NONDESTRUCTIVE EVALUATION OF CORROSION IN VARYING

NONDESTRUCTIVE EVALUATION OF CORROSION IN VARYINGENVIRONMENTS USING GUIDED WAVES

Shruti Sharma1 and Abhijit Mukherjee2

1Civil Engineering Department, Thapar University, Patiala, India2Indian Institute of Technology, Gandhinagar, Gujrat, India

This work reports nondestructive evaluation of reinforcing bars that are corroding in thepresence and absence of chlorides utilizing ultrasonic guided waves. The effect of ratesof corrosion and its progression in the two environments on the ultrasonic signals isdiscussed. Surface and core seeking guided wave modes were used to monitor beamsundergoing accelerated impressed current corrosion. Effective combination of guided wavemodes could relate to the differences in corrosion mechanisms and rates in the two envir-onments. Calibration of the ultrasonic data with the physical condition of the bar in the twoenvironments has been attempted. It is done by conducting destructive tests of mass loss,tensile strength, and pull out strength at different stages of corrosion.

Keywords: chloride corrosion, guided waves, mass loss, reinforced concrete, tensile strength, ultrasonic

INTRODUCTION

Corrosion of steel reinforcement in concrete is the most commondurability problem encountered by reinforced concrete (RC) structures,especially when they are exposed to harsh environments with high carbondioxide or chloride concentrations. After the onset of corrosion, accumu-lation of the voluminous corrosion products at the steel-concrete interfaceinduces tensile stresses in concrete. It leads to cracking, delamination, andspalling that further accelerate corrosion. Moreover, bond between steeland concrete is weakened. Further, corrosion leads to substantial loss ofreinforcement in the form of pitting of the bars which reduces the ductilityof the steel rebars and can lead to catastrophic failure. Thus, damages andthe economic loss caused by corrosion of steel in concrete makes it arguablythe single largest infrastructural problem facing the industrialized countries[1]. Hence, the best way to avoid corrosion induced damages is monitoringof embedded reinforcements in concrete regularly to ensure overall structur-al safety and serviceability.

A wide range of techniques have been reported in the literature that maybe suitably employed for the monitoring of corrosion of steel in concrete struc-tures for the purpose of diagnosing the cause and extent of the reinforcement

Address correspondence to Shruti Sharma, Assistant Professor, Civil Engineering Department, ThaparUniversity, PO Box 32, Patiala 147004, India. E-mail: [email protected]; [email protected]

Research in Nondestructive Evaluation, 24: 63–88, 2013

Copyright # American Society for Nondestructive Testing

ISSN: 0934-9847 print=1432-2110 online

DOI: 10.1080/09349847.2012.699609

63

corrosion. But the existing techniques are electrochemical techniques likehalf cell potential mapping, linear polarization etc. which relate corrosionrate and extent through assessment on surrounding concrete medium. Appli-cation of these techniques to the RC specimens and measurement of on-sitecorrosion survey poses a number of difficulties. None of the methods con-centrate on corrosion monitoring through direct measurements on embeddedsteel which is directly affected by corrosion. The degradation acceleratesunless early remedial action is taken. Removal of concrete to visually inspectthe reinforcement is detrimental to the structure. Assessing the integrity ofreinforcing bars poses practical difficulties because the reinforcements aretypically embedded inside concrete. Nondestructive techniques (NDT) suchas radiography, acoustic emission, X-ray, thermography, etc. cannot be usedeffectively for assessing the integrity of civil engineering installations becauseof large sizes, limited accessibility, and practical problems associated withthe implementation of these specialized techniques. Hence, there is a needfor nondestructive, nonintrusive, in-situ, and real time corrosion monitoringsystem for RC structures.

Guided waves offer a potentially attractive solution for monitoring ofembedded reinforcements in concrete and are gaining popularity amongresearchers. They have the advantages of testing over long distances, sensi-tivity greater than conventional ultrasonic testing procedures, ability to testmultilayered structures, and are relatively inexpensive due to simplicityand lower sensor cost. Furthermore, frequency and mode tuning can be donefor evaluation of different types of deterioration or damage. Corrosion ofreinforcing steel in concrete results in debonding, pitting, and irregularitiesin the bar profile. These phenomena affect wave propagation through thebar differently. Researchers have reported various studies on different aspectsof corrosion detection through experimental and analytical [2] investiga-tions. Wave energy in steel bars embedded in concrete attenuates at highrates due to leakage into the surrounding concrete. But certain modes existthat have attenuation minima with minimal energy loss due to leakage andmaterial absorption. Specific modes that have low attenuation have beenidentified for the inspection of reinforcing bars in concrete [3]. Guidedwaves have also been used to monitor stress levels in prestressed steel strandsusing suitable guided wave modes at a particular frequency [4].

For corrosion monitoring, one encounters the problem of making thereinforcement corrode at a faster rate. Thus, simulation of corrosion of rebarshas been attempted by researchers [5–12]. It has been simulated in variousways such as by introducing debonding between steel and concrete in theform of PVC pipes or by wrapping tape on the bars [5–8]. It is widely con-jectured that debond impedes leakage of waves into concrete. Thus, highersignal strength at the receiving end indicates debond and corrosion. Both lowand high frequency guided wave modes to monitor corrosion damage inreinforced mortar specimens have been attempted [13,14]. In a recentpublication, corrosion simulated by artificially incorporating debonds and

64 S. SHARMA AND A. MUKHERJEE

notches has been compared with actually corroded specimens [15]. It is con-cluded that a combination of notch effect and debond effect is embodied inactual corrosion. This led to a pilot study to ultrasonically monitor corrosionof reinforcing bars undergoing accelerated corrosion in the presence andabsence of chlorides [16]. It was concluded that manifestation of corrosionin the two environments is considerably different. While chloride corrosionleads to pitting predominantly, absence of chlorides is manifested by pre-dominant debonding. Ultrasonic guided wave modes sensitive to the specificeffects were identified.

