A comparison of nondestructive evaluation methods for bridge deck assessment

A comparison of nondestructive evaluation methods for bridge

deck assessment

M. Scotta,*, A. Rezaizadeha,1, A. Delahazab,2, C.G. Santosc,3, M. Moored,4,B. Graybeale,5, G. Washerf,6

aWiss, Janney, Elstner, Assoc., Inc., Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, McLean, VA 22101, USAbWiss, Janney, Elstner, Assoc., Inc., 330 Pfingsten Rd., Northbrook, IL 60062, USA

cWiss, Janney, Elstner, Assoc., Inc., 14 Washington Rd., Suite 501, Princeton Junction, NJ 08550, USAdWiss, Janney, Elstner, Assoc., Inc., 225 Peachtree Street, N.E. Suite 1600, Atlanta, GA 30303, USA

ePSI, Inc., Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, McLean, VA 22101, USAfFederal Highway Administration, Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, McLean, VA 22101, USA

Received 4 February 2002; accepted 1 July 2002

Abstract

Concrete bridge deck deterioration is a significant problem that must be addressed to preserve highway infrastructure investments in

bridges around the world. Reducing the cost of bridge deck maintenance is critical to government and private agencies responsible for

maintenance of bridges. Maintenance challenges increase as many bridges begin to approach the end of their design life and traffic loads

continue to increase. One means of reducing the cost of bridge deck maintenance is to accurately evaluate the condition of the structure and

its constituent materials. Current methods used to evaluate deterioration of bridge decks include acoustic, electrochemical, electromagnetic,

and visual inspection techniques. The purpose of this study was to assess the advantages and limitations of three available evaluation

methods. The evaluation techniques included in this study were ground penetrating radar, the chain drag method, and IE. These techniques

represent an important group of inspection methods currently used to evaluate in-service bridge decks. The bridge deck used in the study

contains significant delaminations but exhibits virtually no outwardly visible signs of these deficiencies, so a detailed visual inspection survey

of the deck was not included. Cores were taken from the bridge deck at selected locations to confirm the accuracy of the results obtained

through each evaluation method. This paper presents the findings from each method and describes their respective advantages and

limitations.

Published by Elsevier Science Ltd.

Keywords: Nondestructive evaluation; Radar; Bridge decks; Concrete; Corrosion-induced delamination

1. Introduction

The highway infrastructure in the United States and

around the world faces a deterioration problem of

unprecedented magnitude. Challenging environmental con-

ditions and increased traffic volumes make bridges more

susceptible than ever to deterioration. Reinforced and

precast concrete bridge decks are subjected to very

demanding conditions, and virtually all bridge decks will

fail to serve out the useful life of the bridge [1]. Accurately

evaluating the condition of a bridge deck over time and

using this information to choose appropriate maintenance

techniques has become critical for the efficient allocation of

bridge deck maintenance resources.

Many nondestructive evaluation (NDE) methods have

been developed for bridge deck assessment but few have

been evaluated in a performance-based comparison.

Further, most of these methods have not been integrated

into a working bridge management system that makes use of

the important data provided by these methods. Four basic

categories of bridge deck assessment methods utilized

include visual inspection, acoustic, electromagnetic, and

electrochemical testing. Methods within these four basic

0963-8695/03/$ - see front matter Published by Elsevier Science Ltd.

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NDT&E International 36 (2003) 245–255

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1 Tel.: þ1-202-493-3119; fax: þ1-202-493-3126.2 Tel.: þ1-847-272-4236; fax: þ1-847-291-9599.3 Tel.: þ1-609-799-7799; fax: þ1-609-799-7088.4 Tel.: þ1-404-577-7444; fax: þ1-404-577-0066.5 Tel.: þ1-202-493-3122.6 Tel.: þ1-202-493-3082; fax: þ1-202-493-3126.

* Corresponding author. Tel.: þ1-202-493-3124; fax: þ1-202-493-3126.

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categories have been implemented in a variety of ways, but

the commonly accepted approaches have narrowed to only a

few widely used methods. These include visual inspection,

chain drag, IE, ground penetrating radar (GPR), infrared

thermography (IR), and half-cell potential.

The primary goal of these methods is to detect corrosion-

related damage in a bridge deck. For most of these methods,

the delaminations in a deck (which result from volume

expanding corrosion processes at the reinforcing steel) are

detected or measured. However, half-cell potential methods

are designed to identify locations in the deck, where

corrosion activity is likely to be ongoing, which may result

in or may already be areas of delaminated concrete.

