1.1 Overview 1
1.2 Problem Statement 1
1.3 Aim and Objectives 2
1.4 Scope and Limitation of the Study 2
1.5 Significant of the Study/ Expected Findings 3


2.1 Introduction 4
2.2 Common Defection in Concrete Structures 4
2.2.1 Void 5
2.2.2 Concrete Surface Crack (Surface Opening) 5
2.2.3 Delamination 6
2.2.4 Bugholes 6
2.3 Testing of Concrete Structures by Destructive Test
2.4 Testing of Concrete Structures by Non-Destructive Test (NDT)
2.4.1 Rebound Hammer Test 8
2.4.2 Ultrasonic Pulse Velocity Test (UPV) 9
2.4.3 Air-Coupled Impact-Echo 10
2.4.4 Electrical Resistivity 11
2.5 Types of Ultrasonic Wave 12
2.5.1 Longitudinal Wave (P-wave) 13
2.5.2 Transverse Wave (T-wave) 13
2.5.3 Surface Wave (R-wave, L-wave) 14
2.6 Non-Contact Sensor
2.7 Previous Studies Related to the Proposed Study
2.8 Summary 16


3.1 Introduction 17
3.2 Flow Chart of General Process 17
3.3 Flow Chart of Numerical Simulation and Analytical Study 18
3.4 Numerical Simulation 18
3.5 Numerical Model 19
3.6 Analytical Study 21
3.7 Summary 21

1.1 Overview
Concrete is a common material to use in construction industry because it is
versatile and globally available with the easy shaping properties. However,
concrete structures are facing many types of deterioration attack. Surface
opening cracks can be one of the most common defection that found on it (Chai,
et al., 2010). Surface opening cracks will lead to serviceability and durability
problems especially in concrete stiffness degradation and reinforcement bars
corrosion other than just appearance imperfection (Kee and Zhu, 2009).
In construction field, it is preferred to use non-destructive testing (NDT)
method in performing diagnosis for concrete cracking. The main purpose is to
always ensure the safety of buildings without further damaging the structure.
Numerous NDT methods have been developed and investigated for concrete
cracking assessment based on wave propagation principal. Surface Rayleigh
wave (R-wave) is one of the elastic wave components popular nowadays to
estimate the crack depth and its location. (Lee, Chai and Lim, 2016).
1.2 Problem Statement
Non-destructive test (NDT) is normally use to identify the compressive strength
and other concrete properties from existing structures. The most challenging
part of the inspection is gaining assess onto the surface to inspect the surface
appearance and the inner condition of the structure. Besides, conventional
elastic wave-based NDT methods are relying on the contact sensor that directly
attach on the concrete surface. The setup for contact sensor (receptor of the wave)
is time-consuming and inconvenient as investigator requires to re-attach the
sensor for each measurement on different location. Moreover, the reliability of
recorded data relies on the coupling condition of the contact sensor that attach
on the concrete surface. This causes the wave signal generates from the contact
source and penetrate through concrete crack unable fully receive by the contact
sensor. Thus, an efficient non-destructive test (NDT) technique is vital to
inspect defect throughout the structure (Degala, Rizzo and Ramanathan, 2009).

Hence, a study of using a non-contact sensor in determining the different
condition of the surface opening crack is proposed. This study emphasizes on
the variations of R-wave properties in term of amplitude, velocity as well as
frequency through the propagation over the concrete that consists of different
depths of surface opening crack and its effects to the consistency of NDT results
in the assessment of concrete surface crack.
1.3 Aim and Objectives
This study is aims to investigate the behavioural changes of Rayleigh wave
propagation through the concrete structure that containing surface opening
crack. The following objectives are listed in order to attain the aim of this study:

i. To determine the optimal distance of non-contact sensor to concrete
surface for the Rayleigh wave based concrete non-destructive test.
To investigate the changes of Rayleigh wave (elastic wave) properties
when propagating in concrete structures with several concrete surface
opening crack depth.
To correlate the results obtained from contact-based and proposed non

contact based Rayleigh wave concrete NDT methodology.
1.4 Scope and Limitation of the Study
This study emphasizes on the identification of feasibility of the non-contact
method in characterising surface opening crack and to determine the optimum
distance between non-contact sensors and the concrete surface. The changes of
Rayleigh-wave (R wave) behaviour in amplitude, frequency, velocity and rate
of attenuation after it propagates over the different depths of concrete crack are
requires to investigate.
Throughout this study, Wave2000 software is suggest to use in the
numerical simulation for different excitation frequency input from contact
source in the detection of different opening surface concrete depth by placing
the non-contact sensors at different distances from the concrete surface. A total
of one hundred twenty different cases are proposed to simulate including twenty
control cases.

