Experimental study on pullout strength of anchor bolt with an embedment depth of 30 mm in concrete under high temperature

Experimental study on pullout strength of anchor bolt with an embedment depth of 30 mm in concrete under high temperature

Nuclear Engineering and Design 229 (2004) 151–163 Experimental study on pullout strength of anchor bolt with an embedment depth of 30 mm in concrete ...

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Nuclear Engineering and Design 229 (2004) 151–163

Experimental study on pullout strength of anchor bolt with an embedment depth of 30 mm in concrete under high temperature Jun Hashimoto∗ , Katsuki Takiguchi Department of Mechanical and Environmental Informatics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 16 December 2002; received in revised form 15 October 2003; accepted 3 November 2003

Abstract Many anchor bolts are used in nuclear-related facilities in order to fast some equipments and pipes. About these anchor bolts, the possibilities should exist in being exposed to high temperature due to accident. However, little information is available regarding the effects of high temperatures on the behavior of anchor bolts. To obtain basic experimental data and to examine the response of cast-in-place anchor bolts to high temperature exposure, pullout strength of an anchor bolt during heating and after heating was examined. The maximum temperature in this test is 500 ◦ C and two different rate of heating were prepared. Tests results show that pullout strength decreases with increasing surface temperature. Regardless of different rate of heating in this experiment, identical ratio of pullout strength of anchor bolt was obtained when the same temperature of concrete surrounding the bolt head was applied. © 2003 Elsevier B.V. All rights reserved.

1. Introduction Some recent researches on high temperature behavior of concrete show that its strength will decrease with further increase of temperature and other mechanical properties also get worse, due to drying shrinkage, thermal stress and so on (Abe et al., 1999; Nagao and Nakane, 1994). It is also reported that the spalling happens on the surface of concrete subjected to elevated temperature (Inoue et al., 1997). In nuclear-related facilities, many anchor bolts are embedded in concrete structures in order to support equipments and pipes. There is a possibility that concrete structures in which anchor bolts are embedded will be exposed to high temperature in a short ∗

Corresponding author. Tel.: +81-3-5734-3155; fax: +81-3-5734-3155. E-mail address: [email protected] (J. Hashimoto).

time, and that the pullout strength of anchor bolts will decrease. However, little information is available regarding the effects of high temperature on the behavior of anchor bolts. Therefore, they are usually designed without consideration for high temperature effects. In this research, pullout tests of cast-in-place anchor bolt were carried out under high temperature up to 500 ◦ C. The objectives of this paper are to obtain basic experimental data and to examine the response of cast-in-place anchor bolts to high temperature exposure. At present, there are formulas to determine pullout strength of a cast-in-place anchor bolt. ACI (1978) proposes the formula as follows; √ PACI = 0.33φ σB × Ac

(1)

Ac = πh (h + d)

(2)

0029-5493/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2003.11.003

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where, PACI is the pullout strength (N), φ is the reduction factor, σ B is the compressive strength of concrete (N/mm2 ), Ac is the effective area of cone failed surface (mm2 ), h is the embedment depth (mm) and d is the diameter of a stud (mm). In “Design Recommendation for Composite Construction” published by the Architectural Institute of Japan (1985), pullout strength is given as follows: √ PAIJ = 0.31Ac σB (3) Another formula proposed by R. Eligehausen (1995), √ PELI = k σB × h1.5 (4) √ where, k is the empirical coefficient (=15.5 N/mm). Note that Eqs. (1) and (3) are in SI unit system.

2. Experimental investigation 2.1. Specimen In all, four tests were carried out, and 80 specimens were tested. The dimensions and details of the specimens are illustrated in Fig. 1. All of specimens are square concrete blocks with a side length of 240 mm considering the cone failed area, and with a depth of 120 mm. An anchor bolt used in this study is JIS standardized M12-bolt with a nominal length of 80 mm, and with a thread length of 30 mm as shown in Fig. 2. This headed stud bolt was embedded at the center of the top surface of all the specimens, and the embedment depth from the top surface of the concrete to the upper surface of the bolt head is 30 mm as shown in Fig. 3. Four square nuts and steel pipes were installed to be fixed with the testing apparatus. Twelve specimens equipped with thermo-couples are to measure the temperatures inside the specimen during heating (see Fig. 1b). Two thermo-couples are to measure the temperatures of anchor bolts and the others are to measure the temperature of the concrete. To fix these thermo-couples, the threaded steel bars (diameter = 3 mm) were used. Ordinary normal concrete was used with design compressive strength of 30 N/mm2 . Six hours after casting of concrete, the specimens were cured by covering wet sand. This curing sand was removed on the day of testing. The materials properties are shown in

Fig. 1. Detail of specimens: (a) typical specimen; (b) specimen with thermo-couples.

