Low temperature fracture behaviour and AE characteristics of autoclaved aerated concrete(AAC)

CEMENT and CONCRETE RESEARCH. Vol. 17, pp. 743-754, 1987. Printed in the U S A 0008-8846/87 $3.00+00. Copyright (c) 1987 Pergamon Journals, Ltd.

LOW

TEMPERATURE FRACTURE BEHAVIOUR AND AE CHARACTERISTICS OF AUTOCLAVED AERATED CONCRETE(AAC)

H. D. Jeong ~, H. Takahashi* and S. Teramura ** (*) (*~)

Research Institute for Strength and Fracture of Materials Faculty of Engineering, Tohoku University, Sendal, JAPAN Onoda ALC Co,Ltd., Tokyo, JAPAN

(Communicated by Z.P. Bazant) (Received April 8, 1987) ABSTRACT For a quantitative understanding of freezing damage on Autoclaved Aerated Concrete(AAC) and fiber reinforced AAC(i.e., RAAC), the influence of water and temperature ( R . T . ~ - 2 0 ~ ) on those materials have been studied by the investigation of ~E characteristics, the fracture mechanics J-integral test and SEM observation. Futhermore, using the AE frequency analysis based on the frequency energy density distribution ratio(EDDR), the micro-fracture process for various test conditions has been interpreted. The AE activities and fracture toughness showed a large difference depending on the water content and temperature. All the AE events emitted during the fracture toughness tests could be classified into 6 groups. Also, the AE sources were considered paying particular attention to the micro-crack formation, the friction of inter-matrix and the fiber breaking behaviors at fracture. Noting that the AE is emitted during the drying process, the drying shrinkage damage was discussed.

Introduction In response to a questionnaire about the utility of Autoclaved Aerated Concrete(AAC) in Japan, many users were concerned about severe freezing damage compared with other building materials. To overcome this weak point, recently many efforts have been carried out to develop AAC showing a high resistance to freezing damage. But, to develop such materials, it is firstly necessary to fully understand about the controlling factors of freezing damage. There have been many studies concerning freezing damage in porous building materials. Most of them, however, are focused on the mechanical properties such as changes of compressive strength, and permanent elongation due to the damage, water absorption through the porous structure, and the critical water content at low temperature(l). However, when considering the development of AAC having the high resistance to freezing damage, it is neccessary to understand the effect of water and temperature on the initiation and propagation mechanism of 743

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Vol. 17, No. 5 H.D. Jeong, et al.

micro- and macro- cracks which result from freezing damage. Also, the fracture toughness variables which relate to the change of resistance to crack initiation should be evaluated using fracture mechanics concepts. Acoustic Emission (AE) techniques have been successfully applied to detect the location and initiation of cracks in concrete(2). Some studies of metals showed that it is possible to characterize the micro-fracture mechanism by the AE event classification using AE frequency analysis(3). The same method is thought to be applicable to the interpretation of crack behaviors in AAC. The object of this work is to clarify the effects of water and temperature on the micro-fracture mechanism by fracture toughness testing and AE measurement using compact tension specimens in various test conditions (O ~ 92% water content, R.T.--20 °C ) for the AAC and reinforced AAC (RAAC) which is developped for high strength and resistance to damage. The effect of fibers on micro-fracture mechanism and fracture toughness variables are also investigated. Furthermore, noting that AE is emitted during the drying process, the drying shrinkage damage is also discussed.

Experimental procedure The materials tested in the present work were: i) an AAC composed of silica, quick lime, cement, and gypsum, and 2) RAAC whose woody fiber (of a needle-leaf ) content is below 2% of the total weight. From these materials, compact fracture toughness test specimens (modified 2TCT) with thickness B=76mm and width W=ll5mm were prepared with crack orientation perpendicular to the blowing direction. The fracture toughness tests were performed at a constant displacement rate of 0.05 mm/min using an Instron testing machine. The block diagramm of AE measurement and frequency analysis system is shown in Fig.l. The AE event classification method was the same as employed in previous work (3). The AE signals emitted during fracture toughness tests were detected using a transducer with a maxlmun frequency response at 400 kHz(NF-3172). The pre-amplified(60dB) AE signal was bandpass-filtered between 5kHz and 500 kHz, and further amplified with a broad-band amplifier which provided an additional gain of i0 or 20dB. o