The non-contact sensors are locating at different heights from the
concrete surface to determine the best receiver result of R-wave signal. In order
to obtain the optimal distance between the non-contact sensor and concrete
surface for different depth of the surface opening crack, the depth of surface
opening crack is increasing from 0 mm (act as control case) to 500 mm by
incremental of 100 mm. The non-contact sensors are locating at five different
distances from the concrete surface, which are 0 mm (contact), 5 mm, 10 mm,
15 mm, and 20 mm from the surface. Four different excitation frequencies are
requiring to generate from the contact source in numerical simulation ranging
from 10 kHz, 15 kHz, 20 kHz and 30 kHz.
Other than that, the simulation results may consist of the mixing of the
sound wave and the R-wave generated from the source. This study also requires
to differentiate or separate between these two types of the wave by looking at
the similarity style based on wave graph generated in simulation. The results
from wave2000 simulation are further analyse by using Fast Fourier Transform
(FFT) method through MATLAB to find out the trend of wave style.
1.5 Significant of the Study/ Expected Findings
This study determines the reliability of non-contact elastic wave-based NDT
method in the surface opening crack assessment. The proposed non-contact
sensor in this study is aiming to achieve a more convenient approach in concrete
quality checking. It requires an easier setup compare to the contact sensor that
need to attach on the concrete surface. Rayleigh wave (R-wave) is use in crack
depth estimation. Also, this study is planning to determine the optimal distance
between the non-contact sensor and the concrete surface. This optimal distance
plays an important role in concrete defect assessment configuration for the
highest reception of R-wave for different surface opening crack depths. The
result of purposed non-contact sensor will also compare and correlate with the
contact sensor with the R-wave type of Non-Destructive Test method. It can be
one of the stepping stone in discovering advanced R-wave based NDT methods
by using the non-contact sensor.

2.1 Introduction
This chapter discuss about the related background study such as concrete
defections, concrete property tests as well as type of elastic wave. Concrete
structures are exposing around the environment and this causes different type of
defections. The condition of defections will become more serious in a long
period and when it reached the critical condition, the structure may corrupt.
Concrete properties tests are dividing into non-destructive test by not damaging
the concrete structure and destructive testing where the concrete structure will
be destroyed during the test (Sakshi Gupta, 2018). Furthermore, several type of
elastic waves that use to identify the concrete defection in cracking also
mentioned in this section and the type of non-contact sensors.
2.2 Common Defection in Concrete Structures
Concrete is a common material in the construction field, but construction errors
or negligence can develop defects in a concrete structure. These defections can
be due to poor construction practices, low-quality control and the environment
issues. Among all the causes, corrosion of reinforcing steel is one of the main
causes in concrete defections. Corrosion of steel bar produce rust and create
tensile stresses which can eventually cause deterioration. The common concrete
structure defections including voids, surface cracks and delamination, as shown
in Figure 2.1. The defects in concrete will affect to the durability of concrete
structures (Zhu and Popovics, 2008).
Figure 2-1: Common Types of Defects in A Concrete (Zhu and Popovics, 2008).

2.2.1 Void
Voids are normally caused by the bad solidification of concrete. Besides,
improper mould, vibration and high water-cement ratio are also the reasons of
void formation. It can weaken the concrete strength and reduces the durability
for concrete. However, void in concrete does not directly give effect to the
properties of the concrete structure, but it possible causes issue in future.
Formation of voids caused by unfinished grout in post-tension ducts result in
corrosion of tendon and may cause structural failure (Zhu and Popovics, 2008).
2.2.2 Concrete Surface Crack (Surface Opening)
Concrete changes in volume are mainly due to fluctuations in moisture content
and variation of temperature in the concrete structure. The contraction that
restraint to change in concrete volume cause cracking and produce surface
openings when the developed tensile stresses are higher than the concrete tensile
strength. Normally, the concrete mix will add more water than the required
amount to hydrate the cement. When the excess water evaporates, concrete
shrinks and produces tensile stresses in hardened concrete, causing the concrete
surface openings. Concrete cover is the concrete layer between rebar and
concrete surface act as a protection of reinforcement from external agents which
can induce steel corrosion by covering it with concrete. However, the concrete
surface opening forms when the cover is broken and the rebars are exposing to
open air that led to reinforcement corrosion. This detrimental problem will cause
a reduction of resisting section in rebar and damage to concreate surrounding
It will combine with mechanical and chemical
degradation cause the defection in concrete structure when the bond between
concrete and rebar become impaired (Andrade, et al., 2016).
Figure 2-2: Concrete Surface Crack (Surface Opening)