Commonly used bridge deck assessment methods are

briefly described here. Visual inspection involves the

assessment of the bridge deck for cracks, spalls and other

irregularities in the deck surface that can be detected

visually [2]. Visual inspection is the most common method

of deck assessment, and requires few tools and minimal

training of inspectors. Subsurface or internal defects in the

deck are difficult to evaluate by this method.

Common acoustic techniques include the chain drag

method and IE. The chain drag method involves dragging a

chain across the bridge deck surface while listening for

changes in the acoustic response. This method involves only

a qualitative assessment, but is rapid and inexpensive. IE is

frequently used when a more thorough acoustic assessment

is needed [3]. The more expensive and time consuming IE

method is quantitative, since the response to an impact is

detected by an acoustic transducer, recorded, and sub-

sequently analyzed for anomalies.

Two electromagnetic methods used for bridge deck

assessment are GPR [4] and IR. GPR systems record the

response to a radar impulse or a series of radar impulses

incident on the bridge deck surface. Careful assessment of the

GPR responses can often yield information about the material

properties of the deck and its condition. IR can be used for

bridge deck assessment, but it is highly susceptible to

environmental conditions. IR records electromagnetic waves

emitted in the infrared portion of the electromagnetic

frequency spectrum. These infrared wavelength emissions

vary in response to environmental conditions as well as the

geometry and emissivity properties of the feature under

investigation. For a bridge deck application, diurnal tem-

perature variations can lead to temperature variations in the

deck that are manifested in thermal gradients on the bridge

deck surface. Under favorable conditions, these gradients

may be related to subsurface bridge deck features. Factors

such as wind, rain, shadows and small diurnal temperature

variation can present challenges for this method [5].

Finally, electrochemical methods that measure corrosion

activity by half-cell potentials are common methods of

bridge deck assessment [6]. These methods can be effective

for locating active corrosion sites, but are time consuming

and expensive to implement. Further, because half-cell

measurements relate to corrosion activity, physical

characteristics of the bridge deck such as delaminations

are not directly assessed.

Three of the six aforementioned methods of bridge deck

evaluation are described and compared in this study. Each

method has advantages and limitations that can be assessed

more thoroughly when they are compared to one another.

Some work comparing bridge deck evaluation methods has

been done in the past [7] but results from these studies have

not been conclusive. The Van Buren Road Bridge (Fig. 1),

which spans Quantico Creek in Virginia west of Interstate

95, was selected as the test bridge for this study.

2. Methods

A comparison of different NDE methods for detecting

bridge deck delamination damage requires reliable ground

truth information. In this study, ground truth data was

derived from cores that were extracted from the bridge deck

and from a reference chain drag study that was conducted by

nondestructive evaluation validation center (NDEVC) staff

using a comprehensive chain drag test. The reference chain

drag survey conducted by the NDEVC tested the entire

bridge deck area, which was subdivided into grid squares for

the survey. Good agreement between chain drag test results

and coring test results were found and details are presented

in Section 2.1 of this paper.

IE testing was conducted at regular intervals over a two-

dimensional grid on the bridge deck. Each IE test result

collected relates to the local condition below the surface of

the bridge deck at that test location. The basic principle of

the IE method involves impacting the surface of the material

with a small diameter impactor and detecting and recording

the response. Appropriate interpretation of the IE data

usually allows the physical condition of concrete to be

assessed. A discussion in Section 2.2 of this paper provides

details on how these assessments are made.

GPR was also applied to the Van Buren Road Bridge

deck, and it was carried out with two different instruments.

GPR data was collected using a commercially available

Fig. 1. Van Buren Road Bridge viewed from the east.

M. Scott et al. / NDT&E International 36 (2003) 245–255246

radar system made by Geological Survey Systems, Inc.

(GSSI) and with a prototype system developed by Lawrence

Livermore National Laboratory (LLNL) for the federal

highway administration (FHWA). Both systems take

advantage of synthetic aperture radar techniques that

capture a series of radar waveform responses at regularly

spaced intervals. The GSSI system features a ground

coupled, broadband antenna pair with a 1.5 GHz center

frequency. The LLNL prototype system, called High Speed

Electromagnetic Roadway Mapping and Evaluation System

(HERMES) utilizes an array of 64 air coupled antenna pairs

that transmit a radar pulse spanning a band of frequencies

centered around 2.4 GHz. Details about the operational

characteristics of each of these systems and the development

of the HERMES prototype are provided in Section 2.3 of

this paper.

