appearance of voids at the interface andtherefore defined the moment of discovery of voids as the moment of initiation of debonding
evolution. Comparing the strain amplitude in the non-heated region (Fig. 5-7(c)), the accumulationof damage starts from 18 cycles but does not reach the level of "debonding" yet.
At the thermal loading boundary, the strain amplitude of HB-M evolves similarly to that of HM, but no significant decrease in HB-T is observed. Thus, the tile/rendered mortar interface does
not reach the conditions for initiating the debonding mechanism, although there is a certain degreeof adhesive loss. The strain amplitude of HB-C decreases in cycles 38-48, increases in cycles 4856, and remains stable with a small amplitude after an abrupt drop in cycle 56. It is essentiallycertain that after 56 cycles the adhesion at the rendering mortar/concrete interface failed over asignificant area. The concrete surrounding the FBG sensor returns to its "free" state, deforming only
slightly under the influence of thermal loads. Interestingly, the phenomenon of temporary increasein concrete strain amplitude (48-56 cycles) prior to adhesion failure was also observed in themortar/concrete specimens in Section 4.2, again at the thermal loading boundary. The mechanism
96Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

is not yet clear, but this phenomenon can be considered as a precursor to adhesion failure.
Fig. 5-8 (a) and (b) show the strain amplitude ratios of concrete to rendering mortar and tile torendering mortar, respectively. The strain amplitude ratios in the non-heated regions remain stableat both interfaces, which is proof of healthy adhesion. For the mortar/concrete interface, the central
amplitude ratio of the thermal load increases from 18 to 48 cycles, with a more significant increase
from cycle 33 onwards. The growth rate of the amplitude ratio reflects the debonding evolution rate.
After cycle 48, the mortar and concrete reach an equilibrium state, which may lead to adhesion
failure due to fatigue if the loading temperature is not further increased. According to
Mahaboonpachai et al. [12], damage accumulated in terms of micro-cracks at the interface when thetemperature on the tile surface is around 80°C. The results of the thermal loading boundary are notdiscussed further due to the disturbing nature of the HB-C data.
For the tile/mortar interface, similar behavior is observed at the central and boundary of thethermal load. The amplitude ratio decreases from 18 to 36 cycles due to the increased motion of themortar and the decrease of the tiles, reaching equilibrium after 36 cycles. Again, it can be consideredthat the maximum damage at this load intensity has been reached.

5.1.5 Response of strain behavior to adhesion state
Three adhesion states can be determined based on the analysis of strain behavior: healthyadhesion, adhesion damage (partial debonding), and adhesion failure (complete debonding).
- Healthy adhesion. The strain behavior in the non-heated regions (N-T, N-M, N-C) representsa healthy bond. The strain behavior between all layer materials maintains some correlation, and astable strain amplitude ratio is maintained.
- Adhesion damage (partial debonding). Adhesion damage occurs at both interfaces in thethermal loading center, manifested by the separation of the strain trajectories of the materials. The
sudden decrease/increase in strain amplitude represents a loss of correlation, and the materials lose
their followability to each other's deformation.
-Adhesion failure (complete debonding). Adhesion failure occurs at the mortar/concreteinterface at the thermal loading boundary. The material exhibits a different strain behavior thanhealthy adhesion, accompanied by a more dramatic change in strain amplitude.

Determining the boundary between adhesion damage and adhesion failure is complex, and thedifference may lie in the intensity of the change in strain behavior. The anomaly exhibited by point
HB-C may be due to an initial defect. If the load intensity is further increased, other test points may
exhibit similar strain behavior.
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Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

Consistency in the overall deformation direction (tensile/compression) at the mortar/concretebond interface represents a healthy adhesion, and loss of consistency implies adhesion damage.
However, due to the buffering effect of the adhesive mortar, the deformation direction of the tiles isless influenced by the mortar, therefore, the correlation between the strain behavior of the tiles andthe mortar needs to be interpreted with care. In addition to comparing the strain behavior in healthy
conditions, more relevant data need to be accumulated. It is certain, however, that with the loss ofadhesion, the material moves closer and closer to its unconstrained "free state."
In summary, strain monitoring makes it possible to clarify the degradation progress of theadhesion. The initial data (healthy state) should be retained for engineering applications. If the strain
behavior shows a loss of follow-through, one should be alert and increase the frequency ofinspections; if the strain behavior shows severe abnormalities, repair and replacement ofcomponents should be considered.

5.2 EXPERIMENT ON LARGE-SCALE WALL SPECIMEN

Previous studies have verified the feasibility of detecting delamination with FBG sensors on
small-scale specimens. By monitoring the interlaminar strain, we can observe the adhesion
degradation process of the four-layer composite (tile-adhesives-rendered-concrete) under PTL aging
cycles. However, the results obtained on a small area of the adhesive interface cannot be directly
used as a design reference in practical engineering, and its corresponding conclusions are flawed.
For example, it is impossible to simulate the constrained behavior of structural components such as
beams and columns. Firstly, due to the dimensional limitations of the small-scale specimens, thedeformation due to thermal loading is only constrained to one dimension, i.e., along the length of
the specimen. It is almost free in the other direction, which does not correspond to the constraint
conditions of an external wall when subjected to solar radiation. Secondly, small-scale specimens
are heavily influenced by drying shrinkage, which interferes with interpreting the experimental
results. Therefore, validating the proposed AT method with the delamination detection method onlarge-scale specimens is necessary.
In this section, a specimen of the concrete substrate, consisting of beams, columns, and walls,
was created to simulate a wall. PTL aging cycles were applied to the specimen, and the wall wasconstrained from both directions in the plane to simulate the actual solar radiation on the outer wall.
To further confirm the reliability of delamination detection using the FBG sensor, we adopted theacoustic diagnosis as the circumstantial evidence for the occurrence of delamination. In addition,
two types of tile adhesives commonly available on the market were tested: bonding mortar and98

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

organic adhesives.

This study contained three objectives, 1) to validate the feasibility of FBG

sensor detection of delamination on a large-scale specimen, 2) to investigate the applicability of the
proposed AT method, and 3) to investigate the difference in the debonding evolution between
cementitious and organic adhesives.

5.2.1 Specimen preparation
As shown in Fig. 5-9, a small concrete wall specimen was prepared as the substrate with overall
dimensions of 505 x 505 mm, surrounded by a beam and column with a cross-sectional area of 100

Figure 5-9 Outline of wall specimen.
x 100 mm and reinforced with 6 mm diameter steel bars. The tiles were laid in the central area used
to simulate the wall, which measured 50 x 305 x 305 mm, with wire mesh inside. The mixture
proportion of the concrete substrate is listed in Table 3-2. Twenty-one ceramic tiles measuring 90 x
45 mm with a jagged surface were laid on the front and back of the substrate using bonding mortar
and organic adhesive, respectively.
Bonding mortars and organic adhesives are two common tile adhesives currently available on
the Japanese market. The former, whose main component is a polymer-cement mortar, has a long
history of use and is still the most widely used. The latter, commercially known as elastic adhesives
and is an organic adhesive, with urethane resin or modified silicon adhesives as the major
component, which has only been used as a tile adhesive in the last 20 years but is rapidly gaining
widespread development and application due to its excellent physical properties and ease of
construction. The same bonding mortar as in Section 5.1 was used in this study. Organic adhesives
come from the same manufacturer, both adhesives are commercially available in premixed type, and99

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

the methods of application and mixture proportion are in strict accordance with the manufacturer's
instructions.
In this study, no rendered layer was applied between the concrete substrate and the bonding
mortar to minimize the differences in drying shrinkage between the materials. Furthermore, theconcrete substrate was allowed to cure in a general indoor environment for two months to ensurethat most of the drying shrinkage had occurred.

