Download - SPIE article Mangual-final version (2-16-2011)

8/7/2019 SPIE article Mangual-final version (2-16-2011)

1/13

Assessment of corrosion rate in prestressed concrete with acoustic

emission

Jes Mangual*a

, Mohamed K. ElBatanounya

, William Vleza

,Paul Ziehla, Fabio Mattaa, Miguel Gonzlezb

aUniversity of South Carolina, 300 Main Street, Columbia, SC, USA, 29208;bMistras Group Inc., 195 Clarksville Rd, Princeton Junction, NJ, USA, 08550

ABSTRACT

Acoustic Emission (AE) sensing was employed to assess the rate of corrosion of steel strands in small scale concrete

block specimens. The corrosion process was accelerated in a laboratory environment using a potentiostat to supply a

constant potential difference with a 3% NaCl solution as the electrolyte. The embedded prestressing steel strand served

as the anode, and a copper plate served as the cathode. Corrosion rate, half-cell potential measurements, and AE activity

were recorded continuously throughout each test and examined to assess the development of corrosion and its rate. Atthe end of each test the steel strands were cleaned and re-weighed to determine the mass loss and evaluate it vis--vis theAE data. The initiation and propagation phases of corrosion were correlated with the percentage mass loss of steel and

the acquired AE signals. Results indicate that AE monitoring may be a useful aid in the detection and differentiation of

the steel deterioration phases, and estimation of the locations of corroded areas.

Keywords: acoustic emission, corrosion, durability, prestressed concrete, reinforced concrete

1. INTRODUCTION

The corrosion of reinforcing steel is a major durability issue for prestressed and reinforced concrete structures in coastal

areas and where de-icing salts are regularly used. The environment in good quality concrete has high alkalinity due tothe presence of sodium, potassium, and calcium hydroxides developed during hydration. The concrete surrounding the

reinforcement acts as a physical barrier and the steel remains passivated. If the alkalinity level is reduced the steelreinforcement becomes susceptible to corrosion.

After the formation of corrosion byproducts in steel reinforcement, the product first accumulates at the bar surface and

tries to fill the closest voids. They then spread throughout the material and mix with the hydrated products of cement

stressing the concrete cover until it relaxes by the formation of cracks. Depending on the degree of hydration and thetype of oxide this corrosive product will have a much higher volume than the original metal. The unit volume of the final

corrosion product Fe(OH)33H2O may be as large as six times the original Fe volume1. This oxide layer will exert stress

to the concrete surrounding the reinforcement which will produce cracks along the length of the steel. The process

ultimately reduces the strength of the bond and may affect the load bearing capacity of the components undergoing

distress, resulting in a decrease of service life2.

1.1 Research significance

Corrosion of reinforcing steel is the most common source of distress in concrete bridges near the marine environment or

components in contact with de-icing salts. Corrosion decreases the cross sectional area of steel strands minimizing their

ductility and increasing stress concentrations at the reinforcement interface3. Billions of dollars are attributed annually

toward repair of corroded structures. The presence of hairline cracks can be detrimental to the structure due to the

available space to accommodate the oxides without expanding the surrounding concrete4. It is therefore important to

seek monitoring methods that can effectively assess corrosion in reinforced concrete members. Acoustic Emission may

be a promising method due to its extreme sensitivity to crack growth caused by expansion of ferrous products 5. In this

paper AE is compared with other corrosion assessment methods.


2/13

*[email protected] ; phone 1 787 246-6158

2. EXPERIMENTAL DETAILS

2.1 Test specimens

Four specimens were tested under accelerated corrosion. Two specimens were notched and pre-cracked at midsection

and two specimens were un-cracked. Concrete specimens with dimensions 4.5 4.5 20 inches (114 114 508 mm)and an embedded 0.5 in. (13 mm) diameter grade 270 low relaxation prestressing strand were cast using concrete having

28-day compressive strength of 6 ksi (41.4 MPa), w/c ratio of 0.4, and No. 57 coarse aggregate. The specimens were air-

cured for 28 days and then removed from the molds. The notched specimens (denoted as Type A) were then loaded in

bending to produce a crack that extended throughout the entire midspan cross-section. Figure 1 shows a sketch of Type

A specimens, which were used to simulate the effect of hairline cracks in prestressed concrete. The remaining un-

cracked specimens (Type B) were used to study corrosion behavior in un-cracked concrete.