The application of a range of NDE methods to a single

bridge deck provides a very rich data set for analysis. This

paper presents a summary of an initial analysis of the

test data. This analysis may be appended or supplemented in

Fig. 2. Data sets from (a) a series of chain drag surveys, (b) an IE survey overlaid with reference chain drag data, (c) reconstructed HERMES data overlaid with

reference chain drag data and (d) raw HERMES data A–A along with raw GSSI data B–B.

M. Scott et al. / NDT&E International 36 (2003) 245–255 247

the future. A preliminary analysis of this data was presented

in Ref. [8]. Results from each NDE technique are

summarized in Fig. 2.

The results in Fig. 2 were collected from the 17.7 m long

center span of the Van Buren Road Bridge deck. Fig. 2(a)

summarizes a series of chain drag tests performed by state

inspectors and a reference chain drag survey performed by

the NDEVC. The NDEVC reference chain drag survey

results are indicated by continuous lines, which enclose the

boundaries of detected delaminations. In Fig. 2(a), the gray

areas in this figure indicate detected delamination locations

as determined by twenty-two pairs of state bridge inspectors

from various states in the United States. Darker shades of

gray indicate stronger agreement between surveys while

lighter shades of gray indicate fewer of the twenty-two

survey results in agreement. Locations of cores extracted

from this section of bridge deck are also indicated. Fig. 2(b)

presents the results from an IE survey of the center span.

Light gray filled circles indicate that a delamination was

detected at a given location while open circles indicate solid

concrete based on IE test results. Dark gray filled circles

indicate suspect IE test results that cannot be definitively

interpreted solely as responses to shallow delaminations.

Fig. 2(c) presents plan view images that were extracted from

a three-dimensional reconstruction of a HERMES GPR data

set. Fig. 2(d) presents raw data collected with HERMES

(section A–A) and the GSSI radar system (section B–B).

2.1. Chain drag and coring

The chain drag test was conducted in accordance with

ASTM D 4580-86 [3]. Two bridge inspectors participating

in the NDEVC visual inspection study [9] are illustrated

carrying out the chain drag procedure in Fig. 3. The chain

drag procedure is often most efficiently done using two

people working together. The two-person team allows the

tasks of dragging the chains over the deck, clarifying defect

boundaries using a rock hammer and recording findings to

be divided. Frequently, this method is employed with a

Fig. 3. Bridge inspectors carrying out a chain drag survey on the Van Buren

Road Bridge.

Fig. 4. Bridge deck cores in the process of being extracted from the Van

Buren Road Bridge.

Fig. 5. A bridge deck core extracted from the Van Buren Road Bridge that

contains a clear indication of a shallow delamination.


single inspector using only a chain to define boundaries. In

addition to acoustic responses, visual indications of surface

distress are often indicated on the data sheets. Fig. 4 shows a

coring machine being used to remove cores from the Van

Buren Road bridge deck. Fig. 5 shows one of the ten

extracted cores (Core No. 7). As shown in Fig. 5, Core 7

exhibits indications of a delamination directly below the top

surface. The cracks observed in this core, and in other cores

that were removed from the deck, were confirmed by

making visual observations of the interior surface of the

core hole and by IE testing.

As shown in Fig. 2(a), the chain drag surveys performed

by state bridge inspectors were not entirely consistent with

the reference chain drag survey performed by NDEVC staff.

A thorough analysis of all of this chain drag data is provided

in Ref. [9]. From Ref. [9], if the NDEVC reference chain

drag survey is used as a basis for comparison, (therefore

leading to an estimate that 19% of the bridge deck is

delaminated) and an allowance of ^5 percentage points is

permitted as a reasonable error, five of the twenty-two

inspection teams had overall delamination estimates that

were in this range. The data also show that location, where

five inspection teams indicated a delamination response

collectively represent a 21% delaminated deck estimate.

Varying the level of inspection team overlap for detected

delamination indications, from one team to twenty-two

teams, results in the delamination percentage estimate

varying from 2 to 69%, respectively. These statistics are

indicative of the variability inherent in the chain drag

method.

