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

Evaluation of Depth of Deterioration of Concrete Structure after Fire Using Stress Wave Method

Chia-Chi CHENG,

Department of Construction Engineering; Chaoyang University of Technology, Taichung County,

Chinese Taiwan

Phone: +886 4 23323000ext 4243, Fax: +886 4 23742325; e-mail: cccheng@cyut.edu.tw

Abstract

The aim of the present research is to evaluate the degree of deterioration by means of

stress waves. In this study, the stress waves generated by the impact of a steel ball on

concrete surface are recorded by a displacement receiver 200 mm away. The received

signal is analyzed in the frequency domain. The frequencies of the dominant response

in the spectrum are used to evaluate the depth of deterioration for concrete specimens

experienced various high temperatures. Two types of specimens were used for

investigation. One is small specimen with dimensions 40*40*15 cm3. These

specimens were heated with the temperatures 300 to 800. The other is a large

concrete plate with the size 240*130*15 cm3. The specimen is heated by fire with the

highest temperature 600. The principal frequencies obtained from the small

specimens were compared to the depth of deterioration evaluated by drilled core. The

cores were sliced into thin disks and the depth of the deterioration was evaluated by

the dynamic elastic modulus of the disks. The test procedures were also applied to the

big plate specimen to obtain the depth of deterioration. The test results show the depth

of deterioration can not be evaluated when it is less than 2 cm. A linear relationship

can be found between the lowest peak frequency and the depth of deterioration.

Keywords: stress waves, concrete, fire, depth of deterioration

1. Introduction

Immediate collapse is usually not the case for reinforced concrete structure damaged

by fire. The damage assessment for RC structures after fire is important for engineers

to decide the strategies of retrofit or repair. For damage assessment, the issue

concerned most would be the depth of concrete deterioration. When the damage of

concrete reaches the surrounding of the steel bars, the bond between the steel bar and

concrete would decrease so would the bearing capacity of the structure member. The

typical way to evaluate the depth of the concrete deterioration is by core drilling. The

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method can not be generally applied because it will locally damage the structure. Thus,

it is important to develop techniques for evaluating the depth of concrete damages

in-situ non-destructively after fire.

The aim of this study is to evaluate the depth of deterioration by stress wave

related method. In this study, the stress waves generated by the impact of a steel ball

on concrete surface are recorded by a displacement receiver 200 mm away. The

received signal is analyzed in the frequency domain. The frequencies of the dominant

response in the spectrum are used to evaluate the depth of deterioration for concrete

specimens experienced various high temperatures.

2. Experimental Design

Two types of specimens were cast in this study. Small block specimens with

dimensions 40*40*15cm3 were designed for studying the effects of variations of

concrete mixtures and oven temperatures to the test responses and establishing the

empirical relationship between the measured response and depth of deterioration. The

large plate specimens with dimensions 240×130×15cm3 were constructed to study the

effect of lateral dimensions to the response. Moreover, as the small block specimens

were placed in an electrical oven for heating and the large plate specimens were

placed in a flame chamber in the simulating fire environment, the frequency-depth

relationship established by the small specimens can be verified by the tests performed

on the large plate specimens. The concrete mixtures were listed in Table 1. There are

four w/c ratios, 0.45, 0.55, 0.6, and 0.65 and five highest oven heating temperatures,

namely 300°C, 400°C, 500°C, 600°C, 800°C used for the block specimens. All the

specimens were exposed to the highest oven temperature for two hours. For the plate

specimens, the w/c ratio is 0.6. In the heating process, the temperature elevation

followed the standard fire curve [1] to the highest chamber temperature 600°C. The

specimen experienced the highest temperature for two hours and naturally cooled

down to room temperature.

The small blocks were cured in water for 28 days, air dried, and heated at the age

90 days. To ensure a complete dryness, the specimens were preheated in oven at 60

for 24 hours before the regular heating process. The specimens were heated by one

side only to simulate the condition in fire site. To achieve that, four blocks with

different w/c ratios were arranged back to back and surrounded by ceramic sheets and

gypsum plates as shown in Figure 1. The plate specimen was also heated on one sided

in the gas oven.