The cooling of the specimens to the required temperature(-20+2 C) was performed in a chamber by controlled injection of liquid nitrogen which is distributed in gas phase. The fracture toughness tests at low temperature were carried out after holding the specimen for two hours at test temperature to avoid a temperature gradient at the specimen midsection. To measure the water content of specimens, reference test pieces were prepared with a diameter D=50 mm, and a height H=64 mm. The water content (wt%) was defined as wt(%) = (W - Wd)/Wd, where W w and W d are the wet weight W and the dry weight, respectively. To achieve the 0 % water content, specimens were placed in the laboratory atmosphere for 2 days and were evacuated with a rotary pump during 2 days. To obtain a high water content in a short time, specimens were immersed in the distilled water under pressure at 7.5 MPa for 25 min using an autoclave. The highest water contents achieved were 85 % and 92 % in AAC and RAAC, respectively. Intermediate water contents were achieved by the atmospheric drying from the condition of the highest water content.

Vol. 17, No. 5 ACOUSTIC EMISSION,

745 LOW TEMPERATURE,

Sensor Charge Amp. (NF-3172) gain 60dB

Bandpass Filter 5-500kHz

FRACTURE, AUTOCLAVED AERATED CONCRETE

Main A m p 10/20dB

Peak detector

Puls-Height histogrammer i (8 levels)

VTR-type data recorder

Recorded tape Fig.

1

Past Fourior Transformation

Block diagramm analysis system.

of

Disk Floppy

temperature

Printer

Acoustic Emission measurment

Results Influence of water,

Personal Computer

printer

and

AE

frequency

and discussion

and woody fiber on AAC properties.

The load vs load-line displacement (P-VL) curves obtained from fracture toughness tests of AAC and RAAC specimens in the various test conditions are shown in Figs.2a and 2b. Table 1 shows a summary of the test results. In the table, Jpmax, which is the value of J-integral at the maximum load, was used as the measure of fracture toughness. From these results, it is found that AAC shows an abrupt rupture after maximum load( A N F curves in Fig.2a), whereas RAAC shows a higher resistance to the crack growth with a higher 1,2 ~

---K-- ~

A-N : Table la

1.O

Fig. 2a P-V L curves for varlous test conditions(AAC)

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l 300

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746

Vol. 17, No. 5 H.D. Jeong, et al.

1,2

RAAC 1.0

I ~ 14 T a b l e

o81

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Fig. 2b P-V$ curves for varlous test conditions(RAAC)

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,-

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,,

l 200

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/ soo

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Table 1 Results of fracture toughness test on AAC (a) and RAAC (b)

Sp. A B C D E F G H I J K L M N

Temp. (°C) R.T. R.T. R.T. R.T. R.T. R.T. -20 -20 -20 -20 -20 -20 R.T. R.T.

wt(%) ( % ) 0 0 48 40 85 85 0 48 40 85 85 85 0 0

(a) Pmax. Jpmax. E (kN) (N/mm)_(kN/mm) 0.49 3.6 2.34 0.54 5.5 2.67 0.46 3.3 0.39 2.7 2.41 0.34 2.6 2.44 0.38 2.9 2.21 0.43 3.9 1.92 1.04 43 2 15 0.66 17 2 30 1.03 50 3 91 I. 11 69 3 05 0.86 60 4 04 0.34 3.7 2 30 0.25 2.3 2 18

(× 10.3 )

Sp. i 2 3 4 5 6 7 8 9 i0 ii 12 13 14

(b) Temp. wt(%) Pmax. Jpmax.

(°c)

( z ) (kN)

R.T. R.T. R.T. R.T. R.T. R.T. -20 -20 -20 -20 -20 -20 R.T. R.T.