0.5 in. diameter

lo-lax 7-wire strand

20.0

5.0

Midspan notch and crack

(Type A specimens only)

Figure 1. Schematic of Type A concrete specimen with embedded steel strand. Dimensions in inches (1.0 in. =

25.4 mm)

In addition to the four test specimens described above, two control specimens were monitored without the impression of

current to assess background noise. Test durations were selected such that the total amount of weight loss did not exceed

2% of the original weight of the steel strand. Previous investigations have noted that after achieving a 3% weight loss ofsteel the remaining load capacity decreased as the percentage of weight loss increased6.

2.2 Accelerated corrosion test setup

The accelerated corrosion setup, illustrated in Figure 2, consists of electrochemical cells developed by placing each

specimen in a plastic vessel with a 3% NaCl water solution, and electrically connecting a copper plate with the steel

strand. The copper plate, placed beneath the concrete specimen supported by neoprene pads, serves as the cathode. The

NaCl solution rests 0.25 in. (7 mm) below the level of the strand. The faces of the concrete are covered with a corrosiveresistant layer to avoid edge effects. Test durations were 2 and 6 days; one Type A and one Type B specimen was tested

for each duration. Specimens A-01 and B-01 correspond to a 2 day potential supply while specimens A-02 and B-02

correspond to a 6 day potential supply.

+

Resistor

AE/ arametric data ac uisition

Power supplyDigital Volt Meter Cu/CuSO4reference electrode

AE sensor

3% NaCl water solution

Figure 2. Setup for accelerated corrosion test (crack present for Type A specimens only).
mailto:*[email protected]:*[email protected]:*[email protected]:*[email protected]


3/13

The dissimilar metals in contact and sharing the diffused chloride create a corrosion cell which accelerates the transfer of

electrons by controlling the potential difference via the rectifiers. The degree of corrosion activity in the cell is affected

by the current that flows between the dissimilar metals. For corrosion propagation, a direct external current is supplied to

the specimens through a rectifier or potentiostat. The anode and cathode are connected to the terminals of the rectifier

and a potential is impressed between them. The current flowing through the system is monitored continuously via aresistor.

Two main stages are of interest for the characterization of acoustic emission; a) corrosion initiation and b) corrosionpropagation. Initiation culminates after the chloride ions permeate the concrete cover and accumulate in the surroundings

of the reinforcement, thereby breaking down the passivity of the rebar. Propagation is defined as the process for which

the rate of corrosion is accelerated and delamination of the concrete cover occurs.

The outcome of the accelerated corrosion test is a percentage weight loss of the reinforcement as a function of time as

stipulated by Faradays law. The amount of corrosion is related to the electrical energy consumed, which is a function of

voltage, amperage, and time interval. It is of interest to relate the rate of acoustic emission activity with the corrosion

rate as determined by the rate of electrical energy consumed.

2.3 Time variant corrosion process

A phenomenological model to represent the corrosion loss of steel in a chloride environment has been previously

proposed7, 8 and a modified version is shown in Figure 3. It consists of two transition periods that are defined as the onset

of corrosion and the nucleation of cracking, represented by Phase I and III. In Phase I the corrosion process is controlled

by the rate of oxygen transport which is eventually inhibited after the chloride ions permeate the concrete cover and

accumulate in the surroundings of the steel reinforcement, thereby breaking down the passivity of the rebar. As the rate

of oxygen is inhibited, the rate of corrosion reaction ceases in Phase II (dormant). In Phase III, the corrosion process is

accelerated as cracks begin to form and delamination of the concrete cover begins.

