research article an experimental study for...

Research ArticleAn Experimental Study for Quantitative Estimation of RebarCorrosion in Concrete Using Ground Penetrating Radar

Md Istiaque Hasan and Nur Yazdani

Department of Civil Engineering, University of Texas at Arlington, P.O. Box 19308, Arlington, TX 76019, USA

Correspondence should be addressed to Md Istiaque Hasan; [email protected]

Received 5 November 2015; Accepted 24 December 2015

Academic Editor: Claudio Mazzotti

Copyright © 2016 M. I. Hasan and N. Yazdani. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Corrosion of steel rebar in reinforced concrete is one the most important durability issues in the service life of a structure. In thispaper, an investigation is conducted to find out the relationship between the amount of reinforced concrete corrosion and GPRmaximum positive amplitude. Accelerated corrosion was simulated in the lab by impressing direct current into steel rebar thatwas submerged in a 5% salt water solution. The amount of corrosion was varied in the rebars with different levels of mass lossranging from 0% to 45%. The corroded rebars were then placed into three different oil emulsion tanks having different dielectricproperties similar to concrete. The maximum amplitudes from the corroded bars were recorded. A linear relationship between themaximum positive amplitudes and the amount of corrosion in terms of percentage loss of area was observed. It was proposed thatthe relationship between the GPR maximum amplitude and the amount of corrosion can be used as a basis of a NDE technique ofquantitative estimation of corrosion.

1. Introduction

Ground Penetrating Radar (GPR) has been widely used inseveral types of nondestructive evaluation of concrete, such asdetecting and locating rebar, finding concrete thickness, anddetecting voids and cracks. GPR has also been usedwith greatsuccess to map deterioration in bridge decks [1–3]. Rebarcorrosion is one of the main reasons for the deterioration ofreinforced concrete. During corrosion, the resulting productsaccumulate over the anodic steel rebar. If there is crack orthe concrete is very porous, the corrosion material dissipatesaround the rebar. So, the property of concrete around therebar changes and the effective diameter of the rebar isreduced. When concrete is very dense around the rebar, thecorrosion product accumulates around the rebar but it cannotdisperse around the rebar into the concrete. The reason isthat high strength concrete is very dense; therefore, the per-meability is very low. Because the corrosion material cannotbe dispersed around the rebar into the aggregate, it stays onthe surface of the rebar. This cumulative accumulation ofcorrosion product creates a high pressure. This pressure is

higher in high strength concrete than low strength concrete.So high strength concrete cracks earlier than low strengthconcrete. ASTM C876 Half Cell Potential is a widely usedmethod for a qualitative estimation of corrosion activity [4].In the past, GPR data was used to predict the deteriorationdue to rebar corrosion [5]. Experimental methods involvingGPR to detect the presence of rebar corrosion have also beenreported, where an accelerated corrosion test was performedon a rebar embedded in concrete in a laboratory environment[6]. The response of GPR waves at three different levels ofcorrosion, NaCl contamination of concrete, depassivationof rebar, and starting of rebar corrosion, was monitored.Another study was performed on accelerated corrosionwhere the peak to peak amplitudes of the direct wave (DW)and the reflected wave (RW) were taken as GPR parameters[7]. These existing studies on GPR use for rebar corrosioninvestigation addressed either detection or qualitative eval-uation of corrosion. A quantitative nondestructive approachto estimate the rebar corrosion in existing concrete will bevery useful, especially in forensic investigation. This willallow the determination of the extent of rebar corrosion in

Hindawi Publishing CorporationJournal of EngineeringVolume 2016, Article ID 8536850, 8 pageshttp://dx.doi.org/10.1155/2016/8536850

