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NBS TECHNICAL NOTE 1199

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Pulse-Echo Method for

Flaw Detection in Concrete
NATIONAL BUREAU OF STANDARDS

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' JJATXONAL BtJF.EATl OF STiUTDAtlDS Pulse-Echo Method for Flaw

Detection In Concrete

Nicholas J. Carino

Mary Sansalone

Structures Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Gaithersburg, Maryland 20899

Ke*f 01=

C(o.

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued July 1 984
National Bureau of Standards Technical Note 1 1 99

Natl. Bur. Stand. (U.S.), Tech. Note 1 199, 39 pages (July 1984)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1984

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
TABLE OF CONTENTS

Page

ABSTRACT 1

1.0 I NTRODUCT I ON 1

2.0 PRINCIPLES OF THE METHOD 2

2.

1

Wave Propagation 2

2.2 Wave Modes 3

2.3 Wave Characteristics 3

2.4 Wave Velocity 4

2.5 Reflection 5

2.6 Diffraction at Crack Tips

2.7 Attenuation and Divergence 8

3.0 INSTRUMENTATION 10

3.1 Pulse Source 10

3.2 Transducer 10

3.3 Oscilloscope 11 3.4 Pu I se-echo Signal Traces 12

4.0 PAST RESEARCH 13

4.1 Pavements and Bridge Decks 13

4.2 Dams 14

4.3 Piles 15

4.4 Reactor Structures 15

4.5 Refractory Concrete 15

4.6 Submerged Structures 16

5.0 RESEARCH NEEDS 16

6.0 REFERENCES 18

iii

PULSE-ECHO METHOD FOR FLAW DETECTION IN CONCRETE

Nicholas J. Car I no and Mary Sansalone

National Bureau of Standards

Washington, D.C. 20234

The basic principles of the pulse-echo method for the detection

of Internal flaws in concrete are presented. As the

heterogeneous nature of concrete poses prob I ems not encountered

In pulse-echo evaluation of metals, progress In this area of

concrete nondestructive testing has been slow. A review of past

research shows that pulse-echo techniques have been used

successful ly to detect flaws within concrete; however, no standardized method currently exists for pulse-echo evaluation of

concrete structures. Based on the current state of knowledge,

areas of needed research are outlined.

Keywords: Acoustics; concrete; integrity testing; nondestructive

testing; pulse-echo method; wave propagation.

1. INTRODUCTION

Despite recent advances In concrete technology, there Is no

satisfactory method for detecting Internal defects In hardened concrete.

The need for this capabi I Ity Is Increasing with the current emphasis on

rehabll Itatlon of existing structures and on qua I Ity acceptance systems for new construction. The routine method for verifying the existence of defects

with In a concrete structure Is the tak Ing of dr I I I ed core samp I es. Th Is

approach has several disadvantages: 1) It Is expensive; 2) It Is

destructive and requires subsequent repair of the holes; and 3) It samples

only a smal I portion of the concrete In a structure.

Whl I e It Is posslb I e to "look" Inside concrete using X-ray or gamma

radiography, such methods are expensive and cumbersome and they may pose

health hazards. Thus, there Is an urgent need to develop a reliable

nondestruct I ve system for f I aw detection that Is economlca I and easy to

operate. The pulse-echo method, which Is based on the principle that

Internal defects wl I I Interact with stress waves, has the potential to f I I I this need.

With the exception of visual Inspection, the use of acoustic methods

Is the o I dest form of nondestruct I ve testing. Str Ik Ing an object w 1th a hammer and I Istenlng to the "ringing" sound Is a common way of detecting

the presence of Internal voids, cracks or de I amlnat Ions. This form of

acoustic Inspection Is known formal ly as "resonant frequency testing." Its

disadvantage is that it Is primarily a qualitative and subjective test.

With the advent of sophisticated electronic Instrumentation In the

I940's, two nondestructive test methods based upon acoustic waves were

developed: the through-transmission method and the pulse-echo method.

Figure 1 Illustrates the basic principles and differences of the two

approaches. In the through-transmission method, one measures the time It takes for a pulse, which Is defined as a short wave train, to travel from a transmitting transducer to a receiving transducer. From the measured
travel time and the known distance between transducers, an apparent pulse

velocity Is calculated. The computed pulse velocity value can be used to

draw Inferences about the Integrity of the medium. This approach has Its

limitations: It requires two accessible surfaces and It offers no

Indication as to the cause of any differences among pulse velocity values

In different parts of a structure. In the pulse-echo method both the pulse

source and the receiving transducer are mounted on the same surface. The

receiver monitors wave reflections from Internal defects or external

boundaries. One measures the round-trip travel time from the Initialization

of the pulse to the reception of the reflected pulse. Thus, If the

velocity of the pulse Is known, the distance to a defect or an Interface

can be calcu lated.

The pulse-echo method Is commonly used to Inspect welds In steel

structures for the presence of cracks, porosity, or slag Inclusions.

Recently medical researchers have used the method to scan the human body,

eliminating the need for harmful X-ray analysis. Pulse-echo methods have

been used for detecting very large structural cracks In concrete dams, for

Integrity testing of slender concrete structures, such as piles, and for

measuring the thickness of plate elements, such as highway slabs. However,

no versatile, standardized method currently exists for general

nondestructive evaluation of concrete structures.

The objective of this paper Is to provide the reader with background

Information on the pulse-echo method and Its use In the evaluation of

concrete structures. We begin with a discussion of the basic principles of

elastic wave propagation and reflection and we explain why concrete poses

problems not encountered In homogeneous materials, such as metals. This Is fol lowed by a review of past research results where pulse-echo techniques

were used successful ly to evaluate concrete structures. Final I y, areas of

needed research are out I Ined.

2. PRINCIPLES OF THE METHOD

The pulse-echo method for nondestructive testing Is analogous to the

sonar system used by ships for sounding ocean depths. Mechanical energy In the form of a short pulse Is Introduced Into the test medium at an accessible surface by a transmitter. As the resulting stress wave

propagates through the medium It Is reflected by material defects or by

Interfaces between phases of d I f f erent dens it les or e I ast Ic modu I I. A

recei v ing transducer coup I ed to the surface of the test object mon Itors

these reflected waves, or echoes. The transducer output Is displayed on an osci I loscope. Using the time base of the display, the travel time of the

pulse can be determined. If the wave velocity in the medium Is known, the

round-trip travel time of each echo can be used to determine the location

of a defect or an interface.

2.1 Wave Propagation

When a stress pulse is applied to the surface of an object, the pulse

propagates Into the object and, at any particular Instant, Its location can be Identified by the wave front. There are three Idealized shapes of wave

fronts: planar, spherical and cylindrical DD. The shape of a wave front

wll I depend upon the characteristics of the source that is used to generate

the stress pulse. A true plane wave Is produced by a flat osci I I ator that
Is mounted In a rigid wa I I and which radiates Into a I Iquld [2], When a stress pulse Is Introduced Into a solid object using a point source (e.g.,

by Impacting the surface of the object over a sma I I contact area) the

resulting wave front Is spherical. A cylindrical wave front Is generated

by a I Ine source; the I Ine forms the axis of the cy I Indrlcal wave. The

wave fronts of spherical or cylindrical waves diverge as they travel away

from their source. However, at far distances from the source the wave

fronts of divergent waves become nearly planar, and their acoustic

properties become very slml lar to those of plane waves [2].