NDEVC reference chain drag data was compared to the

data from cores taken from the bridge deck. All ten of the

cores removed from the bridge deck matched results from

the reference chain drag survey and one of the data sampling

methods implemented for the IE survey (Table 1). Five

cores contained delaminations while the remaining five

cores did not, as shown in Table 1. These results support the

confidence that can be placed in the reference coring and

chain drag tests and allows for a basis of comparison with

other NDE methods. In addition, the comparison between IE

and chain drag results in Section 2.2 indicates the sensitivity

of the IE test to defect proximity. The differences between

IE results at core locations relative to results at adjacent grid

points indicate this sensitivity. This effect may account for

the differences observed between the delamination percen-

tages determined using chain drag and IE, (Table 2).

The quality of chain drag surveys is affected by many

factors. For the results presented in this study, two person

teams of State bridge inspectors with different levels of

experience and using different procedures (based upon the

individual state inspection guidelines that each inspector

was accustomed to) carried out chain drag surveys that

showed variability in results from this technique. Although

this variability is significant, it should be noted that

improved training of inspectors and more accurate docu-

mentation of the results by inspectors could improve

inspection quality [9]. The accuracy of the NDEVC

reference chain drag survey was much higher than a typical

chain drag survey due to the extensive time taken to conduct

it (two days) and the refined two-foot grid that the results

were recorded on.

2.2. Impact-echo

The IE NDE technique was used to determine the

extent of concrete deck delaminations at the Van Buren

Bridge in a comparison with test results from other

bridge deck evaluation methods. The IE testing technique

is a stress-wave method, which uses mechanical energy

to transmit a sound wave into the concrete. A spectral

analysis of reflected compression waves (from internal

interfaces or external boundaries) is then performed to

Table 1

Results from cores and acoustic based bridge deck evaluation methods

Core number Core result NDEVC chain drag result IE (at core location) IE(at grid location)

1 Delaminated Delaminated Delaminated Delaminated

2 Delaminated Delaminated Delaminated Solid

3 Solid Solid Solid Solid



6 Delaminated Delaminated Delaminated Suspect distress

7 Delaminated Delaminated Delaminated Suspect distress


9 Delaminated Delaminated Delaminated Delaminated


Table 2

Percent of bridge deck found delaminated based on the NDEVC reference

chain drag survey and an IE survey at the Van Buren Bridge

NDEVC chain drag (deck

area percent delaminated)

IE (deck area percent

delaminated)

North span 6.3 10.3

Center span 17.7 28.3

South span 20.6 27.3

Total 14.8 22.0


determine the internal characteristics of the concrete at

that location.

When the material is stressed with a suddenly applied

load, stress waves propagate through the material at a finite

speed. This velocity is a characteristic of the material the

waves propagate through. When a concrete surface is

impacted, a transducer mounted on the same concrete

surface receives the longitudinal wave reflections from

discontinuities within the concrete, as shown in Fig. 6.

Since concrete is a heterogeneous material, detecting

reflected stress waves or echoes is very difficult. However,

by using fast Fourier transform (FFT) analysis, the time

domain signal is converted into a frequency domain

function, simplifying signal interpretation. Therefore, by

knowing the compression wave propagation velocity (Cp)

through the material, the frequency spectrum ( f ) can be

evaluated to determine the location of the discontinuity or

the thickness of the material. An assessment of the thickness

of the concrete is made by using the following Eq. 1

T ¼ Cp=2f ð1Þ

where T is the thickness of the medium (m), Cp,

compression wave velocity in the medium (m/s), and F is

the frequency (Hz).

IE testing of the Van Buren Bridge was carried out over

locations on a two-dimensional grid with a two-foot spacing

between grid locations (Fig. 2b). A small diameter steel

sphere (impactor) is used to generate a transient stress-wave

that propagates through the concrete deck. Fig. 7 shows the

IE test being performed using the Docter IE system

manufactured by Germann Instruments of Copenhagen,

Denmark. The Docter system consists of a hand-held testing

unit and a computer-based data acquisition system. The

hand-held unit contains a displacement transducer with a

conical piezoelectric tip attached to a cylindrical brass

backing. The transducer has a nominal resonance frequency

of approximately 1 kHz. The computer-based data acqui-

sition system consists of a portable computer with a data-

acquisition card and a proprietary IE software known as

Echo-Blue. The IE technique can be applied to a variety of

civil infrastructure elements, including plate-like structures.