2 Journal of Engineering

existing concrete structures, thereby estimating the impactof the reduced rebar sizes on member strength, ductil-ity, and serviceability. If needed and feasible, rehabilitationand strengthening of the deteriorated structures could beundertaken. GPR can nondestructively scan existing concretestructures, especially flat extended surfaces (such as slabs,decks, and pavements), at a very fast rate. Coupling thisfast scanning method with a quantitative rebar corrosiondetection technique will be a groundbreaking effort ofsignificant benefit to various relevant parties. The existingresearch on corrosion monitoring with GPR did not addressthe effect of changing electromagnetic property of concreteat different level of corrosion. The change is that concreteproperty at different corrosion level is a result of changingpermittivity and conductivity of the concrete due to thepresence of the corrosion agents and corrosion productsin the microstructure of concrete surrounding the rebar.In the study reported herein, an approach of simulatingcorroded steel rebars in concrete in an experimental settingwas undertaken. From the GPR response of the laboratorysimulated corroded steel rebars, a basis to estimate thequantitative amount of rebar corrosion was developed. Theexisting methods discussed above focused on determiningthe qualitative stages of corrosion. In the study, it is shownthat the physical loss of rebar area due to corrosion can bequantitatively correlated with the changes of GPR response.

2. Experimental Setup

A corrosion tank was prepared to perform the acceleratedcorrosion of three rebars 16mm in diameter. The tank wasfilledwith 5% sodium chloride solution,made of regular tablesalt and tapwater.The 300mm long rebarswere submerged inthe solution to act as anode of an electrochemical cell. A fewextra rebars were also submerged in the salt water solution toact as a cathode. The three rebars were connected in series,ensuring that the first of the three rebars would attract themost electrical current. This was expected to cause mostamount of mass loss in the first rebar. The other two rebars,as they were connected in the series connection, wouldreceive less electrical current, resulting in lesser amounts ofcorrosion. This approach resulted in the varying amounts ofcorrosion in the three rebars as needed for the experiment.A 15-volt DC current source was used to supply electricalcurrent to the electrochemical cell through the anodes andthe cathodes. The experimental setup of the acceleratedcorrosion of the rebars is shown in Figure 1. The resultingcorrosion products can be seen floating on the salt watersolution.

After three days of continuous electrical current throughthe electrochemical cell, the process was stopped and theanode rebars were taken out of the solution and thoroughlycleaned. The rebars were weighed to determine the amountof weight loss due to corrosion. Some parts of the rebarswere outside of the solution tank and did not experience anycorrosion, and theywere excluded in the calculation ofweightloss. It was assumed that the weight loss due to corrosionoccurred uniformly along the corroded length of the rebar.Figure 2 shows the three rebars with varying amounts of

Figure 1: Tank for accelerated corrosion.

Figure 2: Rebars with different degrees of corrosion.

corrosion. An uncorroded rebar is shown on the extremeleft for comparison. Table 1 presents weight losses for eachrebar and the associated rebar cross-sectional area losses.Thecorrosion loss for rebar areas ranged between 0 and 45%.Active corrosion can reduce the rebar diameter by as much as0.0039 in (0.1mm) per year [8]. Therefore, the range of rebarcorrosions studied herein is practical for concrete structures.

3. Underlying Theory and Assumptions

In the electrochemical corrosion process of embedded steelrebars in concrete, the iron ions from the rebar are convertedto iron oxide or rust through the presence of chloride ions,which is a very strong oxidizing agent. These corrosionproducts accumulate around the rebar and the effectivecore area of the rebar gets smaller as the corrosion processcontinues. The corrosion products contaminate the concretein the vicinity of the rebar. This contamination increases thedielectric permittivity as well as the electrical conductivityof concrete. During the GPR scanning of a corroded rebar,the GPR electromagnetic wave travels through the rustcontaminated concrete towards the rebar. But the radarwave has to penetrate through the corrosion product to hitthe surface of the noncorroded core of the rebar and thenreflected back towards the receiver of the GPR antenna.The dielectric constant of iron oxide is higher than that ofconcrete [9]. Steel is a very good conductor and almost totally

Journal of Engineering 3

Table 1: Weight and area loss in corroded rebars.

