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Acoustic emission characteristics of mortar under compression
CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 641-652, 1992. Printed in the USA. 0008-8846/92. $5.00+00. Copyright © 1992 Pergamon Press Ltd.
ACOUSTIC EMISSION CHARACTERISTICS OF MORTAR UNDER COMPRESSION
C.C. Weng*, M.T. Tam** and G.C. Lin** (*) A s s o c i a t e Professor, Department of civil Engineering, National chiao Tung university, Hsinchu, 300, Taiwan, R.O.C. (**) Graduate Research Assistants, Department of civil Engineering, National Chiao Tung University, Hsinchu, 300, Taiwan, R.O.C. (Ref=~d) (R~iv~d Aug.8.1991) ABSTRACT This research is devoted to the study of the characteristics of acoustic emission (AE) signals of mortar under uniaxial compression. A total of 40 specimens were tested, in which the w a t e r - c e m e n t ratio, age, compressive strength and strain were the major parameters of mortar related to the AE signals. In addition, different compressive modes were used in the tests in order to investigate the AE Kaiser effect in mortar. From the test results, good correlation between the AE signals and the major parameters of mortar was observed. The experimental findings may provide useful information for further understanding of the AE characteristics of concrete and reinforced concrete. Introduction In recent years, there has been an increasing need to develop new NDT (nondestructive testing) methods for m o n i t o r i n g the integrity of concrete structures. Among the NDT methods available today, acoustic e m i s s i o n (AE) technology is considered a potential new m o n i t o r i n g tool. The AE technology has the advantage of highly sensible detect a b i l i t y of active microscopic events such as p l a s t i c d e f o r m a t i o n or c r a c k i n g within materials. Acoustic emission signals can be described as t r a n s i e n t elastic stress waves generated by the rapid release of internal strain energy (i). Although the AE t e c h n o l o g y belongs to the c a t e g o r y of NDT methods, it differs from the other methods in that the signals which are picked up are emitted by the m a t e r i a l under stress, i.e., the AE technology is a dynamic detecting m e t h o d (2). Thus, the AE t e c h n o l o g y can be used to detect a growing crack inside a material. In order to utilize the AE technique as a d i a g n o s t i c method in concrete, it is felt that an AE investigation focusing on the basic component of the concrete material, mortar, may provide useful information for future studies. This research is d e v o t e d to the study of AE characteristics of mortar under uniaxial compression. The c o r r e l a t i o n between the variations of the major p a r a m e t e r s of mortar (e.g. w a t e r - c e m e n t ratio, compressive strength, age, strain, etc.) and 641
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the d e t e c t e d A E s i g n a l s are i n v e s t i g a t e d in t h i s study. In addition, s i n c e the K a i s e r e f f e c t is an i m p o r t a n t c h a r a c t e r of a c o u s t i c e m i s s i o n , t h e A E c h a r a c t e r i s t i c s of m o r t a r s u b j e c t e d to t h e u n l o a d and r e l o a d p r o c e s s are also studied.