Structures such as bridge decks, slabs and walls are

considered plate-like structures. Plate-like structures are

defined as those in which the lateral dimensions of the

structure are at least five times its thickness. Unlike other

concrete structures such as beams and columns, in which

reflections from the boundaries (the sides or edges of the

structure) introduce additional peaks in the spectral plot,

plate-like structures are typically characterized by a single

peak from the back side or bottom of the structure when data

is collected from a solid concrete area.

Fig. 8 shows the IE result for a test conducted at Grid

Point B8 (Core No. 4). Fig. 8(a) shows the detected

waveform in the time domain. A FFT analysis is then

performed to convert the waveform into a frequency domain

function, as shown in Fig. 8(b). This spectral plot has a

predominant frequency of 9.3 kHz. This frequency was

determined to be a typical full-thickness result from a solid

section. The frequency associated with the full-thickness of

the slab (9.3 kHz) was determined by calibrating the IE

equipment with nominal deck thickness (obtained from

extracted cores) and Eq. (1). This type of IE response

(shown in Fig. 8) is typical of plate structures, where a

single predominant peak is expected.

Fig. 9 shows the result for a test conducted at Grid Point

C26. The frequency content of the waveform can be

observed in Fig. 9(a). Correspondingly, the transformation

into the frequency domain (Fig. 9(b)) reveals multiple peaks

with high amplitudes and a predominant low frequency at

3.8 kHz. This type of response is characteristic of shallow

(thin) delaminated concrete. A shallow delaminationFig. 6. A schematic illustration of the IE test.

Fig. 7. The IE technique is being carried out on the Van Buren Road Bridge.


response is fundamentally different than a full thickness

response obtained from a solid section. The wave produced

by impacting the concrete surface excites the flexural modes

of vibration of the thin layer of concrete above the

delamination or crack. These modes of vibration correspond

to the hollow sound one hears in a delaminated test area in a

chain drag or hammer tap delamination survey. The

difference in the amplitude of the response in Figs. 8(a)

and 9(a) may be accounted for by the presence of the defect

for the Fig. 9(a) response and the variability associated with

using the manually actuated impactor.

The description of Figs. 8 and 9 shows the typical

analysis method for interpreting the collected IE data from a

bridge deck. Many other features can be observed in typical

data obtained using this technique [10], but are beyond the

scope of this paper.

2.3. Radar

Two types of radar systems were used to survey the Van

Buren Road bridge deck for delaminations. These systems

included a commercially available GPR system built by

GSSI called the subsurface radar system (SIR), and a

prototype system known as HERMES designed and

developed by LLNL for the FHWA. Both radars were

employed to survey all three spans of the Van Buren Road

bridge deck. Results from testing over the center span are

shown in Fig. 2(c) and 2(d) and are discussed in Sections

2.3.1 and 2.3.2.

2.3.1. Survey using the GSSI SIR-2000 system

The GSSI SIR-2000 system was used to scan along the

surface as shown in Fig. 10. Scanning was conducted at

0.61 m (2 ft) intervals across the bridge deck, following

reference lines placed on the deck surface. Data were

collected by passing the ground coupled antenna of the radar

over the locations marked with longitudinal reference lines,

labeled A–J in Fig. 2(b). The system was moved at a steady

rate over the reference lines and a trigger switch on the hand

grip of the system was used to start and stop the data

acquisition. The testing was conducted by collecting a series

of scans that were saved line by line for each span.

The GSSI system used in this survey employed a

broadband ground coupled antenna with a center frequency

Fig. 8. IE test response from a solid concrete location (Core 4), (a) time

domain plot; (b) frequency domain plot.

Fig. 9. IE test response from a shallow delamination location near grid point

C26, (a) time domain plot; (b) frequency domain plot.

Fig. 10. The GSSI’s SIR GPR system as used on the Van Buren Road

Bridge.


of 1.5 GHz. The system consists of an enclosure box with a

small survey wheel that houses a pair of antennas (one

transmitter and one receiver) and a long fiberglass rod fitted

with a handle attached to the box. The PC based controller

and data acquisition unit with LCD screen are built in a

separate sealed unit that can be carried on a shoulder strap.

The two units are integrated together so that the operator can

walk comfortably at normal and consistent speed to survey

the designated area.