Figure 3. Phenomenological model showing phases of corrosion for steel immersed in seawater8.

3. TECHNIQUES USED FOR ASSESSMENT OF CORROSION

3.1 Half-cell potential

A copper-copper sulfate (Cu/CuSO4) reference electrode was selected for half-cell potential measurements. The

methodology consists of comparing the corrosion potential, Ecorr, of the steel with a reference potential and estimating

the probability of corrosion based on the potential reading. The reference electrode is connected to the negative terminal

and the reinforcement is connected to the positive terminal of the voltmeter. This establishes a potential difference whichoffers key information related to structural decay. For the tests described, the reference electrode was placed on top of

the concrete surface near the midsection until the measured values did not fluctuate with time. It is an accepted practice

to use the following values as thresholds9:

if potentials over an area are more positive than -200 mVCSE (half-cell potential measurements in reference

to copper sulfate electrode) then there is 90% probability no corrosion is occurring;


4/13

between -200 mVCSE and -350 mVCSE there is uncertainty as to the corrosion state although breaking of the

passivity may be possible; and

more negative than -350 mVCSE indicates a 90% probability that corrosion of the reinforcement is occurringin that area.

3.2 Polarization and corrosion rate

The potentiostat three-electrode principle consists of an electrical circuit that functions by maintaining the potential ofthe working electrode at a constant level with respect to the reference electrode and adjusting the current at a counterelectrode. These are the steel-strand, copper plate and copper-copper sulfate probe connected to the respective working,

counter, and reference terminals of the potentiostat. The Linear Polarization Resistance (LPR) sweep is performed by

employing the potentiostat and consists of applying a small scanning potential, in the range of 20mV with respect to

Ecorr of the steel, and measuring the resulting current10. The objective of the technique is to obtain the corrosion rate of

the steel after calculations. It is crucial to select the proper reference electrode, polarization area, and equivalent weight

for the anodic site. The small scanning range, typically 0.166 mV/s, is selected to not disturb the natural corrosion

process of the anode. The polarization resistance is the ratio of the applied potential and the resulting current level

(slope of an Ecorricorrcurve). The measured resistance, Rp, is inversely related to the corrosion current, icorr, as shown in

Equation 1:

( )2.303

a c

p

corr a c

b bER

i i b b

= =

+

(1)

where:

Rp = polarization resistance, -cm2

E= change in applied potential relative to corrosion potential Ecorr, mV

i =current response to applied potential spectrum, mA

icorr = corrosion current, A/cm2

ba, bc = anodic and cathodic Tafel slopes, mV

The corrosion current, icorr, is calculated from Equation 1 and is normalized by the area polarized, typically 1 cm2.

Corrosion rate may be calculated from Equation (2). A plot of LPR sweep and Tafel curve for steel under accelerated

corrosion is shown in Figure 4.0.13

corri EW

CRd

= (2)

where:

CR = corrosion rate, milli-inch per year (mpy)

EW= equivalent weight of iron, 27.92 g

d= density of iron, 7.8 g/cm3


5/13

0.000001 0.00001 0.0001 0.001

Current , Log(A)

-0.54

-0.56

-0.58

-0.0006 -0.0003 0 0.0003

Current (A)

-0.54

-0.56

-0.58

Figure 4. Corrosion rate determination using LPR and Tafel.

For the tests described, corrosion potential data of the reinforcing strand was acquired using a potentiostat and associated

software.

3.3 Acoustic emission monitoring

The final method discussed for the assessment of corrosion is acoustic emission. AE sensors can be described as a

piezoelectric crystal that has been suitably packaged for the detection of transient stress waves arising from the rapid

release of energy within a material. A more complete description and related terminology can be found in ASTM

E131611.