The present work aims to study in detail, the corrosion phenomena in thepresence and absence of chlorides using ultrasonic guided waves. An ensem-ble of samples at different stages of corrosion in two environments has beenmonitored ultrasonically in pulse transmission mode. The effect of mech-anism and rate of corrosion in the two environments on the ultrasonic signalsis identified. The corroded bars were subjected to destructive tests of massloss, tensile strength, and bond strength. The ultrasonic voltages weremapped to percentage mass loss, tensile strength, and pullout strength in thetwo environments.

CORROSION IN VARYING ENVIRONMENTS

Corrosion of steel in concrete is an electrochemical process where theanode and the cathode are on the same reinforcing bar. Concrete is alkalinedue to the presence of hydroxides of calcium, potassium, and sodium and itspH ranges between 12 and 13. Due to the high alkalinity of the concretepore water, the steel reinforcing bars are passivated by an iron oxide film thatprotects the steel. But the passivating environment is not always maintained.Presence of oxygen, water, and aggressive ions such as chlorides beyond athreshold depassivates the steel bars, and corrosion is initiated. Corrosionin the absence and presence of chloride ions is referred to as OxideCorrosion (OC) and Chloride Corrosion (CC), respectively.

Oxide Corrosion (OC): The corrosion of steel in concrete in the presenceof oxygen but without chlorides takes place in several steps [1]. Concreteworks as an electrolyte that facilitates the flow of electrons between theanode and the cathode. At the anode on the steel reinforcing bar, iron atomslose electrons to become iron ions (Feþþ). At the cathode, on the same steelbar where metal is not consumed, oxygen in the presence of water, acceptselectrons to form hydroxyl ions (OH-). The hydroxyl ions combine with theferrous ions to form ferrous hydroxide which oxidizes further to hydratedferric oxide commonly called Rust.

The manifestation of OC is in the formation of these corrosion productson the surface of the rebar. Thus, the steel-concrete interface is altered whererust is formed. At the initial stages, the corrosion products increase thefriction coefficient at the interface through the roughening of the bar surface.

NONDESTRUCTIVE EVALUATION OF CORROSION 65

As corrosion progresses, due to higher volume of corrosion products than thecorresponding volume of steel, an outward pressure is generated on con-crete. This might initially improve the interfacial bond. However, the press-ure results in tensile stresses in concrete. As concrete is weak in tension,cracks are developed in it leading to debond.

Chloride Corrosion (CC): Another major cause of depassivation of thereinforcing bar is the presence of chloride ions. Diffusion of chlorides intoconcrete is the major problem in most parts of the world. The chloridesmay be introduced into concrete as internal chloride in a fresh state andas external chloride in a hardened state. Possible sources of chlorides inconcrete are aggregates, mixing water, admixtures (accelerators), deicingchemicals, and use of sea water for construction. The chlorides attack thepassive layer and act as catalysts to corrosion. In the presence of chlorideions, passive layer is locally destroyed and the process of localized corrosionis initiated. They are not consumed in the process but help to break downthe passive layer of oxide on the steel and allow the corrosion process toproceed quickly.

As in the case of OC, ferrous hydroxide is oxidized to rust in the presenceof oxygen and water. The resulting corrosion products and accompanyingprocesses of cracking, spalling, and delamination of concrete occurring inboth types of corrosion are the same, but the rate and the mode of occurrenceof various aspects of corrosion phenomena are different in the two cases. Therate of CC is far higher than OC. Although it also leads to debond as describedin OC, its distinguishing feature is pitting caused by dissolution of iron result-ing in formation of crevices in the bar. It leads to locale loss of area. Thus, inaddition to debond, CC manifests itself in local weakening of the bar.

This study investigates the applicability of ultrasonic guided waves toidentify these two common types of corrosion mechanisms, i.e., oxide andchloride corrosion occurring in reinforcements in concrete. An experimentwas carried out to create the two different types of corrosion in RC samples.The samples were ultrasonically monitored throughout the corrosionprocess. Finally, ultrasonic signals were calibrated with mass loss, tensilestrength, and pullout strength of the reinforcement.

GUIDED WAVES IN BARS IN CONCRETE

In a finite, perfectly elastic media like a reinforcing bar, the ultrasonicwave is reflected from its boundaries, and the energy is contained withinthe bar as a guided wave. The complex effect of the bar boundaries resultsin dispersion of the wave and generates different modes that have predictableproperties such as mode shapes and frequencies. They can be calculated bysolution of the wave propagation equations. The velocity-frequency relation-ships of guided waves can be displayed as dispersion curves [17,18]. Fora cylindrical system like reinforcing bar, waves propagate in three modes


due to dispersive effect of boundaries, i.e., longitudinal (L), flexural (F), andtorsional (T) modes. The modes are numbered according to the formatused by Disperse [18], which closely follows that defined by Silk andBainton [19].

