Study of mechanisms of explosive spalling in high-strength concrete at high temperatures using acoustic emission

Construction and Building Materials 37 (2012) 621–628

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Study of mechanisms of explosive spalling in high-strength concrete at high temperatures using acoustic emission Mitsuo Ozawa a,⇑, Shinya Uchida b,1, Toshiro Kamada c,2, Hiroaki Morimoto a,3 a

Department of Civil Engineering, Gifu University, 1-1 Yanagido, Gifu City, Gifu 501-1193, Japan Department of Civil Engineering and Architecture, Saga University, 1 Honjo, Saga City, Saga 840-8502, Japan c Department of Civil Engineering, Osaka University, 2-1 Yamadaoka, Suita City, Osaka 565-0871, Japan b

h i g h l i g h t s " This study used the AE method for detecting the explosive spalling process in HSC. " It was found that AE events were indeed indicative of the state of the explosive failure process. " b-Value analysis was successfully applied to clarify the fracture process.

a r t i c l e

i n f o

Article history: Received 31 March 2012 Received in revised form 19 June 2012 Accepted 25 June 2012 Available online 7 September 2012 Keywords: Acoustic emission Explosive spalling High-strength concrete Vapour pressure High temperature b-Value analysis

a b s t r a c t Mechanisms behind explosive spalling of high-strength concrete during a heating test were investigated on the basis of acoustic emission (AE) measurements of wet and air-dried specimens. The relation between the measured values of the internal temperature and vapour pressure and AE events produced by micro-cracking were investigated. It was found that AE events were indeed indicative of the state of the explosive failure process. Furthermore, b-value analysis was successfully applied to clarify the fracture process, regardless of the moisture content of the specimens. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Fire poses one of the most serious risks to concrete buildings and structures because it often results in explosive spalling of concrete. There are two mechanisms by which concrete can be damaged by fire. One mechanism involves the thermo-mechanical process, which is directly associated with the temperature field. As the temperature of the concrete surface increases, the temperature gradients give rise to a distribution of compressive stress in a direction parallel to the concrete surface; the distribution leads to the development of tensile stresses in a perpendicular direction. ⇑ Corresponding author. Tel./fax: +81 58 293 2459. E-mail addresses: [email protected] (M. Ozawa), [email protected] (S. Uchida), [email protected] (T. Kamada), [email protected] (H. Morimoto). 1 Tel./fax: +81 95 228 8941. 2 Tel./fax: +81 66 879 7619. 3 Tel./fax: +81 58 293 2403. 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.06.070

When the tensile stress exceeds the tensile strength, explosive spalling of concrete occurs, as shown in Fig. 1 [1,2]. The second mechanism involves the mass transfer of liquid phases (liquid water, vapour, and dry air). When the temperature of the concrete surface increases, the moisture content of the concrete varies with depth from the concrete surface, as shown in Fig. 2. Consequently, the vapour pressure in the vapour zone and humid zone becomes larger than that in the dry zone and moist zone. In particular, the vapour pressure dramatically increases at the boundary between the vapour zone and humid zone. Around the peak of the vapour pressure, since high vapour pressure in concrete generates a large tensile stress, concrete spalling would occur [3,4]. Several studies have investigated the second mechanism by comparing vapour pressure with saturated vapour pressure (SVP) [5–7]. It has also been found that the spalling behaviour of concrete is strongly affected by its water content [8]. These studies have shown that explosive spalling could be minimised through the addition of synthetic fibres (especially polypropylene fibre) to high-strength concrete (HSC) [8–10,4,11–18].

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T Temperature

σ

x

Tension

x

Compression Steel bar

Fire Concrete

Fig. 1. Spalling mechanism: thermal dilation.