Rebar # Initial weight (g) Weight aftercorrosion (g) Weight loss (g) Length of corroded

part of rebar (mm)Average area loss,

%1 472 472 0 0 02 472 394 78 175 223 472 363 109 150 314 472 310 162 150 45

Antenna

Return waveDirect wave

Non-corroded core of rebar Corrosion

product (Iron oxides)

Figure 3: GPR scanning of corroded rebar in concrete.

reflects the incident GPR wave. The power of the reflectionwave from the interface of two different materials dependson the contrast of the dielectric constants of the two media[10]. If the contrast is high, most of the incident wave getsreflected and a small part of the wave goes through theinterface. If the dielectric contrast is low, a small part of theincident wave gets reflected andmost of it travels through theinterface into the second medium. In this study we assumethat the concrete is very severely corroded and the corrosionproducts got dispersed around the rebar through cracks andinterstitial pores. The dielectric constant of iron oxide is 14[9] which is a little higher than the dielectric constant ofconcrete. This close difference indicates that a radar wavecan penetrate through the corrosion contaminated concreteand the rust and eventually reflect back from the surface ofthe noncorroded rebar core. Therefore, it is assumed thatGPR wave can travel through the corrosion products. Thechanged environment in the vicinity of a corroded rebarcan be monitored with a GPR. The schematic diagram ofGPR scanning of a corroded rebar in concrete is shownin Figure 3. The dielectric constant of the concrete betweenthe GPR antenna and the noncorroded core of the rebarincreases due to the contamination of the concrete by externalcorrosion agents and the internal development of corrosionproducts. The reduction of the rebar area and the increaseof the dielectric constant of rust contaminated concrete arethe two major factors that can differentiate the GPR responsefrom a corroded rebar and that from a noncorroded rebar. Itwas expected in the study reported herein that the increase ofdielectric constant and decrease of the rebar size would resultin a decrease in the maximum amplitude of the returningradar wave from the rebar. It is worth mentioning that the

corrosion products and agents not only increase the overalldielectric constant of the concrete but also increase theelectrical conductivity of the concrete which is an importantfactor for GPR signal attenuation. In this study, only the effectof changing dielectric constant is addressed. The corrosionproduct is manly iron oxide and it may affect the magneticpart of the electromagnetic wave. This effect is also notconsidered in this study.

4. GPR Scanning

According to existing research knowledge, water oil emul-sions can be used as a substitute of concrete in geophysicalinvestigation using GPR [11]. This emulsion can be madewith appropriate proportioning of water and oil to match thedielectric constant of concrete. In this study, three differentwater oil emulsion tanks were prepared to simulate concretedielectric constants for embedding the corroded rebars inthese solutions. Sodium lauryl sulphate was used as anemulsifying agent to assure propermixture of oil andwater inthe emulsions.The simulated dielectric constants of the threetanks were measured as 2.73, 5.47, and 9.3 using the two-waytravel time (TWTT) of a GPR scan by placing a steel plateunder the tank.These values fall within the range of dielectricconstants of most concrete. GPR is very sensitive to presenceof moisture because of the very high dielectric constant ofwater. The dielectric constant of concrete mainly depends onthe presence of moisture in the interstitial spaces in concrete.Very dry concrete has a low dielectric constant and freshconcrete or concrete with a high moisture content showshigher value of dielectric constant. A system was designedand made to hang the rebars at particular depth in the oilwater emulsion tanks. The rebar hanging arrangement wassupporting the Plexiglas cover over the emulsion as shown inFigure 4. A thin Plexiglas cover was placed over the emulsiontank to facilitate the movement of the antenna over the liquidsurface. The dielectric property of Plexiglas is similar to theoil used in the emulsion. So the GPR wave is assumed to beunaffected by the layer of Plexiglas at the top of the emulsion.Figure 5 shows comparison betweenGPR data collected fromthe oil emulsion tank and a real concrete beam. It is obviousfrom Figure 5 that the B-Scan and A-Scan from both thesources are compatible.