5.2.2 Aging cycles and delamination detection
As previously mentioned, to investigate the applicability of the proposed AT method under
two-dimensional constraints, the rubber heater used to provide the thermal load was secured to thesurface of the four central tiles by heat-resistant double-sided tape. Fig. 5-10(a) shows the area ofthe specimen subjected to the thermal load and the configuration of the sensors. Fig. 5-10(b) shows(a) arrangement of sensors in the specimen
Thermal Load cycles
Peak tem. Cycle
40℃
1
50℃
1
60℃
1
70℃
1
80℃
1
90℃
1
100℃
1
110℃
20
120℃
Until delamination(b) setup of the experimental device and aging cycles
Figure 5-10 Schematic of the operation of the PTL aging cycle experiment.
100Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

the setup of the experimental device and aging cycles.
The delamination detection point is set in addition to the center of the area subjected to thermalload on the front (bonding mortar) and the back (organic adhesive). There is also a detection point
for the area on the front that is not heated. The detection points are named with symbols: the partsubjected to thermal load is denoted by H and vice versa by NH. For example, H-Tile represents thedetection point on the surface of a tile in the area subjected to thermal load. Ideally, areas subjectedto thermal loads will delaminate while other areas will retain integrity. As described in Section 5.1,
damage in strain monitoring methods is inferred indirectly through abnormal strain response andtherefore requires reference to data from sound sites. At each detection point, two FBG sensors areplaced parallel to the substrate surface and the tile surface. According to the results of the study insection 5.1, delamination was generally the first between the two cementitious materials. As the
specimen was an utterly symmetrical structure, the thermocouple used to calculate the temperaturecompensation was set in a symmetrical position on the FBG sensor to prevent the thermocouple
wires from becoming the starting point for debonding.
A gradual warming approach was still taken to run the PTL aging cycle, as shown in Fig. 510(b). The rubber heater starts at 40 °C and rises cycle by cycle until delamination occurs. Although
the temperature setting of the rubber heater was relatively high, the actual temperature of the tile
surface was about 90 °C at the maximum, as shown in Fig. 5-11. Referring to Section 5.1 for the
degradation speed of the aging cycle with a peak temperature of 80°C, the high-temperature setting
for this work is intended to accelerate the degradation process. Similar to Section 5.1, a secondary
cyclic thermal load was applied to the specimens diagnosed as delaminated after they had cooled,reproducing the temperature history in the sound condition to compare the trajectory of the strain
occurring before and after delamination.
It is worth noting that the ATs on both sides of the specimen are not performed simultaneously
but sequentially. Since the test was conducted from the mortar side, it is possible that the organic
adhesive on the back side was also affected by the test on the mortar side. Still, in any case, sinceno delamination occurred on the organic adhesive side, there was little influence of heat transfer tothe backside.
Acoustic diagnostics are performed at the end of each cycle to assist in monitoring the progressof degradation. As shown in Fig. 5-9(a), the tile undergoing percussion inspection is located above
the tile attached to the FBG sensor. Due to the symmetry of the specimen, tiles in symmetrical
positions are considered to undergo the same degradation process. The same inspector tapped thecenter of the tile with a special hammer, taking an average of three measurements. A microphone
was fixed 30mm above the percussion point to record the percussion audio.
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Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

Figure 5-11 Temperature history of bonding mortar side.

5.2.3 Results of the bonding mortar(1) Temperature monitoring
The temperature history of the detection points on the side where the bonding mortar wasapplied is shown in Fig. 5-11, with a temperature difference of around 40°C between the control
temperature of the rubber heater and the actual temperature of the tile surface. The temperature
variation in the tile without thermal load is minimal compared to the area subjected to the thermalload, and it can be assumed that the adhesion integrity of the area is always maintained. The
framework beam column that provided the constraint in the specimen was barely affected by thethermal load and was essentially the same as the ambient temperature, providing the expected
constraint conditions.
(2) Acoustic diagnosis results
The acoustic diagnostic results at the end of the 0th, 17th, 27th and 37th cycles were conducted.
The sound pressure level (SPL) waveforms of the percussive audio were calculated, and a Fast
Fourier Transform (FFT) was performed. As shown in Fig. 5-12, the maximum absolute value ofthe SPL is defined as the relative maximum amplitude derived from the SPL - time waveform. Soeta
et al. [161] supported the reliability of using the relative maximum amplitude derived from thesound waveform as a diagnostic criterion for stratification. Considering the sensitivity of using themagnitude of sound pressure (Pa) as an evaluation metric, we converted this metric to soundpressure levels. Still, the trend in magnitude was not altered. The SPL - time waveform is calculatedusing the audio processing software "WavePad "as follows:𝐿𝑃

20lg 𝑃/𝑃
102

(4-1)

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

0 Cycle17 Cycle

37 Cycle
Figure 5-12 Value (left), change rate (right) of Figure 5-13 Fourier amplitude spectrum

maximum SPL of impact sound.

of impact sound.

Where 𝐿𝑃 is the sound pressure level (dB), 𝑃 is the sound pressure (Pa), 𝑃 is the reference
acoustic pressure (2×10-5 Pa)
As a diagnostic basis, these two analysis methods of percussive audio have been validated inseveral previous studies in our laboratory [162]. Fig. 5-12 and Fig. 5-13 show the maximum SPL
and the Fourier amplitude spectrum, respectively. Both diagnoses showed that at the end of cycle
37, only the impact sound of the heated area tiles showed an unusual variation. The maximum SPL
increased significantly by more than 4000 dB (approximately 38%) compared to the initial period.
Similarly, the Fourier amplitude spectrum results showed an unusual spectral peak. We determinethat delamination of the heated tiles has occurred at this moment. It is worth noting that in thisexperiment, the peaks of the amplitude spectrum were shifted towards higher frequencies along withthe peeling, but in other experiments, there have been shifts towards the lower frequency side. We
plan to investigate the mechanism in the future.
(3) Strain monitoring
Fig. 5-14 (a) and (b) show the strain history of the detection points in the non-heated andheated zones, respectively. In the non-heated zone, the strain of the tiles is less than the strain of thesubstrate, while in the heated zone, the opposite is true. This can be explained by the temperature
history, where there is a temperature difference between the tiles and the substrate in the heatedzone. In contrast, in the non-heated zone, the temperature of the two mostly agrees. This is due tothe tile joints preventing heat conduction between the tiles, whereas the substrate has a continuityof heat transfer. Furthermore, the tiles used in this study have a lower coefficient of thermal
expansion than the substrate.
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Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE(a) Heated zone(b) Non-heated zone

Figure 5-14 Strain history of concrete and tile surface.
As mentioned earlier, the occurrence of delamination is indirectly inferred from the observed
anomalous strain response. Theoretically, some correlation should be maintained between the strainof the tiles and the concrete in a sound state. Along with the degradation process, this correlation
will disappear the moment delamination occurs, when the tiles lose the restraint of the substrate. To
this end, the absolute value of the strain difference between them was calculated. The “|strain
difference|” is given as follows:
∆𝜀

|𝜀𝑇

𝜀𝐶|

(5-1)

Where ∆𝜀 is the |strain difference|, 𝜀𝑇/𝜀𝐶 is the strain on tile/concrete surfaces.

