New Directions in Concrete Health Monitoring Technology

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NEW DIRECTIONS IN CONCRETE HEALTH MONITORING TECHNOLOGY

By Surendra P. Shah,1 Member, ASCE, John S. Popovics,2 Associate Member, ASCE,Kolluru V. Subramaniam,3 Student Member, ASCE, and Corina-Maria Aldea4

ABSTRACT: The NSF-sponsored Center for Advanced Cement-Based Materials is actively involved in researchaimed at the development of technologies for health monitoring and nondestructive evaluation of the concreteinfrastructure. This paper summarizes pertinent research performed at the center. Basic findings from severalnew laboratory-based nondestructive evaluation techniques for concrete are reported. The described techniquesare based on measurements of mechanical waves that propagate in the concrete. First, ultrasonic longitudinalwave (also called the L-wave or P-wave) signal transmission (attenuation) measurements are shown to besensitive to the presence of damage in the form of distributed cracking in concrete. Next, experimental proceduresthat enable practical one-sided wave signal transmission measurements to be performed on concrete structuresare described. The utility of the signal transmission measurement is demonstrated by two experimental testseries; the depths of surface-opening cracks in concrete slabs are estimated and the extent and nature of autog-enous healing in concrete disks are studied. Finally, an approach by which fatigue-induced damage in concretestructures is nondestructively monitored is described. Vibration frequencies are shown to be sensitive to thepresence of fatigue-induced cracking in concrete specimens; changes in the vibration frequency of a concretespecimen during fatigue tests are related to the remaining fatigue life of the test specimens.

INTRODUCTION

Damage in concrete may take the form of distributed mi-crocracks, such as that caused by freeze-thaw action, or dis-tinct large cracks that extend significant distances within thestructure. Techniques that can detect, localize, and characterizethis damage in a nondestructive fashion, that is, using non-destructive evaluation (NDE) techniques, are of great interestto practicing structural engineers (Rens et al. 1998). The useof mechanical wave-based test methods, such as ultrasonic andsonic approaches, enables inspection well below the surfaceof the concrete structure and offers direct information con-cerning the effective elastic constants and/or presence of flawsor damaged areas. (Note that mechanical waves are also re-ferred to as stress waves.) For example, measurements of thelongitudinal wave (L-wave) velocity have been used to pro-vide information on concrete quality, uniformity, location ofcracked or damaged areas, and strength gain (ACI 1998). Morerecently, the impact-echo method (Sansalone 1997) has beenapplied to locate large voids and delaminations in concreteslabs and the spectral analysis of surface waves (SASW) tech-nique (Kalinski 1997) to identify cracked areas in concretebeams. However, many important problems in concrete struc-tures remain undetectable with these existing wave techniques.For example, existing NDE techniques cannot detect the earlystages of deterioration in concrete owing to freeze-thaw cy-cling, sulfate attack, or steel reinforcing corrosion. The Centerfor Advanced Cement-Based Materials at Northwestern Uni-versity is actively involved in research for the development oftechnologies for health monitoring and nondestructive assess-ment of the concrete infrastructure that aims to improve thestate of the art. Laboratory results on new mechanical wave-

1Dir., NSF Ctr. for Advanced Cement-Based Mat., Northwestern Univ.,2145 Sheridan Rd., Evanston, IL 60208-4400.

2Asst. Prof., Drexel Univ., Dept. of Civ. and Arch. Engrg., Philadel-phia, PA 19104.

3Res. Assoc., NSF Ctr. for Advanced Cement-Based Mat., Northwest-ern Univ., 2145 Sheridan Rd., Evanston, IL.

4Consultant, BAYEX, 39 Seapark Dr., St. Catherines, ON, CanadaL2M 6S5.

Note. Special Editor: Roger Ghanem. Discussion open until December1, 2000. To extend the closing date one month, a written request mustbe filed with the ASCE Manager of Journals. The manuscript for thispaper was submitted for review and possible publication on February 25,2000. This paper is part of the Journal of Engineering Mechanics, Vol.126, No. 7, July, 2000. qASCE, ISSN 0733-9399/00/0007-0754–0760/$8.00 1 $.50 per page. Paper No. 22241.

