Journal of Applied Geophysics 97 (2013) 45–54

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Journal of Applied Geophysics

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Applications of ground penetrating radar (GPR) in bridge deck monitoringand assessment

Amir M. Alani a,b,⁎, Morteza Aboutalebi a, Gokhan Kilic c

a Department of Civil Engineering, University of Greenwich, Chatham, UKb Bridge and Tunnel Engineering, University of Greenwich, Chatham, UKc Department of Civil Engineering, Izmir University of Economics, Izmir, Turkey

⁎ Corresponding author at: Department of Civil EngineChatham, UK. Tel.: +44 1634 883293.

E-mail address: m.alani@gre.ac.uk (A.M. Alani).

0926-9851/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.jappgeo.2013.04.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 October 2012Accepted 18 April 2013Available online 29 April 2013

Keywords:Bridge structuresHealth monitoring and assessmentGPRData interpretation

This paper presents the essence of two case studies by the authors on twomajor bridges in the UK. The first casestudy reports on the applications of GPR and associated work carried out on the Forth Road Bridge near Edin-burgh, Scotland, with the main objective of identifying possible structural defects including damaged rebarand moisture ingress at specific locations of the bridge deck. The second case study focuses on a fullassessment of the Pentagon Road Bridge, in Chatham, Kent, England with particular emphasis on the identifica-tion of possible defects including structural cracks within the deck structure and establishing the layout of theupper and lower rebar positions throughout the bridge. These studies present interesting results in terms of lo-cations of rebar and an accurate estimate of concrete cover condition as well as reporting on a remarkable sim-ilarity in the processed data concerning areas affected by ingress of moisture within the deck structures of thetwo bridges under investigation. It is believed that this paper will be of particular interest to bridge engineersand structural engineering practitioners with enthusiasm for adopting non-destructive testing methods suchas GPR in the health monitoring and assessment of bridge structures. The observed similarities in the processeddata between the two reported case studies present an interesting concept within the general context of the in-terpretation of GPR data, with the potential for use in many other forthcoming cases. The paper also reports onthe adopted method for the GPR survey with emphasis on difficulties and challenges encountered during theactual survey. The presented results benefit from advanced processing and presentation techniques.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Bridge structures are assets and are vital to human life in terms ofthe economy, mobility, environment and development of communi-ties. No doubt assets should be harnessed and looked after, but notin a dispersive and disorganised manner. It should be part of a robustplanned monitoring and maintenance mechanism within the contextof the life cycles of structures. It also is imperative that any assess-ment and monitoring method adopted should be cost effective, effi-cient and fit for purpose.

Depending on the type and needs of a bridge, different approachesshould be adopted in order to generate relevant and useful information(data) accordingly. Most bridge health assessment theories emphasisethat it is important to know how each technique and method worksand is applied, what exactly it is anticipated to achieve and how it isachieved.

Different types of NDT (non-destructive testing) techniques arecommercially available within the context of bridge health monitoring

ering, University of Greenwich,

rights reserved.

and assessment. Naming a few, accelerometers, smart total station,vibration measurement sensors, wireless network systems and GPRhave proved to be of great service to the industry if and when theyhave been adopted appropriately. It is known within the communitythat no NDT technique on its own can produce complete answers toall questions in terms of bridge condition survey, but they all havebeen proven successful in certain applications. EachNDT technique pro-vides different information about the bridge structure. Therefore eachNDT technique can be used to assess different conditions and problemsof the structure. It is needless to emphasise that it is of paramountimportance to choose the right NDT technique related to bridge healthmonitoring needs (Annan et al., 2002; Gentile, 2010; Parrillo andRoberts, 2006).

Applications of GPR have beenwidely appreciated by different profes-sionals and have been successfully implemented and adopted to solvecomplicated engineering and science based problems. Recent develop-ments in GPR technology (equipment and software) and awareness ofscientists and engineers of its effectiveness and varied applicability haveboosted GPR's credibility and utilisation extensively in recent years(Amos et al., 2009; Scotta et al., 2003; Soldovieri et al., 2006). NowadaysCivil Engineers and NDT Archaeologists, Geologists, Geotechnical Engi-neers, Glaciologists, Forensic Investigators, Environmental Scientists and

46 A.M. Alani et al. / Journal of Applied Geophysics 97 (2013) 45–54

Hydrologists amongst others utilise GPR in one form or another to findsolutions for challenging engineering and scientific problems.