Testing was performed by applying anodic current to the prestressing strands via the rectifiers with AE monitored

continuously. Sensors with a 60 kHz resonance and integral 40 dB preamplification were used. Figure 5 is a sketch of

the triangular AE sensor layout for location of activity zones. Multiple sensors can be used to locate the areaof AE activity by triangulation or the activity of a single sensor can be used to provide anindication of the general area of damage12. The AE method has potential for detecting corrosion in reinforcedconcrete structures due to its extreme sensitivity for detection of stress waves caused by crack growth and crack

interference (friction). Crack growth occurs when corrosion products forming on the steel expand the surrounding

concrete13 and crack interference may occur during loading.

The surface of each concrete specimen was cleaned before testing. To provide better coupling quality the smooth part of

the specimen was used for sensor placement. Because the surface of the concrete is rough epoxy was employed to mount

each acoustic sensor.

To measure the fluctuation of passing current as depassivation of the steel occurs, a 10-ohm resistor was connected

between the rectifier and the anode. By solving Ohms law, the current being supplied through the system was obtained.

The resistor was connected to the parametric input of the AE data acquisition system. Acoustic emission featuresrecorded throughout testing included amplitude, absolute energy, signal strength, counts, duration, and hits.

1.0

1.02.0 AE sensor

Figure 5. AE sensor layout on surface specimen (Type B specimen shown). Dimensions in inches (1.0 in. = 25.4mm).


6/13

4. RESULTS AND DISCUSSION

4.1 Corrosion detection

As the corrosion process advances, an increase in AE activity was observed in all specimens just a few hours after

supplying the external potential. Half-cell potential measurements shifted toward a more negative value as the passive

film surrounding the strand was broken. This process is shown in Figure 6, where the cumulative signal strength and

passing current are plotted with respect to the test duration for specimen A-01.During the initial stage of the test, the passing current decreases because the concrete surrounding the steelreinforcement provides resistance and the amount of chloride solution permeating the concrete and reaching the strand is

low. As the passive steel comes in contact with the electrolyte, the resistance is reduced and an increase in passing

current is observed. During this phase, the oxygen transport controls the reaction rate but corrosion will later decrease as

the chloride ions accumulate in the surroundings of the reinforcement and depassivate the steel. The AE signal may be

attributable to microcracking of the concrete as the corrosion product builds up around the reinforcing steel14. The slope

for the onset of corrosion, and for the dormant phase, is similar in both curves. As corrosion progresses, the slopes are

steeper and decrease as the oxygen transport is reduced in Phase II.

2.0E+80.20Figure 6. Signal strength and passing current vs. time for specimen A-01 (2 day test).

Figure 7 shows cumulative signal strength versus time for specimens Type A for the first 48 hours of testing. A steady

increase in signal strength may be observed from the figure and could be attributed to the onset of corrosion. SpecimenA-01 was tested for two days with a potentiostat, whereas specimen A-02 was tested for six days with a DC switching

rectifier. The specimens tested with the rectifier showed slower initiation times than specimens that were continuously

tested with the potentiostat; in addition, higher currents were obtained from the potentiostat-controlled specimens when

compared to rectifier-controlled. The dormant phase of the corrosion process is also noticeable with the transitional

period occurring at approximately 22 hours into testing.

Time (hours)


7/13

A-01

A-03

Time (hr)

0 14 28 42

0.05

0.10

0.15

0.20

0

Figure 7. AE cumulative signal strength vs. time for Type-A specimens.

Steel depassivation is also reflected by a decrease in half-cell potential values. Before placing the specimen in the

electrolyte, the half-cell potential for specimen A-02 was -107 mV, indicating that the steel is in the passive state. Figure

8 shows the cumulative signal strength and half-cell potential for specimen A-02. At five hours into testing, the potential

shifted to the active state reaching -493 mV. AE signals showed a more sensitive response towards detecting thepassivity breakdown when compared to half-cell potential measurements becoming more negative than -350 mV vs.

copper sulfate electrode.