Specific modes can be excited selectively by choosing a frequencybound. Longitudinal waveforms have axial and radial displacements butno angular displacements and can be produced by coupling a compressionaltransducer parallel to axis of the bar [20,21] or by using angled transducerson the sides [7]. Only longitudinal modes were considered in this study asthe flexural and torsional modes experience high attenuation [22]. Guidedlongitudinal waves were produced in the embedded bars by keepingtransducers parallel to the guiding configuration at the two ends of the bars.Different longitudinal modes were produced by varying the excitationfrequencies.

Concrete around the bar allows the energy to leak from the bar to its sur-rounding concrete. The leakage depends on the relative elastic and dampingproperties of the concrete layer. Thus, the steel–concrete interface can becharacterized by ultrasonic investigations [2,7,8,12,23–25]. The discontinu-ities and irregularities produced as a result of pitting results in scattering andreflections of the wave guide resulting in mode conversions and signalattenuation. On the other hand, loss of bond between the steel and concretereduces leakage into the surrounding concrete and the signal strength rises.Therefore, it should be possible to characterize corrosion through ultrasonicinvestigations.

EXPERIMENTAL DETAILS

Setup and Sample Details

RC beam specimens of dimensions 150mm� 150mm� 700mm wereprepared with concrete having proportions of cement, sand, and stoneaggregates as 1:1.5:2.96. Water cement ratio was kept as 0.45. One25mm diameter plain mild steel bar of 1.2m length was placed at the centreof crossection of the beam at the time of casting. The bar projected out by250mm on each side of beam (Fig. 1). Although ribbed bars that offer mech-anical bonding between the bar and the concrete are more popular in con-struction, in this investigation plain bars are used to avoid such mechanicalbonding and to observe how corrosion influences interfacial bond.

In natural environments, corrosion process takes several years to occur.The process can be accelerated in many ways. The commonly used methodsof inducing corrosion in RC specimens can be recalled as salt spray, chloridediffusion, alternate drying and wetting in salt water, and impressing anodiccurrent. However, the quickest method of inducing corrosion is by impress-ing anodic current. In this method, the specimen is immersed in NaCl


solution, and a direct current is passed making the reinforcement bar asan anode and another metal nobler than steel in electro-chemical seriesas cathode. This method was chosen for inducing accelerated corrosion inRC beam specimens.

Reinforcing bar was made anode by connecting positive terminal of thepower supply to the projected bar in the beam. The middle 300mm wasselected for exposure to corrosive environment. A thick cotton gauze wasplaced in this region to keep it moist. Stainless steel wire mesh was wrappedon the cotton gauze. The negative terminal was connected to the wire meshand was made the cathode. A constant voltage of 30V was applied betweenthe two terminals by means of a constant power supply device. A drippingmechanism was fitted on the wire mesh to keep the cotton underneath satu-rated. For CC, 5%NaCl solution was used as drip. In OC, plain tap water wasused instead of NaCl solution. Cotton gauze below the wire mesh uniformlydistributed the water in the central wrapped portion of the beam.

Selection of Excitation Modes and Frequencies for Ultrasonic Testing

For ultrasonic testing of reinforcing bars, a conventional ultrasonic test-ing system consisting of a pulser-receiver device, ultrasonic transducers, dataacquisition card, and display devices was used. The transducers wereattached at the two ends of the bars by means of a holder and a couplinggel between the bar and the transducer. The holders maintained a constantpressure between the transducer and the bar. Driven by the pulser (DPR300), the compressional transducers (Contact Type, Karl Deutsch Make)generates an ultrasonic pulse that propagates through the embedded bar inthe form of longitudinal waves (Fig. 2). The excitation signal consisted ofa compressive spike pulse. The pulse transmitted at the other end of thebar was recorded on the receiving transducer [15].

FIGURE 1. Set-up for accelerated corrosion.


Both the beams undergoing accelerated OC and CC were ultrasonicallymonitored for 28 days. The selection of frequencies for testing was doneusing the software Disperse [18]. They were also validated by experimentallyconfirming the signal fidelity. The modes that are easily distinguishable andhave lowest signal attenuation were selected [4]. For bars embedded inconcrete, which is a layered waveguide system, leakage plays an importantrole. Frequency regions with lowest attenuating modes are the ones with dis-placement profiles centered in the middle of the bar to minimize leakage butat a low enough frequency to avoid substantial absorption [3,13,14]. Disper-sion curves for 25mm diameter bar are shown in Fig. 3. Hence, fundamentalmode, L (0, 1) mode at 100 kHz starting at zero frequency with low attenu-ation was selected (Fig. 3a). Another mode L (0, 7) at 1MHz forms a lowleakage mode corresponding to maximum energy velocity and minimumattenuation (Fig. 3b) was also used for ultrasonic investigations [4].

Another important feature for the selection of modes was mode shapethat determines the radial distribution of displacement and energy density(Fig. 4). It may be recalled that OC and CC had different effects on thebar. While OC mainly affects the surface of the bar, CC leads to pitting insidethe bar. Thus, a mode that has significant surface component would be sensi-tive to oxide corrosion. It is evident that L (0, 1) is such mode, and hence, wename it surface seeking mode (Fig. 4a). The chloride corrosion, on the otherhand, would manifest itself in a mode that progresses mainly through thecore of the bar and has negligible surface component. L (0, 7) is such a modeand it is referred to as the core seekingmode (Fig. 4b). Thus, these two modes

FIGURE 2. Experimental set-up for ultrasonic monitoring of a beam undergoing accelerated corrosion.Note: Condition of beam after 28 days of accelerated corrosion. (Figure appears in color online.)


have been considered in an attempt to distinguish between corrosionmechanisms in two environments of oxides and chlorides. Pulse transmissionwas monitored once each day for 28 days in both OC and CC specimens.The signals had no significant variation after that time. After the completionof exposure, the specimens were subjected to pullout test and the extractedbar was examined for mass loss and tensile strength. The ultrasonic testresults were compared with those of destructive tests. A test matrix wasdeveloped to carry out accelerated CC and OC for 6, 12, 18, and 28 daysto study different extents and stages of both types of corrosion and to corre-late with destructive tests. Three samples for each age undergoing CC andOC were tested to establish the repeatability of experiments.

