Development of a New Experimental Approval for

Bridge Deck Evaluation

By

Dr. Houssam A. Toutanji Department of Civil and Environmental Engineering

The University of Alabama in Huntsville Huntsville, Alabama

Prepared by

UTCA University Transportation Center for Alabama

The University of Alabama, The University of Alabama in Birmingham and The University of Alabama at Huntsville

UTCA Report Number 01327

April 2003

ii

Technical Report Documentation Page

1. Report No FHWA/CA/OR-

2. Government Accession No. 3. Recipient Catalog No.

5. Report Date January 2003

4. Title and Subtitle Development of a New Experimental Approval for Bridge deck Evaluation

6. Performing Organization Code

7. Authors Houssam A. Toutanji

8. Performing Organization Report No. UTCA Report 01327

10. Work Unit No.

9. Performing Organization Name and Address Civil and Environmental Engineering Department University of Alabama in Huntsville Huntsville, AL 35899

11. Contract or Grant No. DTSR0023424

13. Type of Report and Period Covered Final Report: Jan 2001 – Dec 2002

12. Sponsoring Agency Name and Address University Transportation Center for Alabama Box 870205 Tuscaloosa, Alabama 35487-0205

14. Sponsoring Agency Code

15. Supplementary Notes 16. Abstract This research presents the findings of an experimental and analytical study conducted to interpret and characterize the ultrasonic wave signals received from different types of anomalies in concrete bridge decks. The frequency spectra of the recorded waveforms were obtained by using the Fast Fourier Transform technique. Specimens with different artificial voids were tested to develop the relationships between crack depth, crack width, crack type and spectral responses in the concrete bridge decks. The data were collected using an ultrasonic device owned by the National Aeronautics and Space Administration (NASA) and housed at the University of Alabama in Huntsville, Department of Civil and Environmental Engineering. Results show that the instrument can be successfully used to provide information about the internal conditions of concrete and to identify flaws and cracks in concrete materials. 17. Key Words Repair, rehabilitation, nondestructive testing, bridge deck, concrete bridge deck, Fast Fourier Transform, Thin Plate model, Membrane Drum model

18. Distribution Statement

19. Security Classif (of this report) Unclassified

20. Security Classif. (of this page) Unclassified

21. No of Pages 30

22. Price

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Table of Contents Table of Contents..................................................................................................................... iii List of Tables ............................................................................................................................iv List of Figures ...........................................................................................................................iv Executive Summary...................................................................................................................v 1.0 Background ......................................................................................................................... 2 2.0 Ultrasonic Pulse Velocity Technique .................................................................................. 5 3.0 Experimental Procedure ...................................................................................................... 7

3.1 Materials ......................................................................................................................... 7 3.2 Specimen Preparation..................................................................................................... 8 3.3 Instrumentation............................................................................................................... 9

4.0 Theory of Analysis............................................................................................................ 12

4.1 Vibration of Thin Plate ................................................................................................. 12 4.2 Vibration of Membrane Drum ...................................................................................... 13

5.0 Analysis and Results ......................................................................................................... 16

5.1 Horizontal Cracks ......................................................................................................... 16 5.1.1 Time Domain ......................................................................................................... 16 5.1.2 Frequency Spectra Domain.................................................................................... 17

5.2 Vertical Cracks.............................................................................................................. 21 5.3 Crack Size ..................................................................................................................... 23

6.0 Field Tests......................................................................................................................... 24 7.0 Conclusions ................................................................................................................... …27 8.0 Recommendations for Future Research………………………………………………..…28 9.0 References ......................................................................................................................... 29

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List of Tables

Number Page 3-1 Mix design proportions ……..……..……..…..………..……..……..…..………. 7 3-2 Air content values and other properties of the concrete mixes ..……..…………. 8 3-3 Summary of specimen configurations ..…..……..……..…………..……………. 9 5-1 Results of estimated crack sizes …..……..……..…..……..……..………………. 23

List of Figures Number Page 3-1 Details of a test specimen …..……..…..………..……..………………………... 10 3-2 The ultrasonic device used for this project ……….………...……………..……. 11 5-1 Wave forms for concrete slab in time domain for simulated cracks with

different diameters ………………………………………….……...…………….. 17 5-2 Frequency domain spectra using the direct method of concrete specimens

with different artificial cracks (direct method) ………………….……………….. 18 5-3 Amplitude spectrum of concrete slabs with Plexiglas simulated cracks,

placed at different depths (direct method) ..…….…………………………….….. 19 5-4 Amplitude spectrum of concrete slabs with Plexiglas simulated cracks,

placed at different depths (surface method) …….………...…………….……..… 20 5-5 Frequency domain spectra of concrete slabs with no cracks (solid lines) and with

vertical cracks (dashed lines) at different depths, using the indirect method …… 21 5-6 Relationships between peak frequency ratios and normalized crack depth ….…. 22 6-1 NDE tests preformed on Abington Bridge, Huntsville, AL ..…..……………... 25 6-2 NDE tests preformed on Oakwood Bridge, Huntsville, AL ………….………… 25 6-3 NDE tests preformed on Holmes Avenue Bridge, Huntsville, AL …..………… 26

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Executive Summary

Non-destructive evaluation techniques are used to assess the condition of concrete structures, to predict future performance and allow minor repair systems. Some of the best-known non-destructive evaluation techniques use ultrasonic waves. While changes in the frequency spectrum indicate the presence of anomalies in the material they do not predict the severity or the position of the anomalies. This research presents the findings of an experimental and analytical study conducted to interpret and characterize the ultrasonic wave signals received from different kinds of anomalies in concrete bridge decks. The frequency spectra of the recorded waveforms were obtained by using the Fast Fourier Transform technique. Specimens with different artificial voids were tested to develop the relationships between crack depth, crack width, crack type, and spectral responses of concrete bridge decks. The data were collected using an ultrasonic device that was loaned by the National Aeronautics and Space Administration (NASA) and housed at the University of Alabama in Huntsville, Department of Civil and Environmental Engineering. The instrument was originally designed and used by NASA for gas bubble identification. For this research the device was recalibrated to identify voids and cracks in concrete beams and slabs. Concrete samples, whose mixes and dimensions were carefully controlled, were used to validate the instrument for crack/void identification. The sizes of the voids were estimated using the information obtained from the data files in the LABVIEW software. Results show that the instrument can be successfully used to provide information about the internal conditions of concrete bridge decks and to identify flaws and cracks in concrete materials.