For block specimens, the impact tests were performed by 3 mm-dia. steel ball on

the heated surface along the four test lines shown in Figure 2. In the test, a

displacement receiver is placed 20 cm away from the impactor. The displacement was

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recorded in every 1.334 µs. There are 2048 data been recorded, which leads to 0.732

kHz for frequency resolution of the amplitude spectrum. For every test line, three

repetitive tests were performed to obtain the averaged amplitude spectrum.

After performing the non-destructive tests, one drilled core with diameter 8.1 cm

was taken from each specimen. Each core was cut through the depth and sliced into

1.5 cm-thick pieces. The depth of the concrete deterioration after high temperature

was estimated by the dynamic elastic modulus (Ed) of the disks. The dynamic elastic

modulus (Ed) of the disk is estimated by the fundamental flexural vibrational modal

frequency (f) of the disk. F was measured by an accelerometer placed on the side

surface of the disk which was hung by a string, as shown in Figure 3, following the

test method demonstrated in ref. [2]. The vibration is triggered by a 3mm-diameter

steel ball impacting on the center of the disk. Ed was estimated by Eq. (1). In the

formula, the diameter (d) and density (ρ) were measured for individual disk and the

poisson ratio (ν) was assigned as 0.2. The fundamental modal frequency parameter

(Ω0) for the disk with specific height and diameter was obtained from the eigenvalue

of the numerical model constructed by a finite-element program ANSYS.

Ω+=

0

)1(2fd

Ed

πρν (1)

3. Experimental Results

To reduce the effect of wave reflections from the side boundaries of the block

specimens, only the first 512 data was used for spectrum analysis. The waveforms are

added to 8192 points with the rest of the data set to zero to increase the resolution of

the response in frequency domain. The amplitude spectra are normalized by adjusting

the highest peak amplitude to 1. To illustrate the effects of different oven temperatures

to the test responses, the amplitude spectra averaged from three repetitive tests

obtained from the w/c 0.65 specimens are shown in Figure 4. For the specimen

without heating, shown in Figure 4(a), a dominant response can be found at 18.39

kHz corresponding to the major modal vibration generated by the impact. The

principal frequency becomes lower for higher oven temperature. As shown in Figure

4(b) to (f), the lowest dominant frequencies move to 17.39, 14.0, 13.18、11.16 及8.97

kHz, for oven temperatures 300, 400, 500, 600, 800°C. Moreover, there are some

secondary higher amplitude peaks at the frequencies range 20-50 kHz for oven

temperature 300-500°C. For oven temperatures 600 and 800°C, the responses at the

previous mentioned frequency range are suppressed and those at lower frequencies

are dominant.

As the frequencies of the dominant peak are affected by the surface deterioration,

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the frequencies of the peak amplitude larger than 0.8, which are defined as the

dominant frequencies, for all of the test responses are recorded. Figure 5 shows the

distribution of the dominant frequencies with respect to the oven temperatures for all

the specimens with various w/c ratios. In the figure, we can find the lowest dominant

frequency decreases with the oven temperature for all the w/c ratios. For the

specimens with w/c 0.45, 0.55 and 0.6, single dominant frequency near the one of

room temperature is found for oven temperature less than 400°C. In contrast, for w/c

0.65 specimen, multiple dominant frequencies spreading at a large frequency range

are found at oven temperature 400°C. The presence of the multiple dominant

frequencies indicates the existence of micro-cracks which complicates the wave

propagation. Thus, the test results show the concrete with lower strength, such as the

case of w/c 0.65, was damaged at lower temperature. On the other hand, for the oven

temperature 800°C, narrower dominant frequency ranges were found for all the w/c

ratios. It may imply higher frequency contents of the stress waves generated by an

impact can not be propagate through the damaged layer, so the damaged layer acts as

the low-pass filter.