Each corroded rebar was placed in each emulsion tankat three different depths of 1 in (25mm), 2 in (50mm), and3 in (75mm). The placement of the corroded rebar in theemulsion tanks with known dielectric constants was similarto a real corroded rebar embedded in concrete. The GPR


(a) Antenna supported by Plexiglas cover (b) Rebar supporting arrangement

Figure 4: Rebar and antenna supporting system in the emulsion tank.

A-Scan fromoil emulsion tank

A-Scan from rebal concrete beam

−1.0 0.0 1.0

−0.2 0.0 0.2

Rebar reflection

B-Scan from oil emulsion tank

B-Scan from real concrete beam7.00

6.00

5.00

4.00

3.00

2.00

1.00

0.00

(ns)

7.00

6.00

5.00

4.00

3.00

2.00

1.00

0.00

(ns)

0 12

(in)

1.9 2.(ft)

Figure 5: Comparison between the emulsion tank and a real concrete beam.

equipment consists of a mainframe radar wave generator, ahand cart with antenna mount and calibrated wheels, and ahigh frequency antenna.TheGPR scan can be performed andthe scan results can be seen in real time. The GPR antenna

was of a ground coupled 2.6GHz type, which has one of thehighest frequencies among the commercially available GPRantennas. The wavelength of the antenna is 1.8 in (46mm)in free space. The B-Scans (two-dimensional GPR scans)


Bar-1Bar-2

Bar-3Bar-4

2 31Cover depth (in)

0

0.2

0.4

0.6

0.8

1TW

TT (n

s)

(a) Tank 1 (dielectric = 2.73)

2 310

0.2

0.4

0.6

0.8

1

1.2

1.4

TWTT

(ns)

Bar-1Bar-2

Bar-3Bar-4

Cover depth (in)

(b) Tank 2 (dielectric = 5.47)

2 310

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

TWTT

(ns)

Bar-1Bar-2

Bar-3Bar-4

Cover depth (in)

(c) Tank 3 (dielectric = 9.3)

Figure 6: Change of TWTT with cover depth for varying corrosion levels. Note: 1 in = 25mm.

or radargrams for each rebar were recorded for differenttanks and different depths in each tank. For each rebar,nine sets of data were collected where 3 sets were collectedfrom each tank at three different cover depths. Data wasalso collected for the noncorroded rebar to compare withthe corroded rebars. The B-Scans were transferred to theGPR postprocessing software for refinement. During thepostprocessing, background removal filter was applied andthe maximum amplitudes from the rebars and the TWTTwere recorded.

5. Results and Discussion

Two GPR parameters were chosen to relate with the extent ofrebar corrosion.They are two-way travel time (TWTT) fromthe rebar and the maximum positive reflective amplitudefrom the rebar. Two-way travel times were plotted againstthe rebar depth in the corrosion tanks, as seen in Figure 6.It may be observed that the TWTT is increasing with theincrease of the corrosion rate. The reduced rebar diameterdue to corrosion created a longer travel path for the GPR


0

5000

10000

15000

20000

Am

plitu

de

1 in2 in3 in

20 40 600Rebar area loss (%)

(a) Tank 1 (dielectric constant = 2.73)

0

5000

10000

15000

Am

plitu

de

20 40 600

1 in2 in3 in

Rebar area loss (%)

(b) Tank 2 (dielectric constant = 5.47)

0

3000

6000

9000

12000

Am

plitu

de

20 40 600

1 in2 in3 in

Rebar area loss (%)

(c) Tank 3 (dielectric constant = 9.3)

Figure 7: Maximum amplitude versus rebar area loss.

signal and the extra distance traveled was reflected in theincrease of TWTT. Figure 6 shows that the TWTT increasedwith increasing cover depths. Similar behavior was observedin all three tanks. However, the TWTT for a particular coverdepth and rebar corrosion is increasing with the increase ofthe dielectric constant.

