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NONDESTRUCTIVE TESTING OF BRIDGE DECKS AND TUNNEL LININGS USING IMPULSE-RESPONSE

Authors: Jesper Stærke Clausen1, Asger Knudsen

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Synopsis:

The presence of poor bonding at interfaces between, asphalt, membranes and concrete on bridge decks or between

the original concrete and repair patches of tunnel linings often causes a faster deterioration of the different materials,

resulting in for example corrosion of the reinforcement, which can lead to spalling of the concrete cover layer. Visu-

al inspections often only disclose these problems at a late state in the deterioration process and repair or replacement

of portions or the whole structure can be expensive.

Regular inspections combining visual and NDT tools such as the impulse-response technique and verification of the

results by drilling out a few cores can disclose problems at an early state and provides valuable information of the

actual condition of the structure.

The use of the impulse-response technique gives the user an indication of the mobility and stiffness of the structures

and hence a tool to evaluate the presence of conditions such as poor bonding or delaminations in the structure. Large

areas can be tested rapidly and data are valuable for planning future strategies for maintenance or repair of a struc-

ture.

This paper presents some typical case histories with emphasis on the advantages and limitations of the impulse-

response technique.

Keywords:

Bridge Deck, Impulse Response, Mobility, Nondestructive Testing, NDT, Stiffness, Tunnel Lining, Voids index.

1 Ramboll Denmark A/S, dept. of Bridge Maintenance and Material Technology, Bredevej 2, DK-2830 Virum

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Biography of the authors:

Jesper Stærke Clausen, a Senior Engineer and specialist in NDT at Ramboll Denmark A/S. He has been using non-

destructive testing techniques since 1994. He has used the technique on concrete structures such as buildings,

bridges, tunnels, foundations and nuclear plants. The techniques have been used on both existing and new structures

for quality assurance. In recent years the techniques are also used to evaluate the disintegration of natural stones

such as marble and evaluate quality of old bridges with arches made of bricks.

Asger Knudsen,

INTRODUCTION

The development of reliable nondestructive test methods for evaluation of concrete structures such as bridge decks

and tunnel linings has improved the possibility to investigate large areas faster and giving much more knowledge of

the internal condition of the of the structure. Earlier this information was only available in areas where cores were

drilled or breakup was conducted giving only information of the internal conditions at local spots. In the worst case,

the conditions of the structures are revealed at advanced stages of the deterioration process in the form of spalling or

cracking

One of the test methods is the impulse-response test*, which is a nondestructive, stress wave test used for locating

voids, delaminations or poor support in concrete structures.

The introduction of portable computers and small data acquisition cards has improved the data quality and allowed

the possibility to gather large amounts of test data within a short period of time. Due to the large amount of acquired

data, training in the use of test equipment and understanding the structure are important to be able to perform tests at

the right locations in the structure and to make reliable evaluations of the data. The possibility to acquire a large

amount of data might be fascinating, but data are worthless if testing is conducted at the wrong place, distances

between grid points are too large or the settings of the test equipment are incorrect. Therefore, calibration of the

acquired data by drilling cores or breaking up concrete is still an important aspect of the NDT investigation.

In this paper, three case histories, are given showing typical testing cases where the impulse-response method has

been used with success.

METHOD

The impulse-response method is used for a fast screening of large areas of a structure with the purpose to determine

local areas with possible flaws for a later detailed analysis or for invaisive verification. On bridges, the impulse-

response method is able to locate delaminations and voids in the bridge deck or wearing layer, and honeycombing

and delaminations in the substructures in general.

* Available from: Germann Instruments (Denmark or USA)

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A sledgehammer with a built-in load cell in the ham-

merhead is struck against the surface. The impact is

approximately 100 times stronger than that of the

impact-echo method. This greater stress input means

that the plate responds to the impulse response ham-

mer impact in a bending mode over a very much lower

frequency range (0-800 Hz for plate structures), as

opposed to the reflective mode of the impact-echo

method (normally 2-50 kHz). This low strain impact

sends stress waves through the tested element (bridge

deck). The movement or velocity of the surface is

recorded with a velocity transducer (geophone). The

record of the impact force and the velocity of the sur-

face are stored in a laptop. Figure 1 shows impulse-

response testing in progress.

Figure 1 -- Illustration of the impulse-response equipment in

action. A test is conducted, evaluated, and stored within few

seconds.

In the following, a brief introduction is given to the theory behind the impulse-response technique and how to interp-

ret data. For more elaboration on the subject refer to Davis (2003) and Ottosen et al. (2004).

