17th World Conference on Nondestructive Testing, 25-28 Oct 2008, Shanghai, China

Nondestructive Elastic-Wave Tests of Post-tensioned Concrete Girders in

Road Bridge

Krzysztof SCHABOWICZ 1, Jerzy HOLA 1 1 Institute of Building Engineering, Wrocław University of Technology,

WybrzeŜe Wyspiańskiego 27, 50-370 Wrocław, Poland, Phone. +48 71 320 29 00,

Fax: +48 71 322 14 65, e-mail: k.schabowicz@pwr.wroc.pl, jerzy.hola@pwr.wroc.pl

Abstract

This paper deals with the nondestructive elastic-wave testing of post-tensioned concrete girders in a

five-span road bridge built in Poland in the early 1960s. The tests were carried out to assess the technical

condition of the girders and to take a decision about the range of repairs to be done to the girders or about their

strengthening. The Impact-Echo technique and ultrasonic and electromagnetic techniques (in a support role)

were used in the tests. All the bridge’s 20 girders were tested. The main aim of the tests was to detect voids in

the grout filling the tendon sheaths, air voids between the tendons, and zones of air gaps in concrete due to its

poor compaction. The nondestructive test results and the analyses carried out on their basis, corroborated by

exposures, have not been published before.

Key words: road bridge, post-tensioned concrete girders, concrete, nondestructive tests, elastic waves.

1. Introduction

The road bridge over the Warta River (Poland) was built in the early 1960s. It has five

spans supported by four intermediate piers and two abutments. The overall length of the

bridge, including the abutments, is 186.0 m and its width is 9.82 m. In the bridge cross section

in each span there are four prefabricated reinforced concrete girders, each having a length of

30 m and an I-bar cross section height of 210 cm, tensioned with 61×∅5 mm Freysinett wire

tendons. There are five such tendons (differing in their trajectories) in each girder. Figure 1

shows a general view of the bridge from the downstream side. The support zone of the post-

tensioned concrete girders is shown in fig. 2.

After over 45 years of being in service the bridge was to be repaired. Nondestructive

methods were used to assess the technical condition of the main load-bearing structural

components, i.e. the post-tensioned concrete girders. The nondestructive tests covered, among

other things, an assessment of tendon sheath filling with injected grout and the location of air

voids and uncompacted concrete zones. The data were needed to calculate the actual load-

bearing capacity of the bridge and to determine the range of repairs or take a decision to

strengthen the girders. The Impact-Echo technique and supporting ultrasonic and

electromagnetic techniques were employed for this purpose.

Fig. 1. General view of bridge

from downstream side.

Fig. 2. View of support zone

of post-tensioned concrete girders.

2. Range of tests and their short description

All the twenty post-tension concrete girders were tested. Tendon sheath grout filling for

all the five tendons in each girder was investigated using the Impact-Echo technique [1-7].

The tendon and reinforcement runs in the girders were located using the electromagnetic

technique. In 1200 spots in each girder an elastic wave was nondestructively induced by

means of an Impact-Echo inductor and the amplitude image of the wave was recorded over

time. Then the image was transformed using the fast Fourier transform and specialist software

to an amplitude-frequency spectrum of the recorded wave. The spectra were the basis for an

analysis of the on-site test results. Table 1 shows the post-tensioned concrete girder test

parameters predetermined using the Impact-Echo and ultrasonic equipment.

Table 1. Post-tensioned concrete girder test parameters.

Girder

Wave velocity P Cp

[m/s]

Element thickness

T [mm]

Cover thickness

d [mm]

Element thickness frequency

fT [kHz]

Tendon frequency

fd [kHz]

Defective tendon

frequency fv

[kHz] 1 3 4 5 6 7 8

1 3700 186 68 9.95 13.60 27.21

2 3800 180 65 10.56 14.62 29.23

3 3600 177 63.5 10.17 14.17 28.35

4 3900 183 66.5 10.66 14.66 29.32

3. Test results and their analysis

Certain characteristic groups of signals were distinguished on the basis of the Impact-

Echo tendon sheath grout filling test results for the bridge girders.

The exemplary signal shown in fig. 3 is characteristic of concrete free of defects in the

tested place (on the web). In this case the high amplitude peak at a frequency of 9.76 kHz is

for a structural component (girder web) thickness of 177 mm.

Fig. 3. Exemplary elastic wave amplitude-frequency spectrum for component thickness, recorded using Impact-

Echo technique. Theoretical test situation and relation for component thickness, according to [6], are shown in

top right corner.

The signal shown in fig. 4 is characteristic of tendon sheaths fully filled with grout. In this

case the peak at a frequency of 9.16 kHz is for a component thickness of 186 mm. Whereas

the peak at a frequency of 13.18 kHz indicates that the tendon sheath is fully filled with grout,

which was confirmed by the exposure shown in fig. 5.

Fig. 4. Exemplary elastic wave amplitude-frequency spectrum recorded using Impact-Echo technique, indicating

tendon sheath fully filled with grout. Theoretical test situation and relation for tendon sheath fully filled with

grout, according to [6], are shown in top right corner.

Fig. 5. Uncovered tendon sheaths fully filled with grout.