Figure 7 shows the plot of maximum amplitudes ver-sus cover depths for the three tanks having three differ-ent dielectric constants. It is observed that the maximumamplitude decreased with the increase of rebar area lossdue to corrosion in all three tanks with varying dielectricconstants and for all cover depths. In order to developpredictive models for rebar corrosion estimation, regressionequations for the best fit trend lines (shown in Figure 6) were

generated herein. The best fit equations and the correspond-ing correlation coefficients are presented in Table 2. Thecorrelations coefficients are very high, ranging from 0.85to 0.99, indicating high levels of accuracy of the corrosionmodels. It is also observed that the relationship betweenthe GPR maximum amplitude from the corroded rebarsand the amount of corrosion is almost linear. Figure 6shows that the rate of change in GPR amplitudes is greaterat lower cover depths which means that the changes in GPRsignal due to corrosion at lower depths are more prominent.The models from Table 2 show that a relationship existsbetween the maximum positive amplitude and the amountof corrosion if the dielectric constant and the concrete coverare known.


Table 2: Amplitude corrosion relationships for 16mm rebar.

Dielectric constant Cover depth(mm)

Rebar corrosion area loss, 𝑦(%), 𝑥 = maximum

amplitudeCorrelation coefficient

Measured massloss%

Predicted massloss%

% error

2.73

25 𝑦 =𝑥 − 17584

−155.130.97

223145

272844

23−10−2

50 𝑦 =𝑥 − 11000

−94.290.98

223145

263043

18−3−4

75 𝑦 =𝑥 − 7146

−55.010.96

223145

272944

23−6−2

5.47

25 𝑦 =𝑥 − 12568

−133.780.85

223145

293637

3216−18

50 𝑦 =𝑥 − 7058

−69.990.87

223145

313039

41−3−13

75 𝑦 =𝑥 − 3244

−29.730.84

223145

232052

5−3516

9.3

25 𝑦 =𝑥 − 9302

−76.830.98

223145

213443

−510−4

50 𝑦 =𝑥 − 4923

−37.040.99

223145

252945

14−60

75 𝑦 =𝑥 − 3432

−44.620.99

223145

253143

140−4

6. Conclusions

In this study, the variation of GPR response over a rangeof corrosion states of steel rebar embedded in concrete wasinvestigated. The effect of concrete dielectric constant onGPR response was also investigated. Rebars were corrodedin an accelerated environment and a method of simulatingthe corrosion in laboratory using oil water emulsion tankwas used. The corroded rebars were scanned with a 2.6GHzground coupled antenna. It is assumed that the corrosionprocess reduces the diameter of the rebar and increases thedielectric constant of concrete. It is also assumed that theGPRwave penetrates through the corrosion products around therebar and reflects from the uncorroded core of steel rebar.The following conclusions may be made based on the resultsreported herein:

(i) The issue of quantitative nondestructive corrosionestimation of steel rebar in concrete structures hasbeen addressed herein. The method is based on GPRscanning of severely corroded existing concrete sur-faces with embedded steel rebars. GPR scanningis a speedy and labor friendly technique that maybe conveniently and economically used to estimatethe corrosion levels in rebars without any concretedamage. This study involved only the evaluation of

16mm embedded rebars. However, the study can beextended to other rebar sizes as well through thedevelopment of appropriate corrosion models.

(ii) The dielectric constants employed herein (2.73 to 9.3)using emulsion tanks represent most practical con-crete mixes. The dielectric constant of concrete fora specific case should be known for using the corre-lations for estimating rebar corrosion. The constantmay be conveniently found through GPR scanning incombination with some investigative drilling. Herethe relationships between amplitudes and corrosionare different for different dielectric constants and dif-ferent concrete covers. For practical use, where thedielectric constant and concrete cover can changespatially over the concrete surface, a unique modelhas to be developedwhich is independent of dielectricconstant and the concrete cover. To make the modelmore useful, the effect of changing electrical con-ductivity must be included in the amplitude versuscorrosion loss model.