2.2 Wave Modes

There are three primary modes of stress wave propagation through

Isotropic elastic media. These wave modes are characterized by the

oscl I latlon path of the particles transmitting the wave. The first mode of

vibration Is the longitudinal or compression wave mode, In which the

particles transmitting the wave vibrate about their equl

I

Ibrlum position In a direction parallel to the direction of wave propagation. Although

compression waves (commonly referred to as P-waves) propagate In a I I types

of media, a second mode of vibration, the transverse or shear wave mode,

can propagate only In media with shear stiffness, I.e., In solids. In shear waves (S-waves) particle vibration Is perpendicular to the direction

of wave propagation. Where there Is a sol ld/1 Iquld or a solid/gas

Interface, Raylelgh or surface waves (R-waves) can propagate over the

Interface. In this mode of vibration the oscl I latlon path of the particles

Is el I Iptlc. The amp I Itude of particle motion In a surface wave decreases

exponential ly with distance away from a free boundary C3U. Figure 2a

II lustrates the propagation of the wave fronts corresponding to the three

wave modes In a plate. The stress pulse was Introduced Into the top

surface of the plate by a point source; thus the resulting wave fronts of

the P and S-waves are spherical.

2.3 Wave Characteristics

The major parameters associated with stress waves, along with their

corresponding symbols and units, are given In Table 1. The fol lowing

relationship between frequency, f, wavelength, X, and wave velocity, C, Is val Id for a I I types of waves.

f X = C (1)

Other characteristics of stress waves are amplitude of particle motion, A,

acoustic pressure, P, and acoustic Intensity, I. (Acoustic intensity Is

the average rate of flow of energy through a unit area normal to the

direction of wave propagation DID. For compression or shear waves, the

amp I Itude of particle motion depends on the Intensity and the wavelength of

the propagating wave and on the physical properties of the material in

which the wave Is propagating [4]:

(2)
where is the mass dens I ty of the materia I . The amp I I tude of part ic I e motion is extremely sma I L For a compression wave in concrete, with an intensity of 10 Watts/cm (a very high value for pu I se-echo testing of

mater la I s [2]), the amp I itude of particle motion is on the order of onethousandth of a millimeter or less. Displacement amplitude is related to

acoustic pressure by the fol lowing equation [2]:

P = p C u A (3)

where w Is the angular frequency of the wave. For compression waves

acoustic pressure is the normal stress paral lei to the direction of

propagation. For shear waves acoustic pressure Is the shear stress

associated with the propagating wave.

2.4 Wave Velocity

The propagation of stress waves through a heterogeneous bounded solid,

such as a structural concrete member, is a complex phenomenon. However, a basic understanding of the relationship between the physical properties of

a material and the velocity of wave propagation can be acquired from the

theory of wave propagation In infinite isotropic elastic media [5],

In infinite elastic sol Ids the compression wave velocity, C_, is a function of Young's modulus of elasticity, E, the mass density, p, and

Poisson's ratio, v :

E(l - u)

C = / (4)

P ,/ (I - y)(\ - 2v)

In bounded sol ids, such as thin plates or long rods, compression wave

velocity can vary depending on the dimensions of the solid relative to the

wavelength of the propagating wave. For rod-like structures, such as piles, compression wave velocity is independent of Poisson's ratio If the

rod diameter is much less than the wavelength of the propagating wave [4U.

In this case C_ is given by the fol lowing equation:

x

C

P =

For v = 0.2, a typical value for concrete, the compression wave velocity

Is five percent higher In an infinite solid than In a long thin rod.

The shear wave the velocity, C

s

, In an Infinite solid is given by the

fol lowing equation:

c

s

=

|

| 2 p ( I - v )

where G is the shear modu I us of el ast Iclty. For v = 0.2, the shear wave

velocity Is 61 percent of the compression wave velocity.
Surface waves propagate at a velocity, C

R

, which can be determined

from the fol lowing approximate formula C6U:

0.87 + 1.12

Cr = C

(7)

I +

S

For v = 0.2, the surface wave velocity Is 92 percent of the shear wave

velocity.

In many app

I

Icatlons compression wave velocity cannot be measured

directly. However, compression wave velocity Is related to surface wave

velocity by the fol lowing equation:

0.87 +1.12 \ \ - 2v

CR

=

/

I + v V 2(1 - v ) P

Thus, If Polsson's ratio Is known, the measured surface wave velocity can be used to estimate the compression wave velocity C7H. For v = 0.2, the

surface wave velocity Is 56 percent of the compression wave velocity.

2.5 Reflection

When a wave front Is Incident upon an Interface between dlsslml I ar

media, "specular" reflection occurs. (The term specular reflection Is used

since the reflection of stress waves Is similar to the reflection of light

by a mirror.) Stress waves can be visualized as propagating along ray

paths with the geometry of ray reflection being analogous to that of I Ight

rays. The shape of the reflected wave front can be determined by

considering the reflection of Individual rays. In Figure 2b Incident rays

0A and OB are reflected as rays AC and BD, and It Is seen that the

reflected rays behave as though they were radiating from virtual point 0'. Thus, the reflected wave fronts PP and SS are also spherical.

At a boundary between two different media on I y a portion of a stress

wave Is reflected. The remainder of the wave penetrates Into the

underlying medium (wave refraction), as shown In Figure 3a. The angle of

refraction, /3, Is a function of the angle of Incidence, 0, and the ratio

of wave velocities, Co/Ci, In the different media and Is given by Snell's

Law [8]:

sin (8 = £2 sin d (9)

C|

Unlike other types of waves, stress waves can change their mode of

propagation either partial ly or completely when striking the surface of a sol Id at an obi Ique angle [9]. Incident compression waves can be partial ly

reflected as both compression and shear waves and can be refracted as both

compression waves and shear waves, or as a surface wave, depending on the

angle of Incidence. Since shear waves propagate at a velocity less than

that of compression waves, they will reflect and refract at angles,

6 S and /3 S » that are less than the angles of reflection and refraction

of the compression waves, as shown In Figure 3b.
The relative amplitude of reflected echoes depends on the mismatch In specific acoustic Impedances at an Interface, the angle of Incidence, the

size, shape, and or I en tat Ion of a f I aw or an Interface, the distance of a dlscont Inu Ity from the pu I se source, and the attenuation a long the wave

path. The Influence of each of these factors Is considered In the

fol i owing discussion.

