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Chapter 1

Introduction

1.1 Motivation

There has been a tremendous increase in the usage of ultrasonic methods for inspection of

materials. Not all of these methods can be incorporated for the inspection of concrete

because of its inhomogeneous nature. The use of SAFT reconstruction technique for the

positioning and sizing of defect points is fairly new phenomenon in non-destructive testing

of concrete. It utilizes the results obtained from the Impact-echo testing of the concrete

sample. Its better resolution and improved results over other ultrasonic testing method is

used for many other applications including the use in photography for getting higher

resolution digital images by using array of digital cameras. Nowadays, this method is used

for medical ultrasound imaging to get better resolution image of internal organs of human

body. Its vast benefits have always prompted researchers to explore new areas of its

utilization.

1.2 Objectives

The objective of the study can be broadly categorized as-

Understanding the ultrasonic wave propagation in concrete

The importance, need and different methods of Non-destructive techniques

Effect of various subsurface discontinuities on the propagation of stress waves

Understanding the basic method of Impulse-echo technique and studying the

benefits of SAFT method over Impulse-echo method.

Researching the basic reconstruction technique of SAFT algorithm

LS-DYNA modeling of concrete slab with defect and response collection on various

receivers to finally backtrack the location and size of the defect by the use of

MATLAB algorithm.

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1.3 Importance and Need of Non-Destructive Testing

Regular and frequent inspections are essential to ensure the structural integrity of concrete

structures. Non-destructive testing (NDT) methods are well suited for this purpose because

they allow the detection of invisible defects at their early stages of development without

harming the structure itself. In many cases the high repair cost can be significantly reduced

if the damage is detected as early as possible. Several NDT techniques can be employed for

the health monitoring, the detection of cracks, tendon ducts and other built-in components

in the superstructure which generally require one-sided accessibility. This requirement

restricts the applicable NDT methods to ultrasonic testing, impact echo, impulse radar and

in some cases thermography.

It is often necessary to test concrete structures after the concrete has hardened to

determine whether the structure is suitable for its designed use. Ideally such testing should

be done without damaging the concrete. The tests available for testing concrete range from

the completely non-destructive, where there is no damage to the concrete, through those

where the concrete surface is slightly damaged, to partially destructive tests, such as core

tests and pullout and pull off tests, where the surface has to be repaired after the test. The

range of properties that can be assessed using non-destructive tests and partially

destructive tests is quite large and includes fundamental parameters such as density, elastic

modulus and strength as well as surface hardness and surface absorption. Reinforcement

location, size and distance from the surface are also assessed from these tests. In some

cases it is also possible to check the quality of workmanship and structural integrity by the

ability to detect voids, subsurface cracking and delaminations.

Non-destructive testing can be applied to both old and new structures. For new structures,

the principal applications are likely to be for quality control or the resolution of doubts

about the quality of materials or construction. The testing of existing structures is usually

related to an assessment of structural integrity or adequacy. In either case, if destructi ve

testing alone is used, for instance, by removing cores for compression testing, the cost of

coring and testing may only allow a relatively small number of tests to be carried out on a

large structure which may be misleading.

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1.4 Different Methods of Non-Destructive Testing

Non-destructive testing methods may be broadly classified into two sections:

searching techniques

analyzing techniques

The searching techniques must be able to examine the complete volume of the component

under inspection or at least the welding areas. They must combine a high inspection speed

with a high reliability in finding and documenting indications above some registration levels.

The analyzing techniques are used to decide whether an indication found by a searching

technique is really a defect or some type of a form-echo. They need not to have very high

inspection-speeds because they only are used to inspect those parts of a component where

overstepping of the registration levels were found.

These analyzing techniques must be able to

differ between defect- and form-echoes

measure the defects position

classify in "crack-like" or "globular"

evaluate the defect’s size

Thus they must be able to give the input data for the fracture mechanics calculation of the

defects criticality. They should provide the exact location, size and shape of the subsurface

discontinuity as well as the attributes of the components inside the concrete block under

investigation.