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1.0 Background Bridges are among the most expensive components of any transportation system. They are also vulnerable to damage due to excessive loads and environmental attacks. The replacement and repair of deteriorated bridge decks represent a tremendous expense for transportation agencies. According to the US Department of Transportation, over 40 percent of the bridges in the United States are classified as structurally deficient or functionally obsolete, with the estimated cost of repairs running in the billions of dollars (Xanthakos, 1996). The amount of deterioration of concrete structures can be evaluated by means of a visual examination and physical analysis. Some common forms of physical examination are hammer sounding and chain drag surveys. Hammer sounding is used to detect areas of unsound concrete and is frequently used to detect delamination. A chain drag is used to locate voids and delaminated areas by listening for “hollow” sounds. These manual sounding techniques have proven useful in locating cases of delamination, which ranges from mild to severe. However, early deterioration of concrete structures cannot be determined by visual or physical examination processes already mentioned. Moreover, the usefulness of these techniques is impaired by limited inspection speed, highway noise, asphalt overlaid decks, and by subjectivity and skill variability of inspectors. Ground penetrating radar uses the reflections of high frequency electromagnetic waves caused by changes in the electromagnetic properties of the materials being probed (Chen, et al. 1994). The major advantage of radar is the ability to pass through layered media, providing information on bituminous surfacing, concrete deck slabs, the interface between the two, and any discontinuities. Infrared thermography is performed by examining the temperature difference that exists between the solid material and delaminated areas under certain atmospheric conditions. Thermovision cameras are used to detect these temperature differences (Roddis, 1987; Manning and Holt, 1986). A series of photographs is used to map the deck. Electrical methods based on copper and copper sulfate electrode tests are used to measure the ability of concrete to conduct electricity. Lower electrical resistance indicates high chloride contents and establishes an estimate of corrosion activity on the reinforcing bars (Andrade and Gonzalez, 1978). Two other nondestructive techniques available are the ultrasonic pulse echo and ultrasonic pulse velocity (UPV). The impact echo method, a technique based on transient stress waves, has been used successfully to detect flaws in many different types of concrete structures, including plate-like structures such as bridge decks, slabs, walls, beams, columns, layered structures, and hollow cylindrical structures (Kelsden, 1980; Carino and Sansalone, 1986). In the ultrasonic pulse echo technique, a transducer is the sender and the receiver. A pulse of ultrasonic energy is sent through the tested material. The analysis of the frequency domain allows the user to determine the thickness and the depth of a crack. The UPV uses ultrasonic waves. The wave velocity depends on the inertia and elastic properties of the transmission material. It combines an easy test procedure and accuracy, at a relatively low cost (Jones and Gatfield, 1960). This technique can detect areas of internal cracking, internal delamination, and relative strength parameters

3

(Kaplan, 1960; Malhotra and Carino, 1991). The UPV technique is often employed to establish the uniformity and relative qualities of concrete, such as permeability and porosity. The UPV technique has the advantages of being independent of the size or shape of the specimens, and applying equally well to mass concrete pavement or laboratory specimens. The importance of this is evident when determining the elastic properties of concrete in field structures of any size or shape without the necessity of preparing samples. Other advantages of the UPV technique include accuracy, easy test procedure, determination of interior properties of concrete specimens, and nondestructive characteristics at relatively low cost. However, the technique has drawbacks, mostly in the interpretation of the signals received. Nondestructive eva luation using ultrasonic testing methods has been used on concrete for more than a half century. Powers (1938) and Obert (1939) were the first to develop and extensively use the resonance frequency method. Cheesman and Lislie (1949) developed instruments for measuring the ultrasonic pulse velocity. Gericke (1963) proposed a technique called ultrasonic spectroscopy, based on spectral characteristics. Akashi and Amazaki (1987) applied the technique to concrete subjected to alkali-aggregate reaction. Currently, ultrasonic testing is extensively employed to estimate defects in concrete structures. Many attempts have been made to correlate the pulse velocity calculated from transit time with the compressive strength of concrete (Chung and Lauer 1983; Suari and Fernando 1987; Green 1987; Yasunori and Masayasu 1995). It has been found that even if such a relationship could be established, it would be strongly dependent on factors like material type, curing conditions, moisture content, etc. The UPV technique was used to investigate cracks in hardened concrete and to study the damage growth during cyclic loading of concrete (Suari and Fernando 1987). This technique has been used with other non-destructive evaluation (NDE) techniques on bridge decks to detect areas of internal cracking and internal delamination, and to determinate strength parameters (i.e., elastic modulus approximations) (Green, 1987). Yasunori and Masayasu (1995) demonstrated the relationship between crack depth and spectral responses in concrete members based on ultrasonic spectroscopy. Spectral responses were measured by driving sweep-mode input, which was used equivalently to Fourier spectra. Peaks on spectral responses in the low frequency range were associated with the resonance of the members. The evaluation of crack depth was performed by calibration curves relating the crack depth and frequency. These calibration curves were prepared for different concrete mix proportions with the same results. This research was conducted to interpret and characterize the ultrasonic wave signals received from different kinds of anomalies in concrete bridge decks. Controlled laboratory experiments were conducted on hardened concrete slabs containing known artificial internal flaws. Different crack types and sizes were simulated in concrete slabs, such as thin Plexiglas discs, actual air cracks, honeycombed concrete, and fresh and saline water delaminations. The frequency spectra of good concrete were compared to those containing artificial cracks. Crack sizes were estimated using the Thin Plate model or the Membrane Drum model. The effect of crack type, depth, and size on the spectral responses was investigated.