The portion of an Incident plane compression wave that Is reflected at an Interface between two media of different densities or elastic modu I I depends on the specific acoustic Impedances of each medium. The specific

acoustic Impedance, Z, of a medium Is

Z = p C

p (10)

Since C_ Is approximately equal to J E/p

FP (11)

Specific acoustic Impedance values for compression waves In selected

materials are given In Table 2. Equation (10) Is also val Id for shear

waves If the shear wave velocity, C

s

, Is used to calculate acoustic

Impedance,

The Intensity of reflected energy, l p

, Is maximum when the angle of

Incidence Is normal to the Interface and Is determined from the following

equation £4]:

" Z

2

2

l r =lj[ I

:

i +Z

2

where I f Is the Intensity of the Incident energy. As shown by equation

(2), the amp I Itude of the returning echo wl I I be proportional to the square

root of I p

. At an Interface between a sol Id and a gas, such as between

concrete and air, the specific acoustic Impedance of the sol Id Is so much

larger than that of the gas that virtual ly a I I the Incident energy Is

reflected producing a strong echo. For cases of oblique Incidence the

amp I Itude of the reflected waves wl I I depend on the angle of Incidence as we I I as on the acoustic Impedance mismatch [2]. If mode conversion occurs,

the reflected energy wl I I be sp I It between the different wave modes.

Coefficients of reflection for the amplitude of a reflected wave as a function of the angle of Incidence can be determined using the formulas In Reference [23, wh Ich are app I I cab I e for p I ane waves Inc I dent upon p I ane

boundaries. These formulas were used to calculate the reflection coefficients for a concrete/air Interface. Figure 4 shows reflection

coefficients for an Incident P-wave, and Figure 5 shows reflection coefficients for an Incident S-wave. It Is assumed that each Incident wave has

an amplitude equal to unity. Each figure Is composed of three graphs. The

graph in the upper left gives the reflection coefficients for the wave with

the same mode as the Incident wave; for example, in Figure 4, which Is for

an Incident P-wave, the upper left graph shows the reflection coefficients

for a reflected P-wave. The graph In the lower right gives the reflection
coefficients for the mode-converted wave. The lower right graph in Figure

4 shows the reflection coefficients for the S-wave produced by mode

conversion of the incident P-wave. The graph in the upper right gives the

angular relationship between the Incident wave and the mode-converted wave,

which is determined using Sne I Ms Law. The drawing in the lower left gives

an il lustratlve example. In Figure 4 the lower left drawing shows a P-wave

(amplitude equal to unity) incident on a concrete/air interface at an angle

of 30°. The ang I e of the ref I ected P-wave,

p

, Is equa I to the ang I e of

the incident P-wave; thus a horizontal line can be drawn from

p = 30° to

the graph of the reflection coefficients for the reflected P-wave. In this

case R. Is equa I to 0.56, I.e., the amp I itude of the refl ected P-wave is

56/K of the amp I Itude of the Incident P-wave. If a second I ine Is drawn

horizontal ly from dp = 30° to the graph representing Sne I I 's Law, the angle

of the shear wave produced by mode conversion of the Incident P-wave can be

obtained (0 S = 18°). Subsequently, the reflection coefficient for the

mode-converted shear wave, R

s

, can be determined. In this example R_ is

equal to 0.62, I.e., the shear wave has an amp I itude equal to 62$ of the

Incident P-wave.

In the previous discussion It was assumed that reflection and

refraction of wave fronts occurred at planar Interfaces between two

dissimilar media. This analysis Is also applicable to flaws or discontinuities within a medium. The ability of the pulse-echo method to

detect flaws or discontinuities (sensitivity) depends on the wavelength of

the propagating wave and on the size and orientation of the flaw or discontinuity. Waves wi I I diffract or bend around the edges of

discontinuities if the size of the discontinuity Is approximately equal to

or less than the wavelength; therefore, no reflection occurs for flaws that

have lateral dimensions less than the wavelength. Figure 6 shows the

relationship between frequency and wavelength for the range of compression

wave velocities typical ly measured In concrete. To be able to detect flaws

on the order of 0.1 meters It would be necessary to Introduce Into the

concrete a stress pulse that contains frequencies greater than 20 kHz.

2.6 Diffraction at a Crack Tip

When a stress wave Is incident upon a crack within a solid, the

pattern of re-radiated energy Is determined by four processes [10U:

1. Specular reflection from the crack face,

2. Raylelgh waves which travel along the crack surfaces

and radiate energy when reaching the crack tips,

3. Diffracted spherical waves which originate from the

crack tip, and

4. Mode conversion.

Specular reflection, produces the strongest echo signals and Is the easiest

to analyze; however, this reflection has a strong angular dependence and can only be detected If the crack Is oriented at a favorable angle to the

propagating wave and receiving transducer. If a crack Is not oriented at a favorable angle for detection of specularly reflected waves, the signals

received from diffracted waves can be used to detect the presence of cracks

DID. Figure 7a shows the specularly reflected wave front (PP) produced by

a spherical P wave incident upon a crack C12], Ray 0A, which intersects

the crack tip will be diffracted, producing an additional spherical wave
(P^), as shown In Figure 7b. Note that diffraction permits the stress wave

to penetrate the "shadow zone" behind the crack. (A similar situation

would occur for an incident S wave.) Researchers have been able to record

photographic Images of diffraction using photoelastlcity C13, 14], Which

confirm the conditions depicted In Figure 7b. The amp I Itude of particle

motion In a diffracted wave varies with direction and, according to

theoretical calcu I atlons,. the amplitude In the diffracted wave is an order of magnitude less than the amplitude of specularly reflected waves

Do].

2.7 Attenuation and Divergence

As a wave propagates through a solid the acoustic pressure (and thus

the amplitude of particle motion) decreases with path length due to

attenuation (scattering and absorption) and divergence.

In a heterogeneous solid, scattering Is the result of wave reflection,

refraction, diffraction, and mode conversion at each Interface between

dissimilar media. In ordinary concrete the density and the elastic modulus

of the coarse aggregates are general ly higher than those of the mortar;

thus, from equation (11), the specific acoustic Impedance of the coarse

aggregate Is higher than that of the mortar. If the wavelength of the

propagating wave Is less than the size of the aggregate, this mismatch In impedances causes scattering of the incident wave and returning echoes as the waves undergo reflection and refraction from each mortar-aggregate

Interface C7], For higher quality concrete the specific acoustic

Impedance of the mortar approaches that of the coarse aggregate and

scattering Is reduced. In pulse-echo evaluation of concrete lower

frequency waves must be used (I.e., the wavelength-to-aggregate-size ratio

must be increased C15]) to reduce the attenuation of wave energy due to

scattering. However use of lower frequency waves reduces the sensitivity

of the propagating wave to sma I I flaws. Thus there Is an Inherent

limitation In the flaw size that can be detected within concrete.

Although attenuation of wave energy In heterogeneous solids Is

primarily due to scattering, part of the wave energy Is absorbed and turned

Into heat (hysteretic damping) £<?!!• In solids damping is mainly caused by

internal friction.

Due to attenuation, the acoustic pressure of a plane wave decreases

exponent! a I ly with path length and can be determined from the f o I I owing

equation ZZl:

P = Po

e- ad (13)

where PQ

is the initial acoustic pressure, a Is the aftenuatlon coefficient

CdB/m], and d Is the length of the wave path [m]. Since scattering is the

principle cause of attenuation, the value of a depends upon the wavelength

of the propagating wave. Thus, the higher the frequency of the wave that

is used to probe a given material, the larger wi I I be the value of a for

that material. The published data on a -values for concrete are scant.