Table 1. Here, the average of compressive and splitting strength of the concrete cylinder during testing is shown. 2.2. Testing procedure The apparatus for pullout test is illustrated in Fig. 4. The specimen was fixed to the testing apparatus with four bolts as shown in Fig. 5, then its anchor bolt

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Fig. 2. Detail of anchor bolt. Table 1 Materials properties Concrete

Test Test Test Test

1 2 3 4

Anchor bolt

Compressive strength (N/mm2 )

Splitting tensile strength (N/mm2 )

47.0 44.3 33.0 46.0

3.43 3.71 3.12 3.75

Fig. 3. Embedment of anchor bolt.

Yield strength (N/mm2 ) 394

Tensile strength (N/mm2 ) 488

Young’s modulous (kN/mm2 ) 19.8

was connected to the end of the loading beam. To examine the ratio of forces of these four bolts during the pullout test, the strain gauges were fixed on each of bolt (see Fig. 6). This measurement was carried out on four specimens. Fig. 7 shows the detail of connection of the loading beam and anchor bolt. At first, anchor bolt was connected with treaded steel bar by using coupler. Then, this treaded steel bar was connected to loading beam. The tensile load is applied to an anchor bolt by applying loads to the other end of the loading beam. Here, the tensile load from the loading beam was previously measured.

Fig. 4. Apparatus for pullout test.

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Fig. 5. Detail of method to fix specimen to apparatus.

The detail of heating the specimen is illustrated in Fig. 8. To improve thermal efficiency, the insulators were installed around the specimen and the heaters. One side of specimen with anchor bolt was heated by hot air by increasing its temperature using the electric heater with Nichrome wires. Above the specimen, a thermo-couple was employed to measure the air tem-

Fig. 6. Bolt with strain gauge.

Fig. 7. Detail of connection of loading beam and anchor bolt.

perature near the heating surface of specimen (see Fig. 3). This temperature was controlled with the processes of heating as shown in Fig. 9. In each test, pullout strength under normal temperature was examined. To examine the effect of high temperature on pullout strength, the specimens were heated while applying constant tensile load P on the anchor bolt. The tensile load P was decided between 0 N and the average of the pullout strength under

Fig. 8. Method of heating.

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Fig. 9. Loading history: (a) loading history A (T); (b) loading history B; (c) loading history C.

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normal temperature P0 . The pullout test was completed when the specimen fractured. If the specimen did not fracture until the end of heating, it was cooled and the pullout test was carried out after its temperature went down to normal temperature. Here, the specimens were cooled without insulators. With loading history A (T), the specimen was heated up to proposed maximum temperature T at the rate of 10 ◦ C/min Unless the specimen fractured, T was kept for two hours. After that, the specimen is cooled down to normal temperature (Fig. 9a). Seven kind of proposed maximum temperature T was prepared between the normal temperature and 500 ◦ C: 100 ◦ C, 150 ◦ C, 200 ◦ C, 300 ◦ C, 350 ◦ C, 400 ◦ C, 500 ◦ C. The purpose of loading history A (T) is to obtain the relation between the pullout strength and the temperature at the breaking point with relatively faster heating than loading history C. Loading history B, in Fig. 9b, is proposed to examine the effect of the difference of heating process by comparing with the result of loading history A (T)

with T = 500 ◦ C. The temperature was increased up to 500 ◦ C after keeping 300 ◦ C for two hours. The rate of heating is 10 ◦ C/min. For loading history C shown in Fig. 9c, the rate of heating is 0.75 ◦ C/min which is much slower than loading history A. The results of loading history C were compared with the results of loading history A (T) with T = 500 ◦ C, and the effect of the speed of temperature increase was examined.