4

This research can be a stepping-stone for new viable applications of the UPV technique. An ultrasonic velocity instrument that was originally designed by the National Aeronautics and Space Administration (NASA) for gas bubble identification was recalibrated for identification of voids and cracks in concrete beams and slabs. Concrete samples, whose mix and dimensions were carefully controlled, were used to validate the instrument for crack/void identification. Results show that the instrument can be successfully used to identify flaws and cracks in concrete materials.

5

2.0 Ultrasonic Pulse Velocity Technique Ultrasonic waves are mechanical waves that are generated by exciting a piezo-electric crystal with a high voltage pulse. The wave is then transmitted through the tested material, which is in contact with the transducer containing the crystal. When this wave impinges upon a receiving transducer it produces an output voltage. Three basic types of stress waves are created in a solid medium when an ultrasonic pulse is sent through a test object: longitudinal, shear, and surface waves (ACI 1989). There are three possible ways in which the transducers may be arranged: direct, semi-direct, and indirect transmission. In the direct method the transducers are placed on opposite faces of the tested element. In the semi-direct method the transducers are placed at a 90o angle, whereas in the indirect method both transducers are placed on one face of the tested element. Since the transmission path length and direction are not well defined in the indirect method, the results are less satisfactory. Thus, the indirect method does not give any information on deeper concrete; it only collects information on the surface. The velocity of these waves depends upon the elastic properties of the propagating media, and is a characteristic of the test material. In general the velocity is constant for a given material for any wavelength and any frequency. The relationship between the velocities for the different waves is that the longitudinal wave travels the fastest, followed by the transverse wave, and finally the surface wave, which is the slowest. When a mechanical wave is transmitted through a material, the elements of the medium are deformed. The disturbance propagates and carries energy through the medium. The medium deformation depends upon its properties of deformability and inertia. Each element of the material reached by the wave has the same oscillation frequency, as in the case of a non-disperse material. When a mechanical wave hits an interface, according to the interface dimension, the wave can be reflected, transmitted or diffracted. The amount of transmitted energy depends upon the relationship between the impedance of the two materials. The impedance is the product of the density of the material and the wave velocity in the material, and it may be thought of as the resistance of the material to the passage of the wave. Bodies in nature have natural frequencies, which define their modes of vibration. A mode of vibration is a way in which a system can vibrate. The frequencies at which it is possible to realize the vibrations depend on the properties of the system. The lowest frequency is associated with the first mode of vibration and is called the fundamental frequency. Experimentally, the natural frequency of a body can be known if a resonance is induced on the body. A resonance on a body is induced when a vibration that interacts with the body has at least a frequency of vibration equal to a natural frequency of vibration of the analyzed body.

6

The pulse transit time between the two transducers is governed by the fastest wave. The velocity of the ultrasonic wave in the concrete depends on the physical properties of the material, such as elasticity and density. The pulse velocity in an elastic solid is given by:

)21)(1(

)1(vv

vEVp −+

−=

ρ (1)

where Vp is the compressional pulse velocity, E is the dynamic modulus of elasticity, ρ is the density, and v is Poisson’s ratio. This equation can be used to determine the velocity of transmission of longitudinal pulses through a solid of any shape or size, provided the smallest lateral dimension perpendicular to the path of travel is not less than the wavelength of the pulse vibrations. The magnitude of the pulse velocity when it goes through a concrete slab is affected by the following factors:

• aggregate size grading and content • water to cement ratio • degree of compacting • curing conditions and age of concrete • acoustical contact • temperature of concrete • moisture condition of concrete

All tests conducted on the concrete slab materials were carried out in accordance with American Society for Testing and Materials (ASTM) standards.

7

3.0 Experimental Procedure 3.1 Materials Twelve concrete slabs were constructed in four batches, with the same specifications. The mix design ratio of the concrete slabs was cement:sand:gravel:water = 1:2:3:0.55 by weight. ASTM Type II Portland cement was used. A sieve analysis was performed on all aggregates. Crushed stone coarse aggregate was used with a maximum size of 3/4 inches. It was sieved through the standard sieves #1.5, #1, #3/4, #1/2, #3/8, #4 and #8. The fine aggregate was made of 50% beach sand and 50% manufactured sand. The fineness modulus of the fine aggregate was 2.68, which met ASTM standards. The mix design properties of the concrete are shown in Table 1.

Table 3-1. Mix design proportions

Materials used per batch Ratios by Mass

Water to cement ratio 0.55

Aggregate to cement ratio 5

Coarse to fine aggregate ratio 1.5

Aggregate type Normal weight

ASTM cement type II

The concrete slab specimens measured 24x24x6 inches and were cast in wood forms. The forms were filled in two layers; each layer was compacted 25 times with the tamping rod. Simulated cracks were placed horizontally in the center of the slabs. The exact coordinates of the simulated crack position were recorded for later reference. All slabs were covered with plastic sheets immediately after casting to minimize moisture loss. Twenty-four hours later, the slabs were stripped and placed in a water bath for 28 days. Special attention was paid to ensure that all specimens were cured for the same duration and were tested at the same temperature and humidity conditions. The average standard 28-day cylinder compressive strength (f’c) of the concrete was 4,233 pounds per square inch. The unit weight and slump of the concrete were calculated according to ASTM standards. The total air content of the freshly mixed concrete was also determined using ASTM C173-78. The results of these measurements are shown in Table 3-2.