4 / JOURNAL OF ENGINEERING MECHANICS / JULY 2000

J. Eng. Mech. 20

FIG. 1. Experimental Setup for Ultrasonic Through-ThicknessMeasurements

based NDE techniques for concrete are reported here. Futureeffort will be directed toward practical application of the newtechniques to concrete structures.

ULTRASONIC WAVE VELOCITY AND ATTENUATION

An experimental program was carried out to evaluate theeffectiveness of traditional through-thickness ultrasonic in-spection techniques to monitor the presence of distributed mi-croscopic damage in concrete. The goal of the research wasto evaluate whether these laboratory-based NDE techniquescan detect damage in the early stages of development, beforethe damage coalesces to larger cracks. The results reported inthis section are obtained using a conventional ultrasonic im-mersion tank, which is not suitable for testing in-place con-crete structures. However, promising results would encouragethe development of an NDE test that may be applied directlyto the concrete structure using portable immersion scan equip-ment.

Experimental Setup

The ultrasonic tests were applied to a series of prismaticconcrete specimens. To perform an ultrasonic test, the testspecimen was immersed in a water bath, in between a pair of500 kHz, 2.5 cm diameter ultrasonic transducers. The ultra-sonic test configuration is shown in Fig. 1. A pulser unit ex-cites the sending ultrasonic transducer, which emits a pulse of

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FIG. 2. Effect of Reduction of Young’s Modulus of Concrete onThrough-Thickness Velocity (Circles) and Signal Transmission(Squares); Young’s Modulus Reduced by Applied Freeze-ThawCycles

ultrasound that propagates through a short water path (3 mm)before impinging normally on the surface of the concrete. Aportion of the incident ultrasonic energy is transmitted into theconcrete. This pulse then propagates through the thickness ofthe specimen until it impinges on the opposing boundary. Atthe opposing boundary, part of the propagating wave energyis transmitted in the water and is subsequently detected by thereceiving transducer. The received signal is captured, digitized,and stored for later analysis. Knowing the total distance thatthe wave pulse travels, through both the water and the con-crete, and measuring the time delay between sent and receivedpulses, the through-thickness L-wave velocity may be calcu-lated. The difference in the peak-to-peak amplitudes of the sentand received pulse signals gives an indication of the signaltransmission, or conversely the signal attenuation, through thethickness of the concrete. Note that amplitude comparisonscannot provide an absolute measure of signal transmission;however, relative changes in signal transmission may be read-ily obtained. The results of other ultrasonic analyses performedon the specimens, such as frequency-dependent attenuation,are reported elsewhere (Sellick et al. 1998). The ultrasonictests were performed on concrete with varying amounts ofuniformly distributed damage. The damage was imparted tothe concrete by freeze-thaw cycling of concrete specimens.The extent of imparted cracking damage was monitored withdynamic Young’s modulus tests and verified by microscopic(optical) inspection of selected specimens.

Experimental Results

The experimental results are summarized in Fig. 2. Ultra-sonic pulse transmission measurements are more sensitive tothe presence of microscopic damage in concrete than are ul-trasonic pulse velocity measurements. For example, a 25% re-duction in the dynamic Young’s modulus owing to the im-parted freeze-thaw damage results in only a 3% reduction inultrasonic L-wave pulse velocity, but a 31% reduction in signaltransmission (peak-to-peak amplitude reduction). A 50% re-duction in the dynamic Young’s modulus results in a 9% re-duction in ultrasonic L-wave pulse velocity, but an 84% re-duction in signal transmission (Fig. 2). Although the signaltransmission measurements are much more sensitive, they ex-hibit much more variability than the velocity measurements.The results suggest that signal transmission measurements maybe an excellent approach to detect damage in concrete if theinherent application and signal variability problems are over-come.