The application of GPR in structures including highway/road infra-structures, such as in the assessment and monitoring of bridges and tun-nels, is not a new concept. GPR has been used successfully to monitorbridge deckswithin the context of identification and integrity assessmentof rebar, rebar cover length, depth of cracks, settlement, ingress of mois-ture and delamination, layers of materials, cavities, location of rebar andother structural features (beams and columns) as well as bridge abut-ments (leakage, cracks and settlement) (Benmokrane et al., 2004; Fujunet al., 2011; Helwany et al., 2003; Lubowieckaa et al., 2009; Parrillo andRoberts, 2006; Rhazi et al., 2003).

This paper presents two case studies of applyingGPR in assessing theactual condition of two bridges in the UK. The first case study presentswork on the Forth Road Bridge near Edinburgh, Scotland and the secondcase study presents part of a comprehensive investigation on thePentagon Road Bridge in Chatham, Kent, England. This paper presentspart of a larger study which considers other NDT methods in conjunc-tion with each other in order to produce a clearer picture of the healthof bridges.

2. Results for case study 1 — Forth Road Bridge

The Forth Road Bridge, which is a suspension bridge, opened to thepublic in 1964. The location of the Forth Road Bridge (Figs. 1 and 2) isin east central Scotland. The bridge provides access from the capitalcity of Edinburgh to North Queensferry.

2.1. Equipment

The GPR survey was performed using the RIS HI-BrigHT Bridge Highresolution Tomography, see Fig. 3. Designed specifically for the inspec-tion of bridge decks, this high frequency array antenna system is light-weight and maneuverable yet provides high quality, densely sampleddata. Denser sampling produces higher quality tomography, and three-dimensional (3D) images assist considerably in the interpretation ofdata.

The system is composed of an array of eight horizontally polarised2 GHz channels spaced at 10 cm intervals, mounted on a lightweightand highly maneuverable trolley and powered by a large, 24 Ah 12 Vbattery (RIS Fast Wave control box — DAD which allows driving largerarrays at greater speed). The additional speed also allows for greaterstacking (averaging) which gives a better resolution and at the sametime a slightly deeper penetration.

Fig. 1. Location of the F

The DAD FastWave is controlled by IDS K2 FastWave acquisitionsoftware running on a Panasonic Tough Book CF19. The K2 FastWaveSWmakes the collection of radar data simple. It features a signal calibra-tion and diagnostic check for the radar, and offers the facility to insertscan coordinates and interface with GPS.

The RIS Hi-BrigHT was specifically designed to work in conjunctionwith advanced software processing allowing the detection of shallowfeatures and the structure's condition. It was particularly intended forthe concrete assessment of bridges, to detect the thickness of layers,shallow utilities and drainage, location and spacing of rebar, and mois-ture penetration and delamination.

Three areas of the Forth Road Bridge were surveyed. Of these, Area1 is of interest for this report as the area was the subject of a numberof previous independent investigations and recently was renewed.Fig. 4 shows three distinct areas of interest to the bridge owners,which were investigated by the author and the associated industrialpartners (IDS Limited).

The survey was performed by pushing the system in an 80 cm grid,producing 10 cm spacing between scans (Fig. 5). These scans can beinterpreted to produce images and recover information about the con-dition of the structure's constituent materials.

The main objectives of this survey were as follows:

• Estimation of thicknesses of different structural layers of the bridgedeck

• Location of shallow utilities and drainage• Location and spacing of rebar• Possible moisture penetration and delamination.

2.2. Data processing and results

Data processing was performed using the IDS GRED data analysissoftware. The software provides a two-dimensional (2D) tomographyof the underground layers and a 3D view of the surveyed area. Thecapability of merging on the same tomographic map datasets collectedalong both longitudinal and transversal scans considerably increasesthe reliability of the results of the analysis. The software allows thedevelopment of optimised processing macro which can be applied toeither all or subsets of the data. It also features automatic hyperboladetection, layering capabilities, and transfer to CAD.

Fig. 6 represents a B-scan (longitudinal scan) that annotated to iden-tify the features discovered during the radar survey. By performing thisinterpretation on several B-scans side by side it is possible to build up apicture of the conditions inside the bridge.

orth Road Bridge.

Fig. 2. General view of the Forth Road Bridge.