0 0 40 40 92 92 0 0 51 48 92 92 0 0

0.59 0.77 0.64 0.47 0.48 0.59 0.91 0 77 0 93 0 71 0 32 0 52 0 43 0 66

E

(Nlmm)(kNlmm) 20.5 24.5 30.4 20.6 15.6 19.6 34.3 12.7 33.4 22.5 28.4 34.3 18.6 13.7

2.67 2.34 2. i0 2.25 2.66 2.77 2.89 3.12 2.59 2.77 2.23 2.67

( ~i0 -3)

maximum load than AAC at room temperature, independent of water content, as shown in Fig.2b ( 1 ~ 6 curves). However, for the condition of low temperature (-20 °C) with high water content ( 4 0 ~ 8 5 w t % ) , the maximum load and the resistance to crack growth of AAC increased significantly relative to room temperature, whereas the curves for RAAC at low temperature are dependent strongly on the water content. That is, in the case of -20"C with intermediate water content (48~51wt%), the maximum load is higher than that at room temperature with Owt%, whereas a large reduction of load was observed for the highest water content(92wt%)( 11,12 curves in Fig.2b). Figs. 3a and 3b show the relationship among Jpmax, water content and temperature for AAC and RAAC. From these results, it could be known that the

Vol. 17, No. 5 747 ACOUSTIC EMISSION, LOW TEMPERATURE, FRACTURE, AUTOCLAVED AERATED CONCRETE

5C

%

o AAC

RT.

~40

• RAAC

~30

~2o (a) 10

~0

I)

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2'8

80

wt

7~5

100

Fig. 3 Relationship among J p m a x water content and temper s ature for AAC and RAAC at -20 C (a) and R.T. (b).

%

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J p m a x in these materials shows only slight changes due to the water content at room temperature. But, at low temperature(-20°C), the Jpmax of AAC increases significantly and that of RAAC decreases slightly, showing large scatter with increasing water content. From these results, it can be seen that Jpmax of RAAC is about 5 times higher compared with AAC at room temperature. This result can be explained by the effect of fibers. The fiber reinforcement on AAC was already reported by Teramura et al(4), who demonstrated through fracture toughness test on the same materials as in the present study that the superior mechanical properties of RAAC are the results of strong cohesion between the fibers and the matrix, in addition to the flexibility and roughness of the fibers themselves. It is noticeable, furthermore, that a significant increase of J pmax with water content at -20°C occurs for AAC as shown in Fig.3b. This phenomenon is thought to be due to ice formation which leads to an increase of cross section area and strengthening of the matrix itself at -20°C. There is no significant change of Jpmax for RAAC with high water content, which is opposite to AAC.

AE characteristics,

fractographic observation and AE sources.

All AE events emitted during the fracture toughness tests at conditions ( 0--20"C, 0 ~ 9 2 % water content ) could be classified

various into 6

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Vol. 17, No. 5 H.D. Jeong, et al.

groups by the EDDR characterization procedure (3). EDDR is the ratio of an average energy in a selected frequency range (fz-f,)to whole range (~-fL). In this investigation, selected frequency ranges (fz-fl) for EDDR calculation were adopted as in Table 2, based on the distribution of significant frequency components observed in the total AE spectrum, fL and fH are 0 kHz and 500 kHz, respectively. Fig.4 shows the flow chart for classification of the total AE event in terms of their types, which are defined from the interrelationship of various EDDRs. Particular attention was paid to the largest EDDR which exceed the average value. The classified AE event types for all AE signals observed in the fracture tests are shown in Fig.5. Figs.6 and 7 show the P-Vkcurves along with distribution of the classified AE event energies for AAC and RAAC, respectively, for tests at room temperature with 0 % water content. Photos I and 2 show the fracture appearances after the fracture toughness tests. In the AAC, it can be considered that the micro-cracks form at the short ligaments between pore walls before the maximum load is reached, and the main crack propagates by the coalescence of micro-cracks with increasing load. From Fig.7, it can be seen that the fibers play an important role on AE generation. Also, SEM observation of RAAC(Photo 2) shows that fibers with fragments of matrix on their surfaces pulled out from the matrix without breaking of the fiber. This indicates that the cohesive strength between the matrix and the fiber is stronger than that of the matrix itself. Here, the pulling out of fibers is thought to be a source of the high AE activity. But, noting the fact that the surfaces of the fibers pulled out are covered by matrix fragments, a direct friction between the fiber itself and the matrix can not be considered. In this regard, it is possible to consider that the micro-fracture mechanism of the matrix around the fibers consists of two kinds. Fig.8 illustrates the micro-fracture mechanisms that occure in both AAC and RAAC at room and low temperature, including AE sources. One is the micro-crack formation in the matrix around fibers at low loads, the other is the pulling out of the matrix fragment with the fiber from the matrix in a direction perpendicular to the crack growth at