EXPERIMENTAL RESULTS AND OBSERVATIONS

Visual Observations

Beams undergoing accelerated CC showed the formation of large parallelcracks accompanied by oozing out of liquid corrosion product and extensive

FIGURE 3. Dispersion curves for 25mm steel bar embedded in concrete.


rust stains. A longitudinal crack appeared parallel to the bar within 7 days.The cracks initiated at the surface of the beam and progressed along thedirection of the reinforcement. A reddish brown liquid oozed out of thecracks and at the ends of the beam. With the increase in the volume of cor-rosion products, another crack parallel to the bar appeared on another faceof the beam after 15 days. The crack length and width increased withincrease in exposure. At 28 days of corrosion, there were two large and widelongitudinal cracks that divided the entire beam into wedges and the beamwas in a significantly damaged condition (Fig. 5a). The extracted bar showedwidespread and significant pits and irregularities on its surface (Fig. 5c). Theloss of metal was irregular and deep pits were noticed along the whole lengthof the bar.

Contrary to this, in the beam undergoing OC, no visual change wasobserved until 15 days. Then a longitudinal crack appeared parallel to

FIGURE 4. Mode Shapes. (Figure appears in color online.)


the reinforcement accompanied by a small leakage of dark brown cor-rosion products. The crack increased in length and width but extendedto only middle 1=2 length of the beam till 28 days. A significant obser-vation was the appearance of a perpendicular crack at the centre of thebeam. It appeared on the 22nd day and progressed to divide the lengthof the beam into halves (Fig. 5b). The appearance of a small longitudinalcrack and a perpendicular crack in U-shape was confirmed by repeatingthe accelerated oxide corrosion on another RC beam specimen for 30days. This indicates an outward bending pressure on the concrete dueto OC. The observation may be useful in identifying the type of corrosionat site. The extracted bar showed localized rust product formation only inmiddle one third length of the bar but no deep pits or crevices (Fig. 5c).There was no significant loss of metal as confirmed by visual observationand mass loss.

FIGURE 5. Visual comparison of the condition of beam and bars (28 days of accelerated corrosion).(Figure appears in color online.)


Monitoring with Surface Seeking Mode

Figure 6 shows ultrasonic voltage trends of the received signal in both CCand OC specimens using surface seeking mode L (0, 1) at 100 kHz.

In the CC specimen, rise in signal amplitude was observed right from the2nd day indicating loss of bond between steel and concrete and this contin-ued until 12 days. After 12 days of exposure, the signal amplitude continu-ously dropped until it became more or less constant after 21 days. Theultrasonic observations matched closely with the CC mechanism. Initially,the formation of soluble corrosion product caused delamination of the barfrom the surrounding concrete. Increase in pressure caused by corrosionproducts was alleviated due to the release of pressure by its dissolution. Thiscontinued until a considerable amount of delamination took place. Afterthis, the presence of chloride ions resulted in deterioration of bar throughthe local loss of material. It lead to attenuation of the signal and that contin-ued throughout the remaining period of exposure. This indicated that CCbegins with bond deterioration but finally lead to pit formation and localmaterial loss.

In the OC specimen, the received signal amplitude initially dropped for 4days before it starts to rise. This was possibly due to the increase in confine-ment pressure caused by corrosion products resulting in leakage into the sur-rounding concrete. It may be attributed to less soluble corrosion productsdeveloping at a slower rate in OC as against CC. After 4 days, the signalcontinued to rise until the end of exposure indicating increasing delami-nation during the whole period. But the rise in signal strength was slowerthan CC. This shows that for the same period of exposure, OC affected onlythe surface of the bar causing delamination and no severe pitting or area loss.

FIGURE 6. Transmitted pulse amplitude with L(0, 1) Mode (28 days of accelerated corrosion).


Thus, delamination dominated OC and pits were not expected to form.Visual examination of the bar after conducting the pullout test revealed thatthere was localized surface corrosion, small rust stains and a longitudinalcrack in the middle one third length of the beam. Because of this local pressurebuild up in a small length, a transverse crack appeared at the centre of thebeam. Corrosion had not spread in the whole length of the beam as in chloridecorrosion.

Monitoring with Core Seeking Mode

Figure 7 shows the peak to peak ultrasonic voltage ratios of the receivedand input signals in both CC and OC specimens for the core seeking mode L(0, 7) at 1MHz.

In the CC specimen, the beam showed no significant change in volt-age amplitude of the transmitted pulse until 6 days. After 6 days, therewas a continuous drop in the amplitude until it disappeared completelyon the 25th day. This pointed towards drastic non-uniform area loss inthe form of pits on the whole length of the bar. The widespread loss ofarea was confirmed visually by opening the beam. As corrosion pro-gressed, there was increase in loss of energy due to scattering, multiplereflections and mode conversions. Hence, there was a drastic fall insignal amplitude. It is worth mentioning that only signal amplitude of thismode L (0, 7) was studied as there was no shift in arrival time withincreasing exposure.