Fig. 5-15 shows the history of strain differences in the unheated and heated zones, respectively.
At the end of each thermal cycle, i.e., when the temperature of the heated tile has dropped to its
minimum, the strain difference at this moment is defined as the residual strain difference and ismarked in the figures.
The strain differences at both detection points, H and NH, increase with the aging cycles,
indicating degradation progression. However, the adhesion integrity of the non-heated zone wasalso not disrupted until the end of the experiment (which is also evidenced by the acoustic test
results), as no abrupt fluctuations were observed, indicating that the linkage between the strains of
the tiles and the substrate was always maintained. In contrast, there is a sharp increase in the strain
difference at the detection point H, especially in the period from the 33rd to the 37th cycle in Fig.
5-15(a). According to the strain history at point H shown in Fig. 5-14 (a), the strain in the substratestarts to develop overall towards the tension side from the 30th cycle onwards. By the 37th cycle,
the strain relationship between the substrate and the tiles has changed dramatically compared to thebeginning of the heating and combined with the acoustic diagnostic results. There is a highprobability of delamination between the tiles and the substrate at the end of the 37th cycle.
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Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

Figure 5-15 Strain difference between tile and concrete, (a) in the heated zone, (b) in the
non-heated zone, (c) details of 33rd to 37th cycle in the heated zone.
It should be noted that the rubber heater relies on natural cooling, with the residual temperature
at the end of each cycle also rising with the peak temperature. However, the detection point H was
subjected to thermal loading, despite the peak temperature did not change during the phase from33rd to the 37th cycle, and the residual strain difference rose intensely with each cycle. This might
be regarded the process by which delamination occurs. For further investigation, the strain history
for this phase was expanded, as shown in Fig. 5-15(c). The strain on the tiles increased rapidly byapproximately 30 µ before the peak temperature was about to be reached, while the substrate strain
did not follow this change. This indicates that the correlation maintained between them wassuddenly lost at this moment, which is most likely the moment when delamination occurred.
Subsequently, as the thermal load is cycled, the delamination gradually increases until it can be
diagnosed by percussion.
In addition, Fig. 5-16 shows the ratio of the strain amplitude of the tiles to the substrate during
each cycle, calculated according to Equation 4-1. By monitoring the correlation between the strain
tracks of the tile and the substrate, it is possible to conclude that the unheated zone always maintains
a healthy adhesion, with the strain range ratio remaining relatively stable at approximately 0.75
independent of variations in peak temperature. In contrast, the strain amplitude ratio in the heatedzone increased sharply after 34 cycles. This means that the strain amplitude of the tile becomes more
extensive than that of the structure after delamination, as the delamination reduces the substrate
constraint to which the tiles are subjected.
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Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

Figure 5-16 Ratio of the strain amplitudeof the tile to concrete during each aging
cycles.

Figure 5-17 Comparison of the strain
behaviour of the tiles and substrate before
and after delamination.

Finally, to verify the occurrence of delamination and to observe the strain behavior between
the tiles and the substrate after delamination, aging cycles were applied again, reproducing thetemperature history for cycles 12-17 (the period diagnosed as sound) with a tolerance of ± 5 °C. A
comparison of the temperature and strain history is shown in Fig. 5-17. The secondary thermal loads
were named cycles 40-44. It can be observed that despite having a similar thermal load temperature
history, after being diagnosed as delaminated, the strain magnitude relationship between tile andstructure reversed, showing a different strain behavior to that of the sound period. As described in
Section 5.1, the shift in the strain behavior of the tiles and the substrate indicates a loss of association.
It can be considered that delamination has occurred at this moment.
In summary, the proposed AT method can accelerate the degradation of wall specimens. The
whole process of delamination of the tiles from the substrate under thermal loads was successfully
observed by monitoring the strain with FBG sensors. The loss of correlation between the strain
trajectories of the material marks the onset of delamination. In addition, a discontinuous strain
response can be considered as a danger signal, which has never been observed in previous studies.

5.2.4 Results of the organic adhesives
After the test on the mortar adhesive side was completed, the rubber heater was replaced on
the back side, and the same test was conducted on tiles bonded with organic adhesive. The
temperature history of the other side of the specimen, the detection points on the side where theorganic adhesive was applied, is shown in Fig. 5-18. Despite a brief temperature control anomaly(cycle 36), the rubber heater has provided sufficient high-temperature heat load. In contrast to Fig.
106

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

5-11, the organic adhesives reduced the temperature difference between tiles and substrate to analmost negligible level. The acoustic diagnostic results are displayed in Fig. 5-19, where no signsof delamination were diagnosed during the 45 thermal load cycles.
The corresponding strain histories are shown in Fig. 5-20. In contrast to the bonding mortar
specimen, the residual strains in the tiles and substrate gradually shifted towards compression as thepeak temperature increased, probably due to the physical properties of the organic adhesives.
Interestingly, during cycles 38-45, only the strain in the tiles increased with the peak temperature,
while the strain fluctuations in the substrate remained stable. The speculation is that the organic
adhesives soften as the temperature rises, and with good elasticity, the tension from the expansionof the tiles is absorbed internally by the adhesives.
The path of the strain relationship between the tiles subjected to thermal loading and thesubstrate is shown in Fig. 5-21. The absolute value of the strain difference, the residual strain
difference, and the strain amplitude ratio are calculated in the same way as in Section 5.2.3 (1).
Based on the description above, the variation in correlation represents the occurrence ofdelamination, the same as in the acoustic diagnosis. No indication of delamination occurring wasobserved from the strain results. According to the results of the strain amplitude ratio, the strain
between the tiles and the substrate maintained a stable relationship, which represents a soundFigure 5-18 Temperature history of organic Figure 5-19 Acoustic diagnostic results onadhesives side.
the organic adhesive side.

Figure 5-20 Strain history of organic
adhesives side.

Figure 5-21 Strain relationship between thetiles and substrate on the organic adhesives
side.
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Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

adhesion integrity. Thereafter, the thermal load continued to run for a number of cycles, butdelamination was never diagnosed. Hereby, the organic adhesive is thought to be able to resist
delamination of the tiles due to sunlight and reduce the risk of spalling.

5.2.5 Delamination detection of organic adhesives under freezing-thawing cycles
Due to their small modulus of elasticity, organic adhesives make it difficult to obtain damage
information from the usual percussion methods. Although FBG sensors detected no delamination ofelastic adhesive specimens due to high-temperature thermal loading, an attempt was made to induce
delamination of organic adhesives by freeze-thaw cycles, given that similar organic adhesives
exhibit hardening phenomena (increased modulus of elasticity) at low temperatures.

(1) Experimental procedure
The specimens were transferred to a thermostatic chamber, and after they had reached theambient temperature of -10 °C, a cyclic thermal load was again applied in the original area. This
time the temperature of the tile surface subjected to the thermal load was strictly controlled. Except
for the temperature, the duration and pattern of the cycle remain the same: -10 °C to 40 °C (60 min)→ 40 °C (60 min) → 40 °C to -10 °C (60 min) → -10 °C (60 min). The cycle was performed 45
times, and the temperature error was controlled to ±3 °C.

Figure 5-22 Sensor configuration (left) and temperature history of concrete and tile
surface (right).
Unfortunately, the FBG sensor on the tile surface broke, and we replaced an electricalresistance strain gauge in its original position. In contrast, the substrate strain is still monitored withthe original FBG sensor. The detection configuration is shown in Fig. 5-22

(2) Experimental result
108Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

Fig. 5-23 shows the strain histories of the tile and substrate and the strain amplitude ratios foreach cycle. Similar to the Fig. 5-18, there is almost no temperature difference between the twomaterials, but the relationship between the magnitude of the strain results for the two is reversed.
The temperature amplitude is also around 50 °C for cycles 10-25 in Fig. 5-18 and Fig. 5-20. The
strain amplitudes of the concrete measured by the FBG sensors are similar for the same temperatureamplitude, but the strain amplitude of the tiles is significantly reduced (measured with strain gauges).
There are two possible reasons for this: 1) the variability of the strains measured by the different

Figure 5-23 Strain history of concrete and tile surface under freeze-thaw cycles.
sensors, even though both perform temperature compensation calculations; 2) the hardening of theorganic adhesive at low temperatures, which hinders the deformation of the tiles, while the substrate(without joints), the deformation is difficult to suppress by the organic adhesives.
Despite the differences between the sensors, the results of the strain amplitude ratios
demonstrate the correlation between the movement of the tiles and the substrate. Overall, a stablecorrelation between them was maintained, but the strain amplitude of the substrate increased slightly
in the 10th and 42nd cycles, expanding the strain history for these two cycles and observing a
discontinuous strain response, as shown in Fig. 5-23. In the 10th cycle, the substrate contracted byapproximately 50 µ when the temperature reached its trough, and a similar situation was observedin the following cycles. This suggests that delamination could have occurred at this moment, withthe hardening of the organic adhesive at low temperatures, having been momentarily peeled fromthe substrate, the constraint on the substrate surface being released, and the shrinkage instantly
increasing. However, the weakest temperatures provided in this study were insufficient to denature
the adhesive, which softened and returned to elasticity as the temperature increased. The adhesion
integrity was not damaged.
In summary, in terms of resistance to natural aging caused by sunlight, applying organic
adhesives reduces the risk of delamination of tiles compared to bonding mortars. Attention should
also be paid to the embrittlement of organic adhesives due to low temperatures or other factors.
109

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

Although the general level of cold weather does not cause permanent damage to organic adhesives,
there is a risk that transient denaturation at low temperatures can lead to tile delamination.