'Quality in this context refers to the density and elastic modulus of the

mortar phase. Higher quality Is synonymous with higher density and greater

elastic modulus or both.
However, based on available Information D6, 17], It appears that for

frequencies on the order of 100 to 200 kHz, the probable range of values

for a Is 0.3 to 30 dB/m. Figure 8 shows the decrease of the acoustic

pressure of a plane wave as a function of the length of the wave path for

this range of a-values. It Is seen that for the lower limit of a the

loss of acoustic pressure with path length Is sma I I. However, for the

upper I Imlt of a there Is a drastic decrease In acoustic pressure with path

length. Thus, careful consideration must be given to the frequency content

of the wave that Is used to probe large concrete members.

In addition to causing a decrease In the acoustic pressure, attenuation

also affects the frequency content of a pulse propagating In a heterogeneous medium [18^. Transmitted pulses contain a range of

frequencies. In a material such as concrete the higher frequency

components of the propagating pulse wl I I be preferential ly attenuated with

path length. As a result, the frequency spectrum of the pulse Is

continuously shifted to lower frequencies. Thus, both the sensitivity and

the acoustic pressure of a pulse decrease with path length.

For non-planar waves, reduction of the acoustic pressure also occurs

due to spreading of the wave front as It propagates through the test medium

(divergence) C83. For the case of a point source producing a spherical

wave front, divergence causes the acoustic pressure to vary as the Inverse

of the distance from the source C4l. For other types of sources, such as transducers, a "near-field" region exists close to the source within which

the acoustic pressure varies In a complex manner. However, beyond the

near-field , the acoustic pressure will decrease at a rate approximately

equal to that experienced by a spherical wave C2D.

The path of a received signal Includes both the Incident path (source

to flaw) and the reflected path (flaw to receiver). Both incident and

reflected waves undergo attenuation and divergence; thus, the amplitude of

the echo signal wl I I decrease dramatical ly with total path length. The

relative losses caused by attenuation and divergence wl I I depend upon the

characteristics of both the test medium and the propagating wave [2U. For

example, If high frequency (non-planar) waves are used to test a material

with a low attenuation coefficient, such as a fine-grained steel ( a = 1 to

3 dB/m for frequencies In the range of 2 MHz [2]), the decrease In acoustic

pressure due to divergence of the acoustic beam In the far-field wl I I I Imlt

the thickness of the object that can be probed. If the same high frequency

waves are used to test a heterogeneous, porous material such as concrete

( a > 100 dB/m for frequencies In the MHz range [2]), the proportion of

losses caused by attenuation will predominate, severe I y limiting the

penetrating power of the propagating wave.

zThe end of the near-field (the beginning of the far-field) can be

determined based on the size of the source and the frequency of the

propagating wave [3D.
3. INSTRUMENTATION

A typical pulse-echo testing system consists of three essential units:

a pulse source, a receiving transducer, and a cathode-ray oscl I loscope, as shown In Figure 1.

3.1 Pulse Source

Electro-acoustic transducers and mechanical Impact are the two common methods used to Introduce a stress pulse Into a test object. The selection

of a pulse source for a specific application depends upon the size of the

flaws to be detected and on the characteristics of the test medium. In the

Inspection of metals, high-frequency, short-duration, pulses are Introduced

Into a test object by an electro-acoustic transducer. The transducer Is

shock-excited by a short, high voltage pulse, which causes It to vibrate

(ring) at Its own resonant frequency for a few cycles and transmit a pulse

Into the test object C9D. No commercial transducers are satisfactory for

pulse-echo testing of concrete, thus a mechanical Impact source normal ly Is used to generate a stress pulse with sufficient acoustic energy to overcome

the effects of attenuation and divergence. The type of mechanical Impact

source that Is used will determine the energy and the frequency content of

the pulse. The force-time function of the Impact can be approximated as a half-sine curve. The amplitude of the force-time function wl I I affect the

amplitude of particle motion (and thus the acoustic pressure) as the pulse

travels through the test object, and the contact time of the force-time

function wl I I affect the frequency content of the pulse. The notion of

frequency content arises from the principle of Fourier series, whereby the

pulse shape can be approximated by the sum of a number of sine functions of

different frequencies. Many sine curves are required to approximate the

pulse shape produced by a short contact time, and thus the Input energy Is distributed over a wider frequency range. This frequency range will

contro I the mini mum f I aw s I ze that will be capab I e of ref I ect I ng the pu I se: the higher the frequency range, the smal ler Is the flaw that wl I I reflect

acoustic energy.

3.2 Transducer

The transducer Is the critical element In any testing system based on acoustic wave transmission and reception. Transducer response must be

accurate and reliable. Ideal ly a transducer should respond faithfully to

only one physical parameter (e.g., the normal component of surface

displacement or velocity) so that the response can be correctly

Interpreted. However, conventional transducers usually do not measure a single parameter because of the effects of the coupling between transducer

and test object, the electronics, and the characteristics of the

transducer itself D9]].

The majority of electro-acoustic transducers used In high-frequency

pulse-echo systems utilize piezoelectric crystals In the generation and

reception of stress waves. A piezoelectric material Is one which generates

an electrical charge when subjected to a mechanical stress or deformation.

Single crystals of many compounds exhibit piezoelectricity. Traditional ly

natural crystals, such as Roche I le salt and quartz, were used in the

construction of crystal transducers. Although still used In specific

10
applications, natural crystals have been almost completely replaced by

manufactured ceramic materials.

If the elastic I Imlt of the piezoelectric material Is not exceeded, the

charge densities produced by the piezoelectric effect are directly

proportion a I to the strain of the crysta I . Converse I y, a plezoe I ectrlc

crystal subjected to an electrical charge will produce mechanical

deformation (vibration) [203. Thus, a single piezoelectric transducer can be used for both generation and reception of stress waves In pulse-echo

systems. However, when a single transducer Is used, transmitted pulses

must be of short duration, otherwise the ringing of the transducer crystal

will render It Insensitive to echoes returning from flaws or Interfaces.

In addition a transducer must have the capabll Ity to separate echoes which

are close together (resolution). If a second signal arrives while a crystal Is stl I I vibrating due to an earlier excitation, the second signal

wl I I either be mixed In with the first signal making the signal record

difficult to Interpret or It wl I I go unnoticed, depending on the relative

strength of the echo signals C8H. Thus, a crystal Is "b I Ind" for a short

time after each excitation. To prevent ringing and to reduce blind-time a transducer must be heavily damped [9]. However, damping decreases a transducer's sensitivity £8l» The more a transducer Is damped, the

stronger must be the returning echo that wll I cause It to vibrate. Smal I flaws may not reflect sufficient energy to excite the transducer and no

signal wl I I be produced on the oscl I loscope screen. Thus, an acceptable

balance between resolution and sensitivity must be achieved In the design

and construction of a piezoelectric transducer.