ONE-SIDED WAVE MEASUREMENTS

The utility of most existing concrete NDE procedures isbased on the fact that opposite and parallel sides of the struc-ture are accessible for through-thickness measurements. How-

J. Eng. Mech. 200

FIG. 3. Experimental Setup for Wave Transmission Measure-ment

ever, some concrete structures, such as pavements, do not al-low access to opposite surfaces. Efforts to modify many ofthe through-thickness techniques for application to only onesurface have been shown to be unsatisfactory. One-sided andaccurate NDE techniques for concrete structures are there-fore desired. Researchers at Northwestern University havedeveloped several one-sided NDE techniques for concretestructures. As illustrated in the previous section, ultrasonictransmission (the inverse of signal energy attenuation) mea-surements are sensitive to the presence of damage in concrete.However, the practical measurements of signal transmissionon concrete structures, especially those that offer access toonly one side, such as pavements, has been restricted up tothis time because of the disrupting effects of source, receiver,and coupling variability. Here, a one-sided, self-calibratingsurface wave transmission technique is introduced and shownto be practical for concrete and sensitive to the presence ofcracking in concrete slabs. This technique eliminates some ofthe problems that were encountered with the through-thicknessultrasonic transmission measurements reported in the previoussection: (1) the technique can be applied to one surface of anunprepared concrete surface; (2) absolute measurements ofsignal transmission are obtained; and (3) the variability of thesignal transmission values is reduced. The self-calibrating sig-nal transmission technique has been demonstrated to be emi-nently applicable to pavement structures (Popovics et al.1998a).

Experimental Setup

The hardware used to measure one-sided wave signal trans-mission consists of a controlled impact-based stress wavesource, two receiving accelerometers, a digital oscilloscope, anda personal computer. The two receivers are located on the sur-face of the test specimen along a line with the source, awayfrom the impact site. Two stress wave sources and two receiversare placed on the surface of the specimen along a line thatstraddles the crack, as shown in Fig. 3. Transient stress wavesgenerated by the source propagate along the surface of the spec-imen, passing first receiver 1(B) and then receiver 2(C). Thestress wave source is a DC-powered solenoid with a spring-loaded steel shaft; the shaft striker moves perpendicularly tothe surface of the specimen. The nominal maximum axial forceof this solenoid is 11 N and the diameter of the striker is 2.4mm. In this manner, consistent, nondamaging, point sourcestress wave pulses are generated. Frequencies in the range of0–100 kHz are typically generated with this source. Miniatureaccelerometers are used as contact receivers at locations B andC. The accelerometers are coupled directly to the surface ofthe test specimen with a thin layer of wax. The receiving unitsare small and sensitive and have the following properties: re-ceiver contact face area = 25 mm2; mass = 0.7 g; nominal

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FIG. 4. Arrival Features of Typical Signal Received by Accel-erometer

voltage acceleration sensitivity = 1.02 mV/(m/s2); and a nom-inal 10% flat frequency response over 1 Hz to 25 kHz.

When the impact source is applied to location A, the prop-agating waves are detected by the two accelerometers (loca-tions B and C successively) and are sent to separate channelsof an oscilloscope. Each transient signal is collected for alength of 100 ms and digitized with 512 points. A typical signaldetected by an accelerometer is shown in Fig. 4. As a resultof the impact event, waves propagate in all directions; somewave components propagate along the surface of the specimen,while others reflect from free boundaries. Only the direct sur-face-bounded wave components and the first L-wave reflectionfrom the opposing side of the specimen are captured withinthe time window. The digital data are transferred to a personalcomputer with the GPIB interface. Two time domain signalsare collected from each impact event. Both signals are shiftedso that a common signal feature, the first significant peak,coincides with the center of the time window. These shiftedsignals are then smoothed with a Hanning window, and a se-ries of zeroes are added to the end of the signal until the totallength is 8,192 points. This windowed and zero-padded signalis then transformed into the frequency domain with the fastFourier transform (FFT) algorithm, where the resolution in thefrequency domain is approximately 650 Hz. Next, the impactsource is applied at location D and the entire data collectionprocedure is repeated. As a result of the data collection, a totalof four signals are obtained. Further manipulations of the fourfrequency spectra follow, as described below.