5.3 SUMMARY
This chapter describes the feasibility study of the SM method for delamination detection in tile
systems. PTL thermal loads were applied to tile specimens of two different scales and constituent
materials, and the strains of all layers were monitored with FBG sensors. The following conclusions
were derived:(1) The application of FBG sensors for the delamination detection of tile finishes has provenfeasible. Furthermore, the strain tracking of the composite material by the FBG sensor makes it
possible to observe the moment of damage at any of the bonding interfaces. This gives building
managers timely warnings and establishes the lowest-cost tile maintenance program.
(2) The strain response of an interface composed of different materials may differ whenadhesive damage occurs and needs to be inferred by comparison with the strain response of thatinterface when it is in a healthy state. In general, accompanying the degradation, material
deformation will increasingly converge to an unconstrained state. The disappearance of the original
followability between the strain trajectories of the two materials can be used to determine adhesion
damage. In the case of constant load, abnormal strain response, such as a sudden increase or decrease,can be considered a sign of pathology. Early warning can reduce the risk of accidental tile spalling.
(3) In exterior tilings, rendering mortar is the most deformable component. The response of therendering mortar to climate may be the main factor in delamination. Therefore, performance controlof rendering mortars (e.g., reducing the thermal expansion coefficient) may be an effective way to
improve the durability of exterior tilings.
(4) Quality control of the application is critical to the durability of the exterior tilings. Original
construction defects can lead to adhesion failure at low load strengths.
(5) Resistant to natural ageing caused by sunlight, the application of elastic adhesives reduces
the risk of delamination of tiles compared to bonding mortars. However, it is still to be careful offactors that can cause the elastic adhesive to become brittle, such as low temperature conditions.

In summary, this study provides evidence on the feasibility of FBG sensors for tile
delamination detection and describes simple diagnostic approaches. Pre-installation of FBG sensors
in the facade claddings of new building can greatly enhance the execution efficiency of the110

Chapter V DELAMINATION DETECTION WITH FBG SENSORS UNDER PTL AGING CYCLE

preventative maintenance phase. The FBG sensors are installed where the cladding is most likely topeel off, such as around openings in the building and joints between structural components. The
health of these locations is used as early warning information to plan the timing of a comprehensive
inspection rationally. If large strain fields need to be detected, a FBG sensor network could beconsidered for monitoring special buildings and high-risk areas. However, although it is possible todetermine adhesion damage qualitatively, it is currently difficult to interpret the strain behavior incorrespondence with adhesive properties. Further research is needed in combination with otherdiagnostic methods.

111

Chapter VI. CONCLUSION AND FUTURE STRATEGIES

Chapter VI
CONCLUSION AND FUTURE STRATEGIES
6.1 CONCLUSION
This dissertation describes a series of studies conducted to establish an objective-based durability
design procedure for RC building facades with rendered and tiling. A laboratory-scale durability
assessment method is proposed that can be utilized during the durability design phase to evaluatemortar/concrete adhesion with different materials and application procedures. A technique fordetecting delamination of facade claddings with FBG sensors is proposed. This technique contributes
to the efficiency of the execution of the preventive maintenance phase. In addition, key parameters
influencing the durability of the rendering mortar are investigated, as well as the debonding evolution
under the thermal loads. The main conclusions of each chapter's experimental and investigative work
are summarized below.

6.1.1 A laboratory-scale durability assessment method
Previous methodologies for durability assessment of rendered mortars applied to RC buildings
were shown to be inadequate and unreliable. Therefore, based on the actual degradation mechanism,a reliable AT method is proposed based on the PTL aging cycle. The proposed AT method proved to
be a very useful and powerful tool for assessing long-term adhesion between multiple cementitious
materials. It is beneficial for rapid assessment during the product development phase.

(1) The proposed durability assessment method is applicable to the climate zone in which Japanis located, characterized by no clear distinction between dry and rainy seasons, long sunshine hours,
and large daily temperature swings. The probability of damage to facade claddings in similar climatic
zones differs significantly by orientation.
(2) Solar radiation is identified as the main degradation agent, and thermal expansion mismatch
between materials is the main degradation mechanism of facade claddings. The proposed PTL aging
cycle can simulate this degradation mechanism. "Partial" thermal loading is critical. Both thermal loadand sufficient restraint must be provided to reproduce the degradation mechanism under solar radiation.
(3) The pull-off test and the direct shear test can measure the adhesion loss due to thermal loading.
At the central and boundary of the thermal loading area, the shear strength decreases by 34.5% and112

Chapter VI. CONCLUSION AND FUTURE STRATEGIES

56.4%, and the tensile strength by 27.3% and 35.4%, respectively. Therefore, it is considered that thethermal expansion mismatch between the materials generates stresses in both in-plane and out-ofplane directions at the bonding interface, and the shear stress is greater than the normal stress, which
is a realistic rule in general structures.
(4) All experimental results showed that the mortar/concrete specimens obtained the greatest lossof adhesion in part located at the thermal load boundary. The numerical analysis results confirm thatthe maximum concentrated stress value was obtained at this location. This indicates a reasonable
correlation between stress generation and the degradation mechanism of adhesion. Moreover, it canbe assumed that the weakest position of the facade claddings should correspond to the junction of solar
radiation and shadows and not to the position exposed to sunlight directly.
(5) In durability assessment experiments, the adhesion of the specimen ends is decreased due tocurling deformation caused by drying shrinkage. However, this does not affect the results of the
durability evaluation run at average relative humidity conditions. Furthermore, the resistance of therendering mortar/concrete composite to drying shrinkage can be determined by the adhesion loss at
both ends of the specimens.
(6) Assessing the loss of adhesion through bond strength testing can yield statistically significant
durability assessment results. However, it should also consider the dispersion of test results and theconstruction variability of specimens when referring to the assessment results. This is especially true
when comparing two specimens with little differences in durability. A combination of different
adhesion tests is recommended, such as the strain monitoring method mentioned below.
(7) The adhesion test cycle should be designed according to the aging cycle pattern and thespecimens with the weakest adhesion. The aging cycle run should guarantee at least 50% adhesion
loss for the weakest specimens.

A better approach may be to perform a multidimensional test of

durability, i.e., simultaneous comparison of adhesion loss after different aging procedures to obtain acomprehensive durability evaluation.

Although according to the literature survey, temperature and water are the main degradation
agents. But simulating the collaborative mechanism of different degradation agents in nature iscomplicated. Using temperature as the only degradation agent in experimental scale durability
assessment methods is reasonable and valid. For other relevant degradation mechanisms,
representative aging procedures must be designed according to the applied climate zone (e.g., freezethaw cycles corresponding to cold climate, dry-wet cycles corresponding to wet climate, etc.).