The ratio of the transducer diameter, d, to the wavelength of the

transmitted waves, X, determines the shape of the acoustic beam. Figure 9

C2, 21] shows the directional characteristics for d << X , d/X = 1, and for

d/x= 4. Most of the energy In an acoustic beam Is contained within a cone-shaped region which has Its apex at the transducer. As d/X decreases

the apex angle of the cone Increases, I.e., the directionality of the

acoustic beam becomes less pronounced. A transducer's "f lei d-of- view" Is determined by Its directional characteristics, and It Is this f lei d-of-vlew

that wl I I determine the Interrogation pattern that Is requ tred to f u I I

y

probe the Interior of a test object. Increasing d/X reduces the fleldof-vlew by focusing most of the acoustic energy Into a narrow compression

wave beam. A transducer that emits a focused beam (Figure 9a) can be used

much more effectively to probe a test object as It a I lows flaws or

Interfaces to be located more precisely.

3.3 Oscl I loscope

The Introduction of the pulse Into the test object triggers the

oscilloscope's time base circuit, which provides a linear traverse of the

cathode-ray beam from left to right across the screen. The transmitted

pulse and returning echoes are displayed as vertical offsets from the

horizontal traverse (see Figure 1b). ( I n mu 1 1 1-channe I oscilloscopes

transmitted pulses and returning echoes can be displayed on separate

traces.) Using the time base on the display, the time from the start of

the pulse to echo arrival can be determined. In research applications a digital storage oscilloscope Is preferred to enable detailed analyses of

the received signals.

11
3.4 Pulse-echo Signal Traces

As an II lustration of the pulse-echo technique for concrete, selected

echo signals are shown In Figures 10a and 11. These signals were obtained

by using a transducer recently developed at the National Bureau of

Standards which accurately measures normal surface displacement [22U.

Figure 10a Is a plot of a portion of a signal recorded by a digital

oscl I I oscope during a test on a 0.25-m thick, 1.12- by 1.12-m concrete

slab. The receiver was located at the slab center on the top surface. The

stress pulse was generated by dropping a 8-mm steel ba I I onto the slab from

a height of 0.19 m. The separation between the Impact point and receiver

was 0.125 m. In this case It can be assumed that the Impact of the steel

ba I I acts as a point source emitting a pulse of acoustic energy Into the

slab In a I I directions. Figure 10a also Indicates the theoretical arrival

times of various acoustic rays which were calculated based on a measured

compression wave velocity of 3980 m/s and an assumed Polsson's ratio of

0.2. The paths of selected acoustic rays are I I lustrated In Figure 10b.

The first rays to arrive are the direct P, S, and R-waves, which travel

along the top slab surface. The signal due to the arrival of the R-wave Is very strong; thus, clear Identification of the arrival of the PP-ray (a

reflected compression wave due to a compression wave Incident on the bottom

slab surface) Is not possible In this signal record. Mode conversion of a compression wave Incident upon the bottom slab surface results In a reflected shear wave (PS) which arrives after the PP-ray since the velocity

of a shear wave Is only 61 percent of the velocity of the compression wave. The subsequent peaks in Figure 10a are due to the reception of multiply

reflected compression waves. There Is excel lent agreement between the

signal maxima and the computed arrival times of the 4P and 6P-rays.

Figurel 1 is a p I ot of a portion of a recorded slgna I obtained from a test on a 0.5-m thick concrete slab containing a disc-shaped void (0.5 m In diameter) located 0.25 m below the top slab surface. In this test the

separation between the Impact point and the receiver was 0.10 m. Notice

the similarities between this figure and Figure 10a. The theoretical

arrival times of reflected rays were calculated based on reflections from

the concrete/void Interface, a measured compression wave velocity of 3900

m/s, and an assumed Polsson's ratio of 0.2. The agreement between the

signal maxima and the computed arrival times clearly Indicates that the

rays are being reflected by the concrete/void Interface. The 0.10-m

separation between source and receiver (0.025 m less than in the previous

case) provides a greater time lapse between the arrival of the R-wave and

the arrival of the PP-ray. Since the disturbance due to the R-wave has

time to decay before the arrival of the PP-ray, the PP-ray Is easily

Identified In Figure 11.

Although the traditional approach In pulse-echo testing has been to

study the signal record In the time domain, the Fast Fourier Transform

technique can be used to obtain the frequency content of a time domain

digital signal record C2H. The frequency spectrum of a received signal can be used to determine thicknesses of plate elements or to estimate the size

and location of flaws, a spectrum analyzer is employed to transform the

time domain trace of the recorded signal Into a frequency domain plot.

Figure 12 Is a plot of the frequency spectrum corresponding to the time

domain signal record shown In Figure 10a. The peak frequency, 7.81 KHz,

can be used to estimate the slab thickness. In this example the peak

12
frequency predicts a s I ab thickness of 0.255 m which agrees we I I with the

actual thickness of 0.25 m.

4. PAST RESEARCH

The use of acoustic waves for the detection of flaws In solids dates

back to 1929, when Sokolov of Russia first suggested the use of ultrasonic

waves to find defects in metal objects Q9j. Development of practical

nondestructive test methods was slow; It was not until the nearly

simultaneous introduction of pulse-echo flaw detectors in 1942 by Firestone

of the University of Michigan and by Sproule of England that significant

progress was made. Since that time ultrasonic pulse-echo testing of

met a Is, p I ast Ics, and other homogeneous materia Is has developed into an efficient, reliable, and versatile nondestructive test method. The

extension of acoustic pulse-echo principles to less Ideal materials, such

as concrete, has been hindered by the difficulties Inherent In obtaining

and interpreting a signal record from a heterogeneous material, although

some progress has been made In developing systems for measuring the

thickness of plate elements and for Integrity testing of rod-like

structures such as p'les.

4.1 Pavements and Bridge Decks

Resonance and pulse-echo techniques have been used to measure the

thickness of concrete pavements and to detect de I ami nations in bridge

decks. In the early 1960's, Muenow E23] developed a technique to measure

pavement thickness that was based on determining the frequency of the

fundamental mode of vibration of the slab In the thickness dimension.

Continuous compression waves were Introduced Into the slab by a transmitter. The frequency of these waves was systematical ly varied until

a resonance condition^ was obtained. Slab response was monitored by a receiving transducer which was located adjacent to the transmitter.

Transmitted and received signals were displayed separately on a two channel

oscilloscope. The fundamental mode of longitudinal resonance was

identified by noting the frequency, f, at which a marked increase In the

amplitude of the received signal occurred and by measuring the phase angle

between the transmitted and received signals. If the P-wave velocity In the slab is known, the thickness of the slab, t, which equals one-half the

wavelength of the propagating wave, could be calculated (t = A/2 = C/2f).

Resonance methods have also been used to detect de

I

aminat ions In concrete bridge decks. A technique developed In 1 973 by researchers at

Texas A & M University [24] used an oscl I latlng steel plunger to excite the

characteristic vibrations of a de laminated area. As the plunger oscl I I ates

at 60 Hz, the vibrations of the bridge deck were monitored by a receiving

transducer. The location and extent of de laminated areas could be

determined from the relative amplitude of the received signal. The use of

this technique has been I Imlted by the fact that it cannot be used where

concrete decks have asphaltic overlays exceeding approximately five cm.