EDDR EDDRI EDDR2 EDDR3 EDDR4

fl(kHz)

f2(kHz)

0 130 200 330

I00 200 260 380

EDDR =li'E(f) df/(f 2-f i) ~[E(f)df/(fH-f L) E(f) = power spectral density energy. E l ~ E2= EDDR values

Fig. 4. Flow chart for AE event classification based on EDDR values.

Table 2. Selected frequency ranges for EDDR calculation and definition of EDDR.

Vol.

17, No. 5

749

ACOUSTIC EMISSION, LOW TEMPERATURE,

TYPE

FRACTURE, AUTOCLAVED AERATED CONCRETE

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C

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Fig. 6. P-V. curve and classified L AE event distribution. (AAC, 0 wt%, R.T.)

80dB

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~m

increasing loads, which is similar to the shear mode slippage of the matrix. Thus, from those considerations and the fact that the same AE event types are emitted from AAC and RAAC, it can be said that the micro-fracture mechanism of the matrix for the two kinds of materials is the same for the test at room temperature with 0 % water content as shown in Fig.8, illustratively.

750

Vol. 17, No. 5 H.D. Jeong, et al.

1.2

• 1 .l~

TYPE

A

•

o TYPE

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Considering that AE of types A and B are emitted from earlier loads and the AE of type A shows a high activity in RAAC (Fig.7), it appears that the AE source of type B is the micro-crack formation, while type A is friction between the matrix and the fragment of matrix bonded to fibers. Since type C AE is emitted just at and around the abrupt rupture of AAC, the sources of type C are attributable to rapid coalescence of micro-cracks which leads to the propagation of main crack. The distribution of classified AE events emitted for various test conditions is shown Table 3. The table summarizes up to 200 AE events where the time interval for the most specimens corresponds to the point around or after maximum load except for AAC at room temperature with 0 % water content.

Photo i. Fracture appearance of AAC. ( 0 wt%, R.T.)

Photo 2. Fracture appearance of RAAC. ( 0 wt%, R.T.)

Vol. 17, No. 5 751 ACOUSTIC EMISSION, LOW TEMPERATURE, FRACTURE, AUTOCLAVED AERATED CONCRETE

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Fig. 8. Schematic diagramm of micro-fracture mechanisms and AE event sources on AAC and RAAC at various test conditions. (A N F ; AE event types)

In the case of AACs test at -20°C with 85 % water content, AE events of type E were emitted, as shown in Fig.9. As discussed earlier, the formation of ice governs the mechanical properties, and its fracture is also considered as typical AE sources. Here, the AE event of type E, which was observed only under this condition, is attributable to the crack propagation accompanying ice fracture. RAAC at -20°C with 92 % water content showed a lower maximum load than that at R.T. with 0% water content(Fig.10). But, from P-V_curve which shows L the most gradual load reduction after maximum load with increasing displacement compared with other test conditions, it can be found that the specimen for this test condition shows a higher resistance to crack propagation, with the lowest crack growth speed among the all test conditions. The SEM fracture appearance for this condition showed no big difference compared to room temperature,0% water, and consisted of fiber pullout with matrix fragments and flat appearance. In this condition, two kinds of AE events were observed, type A and F. Here, it is considered that the source of type F is related to slow crack propagation due to the continuous micro-cracking of ice and matrix.

Table 3. Distribution of classified AE event counts for various test conditions.

AAC RAAC

Temp.

wt%

A

B

C

D

E

F

R.T. _20°C

0 85

13 69

Ii 48

3 31

0 0

0 52

0 0

R.T. -20°C -20°C

0 0 92

121 16 26

68 122 0

II 35 0

0 37 0

0 0 0

0 0 174

Photo 3 shows the typical fracture appearance of RAAC for a test condition of -20 °C with 0% water content. The fracture appearance is significantly different compared with other test conditions; most fibers are broken without pulling out. This fiber breaking indicates that a low temperature environment at water free condition promotes high cohesive

752

Vol. 17, No. 5 H.D. Jeong, et al.