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Chapter VI. CONCLUSION AND FUTURE STRATEGIES

Nevertheless, each aging procedure has advantages and limitations, and experts should choose
carefully according to the climatic conditions of the proposed application.
In addition, even if the correspondence between the aging cycle and service life cannot beclarified for the time being. The proposed durability assessment methods are also of great value,
especially for new products with no application history. These tests reflect how the material should
behavior under solar radiation. Comparing it to a reference material with a lengthy application history
allows a quick determination of whether the new material has the required durability.

6.1.2 The key parameters affecting the durability of rendering mortar
Under thermal loading, the followability of the rendering mortar and concrete to each other's
deformation determines the long-term adhesion performance. Some key parameters determine thefollowability of the rendering mortar.
(1) Material properties. Dry shrinkage, modulus of elasticity, and coefficient of thermal expansion.
These three physical properties determine the deformation match of mortar to concrete. Specimens
with the same mix proportion underwent 50 aging cycles, and specimens with 1% SRA added reduced
the shear strength loss by about 15% (compared to specimens without SRA). Higher temperatures
result in more drying shrinkage, so rendered mortars with small dry shrinkage exhibit higher durability.
In tile specimens, the rendering mortar/concrete interface undergoes bond failure earlier than thetile/rendering mortar interface, although the latter was subjected to higher temperatures. It is
speculated that this phenomenon is due to the low modulus elasticity of bonding mortar acting as abuffer. Adhesives with low elastic modulus can reduce the tangential and normal stresses at thebonding interface. The performance of organic adhesives in the experiments in section 5.2 regarding
resistance to thermal loads supports this speculation. Similarly, rendering mortars in tile specimens
showed the most intense response to temperature, and lowering the coefficient of thermal expansionof rendering mortars may help to improve durability. In conclusion, the compatibility of material's
physical properties should be considered as significant aspect of durability design.
(2) Applications. Since thermal expansion mismatch is the main degradation mechanism. The
temperature at which the rendered mortar is applied may be a key parameter. The optimum application
temperature is the median temperature of the region throughout the year, and construction in severe
cold and hot weather should be avoided. In addition, studies on curling deformation indicated that theshrinkage gradient developed on the thickness of the rendering mortar is also a key parameter affecting
adhesion. A rendering mortar with a thickness of 15 mm reduced the short-term bond strength by an

114

Chapter VI. CONCLUSION AND FUTURE STRATEGIES

average of 30% compared to a 10 mm rendering mortar. Although the former did not exhibit a more
loss of adhesion than the latter after the aging cycle, the presence of initial defects also reduced theservice life. Applying techniques that allow the mortar to dry uniformly can significantly reduce thiseffect. Reducing the thickness of the mortar for each application, providing moist curing conditions at
an early stage, applying SRA, and adding an overlayer to prevent evaporation can all prevent highmoisture gradients from forming within the mortar.

6.1.3 Debonding evolution of rendering mortars under the thermal load
The debonding evolution of the rendering mortar was studied by periodic bond strength tests andcontinuous strain monitoring, representing debonding on a large scale (40 mm × 40 mm) and a small
scale (0.15 mm × 10 mm).
According to the bond strength test results, the adhesion of the mortar deteriorates with the aging
cycle in an "S-shaped" path, showing three characteristic phases of degradation-stagnationdegradation. A "plateau" period can be observed during the degradation process, during which thebond strength does not decrease with the running of the aging cycle. The debonding evolution enters
the plateau period when the loss of bond strength reaches about 40%. The duration of the plateau
period is related to the load intensity (peak temperature of the thermal load). The higher the peaktemperature, the shorter the plateau period. The plateau before adhesion failure may occur more thanonce, but it is certain that debonding does not evolve at an even rate.
In strain monitoring experiments, debonding evolution is studied on one specimen. The
debonding evolution is inferred from the characteristics of the strain behavior. The debonding
evolution at the small-scale level goes through two phases: damage accumulation and debonding
development. The damage accumulation phase is characterized by gradually separating the strain
trajectories of mortar and concrete but in the same direction of deformation. It is assumed that somemechanical bonds break at the interface, weakening the connections between the materials but
maintaining the ability to follow each other's deformation. Subsequently, these fractured connections
form small areas of debonding, which gradually expand to complete delamination with the aging cycle.
In summary, these results allow the hypothesis to be made that the plateau period in the evolutionof large-scale debonding may represent that some tiny debonded areas are being accumulated and thatthe bond strength test results will not continue to decrease until a certain threshold is reached.
Therefore, for periodic damage detection methods, debonding occurs abruptly. A continuous detection
scheme can improve efficiency and accuracy.

115

Chapter VI. CONCLUSION AND FUTURE STRATEGIES

6.1.4 Detecting adhesion damage in tilings with FBG sensors
The adhesion state of facade claddings must be monitored over time to ensure their protection
and correctly maintain and repair them when they begin to exhibit signs of degradation. The responseof the constituent materials of a facade to climate reflects their adhesion condition, and damage
detection is performed by identifying anomalies in strain behavior. FBG sensors were used to detectadhesion damage of tile specimens composed of concrete, rendering mortar, tiles, and tile adhesives(bonding mortar or organic adhesives).
The experimental findings show that the FBG sensor's ability to track the composites' strain
enables the detection of damage at any adhesive interface. Based on the strain behavior, three bonding
states can be distinguished.
-Healthy adhesion. A constant strain amplitude ratio is maintained, for example, and the strain
behavior between the material's layers maintains some correlation.
-Adhesion damage (partial debonding) The two materials start to lose their ability to follow eachother's deformation when their strain trajectories begin to diverge, and their strain amplitudes abruptly
decrease or increase.
-Adhesion failure (complete debonding). With a considerable shift in strain amplitude, therelationship between the strain behavior of the materials is completely different from the first one.
The principles above are generally relevant to every adhesive interface in a tiling system; however,the strain behavior corresponding to damage may differ amongst materials and must be compared tothe strain response of that interface in a healthy state. In general, the determination of adhesion damage
can be based on the loss of the initial following between the strain trajectories of the two materials.
When the load is constant, an aberrant strain response, such as a quick increase or reduction, indicatesa lesion.
Although this is a qualitative detection strategy rather than a quantitative one, it can alert building
managers when intervention is required. If strain behavior demonstrates a lack of followability, one
should be cautious and increase the frequency of inspections; if strain behavior becomes severely
abnormal, one should consider repairing or replacing the affected components. The strain monitoring
method can prevent premature or delayed maintenance, hence reducing the danger of accidental tile
spalling significantly.

116

Chapter VI. CONCLUSION AND FUTURE STRATEGIES

6.2 FUTURE STRATEGIES
Based on the findings of this study, future development of highly durable facade cladding systems
will concentrate on the following issues:
The general application of the proposed method for assessing durability still faces the following
main hurdles:
1) Adequate identification of degradants.
2) Synergistic interactions between degradants
3) Key criteria for assessment.
4) Mode of performance test execution.
5) The connection between assessments and real service life.

Future studies will establish natural aging stations and create a database of existing structures.
Determining the climate factors under which the material will be used is one of the most crucial aspectsof durability assessment. Solar radiation may be the major degradation agent in many climatic zones,
however, field data should be utilized to optimize the operation mode of the heat-cool cycle. The
repeatability of the deterioration paths derived from artificial aging cycles must be confirmed innatural aging.
The proposed durability assessment method employs bond strength as a crucial criterion.
Nonetheless, the variability of the bond strength test restricts the accuracy of the evaluation. The strain
monitoring method's experimental results suggest new possibilities. In the future, it is anticipated thatbonding performance will be measured by strain monitoring or other tests, along with thedetermination of new critical parameters. The choice of parameters must be based on the material's
physical changes in natural aging. The aim is to create a system that can assess durability based on
numerous criteria, allowing experimenters more selection options.