Literature
Review
T h e i n i t i a l s t u d i e s of a c o u s t i c e m i s s i o n s w e r e r e p o r t e d by Obert in 1941 (3) and by H o d g s o n in 1942 (4). T h e y w e r e i n t e r e s t e d in p r e d i c t i n g r o c k b u r s t in mines. In the e a r l y 1950's, G e r m a n s c i e n t i s t K a i s e r (5) u s e d e l e c t r o n i c i n s t r u m e n t s to d e t e c t t h e s o u n d e m i t t e d f r o m t h e d e f o r m e d metals. He d i s c o v e r e d the i r r e v e r s i b i l i t y p h e n o m e n o n of AE s i g n a l s k n o w n as K a i s e r effect. A E s t u d i e s in c o n c r e t e w e r e found in e a r l y 1960's. A t that time, a c o u s t i c w a v e s w e r e o b s e r v e d from the f r a c t u r e p r o c e s s of c o n c r e t e m a t e r i a l s by e m p l o y i n g r e l a t i v e l y c r u d e d e v i c e s (6). In 1965, R o b i n s o n (7) i n v e s t i g a t e d the AE c h a r a c t e r i s t i c s of m o r t a r a n d c o n c r e t e specimens. He found that in c o m p a r i s o n to the o t h e r N D T methods, the e a r l y a n d t i n y c h a n g e s of c o n c r e t e m a t e r i a l s c o u l d be d e t e c t e d more e a s i l y by u s i n g A E method. In the 1970's, s e v e r a l A E r e s e a r c h e s were s t a r t e d to m o n i t o r m i c r o c r a c k s in concrete. In 1976, M c C a b e et al. (8) i n v e s t i g a t e d h o w the s p e c i m e n size and the c o m p r e s s i v e m o d e affect t h e A E s i g n a l s e m i t t e d from c y l i n d r i c a l c o n c r e t e s p e c i m e n s . They f o u n d t h a t the A E total e v e n t s i n c r e a s e d as t h e s i z e of t h e s p e c i m e n decreased. In a d d i t i o n , t h e y a l s o s t u d i e d the K a i s e r e f f e c t in p l a i n c o n c r e t e a n d o b s e r v e d t h a t the K a i s e r e f f e c t e x i s t e d w h e n t h e a p p l i e d load d i d not e x c e e d 80% to 85% of its u l t i m a t e s t r e n g t h . M o r e r e c e n t l y , A l l i c h e and F r a n c o i s (9) u s e d t h e A E m e t h o d to i n v e s t i g a t e t h e f a t i g u e b e h a v i o r of mortar. T h e y r e p o r t e d t h a t the c h a r a c t e r i s t i c s of A E s i g n a l s w e r e c l o s e l y r e l a t e d to t h e l o n g i t u d i n a l d e f o r m a t i o n of t h e specimen. In 1987, O h t s u (i0) i n v e s t i g a t e d the AE c h a r a c t e r i s t i c s of R C beams and f o u n d t h a t t h e A E c o u n t r a t e i n c r e a s e d e x p o n e n t i a l l y for t h e b e n d i n g - m o d e - f a i l u r e . O h t s u (ii) a l s o u s e d the A E t e c h n i q u e t o g e t h e r w i t h the f r a c t u r e m e c h a n i c s a n d t h e b o u n d a r y e l e m e n t m e t h o d to i n v e s t i g a t e the c r a c k p r o p a g a t i o n in m o r t a r and c o n c r e t e n o t c h e d beams. In 1989, R o s s i et al. (12) s t u d i e d t h e p h y s i c a l m e c h a n i s m s d u r i n g c r a c k i n g of m o r t a r a n d c o n c r e t e by u s i n g the AE technique. T h e i r r e s u l t s s h o w e d t h a t it is p o s s i b l e to utilize t h e A E t e c h n o l o g y to d i s t i n g u i s h c r a c k s at t h e m a t r i x - i n c l u s i o n interface a n d c r a c k s p r o p a g a t i n g in the matrix. In a d d i t i o n to the b r i e f r e v i e w p r e s e n t e d above, a m o r e d e t a i l e d l i t e r a t u r e s u r v e y can be f o u n d from the l a t e s t e d i t i o n (1991) of CRC h a n d b o o k on N D T of c o n c r e t e (13). F r o m t h e e x i s t i n g l i t e r a t u r e s , it is f o u n d t h a t m o r e r e s e a r c h e s on the A E c h a r a c t e r i s t i c s of m o r t a r are s t i l l needed. T h e A E r e s e a r c h of m o r t a r m a y p r o v i d e u s e f u l i n f o r m a t i o n for f u r t h e r u n d e r s t a n d i n g of the AE c h a r a c t e r i s t i c s of c o n c r e t e and reinforced concrete. Materials
and T e s t S p e c i m e n s