2.2.3 Delamination
The delamination of the reinforced concrete and garage decks is another issue
that will affect the life of the concrete structure. Effective maintenance needs to
detect delamination at an early age so that repairs can be carry out before a
serious degradation happens (Zhu and Popovics, 2008). It is hard to be detect
from eyesight during finishing but it can be found by tapping structure and listen
to the sound. Another noticeable form of delamination is its bump shape that
forms at the concrete surface from trapped air and bleed water. This defection
is due to the finishing of the surface is done before completion of bleeding
process. The water or air particles are trap inside and when the concrete structure
is dry, a hole is formed in the concrete. This condition will weaken the concrete
strength since the concrete is not fully being occupy. Delamination can also
define as parallel surface separation between the substrate coating or horizontal
splitting in concrete slab that caused by either rebar corrosion or freezing and
thawing condition in corroded reinforcing mesh (Shokouhi et al., 2011).
2.2.4 Bugholes
Bugholes are the minor even or uneven hollows that forms on the concrete
surface due to the migration of entrapped air bubbles on concrete surface during
placement and consolidation. Also, vibrators with high amplitude will result in
the production of bugholes quantities (Liu and Yang, 2017). This condition
normally can be found in vertical cast-in-place concrete such as shear wall and
column. The quantity of the size of bugholes depends on the form-facing
material and condition, concrete mix characteristics, and consolidation process.
When the bugholes is big enough, it will cause cracking in the concrete.
Figure 2-3: Delamination in large specimen from (a) above, (b) below
tendon duct (Shokouhi et al., 2011)
(a) (b)

Figure 2-4: Compression Test -Destructive test (Sakshi Gupta, 2018).
2.3 Testing of Concrete Structures by Destructive Test
The destructive test is an investigation of concrete properties by partially or
completely damage it during the testing process. Throughout the destructive test,
the concrete specimen might be a crush in order to determine its mechanical
properties such as concrete hardness and strength. This method is suitable for
the large-scale production of concrete specimens because it requires a lot of
experiment specimen. The destructive testing method is use in determining the
service life of concrete and detect the weakness of design which may cause any
failure during the construction.
There is some type of destructive test methods such as tensile testing,
bending testing and compressive testing by using the Universal Testing Machine
(UTM). Moreover, the hardness of concrete can investigate by Brinell test and
Rockwell, where Pendulum test and Drop weight test can use to generate a result
for impact testing of concrete (Sakshi Gupta, 2018). Although the destructive
test will destruct and destroy the concrete specimens during the testing process,
it is cheaper and economical than other methods. The concrete specimen cannot
be reused again after the testing and this may cause a waste of material. However,
this method able to identify the mechanical properties such as fracture strength,
modulus elasticity with a lower material cost compares to the high machinery
cost that uses in the non-destructive test (Sakshi Gupta, 2018).

2.4 Testing of Concrete Structures by Non-Destructive Test (NDT)
It is important to test concrete structure after casting in order to determine its
strength in the construction site. The concrete testing should be done without
damaging because it is already constructed as an end product. For instance, the
concrete structure such as column is cast on-site and it requires to undergo a
quality test to ensure it is safe to use. Non-Destructive Test (NDT) is suitable
for the assessment of final product that including density, elastic modulus and
strength as well as hardness, and reinforcement (Jedidi and Machta, 2014).
Besides, NDT can be testing for the existing concrete structure to
determine its durability and strength without destructing it. This method can be
used for the quality control process to ensure the building is always under safe
and good condition. For the existing concrete building, the destructive test
method cannot use to check the building condition by damaging it. Crack
detection is important in the operation of concrete structures from time to time
(Zhao and Li, 2018). These deteriorations can be fix efficiently whenever it is
found by NDT. There are several common developed NDT in the construction
field which mainly divided into contact and contactless categories.
Table 2-1: Examples of Contact and Non-Contact Methods in NDT