1.5 Cracking and Subsurface discontinuities in Concrete

Cracking affects the appearance of concrete. In some cases it affects its structural adequacy

and durability. In reinforced concrete, cracking allows easier access to air and moisture

which can cause steel to rust and eventually weaken the concrete. A common problem in

repair and rehabilitation of concrete structures is to determine the extent of cracking within

a structure. In plate-like structures such as bridge decks, slabs and infill walls in frames,

cracking often occurs in the form of delaminations in the plane of the reinforcing bars. For

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example, in reinforced concrete bridge decks, chloride-induced corrosion of reinforcing bars

leads to bursting forces which produce cracks around the bars. These cracks propagate in

the plane of the bars due to the larger bursting forces caused by continued corrosion and

forces caused by expansion of water which penetrates the cracks and undergoes freezing

and thawing. In reinforced concrete infill (or shear) walls in frame structures, cracking

around bars leading to delaminations can be caused by cyclic loading in an earthquake.

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Chapter 2

Various Ultrasonic Testing Methods

2.1 Transmission Technique (Basic ultrasonic testing)

Ultrasonic testing is an established non-destructive technique for the detection of defects

and the characterization of materials. While most applications of this method are centered

on the inspection of metals, the ultrasonic pulse transmission technique has also been used

for a long time for testing concrete elements. For measurement, a transmitting and a

receiving transducer are placed on opposite sides of the test object, and an ultrasonic pulse

is sent through the concrete. The velocity and attenuation of the pulse are then used for

empirical correlation with strength and other characteristic parameters of the concrete.

For applications such as the detection of tendon ducts or small flaws, the transmission

technique is usually not well suited because it lacks sensitivity. The reason for this is the

strongly inhomogeneous nature of concrete. Even at low frequencies the ultrasonic

wavelength is in the order of magnitude of embedded aggregate and pores. This causes

strong scattering which attenuates the transmitted pulses. To reduce this problem, signals

of low-frequency content in the range of 20–100 kHz are employed in the transmission

technique to minimize interaction with the coarse concrete structure. But simultaneously

the sensitivity of detection of the target objects is also decreased.

2.2 Problems arising in testing concrete by ultrasonic techniques

Non-destructive testing employing ultrasonic pulses/Impulse waves have been used for a

long time for the detection of internal objects in metals and other materials. Specifically

ultrasonic pulse-echo testing, allowing for one-sided access of a component, is used for

inspection tasks on a regular basis. The application to concrete, however, faces problems

induced by the inhomogeneous concrete structure. Aggregate and pores have acoustical

properties much different from those of the cement matrix, which gives rise to scattering,

attenuation and mode-conversion of propagating ultrasonic waves. Among the

consequences are pulse attenuation and structural noise, which can mask reflections of

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objects to be detected. The presence of water molecules or air gaps inside the concrete can

change the speed and other characteristics of the P- waves passing through it. This causes

faulty reading of the receiver signals and thus incorrect results.

2.3 Impact Echo Technique- Basics

This is the most common NDT method to inspect the subsurface discontinuities and

components of the concrete material. In the impact-echo method, a transient stress pulse is

introduced into a structure by mechanical impact at a point on the surface. This pulse has

the approximate shape of a half pulse sine curve. This pulse travels into the plate as

longitudinal (P-) and transverse (S-) waves and along the surface as a Rayleigh (R-) wave. The

P and S waves propagate into the structure along spherical wave-fronts and are reflected by

internal cracks or voids or interfaces and by the external boundaries of the structure. An

array of displacement transducer located close to the impact point is used to monitor the

surface displacements caused by the arrival of these reflected waves. These waves are, in

turn, reflected at the free surface, and they propagate back into the test object to be

reflected again by internal interfaces or boundaries. Therefore, a transient resonance

condition is set up by multiple reflections of the waves between the free surface and

internal defects or external boundaries. P-waves are of primary importance in impact-echo

testing of plate structures, because the displacements caused by P-waves are much larger

than those caused by S-waves at points located close to the impact point.