Figure 8 shows a plot of the cumulated signal strength and half-cell potential measurements versus test duration. The

initial period of high AE activity (zero to 28 hours) suggests the onset of corrosion in the prestressing strand,

corresponding to the first phase in the phenomenological model7,8, this is confirmed with decreasing values of half-cell

potential and an increase in corrosion rate. Later (28 to 112 hours), the rate of steel mass loss is reduced and AE activitydecreases as oxygen transport is inhibited due to chloride presence. The second period of high AE activity (beyond 112

hours) is likely to be associated with concrete cracking due to stress exerted by the corrosion product expansion. This

process is coupled with decreasing values of the half-cell potential.

-550

-500

-450

-400

-350

-300

0.0E+0

5.0E+7

1.0E+8

1.5E+8

2.0E+8

0 200000 400000

Cumulative Signal Strength

Half-cell Potential

Time (hr)

0.20

0.15

0.10

0.05

0CumulativeSignalStrength(

mVs

)

Half

-CellPotential(mV)

0 28 56 84 112 140

Figure 8. Signal strength and half-cell potential vs. time for specimen A-02.

Corrosion rate values are only useful after the steel depassivates, performing LPR sweeps on passive steel will provide

erroneous results. In Figure 9 cumulative signal strength versus time is compared to corrosion rate versus time during a 2

day test. During the onset of corrosion, the corrosion rate and slope of the cumulative signal strength curve increase. A

maximum corrosion rate is obtained after culminating Phase I, and as the process approaches the dormant phase, both

the slope of the cumulative signal strength curve and corrosion rate gradually decrease.

Time (hours)

A-02

Time (hours)

Time (hours)


8/13

0

75

150

225

300

375

450

0.0E+0

4.0E+7

8.0E+7

1.2E+8

1.6E+8

Signa l Strength

Corrosion Ra te

CorrosionRate(

mpy

)

0 14 28 42

0.16

0.12

0.08

0.04

0CumulativeSignalStrength(

mVs

Time (hr)

Figure 9. Signal strength and corrosion rate behavior vs. time (specimen A-01).

Location of corroded areas was achieved through triangulation of the AE signals using the source location capability of

the software; wave speeds were calculated by performing pencil lead breaks on the surface of the concrete

andcalculating the travel time between sensors. Figure 10 shows the location of events at 25, 50, 100, and 125 hours of

testing for specimen B-02 (un-cracked specimen). Each graph, labeled (a) through (d), represents the coordinate systemfor the specimen. The bold dots represent the three sensors in the triangular configuration shown in Figure 5; the lighter

dots represent located AE events. The located events indicate that the corrosion process is distributed throughout the un-

cracked specimens.

4.2 Effect of concrete cracks on rate of corrosion

Corrosion rate and half-cell potential for specimen B-02 (un-cracked specimen) are plotted in Figure 11. In the first few

days of testing half-cell potential values were overly positive indicating that a remainder of the passive film was still

present, therefore, corrosion rates with respect to Ecorr yielded no data. Specimen B-01 (un-cracked), which was testedfor 2 days, showed positive values throughout the test and began to decrease at the end of the test. Linear polarization

measurements for specimen B-02 were achievable on the third day of testing, in which half-cell potential measurements

shifted to more negative values as a result of chloride ingress. The relationship between corrosion rate and half-cell

potential can be seen; where an increase in corrosion rate will be reflected in a more negative half-cell potential value.

Time (hours)


9/13

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

0 5 10 15 20 0 5 10 15 20

0 5 10 15 20 0 5 10 15 20

(a) (b)

(c) (d)

Y position vs. X position

Figure 10. Locating active zones for specimen B-02 at: (a) 25 hrs, (b) 50 hrs, (c) 100 hrs and (d) 125 hours into

testing. Dimensions in inches (1 in. = 25.4 mm).