JA resonance condition Is produced when standing waves are set up, that Is,

when waves are repeatedly reflected within a medium In such a way that

their wavelengths are superimposed exactly in phase C9H.

13
In 1964, Bradfleld and Gatfleld [25] of England reported the

development of a pulse-echo technique for measuring the thickness of

concrete pavements. Using two 100 kHz resonant transducers (16 cm tal I, 10

cm w Ide, and 25 cm I ong) they were ab I e to measure the th Ickness of a 12-

Inch concrete specimen with an accuracy of approximately 2%, However this

system could not be field tested [24] due to the Impractical Ity of the test

set-up. Besides being bulky, the transducers were coupled to the concrete

by a large plastic block which required a large smooth flat concrete

surface for good coupling. Difficulties were also reported In obtaining

reflections from rough textured bottom pavement surfaces.

In 1968, Howklns, et al, at I IT Research Institute [26] Independently

Investigated available resonance and pulse-echo techniques In an attempt to

Identify a feasible method for pavement thickness measurements. Tests

using the resonance technique proposed by Muenow [23] and the pulse-echo

technique proposed by Bradfleld and Gatfleld [25] were performed. Although

It was felt that the resonance technique was, In principle, a good

approach, significant reservations were stated concerning the reliability

of Muenow's resonance method. Using a pulse-echo technique similar to that

developed by Bradfleld and Gatfleld, the I IT researchers were able to

measure the thicknesses of 7 and 10- In thick port I and cement concrete slab

specimens, for simple support conditions and for slabs supported on a 4-ln

thick gravel base course, and a 5-ln thick, simply supported bituminous

concrete slab specimen with an accuracy of + 2%, However, It was concluded

that the transducer arrangement was not practical for field use.

A pulse-echo system was developed at Ohio State University In the late

I 960's [21~] to measure pavement thickness by monitoring the travel time for

an ultrasonic pulse to propagate through the thickness of the concrete and

return to the receiving transducer at the top surface after being reflected

by the concrete pavement-subbase Interface. A large transmitter was needed

to Introduce sufficient acoustic energy Into the test medium to overcome

wave attenuation problems due to coarse-grained aggregates and to obtain

coherent reflections from rough pavement-subbase Interfaces. The

transmitter was circular In design, with a 46 cm outer diameter, a 15 cm

Inner diameter, and a 200 kHz resonant frequency. The receiving transducer

was placed In the center of the transmitter. Accuracies of plus or minus

3% at more than 90$ of the test locations were obtained. The accuracy and

good performance of the Ohio State thickness gage was confirmed In

Independent field tests conducted In I 976 by Weber, Gray and Cady [28].

However, Weber, et a I concluded that the Ohio State Instrument wll I nave to

be redesigned to better withstand the rigors of field use before It can be

considered as a practical nondestructive method.

4.2 Dams

Engineers In India [29] used a sonic pulse-echo method to estimate the

depth and extent of large horizontal cracks that developed In the Koyna dam

during a major earthquake In 1 967. Very low frequency acoustic waves (200-

600 Hz) were Introduced Into the concrete by mechanical Impact with a free

fal I Ing steel hammer. This range of sonic frequencies al lowed detection of

cracks on the order of 15 m and larger. The energy generated by the

mechanical Impact was sufficient to obtain reflections from cracks located

100 m away from the point of Impact. After existing cracks were grouted

the pulse-echo technique was used to assess the degree of grout

14
penetration. The assessment was made qualitatively by comparing the echo

amp I Itudes In slgna I records obtained before grouting to those obtained

after grout I ng.

4.3 Piles

Since the early I 970's pulse-echo principles have been widely used

for Integrity testing of concrete piles \J , 30-393. The behavior of

stress waves In slender, rod- 1 Ike structures, such as piles, Is we I I known.

If a pulse Is generated by mechanical Impact at one end of a rod, the

resulting wave front Is Initial ly spherical but quickly becomes planar as the pulse propagates down the long slender rod. Plane wave reflection

occurs at the bottom surface, and the reflected wave front travels back up

the length of the rod to be picked up by the receiving transducer. Thus

the rod acts as a "waveguide." As a result, echo signal analysis Is

relatively uncomplicated. Stelnbach and Vey [7] were among the first to

apply pulse-echo techniques to field evaluation of piles. A compression

pulse was Introduced Into a concrete pile at the top surface by mechanical

Impact and returning echoes were monitored by an acce I erometer mounted on the same surface. The signal record could then be used to detect partial or complete discontinuities, such as voids, abrupt changes In cross

section, very weak concrete, and soil Intrusions, as we I I as the

approximate location where such Irregularities existed. In the absence of

major Imperfections the location of the bottom of a sound pile could be

determined. However, little specific Information could be obtained as to

the extent of defects or the relative soundness of concrete at the location

of an irregularity. The success of the method is dependent upon the

damping characteristics of surrounding soil; a high degree of damping can severely weaken returning echoes.

4.4 Reactor Structures

In 1976 Sutherland and Kent L"15H of Sandia Laboratories used the

ultrasonic pulse-echo method to study the safety of concrete reactor

substructures subjected to the thermal energy of a hypothetical core

meltdown. The relative position of a concrete-gas interface subjected to a high heat flux from a plasma jet was monitored es a function of time to

determine the erosion rate of the concrete substructure. Erosion rates

were determined successfully for plain structural concretes with aggregate

size variations from 3 to 75 mm.

4.5 Refractory Concrete

Claytor and El I ingson [16D used the pulse-echo method to measure the

thickness of 30.5 cm thick refractory concrete specimens. It was found

that for frequencies below 100 kHz the use of a single transducer as both

the transmitter and receiver was Impractical because the ringing of the

transducer obscured the echo signal. When two transducers were used In a transmittei—receiver mode, ringing was not a problem; however, the

transmitting transducer generated strong Raylelgh waves which interfere

with the reception of the echo signal by the receiving transducer. To

reduce Interference due to Raylelgh waves large-diameter (17.8 cm)

transducers were constructed. As the response of a transducer Is an averaged phenomenon over the contact area, the sensitivity of a larger

diameter transducer to local ized surface disturbances (Raylelgh waves) wl I I

15
be reduced. To further reduce Interferences due to Rayleigh waves a glycerin buffer was used between the transducers and the concrete

specimens. The compression waves produced by mode conversion of the

propagating Rayleigh waves radiate Into the glycerin at an angle of 36°.

Thus, the echo signal, which is incident on the glycerin at a much steeper

angle (depending on the spacing of the transducers and on the orientation

of the reflecting surface) Is received with greater efficiency.