8

Table 3-2. Air content values and other properties of the concrete mixes

Air Content ((%)

Batch 1 Batch 2

Beach sand 6 % 7 %

River sand 4 % 5 %

Coarse aggregate 7 % 5 %

Moisture content of concrete 8.3 % 12.5 %

Properties of the fresh concrete

Batch 1 Batch 2

Unit weight 319 lb / ft 3 312 lb/ft3

Slump test 1.5 in. 2 in.

Air content 4.5% 5.1 %

3.2 Specimen Preparation To obtain more consistent results, it is important to adhere to the same mixing procedure for all concrete batches. The procedure used in mixing all specimens is discussed below. All the metal instruments and tools were kept moist (mixing drum, trowels, mixing trays, and rods). No standing water was allowed in the mixing drum or pans. The coarse and fine aggregates were placed in the mixer, and allowed to mix for approximately one minute. Portland cement was placed into the mixing drum, followed by half of the mixing water. After two minutes of mixing, the rest of the water was added and mixing continued for another four to five minutes. When mixing was done, the handle of the concrete mixer was lowered and the contents were poured into a moistened pan. To insure a good contact between the transducers and the test specimen, lithium grease was used as a coupling agent. The objective of the grease was to transmit the pulse. The quantity of coupling used in each measurement was a small spoon of grease. It was important that the transducers were in complete contact with the test medium. Grid lines were drawn on the slab to indicate with more precision the location where the transducers were placed. Each measurement was repeated at least ten times. The specimen configurations and the different internal conditions in the concrete slabs are summarized in Table 3-3. One slab was constructed as plain concrete with no artificial crack. Seven slabs were cast with artificial cracks of Plexiglas. The thin Plexiglas plate was used to simulate an artificial crack or a foreign object inside the concrete slab. To predict the depth of these artificial cracks, the Plexiglas thin plates of the same size were placed at different depths. Air voids and honeycombed concrete were also positioned in two slabs. Fresh and saline-water delaminations were introduced at the center of two slabs. Fresh and saline water were

9

placed in a thin plastic Petri-dish, measuring six inches in diameter and 0.35 inches in thickness.

Table 3-3. Summary of specimen configurations

Specimen No. Specimen Type Dimension of Crack Position

PC Plain concrete _ _

CPI Crack (Plexiglas) 3.5 in. F x 0.35 in. thick 1 in. from top

CPII Crack (Plexiglas) 3.5 in. F x 0.35 in. thick 2 in. from top

CPIII Crack (Plexiglas) 3.5 in. F x 0.35 in. thick 3 in. from top

CPIV Crack (Plexiglas) 3.5 in. F x 0.35 in. thick 4 in. from top

CPV Crack (Plexiglas) 3.5 in. F x 0.35 in. thick 5 in. from top

CPVI Crack (Plexiglas) 3 in. F x 0.35 in thick Center

CPVII Crack (Plexiglas) 3.5 in. F x 0.35 in thick Center

HC Honeycombed concrete 3 in. F x 2.5 in thick Center

AC Air crack 6 in. F x 0.35 in thick Center

PW Plain water-filled crack 6 in. F x 0.35 in thick Center

SW Saline water-filled crack 6 in. F x 0.35 in thick Center

Two concrete slabs were prepared from the same batch of concrete, and were utilized to analyze and interpret the waveforms at different vertical crack depths. Each concrete slab measured 12 in. long, 12 in. wide, and 6 in. thick. Cuts were made vertically, as shown in Figure 3-1. Measurements of the waveform were registered by transducers placed on the surface of the concrete slabs. The depth of the cracks ranged from zero to 3.5 inches. Variables such as the temperature, curing condition, relative humidity, size, grading type, content of aggregate, water to cement ratio, degree of compacting, and the age of the concrete were controlled during the experimental procedure. Strict control of these variables allowed better control of the elastic properties of the concrete. 3.3 Instrumentation The ultrasonic velocity instrument that was used in this research was originally designed by NASA for gas bubble identification. In this project, the instrument was recalibrated to identify voids and cracks in concrete beams and slabs. The instrument essentially operated under the assumption that the bubbles acted as non- linear mixers. Every material resonates at a particular frequency. A resonance frequency consists of a sound wave that excites the atoms within it. The instrument excites the characteristic resonance frequency of gaseous emboli. The resonance is proportiona l to the diameter of the bubble, and non-gaseous bubbles have a

10

different resonance frequency than those of gas. This provides a means of distinguishing between air bubbles and other small objects.

Figure 3-1. Details of a test specimen

This instrument is a non- invasive instrument for detecting voids. The instrument consists of two ultrasonic transducers, amplifiers for the transducers, a computer function generator, and signal conditioning electronics including a mixer and low pass and high pass filters. One transducer is referred to as a pump, transmitting a low frequency signal. It is this signal that is used to excite the bubbles which resonate at a frequency related to their diameter. The user sets the frequencies to sweep, and hence bubble sizes to interrogate, but is limited to the range 20 kilohertz to 180 kilohertz by the existing pump transducer. This instrument works under the principle that all materials have a specific frequency at which they resonate and these frequencies are different for different materials. In this methodology a range of frequencies is applied to the test samples. This range is the range of all the different materials that were artificially placed in the samples. The instrument is set to ignore the noise signal and transmits only the original signal from the block. The gains and all other settings of the instrument are set so no voids are found in the concrete block when it has no voids in it. In this experiment the block without voids is referred to as the control specimen, as it is the controlling block.