47A.M. Alani et al. / Journal of Applied Geophysics 97 (2013) 45–54

The exact data processing procedure included; background removal,set start time/zero position, leading to some filtering and sometimesadjusting the gain.

The B-scan presented is in fact a longitudinal section down thecentre of the perceived damaged area, identified by the red line inthe horizontal section (C-scan) shown in Fig. 7.

In the section represented by the red line (at approximately 25 cmdepth) the concrete in “good” condition is represented by lighter con-trast and the rebar can be clearly seen. The area of possible moistureis also identified by a patch of reduced contrast.

The images in Fig. 8 show the expansion of a possible moistureaffected area at reducing depths:

The deteriorated area depicted in Fig. 8 corresponds to the zonewithhigher signal attenuation in the radargram presented below (Fig. 9). Inthe first instance, one may interpret this deteriorated area as an area ofsubsidence in the bridge deck as there is a change in the level of rebarwithin this area. However, when this area was excavated later it wasrevealed that there is no structural subsidence whatsoever. This changeof signal attenuation is basically due to the presence of moisture whichhas penetratedwell below the upper rebar layer of the bridge deck. Thepresence of moisture was confirmed during the excavation. A similarphenomenon presented itself during the processing and interpretationof the data collected from the Pentagon Road Bridge, the second casestudy presented in this paper.

Due to the density channels of the RIS Hi-BrigHT, it is possible torecover large quantities of information during a radar acquisition.This gives a high level of confidence in the data acquired as well asenabling images of exceptional quality to be produced, which aidsdata interpretation. It also allowsmore advanced processing techniques

Fig. 3. Full view of the GPR system RIS Hi-BrigHT.

to be performed such as the mathematical calculation of the areas withhigher than average attenuation (absorption of the radar signal), pro-ducing a 2D map of the moisture levels within the bridge.

Fig. 10 shows a particular area of the survey in which a patch ofincreased attenuation is clearly visible in the produced C-scan. Thisconfirms the results already produced above.

Fig. 11 depicts an AutoCAD illustration of the interpreted data inline with the objectives of the exercise. The following points clearlyaddress the set objectives of this study:

• Location and spacing of two separate layers of rebar: (a) the top layerof rebar in both directions; and (b) the deeper layer of rebar in onedirection only.

• Area of moisture penetration close to the surface.• Larger area of moisture penetration underneath the surface damage.

3. Results for case study 2 — Pentagon Road Bridge

The Pentagon Road Bridge (Fig. 12) was constructed in 1975 andcarries an access road from Rope Walk to the Pentagon ShoppingCentre in Chatham, Kent. The bridge is a four span simply supportedconcrete deck of beam and slab construction. At the west high end,the bridge links the access road to the rooftop car park of the shoppingcentre. The end support is a leaf pier that is shared with the access road.There is a lower access road for buses that is a brick faced concrete abut-ment at the east, low end of the bridge. At the east end there was a

Fig. 4. Areas under investigation.

Fig. 7. Area 1 C-scan (horizontal scan at depth 25 cm).Fig. 5. Details of the survey arrangement for Area 1.

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pedestrian walkway beneath the deck but it has been removed. Thebridge foundations are spread footings.

Figs. 13 and 14 depict a plan view and a 3D model of the bridgerespectively.

3.1. Visual inspection

Previously, a visual inspection was carried out with a camera, LeicaDisto laser height measuring instrument, crack gauge, hammer, torch,ladder and a 22 m ascendant telescopic hoist in 2009 by Jacobs Engi-neering U.K. Limited. From the report, the faults and recommendationsare a number of known defects on the bridge with the rebar visible insome places due to concrete deterioration. Concrete repairs are requiredto theWest End Leaf Pier Support, Piers 1 and 2 and all of thedeck spans.The transverse beam at the west end of Span 4 is in very poor conditionand requires an assessment of its adequacy prior to any remedial worksbeing carried out. The surfacing on the deck is nearing the end of its lifewith areas of reinstatement, some in poor condition, potholes and fret-ting. All of the bearings to the main spans that could be seen were inpoor condition. The report recommended that all of the bearings needreplacement. The transverse joints and the longitudinal joint have faultsthat indicate that there is leakage at each. The joints should be repaired.The parapets are damaged with areas of corrosion and require repairand painting.