Theoretical Basis of Transmission Measurements

In the frequency domain, one can represent a stress wavesignal sent by the source at location A and received by thenearest accelerometer at location B as a simple product ofterms

V = S d R (1)AB A AB B

where VAB = FFT of the captured time domain signal; SA =generating response term; RB = receiving response term; anddAB = signal transmission function between locations A and B(Achenbach et al. 1992). Similarly, the stress wave signal sentby the same impact event at location A and received by thefar accelerometer at location C is given by

V = S d d R (2)AC A AB BC C

The Si and Ri terms contain undesired variability caused byvariation of impact events, accelerometer coupling, etc. Thisvariability masks the desired transmission response betweenthe two accelerometers dBC. Thus, we are interested in deter-


J. Eng. Mech. 200

FIG. 5. Experimentally Obtained Surface Wave Transmissionfrom 102 mm Thick Concrete Free Slab with Varying DamageConditions

mining dBC by eliminating the Ri, Si, and extraneous dij terms.This can be accomplished by collecting another complemen-tary set of stress wave signals, which is sent from the otherside of the receiving accelerometer pair along the same line;that is, the stress wave source is moved to location D. Thesignals sent from D and received at C and B are expressed asVDC and VDB, respectively, using the same convention as be-fore. Thus four signals are obtained: VAB, VAC, VDB, and VDC.Simple manipulation of the Vij terms results in an expressionfor the transmission between locations B and C

0.5d ( f ) = (V V /V V ) (3)BC AC DB AB DC

Eq. (3) is valid assuming that the material is globally iso-tropic, that the coupled accelerometers at locations B and Chave negligible effects on the passing surface waves, and thatthe contribution of other waves (e.g., direct and reflectedL-waves) is negligible. These assumptions have been demon-strated to be correct for this experimental setup (Popovicset al. 2000). The term dBC is a function of frequency and canbe visualized as the ratio of the amplitude of the signal fromthe far accelerometer to that of the near accelerometer. Thus,a transmission value of 1 indicates no amplitude loss (com-plete transmission) as the wave propagates between points Band C, whereas a value of 0 indicates complete signal ampli-tude loss (no transmission). In the case of a point source ofstress waves dBC values should be less than 1, even for a per-fectly transmitting material, since a considerable signal am-plitude loss will result from beam spreading.


The sensitivity of dBC measurements to the depth of surface-breaking cracks in concrete is demonstrated first. Transmissionmeasurements were performed on a free 10 cm thick concreteslab under three different path conditions between the accel-erometers at B and C: (1) across an undamaged path; (2)across a 1 cm deep notch cut into the surface; and (3) acrossthe same notch after the slab was subjected to flexure until acrack emanating from the notch propagated several centime-ters into the slab. Fig. 5 shows the obtained signal transmissioncurves within a frequency range of 0 to 200 kHz for the threecases. Reliable results could not be obtained for higher fre-quencies. Clearly, dBC suffers a severe reduction in value fornearly all frequencies when the surface waves pass across thenotch. A further reduction in all frequencies is noted whencracking is introduced additionally to the surface wave path.Thus, dBC is sensitive to the presence of near-surface damagein concrete.

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FIG. 6. Water Permeability Coefficient as Function of Time; In-itial Crack Width 5 205 Microns

Signal transmission measurements are now applied to mon-itor the autogenous healing of cracks in concrete. Autogenoushealing refers to the ability of cement to heal cracks and wasfirst reported by Abrams (1925). Possible reasons for healingare chemical precipitation of calcium hydroxide and calciumcarbonate, mechanical blocking, obstruction of narrow crackareas with ultrafine material, and swelling and rehydration ofthe hardened cement paste on the crack walls. Feedback-con-trolled splitting tensile tests were used to induce microcracksof designed widths in the concrete specimens (Aldea et al.2000). The water permeability of cracked samples was thenevaluated by the low pressure water permeability test. Stresswave transmission measurements were used to quantify theextent of healing of cracked concrete. In this case, the depthof the crack remains constant, but the crack width or the in-terface condition between opposing crack faces vary. Crackedsamples were subjected to signal transmission measurements,then vacuum saturated according to ASTM C 1202-94 and setup for the water permeability test (WPT) (Aldea et al. 1999).Besides mechanical tests, NDE techniques based on ultrasonicpulse velocity (UPV) measurement have been used to assesscrack healing (Munday et al. 1974; Abdel-Jawad and Haddad1992). Although UPV measurements can detect the occurrenceof crack healing, it has been shown that this method cannotaccurately determine the extent of crack healing.