T Temperature

σ

x

m ¼ log a

ð1Þ

log N ¼ a  bm

ð2Þ

where m is the magnitude as defined in seismology; m is equivalent to the log scale of the amplitude ‘a’ of the AE signal. N is the number of signals with a magnitude greater than m, and the coefficient b is the negative slope of the log N–m plot. AE techniques have been used for damage assessment of materials [26–36]. Specifically, the coefficient exponent b (the so-called b-value) changes with the type of damage. In the initial stages of micro-cracking, a large number of low-amplitude AE signals are dominantly generated; in the later stages, fewer signals are generated, but with higher amplitudes. This implies a progressive decrease in the b-value as the specimen approaches impending failure. This is the core of the so-called ‘b-value analysis’ used for damage assessment. AE has previously been used to monitor the explosive spalling behaviour of HSC during heating tests [37–39]. This paper describes the use of AE for monitoring the explosive spalling of HSC during a heating test. The monitoring of the explosive spalling process and comparison of the measured vapour pressure and temperature for different AE events can help elucidate the mechanisms of the explosive spalling process. Further, the applicability of b-value analysis to the estimation of the explosive spalling behaviour of concrete under high temperatures is studied. 2. Materials and methods 2.1. Materials

Vapour pressure (Tension)

x

Vapour zone Dry zone Moist Zone

As listed in Table 1, early strength Portland cement with a specific surface area of 4550 cm2/g and a density of 3.13 g/cm3 (chemical composition is shown in Table 2) was used for preparing the concrete material; the water/cement ratio was 0.30. Crushed river stone with a maximum size of 25 mm was used as the coarse aggregate. The main component of the superplasticizer (SP) admixture was a polycarboxylic acid polymer. The fresh concrete properties and the mechanical properties of the hardened concrete at 28 days were measured, as shown in Table 3.

Humid zone 2.2. Specimens and curing conditions

Fire

Concrete Steel bar

Two types of curing conditions for specimens are mentioned in Table 4. Two specimens were considered for each of the conditions. The dimensions of each specimen were 400  400  100 mm. As shown in Fig. 3, specimens were tested directly after wet curing (wet) or after further air-drying. The concrete specimens were cast and left in the formwork for 1 day. The specimens were then wet cured at 20 ± 2 °C for 64 days. After wet curing, some specimens were air-dried in controlled conditions (20 °C, RH 40%, 118 days).

Fig. 2. Spalling mechanism: vapour pressure. 2.3. Estimation of water content [40]

For studying the cracking of concrete, acoustic emission (AE) is known to be particularly useful [19–24]. The Gutenberg–Richter (GR) law is widely used to describe the amplitude distribution of AE signals [25].

Before the heating test, the distribution of the water content of the specimen was determined with six ceramic RH sensors placed at depths of 0, 4, 6, 8, 10, and 52 mm from the surface (Fig. 4). The water content of concrete was estimated as humidity from the RH values determined by using non-heating-type ceramic humidity sensors. As shown in Fig. 5, this sensor consisted of a humidity-sensing element (ceramic) and a resis-

Table 1 Mixture proportion. Water cement ratio

0.3

Unit weight (kg/m3) Water

Cement

Fine aggregate

Coarse aggregate

Admixture

132

440

840

1060

22

Material

Description

Density (g/cm3)

Water Cement Fine aggregate Coarse aggregate Admixture

Tap water Early strength Portland cement River sand River stone Super plasticizer and air entraining agent (Poly-carboxylic acid cross-linked polymer)

1 3.13 2.61 2.62 1.01

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M. Ozawa et al. / Construction and Building Materials 37 (2012) 621–628 Table 2 Chemical composition of early strength Portland cement. SiO2

Al2O3

Fe3O3

Chemical composition (%) 20.3 5.29 2.44

Measuring range

Resistance thermometer

CaO

MgO

SO3

Ig.loss

64.79

1.58

3.01

1.04

Unit (mm)

20

90%RH

(0

50oC) Sensitivity

±3%( 10oC ( 30%

Table 3 Fresh and hardened properties of concrete. Air (%)

Compressive strength at 28 days (MPa)

Elastic modulus at 28 days (GPa)

30

20

1.8

83.5

41

Fig. 5. RH sensor.

RH con (%) (CalibratedReference humidityRH in (%) concrete)

Slump (cm)

Table 4 Outline of specimens. Type of concrete

Curing condition

Size of specimen(mm)

Number of pieces

HSC

Wet Air dried

400  400  100 400  400  100

2 2

2. Air-dried specimen

Wet curing (20 oC,64 days) Heating test

80% )

Ceramic RH sensor

Placing temperature (°C)

1.Wet specimen

50oC )

100 80 60 40 20

Experimental period:400days 0 0

20

40

60

80

100

Concrete (%) RH meRH(%) (Measured humidity in concrete)

Controlled room curing (20oC,R.H.40%,118 days)

Fig. 6. Relationship between humidity in concrete and ambient air humidity in equilibrium.