In t h i s study, the m o r t a r s p e c i m e n s w e r e m a d e by u s i n g the Type I P o r t l a n d c e m e n t and the g r a d e d s t a n d a r d s a n d for c o m p r e s s i v e s t r e n g t h t e s t a c c o r d i n g to A S T M Ci09 (14). The c e m e n t to s a n d r a t i o was 1:2.75. T h e s p e c i m e n s are cubes of d i m e n s i o n s of 5 0 x 5 0 x 5 0 mm. A t o t a l of 40 s p e c i m e n s w e r e tested. T a b l e 1 g i v e s t h e n u m b e r of s p e c i m e n s t e s t e d for d i f f e r e n t c o m b i n a t i o n s of t h e m a j o r p a r a m e t e r s of
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mortar. It is s e e n from the table t h a t four s p e c i m e n s w e r e t e s t e d for e a c h c o m b i n a t i o n of w a t e r - c e m e n t r a t i o and t h e a g e of specimen. The w a t e r - c e m e n t r a t i o of the s p e c i m e n s v a r i e d f r o m 45% to 70%. The s p e c i m e n s w i t h two d i f f e r e n t ages (7 and 28 days) w e r e tested. For t h e s t u d y of the K a i s e r effect, two s p e c i m e n s w e r e t e s t e d for each g r o u p of c o m b i n a t i o n to observe the r e p r o d u c i b i l i t y of the experiment. TABLE 1 Specimens for AE study of Cc~pression Test of Mortar
Subjects of Study
AE Sig~al~ vs. Water-Cement Ratio, Age, Compressive Strength and Strain
Age (~a~) Water-Cement Rat{o (%)
Kaiser Effect in Mortar
28
28
45
50
60
70
45
50
60
70
45
50
60
70
4
4
4
4
4
4
4
4
2
2
2
2
Nmt~r of
AE M o n i t o r i n g
System
T h e A E T m o d e l 5500 a c o u s t i c e m i s s i o n d e t e c t i n g s y s t e m of the A c o u s t i c E m i s s i o n T e c h n o l o g y C o r p o r a t i o n w a s u s e d in t h i s study. A p i e z o e l e c t r i c t r a n s d u c e r (ACI75L A E sensor) w i t h r e s o n a n t f r e q u e n c y at 175 k H z w a s u s e d to d e t e c t the A E signals. T h e g a i n s of t h e p r e a m p l i f i e r and t h e m a i n a m p l i f i e r w e r e set to b e 60 a n d 30 dB, respectively. A b a n d p a s s filter w i t h f r e q u e n c y r a n g e b e t w e e n 125 to 250 k H z w a s used. T h e t h r e s h o l d level for A E d e t e c t i o n w a s set to 0.i volt. D u r i n g the tests, the AE s e n s o r was c a r e f u l l y a t t a c h e d on each s p e c i m e n at the s a m e l o c a t i o n to m a i n t a i n the s a m e e x p e r i m e n t a l c o n d i t i o n s all t h r o u g h the tests. Experiments F i g u r e 1 shows a s c h e m a t i c b l o c k d i a g r a m of t h e t e s t s e t u p used in t h i s study. T h e c o m p r e s s i o n test was c a r r i e d o u t w i t h a u n i v e r s a l t e s t i n g machine, a n d the s t r a i n r e a d i n g s w e r e r e c o r d e d by u s i n g a d i g i t a l s t r a i n indicator. To study the r e l a t i o n s h i p s b e t w e e n the A E s i g n a l s a n d the s p e c i m e n w a t e r - c e m e n t ratio, age, c o m p r e s s i v e s t r e n g t h a n d strain, t h e f o l l o w i n g e x p e r i m e n t a l p r o c e d u r e w a s used: (i) M o r t a r s p e c i m e n s w e r e c a r e f u l l y p r e p a r e d a c c o r d i n g to the r e q u i r e m e n t s of A S T M Ci09. (2) A f t e r 7 or 28 days, t h e s p e c i m e n s w e r e t a k e n o u t f r o m the m o i s t r o o m a n d n a t u r a l l y d r i e d in air. Then, a s t r a i n g a g e (TML PL-10) w a s g l u e d on t h e specimen. (3) T h e A E s e n s o r w a s a t t a c h e d to t h e s p e c i m e n w i t h a c o u s t i c a l i m p e d a n c e m a t c h i n g couplant. (4) T h e A E m o n i t o r i n g s y s t e m a n d t h e s t r a i n i n d i c a t o r w e r e prepared. (5) T h e l o a d i n g r a t e of t h e u n i v e r s a l t e s t i n g m a c h i n e at 7.5 k N / m i n w a s set. Then, t h e c o m p r e s s i o n t e s t and t h e r e c o r d i n g of the AE s i g n a l s and s t r a i n r e a d i n g s w e r e b e g a n u n t i l t h e s p e c i m e n failed.