Water permeability was monitored up to 90–100 days forall the specimens tested. Water flow drastically decreases withtime, and, consequently, the permeability drastically decreasesas shown in Fig. 6. In general, permeability decreases by sev-eral orders of magnitude, although the decrease in permeabilitycoefficient depends on initial crack opening. The results of thepresent research showed that, for all the cracked specimens,the permeability tended to decrease to the range correspondingto that of the uncracked material. This behavior reveals theability of cracked concrete to seal itself in time.

One-sided stress wave transmission measurements wereused to characterize the material immediately after inducingthe crack, and then after 100 days of WPT. The two receiverswere attached with wax to the surface of the specimen andlocated on each side of the crack, along a diametral line per-pendicular to the crack. Signal transmission values are sensi-tive to the presence the crack for all the specimens tested, asshown in Fig. 7. A drastic drop in transmission value, com-pared with the uncracked material, is seen in the case of initialcracks wider than 100 microns. The transmission measure-ments clearly distinguish a crack width threshold: crack widthsless than 100 microns show much greater signal transmissionthan those above 100 microns. However, the transmissionmeasurements are unable to clearly distinguish between crackwidths within the range above 100 microns. Transmission

J. Eng. Mech. 2

FIG. 7. Signal Transmission Immediately after Inducing Crackbut before WPT for Varying Initial Crack Width

FIG. 8. Signal Transmission Immediately after Inducing Crackand Again after 100 Days of WPT for Varying Initial Crack Width

measurements performed after 100 days of WPT show a sig-nificant change for the cracked samples: the transmission val-ues increase and become closer to that corresponding to theuncracked sample (Fig. 8, dotted line). The observed recoveryin signal transmission suggests significant closing (filling) ofcracks in each case.

Both permeability and transmission measurements confirmautogenous healing of the cracks. Yet the recovery in signalwith crack healing is not as spectacular as that observed forpermeability; the signal transmission of cracked concrete afterWPT never recovers to the value of uncracked concrete. Thissuggests that, although the healed cracks are probably filledwith products that drastically reduce water flow, these productsare more mechanically compliant than sound material.

VIBRATION MEASUREMENTS

Vibrational frequency analysis may be used to evaluate theglobal condition of a structure (Pandey and Biswas 1994).When a structure is dynamically excited, the resulting me-chanical vibration comprises different modes of vibration, eachwith its own characteristic natural frequency, amplitude, andmotion. The frequency of a given mode is a function of thegeometry and boundary conditions of the specimen and themass density and elastic constants of the material. Damageimparted to the structure results in a change in the apparentelastic contacts, resulting in a change in the natural frequenciesof vibration (Popovics et al. 1998b). The natural frequenciesof vibration can be accurately tracked while a concrete spec-imen is subjected to test loading (Subramaniam et al. 1998).In this section, it is shown that the natural frequencies may beused to predict the remaining life of a concrete specimen sub-jected to fatigue loading.


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Experimental Test Setup

Prismatic concrete beams with 46 cm length, 5 cm depth,and 10 cm height were used in this study. The experimentalsetup for vibration analysis is shown in Fig. 9. A 4.75 mmdiameter steel ball was used for impacting the specimen. Thestructural response of an impact was measured using two ac-celerometers strategically positioned on the beam. The accel-erometers are the same as those used in the experiments re-ported in the previous section. The transient response of eachaccelerometer was sent to two separate channels of a digitaloscilloscope. The signals have a length of 5 ms and were dig-itized using 4,096 points. Using the GPIB interface system,the digital data were transferred to a personal computer. Thedata were then zero-padded to 8,192 points and the frequencydomain (amplitude spectrum) was obtained using an FFT al-gorithm. The peaks in the amplitude spectrum of the frequencyresponse correspond to the various resonance modes of vibra-tion. The frequencies corresponding to these peaks were re-corded. Fig. 9 also shows a typical time domain response andthe associated frequency amplitude spectrum for a concretebeam. Note that the frequency values at the peaks in the fre-quency domain are readily discernable. Data obtained usingthis procedure produced repeatable results on the same spec-imen, and the values were consistent for different specimens.Changes in measured value of frequency, due to surroundingnoise or inconsistent impact, are negligible.