(iii) The two-way travel time of GPR waves increases withthe increase of rebar corrosion. The rate of change oftwo-way travel time of GPRwaves is higher at smallerconcrete cover depths. At smaller concrete cover,


the loss of energy in the radar wave is less. Therefore,when the radar wave is reflecting from the surfaceof the corroded rebar, there is a small amount ofloss of energy. This is evident by high magnitude ofamplitudes from the rebar at 25mm cover depth level.When the cover depth is higher, the radar wave has totravel a longer path to and from the rebar towards theantenna. In the process, the loss of energy in the radarwave is high. This is evident by the low magnitude ofamplitude at 75mm cover depth.

(iv) The relationship between the maximum GPR ampli-tudes from corroded rebars and the corrosion loss ofthe rebar is observed as linear. However, this studyis done with only four different levels of corrosion.More points need to be considered to confirm the lin-earity of the amplitude versus mass loss relationship.Maximum amplitudes decrease with the increase ofcorrosion. The rate of decrease of amplitude is higherat lower concrete covers.

(v) Themaximum amplitude from a rebar for a particularstage of corrosion was decreasing with the increase ofdielectric constant.

(vi) This experimental study can be used as a basis forquantitative estimation of concrete rebar corrosionextent in the field using GPR. Extensive experimentalstudy on real concrete samples with corroded rebarsneeds to be done to verify the method of estimatingcorrosion.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

References

[1] I. Hasan and N. Yazdani, “Investigation of inadequate concretecovers in a new bridge deck using ground penetrating radar,”in Proceedings of the 93rd Annual Meeting of the TransportationResearch Board, no. 14-4191, Washington, DC, USA, January2014.

[2] J. Hugenschmidt, “Concrete bridge inspection with a mobileGPR system,” Construction and Building Materials, vol. 16, no.3, pp. 147–154, 2002.

[3] N. Gucunski, C. Rascoe, R. Parrillo, and R. L. Roberts, “Com-plimentary condition assessment of bridge decks by high fre-quency ground penetrating radar and impact echo,” in Proceed-ings of the 88th Annual Meeting of the Transportation ResearchBoard, Washington, DC, USA, January 2009.

[4] ASTM, “Standard test method for half-cell potentials ofuncoated reinforcing steel in concrete,” ASTM C876-09, Amer-ican Society for Testing andMaterials,West Conshohocken, Pa,USA, 2014.

[5] S. S. Hubbard, J. Zhang, P. J. M. Monteiro, J. E. Peterson, andY. Rubin, “Experimental detection of reinforcing bar corrosionusing nondestructive geophysical techniques,” ACI MaterialsJournal, vol. 100, no. 6, pp. 501–510, 2003.

[6] W.-L. Lai, T. Kind,M. Stoppel, andH.Wiggenhauser, “Measure-ment of accelerated steel corrosion in concrete using ground-penetrating radar and a modified half-cell potential method,”Journal of Infrastructure Systems, vol. 19, no. 2, pp. 205–220,2013.

[7] S. Hong, W. W.-L. Lai, G. Wilsch et al., “Periodic mapping ofreinforcement corrosion in intrusive chloride contaminatedconcrete with GPR,” Construction and Building Materials, vol.66, pp. 671–684, 2014.

[8] ACI Committee 222, Corrosion of Metals in Concrete, AmericanConcrete Institute, Detroit, Mich, USA, 1996.

[9] Dielectric Constants of Various Materials—Clipper Controls,January 2015, http://www.clippercontrols.com/pages/Dielec-tric-Constant-Values.html.

[10] J. J. Daniels,Ground Penetrating Radar Fundamentals, PreparedAs an Appendix to a Report to the US EPA, Region V, 2000.

[11] C. Warren and A. Giannopoulos, “Creating finite-differencetime-domain models of commercial ground-penetrating radarantennas usingTaguchi’s optimizationmethod,”Geophysics, vol.76, no. 2, pp. G37–G47, 2011.

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