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loading
Strain Gage
Preamplifier FIG. 1 AE Setup for Compression Test of Mortar
Sensor
[
Mainframe
Strain Indictator
l_,oadi_ng
T o s t u d y the K a i s e r e f f e c t in m o r t a r s p e c i m e n s , t h e s a m e p r o c e d u r e s t a t e d a b o v e w a s u s e d e x c e p t that: (I) T w o d i f f e r e n t l o a d i n g r a t e s of 7.5 k N / m i n a n d 15 k N / m i n w e r e used. F r o m t h e s e tests, t h e i n f l u e n c e of the l o a d i n g r a t e on K a i s e r e f f e c t c a n be o b s e r v e d . (2) T h e l o a d w a s f i r s t i n c r e a s e d to 20 kN and w a s h e l d at t h i s load level for a s h o r t p e r i o d of time. Then, u n l o a d e d to 5 kN. (3) R e l o a d e d to 40 kN, h e l d t h e load, t h e n u n l o a d e d to 5 kN. (4) R e p e a t e d the r e l o a d a n d u n l o a d p r o c e s s to h i g h e r l o a d s of 60, 80, or i00 k N u n t i l t h e f a i l u r e of the specimen. Results Correlation Compressive
and Discussion
Between AE Total Events S t r e n g t h of M o r t a r
and Water-Cement
Ratio
and
F i g u r e 2 s h o w s t h e c o r r e l a t i o n b e t w e e n the v a r i a t i o n of the A E t o t a l e v e n t s a n d t h e w a t e r - c e m e n t r a t i o (W/C) of the m o r t a r s p e c i m e n s (28 days). F r o m t h i s figure, it is s i g n i f i c a n t to o b s e r v e t h a t as 5000
4000
•
E 3o00
~
Age = 28 days
to
A
t
t~J
FIG. 2 Correlation Between AE Total Events and Water-Cement Ratios of Mortar (28 days)
:
•
o
2000 A
1000
o 4O
'
5O
'
6O
Water-Cement
7
Ratio ( ~ )
'0
80
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the w a t e r - c e m e n t r a t i o increased, t h e total n u m b e r of AE e v e n t s d e c r e a s e d . A n a p p r o x i m a t e l y linear r e l a t i o n s h i p e x i s t s b e t w e e n these two parameters. F i g u r e 3 shows t h e c o r r e l a t i o n b e t w e e n the c o m p r e s s i v e s t r e n g t h of m o r t a r s p e c i m e n s (28 days) a n d the AE total events. It can be o b s e r v e d f r o m t h i s f i g u r e t h a t the AE total e v e n t s i n c r e a s e d as the c o m p r e s s i v e s t r e n g t h increased, s i m i l a r to the p h e n o m e n o n o b s e r v e d f r o m Fig. 2, an a p p r o x i m a t e l y linear r e l a t i o n s h i p a l s o e x i s t s b e t w e e n the AE t o t a l e v e n t s a n d the c o m p r e s s i v e s t r e n g t h of mortar. 5000
4000
Age = 28 days &
FIG.