3. Test results and discussion 3.1. Response of testing apparatus during experiment In this experiment, simple method of using four bolts to fix specimen to the testing apparatus is adopted. Fig. 10 indicates the relation of the ratio of strain of four bolts used to fix the specimen to the testing apparatus. It can be observed from Fig. 10a

1.2

Ratio of strain

1 0.8 0.6 0.4 0.2 0 0 (a)

Time of load applied to anchor bolt

(b) Fig. 10. Ratio of strains of four bolts: (a) time vs. strain of four bolts; (b) ratio of strains of four bolts.

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Fig. 11. History of temperature and tensile force.

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Table 2 Pullout strength under normal temperature (kN)

P0 PELI

Test 1

Test 2

Test 3

Test 4

18.3 17.5

19.8 17.0

14.8 14.6

17.3 17.3

that the ratio of strain of four bolts was constant during pullout testing, even though the load applied to anchor bolts was increased. The measured ratio of strain of bolts of four specimens is also shown in Fig. 10b. From this figure, it can be seen that, as to two specimens, the load applied the anchor bolt was evenly distributed to the four bolts. On the other hand, the load was unevenly distributed as to the others, and the minimum strain was 70% of the maximum strain. Table 2 indicates the value calculated with Eq. (4) PELI and the average of the pullout strength under normal temperature P0 in this experiment. There is little difference between them. From this result, an inclination difference of reaction is insignificant. Therefore, the method in this testing to fix specimen with the apparatus adopted in this experiment is acceptable. 3.2. General response of specimen Fig. 11 indicates the history of the heating temperature and the tensile load applied to the anchor bolt. This figure is a part of the results of this experiment, and the heating temperature shows the value by the thermo-couple above the specimen which measures the air temperature near the hearting surface of specimen (see Fig. 3).

500 ◦ C (Fig. 11d–f). The others did not fracture during heating, and the pullout strength after heating was 50% of it under normal temperature. Here, as for the pullout strength of anchor bolt which was examined after heating, there were little differences whether the tensile load had been applied to anchor bolt during heating or not. In other words, the tensile load, which had been applied to anchor bolt during heating, had no influence on the pullout strength after heating. The increase of heating temperature of two specimens tested with T = 400 ◦ C was terminated due to spalling of their heating surface at around 390 ◦ C of heating temperature, as shown in Fig. 11g. Note that, these specimens were unloaded yet, in order to examine the pullout strength after heating, when the spalling happened. It was found that the pullout strength was greatly decreases because of spalling. 3.2.2. Loading history B Three specimens were prepared and tested with loading history B. The tensile load was applied with P = 0.4P0 and 0.5P0 , and no specimens which fractured were observed during heating (Fig. 11h). The pullout strength after heating was 50% of it under normal temperature, and almost equal to the pullout strength after heating with loading history A with T = 500 ◦ C. 3.2.3. Loading history C The results of loading history C are shown in Fig. 11i–l. The specimens were tested under tensile o

o

10 C/min (fractured with constant temperature)

1.2

o

0.75 C/min

1 0.8 P/P0

3.2.1. Loading history A (T) As shown in Fig. 11a and b, the specimens which were tested by using loading history A, with T = 150 and 300 ◦ C, fractured when heating temperature was kept 150 and 300 ◦ C, respectively. The anchor bolts of both specimens were loaded with tensile load P = 0.7P0 . In the case of P = 0.5P0 with T = 300 ◦ C, the specimens did not fracture during heating, and the pullout strength after cooling was 70% of it under normal temperature. While, with T = 500 ◦ C, the specimens were tested under tensile load 0.15P0 ∼0.8P0 . When the tensile load applied to the anchor bolts was 0.2P0 ∼0.8P0 , the anchor bolts were pulled out during increase of heating temperature from 200 to

10 C/min (fractured with increase of tepmerature)

0.6 0.4 0.2 0

0

100 200 300 400 o Heating Temperature ( C)

500

Fig. 12. Relation of pullout strength and heating temperature.