Vibration Mode Identification

Each structure may support an infinite number of vibrationalmodes. Since there are far too many modes to track in a struc-ture, it is easiest to identify the modes that are most prominentor have the highest amplitude for a given excitation. Eachmode is associated with a certain type of motion of the body,and each may become more or less prominent depending uponthe location of impact and sensors. The optimal sensor loca-tions for a given mode of vibration are determined from themode shape of the specimen. In simple beams, there are fourmain modes of vibration: strong-axis bending, weak-axisbending, longitudinal, and torsional. The fundamental modeshapes and optimal sensor locations are shown in Fig. 10 forthe strong-axis bending and longitudinal modes. Only thestrong-axis bending and the longitudinal modes were investi-gated. The first five bending and the first three longitudinalresonance frequencies of the beam can be identified using theexperimental setup.

Closed-form solutions for the natural frequencies for eachmode do not exist for finite-length beams. Therefore, the dy-namic response of the beam specimen was numerically cal-culated using a dynamic three-dimensional finite-element pro-gram within the IDEAS software package. With such aformulation, the specific modes shapes can be identified andassociated with their natural frequencies. The beam was dis-cretized using four-noded tetrahedral elements having threedegrees of freedom at each node. The beam was meshed using3,000 elements and traction-free boundary conditions wereprescribed. The elastic material parameters for the analysiswere obtained by testing companion cylinder specimens.


To study the influence of cracks on the global resonancefrequencies of the specimen, a crack was introduced in a con-trolled manner by testing the beam in a three-point bendingconfiguration using a closed-loop servo-hydraulic testing ma-chine. The two linear variable displacement transducers(LVDTs) mounted at the bottom of the beam measure the dis-placement of the bottom fiber of the beam and approximate


J. Eng. Mech. 20

FIG. 9. Experimental Setup for Vibration Frequency Measure-ments

FIG. 10. (a) Bending (Strong-Axis) and (b) LongitudinalModesof Vibration and Optimal Source and Receiver Location to Ex-cite These Modes

the crack mouth opening displacement (CMOD) after thecrack initiates. In the test, the average deformation of the twoLVDTs is used as the feedback signal, wherein the averagedeformation of the two LVDTs is increased at a constant rate.For beams tested in three-point bend configuration the beamfails primarily due to propagation of a single flexural crack atthe midspan. The crack initiates in the prepeak part of the loadresponse, and the postpeak represents steady crack propaga-tion. There is a progressive decrease in the structural stiffnessdue to crack propagation as evidenced by the decrease in theslope of the loading-unloading curves in the postpeak. Theeffective crack length at different points in the postpeak canbe estimated from the structural stiffness using the principlesof linear elastic fracture mechanics (LEFM). The details of thisprocedure are provided in the literature, e.g., Shah et al.(1995).

Resonance frequencies were measured at the bottom of eachunloading-reloading cycle in the postpeak part of the load re-sponse. The specimen was removed from the test machine andplaced on a bubble wrap to simulate the free boundary con-dition. The influence of crack length on the resonance fre-quencies is shown in Fig. 11. The resonance frequencies havebeen normalized with respect to the values at the beginningof loading. Almost all modes show a consistent decrease withincreasing crack length although the actual change varies. Thefirst bending and the first longitudinal modes are affected themost, while the second longitudinal mode does not show anychange. The bending modes show a larger drop in frequencyowing to crack propagation when compared with the longi-tudinal modes. The higher order modes show a lesser changein the normalized frequency than the lower order modes. Sim-

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FIG. 11. Influence of Crack Length on Frequencies of Resonance (Normalized): (a) Bending (Strong) Modes; (b) Longitudinal Modes

FIG. 12. Fatigue Response of Beam Specimen: (a) MechanicalStiffness; (b) Crack Length with Number of Cycles of FatigueLoading

ilar trends were consistently observed in all the beams thatwere tested.