An impulse-response examination cannot be used alone to evaluate a concrete or a road structure. The method is

primarily considered as a relative method used for screening of large surfaces. The method should always be sup-

plemented and verified with other examinations. These verifications can be done by means of drilling cores, break-

ing up concrete or the use a borescope. Screening with the impulse-response gives the possibility to identify poten-

tially damaged areas.

From the spectrum (obtained using a fast Fourier transformation) of the hammer force and the spectrum of velocity,

the surface mobility plot is computed by dividing the Velocity spectrum (signal from geophone) by the force spec-

trum (signal from hammer load cell). An example of the mobility plot is shown in Figure 2. The vertical axis is in

the units of velocity per unit force and the horizontal axis is frequency.

Figure 2 -- Example of a mobility plot from a testing point from an impulse-response test.

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For the evaluation of impulse-response data the parameters listed in Table 1 are calculated from the mobility plot.

Table 1 also summaries the interpretation of these parameters on the basis of past experience with plate-like struc-

tures.

Table 1 -- Parameters used for evaluation of impulse-response data.

Average mobility The average mobility is shown as the dotted line in Figure 2.

Interpretation: The average mobility between 100 and 800 Hz is directly related to the

density, support, and thickness of the structure tested. A reduction of the thickness of the

plate results in an increase in the average mobility. If the top layer of a road surface is

completely or partly delaminated from deeper layers, the average mobility increases

because the mobility corresponds to the thin delaminated layer. A high value compared

with other measurements in an apparently homogeneous area is therefore an indication of

a delamination.

Stiffness Stiffness is the inverse of the slope below 80 Hz on the mobility plot (shown as the solid

line on Figure 2). The slope of the initial portion of the mobility plot is the flexibility

(compliance) of the structure.

Interpretation: The stiffness of the structures at the test point is a function of the concrete

quality, thickness, and support of the structure. The value is used on a comparative basis

to evaluate differences in the structure.

Mobility slope If honeycombed concrete is present in the structure the attenuation of the response is

reduced and the stability of the mobility-plot is affected within the range of 50 Hz to 1

kHz. This leads to a non stable mobility plot with an increasing trend as shown in Figure

2 (right side).

Interpretation: A higher average slope or non stable mobility plot indicates a higher

probability for honeycomb in the concrete structure.

Voids Index The ratio of the maximum initial mobility to the average mobility.

Interpretation: If delaminations are present or there is a lack of support of the structure,

the peak mobility below 100 Hz is much higher than the average mobility (See the left

side of Figure 2). If the value of the Voids Index is larger than 2-4 it’s an indication of

an area with a potentially poor condition (delamination or no support).

Figure 3 --Typical response due to flaws compared with sound responses. If a delamination or void is present under a

structure, a high mobility is measured at low frequencies (left figure). If honeycomb in concrete is present, an increas-

ing slope in the mobility plot is seen (right figure).

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The impulse-response method does not give any depth indications for the flaws. Only relative values of the various

parameters can be evaluated for the structure in question, which give an indication of “good” and “bad” areas. Other

tests or invaisive verification are needed to confirm the interpretation from the measured mobility plots.

CASE HISTORY– EMPHASIS ON DELAMINATIONS

1. Examination of a bridge soffit.

The soffit of an approximately 35 year old bridge was examined due to the suspicion of severe delaminations below

the edge beams at both sides of the bridge deck. A twin bridge parallel to the one in question was repaired 6 years

earlier and the repair was much more extensive than expected. The real extent of the damage was not discovered

before the repair was initiated, and the repair costs exceeded the replacement cost of the whole bridge. To avoid the

same mistake, NDT was incorporated in the test program to give more specific details about the actual condition of

the bridge deck.

A visual inspection revealed the presence of water leakage up to 2 m from the edge beams, see Figure 4. The use of

thermography disclosed that some areas around the drainpipes had shallow delaminations, see Figure 5.

Figure 4 -- Examples of water leakage under the bridge.

Figure 5 -- Damage is visible around the drainpipes and the thermogram (infrared picture) to the right shows the

extent of the delamination (marked with a cirle)

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The bridge crosses a highway with three lanes in each direction. It was decided to use the impulse-response method

to test the soffit of the bridge in the areas that showed signs of severe leaking. A grid with a spacing of 1 m x 1 m

was marked on the soffit and testing was made from a mobile lift with a platform that is 4 m long and 1.5 m wide.

Due to the dimensions of the platform, testing was conducted in 2-3 rows only in each test series and the data had to

be assembled afterwards. The software, however, enables the user to enter a start and end point in the grid and there-

fore the assembling is done easily.