Data was displayed as a two-dimensional image in real

time to confirm that the acquisition was proceeding as

expected. An example of typical data obtained by the

system during the testing is shown in Fig. 2(d), section

B–B. The data is presented as a two-dimensional GPR B-

scan image such that the horizontal axis is the spatial

dimension along the length of the bridge deck and the

vertical axis corresponds to the time required for the

reflected radar pulse to return to the antenna. For this data,

gray scale intensity represents the amplitude of the radar

reflection at a given location and time. High positive

amplitudes are indicated by light shades up to white and low

to negative amplitudes are indicated by dark shades down to

Fig. 11. Test data collected near Core 7 using (a) chain drag and IE (b) HERMES and (c) GSSI radar.


black. It should be noted that the GSSI data in Fig. 2(d) B–B

is keyed to a color bar with alternating gray and white areas,

positioned directly above the data. This color bar indicates

delamination locations based on the NDEVC reference

chain drag survey. The white portion of the color bar

indicates locations where the chain drag test showed there

was solid concrete and the gray portion indicates locations

where the chain drag test indicated delaminated concrete

was present. The results indicated by the color bar were

obtained from locations that correspond to the path of the

GSSI antenna that was followed to collect the data displayed

below the color bar.

Examination of the amplitude and phase of radar

reflections in delaminated and solid concrete locations

does not reveal a consistent response that distinguishes

delaminated concrete areas from solid concrete areas. The

dominant response is to transverse reinforcing steel in the

bridge deck, positioned orthogonal to the direction of

antenna motion. The response to this steel is indicated by

high amplitude reflections produced, where reinforcing steel

is directly beneath the antenna and trailing parabolic shaped

tails on either side of the steel as the antenna approaches and

passes it. Other notable features include distinct parabolic

features near the extreme left and right ends of the data

corresponding to responses to the bridge joints.

Observations of similar features in the GSSI data can be

made in Fig. 11(c), where a short section of GSSI data from

the middle span of the Van Buren Road Bridge, (passing

over a delamination near Core 7) is presented. The data in

this figure was collected as the GSSI antenna was moved

along the path indicated by the dashed line in Fig. 11(a). As

this GSSI data set is traversed from left to right, the top layer

of reinforcing steel (indicated by parabolic shaped

responses near the top of the figure) is detected at response

times that vary by a few tenths of a nanosecond. This may be

indicative of a physical change in the reinforcing steel

depth, but it may also be indicative of local material

property variation [11]. Another important feature in the

GSSI data is the variation in the amplitude of the response to

reinforcing steel. These variations can be observed in both

Figs. 2(d) and 11(c). A definitive reason for this amplitude

variation could not be determined, but these variations are

probably explained by attenuation with depth. An observed

reduction in the magnitude of the radar response to

reinforcing steel detected at later relative times supports

this hypothesis. It should be noted that the path traversed by

the HERMES response shown in Fig. 11(b) came from data

collected along a nearby but different path than the GSSI

radar data, as shown by the dotted line.

2.3.2. Survey using the HERMES system

HERMES is a unique prototype GPR system that was

designed and built as a specialized bridge deck inspection

tool. HERMES is capable of surveying a deck surface one

lane at a time at a scan width of about 1.9 m at normal traffic

speeds. The antennas are mounted in the back of a trailer

that can be towed at highway speeds over the bridge deck

while collecting and acquiring data. The HERMES

controller unit powers the antennas to launch ultra-wide

band microwave pulses. The antenna controller unit is

coupled with a distance encoder wheel that determines the

antenna’s firing time sequence as well as the trailer’s

position over the bridge deck. The HERMES shown in

Fig. 12 is collecting data over a bridge deck in Tennessee.

HERMES was used to collect data over the Van

Buren Road Bridge deck, providing coverage of the

majority of the bridge deck area. HERMES uses software

to manipulate the data collected by each of the antennas

in its array. The collected data from each antenna is very

similar to the data collected by a typical GPR system, as

shown in Fig. 2(d) section A–A. Parabolic shaped

responses to transverse reinforcing steel can be clearly

observed in the HERMES response, in a similar data

display to the GSSI data in Fig. 2(d) section B–B. One

notable difference between the two system responses is

the sharper definition of the reinforcing steel in the raw

data from the GSSI system (observed as a narrower

parabolic shape), relative to the raw data from HERMES.