The above inspection and subsequent report did not cover asub-surface investigation and assessment of the bridge structure. Thefirst author's research team at the University of Greenwich requested

Fig. 6. B-scan depiction of a section of

access to the bridge via the Highway Engineering Department ofMedway City Council (the owners of the bridge) in order to carry outa full survey of the bridge using their GPR system. For that purposethe following objectives were set:

• To locate the position of the upper rebar• To estimate the depth of rebar cover throughout the bridge deck• To locate the position of the lower rebar• To identify possible areas of moisture ingress below the deck surface• To identify any other structural features and/or defects.

3.2. The survey

The GPR survey was carried out on 20th January 2011. The weatherwasdry and sunny and the temperaturewas recorded as 10 °C. This sur-veywas fully supported by theMedway City Council's Highway divisionin terms of access to the bridge, the provision of heavy plant (cherrypicker and scissor lifts) and the much needed traffic control on theday, as the bridge is heavily used by members of the public.

The survey was performed by marking a grid on the ground usingchalk or temporary paint and pushing the radar across the grid instraight lines. The location of the grid is referenced by recording thecoordinates with respect to a fixed location. The process of referencingis very important and should be carried out with care and diligence inorder to map the survey area accurately. Due to the frequency of theantennas and the size of the array, it was recommended that multiplesurveys be performed covering moderate areas, rather than one largesurvey. For optimum results it is recommended to push the radar

the bridge with identified feature.

Expansion of the deteriorated area Expansion of the deteriorated area

Depth 5cm

Depth 10cm

Depth 18cm Depth 22cm

Renewed area Rebar gridDeteriorated area

Fig. 8. Horizontal cut at 5, 10, 18 and 22 cm below the bridge deck surface.

49A.M. Alani et al. / Journal of Applied Geophysics 97 (2013) 45–54

in both the transversal and longitudinal axes. The GPR test wasperformed on bridge deck covering an area around 7 m × 60 mapproximately.

Over the bridge deck, a total of 54 longitudinal and 182 transversalarray scans were cautiously collected.

3.3. Results of data processing

The data processing was similar to case study 1 detailed above. Alsosimilar to case study 1, the capability of merging on the same tomo-graphic map of datasets collected along both longitudinal and transver-sal scans considerably increases the reliability of the results of theanalysis.

Fig. 9. Area 1 different rebar layers detected on a longit

The process involved the following steps:

Data filtering, estimate of the propagation's velocity on EM wavesin the structure through hyperbolic fitting and migration such asfocusing.

The processed data sets run the automatic extraction of the shallowerlayer of rebar, the analysis of rebar backscattered signal and manualextraction of the deeper layer of rebar. Acquired radar data is saved asrawdata and is processed to display as a ‘B-scan’ that represents a verticalslice through the surveyed area.

Due to the scale of the operation and for the sake of accuracy, it wasdecided to divide the deck surface into 8 separate but interconnectedzones and carry out the GPR survey separately. This also was necessary

Identified structural feature at 60cm below the deck and beyond

udinal radar section (depth against distance (m)).

Fig. 10. Area 1 C-scan with increased attenuation.

Deeper Rebar

Deeper moisture ingress Surface damage

Shallow Rebar

Fig. 11. Schematic 3D drawing with AutoCAD.

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to allow the traffic to flow at certain intervals. Fig. 15 depicts the pro-posed zone configuration of the bridge for survey purposes:

Fig. 16 depicts a set of vertical sliced data at depths of 6, 16, 26, 38 and50 cm from the surface of the deck respectively. Certain consistent

Fig. 12. Pentagon Road Bridge i

features and their respective appearances have also been highlighted(red circles). These features then have been superimposed on a com-putermodel depiction of the bridge in order to demonstrate their scalesin 3D.

n Chatham, Kent, England.

Fig. 13. Plan of the Pentagon Road Bridge.

Fig. 14. 3D model of the Pentagon Road Bridge.

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A possible explanation for the appearance of the highlighted fea-tures could be ingress of moisture through the surface of the deck.These features associate with surface cracks that were observed

Fig. 15. Aerial photo of the bridge with the

during the visual assessment of the bridge. Although the feasibilityof closing the bridge for intrusive inspection and repair was verylow, recommendations were made to the owners of the bridge to ex-pose one of the highlighted areas in order to verify the suggestedmois-ture presence and possible delamination of the concrete at lower levelsof the bridge structure. The owners have taken the above recommenda-tion on board and have agreed for this to be included in the nextplanned maintenance survey of the bridge. Nevertheless, this featureis compatible with other case studies and laboratory experiments thatthe team has carried out previously.