Contact methods Non-contact methods
Rebound Hammer Test Air-Coupled Impact-Echo
Ultrasonic Pulse Velocity Test (UPV) Electrical Resistivity

2.4.1 Rebound Hammer Test
The rebound hammer test is one of the contact NDT method because it punches
on the concrete surface without damaging the whole concrete specimen. This
method is widely used to evaluate concrete quality in term of surface hardness
and compressive strength of concrete. This device is portable and less expensive
comparing to other NDT methods which might requires complex set-up for the
test. The values that obtain in the direction of non-horizontal is causes by
gravitational forces. To encounter this effect, the non-horizontal rebound values
must be standardising to the horizontal direction (Sanchez and Tarranza, 2015).
Besides, this method is considered easy to use and causes a small amount of

impact which do not damage the concrete specimen. Investigator only requires
to press on the spring-loaded piston to the concrete surface. When the hammer
is press down, it will release onto the plunger and rebound. The rebound height
of the piston is the result as an index of surface hardness. This surface hardness
test is useful in situations that do not require correlations and any setup.
Rebound hammer test is suitable for existing buildings evaluation of strength
especially in early strength of concrete (Sakshi Gupta, 2018).
2.4.2 Ultrasonic Pulse Velocity Test (UPV)
Ultrasonic pulse is a longitudinal wave pulse velocity method in NDT due to
the reasons of cost-effectiveness and simplicity in evaluating concrete
properties. The principle of this test is to determine the depth of a surface
opening in concrete by using elastic stress wave time-of-flight technique nondestructively. It is referring to the measurement of the time transfer of an
ultrasonic longitudinal waves pulse over route distance between source and
sensors to measure the material depth and presence of concrete defection such
as concrete crack. Besides, it also can used to determine the mechanical
properties of concrete model such as Young modulus, Poisson ratio and even
the concrete compressive strength. UPV test can be conducting in both direct
(through thickness) and indirect (surface) transmission of wave movement
depending on the accessibility as shown in figure 2.5. Among of them, direct
transmission is common use as pulse amplitude due to its receiving transducer
is greater. The pulse transducer and the sensors require to locate on the opposite
sides pointed directly with one another. Ultrasonic waves will propagate in P
wave (longitudinal), S wave (shear) and surface wave (Petro and Kim, 2011).
Figure 2-5: Configurations of ultrasonic pulse velocity method: (a) direct
transmission; (b) indirect transmission (Petro and Kim, 2011).

2.4.3 Air-Coupled Impact-Echo
Impact-echo is use to detect location and range of concrete defection such as
cracks, delamination, voids and honeycombing by 2D scanning. It is an elastic
wave-based NDT technique that produces by contact or non-contact source.
The refection of generated transient wave in the internal interfaces is then
measure. Air coupled sensor is a non-contact sensor that use to detect the
transient surface motion and the time-domain signal will be transform into the
frequency domain. The depth of concrete crack and honeycomb defection can
be determined by the Lamb waves using air-coupled pitch-catch configuration
(In et al., 2014). This method is a point inspection method which mean that it
will be time-consuming and high labour intensive to investigate large concrete
specimen. The air-coupled sensors able to eliminate sensor coupling problems
and thus perform more consistent measurement results compare to contact
sensor (Kee and Zhu 2009). In order to improve efficiency, the contactless
automated impact-echo scanning system enable by eliminating contact between
sensor and concrete (Zhu, 2008). Illustration of Air-coupled impact-echo
technique is shown in Figure 2.6.
Figure 2-6: Air-coupled impact-echo method (Zhu, 2008)

2.4.4 Electrical Resistivity
Concrete is a resistive aggregate which able to conduct electricity through its
porous cement part. It also acts as an interstitial medium for the transfer of
electrical ion. The conduction through the concrete structure is classify as
electrolytic condition that connects to the circulation of the electricity network.
Electrical technique from geophysical science is easy to use on-site constraints.
Electrical resistivity of concrete is sensitive to the volume and connectivity
degree of porosity. It is able to characterise the concrete damage, especially
concrete crack. Also, it is sensitive to the moisture degree of concrete on the
assessment of the crack depth and opening (Lataste et al., 2003). Resistivity that
determines by measuring electrical is complicated due to the material properties
of concrete. Concrete is a porous material and its electrical properties are
depending on the degree of saturation, where it is an insulator in dry concrete
condition and it acts as the conductor in saturated concrete condition. Fourprobes device is shown in Figure 2.7, and this device is moving over the
concrete surface to determine the electrical properties of that location. In order
to diagnose a better result of crack depth, 3D finite elements CESA-LCPC
software is used to model electrical conductivity of concrete. This electrical
method represents a technique help to access damage degree of concrete
structures without damage it.
Figure 2-7: Setup Four-probes square array principle (Lataste et al., 2003)