FIGURE 7. Transmitted pulse amplitude with L(0, 7) Mode (28 days of accelerated corrosion). (Figureappears in color online.)


Ultrasonic monitoring of the OC beam with L (0, 7) mode showed nodrop in signal until 12 days. Then it starts dropping slowly but the fall wasnot as drastic as observed in CC. In 28 days of exposure of OC specimen,a nominal fall of 0.5V was observed as against 3.5V signal amplitude fallin CC specimen. This clearly differentiates the rate as well as the corrosionmechanism in the two environments. OC is a slow process marked bydelamination for a significant period of exposure before the degradation ofthe core begins. Pitting is marginal in this case.

DETAILED STUDY OF CORROSION MECHANISM

A test matrix was developed for carrying out accelerated corrosionand ultrasonic monitoring in two environments. This was done to cali-brate the ultrasonic signals with different ages and stages of acceleratedchloride and oxide corrosion by performing destructive tests such asmass loss, pullout strength, and tensile strength. Further tests for 6, 12,18, and 28 days confirmed the repeatability of ultrasonic tests. Ultra-sonic signals in CC specimens disappeared in 28 days so tests were lim-ited to this period to facilitate comparison of OC and CC environments.To study the complete corrosion process in oxide environment, one OCbeam specimen was ultrasonically monitored till the signals vanished for130 days.

CORROSION IN CC ENVIRONMENT

From the study of CC ultrasonic signal plots with both core and surfaceseeking modes until the signals vanished, three distinct zones were observed(Figs. 8 and 9), as follows:

1. First six days had a rising L (0, 1) signal but steady L (0, 7) signal indicatingpredominant surface changes but inappreciable core changes. This zoneis referred as ‘Delamination Zone.’

2. From six to twelve days, the L (0, 1) signal continued to rise but L (0, 7)signal fell significantly. This indicates that the corrosion makes inroadsdeeper in the bar and it was not restricted to the surface only. Thus, bothmodes of corrosion were present. This zone is referred as ‘TransitionZone.’

3. Beyond the twelfth day, both L (0, 1) and L (0, 7) signals fell continuouslyexhibiting the same trend. At this stage, pitting of the reinforcing bar waspredominant and this zone is referred as ‘Pitting Zone.’

It may also be noted that the strength of L (0, 1) was significantly higher thanthat of L (0, 7). Thus, L (0, 1) mode had lower attenuation and it would travellonger distance through the bar.


From the CC ultrasonic plots, the mechanism of corrosion of reinforcingbar in presence of chlorides can be well understood. CC begins with thedelamination of bar from the surrounding concrete marked by increased

FIGURE 8. Trends of transmitted pulse with L(0, 1) mode at different stages of CC. (Figure appears incolor online.)

FIGURE 9. Trends of transmitted pulse with L(0, 7) mode at different stages of CC. (Figure appears incolor online.)


signal amplitudes. The delamination zone marks the onset of corrosionphenomenon in a bar undergoing accelerated CC and is well picked upby the surface seeking mode. As corrosion progresses, it is marked bylocal loss of area in the form of pitting and crevices along with debondingof bar. It begins in the transition zone shown by signal rise in L (0, 1)mode and signal attenuation in L (0, 7) core seeking mode. It marks theprogress of corrosion causing surface modification as well as non-uniformarea loss in the form of pits. As exposure increases, there was a drastic fallin signal amplitude in both L (0, 1) and L (0, 7) modes due to increase inloss of energy caused by scattering, multiple reflections and mode conver-sions. The most likely cause of this phenomenon is that corrosion affectsthe waveguide. Corrosion reduces the diameter of the bar non-uniformly.Thus, the waveguide is disturbed and scattering takes place from therough surface.

The trends of peak-peak voltage ratios were same in the samples of dif-ferent extents of corrosion in L (0, 1) mode (Fig. 8). There was a rise in signalstrength up to 12 days. This signifies that corrosion begins with delamination.The drop in signal amplitudes beyond this point confirms the start of pittingphenomenon. The signal strength diminishes from this point till it completelyvanishes. It may be noted that the 12 day sample had an early fall in signalstrength in comparison to the other samples. This signifies premature pittingwhich was also captured by the L (0, 7) mode. The L (0, 7) mode that is sensi-tive to bar profile changes indicated same trends for 6, 12, 18 and 28 days ofcorrosion (Fig. 9). In this case too, the 12 day samples exhibited early depar-ture from other trends. Thus, there is a good correlation between L (0, 1) andL (0, 7) mode results. Hence, the mechanism of corrosion occurring dueto chlorides is well picked up by surface and core seeking modes usingultrasonics.

Corrosion in OC Environment

In case of beam undergoing OC, the similar three zones were observed inultrasonic signal plots as in CC plots, but the process was spread over a largeduration (130 days), being a slow process (Figs. 10 and 11):

1. L (0, 1) signal continuously rose but very slowly for 50 days indicatingpredominant surface changes but inappreciable core changes (Fig. 10).From 1st to 22nd day, there was significant delamination and hence, thiszone is called the ‘Delamination Zone.’