1.2

AAC

,c

/

/

(eswtO~ -20~j)

~ .

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eelff °~5~

5-e g ,,,

Vol. 17, No. 5 ACOUSTIC EMISSION, LOW TEMPERATURE,

753 FRACTURE, AUTOCLAVED AERATED CONCRETE

strength between the matrix and fiber, leading to the increase of maximum load as shown in Fig.2b and Table lb. Also, for this test condition, a new AE event type(typeD) was emitted. Here, the source of type D AE is identified as due to the fiber breaking.

Drying shrinkage damage of AAC and AE In this section, noting that the AE of high activity are emitted from unstressed specimen at room temperature with intermediate water content (40 50 wt%), the drying shrinkage damage in AAC is discussed: Fig.ll shows the AE generated when an RAAC specimen was drying from 92 % water content in an ambient temperature environment. AE and changes in the compliance of the same specimen were measured simultaneously to determine whether or not micro-cracks are generated by the drying process. It can be seen from Fig.ll that AE starts to be emitted at 60 % water content and diminishes near 40 wt%. The same phenomenon was also observed in AAC. Some reports on the relation between Young's modulus and water in rocks showed that the presence of water leads to a decrease of strength and Young's modulus due to a reduction of friction in rock mineral constituents (5,6). From a comparison of Young's modulus values between the as-cast and the highest water content conditions, as shown in Fig.12, it can also be considered that the same phenomenon occurs in AAC.

1o0

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~

RAAC

water

• I i | |V|IET IOAE EVENTS

content

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1oo i [ TEIIrlN

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754

Vol. 17, No. 5 H.D. Jeong, et al.

1.2

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1 RAAC~as rec~ved 3 AAC I

2 ~)after drying

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13. Comparison of P-V L curves ( ascash and dried specimen ).

Photo 4. Micro-crack observed in dried specimen.

However, the permanent changes of Youn~s modulus observed in specimens ( about 0.4 kN/mm in two kind of materials ) dried from the condition of high water content indicates that another factor may be involved in the change of Young's modulus. This factor may be micro-cracks formation. This view is strongly supported by the results of Fig.13 and Photo 4. From Fig.13 which shows the difference of P-V L curves with and without the drying damage, it can be concluded that the maximum load of the specimens dried from the highest water content decreases in comparison with the as-cash specimens. Futhermore, the Jpmax of the dried specimens( M,N of Table la and 13,14 of Table Ib) also snows lower values compared with as-cash specimens. These phenomenon agree with Atkinson's work which revealed, through the investigation of the effect of thermally induced micro-cracks on fracture toughness in rocks, that the decrease of fracture toughness is the result of micro-crack formation in the matrix(7). Photo 4 shows a crack observed in a dried RAAC specimen. From these results, AE emitted during the drying process can be related to microcracking. Generally, the freezing damage such as cracking is known due to the interaction of temperature and water. However, from the present results,it can be said that the drying process alone can generate micro-cracks in the matrix. Conclusions This study demonstrates the effects of water and temperature on the mechanical properties of AAC and reinforced AAC. The micro-fracture mechanisms were interpreted using AE frequency analysis. It is suggested that micro-cracking during the drying process produces the drying shrinkage damage in AAC. References i. Folker H. Wittman; AAC, Moisture and Properties, Elsevier (1983). 2. A.G.Evans and J.R.Clifton; Cement and Concrete Research 6,535(1976). 3. H.D.Jeong and H. Takahashi; Proc.of 2nd Conf. of Asian-Pacific Congress on Strength Evaluation,329(1986). 4. S.Teramura, K,Tsukiyama and H.Takahashi;Ref.3, 341. 5. T.Hashida and H.Takahashi; ASTM, J. of Testing and Evaluation, 13, 77(1985) 6. C.Wada and H.Takahashi;J. of Mining and Metallugical Institute of Japan 109, No.i175, 9(1986).(in Japanese) 7. P.G.Meredith and B.K.Atkinson; Phys.Earth Planet. Inter.,39, 33(1985).