The depth of the concrete deterioration is evaluated from the dynamic modulus

of the sliced disks obtained from the drilled cores. Figures 6(a) and (b) show the

amplitude spectra obtained from the solid and damaged disks. The dominant peak

frequency is about 11 kHz for the solid disk corresponding to the fundamental

flexural vibrational mode. For the badly damaged disk, such as the case shown in

Figure 6(b), multiple peaks are found in spectra due to the existence of cracks inside

the disk. When the disk has only minor damages, one dominant peak was found but

the peak frequency is usually lower than 10 kHz indicating lower dynamic modulus.

Thus, concluded from the test results, the depth of concrete deterioration is defined by

the deepest position of the slice with the spectrum having multiple dominant peaks or

mono-peak frequency less than 10 kHz. Table 2 shows the estimated depths of

deterioration for all the specimens. In Table 2, the second column shows the lowest

dominant frequency obtained from the specimen-impact-tests and the third column

shows the estimated depth of deterioration. The relation between the lowest dominant

frequency and the depth of deterioration is shown in Figure 7. Because the lowest

dominant frequency are all around 17 kHz for the depth of deterioration less than 2

cm, the modal vibration of the specimens still controls the response for these cases

with shallow damages. For the depth larger than 2 cm, the depth of the lowest

dominant frequency decreases with the depth of deterioration and the corresponding

empirical formula is shown in Eq. (2).

521.28991.1 +−= fd R2=0.8463 (2)

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The same experimental process is applied to the large concrete plate specimen.

As the spectra are unlikely affected by the mode shape of the specimen, the

displacement waveforms did not set to zero after the 512 data point like those for

small block specimens. As shown in Figure 8, there are six positions been tested after

fire exposure. Three positions labeled as A, B, and C are at the area without surface

spalling and the three positions labeled as BR1, BR2, and BR3 are at the area with

surface spalling. The orientation of the test lines are also indicated in Figure 8. The

depth of deterioration estimated by Eq.(2) is compared with the real depth estimated

by the core slicing technique. The results and the errors are shown in Table 3. It was

found the error in estimation is less than 1.1 cm for the cases with lowest dominant

frequency less than 10 kHz. For the other cases, the depths of damage are all around

10 cm but the lowest dominant frequencies are not very stable and usually higher than

expected. It was suspected the orientation between the test line and the major cracks

may affect the value of the lowest dominant frequencies.

4. Conclusions

The test results show this innovational test method is easy in operation and has

large potential in estimating the depth of the deterioration larger than 2 cm. As

whether the depth of deterioration is larger than the concrete cover is the major

concern for engineers, the method may be proper for fire-site evaluations. However,

more studies are needed for obtaining the correct lowest dominant frequency and the

effect of internal reinforcement to the test response.

Acknowledgements

The research was sponsored by the National Science Council of the Republic of China,

Taiwan, under contract no. NSC 95-2211-E-324-050-.

Reference

[1] Lin, Y. and Su. W.C., “The Use of Stress Waves for Determining the Depth of

Surface-Opening Cracks in Concrete Structures,” Materials Journal of the

American Concrete Institude, Vol.93, No.5, pp.494-505 (1996).

[2] Leming, M. L., Nau, J. M., and Fukuda, J., “Nondestructive determination of the

dynamic modulus of concrete disks,” ACI Mater. J., 95(1), pp. 50–57 (1998).