Heating test Fig. 3. Curing condition.

After the preconditioning tests, the water content of mortar was estimated for each RH precondition. Each mortar specimen was crushed to a size of 10 mm or smaller. After crushing, the mortar particles were dried in a furnace at 110 °C for 24 h and then weighed. It was confirmed that the RH of mortar after water curing was actually 100%. The water content was determined by using the expression.

Resistance thermometer 400

W n ¼ ðW wet  W d Þ=W d  100

400

Ceramic RH sensor

where Wn is the water content (%), Wwet is the weight of wet mortar before heating (g), and Wd is the weight of dry mortar after heating (g). The water content of concrete was evaluated from the change in the internal RH by using the relationship shown in Fig. 7, which is represented by the following empirical relationship:

50

Resistance thermometer

W n ¼ 0:000921  ðRHcon Þ2  0:0358  ðRHcon Þ þ 4:01 10

4 6 8

RH sensor

100

ð4Þ

ð5Þ

where Wn is the water content (%) and RHcon is the calibrated humidity in concrete (%).

Unit (mm)

tance thermometer for temperature corrections. The measurable relative humidity range was 20–90%, with a sensitivity of ±3%. Note that the sensor gives different results in atmospheric air and in concrete (which is an alkaline solid) under the same humidity conditions. For this reason, a preconditioning test was conducted to confirm the operation of the sensor in concrete and to obtain a calibration curve. As shown in Fig. 6, the relationship between the humidity in concrete and the ambient air humidity at equilibrium can be approximated by the following linear equation.

RHcon ¼ 1:45  RHme  26:1

ð3Þ

Water content ratio (%)

Fig. 4. Location of RH sensor in specimen.

10 8 6 4 2 20

40

60

80

100

RH in specimen (%) where RHcon is the calibrated humidity in concrete (%) and RHme is the measured humidity in concrete (%).

Fig. 7. Relationship between water content ratio and internal relative humidity.

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10

Thermo-couple Water content (%)

200

110

400 110

8 6 4 2 0

0

10

20

30

40

50

Depth from surface (mm) Fig. 11. Distribution of water content.

200

Unit (mm)

Miniature transducer gauge

Thermo-couple

Temperature (oC)

Miniature transducer gauge

No.1 No.2 No.3 No.4 No.5

200

2 1

Depth 8mm 0

10

8 25

3

100

0

100

4 Vapor pressure

20

30

40

Vapour pressure (MPa)

Fig. 8. Size and shape of specimen.

Spalling 300

0 50

Heating time (min.) 350

25

Fig. 12. Vapour pressure, temperature vs time (wet).

Electric furnace

Spalling

Unit (mm)

AE sensor

Temperature (oC)

Preamplifier

4 Vapour pressure No.1 No.2 No.3 No.4 No.5

200

3 2

100 1

Depth 8mm 0

0

10

20

30

40

Vapour pressure (MPa)

Fig. 9. Heating test set-up.

300

0 50

Heating time (min.)

Specimen

Fig. 13. Vapour pressure, temperature vs time (air dried).

Preamplifier

Fig. 10. Overview of AE measurement.

2.4. Heating tests

Vapour pressure (MPa)

4

2 1 0

As shown in Figs. 8 and 9, specimens with dimensions of 400  400  100 mm were used. Four steel pipes (inner diameter: 2 mm; length: 110 mm) and one pipe of 200 mm length were embedded in the specimens at a depth of 8 mm from the surface to measure the internal vapour pressure; all pipes were parallel to the

SVP Wet specimen Air-dried specimen

3

0

100

200

300

o

Temperature ( C) Fig. 14. Maximum value of vapour pressure and SVP (wet and air dried).