After testing was done and data were assembled and analyzed on-site, three locations were selected for drilling cores

for verification of the data. The contour plot of average mobility and locations of the three selected spots where

cores where drilled are shown in Figure 6. The bridge is not perpendicular to the highway and therefore the plot

looks a little awkward at the edge beams of the bridge.

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Testpoint/meter

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US-broplade Average Mobility 5/10/2008

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idle lane

The outer lane

Centrestrip

Figure 6 -- Contour plot of average mobility from the soffit of the bridge deck. The location of the 3 cores is shown

with arrows.

The mobility plot for each dataset corresponding to core locations 2 and 3 are shown in Figure 7 and Figure 8. Note

the difference in the y-axis (mobility values). The data from location 2 show a peak mobility of 70 (average is 28)

and at location 3 the peak limit is only 10 (average is 5). With reference to Table 1 this indicates, that the thickness

of the plate in location 2 has been “reduced”, and this reduction of the thickness of the plate results in an increment

in the average mobility. The “reduction” of the thickness is due to a delamination in the concrete structure, see Fig-

ure 9.

Prior to drilling the cores, the reinforcement and cable ducts with pre-stressed cables in the bridge deck were located

using ground penetrating radar with a 1.6 GHz antenna.

Cores for calibration

1 3 2

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Figure 7 -- Mobility plot from core 2. Note the peak mo-

bility on the y-axis is approximately 70.

Figure 8 -- Mobility plot from core 3. Note the mobility

peak on the y-axis is approximately 10.

Figure 9 — The three cores taken from the soffit of the bridge. The concrete surface is to the left in each photo. The

presence of the delaminations at locations 1 and 2 resulted in the higher average mobility values shown in Figure 6.

The cores from each location are shown in Figure 9. Examination of the bore holes showed that the cores, which

came out in more than one piece, all had internal cracks before the coring. The cores confirmed the evaluation of the

impulse-response data.

The impulse-response examination concluded that the soffit of the bridge deck was delaminated up to approximately

2 m from the edge beams of the bridge deck. Based upon the impulse-response examination, experience from the

adjacent bridge, and, a cost-benefits analysis of various repair strategies or a replacement of the bridge, it was de-

cided to replace the bridge and only reuse the foundation.

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2. Examination of bridge deck surface

The road surface of a bridge crossing a marsh area with two small creeks was examined using the impulse-response

method due to the suspicion of flaws in the membrane between the asphalt layer and concrete structure. NDT was

incorporated in the test program to give more specific information about the relative condition of the bridge deck in

selected areas and due to the limited time available on-site. See Figure 10.

Figure 10 -- The bridge and testing in action with the impulse-response equipment.

Three grids each with spacing of 0.5 m x 1.0 m and up to approximately 52 m long were made on the northern part

of the bridge. Two grids were located in the south-going lane and one in the north-going lane (referred to as Grid 1,

Grid 2 and Grid 4). One other grid was located in the southern part of the bridge (referred to as Grid 3). See Figure

11 and Figure 12.

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Figure 11 – Aerial view of the northern part of the bridge.

The location of the 3 grids is highlighted.

Figure 12 -- Aerial view of the southern part of the

bridge. The location of the grid is highlighted.

After testing of each grid was done and data had been analyzed on-site, three locations were selected for drilling

cores to verify the data interpretation. Voids Index larger than 2-4 it’s an indication of an area with a potentially

poor condition. Location D has a Voids Index in the range 1-2 (low), location E has a Voids Index in the range 4-5

(high), and location F has a Voids Index in the range 3-4 (medium). The cores drilled from location D and E are

shown in Figure 16.

Figure 13 show the Voids Index contour plot and the location of the three selected test points where cores where

drilled for grid no. 2.

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Dist. in m from southern joint

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Edge beam

Felt 2 Voids Index 13/5/2008

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F

Figure 13 -- The Voids Index contour plot from grid 2 on the bridge deck. The locations of the three cores are shown

as points D, E and F.

Grid 2

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Figure 13 also indicates the location of the supporting columns/beams under the bridge (See Figure 10). Their loca-

tions are marked with solid arrows and they are exactly 9 meters apart.

The mobility plot for each dataset at the locations of cores D and F are shown in Figure 14 and Figure 15. Note the

difference in the scale of the y-axis. The plot from location D shows a maximum mobility of 18. This plot is a typi-

cal plot for a solid structure. In the plot from location F, there is a distinct peak with a value about 130. This plot is

typical for a delaminated structure.

The Voids Index is equal to the ratio of the maximum initial mobility to the average mobility. The Voids Index is

shown at the bottom of each plot in Figure 14 (Voids Index of 1.4) and Figure 15 (Voids Index of 4.1).