Fig.2 .1 - Schematic measurement setup for one-dimensional aperture scans

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The stress waves are generated when pressure or deformation is applied suddenly. These

waves consist mainly of three types-

1. P- waves : Longitudinal waves propagating parallel to the propagation direction

2. S- waves : propagate perpendicular to the propagation direction

3. Rayleigh (R-) waves : surface waves propagation only near surface

The velocity of P- waves inside concrete material depends on the Young’s modulus, mass

density and Poisson’s ratio of the concrete sample.

Vp =

Where, for M20 concrete grade (taken for analysis) the values of different variables are,

E = Elastic / Young’s modulus of concrete = 2.236 e10 Pascal

Ρ = Mass density of concrete = 2400 kg/m3

ν = Poisson’s ratio of concrete = 0.2

This method is effective for locating large voids or delaminations in plate like structures, e.g.

pavements or bridge decks, where the defect is parallel to the test surface. A mechanical

impact produces stress waves of 1 to 60 kHz. The wavelengths of from 50 mm to 2000 mm

propagate as if in a homogeneous elastic medium.

2.4 Method of determination of distance of reflecting surface by Impact-Echo method

The mechanical impact on the surface generates compression, shear and surface waves.

Internal interfaces or external boundaries reflect the compression and shear waves. When

the waves return to the surface where the impact was generated, they create displacements

in a transducer and subsequently a display on a digital oscilloscope. The resulting voltage-

time signal is digitized and transformed, in a computer, to amplitude vs. frequency plot. The

dominant frequencies appear as peaks on the frequency spectrum. The dominant frequency

is not necessarily the thickness signal. Using each of the frequencies identified as peaks on

the frequency spectrum, the distances to the reflecting surfaces can be calculated from

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d =

Where,

d = distance of the reflecting surface,

f = dominant frequency,

V = velocity of compression waves in the test material.

If the receiver is placed close to the impact point the reflected signals may not be seen

because the transducer is still ringing due to the impact.

Fig. 2 .2 Schematics of Impulse echo testing of the concrete slab

The data which is gathered by the array of receiver points are in this report Z- displacement

vs. time characteristics and fast Fourier transform is not required. The received data is put

for analysis in the SAFT reconstruction algorithm.

2.5 Range and limitations of impact-echo testing method

Velocity of the P-wave which causes most of the displacement in the receivers is frequency

times wavelength of the signal. Thinking in terms of a multiple of the wavelength-

Velocity = frequency × wavelength

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V= f *λ

Where,

λ is wavelength and f is the frequency of the stress pulse.

For impact test to work i.e. the technique can only investigate cracks and defects which are

in order greater then half of the wavelength of the impact/input pulse. Recent research has

shown that the ‘near field’ detection capability of impact-echo is,

Minimum depth of detectable target = λ/2

In order to determine λ, the velocity of P- wave through the concrete is to be known. The P-

wave velocity through the concrete is 3200 m/s (rounded value) for M20 grade concrete.

And, λ = 3200/frequency in Hertz

When using impact-echo equipment, one selects the excitation frequency in order that the

appropriate size of spherical hammer is chosen. For example, in this report, the impact time

of 10 µs is used to detect the defect i.e. a 100 KHz excitation frequency hammer (half sine

pulse) is chosen, the near field minimum depth resolution would be

λ/2 = [ 3200/(100 KHz × 2) ] = 0.016 meter

This means this method can only work for crack depths greater than 16 mm for the taken

sample. A check needs to be undertaken on actual impact frequency achieved as the surface

of the concrete may crumble. If the surface crumbles, even a little, on impact-

Contact time increases

Lower frequency of excitation is achieved

Longer wavelength signal is generated

Lower “near field” resolution is achieved.