-560

-555

-550

-545

-540

-535

0

50

100

0 50 100 150

-

Series2

Time (hr)

orros on a e

Half-Cell Potential

Figure 11. Corrosion rate and half-cell potential for specimen B-02.

The results from Figure 11 may be compared to the half-cell potential values displayed in Figure 12, where both types of

specimens (cracked and un-cracked) are compared.

Figure 12 (a) shows half-cell potential readings for Type A specimen (cracked). Free chloride passage quickly lowers thepotential of the steel anode and maximizes the amount of current supplied. Figure 12 (b) shows the slow shift in potential

towards the negative region for Type B specimens (un-cracked). It may be concluded that the half-cell potential for un-

cracked specimens remains positive at least 2 days into testing, depending on the concrete cover width and supplied

current density. When compared to Type B (un-cracked) specimens, the corrosion rates of all crack-induced specimens

were above the active threshold.

Time (hours)


10/13

0 50 100 150Half

-CellPotential(mV)

Time (hr)

-700

-500

-300

-100A-01

A-03

Time (hr)

-700

-400

-100

200

500

0 50 100 150

B-01

B-03

Half

-CellPotential(mV)

Figure 12. Half-cell potential values for: (a) Type A (cracked), and (b) Type B (un-cracked) specimens.

Comparing the magnitudes of cumulative signal strength indicated that a higher magnitude of cumulative signal strengthwas observed for un-cracked specimens. This is shown graphically in Figure 13 by plotting the cumulative signal

strength for specimen A-02 and B-02 where the magnitudes of the vertical axes differ. Cracked specimen A-02 quickly

diffuses the chloride and begins the initiation phase. Specimen B-02 slowly begins the initiation process and maintains

an increasing slope until the conclusion of the test. Type A specimens present lower magnitude partially due to the effect

of crack presence on the reflection of the AE signal. This effect was confirmed by performing pencil lead breaks onboth sides of the midsection crack.

0

0.05

0.1

0.15

0.2

0.25

0

6

12

18

24

30

0 50 100 150

Cum.inalstrenthA

-03

um.sgnastrengt

-

Time (hr)

B-03

A-03

Time (hr)

Figure 13. Cumulative signal strength versus time for cracked and un-cracked specimens (zero to 6 days).

4.3 Steel mass loss model

After testing, each specimen was broken apart and the steel strand was removed, cleaned and re-weighed as stated by

ASTM G115, 16. Inspection of the corroded prestressing strand depicted higher surface loss on the side closest to theelectrolyte solution. The experimental mass loss calculations presented in Table 1 were achieved by obtaining the

percentage mass reduction in the steel embedded inside the concrete. As stated by Faraday, the amount of mass loss is

dependent upon time, the amount of current being charged, and the number of electrons transferred. Although Faradays

law realistically portrays the dissolution of steel in an acidic solution it proves difficult to estimate the amount of

reinforcement steel mass loss. This arises by the fact that the phenomenological model governs the corrosion process of

steel embedded in concrete surrounded by a thick cover, and Faradays formula linearly depicts the dissolution of iron inan acid media. Also, the level of the steel strand was kept above the solution level rather than immersed.

For the application of a variable current under potential control, the trapezoidal rule was employed to integrate the

current density as a function of test duration, Equation 3;

Mass loss =Fz

tiM

(3)

Time (hours) Time (hours)

Time (hours)

A-01

A-02

B-01

B-02

B-02

A-02


11/13

where:

Mass loss of embedded steel, g

M= molecular weight of iron, 54.8 g/mol

i = passing current, A

t= test duration, sz= electrons transferred, 2

F= Faradays constant, 96,487 C/mol

Table 1 Mass loss measurements and Faradays theoretical model.