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Area

Area Area

4 3

90

2 80

1

Vapour pressure

70 600 500 Area 400 300 Temperature. 200 100 0 0 10

Area

0 300

Area

Area

200 Cumu.AE events

100

Spalling

30

20

40

Temperature(°C)

Cumulative AE events

Amp.(dB)

100

Vapour pressure(MPa)

Area

0 60

50

Heating time (min.) Fig. 15. Maximum vapour pressure and AE behaviour (wet).

Area

Area

Area

4 3

90

2

80

Vapour pressure

1

70 600 Area 500 Area 400 300 Temperature. Cumu.AE events 200 100 0 0 10 20

0 300

Area

Area 200 100

Spalling 30

40

Heating time (min.) Fig. 16. Maximum vapour pressure and AE behaviour (air dried).

50

0 60

Temperature(°C)

Cumulative AE events

Amp.(dB)

100

Vapor pressure(MPa)

Area

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M. Ozawa et al. / Construction and Building Materials 37 (2012) 621–628

heated surface. The steel pipes were connected to pressure transducers located outside the furnace. The measurable pressure range of these transducers was 0–5 MPa. Hydraulic jack oil was filled in the steel pipes. A thermocouple was placed in the central zone of the specimen at a depth of 8 mm from the heated surface. An electric furnace with a power rating of 56 kW was used for heating the specimens. In all heating tests, the temperature was increased at a rate of 20 °C/min.

Unit (mm) 14-16 12-14 10-12 8-10 6-8 4-6 2-4 0-2

2.5. AE technique [27] As shown in Fig. 10, the AE system consisted of transducers, preamplifiers, and a data acquisition system. Four transducers were placed on the top surface of each prismatic specimen. The signals were amplified by 40 dB by using a preamplifier and fed directly to a four-channel AE acquisition system interfaced with a personal computer. The resonant frequency of the AE transducer was 150 kHz. An AE event was defined as an event with an amplitude greater than a threshold level (70 dB). The acoustic energy was defined as the total energy detected by the transducer, that is, the integral of the absolute amplitude of the signal over time for the duration of the AE signal. While the consideration of acoustic events is helpful for event ‘counting’, the acoustic energy allows the user to obtain more information about the magnitude of an acoustic event in a single parameter.

Fig. 17. Heating face (wet).

Unit (mm) 10-12 8-10 6-8 4-6 2-4 0-2

3. Results and discussion 3.1. Distribution of water content of specimens before heating test As shown in Fig. 11, the water content of the wet specimen was about 9.5% and was uniformly distributed from the heated surface to the depth of 50 mm. For the air-dried specimen, the water content was about 4.0% at the heated surface, while it was 9% at the depth of 50 mm.

Fig. 18. Heating face (air dried).

3.2. Heating tests

3.2.2. Vapour pressure and SVP The maximum vapour pressure and saturated vapour pressure were compared for the wet and air-dried specimens during the heating experiment. As shown in Fig. 14, in the case of the wet specimen, the vapour pressure was very close to the SVP. In contrast, for the air-dried specimen, the vapour pressure was smaller than the SVP. This result is often attributed to the partial pressure of the dry air that is enclosed in the pores [5–7]. 3.2.3. Amplitude of AE events related to vapour pressure The AE characteristics (acoustic events, cumulative acoustic events, and amplitudes) are shown in Figs. 15 and 16, against the vapour pressure and internal temperature at the depth of 8 mm from the heating surface for the wet and air-dried specimens, respectively. In the wet specimen (Fig. 15), at temperatures lower than 50 °C (Area I), no AE events were recorded. From 50 °C to 100 °C (Area II), a small number of events were recorded, mostly with amplitudes less that 80 dB. This result suggested that water moves to the inner parts of the concrete to create vapour and humid zones. From 100 °C to 200 °C (Area III), the occurrence of AE events increased together with the vapour pressure. Therefore, it was concluded that the occurrence of AE events with amplitudes of 90 dB or higher was associated with the vapour pressure in Area III. It was inferred that water moved to the inner part of the concrete and created dry, vapour, and humid zones and that the occurrence of micro-cracking was associated with thermal shrinkage and the vapour pressure.