2. From 22nd day onwards, L (0, 1) signal continued to rise (Fig. 10) but L(0, 7) signal started falling though not very significantly as in case ofCC (Fig. 11). Oxide corrosion is a slow phenomenon resulting in surfacemodification as shown by L (0, 1) mode along with very slow ingress intothe core from 22nd day to 50th day. In this zone, both modes of corrosionare present. This zone is referred as ‘Transition Zone.’


3. Beyond the 50th day, both L (0, 1) and L (0, 7) signals fell continuouslyexhibiting the same trend. At this stage, pitting of the reinforcing bar ispredominant and this zone is referred as ‘Pitting Zone.’

FIGURE 10. Trends of transmitted pulse with L(0, 1) mode at different stages of OC. (Figure appears incolor online.)

FIGURE 11. Trends of transmitted pulse with L(0, 7) mode at different stages of OC. (Figure appears incolor online.)


4. Visual observation of the OC extracted bar after 130 days showed a pits ina very localized length of about middle one third length. There was onlysurface modification in the rest length of the bar (Fig. 12).

5. As observed in case of CC, it was also noted that the strength of L (0, 1)was significantly higher than that of L (0, 7) indicating lower attenuationof L (0, 1) mode.

Hence, it can be concluded that OC is a slow process mainly resulting inthe formation of rust product which is not widespread. It begins with slowdelamination of the bar from the surrounding concrete for same period ofexposure as CC specimen. Then slowly corrosion progresses towards the coreand pitting starts, but the pits are localized and the process takes a long time.

Same trends of peak to peak voltage ratios are observed for differentperiods of exposure in both L (0, 1) and L (0, 7) modes. There was rise insignal strength throughout the period of exposure in L (0, 1) mode indicatingdelamination for 6, 12, 18, and 22 days (Fig. 10). There was no significantsignal change observed initially for 22 days by the core sensitive L (0, 7)

FIGURE 12. Condition of OC Beam (130 days of accelerated corrosion). (Figure appears in color online.)


mode. It indicated no appreciable core changes (Fig. 11). After 22 days, fall insignal strength was observed indicating ingress of corrosion into the core. Butthis fall was not as appreciable and drastic as observed in CC corrosion. OC is aslow phenomenon resulting in surface modification as shown by L (0, 1) modealong with slow ingress into the core from 22nd day to 50th day. After thistransition zone, from 50th day onwards, pitting begins till the signal vanishesin 130 days. Hence, OC is well evaluated by both L (0, 1) and L (0, 7) modes.

DESTRUCTIVE TESTING AND CALIBRATION OF ULTRASONICRESULTS

The bars were subjected to a series to destructive tests after the period ofexposure was completed. The pullout strength of the interface was deter-mined by securing the specimen in a universal testing machine (UTM) andapplying a tensile force on the bar at a rate of 0.02mm=sec. Tensile testingand mass loss of the extracted bar were performed to establish the effect ofcorrosion on the mechanical properties. The bar was cleaned with wirebrush and then acetone to remove all the corrosion products. It was weighedto evaluate its residual mass. The bar was then tested for tensile strength inUTM to determine its residual strength.

Beams in CC Environment

Destructive Testing Results After 6 days of exposure, the pullout strengthalmost doubled (Table 1). This was due to the increase in bond as a resultof pressure generated by initial formation of corrosion product around theextruded mild steel bar. As the corrosion progressed, pullout strengthdropped due to debonding of the bar from the surrounding concrete. For28 days of exposure specimen, the pullout strength was 28% lower in com-parison to 6 days exposure specimen. After the pullout strength, the barswere dug out of the beam. The corrosion products were spread in the wholelength of the beam accompanied by reddish brown stains and longitudinalcracks. The 28 day bar had lost 5.15% of its mass and 32.8% of its tensilestrength to that of healthy bar. The higher percentage of strength in compari-son to the mass indicates that the loss of mass is not uniform and is rather

TABLE 1 Destructive Testing Results for CC specimens

Exposure (days)

Property 0 6 12 18 28

Mass Loss (%) 0 0.141 4.19 5 5.96Pull Out Strength (kN) 25 55 50 47.50 20Tensile Strength (N=mm2) 671 660 552.36 511.6 451


local. The visual inspection also confirmed that the bar had experiencedsevere and widespread pitting.

As seen from the destructive test results for different days of exposure forCC specimens, loss in mass (ML), pullout strength (PO), and tensile strength (ft)was observed with the increase in the days of exposure. From the mass lossresults, it can be seen clearly that the mass loss until the 6th day was marginal.There was a sudden increase in mass loss from 6th to the 12th day. After 12thday, increase in mass loss continued but at a slower rate. This signifies that theinitial delamination corrosion that takes place at the surface of the bar does notlead to a significantmass loss. At the onset of pitting, after the 6th day, therewasa significant loss of mass. This was also corroborated by the sudden loss oftensile strength of the bar between 6th and 12th day. The loss of tensile strengthindicated the loss of cross-sectional area of the bar.

The pullout strengths of the beam indicated that there was increase from0th to the 6th day. It may be recalled that the bars used in this experimentwere plain extruded bars without any ribs. Thus, the initial bond at thesteel–concrete interface is only through friction and there was no mechanicalbond. Creation of corrosion products at the surface exerted an outwardpressure on the concrete. This improved the bond initially. But as corrosionprogresses, the soluble corrosion products get washed away creating loss ofinterfacial bond and fall in pullout strengths.