2.6 Parts of the Impact echo System

The Impact echo system consists of three components-

1. Impact source

2. Receiving transducer (receiver)

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3. Portable computer with a data acquisition card

2.6.1 Impact source

The choice of the impact source is very important. These days, hardened steel spheres on

spring-steel rods are used as an impact source. The force time characteristics of the impact

force can be approximated to be a half pulse sine curve. The duration of the impact

determines the frequency content of the stress pulse that is generated. A shorter duration

of impact produces a broader range of frequencies in the waves; however, the amplitude of

each component frequency is lower. The impact duration determines the size of the defect

which can be detected by impact-echo testing because the frequency content of the pulse is

determined by that. As the duration decreases, the pulse contains higher frequency (shorter

wavelength) components, and smaller defects or interfaces can be detected. Shorter-

duration impacts are needed to locate shallower defects. However, waves produced by

shorter-duration impacts (low frequency and low energy) will have limited penetrating

ability in concrete.

2.6.2 Receiving Transducer

The receiver consists of a transducer which detects the reflected pulse i.e. detects the

movement of the surface and provides the displacement vs . time characteristics of the

receiver points. These movements are converted into electrical pulses and are transferred

to the computer system with data acquisition capabilities.

2.6.3 Portable computer with a data acquisition card

A portable computer-based data-acquisition system is used to capture the output of the

transducer, store the digitized waveforms, and perform signal processing and analysis.

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Chapter 3

Ultrasonic SAFT (Synthetic Aperture Focusing Technique)

3.1 Basics of SAFT Technology

Synthetic aperture techniques were originally conceived for radar systems in the 1950s and

were initially implemented using digital computers in the late 1970s and more advanced

techniques were introduced in the late 1980s. Ultrasonic SAFT (Synthetic Aperture Focusing

Technique) reconstruction is an imaging method utilizing the information content of several

pulse-echo measurements. The measurements are recorded on the surface of the concrete

on a one or two-dimensional grid, also called aperture. Since transducers or transducer

arrays of the size of a whole aperture are presently not manageable, one or more

transducers need to be moved to scan the grid.

The set of measured signals obtained at the receivers by numerous reflections and

scattering of source pulse inside the material (acquired by Impact-echo measurement) is

then processed using SAFT reconstruction. The underlying algorithm coherently

superimposes the signals for each image element, thus synthesizing a transducer of the size

of the aperture with variable focusing to each image element. Linear apertures lead to two-

dimensional images (B-scan) and planar apertures to three dimensional images (C-scan).

Fig. 3 .1 Position of aperture and imaging results

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SAFT is a signal processing tool that aims at improving the accuracy of ultrasonic signals,

thus leading to better sizing capabilities. SAFT reconstructions provide detailed information

about the imaged concrete section and can therefore be used for detection and localization

tasks. The superposition process reduces structural noise, which can be a severe problem in

single pulse-echo measurements (A-scan). But since physical limitations and wave

propagation specialties such as mode conversion may cause interfering indications, the

resulting images need to be interpreted, utilizing additional information if necessary.

3.2 Data acquisition in SAFT

There are various methods by which data can be gathered for analysis in the SAFT

reconstruction technique-

1. Fixed source and moving receiver

2. Moving source and receiver (same source as receiver)

3. Moving source and moving receiver

4. Fixed source and array of fixed receivers.

The method used in the analysis here is fixed source and fixed array of receivers. Since the

array of receivers doesn’t cover the whole top surface of the concrete sample, only the part

of cross sectional area that is below the receiver aperture is taken for the analysis.

Synthetic apertures offer a more flexible way of focusing. A synthetic aperture imitates a

large transducer by sampling its area at many points. This can be done either by an array of

transducers measuring simultaneously, or by a single transducer approaching the aperture

points in succession. Moved arrays, a combination of both, are also possible. The apertures

considered here are linear or planar, representing a large line. The planar aperture consists

of a grid of N points in the X- direction and M points in the Y direction; the linear aperture is

obtained for M=1.

For focusing the pulse-echo measurements at the synthetic aperture, the received signals

are processed using the SAFT algorithm (synthetic aperture focusing technique). The SAFT

algorithm focuses the received signals to any point of the reconstructed image by coherent

superposition. In this way, a large virtual receiver with variable focus is synthesized.