11

Figure 3-2. The ultrasonic device available for this project

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4.0 Theory of Analysis The Vibration of Thin Plate and the Vibration of Membrane Drum methods were used in this research to predict crack sizes in the concrete slabs. To make this report self-contained, a brief description of these methods is included in the following sections. 4.1 Vibration of Thin Plate A plate is considered thin when its thickness is much smaller than its diameter. The deformation is assumed to be uniform throughout its thickness and is a function of the radial and axial position of the plate. The Vibration of the Thin Plate model considers a thin plate as a symmetrical element that has two dimensions (depth and diameter). In this research the thickness to diameter ratios used were 0.9:9, 0.9:15, 0.9:17.5. When a thin plate is subject to small deformations, the restoring force is dependent upon the stiffness of the plate. When a wave propagates through a thin plate it generates vibrations in the plate. A longitudinal wave, which has a perpendicular incidence to the greatest surface area of the plate, causes a flexing effect on the plate. The flexing effect of the plate causes a compression force upon the surface of incidence, and at the same time, the underside of the plate suffers an expansion force on its surface. The flexural motion of the thin plate is analyzed on the basis that every straight line perpendicular to the middle surface of the plate remains straight after deformation and perpendicular to the deflected middle surface; this surface is known as neutral axis. This implies that the total longitudinal force on the neutral axis is equal to zero. Nevertheless, a curvature moment and shear force produces the curvature effect on the plate (Landau and Lifshitz, 1986). The relationship between the curvature moment and the shear force is used to find an equation of moment for transversal waves propagating through a thin plate [Landau and Lifshitz, 1986; Lopez, 1983) as follows:

ρ ∂2y2

∂t2+

Ee2o

12(1−ν2)∆2 y = 0 (2)

where: ρ = the mass density of the plate y = the vertical displacement of a point in the plate E = the Young’s modulus of the plate eo = the thickness of the plate v = the Poisson’s ratio of the plate

? = the differential operator

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Four arbitrary constants are needed to complete the solution of this fourth order differential equation. Variable separation method is used to solve equation (2). The solution can be expressed as a product of two functions: one that depends on time and the other depends on position. Using the relationships between the Bessel function and the boundary conditions of the plate, equation (2) can be solved and the natural frequency of a the thin plate can be given by the following equation (Landau and Lifshitz, 1986; Lopez, 1983)

Fon = n

12 2

π eο/a2

E/ρ(1−ν2) (3)

where Fon is the natural frequency of the plate at n where n is the number of mode of vibration, and a is the radius of plate (crack radius size). From equation (3), it is easy to note that a relationship exists between the natural frequency of the plate and its diameter (Lopez, 1983). Thus, the natural frequency of an object can be known experimentally if a resonance is generated in the system. 4.2 Vibration of Membrane Drum When the center of a membrane drum is hit, symmetrical modes of vibration are generated and modified due to the change of the pressure inside the enclosed space of the drum. The vibration modes of a drum membrane are affected by friction forces on the membrane and changes in the pressure of the material contained in the drum. The friction force affects the wave amplitude, which decreases exponentially depending on the damping constant, which increases with frequency. Changes in the pressure in the drum can be analyzed as uniform when the radial velocity of the transverse wave is small compared to the wave velocity of the sound in the air. In this case, changes in the pressure of the drum depend only on the average displacement, y, of the membrane. Assuming that all transformations are adiabatic, the wave equation of an element of a membrane is given as follows (Lopez, 1983; Kinsler et al., 1982; Feather 1963; Arfken and Weber 1996):

∂2y

∂t2+

γPoµsVo

π a2 y = c2

∂2y

∂r2+ 1

r∂ y∂r

(4)

where: y = the transverse displacement of a point of the membrane

14

y = the average of transverse displacement Po = the equilibrium pressure Vo = the equilibrium volume inside the drum µs = the surface density a = the radius of the membrane of the drum c = the transverse velocity of the wave For a mode of vibration, the solution of Equation (4) is given as follows:

( ) ( )( )2

20

kakaJ

kaJ α−= (5)

where α is a constant that measures the restoring force of the membrane and is equal to:

0

04

TVPa γπ

α = (6)

a is the radius of the membrane, Jo and J2 are Bessel functions of order 0 and 2, respectively, k is the wave number, and T is the tensile strength of the membrane. The solution can be found graphically using the Bessel function (Lopez, 1983) and thus, for a special value of α, different modes of vibration (ka) can be found. The fundamental resonance frequency of a membrane is:

( )0101 2ka

ac

Fπ

= (7)

Solving for Equation (7), the radius a (which can be modeled as the radius of a crack), can be determined.

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5.0 Analysis and Results 5.1 Horizontal Cracks 5.1.1 Time Domain When an ultrasonic pulse passes through a concrete slab, the received signal corresponds to a superposition of the different vibrations generated and induced in the concrete slab. The device used in this research sends a longitudinal pulse with a certain bandwidth, and when this pulse travels through a material and hits a discontinuity (a crack), part of the energy is reflected or scattered and the remainder continues to travel through or around the crack. The superposition of these waves is depicted in the time-domain responses. The amplitude of the waveforms change due to many factors such as pressure on the transducers, coupling, absorption, and interference. To eliminate these factors and compare the different graphs, the signal's amplitude was normalized. The normalization was done with reference to the maximum amplitude value in the graph. In time-domain, it was difficult to characterize the waveforms of different slabs. With the inclusion of artificial cracks in the concrete slabs, the waveforms seemed to spread and their shapes became less uniform as compared to that of plain concrete, as may be seen in Figure 5-1. This is due to the fact that the device sends a longitudinal pulse with a certain bandwidth and when this pulse travels through a material and hits a discontinuity (a crack), part of the energy (wave) is reflected, another is scattered, and the remainder continues to travel through or around the crack. The superposition of these waves is depicted in the time-domain responses (Figure 5-1). Although results did not show correlation between the wave responses and the types of cracks in time-domain, it was clear that specimens with artificial cracks exhibited different shapes of waveforms than those with no cracks.