4.6 Submerged Structures

Smith4 demonstrated that Rayleigh waves can be used to detect surface

opening cracks In submerged concrete structures, such as concrete tanks and

offshore structures. Two 0.5 MHz, 25-mm diameter, compression wave

transducers were used as transmitter and receiver. When a transmitted

compression wave strikes the surface of a solid at a critical angle

(defined by Snell's Law) mode conversion occurs producing a Rayleigh wave

which propagates along the solid-liquid Interface. As the Rayleigh wave

propagates, mode conversion also occurs, producing a compression wave which

radiates Into the liquid at the same critical angle and is picked up by the

receiving transducer. The distance between the two transducers could be

adjusted to optimize the amp I itude of the received signal. If the path of

the propagating Rayleigh wave Is crossed by a crack, reflection occurs and no signal will be picked up by the receiving transducer. If a crack is

favorably oriented (a crack at 90° to the propagating wave Is the best

orientation) the compression waves produced by mode conversion of the

reflected Rayleigh wave will be picked up by the transmitting transducer.

Analysis of the received signals obtained from a complete scan, I.e., from

moving the transducers para I lei to and over the surface of the test object

In a prearranged pattern, a I lowed the location of surface opening cracks to

be determined.

5. RESEARCH NEEDS

A review of the literature has shown that pulse-echo techniques have

been used success fu I I y to detect f I aws w ith i n concrete. In particular,

pulse-echo principles have been widely used for Integrity testing of

concrete piles. The technique and the Instrumentation for the evaluation

of this type of structures are fairly well established. Although limited

success has been achieved In other specialized applications, no

standardized method currently exists for general nondestructive evaluation

of concrete structures. Based on the current state of knowledge, the

fol lowing research areas need to be Investigated If an efficient and

re I lable system for flaw detection In concrete Is to be developed:

1. If the pulse-echo technique Is to gain widespread acceptance, It will

be necessary to standardize the key elements of the method. Standard

elements of a pulse-echo system should Include a reproducible Input pulse

with a we I I defined frequency content, instrumentation for accurate and

reliable reception and display of reflected signals, and analysis

techniques for efficient Interpretation of the signal record.

^Smlth, R. L., "The Use of Surface Scanning Waves to Detect Surface Opening

Cracks In Concrete," to be presented at Int. Conf. on In SItu/NDT Testing

of Concrete, Oct. 2-5, 1 984, Ottawa, Canada.

16
2. The characterization of flaws by frequency spectrum analysis

(spectroscopy) should be Investigated. Using the frequency distribution

Information may be obtained about the size, location, and orientation of

Internal defects. (The reader Is referred to reference [40]] which contains

an Introduction to the method and an extensive abstracted bibliography.)

3. A better understanding of elastic wave propagation In heterogeneous

solids Is essential. An analytical model Is needed to describe the

scattering of elastic waves by flaws and discontinuities. (For a review of

proposed scattering theories the reader Is referred to Ref. [413. ) A

numerical model based on a technique such as finite element analysis also

shou I d be cons Idered, perhaps as an alternative to an analytical mode I

.

Such a model, after experimental verification, wl I I enable parametric

studies to evaluate the significance of the variables which affect the

Interaction between stress waves and flaws.

4. As has been discussed, the basic approach In acoustic nondestructive

eva I uat Ion methods is to mon I tor the response of a test object under the

stimulus of a pulse source with known Input characteristics. Changes In the elastic properties and Integrity of the object will be manifested as changes In the received signal. Mathematically, the response, h(xj, of a medium to a given stimulus can be evaluated as the convolution 5 of the

Impulse response function (transfer function), g(x), of the medium with the

stimulus (source function), f(x). The transfer function of the medium Is a function of Its elastic and geometric properties. Alternately, by the

process of deconvol utlon, the transfer function may be obtained from the

known stimulus and the measured response. Research Is needed to evaluate

the effect that changes In the physical characteristics of heterogeneous

and porous sol Ids have on the transfer function. The response of test

objects, such as finite plates containing Internal cracks and voids, to

well-defined Input functions needs to be measured experimentally so that

transfer functions can be determined. By observing changes In transfer

functions with systematic changes In the medium, techniques can be

developed that wl I I a I low one to Infer changes In the characteristics of

the medium from changes In the transfer function.

If the difficulties Inherent in obtaining and Interpreting a signal

record from concrete can be overcome, the pulse-echo method wl I I provide a versatile, economical, and safe technique for the detection of flaws within

concrete structures. It Is further anticipated that the development of a standardized pulse-echo technique wl I I lead to research in the area of

acoustic Imaging (holography). Holographic techniques may eventually make

It possible to obtain a three-dimensional view of the Internal structure of

reinforced concrete elements.

D The convol utlon of two functions f(x) and g(x) Is also a function of x,

say h(x), and Is given by the fol lowing formula:

f(u) g(x-u) du = f(x) * g(x)

where * denotes convolution. If g(x) and h(x) are known, the process of

determining f(x) Is called deconvol utlon, and f(x) Is called the Inverse

function. (The reader Is referred to Reference C42H for a complete

explanation of convolution and deconvol utlon.)

17
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*

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*

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18
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19
29. Nand, K., Desal, P., Marwadi, S., Wedpathak, A., and Guha, S., "A

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32. Bredenberg, H., ed.. Proceedings of the International Seminar on the

Application of Stress-Wave Theory on Piles, Stockholm, June, I980, 344

pp.

33. Forrest, J., "In-Sltu Measuring Techniques for Pile Length," TN-1475,

Civil Engineering Laboratory, Naval Construction Batta I Ian Center,

Port Hueneme, California, March, 1 977, 26 pp.

34. Hlggs, J. S., "Integrity Testing of Piles by Shock Method," Concrete,

Oct., 1979, pp. 31-33.

35. Stain, R. T., "Integrity Testing," CI vl I Engineering (London), April,

I 982, pp. 53-59.

36. Paquet, J. et Br lard, M., "Controle non destrucht des pleux en beton,"

Annales de L'lnstltut Technique du Batlment et des travaux Publics,

No. 337, March, I 976, pp. 49-80.

37. Frlstch, D., "Auscultation des pleux et des parols moulees en beton,"

Bui I I ten de Liaison des Laboratories des Ponts et Chaussees, Vol. 78,

Jul.-Aug., I 975, pp. 65-69.

38. Prelss, K., Weber, H., and Calserman, A., "Integrity Testing of Bored

Piles and Diaphragm Wa I Is," The Civil Engineer In South Africa, Vol.

20, No. 8, Aug., 1978, pp. 191-196.

39. Vankuten, H. and Mlddendorp, P., "Testing of Foundation Piles," Heron,

Vol. 26, No. 4, 1981, pp. 3-42.

40. Fitting, D. and Ad I er, L., Ultrasonic Spectral Analysis lox

Nondestructive Evaluation, Plenum Press, New York, 1981, 354 pp.

41. Kraut, E. A., "Review of Theories of Scattering of Elastic Waves by

Cracks," IEEE Transactions on Sonlcs and Ultrasonics, Vol. SU-23, No.

3, May, 1976, pp. 162-166.

42. Bracewell, R. N., The Fourier Transform and Its App I Icatlons. 2nd

ed., McGraw-Hill Book Co., 1978, 444 pp.