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Resulting higher resolution image is two dimensional (2D SAFT) for the case of the linear

aperture, and three dimensional (3D SAFT) for the planar aperture. Two-dimensional SAFT

images are called B-scan; 3D SAFT images are often called D-scan sections capturing data

through the three-dimensional data field.

3.3 SAFT Reconstruction Algorithm

Impact signal (force vs. time characteristics) which is given by the source moves in the

model and due to the presence of the defects it is transmitted back and reflected in

arbitrary direction. The receivers that are present at the top of the model surface then pick

up this reflected signal from the defect as well as back wall echo that is coming from the

reflection by the back-wall of the model. These signals now vary from the signals that would

normally arise in the case of no defect or discontinuity. These signals are in the form of

amplitude vs. time. The signals received by the array of line receivers are now used for the

backtracking of the defect position. During reconstruction one has to calculate back the

signals received at the surface at receiver point (x, y) into that region inside the material (α,

β, z) with z as the depth coordinate. Here the reconstruction algorithm comes into picture

(W. Muller, V. Schmitz and G. Schafer, Reconstruction by the synthetic aperture focusing

technique (SAFT), 1986). Suppose the source (also the receiver) is moving towards the

defect zone in succession. Here the depth and width plane is messed and a grid of node

points is created. The integral to be solved now is-

A2 (α, β, z) = ∫∫ [ *A1 [ ]

Where,

A2 (α, β, z) is the image of the reflecting surface inside the sample at co- ordinate position of

(α, β, z).

r =

If sound velocity (P- wave velocity that causes maximum displacement) inside the material is

considered to be c, then t can be replaced by the expression;

t = * [ ]

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The two expressions are put in the main integral and solved. The result is the 3-dimensional

amplitude distribution inside the specimen, a so called D-scan.

Reducing the problem to the 1-dimensional case, so scanning only in one line (x-direction),

one has input data A(x, t) and equation reduces to

A2 (α, z) = ∫∫ 1/r *A1 [ ]

In this case the result is the amplitude distribution in an area below the scanned line and

perpendicular to the scanned surface, performing a side view, a so called B-scan.

In both cases, 2-dimensional and 1-dimensional SAFT, the data are picked up at discrete

points at the nodal points of the mess. They are reconstructed into discrete pixel points, and

the amplitude versus time-values is stored digitally, i.e. for discrete and limited time

intervals. Therefore one can overcome the integrals in the equations by simply performing

summations. The final reconstruction algorithm for 1-dimensional SAFT (LSAFT) for one

receiver is

A2 = ]

In other words,

To determine the image location of the defect point (r’j) by L-SAFT algorithm, the image

point I(r’j ) at the point r’j is calculated by using the time taken for the ultrasound to traverse

the distance | (rsi – r’j) | and again a distance of |(rs –r’j)| ,denoted by tij i.e. the time

taken to traverse from source to the image point and again from image point to the

receivers. This is calculated for each received signal position rsi. The set of values of

received signals S(rsi, tij) are backtracked and summed to determine I(r’j). Therefore if the

point r’j is associated with a reflector or defect, a coherent summation will occur resulting in

a large value of I; if this point is not associated with such a reflector no coherent summation

will occur, resulting in a small value for I. Expressed more precisely, the SAFT image field is

I(r ‘) where

I(r’j) = 1/n

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Where,

tij = t0/2 + 2/c( | (rsi-r’j) |+|(rs –r’j)| )

Here, n is the number of receivers, c the velocity of ultrasound in the medium, time t0 is the

impact time of the source and rsi is the receiver location and rs is the source location.

Fig3.2 Parameters involved in the calculation of SAFT image field amplitude

3.4 Imaging Properties

The result of the reconstruction algorithm is a focusing of the received ultrasonic echoes

back to the positions of the reflectors respectively, and because the area inside the scanned

aperture where echoes of the reflectors are received increases with increasing depth of the

reflectors, one gets a simultaneous focusing into all depths with just the same lateral

resolution.

The lateral resolution received for pulse echoes is half the ultrasonic wavelength or half the

probe width respectively. The axial resolution depends on the pulse length transmitted, in

most cases about 5 periods of the ultrasonic frequency used.