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Figure 5-1. Waveforms for concrete slab in time domain for simulated cracks with different diameters

5.1.2 Frequency Spectra Domain Data was transformed from time-domain to frequency spectra domain using the Fast Fourier Transform technique in which it was possible to relate the maximum peak frequencies of the waveforms to the types of cracks. Figure 5-2 shows the spectral frequency responses of the same specimens analyzed in Figure 5-1, for a plain concrete specimen compared to specimens with various types of artificial cracks. The shapes of the Fourier frequency spectrum of the waveform were different in each specimen, and each specimen exhibited a different maximum frequency peak. The specimen with Plexiglas exhibited a maximum peak of around 16 kHz, whereas the plain concrete specimen showed a maximum peak of around 8 kHz. The specimen with fresh water- filled crack exhibited a maximum frequency

18

of around 7.5 kHz whereas the saline water- filled crack specimen exhibited a maximum peak of 10 kHz. It should be noted that each point measurement was repeated ten times, resetting both transducers. The final response was the average of the ten measurements. All graphs

were normalized with respect with the transducer resonance frequency of 54 kHz. Figure 5-2. Frequency domain spectra of concrete specimens with different artificial cracks (direct

method)

Air Crack (AC)

0 5 10 15 20 25 30

fp

Honeycombed Concrete (HC)

0 5 10 15 20 25 30

fp

Plexiglass Crack (PCIII)

0 5 10 15 20 25 30

Frequency (kHz)

fp

Saline Water-Filled Crack (SW)

0 5 10 15 20 25 30

fp

Plain Water-Filled Crack (PW)

0 5 10 15 20 25 30

fp

Plain Concrete (PC)

0 5 10 15 20 25 30

fp

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Figure 5-3 shows the amplitude spectrum of simulated cracks with plexiglass discs placed at different depths using the direct method. These discs have the same of 6- inch diameter and 0.35 inch thickness. The cracks were located at different depths, one, two, three, four, and five inches from the top surface of the concrete slab. Results show that using the direct method, concrete slabs with the same crack type and size located at different depth exhibited similar maximum peak frequencies. The maximum peak frequency was around 15 kHz, regardless of the depth of the crack (Figure 5-3).

Crack at depth of 1.0 in

0

1

0 5 10 15 20 25 30 35

fp

Crack at depth of 2 in

0

1

0 5 10 15 20 25 30

fp

Crack at depth of 3 in

0

1

0 5 10 15 20 25 30

fp

Crack at depth of 4 in

0

1

0 5 10 15 20 25 30

fp

Crack at depth of 5 in

0

1

0 5 10 15 20 25 30

Frequency (kHz)

fp

Fig. 7–Frequ ency domain spectra of conc rete slabs with plexiglass simulated c rack s,measured 6 in. in diameter and 0.35 in. in thickne ss, placed at differen t depth (directmethod).

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Using the surface method (indirect method), there was no apparent relationship between the peak frequency and the crack depth. The multiple peaks show that the device identifies discontinuity. However, this characteristic is not enough to quantitatively relate peak frequency amplitude to crack depth. On the other hand, when the cracks were deeper inside the concrete (more than half the depth of the slab) the amplitudes were much lower (see Figure 5-4). This was expected since in the surface method the ultrasonic pulse propagates the concrete in the layer near the surface and thus does not give any information on deeper concrete; it only collects information on the surface.

Figure 5-4. Amplitude spectrum of concrete slabs with Plexiglas simulated cracks, placed at different depths (indirect method)

0 5 10 15 20 25 30

Crack at 0 depth (no crack)

0 5 10 15 20 25 30

Crack at 1 in depth

0 5 10 15 20 25 30

Crack at 2 in depth

0 5 10 15 20 25 30

Crack at 3 in depth

0 5 10 15 20 25 30

Crack at 4 in depth

0 5 10 15 20 25 30

Frequency (kHz)

Crack at 5 in depth

Figure 5-3. Amplitude spectrum of concrete slabs with Plexiglas simulated cracks, placed at different depths (direct method)

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5.2 Vertical Cracks The variation in the spectral responses of the concrete slabs with no cracks (solid lines) and with vertical cracks at different depths (dashed lines) is shown in Figure 5-5. The maximum peaks were analyzed and showed that with increasing the crack depth, the peak frequency shifted to lower numerical values.

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Figure 5-5. Frequency domain spectra of concrete slabs with no cracks (solid lines) and with vertical cracks (dashed lines) at different depth, using the indirect method

The relationship between the h/H (h being the depth of the crack and H being the height of the specimen) and peak frequency ratios fh/fH (fh being the peak frequency of depth h and fH being the peak frequency of plain concrete) are shown in Figure 5-6. The peak frequency ratio decreases with increasing depth of the crack. Therefore, peak frequency responses are

0

0.5

1

0 5 10 15 20 25 30

no crackto 0.75 in

0

0.5

1

0 5 10 15 20 25 30

no crackto 1.25 in

0

0.5

1

0 5 10 15 20 25 30

no crackto 1.75 in

0

0.5

1

0 5 10 15 20 25 30

no crackto 2.5 in

0

0.5

1

0 5 10 15 20 25 30

no crackto 3.5 in

Frequency (kHz)

23

associated with crack vertical depth, which can be estimated from curves that relate the peak frequency ratios to normalized crack depth (h/H).