The cores marked D and F are shown in Figure 16. Examination of the bore holes showed that the cores, which

came out in more than one piece all had internal cracks prior to the coring. The cores confirmed the interpretation of

the impulse-response data.

Figure 14 -- Mobility data from core D. Note the mobility

peak in the y-axis is approximately 7.

Figure 15 -- Mobility data from core F. Note the mobility

peak in the y-axis is approximately 130.

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Figure 16 -- Left side shows core D (top) and the bore hole, no cracks are visible. Right side shows core F (top) hori-

zontal cracks are visible at the bottom of the bore hole, see arrow.

The impulse-response examination concluded that the area within 1 m from the edge of the bridge was largely dela-

minated. Some local areas in the wheel tracks were also delaminated. The cores showed that delaminations were

present preliminary in the concrete base of the bridge deck.

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3. Preliminary examination of ceiling of submerged tunnel

The ceiling of a submerged tunnel had been repaired approximately 15 earlier because the ceiling was not water

tight. During the repair 15 to 25 cm of concrete had been replaced and injections were made in large areas of the

remaining concrete. Just few years after the repair, water began to penetrate through the ceiling again. Geological

studies have afterwards revealed that settlement took place in the northern part of the tunnel, which was built on an

artificial sand cushion.

A new repair strategy for the ceiling was required and hence the location and size of damaged areas of the ceiling

had to be determined. The use of NDT was incorporated in the test program to give information about the general

condition of the ceiling. The ceiling was covered with an approximately 2 to 4 cm thick layer of fireproofing con-

crete, which is quite soft and made the testing much more difficult. Photos from the tunnel during impulse-response

testing are seen in Figure 17.

Figure 17 -- The ceiling of the tunnel and testing in action with the impulse-response equipment.

Two grids each with a spacing of 1.0 m x 1.0 m, 9 m wide and up to 15 m long, were made on the ceiling of the

south going tube of the tunnel.

After testing of both grids and analyzing the data on-site, two locations were selected for drilling cores for verifica-

tion of the data interpretation in the grid no. 2. Figure 18 shows the contour plot of the average mobility plot and the

location of the two points where cores where drilled. Core no. 1 is located in a point, where the average mobility is

high, indicating a delaminated concrete. Core no. 2 is located in a point, where the average mobility is low, indicat-

ing a sound concrete.

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Figure 18 – Contour plot of the average mobility from grid no. 2. The location of the two cores is marked with solid

dots.

1

2

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The cores marked 1 and 2 are shown in Figure 19. Core 1 was located in the middle of a delaminated area as indi-

cated by the high mobility. Core 2 was located at the edge of a delaminated area, as indicated by the low mobility.

Unfortunately no photos are available from bore hole 2. Examination of the bore holes showed that the cores, which

came out in more than one piece, all had internal cracks before coring. The cores confirmed the interpretation of the

data acquired from the impulse-response tests.

Figure 19 -- Left side shows core 1 (top) and the bore hole. Severe cracks are visible, see arrow. Right side shows core

2, some spalling is visible one side of the core.

The impulse-response examination concluded that large areas of the tested ceiling were delaminated. The cores

showed that delaminations were present throughout the concrete from the repair 15 years earlier.

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CONCLUSIONS:

Because of the limited time available for on-site testing of bridges or tunnels due to mainly the high costs for regula-

tion of the traffic, the use of nondestructive testing has set new standards for how large an area of a structure and

how much valuable information that can be acquired within just 1 day of testing. However, it is vital that the staff is

well trained in the underlying theory as well as the operation of the equipment and has knowledge about the struc-

tures being tested, otherwise, lots of worthless data are collected.

Verification of the data interpretation at each site is vital to ensure that the data have been evaluated correctly.

This paper evaluates the impulse-response method in some typical testing cases, where the method has been used to

locate delaminations in two bridge decks and in the ceiling of a submerged tunnel. The data has been presented in

various contour plots and mobility plots of impulse-response parameters. For each case the interpretation of the data

has been verified by drilling cores.

References:

Davis, A.G.: ”The non-destructive impulse response test in North America: 1985-2001”, NDT & E International 36

(2003), 185-193, Elsevier Science Ltd.

Ottosen, N.S, Ristinmaa, M & Davis, A.G,: ”Theoretical interpretation of impulse response test of embedded con-

crete structures”, Div. of Solid Mechanics, Lund University, Lund, Sweden, ASCE Journal of Engineering Mechan-

ics, V. 130, no. 9. Sept. 2004.