A major shortcoming of the method is the inherent assumption that the pulse reflected

from a flaw has a spectral content that is independent of the flaw's location relative to the

transducer.

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Chapter 4

Modeling in LS-DYNA and SAFT Reconstruction Results

4.1 Specimen Specification

The concrete used in the study is M20 grade concrete with the dimension of the concrete

block taken as,

Length = 300 mm

Breadth = 300 mm

Depth = 50 mm

The velocity of the P- wave calculated by the formula comes out to be 3217.5 m/s for the

M20 concrete.

4.2 Various steps of Modeling of concrete block (without defect) in LS DYNA

This model helps us understand the back wall echo characteristics of the concrete block

without defect. This is taken for comparison with the analysis involving defected concrete

plate. Various steps and attributes of the modeling process are given below:

1. The dimension of the rectangular concrete bock is defined in the messing option

(about 20 elements per wavelength for X and Y; for Z, 3 or 4 more elements

depending on the thickness) and the messing in three axes is defined. In the model, 1

mm messing is used in all the three (X, Y and Z) directions. In this step, cuboidal

concrete block is made with required messing. The element chosen here is 8-node

point solid cubes.

2. Boundary condition of the model is defined in this step. The degree of the freedom

of the boundary nodes is fixed here. In the model the concrete block is assumed to

be simply supported on all sides. The translational motion is restricted in local X-, Y-

and Z- axes. The rotational movement is restricted about Z- axis for the boundary

node points. So, the entire lower edge of the block is constrained for movement.

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3. Next step is defining the material. Material is assumed to be following Hooke’s law

of elasticity. The mass density (2400 kg/m3), Young’s modulus of elasticity (2.236 e10

Pascal) and Poisson’s ratio (0.2) value for the M20 grade concrete is taken.

4. For the type of integration process, a fully integrated quadratic 8 node element

(solid element) with nodal rotation is taken.

5. The impulse load to be applied at the center is defined. The impulse signal is taken to

be half sine pulse with impact time of 25 µs for testing concrete block with no

defect. The time (second) and force (Newton) characteristics are as in the figure. The

scale factor for the abscissa and ordinate values is taken to be 1.

Fig.4 .1 Impact pulse (Half sine) with impact time of 25µs used for inspecting concrete slab (without

defect)

6. The position of the node where impact pulse is to be applied is defined. The co-

ordinate of the node where the source is located is specified in the model. The node

number of the impact source node is also noted.

7. Next, the impulse load previously defined (half sine pulse) is applied at the impact

source co-ordinate. The node number of the impact node is supplied. The direction

of the applied impulse is provided here only. For the model the direction of the input

impulse is negative z direction (as seen in above figure). This is done because the

impulse applied is compressive in nature.

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8. The termination time of the calculation is provided next. Termination time for the

model is taken to be 300µs.

9. Now, the node point values of all the receiver points are noted down where we want

to get the Z-displacement (many other variables can also be obtained) and time

characteristics. All the receiver node coordinates are entered in the model.

10. The nodout option is selected and the time interval between the outputs taken as

2e-7 seconds is specified for the receiver results. Then the model is saved to be run

in LS-DYNA. The LS-DYNA solver gives output nodout file. Various plots of time vs Z-

displacement for different receiver points are drawn from here to be studied in SAFT

algorithm.

Fig4.2 T he time vs. Z- displacement characteristics of the receiver node located 4 mm (Series1) and 12 mm

(Series2) from the impact source point of concrete slab (no-defect)

4.3 Modeling of Concrete block with defect

The concrete block with the defect of 40 mm width placed at a depth of 35 mm from the

top surface of the concrete model is evaluated next. The impact duration of the input

signal is taken to be 10 µs. The calculation is done with time interval between the

outputs taken as 2e-7 seconds.