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load from 0.4P0 to 0.8P0 . When the tensile load was over 0.5P0 , the anchor bolt was pulled out during increase of heating temperature. However, some specimens did not fracture during heating or fractured during cooling to normal temperature, when the tensile load was under 0.55P0 . The pullout strength of the specimens tested after heating was about 70% of it under normal temperature.

159

3.3. Pullout strength during heating The relations between the pullout strength and heating temperature of the specimens which fractured during heating are plotted in Fig. 12. The vertical axis stands for the pullout strength normalized by the average of the pullout strength of anchor bolt with an embedment depth of 30 mm under normal temperature.

Fig. 13. Typical failure types after tests: (a) specimen tested in normal temperature; (b) specimen tested with loading history A; (c) specimen tested with loading history C; (d) specimen tested after cooling.

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Depth from heating surface (mm)

During heating up to 500 ◦ C at the rate of 10 ◦ C/min, the pullout strength greatly decreased when the heating temperature went above about 300 ◦ C, and it decreases to about 20% of it under normal temperature. In the case of heating at the rate of 0.75 ◦ C/min, the pullout strength decreased when the heating temperature went above 150 ◦ C. In other words, with the same ratio of tensile load to anchor bolt, the temperature at the breaking point was about 100 ◦ C lower than it of heating at the rate of 10 ◦ C/min. It was found that the pullout strength decreases to only 50% of the pullout strength under normal temperature. Failure modes of the specimens are shown in Fig. 13. Typical anchor bolt of the specimens tested under normal temperature, was pulled out due to radial cracks around the anchor bolt (Fig. 13a). With regard to the specimens which fractured during heating with loading history A with T = 300 and 500 ◦ C, the surface installed an anchor bolt exploded into pieces with a loud sound as shown in Fig. 13b. On the other hand, as for the specimens tested with loading history A with T = 150 ◦ C and loading history C, the whole heating surface of them broke into pieces and the anchor bolts were pulled out (Fig. 13c). Fig. 14 indicates the temperature distribution inside the specimens at the breaking point. From this figure, it can be observed that the difference of temperatures inside the specimens with heating at the rate of 0.75 ◦ C/min is much smaller than it with heating

o

10 C/min (fractured with increase of tepmerature) o

10 C/min (fractured with constant temperature)

1.2

o

0.75 C/min

1

P/P0

0.8 0.6 0.4 0.2 0

0

100 200 300 400 o Temperature of Bolt Head ( C)

Fig. 15. Relation of pullout strength and temperature of bolt head. Loading History A(T) Loading History B Loading History C

1.4

o

Loading History A(400 C) (Spalling at heating stage)

1.2

P/P

0

1 0.8 0.6 0.4 0.2 0

0

100 200 300 400 500 o Applied maximum temperature( C)

20 40 60 Loading History C o

Loading History A (500 C)

100 120

o

Loading History A (150 C)

0

50

600

Fig. 16. Relation of pullout strength and applied maximum temperature.

0

80

500

100 150 200 250 300 350 400 o

Temperature ( C) Fig. 14. Temperature distribution inside specimens at breaking point.

20

40min. 50min.

40 60 80 100 120

0

100

200

300 o

Temperature ( C)

400

500

0min. 120min. 240min.

30min. 0

Depth from heating surface (mm)

Depth from heating surface (mm)

Depth from heating surface (mm)

0min.

0min.

20 60min. 40

90min.

60 120min. 80 150min. 180min.

100 120

(b)

0

100

200

0 20 40 60 360min. 80 480min.

100

300 o

Temperature ( C)

400

500

120

(c)

600min. 0

100

200

300

400

500

o

Temperature ( C)

Fig. 17. Temperature transition inside specimens: (a) loading history A (500 ◦ C); (b) loading history B; (c) loading history C.