This is due to the wide beamwidth HERMES employs

relative to GSSI to gain advantages for imaging using

wavefield backpropagation techniques. In the case of the

HERMES data, high amplitudes are displayed as white

and low amplitudes are displayed as increasingly dark

shades of gray. The data from all 64 antennas in the

array can be reconstructed using a procedure called

wavefield-backpropagation [12] that mathematically esti-

mates the location of the reflection sources imaged in the

original raw radar data. When this type of calculation is

done in three dimensions using HERMES data, a three-

dimensional volume of data can be generated to allow

interior bridge deck features to be imaged with a much

more realistic appearance than available by conventional

GPR systems.

The end result is data that can be examined in a variety of

ways. An example is presented in Fig. 2(c), where a

horizontal slice from a reconstruction obtained at the level

of the reinforcing steel is displayed as an overlay on

Fig. 12. Photograph of HERMES collecting data over a bridge deck in

Tennessee.


reference NDEVC chain drag data. This slice of recon-

structed data highlights the response to reinforcing steel in

the deck that are aligned with the skew of the deck. The high

magnitude reflections from the reinforcing steel are

displayed as light gray up to white while the background

reflections from other features are displayed in dark gray to

black. Note that the responses in the horizontal layer from

the reconstruction do not show a direct correspondence

between locations where HERMES data indicates a strong

reflection and areas where delaminations were detected by

the NDEVC reference chain drag survey. Additional raw

HERMES data is provided in Fig. 11(b). This data was

collected along a path that traversed Core 7, and is marked

with a dotted line in Fig. 11(a). The area, where the data was

collected traversed a delaminated area, according to the

NDEVC reference chain drag survey, and was later

confirmed to be delaminated based on observations of the

extracted core. The data presented in Fig. 11 shows a B-scan

image of the HERMES waveforms from this collected data

set. An arrow pointing downward in the figure defines the

location of the extracted core relative to the collected data.

An examination of these waveforms does not indicate a

significant amplitude change or phase shift in the delami-

nated area that included Core 7 and the surrounding area.

These findings are consistent with laboratory test results

[13], where simulated delaminations were not detectable

using the current generation HERMES technology. The

response to aluminum marking tape that was placed on the

deck is observed to the right of the core location in Fig. 11

and should not be mistaken for a response to any other

bridge deck feature.

3. Conclusion

Nondestructive evaluation techniques that have been

developed for the purpose of inspecting bridge decks

were tested on a concrete bridge deck for a comparison

study. Techniques included in the study were primarily

directed at detecting corrosion-induced delaminations

below the surface of the concrete. These techniques

included the chain drag method, IE, and two types of

GPR. Ground truth data based on a careful implemen-

tation of the standard chain drag method were confirmed

by examining concrete cores that were extracted from the

deck. The IE and GPR methods were compared to this

original reference data. Although the chain drag method

was used as a reference, it was also investigated to assess

its reliability. The results showed that current technology

implementing acoustic techniques, such as the chain drag

method and IE, are generally consistent with results from

coring when they are carefully performed. However,

there are significant drawbacks to the two acoustic

methods implemented in this study. The chain drag

method is susceptible to inconsistencies and errors due to

subjective interpretations an inspector must make to carry

out this method. The more quantitative IE method

reduces the chances of subjective errors but it is

extremely time consuming and results may not always

be conclusive. The drawbacks of these acoustic methods,

which also include a reduced or often complete inability

to investigate asphalt-overlaid decks, motivate the

investigation of the radar methods that were also

included in this study.

The two GPR systems that were included in this study

were investigated because they represent a potentially faster

and easier way of conducting bridge deck investigations.

Although the technology is continuing to be improved, the

current state of the art commercial systems and even the

prototype system used in this study did not produce

consistent responses to delamination features in the tested

bridge deck. The FHWA and a state pooled fund study are

supporting the development of a new system called

HERMES II that may have a capability to detect delamina-

tion features and may also allow for more rapid bridge

inspections in the future. This technology may also mature

to allow delaminations to be detected through asphalt

overlays.

Acknowledgements

The authors acknowledge the support provided by the

Federal Highway Administration’s Turner-Fairbank High-

wayResearchCenter(TFHRC).Contributions tothis research

were also made by Dr Brent Phares (Iowa State University)

and Dennis Rolander (Wiss, Janney, Elstner Associates) and

they are gratefully acknowledged for their efforts.

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A comparison of nondestructive evaluation methods for bridge deck assessment

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