Wet Air dried

1.5

log N

3.2.1. Build-up of vapour pressure The evolution of the vapour pressure with time under different curing conditions is shown in Figs. 12 and 13. Explosive spalling in wet curing specimen occurred when the pressure of around 3.2 MPa (47 min.) at the depth of 8 mm. Whereas, in air dried specimens, explosive spalling were generated when the pressure of around 3.4 MPa (44 min.) at the depth of 8 mm.

1 0.5 0 70

80

90

100

m = log10a Fig. 19. b-Value analysis.

Above 200 °C (Area IV), the build-up of vapour pressure and explosive spalling occurred at 44 min and 47 min, respectively. The vapour pressure at the onset of spalling was 2.6–3.2 MPa, and the AE events had amplitudes of 90 dB or higher. For the air-dried specimen (Fig. 16), few events were recorded below 50 °C (Area I). From 50 °C to 100 °C (Area II), more events were recorded compared to the case of the wet specimen, with amplitudes ranging from 70 dB to more than 90 dB. The above-mentioned result (of water moving to the inner parts of the concrete and creating dry, vapour, and humid zones) was considered. The occurrence of AE events was associated with the vapour pressure and thermal shrinkage cracking. The temperature reached 100 °C at 28 min in the air-dried specimen, which is earlier compared to the case of the wet specimen (100 °C at 30 min). From 100 °C to 250 °C (Area III), the number of AE events continued to increase, together with the vapour pressure. Therefore, the occurrence of AE events with amplitudes higher than 90 dB was associated with the vapour pressure in Area III. It was inferred that water moved to the inner parts of the concrete and created dry, vapour, and humid zones and that the occurrence of micro-cracking was associated with thermal shrinkage and the vapour pressure.

M. Ozawa et al. / Construction and Building Materials 37 (2012) 621–628

Beyond 250 °C (Area IV), the vapour pressure increased and explosive spalling occurred at 43 min and 45 min, respectively. The vapour pressure at the onset of spalling was 3.0–3.5 MPa, and AE events showed amplitudes of 90 dB or higher. Overall, it was demonstrated that the AE technique could be used to monitor explosive spalling before and after the vapour pressure reaches the maximum value during the heating of specimens, which the maximum vapour pressure depends on the water content. 3.2.4. Heated surface As shown in Figs. 17 and 18, specimens suffered explosive spalling at their heated surface, and the explosive spalling caused very severe damage; the maximum spalling depths were 15 mm and 12 mm in the wet and air-dried specimens, respectively. 3.2.5. b-Value analysis The amplitude distribution of AE events in the wet and air-dried specimens over the course of the heating tests is shown in Fig. 19. By applying the GR law to the AE results, the following empirical relationships are derived:

log N ¼ 6:19  0:062 m ðR2 ¼ 0:842Þ

ð6Þ

log N ¼ 6:19  0:061 m ðR2 ¼ 0:870Þ

ð7Þ

The b-values obtained for the wet and air-dried specimens, 0.062 and 0.061, are nearly equal. Therefore, the b-value can be used to estimate the fracture process of concrete at high temperatures, regardless of the moisture content. 4. Conclusions This study used the AE technique for detecting and studying the explosive spalling process in high-strength concrete. The following were the main conclusions:  The maximum value of vapour pressure in the wet and air-dried specimens was 3.2 MPa and 3.5 MPa, respectively.  The wet and air-dried specimens suffered explosive spalling at their heated surface, and the explosive spalling caused very severe damage; the maximum spalling depths of the wet and air-dried specimens were 15 mm and 12 mm, respectively.  The results of this study showed that the vapour pressure was responsible for the AE events recorded, thus showing that the AE technique is applicable to the monitoring of explosive spalling.  Overall, the AE technique can be used to monitor the explosive spalling failure process in specimens with different water content, before and after ‘the maximum vapour pressure was reached during heating.  The b-values obtained for wet and dry specimens, 0.062 and 0.061, were nearly equal. Therefore, the b-value also can be used to estimate the fracture process of concrete at high temperatures, regardless of the moisture content.

Acknowledgements This study was supported by the Grant-in-Aid for Scientific Research C (General) from the Japan Society for the Promotion of Science (2007, No. 17560406; Head: Prof. H. Morimoto). The authors express their gratitude to the organization for its financial support. They also appreciate the support received from the Japan Insulation Co. Ltd.

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