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Fig.4 .3 Cross- section of the concrete block with defect situated at 35 mm from the top surface

In LS DYNA, the model is made with source at the top center of the block and receivers put

at 2 mm distances in negative x directions. Mess size of 1 mm is taken for X- and Z- axes. The

termination time is taken to be 200 µs. We calculate the response only in left side of the

source exploiting the symmetry of the section. Only the node points which lie directly below

the receiver points are taken into consideration. So, we assume the mess having 21 node

points in X- direction and 50 node points in the Z- direction. This is taken because the

aperture of the receiver array extends only up to the position of the last transducer. The

output at the receivers consists of Z- displacement vs. time characteristics of its node points.

This plot is now used to backtrack the position of the defect. The rest of the modeling is

done as before.

The output at the receivers has some initial displacements caused by Rayleigh waves and it

needs to be overlooked for the SAFT reconstruction. Various peaks in the output (excluding

Rayleigh wave part) are multiple reflections of the P- waves from the defect. The first crest

corresponds to double reflection of P- wave (2 P) inside the material under investigation and

so on.

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Fig.4 .4 –concrete block model with defect size of 40 mm (situated at 35 mm from top surface) to be

simulated in LS-DYNA

The response of various receivers (Z- displacement vs. time characteristics) is obtained from

running simulation in LS-DYNA.

Fig4.5 The time vs. Z- displacement characteristics of the receiver node located 20 mm (Series1) and 2 mm

(Series2) from the impact source point of concrete slab (with defect at 35 mm depth)

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4.4 Reconstruction algorithm in MATLAB™

Small code is written in MATLAB program for reconstructing the different data points

obtained at various receivers for concrete slab (with defect). The code is written in the

APPENDIX.

The MATLAB code (reconstruction algorithm) makes an array of the size of the messing of

size 21*50 in the X-Z direction. Then it assigns the co-ordinate value to the receiver points.

The velocity of the P-wave and the impact time of the input impulse signal are supplied to

the code. Now, for every nodal point in the cross-sectional part below the receivers, the

distance of the node point with the source and again with each of the receivers is

calculated. The sum of the distances is divided by the velocity of the P- wave through

concrete to get the time the input pulse takes to get to node point and back to the receiver.

In this time half the impact time of input pulse is added according to SAFT algorithm. This

time is then looked up in the receiver output characteristics (Z-displacement vs. time) to get

the value of the amplitude of the Z- displacement. The same is done for all the receivers for

a single nodal point and added to get the resultant amplitude value of the Z- displacement.

The Z- displacement value thus obtained is divided by the number of receivers to get the

amplitude value of Z- displacement value for the particular node. Now, the same is done for

all the nodal points in the array nodal points. Thus we get the Nodal point vs. Z -

displacement characteristics. This is plotted to check for what value of nodal points the Z-

displacement value coherently superimposes constructively to a get large value (clearly

greater from the rest nodal points). This gives the co-ordinate of the nodal points at which

defect is present.

The node points in the grid (21*50) are given integer values ranging from 1 to 1050 (total

number of node points in the grid). These values are assigned along rows i.e. the leftmost

and topmost node point is assigned node number 1 and the one just right of it number 2.

This way whole grid is assigned node numbers from 1 to 1050. Now, the MATLAB algorithm

gives the plot of the node point vs. superimposed Z- displacement values. The node

numbers for which the Z- displacement is maximum is from 480 to 710 (from the plot). The

average of the two values gives the average node number of the defect. So, checking the

depth of node number numbered 595 we get the depth of the defect as 28.33 mm (node

number/21 gives the z- coordinate of the node). Comparing with the actual defect position

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which is at 35 mm from the top surface the result is satis factory. Keeping in mind the flaw in

the model that the receiver’s aperture width is equal to the defect’s width, the result is

acceptable.

Fig. 4 .6 Plot of the reconstructed Z- displacement vs. the node number

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Chapter 5

Results and Scope

The concrete block model with defect at 35 mm from the top surface had a design flaw. The

extent of the receiver array (20 mm in negative x-direction) was kept same as the width of

the defect below the aperture. After this also the result calculated by algorithm (28.33 mm)

had only 19% error. If the model had been equipped with more receivers for collecting data

or at least the width of the aperture had been twice the width of the defect the error could

be even more minimized. Usually the SAFT algorithm is done on data collected by moving

receiver and applying ‘delay and sum’ technique to it. Unfortunately, the method of SAFT

analysis discussed in report is less in use.