Correlation of Ultrasonic Voltages with Destructive Tests A correlation wasattempted to facilitate nondestructive estimation of the physical conditionof the bar. It was important to choose the right mode of ultrasonic wave. Itis indicated earlier that L (0, 1) mode is capable of detecting corrosion atearly stage. However, it is not very suitable for mapping its output to thephysical condition of the bar because the same voltage ratios may beobtained in very different physical conditions of the bar. In case of L (0, 7)mode, the signal decreases monotonically with the deterioration of thebar. Thus, it has one-to-one correspondence between the ultrasonic signaland the physical condition of the bar.

Figure 13 shows the comparison between ultrasonic signals anddestructive testing results of mass loss (ML), TS (ft), and PO strength. It is clearthat L (0, 1) may indicate very the same voltage ratios for very different masslosses. L (0, 7) mode, on the other hand, had a monotonically decreasingvoltage ratio with mass loss (Fig. 13a). The same trends are seen with othernondestructive parameters of pullout (Fig. 13b) and tensile strengths (Fig. 13c)as well. Thus, the calibration of the ultrasonic data with the physical state ofthe bar has been attempted with L (0, 7) mode only.

Mass Loss (ML)

The signal strength was highest at the initial stage and it reduced sharplywith the loss of mass finally getting asymptotic once again. Thus, a hyperbolic


curve (Fig. 13a) has been fitted between the voltage ratio and the mass lossgiven by the following equation:

Mass LossðMLt;%Þ ¼ MLmax=ð1þ 100R0RÞ ð1Þ

where

R0¼ Initial peak-peak voltage ratio of the transmitted pulse w.r.t. initialpulse

R¼ peak voltage ratio of the transmitted pulse w.r.t initial pulse at aparticular instant

FIGURE 13. Correlation of destructive CC results with peak to peak voltage ratio (R). (a) % Mass loss vspeak-peak voltage ratio(R); (b) Pullout load� vs peak-peak voltage ratio; and (c) Tensile strength�� vspeak-peak voltage ratio. (Figure appears in color online.)


MLt¼Mass loss (%) anticipated at a particular instantMLmax¼Maximum % mass loss anticipated for a particular extent=degree of

corrosion

Pull Out Strength (PO)

It has been already indicated that the bond strength increased initiallyand then reduced due to corrosion. It was felt that to develop a reliablerelationship between the voltage ratio and the bond strength, more datapoints would be essential (Fig. 13b).

Tensile Strength (TS (ft))

The tensile strength of the bar reduces monotonically with corrosion.Thus, straight line fit (Fig. 13c) has been attempted for calibrating the residualstrength with voltage ratio approx. given by,

ft ¼f0R0

R ð2Þ

where

ft¼ Tensile Strength at any instant (N=mm2)f0¼ Initial Tensile Strength (N=mm2)

Beams in OC Environment

Destructive Testing Results In OC beam specimens (Table 2) as the exposureincreased, the pullout strength continuously increased with the progress ofcorrosion. This was due to the formation of rust product having largevolumes in comparison to the surrounding steel bar resulting in increasingbond strengths. Throughout the slow oxide corrosion process, only surfacemodification caused by corrosion products formation takes place. Anincrease in pullout strength by 51% of the healthy beam was observed after28 days of exposure which increased by 2.6 times in 130 days of corrosion. Itis because of the increase in bond of the plain bar to the surrounding con-crete as compared to a healthy bar due to rust product formation in oxide

TABLE 2 Destructive Testing Results for OC Specimens

Period of exposure (days)

Property 0 6 12 18 28 130

Mass Loss (%) 0 0.1025 0.115 0.12 0.316 2Pull Out Strength (kN) 25 32 34 49 51.5 65Tensile Strength (N=mm2) 671 666.7 658.34 650.13 598.23 576.3


corrosion. For the same exposure time, in the beam undergoing CC, cor-rosion was large, widespread and extensive and the rust product was washedaway by chloride solution through the cracks results in release of pressureand decrease of bond. Hence, it experienced loss in PO.

Also, the extracted OC bar observed only nominal loss in mass andtensile strength with the increase in corrosion. In 28 days, the bar experi-enced only 0.316% and 10.88% loss of its mass and tensile strength respect-ively. In 130 days, % mass loss and tensile strength increased marginally to1.5% and 14.1%, respectively. The higher percentage of strength andresidual mass indicates that oxide corrosion does not lead to severe pittingand loss of metal. The visual inspection also confirmed that the bar hadnot experienced large and widespread pitting and area loss.

As seen from the destructive test results for different days of exposure forOC specimens, nominal loss in mass (M) and tensile strength (TS (ft)) isobserved with the increase in exposure to oxide environment. From the massloss results, it was seen clearly that the mass loss is marginal. This indicatesthat delamination corrosion that takes place at the surface of the bar does notlead to a significant mass loss. This was also supported by insignificant loss ofTS of the bar. The PO of the beam indicate that there has been an increasefrom 0th to 130th day continuously. The bars as indicated were plainextruded bars where the initial bond at the steel–concrete interface was onlythrough friction and there was no mechanical bond. Creation of corrosionproducts at the surface exerts an outward pressure on the concrete, thusimproving the bond.