Specimen no. Original mass (g) Mass loss (%) Mass loss (%) [Eq. 3]

A-01 388.6 1.12 2.76

A-02 392.3 0.90 1.57

B-01 392.8 0.15 0.42

B-02 393.4 0.73 2.92

Figure 14 (a, b) shows the corroded steel strands for cracked specimens A-01 and A-02, respectively. Although

specimen A-01 was continuously tested with the potentiostat during a 2 day period, the amount of shed steel was greater

than specimen A-02. When compared, specimen A-01 lost 1.12% of its total initial mass under an average passing

current of 230 mA; whereas specimen A-02 lost 0.90% of the initial mass during a 6-day period and a 40 mA average

current controlled by a rectifier. Therefore, the amount of passing current compensates for the short test duration ofspecimen A-01. The accuracy and energy capabilities of the potentiostat influence the results and are also seen in the AE

data; specimens under potentiostat-control show an increase in cumulative signal strength earlier than rectifier-controlled

specimens and the current density imparted is higher in these specimens.

(a) (b)

1 inch 1 inch

Figure 14. Corroded steel strand after finalizing test for (a) specimen A-01 and (b) specimen A-02.

4.4 Summary

The study described provides insight into the capabilities of Acoustic Emission (AE) monitoring for detection of damagedue to corrosion in reinforced concrete elements. Small scale concrete specimens were cast with an embedded 0.5 in. (13

mm) diameter prestressing strand. Two specimens were pre-cracked at midspan and two were un-cracked. The

corrosion process was accelerated in a laboratory environment by applying a potential to the strand. Passing current,half-cell potential, corrosion rate, and AE data were compared to assess the potential of AE monitoring for detecting the

onset of corrosion and its progression. Acoustic emission proved to be reliable in detecting the initiation phase of un-

cracked concrete, while half-cell and corrosion rate data does not show this trend until the steel is fully depassivated.

Signal strength for the un-cracked specimens increased at a much lower rate than for the cracked specimens. The


12/13

presence of a crack permits chloride diffusion, thereby decreasing time to depassivation and causing localized damage.

The magnitude in terms of cumulative signal strength in the un-cracked specimens was much higher than for the cracked

specimens. It is hypothesized that steel expansion leading to the formation of cracks at the concrete bond for un-cracked

specimens may be more energetic due to absence of the pre-crack to liberate this pressure.

5. CONCLUSIONS

The following conclusions are drawn:

Progression of corrosion is a faster process in Type A (cracked) specimens because chloride is diffused more

readily, lowering the potential of steel, whereas a decrease in half-cell potential is not noticeable in Type B (un-

cracked) specimens until day 3. Even though the electrochemical monitoring of the Type B specimens yielded

no reasonable results in the first days of testing, AE displayed a slow growing signal.

Analysis of the passing current and the corrosion rates obtained by performing linear polarization generally

agree with the shape of the cumulative signal strength graph. The passing current and corrosion rate values in

laboratory conditions will fluctuate depending on the passivity of the steel, the amount of chloride ingress, and

debonding due to corrosion cracks at the steel-concrete interface.

Uniform concrete reduces the amount of current which may be applied, delaying the detection of corrosion with

conventional methods. Therefore, half-cell potential data and corrosion rate do not give indications while thesteel is still passive.

Increased AE activity with higher energy was observed in Type B specimens, believed to be associated with the

absence of an external crack to release the expansive pressure.

When comparing the experimental mass loss for Type A (cracked) specimens, it may be concluded that the 6

day test fully encompasses Phases I and II of the model, an increase in AE hits and a dormant phase beingclearly visible. The cumulative signal strength obtained from Type B (un-cracked) specimens show that, at the

end of the 6-day test, the corrosion process approached the dormant phase of the model.

Monitoring concrete members containing prestressing steel with AE, by means of an accelerated corrosion cell,

yielded pertinent information for the identification of pitting corrosion initiation and progression. The presence

of corrosion activity was associated with AE data and represented through AE features. The position of the

activity was determined using triangulation.