9,16 kHz

13,18 kHz

9,76 kHz

Frequency [kHz]

Am

plit

ude

Frequency [kHz]

Am

plit

ude

The signal shown in fig. 6 is characteristic of tendon sheaths not fully filled with

grout. The peak at a frequency of 8.79 kHz is for a component thickness of 177 mm. The peak

at a frequency of 35.43 kHz indicates a place where the tendon sheath is not filled with grout.

In such places the concrete cover was removed by drilling a hole or chiselling in order to

verify the nondestructive test result. Such an exposure is shown in fig. 7. Voids in the injected

grout filling were typically about 10 cm long and they occurred in four tendons in three

girders.

Fig. 6. Exemplary elastic wave amplitude-frequency spectrum recorded using Impact-Echo technique, indicating

tendon sheath not fully filled with grout. Theoretical test situation for tendon sheath not fully filled with grout,

according to [6], is shown in top right corner.

Fig. 7. Uncovered tendon in girder web. Tendon sheath not fully filled with grout and tendon wires covered with

rusty tarnish.

Simultaneously conducted ultrasonic tests, during which ultrasonic pulses were passed

through the webs and flanges of the post-tensioned concrete girders to assess the uniformity

of the concrete, showed longer pulse passage times in some areas. This was the case in as

many as 11 girders. Therefore additional Impact-Echo tests were carried out in those areas.

The tests indicated honeycombing, as shown in fig. 8. The characteristic high amplitude peak

at a frequency of 10.74 kHz indicates a component thickness of 177 mm. The “packet” of

lower amplitude peaks with different frequencies indicates uncompacted concrete. The test

results were confirmed by the exposure shown in fig. 9.

35,43 kHz

8,79 kHz

Frequency [kHz]

Am

plit

ude

Fig. 8. Exemplary elastic wave amplitude-frequency spectrum recorded using Impact-Echo technique, indicating

uncompacted concrete near tendon.

Fig. 9. Uncovered tendon in girder flange. Air voids visible near tendon.

Difficulties in recording ultrasonic pulses (using the ultrasonic technique) were

encountered in a few test areas. The Impact-Echo signal recorded in such places made it

possible to locate air voids there, as shown in fig. 10.

Fig. 10. Exemplary elastic wave amplitude-frequency spectrum recorded using Impact-Echo technique,

indicating void between tendons.

As the exposure in fig. 11 shows, the sheath of tendon 2 is fully filled with grout, but tendons

3 and 5 are located close to it, i.e. in a location inconsistent with the design. The location of

the tendons was changed during construction. As a result, relatively narrow air voids formed

between tendons 2-3 and 2-5, which were not filled with concrete.

10,74 kHz

18,06 kHz 33,66 kHz

Frequency [kHz]

Frequency [kHz]

Am

plit

ude

Am

plit

ude

Fig. 11. Uncovered tendons in central part of one girder. Tendon sheaths are filled grout, but tendons 3 and 5 are

situated (inconsistently with design) too close to tendon 2.

4. Conclusion

The results of the nondestructive elastic-wave tests and the exposures verifying them have

shown that there are voids in the grout filling the tendon sheaths, air voids between the

tendons and air voids in the (uncompacted) concrete in the road bridge built in Poland in the

1960s. The Impact-Echo was found to be very helpful in detecting the defects. The test results

were used to calculate the actual load-carrying capacity of the bridge and to determine the

scope of repairs to the post-tensioned concrete girders. Thus the tests have contributed to an

improvement in the durability and service safety of the bridge.

References:

[1] ASTM C 1383, “Test Method for Measuring the P-Wave Speed and the Thickness of

Concrete Plates using the Impact-Echo Method,” 2000, ASTM Standards Vol. 04.02.

[2] Garbacz A., “Nondestructive investigations of polymer-concrete composites using stress

waves – repair efficiency evaluation”, Politechnika Warszawska, Warsaw, 2007.

[3] Jaeger, B.J., Sansalone, M.J., and Poston, R.W., 1997, “Using Impact-Echo to Assess

Tendon Ducts,” Concrete International, Vol. 19, No.2, February, pp. 42-46.

[4] Lin, J. M., and Sansalone, M., 1997, “A Procedure for Determining P-wave Speed in

Concrete for Use in Impact-Echo Testing Using a Rayleigh Wave Speed Measurement

Technique,” Innovations in Nondestructive Testing, SP-168, S. Pessiki and L. Olson,

Eds., American Concrete Institute, Farmington Hills, MI, pp.137-165

[5] Ottosen N. S., Ristinmaa M., Davis A.G., “Theoretical Interpretation of Impulse Response

Tests of Embeded Concrete Structures”, Journal of Engineering Mechanics, ASCE, Vol.

130, No. 9, September, 2004, pp 1062-1071.

[6] Sansalone M., Street W., “Impact-Echo Nondestructive evaluation of Concrete and

Masonry”, Bullbrier Press Ithaca, N.Y., 1997.

[7] Sansalone, M, 1997, “Impact-Echo: The Complete Story,” ACI Structural Journal, Vo. 94,

No. 6, November-December, pp. 777-786.

tendom 5 tendom 3

tendom 2

tendom 3

tendom 2