Figure 5-6. Relationships between peak frequency ratios and normalized crack depth

5.3 Crack Size The simulated cracks in this investigation were of cylindrical shape, with thicknesses that were much smaller than their diameters. The sizes of the cracks were estimated by modeling a crack either as a thin plate or as a thin drum. The model of vibration of a Thin Plate was used in the case of solid cracks, such as specimens with Plexiglas. The model of vibration of a Membrane Drum was used for nonsolid cracks such as specimens with air cracks, fresh water- filled cracks and saline water- filled cracks. The two main assumptions made in the case of solid cracks were: (1) all cracks were assumed to be thin plates with thicknesses much smaller than diameters, and (2) peaks in frequency spectra were assumed

0 . 75

0 . 8

0 . 85

0 . 9

0 . 95

1

0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7

Ve rti ca l C r ack D ep t h ( h / H)

Fit t i ng li ne

Peak

Fre

quen

cy (f

h/f H

)

24

to correspond to resonance frequencies of the plates. In the case of nonsolid cracks, the crack was assumed to be a thin drum. These two theories were briefly described earlier and are described in more detail elsewhere (Landau and Lifshitz, 1986; Lopez, 1983; Lopez, 1983; Kinsler et al., 1982; Feather, 1963; Arfken and Weber, 1996). The analytical values calculated using the Vibration of Thin Plate and the Drum models were compared to experimental values. A summary of the results is presented in Table 5-1. Results show that the predicted radius size of different cracks compared well with the actual radius sizes. The difference between the predicted values and those of actual ranged between two and 30%.

Table 5-1. Results of estimated crack sizes

Specimen No.

Peak Frequency (kHz)

Estimated Radius (in)

Actual Radius (in)

Difference (%)

CPI 15.55 1.86 1.77 5.1

CPII 16.30 1.76 1.77 6.7

CPIII 15.48 1.80 1.77 2.0

CPVI 6.49 1.39 1.48 5.6

CPVII 8.74 1.2 1.72 30.2

AC 7.99 2.42 3.0 18.0

PW 7.44 3.11 3.0 5.3

SW 10.48 2.40 3.0 18.7

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6.0 Field Tests To verify the laboratory and theoretical data, three bridges in Huntsville, Alabama were tested. They were the Abington Bridge, Holmes Avenue Bridge, and Oakwood Bridge. According to city engineers, Abington is the oldest of the bridges with the lightest traffic. We did not have to stop traffic on Abington Bridge since the traffic was light and the test was conducted very early in the morning. Holmes Avenue Bridge and Oakwood Bridge are between 10 and 15 years old. Holmes Avenue Bridge also had light traffic and was tested early in the morning; thus there was no need to close traffic. However, Oakwood Bridge was partially closed to traffic for a couple of hours until the tests were completed. Over 30 data points were collected from each bridge. Each bridge was tested to identify maximum flaw sizes and flaw/anomaly types in the bridge. The results were compared to the frequencies of sound concrete blocks without any flaws. The three bridges produced different results. The frequencies obtained from Oakwood Bridge compared well with those of sound concrete (without any flaws or cracks). However, Abington Bridge compared well with concrete blocks with anomalies such as corroded steel and large air voids. Holmes Avenue Bridge compared well with concrete blocks with artificial flaws such as cardboard and Plexiglas. To verify these findings, cored samples must be taken from each bridge. The Principle Investigator of this project is working with the City of Huntsville to obtain core samples from each bridge. Tentatively, the City has agreed to take core samples from Abington Bridge. The results of the tests of the cored samples will be reported in a supplementary document supporting this report.

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Figure 6-1. NDE tests preformed on Abington Bridge , Huntsville, Alabama

Figure 6-2. NDE tests preformed on Oakwood Bridge , Huntsville, Alabama

26

Figure 6-3. NDE tests preformed on Holmes Avenue Bridge, Huntsville, Alabama

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7.0 Conclusions This report presents preliminary findings of an experimental and analytical study conducted to interpret and characterize the ultrasonic wave signals received from different kinds of anomalies in concrete bridge decks. Different crack types and sizes were simulated in concrete slabs, such as thin Plexiglas discs, actual air cracks, honeycombed concrete, and fresh and saline water delamination. The spectral responses received from concrete slabs with and without artificial cracks were compared. Crack sizes were estimated using Thin Plate and Membrane Drum vibration models. The following conclusions were drawn from this study:

1. Time-domain waveform signals received using ultrasonic wave velocity techniques do not provide clear information on the internal conditions of concrete specimens, whereas frequency spectra waveforms may provide more information on internal conditions.

2. Using the surface method (indirect method), the amplitude or the intensity of the waveforms was higher in specimens with cracks close to the surface. When the cracks were located deeper in the concrete, more than half the depth of the slab, the amplitude or intensity was much lower (Figure 5-4).

3. Using the direct method, concrete slabs with the same crack type and size but located at different depths showed similar maximum peak frequencies.

4. For vertical cracks, the ratios of peak frequencies responses can be correlated with crack vertical depth. Crack depth can be estimated from curves that relate the peak frequency ratios to the normalized crack depth (Figure 5-6).

The crack size in concrete slabs can be estimated using the Thin Plate or the Membrane Drum model. Results show that the predicted radius sizes compared well with the actual radius sizes.

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8.0 Recommendations for Future Research The laboratory results were verified theoretically, however, there is a need to conduct a more comprehensive field test to verify the project’s reliability and confidence. To verify the field test documented in section 6 of this report, core samples should be taken from each of the tested bridges, and the cores should be tested. Tentatively, the City of Huntsville has agreed to core samples from the Abington Bridge, which should provide verification of the laboratory and theatrical data for that structure. To gain the most benefit from this study, the following recommendations are made:

1. Take core samples from the Holmes Avenue and Oakwood bridges to verify the laboratory and theatrical results. At least five samples should be cored at different locations on each bridge deck.