In the conclusion, the key parameters determining the durability of the rendered mortar arepresented. These parameters are summarized from the experimental findings. The plan for further
action is to quantify the impact of these criteria. In facade construction, rendering mortar is the mostmalleable and controllable substance. One of the ongoing projects investigates the optimal physicalinteraction between material layers based on durability evaluation tests. Furthermore, initial defects
caused a significant majority of adhesion failures, and long-term goals include standardizing the117

Chapter VI. CONCLUSION AND FUTURE STRATEGIES

application procedure in durability design.
The strain monitoring method has proved to be an effective technique for damage detection oftilings. However, it is not yet possible to quantify the strain behavior of the material, and only the mostfundamental property judgments are possible. Future plans include investigating strain behavior invarious aging environments and actual structures in conjunction with other nondestructive testing
techniques, such as the UT and AE methods. The objective is to quantify the relationship between
strain responses and adhesion, while further investigating the reasonability of the hypothesis about thedebonding evolution.

118

Appendix A
Cohesive Zone Material (CZM) Mode
Mixed-Mode Debonding law in AYSYS

Figure A-1 Mixed-Mode Debonding law in ANSYS, according to [143].
In mixed-mode debonding the interface separation depends on both normal and tangential
components. The equations for the normal and the tangential contact stresses as follows [143]:
𝑃

𝐾 𝑢 1

𝑑

(1)

𝜏

𝐾𝑢 1

𝑑

(2)

and

where:
P = normal contact stress (tension)τt = tangential contact stressKn = normal contact stiffness
Kt = tangential contact stiffness
un = contact gap
ut = tangential slip distancedm = debonding parameter
The debonding parameter is defined as:𝑑with dm = 0 for Δm

∆
∆

𝑋

(3)

1 and 0 < dm 1 for Δm > 1, and Δm and X are defined below.

∆

(4)
119

𝑋

(5)

where:𝑢= contact gap at the maximum normal contact stress (tension)𝑢 = tangential slip distance at the maximum tangential contact stress𝑢= contact gap at the completion of debonding𝑢 = tangential slip distance at the completion of debonding
The constraint on X that the ratio of the contact gap distances be same as the ratio of tangential
slip distances is enforced automatically by appropriately scaling the contact stiffness values.

For mixed mode, debonding is complete when the energy criterion is satisfied:
1

(6)

𝐺

𝑃𝑑𝑢

(7)

𝐺

𝜏 𝑑𝑢

(8)

𝐺

𝜎

𝑢

(9)

𝐺

𝜏

𝑢

(10)

With

where:
Gn / Gt = The normal / tangential fracture energies
Gcn / Gct = The normal / tangential critical fracture energies
σmax = Maximum normal contact stressτmax = Maximum equivalent tangential contact stress120

Appendix B
Effect of Curling Behavior of Mortar Due to Drying
Shrinkage on Mortar-Concrete Adhesion System

A significant shear strength loss at the free edge of the mortar/concrete adhesion specimens wasobserved in the experimental studies in Chapters 3 and 4, even though this part of the specimen was
not affected by thermal loading. As described in Section 2.21, this phenomenon is most likely due todrying shrinkage. The difference in moisture between the upper and lower surfaces of the mortar
causes it to deform in an out-of-plane direction at the unconstrained free edges in the form of a"curling" (Fig. 2-6(c)). Curling deformation is considered an adhesion failure mechanism, and it isnecessary to evaluate the potential impact of curling deformation on the durability of mortar/concreteadhesion in establishing durability assessment methods.
Curling deformation leads to the generation of confining stresses at the bond interface. Previous
studies have focused on cracking due to in-plane stresses, and researchers have described cracks dueto the inhibition of drying shrinkage [81] [82]. However, stresses in both directions (in-plane and outof-plane) affect the bond between mortar and concrete. For this reason, predicting the restraining
stresses generated at the bond interface in both directions is necessary. Furthermore, the study focusedon the concrete slab and pavement slabs [79] [82] [163] [164], and the overlayer applied to theseelements is very different from rendered facades. Therefore, it is necessary to reinvestigate the effectof curl deformation on the façade claddings.
Appendix B describes the series of studies that were conducted, the curling of mortar in the freedrying shrinkage and restrained shrinkage states was investigated using laser sensors and digital image
correlation (DIC) technique, respectively. The stresses at the mortar-substrate adhesive interface due
to curling were predicted by numerical simulation. The purpose of this study is to understand thecurling behavior of rendering mortars, thereby assessing its effect on the mortar-concrete adhesion
system.

121

B.1 FREE CURLING OF RENDERING MORTAR
B.1.1 Experimental procedure(1) Specimen preparation
Two mortar materials were selected for investigating the curing behavior under the non-restraint
condition. One is a standard formulation for rendering (PL), according to the Japanese ArchitecturalStandard Specification — JASS15 Plastering Work. It was the reference mortar upon SR specimens
which inclusion of SRA. SRA is widely used as an admixture to reduce the drying shrinkage of
cementitious materials and is beneficial in reducing curling [153]. As described earlier, curling resultsfrom the drying shrinkage gradient in the thickness of the mortar. Therefore, the thickness of
specimens and the amount of drying shrinkage (with/without SRA) as two parameters to prepare
multiple specimens with different curling deformations. Table B-1 reports the mixture proportion andspecimen specifications.
The mortar specimens had dimensions of 10/20/30 mm × 100 mm × 400 mm, as shown in Fig.
B-1. The specimens were demolded on the third day after casting and subsequently curing in water at
20 ℃ for 21 days.
Table B-1 Specifications of mortar specimens
Specimen

Parameter
SRA

Cement (C)Fine aggregate

ID

Thickness

PL-1

10mm

PL-2

20mm

PL-3

30mm

Ordinary

S: Crushed

SR-1

10mm

Portland cement

sand

SR-2

20mm

SR-3

30mm

S/C (by
Weight)

W/C
(%)

2.5

50

-

1%a

a % by weight of the gross mass

(2) Measurements with Laser Sensors
The most common method to quantify curl deformation is to measure the vertical displacement(usually at the center and corner) of a specified detection point on the specimen (away from thesubstrate direction). Fig. B-1 shows the device for measuring curling behavior, that is, thedisplacement of the upper surface of the mortar in the out-of-plane direction. To reproduce the curlingof exterior wall mortar, the upper surface of specimens was open to the air, and the other sides were122

Figure B-1 Displacement measurement system.
protected from drying by adhesive aluminum tapes. Ideally, the specimen will have a parabolic shapeon the section after being curled. Five symmetrical measuring points were set on the central axis of
the drying surface, and a thin copper plate of 10 mm2 was attached to each measuring point. The fivelaser sensors, whose accuracy is 0.2 μm/m, are used to measure the displacement in the verticaldirection. The bottom surface was supported by two steel sheets, each 25 mm from the center of thespecimen, to limit the position of the measuring points. Therefore, specimens should only occur underself-restraint caused by the drying shrinkage gradient.
To observe a more pronounced curl, the specimens were placed in an incubator at 40 ℃ and 3%
RH for accelerated drying after the initial value was measured. The displacement and mass loss werethen measured periodically over 72 hours.