The project needs to go further to get 3-D images of the reconstructed defect positions and

analysis of the same.

In future, the SAFT reconstruction technology is more to be used in health monitoring of all

concrete superstructures. Its imaging technique can further increase the probability by

which the cancer cysts and other anomalies of the human body are detected. Its technique

can also be used to detect underwater characteristics of large water bodies. Its fairly simple

and detailed imaging properties will make other ultrasonic techniques for concrete defect

detection obsolete.

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REFERENCES

[1] http://www.ndt.net/article/concrete/concrete.htm Comparison of Pulse echo method for

testing concrete

[2] http://www.ndt.net/article/civil497/mschic/mschic.htm Towards SAFT-Imaging in

ultrasonic inspection of concrete

[3] http://www.ndt.net/article/kroggel/kroggel.htm Detection of thickness, voids, honeycombs and tendon ducts utilizing Impact echo technique.

[4] http://www.ndt.net/article/ndtce03/papers/v051/v051.htm Ultrasonic imaging of concrete

elements: State of the art using 2D synthetic aperture

[5] Mary Sansalone. Impact Echo: The complete story. ACI Structural journal: Title no.94-

S71, November-December 1997.

[6] M. Schickert, Progress in ultrasonic imaging of concrete, Materials and Structures 38

(November 2005), pp. 807-815, 6 April 2005

[7] Meng-Lin Li,Wei-Jung Guan, Improved Synthetic Aperture Focusing Technique with

Applications in High-Frequency Ultrasound Imaging, IEEE transactions on ultrasonics,

ferroelectrics, and frequency control, vol. 51, no. 1, january 2004

[8] Martin Schickert, Wolfgang Hillger, Automated ultrasonic scanning and imaging system

for application at concrete structures.

[9] Jorgen Arendt Jensen, Svetoslav Ivanov Nikolov, Kim Lokke Gammelmark,

Morten Hogholm Pedersen, Synthetic aperture ultrasound imaging, Ultrasonics 44 (2006)

e5–e15

[10] J. Opretzka, M. Vogt and H. Ermert, A Model-Based Synthetic Aperture Image

Reconstruction Technique for High-Frequency Ultrasound, 10.1109/ULTSYM.2009.0094

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25

[14] C. Cheng, M. Sansalone, The impact-echo response of concrete plates containing delaminations: numerical, experimental and field studies, Materials and Structures, 1993, 26,

pp. 274-285 [15] www.lstc.com

26

APPENDIX

MATLAB code for the detection of effect by SAFT reconstruction technique

% loading the z- displacement vs. time characteristics of the entire receiver points below the

receiver aperture

load lt

for i = 130:2:148 % applying coordinate value to the receiver points

r1(i/2-64) = i;

r2(i/2-64) = 0;

end

% creating array for node points below the receiver aperture

d = zeros(50,21);

% Velocity of P- wave through concrete in mm/sec

v = 3200000;

% Impact time for input impulse in seconds

to = 10e-6;

for i = 2:51

for j = 130:150

temp = sqrt((150-j)^2+(i)^2);

for k = 1:10

temp2 = sqrt((r1(k)-j)^2+(r2(k)-i)^2) + temp;

time = (temp2/v) + to/2;

disp = getvalue(time,k,lt);

d(i-1,j-129) = d(i-1,j-129) + disp;

end

end

end

for i = 1:50

for j = 1:21

27

amp(((i-1)*21)+j) = d(i,j)/10;

end

end

for i = 1:1050

num_node(i) = i;

end

plot(num_node,amp);

getvalue function is defined here-

function [temp] = getvalue(temp2,k,lt)

for i = 1:596

if i ~=596

if lt(i,1) <= temp2 && lt(i+1,1) >= temp2

temp = lt(i,1+(k));

break

end

elseif i == 596

temp = lt(596,1+(k));

break

end

end

end

END OF THE CODE