2. Collect data from other bridges in Alabama and take core samples for verification of

results.

3. NASA is investigating the possibility making the nondestructive device portable by reducing its size, which would make field tests much easier. It would be worthwhile to keep in touch with NASA about this opportunity.

4. More laboratory tests should be done to study the effect of different concrete mixes.

In addition, it is important to increase the number of tested samples to produce statistically reliable data.

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9.0 References

Xanthakos, P., “Bridge Strengthening and Rehabilitation,” Prentice Hall, 1996. Chen, H. L., Halabe. U. B., Sami, Z., and Bhandarakar, V., "Impulse Radar ReflectionWave

forms of Simulated Reinforced Concrete Bridge Decks", Journal of Materials Evaluation, December 1994, pp. 1382-1388.

Roddis, W. M. K., " Concrete Bridge Decks Assessment Using Thermography and Radar", Master Thesis, Department of Civil Engineering, MIT, Cambridge, MA, Jan. 1987.

Manning, D. G., and Holt, F. B., " Deck Assessment by Radar and Thermography", Transportation Research in Concrete, No. 1083, 1986, pp. 13-20.

Andrade, C., and Gonzalez, J. A., "Quantitative Measurements of Corrosion Rate of Reinforced Steels Embedded in Concrete, Using Polarization Resistance Measurements", Corrosion, 29, 1978, pp.515.

Kelsden J. A, " Measuring Steel - in-Concrete Corrosion with an Electrical Resistance Probe", Paper 123, Corrosion/73, NACE, Houston TX, 1980.

Carino, N. J., and Sansalone, M., "A Point Source-Point Receiver, Pulse Echo Technique for Flaw Detection in Concrete", ACI Journal, Vol. 83, No. 2, Mar-Apr 1986, pp. 199-208.

Jones, R., and Gatfield, E. N., " Testing Concrete by Ultrasonic Pulse Velocity Technique", D. S. I. R. ROAD Technical Paper No. 34, March 1960, pp. 3-8.

Kaplan, M. F., " The Relationship Between Ultrasonic Pulse Velocity and Compressive Strength of Concrete Having the Same Workability but Different Mix Proportions", Magazine of Concrete Research, Vol. 12, No. 34, March 1960, pp. 3-8.

Malhotra. V. M., and Carino N. J., "Handbook on Nondestructive Testing of Concrete", CRC Press, 1991, pp. 169-187.

Powers, T. C., " Measuring Young's Modulus of Elasticity by Means of Sonic Vibrations", Proceedings, ASTM, V. 38, 1938, pp. 460.

Obert, L., "Sonic Method of Determining the Modulus of Elasticity of Building Materials Under Pressure", Proceedings, ASTM, Vol. 29, 1939, pp. 987.

Cheesman, W. J., and Lislie, J. R., "Ultrasonic Method of Studying Deterioration and Cracking in Concrete Structures", Proceedings ACI Journal, Vol. 46, No. l, Jan. 1949, pp. 17.

Gericke, O. R., "Determination of the Geometry of Hidden Defects by Ultrasonic Pulse Analysis Testing", Journal of the Acoustics Society of America, Vol. 35. No. 3, 1963, pp. 364-368.

Akashi, T., and Amazaki, S. "Evaluation of Concrete by Ultrasonic Spectroscopy", Cement and Concrete, Vol. ll, No. 489, 1987, pp.12-30.

Chung, H. W., and Lauer, K. S., "Diagnosing in Situ Concrete by Ultrasonic Pulse Technique", Concrete International , Vol. 5, No. 10, Oct. 1983, pp. 42-49.

Suari, W., and Fernando V., "Ultrasonic Pulse Attenuation as a Measure of Damage Growth During Cyclic Loading of Concrete", ACI Material Journal, Vol. 84, May-June 1987, pp. 185-193.

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Green, R. E. Jr. "Ultrasonic Nondestructive Materials Characterization", Materials Analysis by Ultrasonic, edited by A. Vary, Noyes Corp., Park Ridge NJ, 1987, pp. 1-12.

Sakata Yasunori, and Ohtsu Masavasu. " Crack Evaluation in Concrete Members Based on Ultrasonic Spectroscopy", ACI Material Journal , Vol. 92, No. 6, Nov-Dec 1995, pp. 686-698.

Landau, L. D and Lifshitz, E. M, " Teoria de la Elasticidad ", Academia de Ciencias, U.R.S.S. Vol. 7. 1969, pp. 69.

ACI Committee 228, “In-Place Methods for Determination of Strength of Concrete,” Manual of Concrete Practice, ACI 228.1R, American Concrete Institute, Detroit, Michigan, 1989.

Landau, L. D. and Lifshitz, E. M., Theory of Elasticity, Academy of Science, Moscow, 1986, Vol. 7, 69p.

Lopez, R. M., Acoustic Appendices, Publication Department at the University School of Technical Engineering (in Spanish), Madrid, Spain, 1983, 814p.

Lopez, R. M., Acoustic, Publication Department at the University School of Technical Engineering (in Spanish), Madrid, Spain, 1983, pp. 403-407.

Kinsler, L. E., Frey, A. R., Coppen, A. B., and Sanders, J.V., Fundamentals of Acoustics, Third Edition, John Wiley & Sons, 1982, 89p.

Feather, N., An Introduction to the Physics of Vibration and Waves, Edinburgh University Press, Chicago, 1963, 304p.

Arfken, G. B., and Weber, H. J., Mathematical Methods for Physicists, Fourth Edition, 1996, pp. 627-635.