B.1.2 Experimental result(1) The curling deformation
Fig. B-2 shows the displacement of the drying surface of the mortar specimen in the out-ofplane direction. Based on the displacement results of the five symmetrical measurement points, thespecimens were considered to exhibit significant curling. At point C, corresponding to the central
portion of the specimen, as the drying progresses, it is displaced downward, and the others are
symmetrically displaced upward. The displacement was significantly reduced from the ends to the
middle (A and E at both ends, B and D at the midpoint). Measuring points A and E, B and D are in acentral symmetrical position, and the displacement of the two is in an approximately symmetrical state.
On the whole, the measurement points distributed in symmetrical positions on both sides have123

PL (without SRA)SR (with SRA)(a) Specimen thickness of 10 mm(b) Specimen thickness of 20 mm(c) Specimen thickness of 30 mm
Figure B-2 The displacement of each point changes with time.

similar displacements. In the following results, the difference in displacement between the ends (A, E)
and the specimen center (C) is used to calculate the curl amount. The curling deformation is given as
follows:
𝐶

𝑑𝐴

𝑑𝐸 /2

𝑑𝐶

(B-1)

Where C is the curling deformation index [mm], dA, dE and dC is the displacement at point A, E
and C [mm].
124Figure B-3 Curling deformation over 72 hours.

Figure B-4 Mass loss over 72 hours.

Fig. B-3 shows the curling deformation index of each specimen over 72 hours. As the thickness
increases, the curling deformation of the specimen decreases. This indicates that the thinner specimens
produced a more significant shrinkage gradient due to the faster drying speed, and the non-dried partfunctions as a restraint in the thick specimens. The thin specimen (PL-1) reaches the maximum curling
deformation at 36 hours. Then the deformation of the specimen fell back since the drying speed inside
the specimen began to become uniform, and the difference in the amount of shrinkage between thesurface and the bottom surface was reduced. Compared with PL specimens, the curling deformationof mortars containing SRA was significantly reduced.

(2) Mass loss rate
The mass loss rate (water loss) of mortars is shown in Fig. B-4. Since the specimens only
evaporated water through the upper surface in the initial drying stage, it is considered that the massloss of the specimen is dependent on the thickness. The 10mm thin specimens showed the largest
decrease in mass moisture content. In particular, the specimens containing SRA showed a moresignificant mass loss rate regardless of the thickness. This phenomenon may be related to themechanism of action of SRA. The SRA used in this experiment is not a type that has a water retention
effect but a type that reduces capillary tension.

B.1.3 Numerical analyses(1) FE Model
A simple numerical simulation method for curling is proposed based on the 3D FEM analyses
with the program ANSYS Workbench. The results obtained by numerical analysis have been comparedwith the experimental results to verify mutually.
125

The models have the same dimensions as the specimens. Three thicknesses (10mm / 20mm /30mm) were set, as shown in Fig. B-5. Corresponding to the positions of the two supporting steel
sheets in the displacement measurement (each 25 mm from the center of the bottom surface of themodel), two lines of 100 mm length were fixed as constraints. The material parameters are the sameas in Section 3.2.1 (Table 3-1).
Since all specimens have the same drying area, the same type of specimens needs to simulate thesame shrinkage gradient to reproduce the curl. For this purpose, we split the mortar model from 1/10of the thickness into two elements, with the upper 1/10 as a shrinkage element. The drying shrinkage

Figure B-5 Free shrinkage model.
is controlled by adjusting the thermal expansion coefficient of the shrinkage element material.
According to Fig. B-3, both PL-1 and SR-1 specimens reached maximum deformation in the freecurling test. We applied a shrinkage strain of 1300 μ and 800 μ to the shrinkage element based on thedisplacement of the PL-1 specimen at 36 hours and the displacement of the SR-1 specimens at 72
hours, respectively.

(2) Analysis results
The experimental and numerical displacement comparison results are shown in Fig. B-6.
Corresponding to the measurement points of displacement in the specimen, the displacement path isthe long central axis of the upper surface (shrinkage surface) of the model. Fig. B-6(a), (b) shows theresults of the model given a shrinkage strain of 1300 μ and 800 μ, compared to the maximum
displacement results of the PL and SR specimens, respectively. The numerical results obtained for themodels with different thicknesses at quantitative shrinkage strains are well agreed with theexperimental results, which indicates that all specimens almost reached peak curling within 72 hours.

126

1.4
1
0.8
0.6

Displacement [mm]Displacement [mm]1.4

Theoretical-20mm
Theoretical-30mm
Theoretical-10mm
Experimental-PL-1
Experimental-PL-2
Experimental-PL-3

1.2

0.4
0.2

1
0.8
0.6
0.4
0.2

0

0

-0.2

-0.2

0

100
200
300
Coordinates from the surface [mm]400(a) 1300 μ shrinkage model with PL specimen

Theoretical-20mm
Theoretical-30mm
Theoretical-10mm
Experimental-SR-3
Experimental-SR-2
Experimental-SR-1

1.2

0

100
200
300
Coordinates from the surface [mm]400(b) 800μ shrinkage model with SR specimens

Figure B-6 Displacement results on the axial path, comparison between the experimental
and numerical response.
In this analysis, we did not give the overall distribution of drying shrinkage strain in the specimen
as in reality. It just gave a shrinkage strain to some elements of the upper surface, but the experimental
results can be reproduced by this analysis, which verified the model and supported the ability tocharacterize the curl using displacement measurements. In addition, when the same shrinkage strain
is input, the experimental results are reproducible for specimens of different thicknesses. This indicatesthat the internal shrinkage gradients of the driving curling forces of the mortar are similar, which may
be due to the severe drying conditions.

B.2 STUDY OF RESTRAINED SHRINKAGE BY NUMERICAL ANALYSES
B.2.1 FE Model
To understand the influence of curling on the mortar-concrete adhesion system, modeling the
restrained condition to estimate the stress state at the adhesion interface when the concrete substrate
restrained the curling of the mortar. The model consists of three components, concrete with dimensions

Total Deformation / Unit: mm

Figure B-7 Restrained shrinkage model.
127

Figure B-8 Maximum deformation ofmortar.

of 100 mm × 100 mm × 400 mm, mortar with dimensions of 10 mm × 100 mm × 400 mm, and theadhesion interface; the mesh adopted is shown in Fig. B-7. The cohesion zone model (CZM) based
adhesion material was set on the adhesion interface elements. A detailed description is given in Chapter
3 and Appendix A of the dissertation. The interfacial stress state of the mortar subjected to thermalloading has been investigated using the same model.

B.2.2 Interfacial stress estimation under restraint conditions
Based on the numerical results of the free curling with the same thickness (10mm), the same
method was used to apply the corresponding shrinkage strain to the restrained curling model. After
applying a strain of 1300 μ to the shrinkage element, the deformation result of the mortar Fig. B-8 andthe equivalent stress distribution at the adhesive interface is shown in Fig. B-9(a). The stresses areconcentrated at the corners and free edges of the interface. Previous studies confirmed the correlation
between stress generation and the adhesive strength reduction mechanism. We believe that the effectof this degree of stress generation on the mortar-concrete adhesion system is not negligible.
As previously mentioned, SRA has the effect of reducing the curling of the mortar. According to
the numerical results of the SR-1 free curling model, a strain of 800 μ was applied to the shrinkageelement of the model, and the equivalent stress distribution at the adhesive interface is shown in Fig.
9(b). The interfacial stress decreases as the curling deformation decreases. Hereby, SRA is thought tobe able to suppress the reduction in the mortar adhesion to the substrate due to curling deformation,
which agrees with that reported by Chilwesa et al [82].

(a) Surface shrinkage strain 1300 μ(b) Surface shrinkage strain 800 μ

Figure B-9 Equivalent (Von-Mises) Stress (MPa) in the interface.

B.3 STUDY OF RESTRAINED SHRINKAGE BY DIC
Although the numerical simulation results have shown the potential of curling in reducing theadhesive strength of mortar to concrete, the correlation between the interlayer peeling phenomenon
128

and curling still needs to be supported by experimental data. To this end, we monitored the peeling ofa mortar cast on a concrete substrate by Digital Image Correlation (DIC) technique, namely monitoring
the relative displacement between the mortar and the substrate in the out-of-plane direction.