43. Carl In, B., Ultrasonics , 2nd ed., McGraw-HM I Book Co., New York,

I960, 309 pp.

20
44. Bel I, F., ed. Foundation Eng I neer 1 ng In Difficult 6/rpun.d, Butterworth

and Co., Ltd., London, 1978, 598 pp.

45. Bowles, J., Foundation Analysis and Design,. 2nd ed., McGraw-Hill Book

Co., New York, 1977, 750 pp.

21
TABLE 1. Parameters of Acoustic Waves

Quantity

Mass Density of Material

Acoustic Wave Velocity

Wavelength

Frequency

Angular Frequency (2irf)

Amplitude of Particle Motion

Particle Velocity

Acoustic Pressure

Acoustic Intensity

Symbol Units

p kg/m3

c m/s

X m

f 1/s or Hz

CO rad/s

A m

V m/s

P N/m2

1 W/m2

22
TABLE 2. Specific Acoustic Impedances

Material Density

P

(kg/m3

)

Compression Wave

Velocity

C

(m/§)

Spec ific Acoustic

Impedance

(kg/(mz-s))

Air 1.205 343 0.413

Concrete 2300 3000 to 4500 6.9 to 10.4 x 106

Granite 2750 5500 to 6100 15.1 to 16.8 x 106

Limestone 2690 2800 to 7000 7.5 to 18.8 x 106

Marble 2650 3700 to 6900 9.8 to 18.3 x 106

Quartz I te 2620 5600 to 6100 14.7 to 16.0 x 106

Soils 1400 to 2150 200 to 2000 0.28 to 4.3 x 106

Structural

steel 7850 5940 46.6 x 106

Water 1000 1480 1.48 x 106

Reference

43

44

44

44

44

45

2

2

°The mass density of concrete depends on the mix proportions and the

specific gravities of the mix Ingredients. The given density is for an average, norma I weight concrete.

23
OSCILLOSCOPE

PULSE

GENERATOR

TIMING MEASURING

CIRCUIT

RECEIVERAMPLIFIER

TIME DISPLAY

TRIGGER

PULSE SOURCE WITH

TRIGGERING DEVICE

RECEIVER

a

FLAW

mm

a) Through-Transmission Method b) Pulse-Echo Method

1. Schematic I I lustration of two acoustic nondestructive test methods:

a) the through-transmission method; and b) the pulse-echo method.

24
INCIDENT

WAVE

REFLECTED

WAVE

INCIDENT

COMPRESSION

WAVE

REFRACTED

WAVE

REFLECTED

SHEAR WAVE

REFLECTED

COMPRESSION

WAVE

REFRACTED

COMPRESSION

WAVE

REFRACTED

SHEAR WAVE

sin e sin P sin 6 sin P sin s sin P<

Ci C2

a) Reflection and Refraction

'p1 ^p2 ^s1

b) Mode Conversion

s2

2. a) Spherlca I wave fronts due to impact on the top surface of a p I ate; and

b) wave fronts after reflection from the bottom surface.

25
(a) R J LO R

«•'.«' \ "' 1

:<* - «••.'

«• ».

•" V \eA-\9

y

s p

•«••.«

-v- • «'«';•

4 ; «

• . .

•

#• « • 4 •'•.'«7

' * *.' *",' *'. *:*.

A B

3. The behavior of a compression wave incident upon an interface between two

dissimilar media; a) reflection and refraction; and b) mode conversion.

26
1 1 1 1 I T

Reflected P-wave

pd.o),no 562)

30°

4. Reflection coefficients at a concrete/a tr Interface for an Incident P wave

as a function of the Incidence angle (Potsson's ratio = 0.2).

27
5. Reflection coefficients at a concrete/air interface for an incident S wave

as a function of the incidence angle (Poisson's ratio = 0.2).

28
1000

500

200

100 -

N

z

5 50

O

z

HID

o

HI

a,

Li.

20 -

10 -

5

-

2 -

10.01

I III I I 1

1

1 I I I I I I 1

1

1 I I 1 MIIL

-

N. \^ \\ /—Cp = 5000 m/s -

>v \ \/\ / /—Cp = 4000 m/s -

— ^v

^ N^^Ov / /~ Cp = 3000 m/s —

—

\ \VsV x-Cp = 2000 m/s

—

I III ' ' i ' I i i 1 \i \j n\ l i i i 0.02 0.05 0.1 0.2 0.5 1 WAVELENGTH, A (meters)

10

6. Relationship between frequency and wavelength for the range of compression

wave velocities typically measured In concrete.

29
7. Diffraction effects due to a sharp edge: a) incident ray OA reaches the

edge; and b) spherical wave front (Pd) emitted from the edge.

30
-I 1 1

a =0.3 dB/m

J I L 0.4 0.8 1.2 1.6 2.0 2.4 2.8

PATH LENGTH (m)

8. Attenuation of plane waves in concrete for frequencies in the range of 100

to 200 kHz.

31
9. Directional characteristics of transducers with various ratios of diameter

to wavelength; a) d/X = 4; b) d/x = 1; and c) d << X ; (Polsson's ratio = 0.2).

32
0.02 -

w

O

>

-0.03 -

a) Oscillogram

TIME (us)

f- h 0.125 m

, . . .9 "rsp

500

0.25 m

b) Ray paths

10. Pulse-echo results due to impact of a steel bal I on the top surface of a 0.25-m thick concrete slab; a) oscillogram; and b) ray paths.

33
0.0

V)

o

>

-2.0 -

500

TIME (us)

0.25m

0.5ml

—H h-0.1 m

_SJ -

void

|— 0.5mH

11. Oscillogram obtained during a test on a 0.5-m thick concrete slab

containing a disc-shaped void 0.25 m below the surface.

34
7.81 kHz

HIQ

Z

O

<

10 20

FREQUENCY (kHz)

12. Frequency spectrum from the transformation of the time domain sign,

shown In Figure 10a.

35
NBS.114A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS TN 1199

2. Performing Organ. Report No 3. Publication Date July 1984

4. TITLE AND SUBTITLE

Pulse-echo Method for Flaw Detection in Concrete

5. AUTHOR(S)

Nicholas J. Carino and Mary Sansalone

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Same as item 6

10. SUPPLEMENTARY NOTES

2 Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The basic pr

of internal

ous nature o echo evaluat

nondestructi

shows that p

detect flaws

currently ex Based on the

are outlined

inciples of the pulse-echo method for the detection

flaws in concrete are presented. As the heterogenef concrete poses problems not encountered in pulseion of metals, progress in this area of concrete

ve testing has been slow. A review of past research

ulse-echo techniques have been used successfully to

within concrete; however, no standardized method

ists for pulse-echo evaluation of concrete structures

current state of knowledge, areas of needed research

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

acoustics; concrete; integrity testing; nondestructive testing;

pulse-echo method; wave propagation.

13. AVAILABILITY

[A] Unlimited

| | For Official Distribution. Do Not Release to NTIS

[~y~] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. ^ 20402.

12 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

39

15. Price

USCOMM-DC 6043-P80
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