Correlation of Ultrasonic Voltages with Destructive Tests A correlationbetween the ultrasonic peak-peak voltage ratios (R) was done to facilitatenon-destructive estimation of the physical condition of the bar. Similar toCC, L (0, 7) mode was mainly capable of detecting oxide corrosion through-out by mapping its output to the physical condition of the bar. In L (0, 1)mode, the voltage ratio increased initially as corrosion progresses with thedeterioration of the bar in the form of delamination. Then amplitude showsdrop after 50–60 days. Due to the variation in voltage trends with corrosion,L (0, 1) mode was not considered for calibration. With L (0, 7) mode, noinitial change for 14 days was observed in voltage ratios when debondingpredominates but after this period, the signal amplitude decreased with thedeterioration of the bar. Hence, L (0, 7) mode was used to relate the ultra-sonic signal voltages with the physical condition of the bar.

Figure 14 shows the comparison between ultrasonic signals anddestructive testing results of ML, TS, and PO. L (0, 7) mode shows amonotonically decreasing voltage ratio with mass loss (Fig. 14a). Thesame trends were seen with other non-destructive parameters of PO(Fig. 14b) and TS (Fig. 14c) as well. Thus, the calibration of the ultrasonicdata with the physical state of the bar has been attempted mainly with L(0, 7) mode.


Mass Loss (ML)

The signal strength of L (0, 1) mode initially increases and then startsfalling as corrosion progresses. Since the signal strength shows variationin trends with the degradation of the bar, L (0, 1) mode was not used forcalibration of OC. With L (0, 7) mode, as corrosion progressed, signalstrength falls causing loss in mass, and hence a linear relation between Rand % mass loss has been established given by (3), which is shown inFig. 14a:

Mass Loss MLtð%Þ ¼ MLmaxð1� 100R=R0Þ ð3Þ

FIGURE 14. Correlation of destructive OC test results with peak to peak ratio (R). (a) % Mass loss vspeak-peak voltage ratio(R); (b) Pullout load vs transmitted pulse amplitude; and (c) Tensile strength vstransmitted pulse amplitude. (Figure appears in color online.)


where

R0¼ Initial peak-peak voltage ratio of the transmitted pulse w.r.t. initialpulse

R¼ Peak-peak voltage ratio of the transmitted pulse w.r.t. initial pulse ata particular instant

MLt¼Mass loss (%) anticipated at a particular instantMLmax¼Maximum % mass loss anticipated for a particular extent=degree of

corrosion

Pull Out Strength (PO)

It has been already indicated that the bond strength increased as cor-rosion progressed with fall in signal strength in L (0, 7) mode. But theincrease in bond strength was very nominal and a scatter of data pointswas obtained. Hence, it was felt that to develop a reliable relationshipbetween the voltage ratio and the bond strength in OC as in case of CC, moredata points would be essential (Fig. 14b).

Tensile Strength (TS (ft))

The tensile strength of the bar reduces with corrosion indicated by fall instrength of L (0, 7) mode. A linear fit was attempted between R and ft in L(0, 7) mode as given in (4). A relationship of TS with L (0,1) mode isnot attempted since the signal shows deviation in trends with progress ofcorrosion:

Tensile Strength ftðN=mm2Þ ¼ f0R=R0 ð4Þ

where

ft¼ Tensile Strength at any instant (N=mm2)f0¼ Initial Tensile Strength (N=mm2)

Thus, it is confirmed that ultrasonics are capable of discerning the typeand mechanism of corrosion of bars in RC structures. Also calibration ofthe ultrasonic voltages with some parameters relating to the physicalcondition of the bar is attempted. It is worth mentioning that calibration ofultrasonic data is an initial attempt based on initial results obtained in limitedtests and they would require further confirmation and fine tuning.

CONCLUSIONS

Ultrasonic guided wave monitoring utilizing specific core and surfaceseeking modes successfully identifies the type, rate, and mechanism of


corrosion in a reinforcing bar in concrete subjected to different exposureconditions. In general, huge pitting and non-uniform area loss highlightedby severe signal attenuation marks chloride corrosion is well picked up bycore seeking mode. It begins with delamination shown by signal rise withsurface seeking mode. In oxide corrosion, the rate of corrosion is slow, loca-lized, and marked by slow bond deterioration as depicted by signal strengthrise in surface seeking mode. Pitting is insignificant as shown by very slowsignal fall in core seeking mode in OC. Thus, through a judicious selectionof ultrasonic modes different types of corrosion in RC structures can be suc-cessfully identified. Bars at different stages of corrosion were ultrasonicallymonitored in both oxide and chloride environments to explore the abilityof ultrasonics to predict the level of deterioration of the bars. It is done suc-cessfully by correlating ultrasonic voltage ratio with destructive parametersof mass loss, tensile strength and bond strength in the two common corrosionenvironments. Thus, a mapping between the physical condition of the barwith the voltage ratios is attempted in the form of algebraic equations. Theseshould facilitate nondestructive evaluation of reinforcements embedded inconcrete. However, the relationships presented here are based on limitedearly results and they should be subjected to scrutiny with more tests.Although the use of guided waves show promise and are effective in identify-ing the presence of corrosion in rebars in widely varying environments, themethod needs access to the ends of rebars. At site, bars that are most suscep-tible to corrosion need to be exposed at the ends to perform the test. Also thesignal-to-noise ratio should be above the ground noise level.

ACKNOWLEDGMENTS

The fund received from the Department of Science and Technology,Government of India, and All India Council of Technical Education is grate-fully acknowledged. Dr. Tribikram Kundu, Professor, Department of CivilEngineering and Engineering Mechanics, University of Arizona, Tucson,AZ, USA, visited the health monitoring laboratory in Thapar University onan NSF grant. The authors acknowledge his advice on the study.

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