It is important to emphasise that the survey of the bridge was a rel-atively challenging endeavour (due to size and accessibility) yielding asignificant quantity of data requiring considerable time for processingin compliance with the set objectives of this investigation. Presentationof all the results/data was outside the scope of this paper for obviousreasons. However, a selected quantity of data have been chosen to high-light the effectiveness of GPR in the assessment of bridge structures inidentifying possible defects that otherwise are not easily identifiableby adopting conventional methods. To this effect, in this case study itwas decided to limit the presentation of processed data to Zone 5 ofthe bridge, Fig. 15.

The processed data and subsequent interpretations are rather con-clusive in terms of the identified structural features and components.Figs. 17 and 18 illustrate clearly the locations of the upper rebar aswell as the lower rebar with possible areas which may have beenaffected by moisture ingress. Fig. 18 also depicts the lower structuralfeatures (at around 60 cm from the surface of the deck — see Fig. 8)believed to be lower beam structures. This is despite the fact thatthe rebar spacing arrangement in this bridge was very close togetherat both upper and lower levels.

The apparent high signal attenuation area depicted in Fig. 17exhibits similar behaviour to the previous case study discussed above.The apparent drop of the lower rebar of the Pentagon Road Bridge(depicted within the green frame) at first instance can be associatedwith structural subsidence of the lower rebar. As discussed under casestudy 1, a similar feature was revealed to be associated with moistureingress within the lower structure of the bridge deck. This similarityprovided an opportunity for the team to interrogate that particulararea within Zone 5 of the Pentagon Road Bridge with closer scrutiny.Further tests (different frequency antenna systems and closer visualinspections — Alani et al., 2012) identified a number of micro-crackson the surface and the lower areas of the bridge deck. It was concludedthat this drop at the lower rebar level is due to the strong attenuation of

proposed zones for survey purposes.

2D Scan 3D ModelDepth = 6cm

Depth = 16cm

Depth = 26cm

Depth = 38cm

Depth = 50cm

Fig. 16. Vertical sliced processed data (left) and computer modelled depiction of possible moisture ingress (right) at depth ranges from 6 cm to 50 cm.

52 A.M. Alani et al. / Journal of Applied Geophysics 97 (2013) 45–54

signals which in turn can be associated with moisture ingress andpossible delamination of the bridge deck structure.

As was alluded to earlier (depicted in Fig. 18, blue framed area),certain structural features at around 60 cm below the bridge deckwere identified (see Fig. 8). These features were not available withinthe limited number of drawings that the bridge owner and sub-contractors had access to. It is of significant value to notice thatthese features are associated with the high signal attenuation areaof the bridge deck. This finding has now led the team to expand theinvestigation to another level in order to explain the possible causesof achieving these results in the first place and also the extent andsignificance of these structural features.

4. Conclusions

In terms of achieving the set objectives of the investigationspresented in this paper, the presented results are conclusive. As a resultof diligent and careful planning, survey (referencing), data acquisition,data processing and interpretation itwas possible to obtain the requiredanswers to a number of challenging questions and beyond. Results ofthese investigations in conjunction with other non-destructive testingmethods adopted (IBIS-S with interferometric capabilities and Acceler-ometer sensor system)produced vital informationwithin the context ofthe structural integrity of the two bridges under investigation. Oncemore it was demonstrated that if the “right” equipment and trained

Fig. 17. Processed data and possible explanations (interpretation) on one of the survey lines within Zone 5 of the bridge (depth against distance (m)).

Fig. 18. Processed data and possible explanations (interpretation) on another survey lines within Zone 5 of the bridge with deeper penetration (depth against distance (m)).

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staff are employed there is a high chance of success in achievingchallenging objectives. No doubt, GPR is effective and conclusive if it isused correctly and appropriately. This study also demonstrates thevalue of lessons learned and knowledge acquired in one investigationbeing applied to other investigations.

Acknowledgement

The authors would like to express their thanks to the HighwayDivision of Medway City Council, Kent, England primarily for providingfull access to the bridge and also for their dedicated help in the planningand operation of the survey throughout.

The authors would like to thank IDS Limited for their immensesupport throughout these studies and beyond.

The authors also would like to thank the technical staff of theDepartment of Civil Engineering at the University of Greenwich fortheir assistance during the survey.

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