co
3
Correlation Between AE Total Events and Compressive Strengths of Mortar (28 days)
•
3000
kJ
0
2000
1000
o
i
i
Compressive Strength (MPo)
(i) (2)
T h e p h e n o m e n a o b s e r v e d from Figs. 2 and 3 are p o s s i b l e because: S p e c i m e n s w i t h h i g h e r w a t e r - c e m e n t r a t i o are g e n e r a l l y w e a k e r in c o m p r e s s i v e strength, w h i c h m a y c a u s e e a r l i e r f a i l u r e of the s p e c i m e n s and r e s u l t in a d e c r e a s e of the A E t o t a l events. H i g h e r w a t e r - c e m e n t ratio u s u a l l y m e a n s h i g h e r p e r c e n t a g e of p o r o s i t y w i t h i n the specimen, w h i c h m a y f a c i l i t a t e the g r o w t h of c r a c k s a n d r e s u l t in e a r l i e r f a i l u r e of the specimen. Thus, f e w e r A E events w e r e detected.
AE Signals
vs.
Compressive
Strains
F i g u r e 4 shows a t y p i c a l c o r r e l a t i o n b e t w e e n the AE total events a n d t h e m e a s u r e d c o m p r e s s i v e s t r a i n s as a f u n c t i o n of time. The test r e s u l t s i n d i c a t e d t h a t the AE total e v e n t s h a d a g o o d c o r r e s p o n d e n c e to t h e m e a s u r e d strains. It is seen from the f i g u r e t h a t d u r i n g the f i r s t t w o - t h i r d s of the c o m p r e s s i o n process, b o t h of the AE total e v e n t s a n d the c o m p r e s s i v e strains i n c r e a s e d l i n e a r l y and m a i n t a i n e d at a r e l a t i v e l y low level. However, w h e n the s p e c i m e n was a p p r o a c h i n g f r a c t u r e , the m e a s u r e d strains w e r e found to i n c r e a s e r a p i d l y and n o n l i n e a r l y , and so did the AE total events. T h e g o o d c o r r e l a t i o n s t a t e d above a l s o e x i s t e d b e t w e e n the v a r i a t i o n s of the AE c o u n t r a t e and the m e a s u r e d c o m p r e s s i v e strains, as s h o w n in Fig. 5. B a s e d on t h e s e e x p e r i m e n t a l findings, it is b e l i e v e d t h a t the AE t e c h n o l o g y has g r e a t p o t e n t i a l for m o n i t o r i n g t h e s t r a i n h i s t o r y of m o r t a r u n d e r u n i a x i a l c o m p r e s s i o n .
that
In a d d i t i o n to the o b s e r v a t i o n s m a d e above, it w a s a l s o found for t h e s p e c i m e n s t e s t e d in this study, t h e r e e x i s t s a c o m m o n
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t r e n d of the v a r i a t i o n of the AE total e v e n t s as a f u n c t i o n of time. Fig. 6 shows a t y p i c a l v a r i a t i o n of the AE total e v e n t s a l o n g the e n t i r e c o m p r e s s i o n process. T h e c u r v e s h o w n in t h e f i g u r e c a n be d i v i d e d into t h r e e s t a g e s w h i c h include: (a) Stage 1 (initial stage): A t the b e g i n n i n g of t h e test, large n u m b e r of A E e v e n t s a p p e a r e d w i t h i n a s h o r t p e r i o d of time. When t h e h e a d of u n i v e r s a l t e s t i n g m a c h i n e c a m e i n t o c o n t a c t w i t h the specimen, cracks propagated rapidly between the weak boundaries w i t h i n t h e specimen, w h i c h r e s u l t e d in l a r g e n u m b e r of A E signals. (b) S t a g e 2 (stable stage): As the c o m p r e s s i o n p r o c e s s continued, the c r a c k s w i t h i n the s p e c i m e n g r e w steadily, and the AE e v e n t s i n c r e a s e d gradually. (c) S t a g e 3 (unstable stage): W h e n the s p e c i m e n w a s a p p r o a c h i n g its u l t i m a t e strength, the g r o w t h of the c r a c k s b e c a m e unstable. Correspondingly, the AE e v e n t s i n c r e a s e d a b r u p t l y , as shown in Fig. 6. F r o m the d e s c r i p t i o n s m a d e above, it is n o t e d t h a t the rapid i n c r e a s e of A E e v e n t s in stage 3 m a y c o n t a i n p o t e n t i a l i n f o r m a t i o n for j u d g i n g t h e s t a b i l i t y of a m o r t a r s p e c i m e n u n d e r u n i a x i a l compression. T h i s p h e n o m e n o n m a y be t r e a t e d as a c r i t e r i o n t h a t t h e s p e c i m e n will s o o n failed. 5000