REFERENCES

[1] Li, Z., Zdunek, A., Landis, E., and Shah, S., Application of Acoustic Emission Technique to Detection of

Reinforcing Steel Corrosion in Concrete, ACI Materials Journal, 95(1), 68-76 (1998).

[2] Jaffer, S., and Hansson, C., Chloride-Induced Corrosion Products of Steel in Cracked-Concrete Subjected to

Different Loading Conditions, Cement and Concrete Research, 39(2), 116-125 (2009).

[3] Yoon, D., Weiss, W., and Shah, S., Assessing Damage in Corroded Reinforced Concrete Using Acoustic

Emission, Journal of Engineering Mechanics, 126 ( 3), 273-283 (2000).

[4] Alonso, C., Andrade, C., Rodriguez, J., and Diez, J., Factors Controlling Cracking of Concrete Affected by

Reinforcement Corrosion," Materials and Structures, 31(7), 435-441 (1998).

[5] ElBatanouny, M. K., Mangual, J., Ziehl, P., and Matta, F., Detecting Corrosion of Steel Prestressing StrandsUsing Acoustic Emission, Conference of the Prognostics and Health Management Society (PHM), (2010).

[6] Auyeung, Y., Balaguru, P., and Chung, L., Bond Behavior of Corroded Reinforcement Bars, ACI Materials

Journal, 97(2), 212-220 (2000).

[7] Ohtsu, M., and Tomoda, Y., Phenomenological Model of Corrosion Process in Reinforced Concrete Identified

by Acoustic Emission, ACI Materials Journal, 105(2), 194-199 (2008).

[8] Melchers, R., and Li, C., Phenomenological Modeling of Reinforcement Corrosion in Marine Environments,

ACI Materials Journal, 103 (1), 25-32 (2006).


13/13

[9] ASTM C876, "Standard test method for half-cell potentials of uncoated reinforcing steel in concrete", American

Standard for Testing and Materials, 1-6 (1991-Reapproved 1999).

[10] ASTM G59, "Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements,"

American Standard for Testing and Materials, 1-4 (1997-Reapproved 2003).

[11] ASTM E1316, "Standard Terminology for Nondestructive Examinations," American Standard for Testing and

Materials, 1-33 (2006).

[12] Ridge, A., and Ziehl, P., Evaluation of Strengthened Reinforced Concrete Beams: Cyclic Load Test and AcousticEmission Methods, ACI Structural Journal, Technical paper 103-S84, 832-841 (2006).

[13] Seah, K., Lim, K., Chew, S., and Teoh, S., The Correlation of Acoustic Emission with Rate of Corrosion,

Corrosion Science, 34(10), 1707-1713 (1993).

[14] Zdunek, A., and Prine, D., "Early Detection of Steel rebar Corrosion by Acoustic Emission Monitoring",

Corrosion, Paper no. 547, ITI technical report no. 16, 1-9 (1995).

[15] ASTM G1, "Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens, American

Standard for Testing and Materials, 1-9 (2003).

[16] Austin, S., Lyons, R., and Ing, M., Electrochemical Behavior of Steel-Reinforced Concrete During Accelerated

Corrosion Testing, Corrosion, 60(2), 203-212 (2004).

ACKNOWLEDGEMENTS

This work is performed under the support of the U.S. Department of Commerce, National Institute of Standards and

Technology, Technology Innovation Program, Cooperative Agreement Number 70NANB9H9007.
http://webstore.ansi.org/RecordDetail.aspx?sku=ASTM+G59-97(2009)http://webstore.ansi.org/RecordDetail.aspx?sku=ASTM+G59-97(2009)http://webstore.ansi.org/RecordDetail.aspx?sku=ASTM+G59-97(2009)http://webstore.ansi.org/RecordDetail.aspx?sku=ASTM+G59-97(2009)http://webstore.ansi.org/RecordDetail.aspx?sku=ASTM+G59-97(2009)

Download - SPIE article Mangual-final version (2-16-2011)

Top Related