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(a)

10min. 20min. 30min. 0

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at the rate of 10 ◦ C/min. The difference of temperatures inside the specimens tested with loading history A with T = 150 ◦ C is also small. Considering the specimens whose heating surface spalled around 390 ◦ C, the fractures of heating surface due to spalling may have effect on the pullout strength of anchor bolt when the difference of temperatures inside the specimens during heating is large. In case of the specimens with the small difference of temperature, the deterioration of the concrete of heating surface due to heating for hours probably influence the decrease of the pullout strength. From Fig. 14, it can be observed that the temperature of the bolt head with the identical tensile load to anchor bolt is the same in spite of the difference of heating temperature. The relation of the pullout strength and the temperature of bolt head as breaking point is shown in Fig. 15. From this figure, regardless of the difference of the rate of heating, identical ratio of pullout strength of anchor bolt was obtained when the same temperature of concrete surrounding the bolt head was applied. It can be also observed that the relation of the pullout strength and the temperature of bolt head of the specimens which fractured during the constant temperature is similar.

surface bears thermal stress during heating due to the great difference of temperature, which causes the micro crack inside the concrete. This micro crack is thought to be the reason of the decrease of the pullout strength after heating. Besides, there is little difference of not only the pullout strength but also the temperature distribution at the end of heating between loading history A with T = 500 ◦ C and loading history B. Failure mode of the specimens which fractured after heating was the same with it tested under normal temperature (see Fig. 13d). When the concrete is cooled, the thermal impact stress is applied on its surface. Radial cracks around the anchor bolt could be observed on the heating surfaces of some specimens during cooling to normal temperature. It is believed that these cracks, developed due to thermal impact stress, may cause the decrease of the pullout strength of cast-in-place anchor bolt.

4. Conclusion

3.4. Pullout strength after heating

In this research, the pullout testing of a cast-in-place anchor bolt in concrete under high temperature up to 500 ◦ C was carried out. From this experimental investigation, it is concluded:

Fig. 16 shows the relationship between pullout strength after heating and the applied maximum temperature T. It can be seen that, in the case of heating at the rate of 10 ◦ C/min, the pullout strength decreases in proportion to an increase of proposed maximum temperature T. The pullout strength decreases to about 50% of it under normal temperature after heating up to 500 ◦ C. As was stated above, the pullout strength of specimens tested with loading history A at temperature 500 ◦ C is identical to the specimens tested with loading history B, but it is slightly smaller than the specimens subjected to with loading history C. The temperature transitions inside the specimens at the earlier stage of heating are shown in Fig. 17. It can be observed that the differences of temperatures inside the specimens tested with loading history A with T = 500 ◦ C and loading history B are much greater than it tested with loading history C at the earlier stage of heating. The concrete of the heating

(1) Pullout strength of a cast-in-place anchor bolt decreases with increasing heating temperature while a cast-in-place anchor bolt in concrete was heated up to 500 ◦ C. The pullout strength decreases to about 20 and 50% when the cast-in-place anchor bolt was heated at rate of 10 and 0.75 ◦ C/min, respectively. (2) Comparison with the results of different rate of heating indicates that identical ratio of pullout strength of anchor bolt was obtained when same temperature of concrete surrounding the bolt head was applied. (3) In the case of a cast-in-place anchor bolt, tested after being heated up to target temperature and cooled to normal temperature, pullout strength of anchor bolt decreases with increasing the applied maximum temperature. The relation of pullout strength and heating temperature is linear, and the pullout strength decreases to 50% of it under normal temperature.

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References Abe, T., Furumura, F., Tomatsuri, K., Kuroha, K., Kokubo, I., 1999. Mechanical properties of high strength concrete at high temperatures, J. Struct. Construct. Eng. (Trans. AIJ) 515, 163– 168. ACI Committee 349, 1978. Proposed Addition to Code Requirements for Nuclear Safety Related Concrete Structures (ACI 349-76), ACI J. 75 (8), 329–335. Architectural Institute of Japan, 1985. Design Recommendation for Composite Constructions. pp. 177–238.

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Eligehausen, R., Balogh, T., 1995. Behavior of fasteners loaded in tension in cracked reinforced concrete. ACI Struct. J. 92 (3), 365–379. Inoue, A., Suzuki, S., Yanagi, K., 1997. Effect of curing method and moisture content of concrete for fire resistance of normal-weight concrete and light-weight concrete, Summaries of Technical Papers of Annual Meeting, Architectural Institute of Japan, pp. 793–794. Nagao, K., Nakane, S., 1994. Experimental studies concerned with properties of concrete subjected to elevated temperature, J. Struct. Construct. Eng. (Trans. AIJ) 457, 1–10.