B.3.1 Experimental procedure
A 100×100×400 mm concrete block was used as the substrate for the specimen. Two common
Japanese exterior wall finishes were investigated, mortar coating (M-1 specimen) and tile finish (T-1
specimen). All materials used in the experiments are described in the previous chapters. Materials for
specimen M-1 are referenced in Tables 3-2 and 3-2 (w/c of mortar is 0.5), and specimen T-1 was tiled
on top of the rendering mortar of specimen M-1. The bonding mortar and tiles used were the same as
those used in Section 5.1.
The concrete substrate was cured in water for over two months. Since the restrained curling tests
were performed under the same conditions as those of free curling tests, in an incubator (40 °C and
R.H. 3%), the concrete was sufficiently dried in the same environment for more than one week before
plastering to reduce the effect on curling. A mortar layer of 400 mm2 and 10 mm thickness was poured
on these substrates. After plastering, the mortar was cured in a 20 °C and R.H. 60% environment with

Figure B-10 DIC Specimen dimensions.

Figure B-11 Measurements of the displacement field at the interface between the mortar
layer and the concrete in the vertical direction, DIC procedure.
129

a wet sheet covering the exposed surface for seven days. After seven days, the mold was removed, andthe four sides of the mortar layer were insulated by adhesive aluminum tapes. The above is the
preparation process of specimen M-1, on which the tiles (6×45×45 mm) are laid is T-1, as shown in
Fig. B-10.
The DIC technique is based on the comparison of two images with two random gray levels
recorded before and after a displacement. The first picture is called “reference” and the second“deformed” [81]. In order to ensure the accuracy and relevance of the results obtained by this method,these principles were followed when acquiring the images for post-processing:(1) Provide a suitable light source and ensure that the light source is consistent for each shot.
(2) The relative position of the camera to the photographed object remains constant.
(3) The parameters of the camera are the same for each shot.
The photographic procedure is shown in Fig. B-11. During the test, a LED light provides a stable
light source. The camera is remotely controlled by a mobile device to ensure no change in the position

Figure B-12 AOI and subset size (40 × 40 pixels).
M-1

T-1

Figure B-13 Strain distributions [μ] (in the out-of-plane direction) exhibited by specimens
M-1 and T-1 at each moment.
130

of the light source, the camera, and the specimen. In order to improve the efficiency of image postprocessing, the photographed surface of the specimen was spray painted to obtain a random ink dot
pattern. From the beginning of drying, images (8278 × 6208 pixels) of the plane were recorded every
24 hours for three days.

B.3.2 Post-processing result
In this study, DIC is carried out using Vic-2D v6, a software developed by Correlated Solutions,
to measure displacement fields. In the post-processing software, the AOI (Area-of-Interest) and thesize of the subset for the operation are selected. Each subset is the smallest feature image unit toestimate the deformation by matching the "deformed" image to the subset with the same characteristics
in the "reference" image. The aim is to investigate the peeling behavior of the mortar-concrete adhesiveinterface, as shown in Fig. B-12, where the selected AOI contains the coating and the interface, andthe subset size of 40 × 40 pixels is the result of several tests.
Fig. B-13 shows the strain distribution along the out-of-plane direction as calculated by the postprocessing software. After 48 hours of drying, it can be observed that the M-1 specimen has a clear
tendency to peel at the free edge of the adhesive interface between mortar and concrete. Compared to
the M-1 specimen, no significant strain in the out-of-plane direction was observed in the AOI range of
the T-1 specimen. The strain distribution at 72 hours of drying was almost identical to that at 24 hours.
That is, virtually no interlaminar peeling occurred in the T-1 specimen. Based on the different material
properties of ceramic and cementitious materials, the main object of evaporating moisture from T-1
specimens is the adhesive mortar layer through the tile joints and thin sides. Therefore, the strain may
have occurred mainly in the in-plane direction.

B.4

EFFECT OF CURLING DEFORMATION
DURABILITY ASSESSMENT METHOD

ON

THE

PROPOSED

The studies in Chapters 3 and 4 clarified that curl deformation resulted in a loss of shear strength
at the end of the specimens. Therefore, there is a need to clarify the effect of curl deformation on
durability evaluation, that is, to determine the influence of ambient humidity on durability assessment.
To compare the damage of mortar-concrete adhesion due to both drying shrinkage and thermal loading,the interface stress states due to these two factors were compared by numerical analysis, as shown in
Fig. B-14. The curl deformation is the result of a shrinkage strain of 1300 μ. The thermal loading131Partial thermal load (downward curling)

Drying shrinkage (upward curling)(a) FE Model(b) Deformation(c) Equivalent (Von-Mises) Stress in interface
Figure B-14 Comparison of partial thermal loading and drying shrinkage effect on the
stress state at the mortar/concrete interface.
conditions are the same as in Section 3.2.2 (partial thermal load at a peak temperature of 70 °C).
According to the stress results, there is almost no deformation in the middle of the mortar underthe most severe drying conditions. In other words, even if the thermal load causes more severe drying
shrinkage, it only reduces the bond strength at both ends and has little effect on the middle part.
Because the proposed method for assessing durability is based on the loss of bond strength in theheated part (the part inside the dashed box), the adhesion loss at the end does not affect the results ofthe durability assessment. Although the results of the DIC experiment showed that the central part of
the M-1 specimen also tended to deform in an out-of-plane direction, considering that this study was
conducted under extremely severe drying conditions (40 °C, 3% RH), the durability assessment was
conducted at 20 °C, 60% RH. It is considered that the actual influence of drying shrinkage in the
durability assessment experiment is much less.
Conversely, the durability assessment method established for rendering mortars does not consider
curl deformation as a degradation mechanism. This is because the rendered facades are minor affectedby curling deformation. First, rendering mortars are generally applied in two or three passes, avoiding
the formation of significant shrinkage differences in thickness. Secondly, the render layer is usually132

covered with decorative coatings, such as paint and tile. In addition, there are cases where a spray-on
SRA specific to the facade is applied. All of these measures have been proven to inhibit thedevelopment of curling.

B.5 SUMMARY
Rendering mortar curls due to the drying shrinkage gradient present in the thickness and thecontinuous monitoring of the displacement field by the DIC technique enables a clear observation of
the mortar peeling due to curling, which can be considered as the mechanism of the adhesion between
mortar and concrete decreases at the free edges.
The proposed numerical analysis method can simulate the curling and effectively understand thestress state generated at the interface. It is considered that curling deformation is a factor to affect theadhesive unity of mortar and concrete.
This study tried two methods to reduce the curling deformation, applying SRA and overlayer.
Based on the displacement measurements and DIC monitoring results, both methods effectively
suppress curling. The inhibition capacity has not been investigated in detail, but applying similar
techniques that can keep the mortar uniformly dry in construction may be effective in preventing thedecrease of bond strength at the free edges, and plan to develop them in the future.
This study contributes to understanding the behavior of mortar-concrete adhesion systems,
thereby improving the durability of mortar coating in actual RC structures. However, this study was
conducted under severe drying conditions, and further research is needed in this area if the effect ofcurl deformation on long-term adhesion is to be fully understood and appreciated. For example,
establishing a milder experimental environment to understand curling behavior further and monitoring
interfacial stresses while performing mechanical property tests.

In summary, the proposed durability assessment method does not need to consider the additional
effect of drying shrinkage. The deformation of the specimen in either in-plane or out-of-plane direction
is driven by the partial thermal load providing a much greater force than the drying shrinkage.
Therefore, while materials with low shrinkage may show a higher durability rating, this result must be
due to the resistance of the material to differential movement due to high temperatures and is acombination of all relevant properties of the material (i.e., shrinkage, modulus of elasticity, tensile
strength, creep, and interfacial adhesion interactions, etc.). Therefore, the durability assessment

133

method proposed in this study can operate in a general humidity environment, and the possibility oferrors in durability assessment results due to drying shrinkage is negligible.

134

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