Low cycle flexural fatigue tests were performed at four dif-ferent load ranges. In all the tests, load was applied betweentwo fixed load levels in a sinusoidal waveform at a frequencyof 2 Hz. All fatigue tests were performed in load control. Thelower limit of the load cycle was kept equal to 5% of theaverage quasi-static peak load value. The four different upperlimits used in this study correspond to 94, 89, 83, and 74% ofthe average quasi-static peak load.

The change in flexural stiffness during fatigue loading isshown in Fig. 12(a). The flexural stiffness represents the secantstiffness computed between the minimum and maximum loadlevels. The damage evolution in concrete during flexural fa-tigue, in terms of the measured stiffness, follows a three-phasetrend. The curve is S-shaped; a measurable change in the firstfew cycles is followed by a region of almost linear change inthe displacements. This is subsequently followed by a sharpdecrease in stiffness prior to failure. Similar responses wereobtained for all four load ranges that were tested. The cracklength computed from the observed change in stiffness usingthe principles of LEFM (Shah et al. 1995) is shown in Fig.12(b). The crack growth during fatigue loading also follows athree-phase process.

The damage imparted to the specimen was monitored usingthe vibrational resonance frequency throughout the fatigueloading process. Fig. 13 shows a plot of the change in thefundamental longitudinal vibrational resonance frequency withthe number of cycles of fatigue loading. The mechanical stiff-ness is also plotted on the double Y-axis graph (on the right-

J. Eng. Mech. 20

FIG. 13. Response of Vibration Frequency and MechanicalStiffness with Number of Cycles of Fatigue Loading

hand side). It can be seen that the vibrational frequencies ac-curately represent the observed trend in the change inmechanical stiffness of the specimen; the reduction in the fre-quency is also a three-phase process. There is hence a strongconnection between the measured mechanical compliance andthe resonance frequencies during the fatigue process.

It is observed that the decrease in the mechanical stiffnessof the specimen in the second, linear phase (region II) of thefatigue process correlates well with the fatigue life; a largerdecrease in stiffness is associated with a smaller fatigue life.Fig. 14 shows a correlation between the slope of decrease inthe fundamental longitudinal resonance frequency in region IIand the fatigue life of the specimen. Each symbol in the graphcorresponds to a different specimen tested in fatigue. There isa very good correlation between the rate of decrease in theresonance frequency in region II of the fatigue response andthe fatigue life of specimen. The slope of reduction in resonantfrequencies in region II can hence be used to predict the fa-tigue life of the cylindrical specimens from the equation

Log(N ) = 21.0027 ?Log(slope of reduction off

resonance frequencies) 1 1.9055 (4)

An analogous equation could be established for assessing fa-tigue-induced damage in structures subjected to repeated load-ing of high stress amplitude, such as airport pavement slabs,structural beams, and bridge girders, if an appropriate vibra-


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FIG. 14. Correlation between Rate of Reduction in Frequencyof Vibration during Second Phase (Region II) of Fatigue Re-sponse and Fatigue Life (Nf ) of Specimen

tion mode of the particular structure can be identified, isolated,excited, and monitored over a portion of its fatigue life.

SUMMARY

The presented results demonstrate the potential of NDEtechniques for concrete that have been developed at the Centerfor Advanced Cement-Based Materials. Future effort will bedirected toward practical application of the techniques to mon-itor the condition of existing concrete structures. Specifically,the writers conclude the following. Through-thickness ultra-sonic signal transmission (attenuation) measurements are muchmore sensitive to the presence of distributed microcracking inconcrete than ultrasonic velocity measurements. One-sidedmechanical wave signal transmission measurements may beapplied reliably to an unprepared surface of a concrete struc-ture by making use of a self-calibrating test measurementscheme. The self-calibrated transmission measurements aresensitive to the depth or width of cracks in concrete; largercrack depths or widths cause reduced signal transmission. Thestructural vibration frequencies of a concrete structure are sen-sitive to the presence of damage, for instance, that caused byfatigue loading. The relationship between reduction in vibra-tion frequency and imparted damage can be used to predictthe fatigue life of a structure, assuming an appropriate andsensitive vibrational mode of a structure is identified, isolated,excited, and monitored over a portion of its fatigue life.


J. Eng. Mech. 200

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