5000
/ 4000
-
- -
- Strain Total Events
-4000
," ,' i
4--"
/
i
~sooo-
3000 o
b3
. -" "" -jz
02000
o
/
-2000
1000
1000 FIG. 4 A Typical Correlation B e t w e e n
AE Total Events and Compressive Strains of M o r t a r (W/C = 60%,
0 0
i;o
~;0
4~o
3;o
Time 3000
s~o ....600 ~-
- '700
8~o
. . . . . . . . . . . . . . . . . .
25ooj ....
d3
Strain Count
g00o 28 5000
b 4000 Rote
,I/
~2000-
d~ i -5000 0 x
c 1500co
-2000 o
days)
(sec)
c 0
1000-
~f
1000
m 500o o 0
0
16o
260
s~o
460
5;o
Time
(sec)
o;o
7;0
8;0
0 9o0
FIG.
5
A Typical Correlation Between AE Count Rates and Compressive Strains of M o r t a r (W/C = 60%, 2 8 days)
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ACOUSTIC EMISSION, MORTAR, COMPRESSION
6000
-
-
647
-
5000
4000
-
FIG. 6 Typical
Variation
t
of
AE T o t a l
Events During Three Stages of Compression Process
c
> 3000 % ~- 2000-
1000"
Stage
1
/
' ~6o
~
-
f
I
Stage 2
I
260
l
s6o
4oo~-- 560
II
600
Stage 5
~o
800
900
Time (sec)
Variation
of A E A m p l i t u d e
During
Compression
Process
B a s e d on the t h r e e stages of the c o m p r e s s i o n p r o c e s s s t a t e d above, it is felt t h a t a study on the v a r i a t i o n s of the AE p e a k a m p l i t u d e for each of the t h r e e s t a g e s m a y p r o v i d e f u r t h e r insight t h e AE r e s p o n s e a l o n g the e n t i r e l o a d i n g history.
to
In o r d e r to e s t a b l i s h the AE p e a k a m p l i t u d e d i s t r i b u t i o n curve for e a c h stage, a n e w t e r m c a l l e d "event ratio" is introduced. The event r a t i o is d e f i n e d as the n u m b e r of AE events for c e r t a i n peak a m p l i t u d e d i v i d e d by the total n u m b e r of AE events w i t h i n a c e r t a i n stage. By c a l c u l a t i n g the e v e n t r a t i o for each d i f f e r e n t p e a k amplitude, three c u r v e s for the t h r e e stages of c o m p r e s s i o n p r o c e s s were established. Figs. 7(a) and (b) s h o w two t y p i c a l d i a g r a m s of event r a t i o vs. p e a k a m p l i t u d e for m o r t a r s p e c i m e n s w i t h d i f f e r e n t w a t e r - c e m e n t r a t i o s and ages. F r o m f i g u r e s 7(a) and (b), an i m p o r t a n t p h e n o m e n o n w a s found w h i c h s h o w e d t h a t the p r o p o r t i o n s of the h i g h p e a k a m p l i t u d e AE events (higher t h a n 75 dB) in the initial and u n s t a b l e s t a g e s (stage 1 and 3) w e r e m u c h h i g h e r t h a n that in the s t a b l e stage (stage 2). This p h e n o m e n o n e x i s t e d in all test r e s u l t s of this s t u d y no m a t t e r what t h e w a t e r - c e m e n t r a t i o or age of the s p e c i m e n was. In addition, it w a s f o u n d t h a t the p r o p o r t i o n of the lower p e a k a m p l i t u d e AE events (50 to 60 dB) in the stable stage (stage 2) was g e n e r a l l y h i g h e r than t h a t in t h e u n s t a b l e stage (stage 3). T h e c h a r a c t e r i s t i c s of the AE p e a k a m p l i t u d e d i s t r i b u t i o n p r e s e n t e d a b o v e s h o w that there exists a good c o r r e l a t i o n b e t w e e n the h i g h a m p l i t u d e A E s i g n a l s (higher t h a n 75 dB) and the u n s t a b l e growing of c r a c k s in the m o r t a r specimens.