Table 1 concrete mixtures for laboratory specimens

concrete mixtures(kg/m3)

6

w/c water cement fine aggregate coarse

aggregate

0.65 241 372 592 1044

0.6 235 392 592 1044

0.55 227 415 592 1044

0.45 208 469 592 1044

Table 2 The lowest dominant frequency and the measured depth of deterioration for

small block specimens

w/c 0.45 w/c 0.55

ambient temp.

lowest dominant

freq.

depth of deterioration

ambient temp.

lowest dominant

freq.

depth of deterioration

(°C) (kHz) (cm) (°C) (kHz) (cm)

23 19.491 - 23 18.3014 -

300 18.9419 - 300 18.1184 -

400 18.6674 - 400 17.4778 0.8

500 17.8439 1.8 500 17.294 1.9

600 16.1967 1.8 600 11.6214 2.6

800 10.0658 9.8 800 9.1507 9.8

w/c 0.55 w/c 0.65

ambient temp.

lowest dominant

freq.

depth of deterioration

ambient temp.

lowest dominant

freq.

depth of deterioration

(°C) (kHz) (cm) (°C) (kHz) (cm)

23 18.4844 - 23 17.9354 -

300 17.4778 - 300 17.4778 -

400 17.1118 0.8 400 13.9091 0.8

500 16.83 1.5 500 15.922 1.8

600 12.2619 4.3 600 11.0723 8.5

800 9.88275 9.7 800 8.51015 10.5

Table 3 Comparison between the estimated and measured depth of deterioration

Test position

Lowest dominant freq.(kHz)

depth from eq. (2) (cm)

depth from drilled

core(cm) errors(cm)

A 10.98 6.66 8.61 2.0

B 14.64 -0.63 11.24 11.9

C 10.98 6.66 10.11 3.5

7

BR1 11.71 5.20 9.99 4.8

BR2 9.52 9.57 8.43 -1.1

BR3 7.32 13.95 13.72 -0.2

Figure 1 Four blocks with different w/c ratio were arranged back to back and

surrounded by ceramic sheets and gypsum plates before put into the oven

Figure 2 Positions of four test lines for small

block specimen

Figure 3 Experimental setup for measuring

the dynamic elastic modulus

0-0.6518.39

00.20.40.60.8

11.2

0 10 20 30 40 50 60

Frequency (kHz)

Am

plit

ude

(

300-0.6521.87

17.75

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60Frequency (kHz)

Am

plit

ude (

400-0.6517.8414.00

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60Frequency(kHz)

Am

plitude (

gypsum

Ceramic

sheet

8

500-0.6513.1815.37

24.07 49.6043.83

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60Frequency (kHz)

Am

plitude

(

600-0.6511.1613.09

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60Frequency (kHz)

Am

plit

ude

(

800-0.658.97

11.44

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60Frequency (KHz)

Am

plitude

(

Figure 4 Amplitude spectra for w/c 0.65 specimens experiencing different temp.

w/c 0.45

0

10

20

30

40

50

0 200 400 600 800

Temperature()

Fre

quency

(kH

z) (

w/c 0.55

0

10

20

30

40

50

0 200 400 600 800

Temperature()Fre

quency

(kH

z) (

w/c 0.60

0

10

20

30

40

50

0 200 400 600 800

Temperature()

Fre

quen

cy(k

Hz)

(

w/c 0.65

0

10

20

30

40

50

0 200 400 600 800

Temperature()

Fre

quency

(kH

z) (

Figure 5 The distribution of the dominant frequencies w.r.t. the oven temperatures for

the specimens with various w/c ratios

0.45-23-1

11.71

0

1000000

2000000

3000000

4000000

0 20 40 60 80 100

Frequency(kHz)

Am

plitude

0.45-500-1

6.5813.54

0

500000

1000000

1500000

2000000

0 20 40 60 80 100

Frequency(kHz)

Am

plitu

de

(a) (b)

Figure 6 The amplitude spectra obtained from (a) the solid and (b) the damaged disks.

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

y = -1.991x + 28.521R2 = 0.8463

02468

101214

0 5 10 15 20

Frequency (kHz)

Dept

h (c

m)

(

Figure 7 The relation between the depth

of deterioration and the lowest dominant

frequency

Figure 8 Test positions labeled as A, B,

C, BR1, BR2, and BR3 are at the area

with surface spalling.