Figure 2-8: Type of Ultrasonic Wave
2.5 Types of Ultrasonic Wave
Several type of NDT that use in identification of concrete defections are
referring to ultrasonic wave propagation. For instance, ultrasonic pulse velocity
(UPV) method evaluates the presence of concrete crack by using the Rayleigh
wave that travel along the concrete surface (Petro and Kim, 2011). Ultrasonic
wave is a type of wave with high frequency of more than 20 kHz. It is an elastic
wave that can be further separates into the longitudinal wave, transverse wave
and surface wave depending on their wave propagation direction. In order to
determine concrete properties or concrete failure, the wave is generated and
flow through the concrete with different speed. The common phenomenon to
wave propagation investigation must be considered the wave will scatter in
heterogeneous medium (Yu et al., 2018). Longitudinal wave propagates faster
than transverse wave after the arrival of the Rayleigh wave.

Figure 2-9: Longitudinal Wave
2.5.1 Longitudinal Wave (P-wave)
Concrete is made up of cements and aggregate with different size unlike the
homogenous metallic material. A longitudinal wave is also known as
compressive wave that can be used to determine the depth of the concrete crack.
Longitudinal wave velocity of a concrete structure can be determined from the
frequency spectrum analysis. It travels in the fastest speed but the amplitude is
relatively small when it propagates near to concrete surface (Petro and Kim,
2011). This wave is categorised under a series of compressive particle with
rarefaction air. It can define as the type of wave when the motion of particle is
parallel to the propagated wave direction. The longitudinal wave can propagate
in solids, liquids and gases state hence it is widely to apply in the detection of
concrete properties and prevent concrete failure.
2.5.2 Transverse Wave (T-wave)
The transverse wave can also name as shear wave in ultrasonic wave
classification. The direction of particle displacement is perpendicular to the
propagate wave direction. The amplitude of the wave is different when it passes
through an obstacle such as concrete. The highest amplitude of the wave is the
crest, whereas the lowest region is the trough. Transverse waves can only
propagate in solid-state because the distance between molecules or atoms in
liquids and gases state is too big. The attraction between molecules is not strong
enough to allow the transmission of a transverse wave.
Figure 2-10: Transverse Wave

2.5.3 Surface Wave (R-wave, L-wave)
There are two type of the surface waves which are Rayleigh wave and Lamb
wave. Rayleigh wave is suitable in non-destructive evaluation of concrete due
to its low attenuation and higher possession energy than other bulk waves. It is
easier to identify from a waveform due to its high amplitude and slower
travelling speed than P-waves. The geometric spreading of the cylindrical wave
trend enables it to travel in a longer distance compare to longitudinal and
transverse wave that only propagate in one direction (Petro and Kim, 2011).
Furthermore, this cylindrical particle movement will avoid the steel bar in
concrete and this will not disturb the wave propagation due to its insignificant
shear properties (Chai et al., 2010). Surface wave carries the largest energy in
the waveform and discriminates the wave components of longitudinal and
transverse in the waveform. These features allow it to undergo detection at long
propagation distances throughout the concrete (Lee, et al., 2016). Only the nearsurface crack can be detected because Rayleigh wave only propagates from the
surface with a penetration depth of around one wavelength.
On the other hand, if a surface wave of around three wavelengths is applied to
the concrete specimen, this type of wave is named as Lamb wave. The velocity
of wave propagation depends on type and thickness of the material, frequency
and the type of wave.
Figure 2-11: Surface Wave (R-wave)
Figure 2-12: Surface Wave (L-wave) (IAEA, 2018)