Correlation of M o r t a r
Between
Hiqh Amplitude
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F r o m t h e a b o v e discussion, it is k n o w n t h a t the h i g h a m p l i t u d e AE s i g n a l s h a v e a g o o d c o r r e l a t i o n with the g r o w t h of u n s t a b l e cracks in m o r t a r specimens. In this section, a study is m a d e to c o r r e l a t e the v a r i a t i o n s of the h i g h a m p l i t u d e AE events and the w a t e r - c e m e n t ratios of t h e specimens.
648
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F i g u r e s 8(a) and (b) show the c o r r e l a t i o n b e t w e e n the variations of t h e w a t e r - c e m e n t r a t i o and the "high a m p l i t u d e e v e n t ratio". The h i g h a m p l i t u d e e v e n t ratio is d e f i n e d as the n u m b e r of h i g h peak amplitude A E e v e n t s (higher than 75 dB) d i v i d e d by the total number of A E events. F r o m these figures, it is o b s e r v e d t h a t the v a r i a t i o n of t h e w a t e r - c e m e n t ratio was l i n e a r l y p r o p o r t i o n a l to the v a r i a t i o n of t h e h i g h a m p l i t u d e A E event ratio. This o b s e r v a t i o n is applicable to s p e c i m e n s t e s t e d at 7 and 28 days. F u r t h e r m o r e , Fig. 8(c) shows a c o m p a r i s o n b e t w e e n the h i g h a m p l i t u d e A E e v e n t r a t i o s o b t a i n e d from s p e c i m e n s t e s t e d at 7 and 28 days. It w a s o b s e r v e d that the h i g h a m p l i t u d e e v e n t r a t i o s for the 2 8 - d a y s m o r t a r s p e c i m e n s were less than t h a t of t h e 7-days. This p h e n o m e n o n is p o s s i b l e b e c a u s e that a l o n g w i t h the p r o c e s s of h y d r a t i o n , t h e p o r o s i t y w i t h i n the m o r t a r s p e c i m e n d e c r e a s e d g r a d u a l l y w i t h time. Therefore, the h i g h a m p l i t u d e event r a t i o b e c a m e fewer for t h o s e s p e c i m e n s t e s t e d at longer age.
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Vol. 22, No. 4
ACOUSTIC EMISSION, MORTAR, COMPRESSION
649
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Vol. 22, No. 4
to be o b s e r v e d m a x i m u m load.
ACOUSTIC EMISSION, MORTAR, COMPRESSION
u n t i l the newly This phenomenon
651
a p p l i e d load e x c e e d s t h e p r e v i o u s is k n o w n as K a i s e r effect.