2.6 Non-Contact Sensor
For the application of the non-contact sensor method in the assessment of the
concrete defections, two approaches that commonly to be chosen which are laser
optical detection and acoustic detection with an air-coupled sensor. Based on
the study by Popovics et al. (2008), the laser detection of the R wave propagates
in concrete with an irregular surface. It is more effective to determine the Rwave in concrete with a laser since the its propagation is faster on the surface
compared with the body wave for one side measurement. A rough concrete
surface will increase the difficulty in detection of signal on different locations.
On the other hand, air-coupled sensor is mainly use for inspection of
wood and control of paper quality. Most of the transducers in the concrete
assessment are based on piezoelectric and electrostatic models. Piezoelectric air
transducers are inherently resonant devices, and require backing to acquire
suitable damping coefficients. This type of transducer is usually use over a
narrow bandwidth in order to match the layer limits. Electrostatic transducer
provides a wider bandwidth and higher sensitivity compared to piezoelectric.
The applications of air coupled sensors are found that the frequency range can
be covered by distinct kinds of transducers as shown in Figure 2.13. It is useful
in detecting low frequency of the application of air coupled sensor. There are
few parameters need to be considered before application of air coupled sensor
such as its sensitivity, frequency response and directional property (Zhu and
Popovics, 2008).
Figure 2-13: Frequency Range of Various Air Coupled Sensors
(Zhu and Popovics, 2008)

2.7 Previous Studies Related to the Proposed Study
Several studies are conducted previously to study the Non-Destructive Test
(NDT) in determination of concrete defection to ensure the safety of structure
all the time. Table 2.2 shows the summary of some studies related to this topic.
Table 2-2 Summary of Related Previous Study

Authors Previous Study Title Remarks
Zhu, J., 2008. Non-contact NDT of concrete
structures using air coupled
Informative study on
non-contact sensor by
air coupled method.
Chai, H., Momoki,
S., Aggelis, D. and
Shiotani, T., 2010.
Characterization of Deep
Surface-Opening Cracks in
Concrete: Feasibility of
Impact-Generated Rayleigh
Informative study on R
wave propagation over
concrete surface
Lee, F., Chai, H.
and Lim, K., 2016.
Assessment of Reinforced
Concrete Surface Breaking
Crack Using Rayleigh Wave
Informative study on R
wave propagation by
Wave2000 software

2.8 Summary
In summary, this chapter describes about the common type of concrete
defections and the methods in determining the defections. Common defections
in the concrete structure are due to void, surface opening crack, delamination,
and bugholes. Furthermore, destructive test (DT) and non-destructive test (NDT)
for concrete condition evaluation are explained for further understanding
between these general types of tests. Four different NDT methods are discussed,
namely Rebound hammer test, Ultrasonic Pulse Velocity Test (UPV), AirCoupled Impact-Echo and Electrical Resistivity. Moreover, the ultrasonic wave
that divided into longitudinal wave, transverse wave and surface wave are
included in this chapter as well. The contact sensor and previous related studies
are explained in the last part of this chapter.

Design methodology to fulfill the aim and objectives of study.
Study journal that related to this topic (Literature Review).
Conduct numerical simulation by Wave2000 software.
Fast Fourier Transform (FFT) Analysis by MATLAB.
Compare and analyse result from numerical simulation.
3.1 Introduction
This chapter discuss about the procedures in the numerical study of impactinduced elastic wave (Rayleigh wave) based non-destructive test on concrete
structure with different depth of surface opening cracks. In overall, the
procedures are dividing into numerical simulation by Wave2000 software and
analytical study by Excel spreadsheet and MATLAB.
3.2 Flow Chart of General Process
The general process of this numerical study is shown in Figure 3.1. Firstly, the
aim and objectives are determined. Journal and researches related to impactinduced elastic wave, non-destructive test (NDT) and concrete failure are
studied in order to understand the background of this project. Wave2000
software analysis is chosen to run numerical simulation throughout the study in
generating data results. The data will then compare and analyse in Excel
spreadsheet and MATLAB to draw a conclusion for this study.
Figure 3-1 General Process of study

Create model geometry of concrete and steel bar.
Define material properties.
Set boundary condition.
Insert vertical crack into the model.
Locate and define the source configuration.
Define Resolving Wavelength, Simulation time
and Point/Cycle in Job Parameter.
Run simulation.
Group the simulation
results based on
discuessed parameters.
Comparison of the
result of contact and
non – contact based
Perform Fast Fourier
Transform (FFT) by
3.3 Flow Chart of Numerical Simulation and Analytical Study
Figure 3.2 illustrates the procedures to conduct numerical simulation and
analytical study. The steps in the numerical simulation are repeating for
different parameters such as excitation frequency from contact source, depth of
the surface opening crack, and distance of sensors from the concrete model
surface before proceeding to analytical study.
3.4 Numerical Simulation
Throughout this study, Wave2000 software is suggest to investigate the effects
of the surface opening crack depth of concrete to elastic R-wave properties
numerically. According to CyberLogic Incorporation (2019), Wave2000 can
present the propagation of wave trend that pass through a concrete model with
different depth of surface opening by solving the 2D elastic wave equation. It
can generate results for simulating the received acoustic waveforms and able to
explore the number of experimental configurations in order to determine the
best sensitivity in wave recipient. The wave equation based on finite differences
method that use to solve 2D acoustic wave is shown in Equation 3.1:
Figure 3-2 Process of numerical simulation and analytical study

Where is the material density (kg/m
3), is the first Lame constant (N/m2),
is the second Lame constant (N/m
2), is shear viscosity (Ns/m2), is bulk
viscosity (Ns/m
2), is the gradient operator, is the divergence operator,
represents the partial differential operator, is the time (s) and is a twodimensional column vector whose components are the and components of
the displacement of the medium at location ( , ), as indicated in Equation 3.2:
Where ‘ denotes matrix transpose.
Sine Gaussian pulse is selected in time function equation and the specific
expression used for waveform follows Equation 3.3:

Where ( )=0 for 0 and > , is the amplitude, the time constant
is inversely proportional to the bandwidth (decreasing the time constant
increases the bandwidth), is the nominal centre frequency of the waveform,
and is the time interval for which the signal is defined.

3.5 Numerical Model
Six different depths of a concrete surface opening crack to be investigating with
four different excitation frequencies from direct contact source. This numerical
model setup is varying the distance of the sensor from the concrete surface to
determine their correlation. A 2D simulation concrete model with the dimension
of 1000 mm (W) x 1000 mm (D) is created in the simulation and shown in
Figure 3.2. The concrete model is set as an infinitive boundary condition at
bottom, left and right sides of the model to prevent the reflective wave.

Figure 3-3: Schematic figure of simulation model (dimension in mm).
The properties of the material that use in the simulation of the concrete
model and steel model are listed in Table 3.1. Geometry and material of the
model have remained uniform throughout all the simulation cases. Elastic
properties of concrete and rebar are assumed to be homogeneously.
Table 3-1 Properties for Model Material
Lame constant related stress to strain in an elastic material, Equation 3.4 and 3.5
are used to calculate the parameter of First and Second Lame constant:
where is Lame first constant, is Lame second constant, is modulus of
elasticity and

Material Concrete Steel
3) 2300 7900
Modulus of elasticity, E (GPa) 24.5 200
Poisson Ratio, 0.2 0.3
0.1 6.0
Shear viscosity, (Pa*s) 0.001 0.1

The simulation begins with a control set on concrete without surface
opening (concrete crack). This control set will be comparing with the concrete
model of different surface opening in order to identify the change in wave
transmission behaviour between them. The simulation is repeating with
different parameter including excitation frequency of the source, crack depth,
and distance between sensor and concrete model. Table 3.2 shows the
parameters that varies in this wave motion simulation.
Table 3-2: Parameter Varies in Simulation

Type of
Frequency of
Source, f (kHz)
Surface Opening
Crack Depth, d
Distance between
Sensor and
Concrete, h (mm)
Contact 10, 15, 20, 30. From 0 to 500 at an
incremental of 100.
From 0 to 20 at an
incremental of 5.

3.6 Analytical Study
Data and results of waveform propagation receive from sensor are extract from
Wave2000 simulation. The data are using to plot amplitude, frequency and
propagation velocity by using Excel spreadsheet. Besides, MATLAB software
is run to perform Fast Fourier Transform (FFT) in order to obtain the dominant
frequency of the R-wave for analysis. The results of numerical simulation are
performing in term of R-wave to show the variation of elastic wave
characteristics interacting with different parameter that discussed previously.
The main finding is to obtain the optimum distance between the sensor and
concrete surface in order to show the highest wave reception, accurateness and
reliability. The result of purposed non-contact sensor will also compare and
correlate with the contact sensor.
3.7 Summary
This chapter is generally discussing about the numerical simulation by using
Wave2000 software with a total of one hundred twenty cases simulating in
different parameter. The results are then proceeding to analytical study by
distributes data in Excel spreadsheet and Fast Fourier Transform (FFT) method
in MATLAB software. The data and results obtain will continue to analyse and
compare between contact and non-contact based NDT.

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