F i g u r e 9 shows a typical AE r e s u l t o b t a i n e d f r o m t h e u n l o a d and r e l o a d process. In this figure, the d o t t e d lines r e p r e s e n t the l o a d i n g history. It is seen that at the b e g i n n i n g of t h e test, very h i g h A E c o u n t rates appeared. Then, w h e n the l o a d w a s h e l d at the c o n s t a n t level of 20 kN for a short p e r i o d of time, the AE signals w e r e f o u n d to d e c r e a s e rapidly. W h e n the AE s i g n a l s a l m o s t d i s a p p e a r e d w h i c h i n d i c a t e d t h a t c r a c k s w i t h i n the s p e c i m e n s h a d s t o p growing, the u n l o a d p r o c e s s began. Then, the s p e c i m e n w a s r e l o a d e d to a h i g h e r level, 40 kN. F r o m the AE r e s u l t s s h o w n in Fig. 9, it is seen that AE s i g n a l s w e r e rare w h e n the n e w l y a p p l i e d load d i d n o t e x c e e d the p r e v i o u s m a x i m u m load. However, m a n y A E s i g n a l s a p p e a r e d again w h e n t h e n e w l y a p p l i e d l o a d i n g p a s s e d the p r e v i o u s m a x i m u m . T h i s p h e n o m e n o n c o n t i n u e d u n t i l the load was i n c r e a s e d to 60 kN. T h e above o b s e r v a t i o n i n d i c a t e d t h a t the K a i s e r e f f e c t e x i s t s in m o r t a r s p e c i m e n up to the load level of 60 kN ( a p p r o x i m a t e l y 80% of its u l t i m a t e strength). F r o m Fig. 9, it was also n o t e d t h a t w h e n the a p p l i e d load a p p r o a c h e d the u l t i m a t e s t r e n g t h of the specimen, A E s i g n a l s w e r e f o u n d to i n c r e a s e c o n t i n u o u s l y even w h e n the load w a s h e l d at the c o n s t a n t level of 80 kN. This m e a n s t h a t the c r a c k s w i t h i n the s p e c i m e n s w e r e growing, i.e., the s p e c i m e n w a s in an u n s t a b l e condition. As the last u n l o a d and r e l o a d p r o c e s s s h o w n in Fig. 9, m a n y A E s i g n a l s a p p e a r e d b e f o r e the p r e v i o u s m a x i m u m load was e x c e e d e d , w h i c h i n d i c a t e d that the K ~ i s e r e f f e c t d i d not exist a n y more. In a d d i t i o n to the o b s e r v a t i o n s m a d e above, Figs. 10(a) and (b) c o m p a r e the K a i s e r e f f e c t for two s p e c i m e n s t e s t e d w i t h d i f f e r e n t l o a d i n g rates, 7.5 k N / m i n and 15 kN/min, r e s p e c t i v e l y . T h e loading p a t t e r n u s e d in t h e s e tests w e r e also d i f f e r e n t f r o m t h a t shown in Fig. 9. F r o m Figs. 10(a) and (b), it was o b s e r v e d that: (i) In spite of the d i f f e r e n c e of the l o a d i n g r a t e u s e d in the tests, K a i s e r e f f e c t e x i s t e d in b o t h s p e c i m e n s w h e n the a p p l i e d load w a s w i t h i n about 80% of the u l t i m a t e s t r e n g t h of the specimen. (2) As c o m p a r e d to the l o a d i n g p a t t e r n u s e d in Fig. 9 w h e r e the load w a s h e l d at a c o n s t a n t level for a s h o r t p e r i o d of t i m e before unloading, the t e s t r e s u l t s h o w e d t h a t the K a i s e r e f f e c t still e x i s t e d r e g a r d l e s s the c h a n g e of the l o a d i n g p a t t e r n .
Summary
and C o n c l u s i o n s
B a s e d on the r e s u l t s of this study, the f o l l o w i n g s u m m a r y and c o n c l u s i o n s are made: (i) A n a p p r o x i m a t e l y linear r e l a t i o n s h i p b e t w e e n t h e v a r i a t i o n of t h e AE total e v e n t s and the w a t e r - c e m e n t r a t i o of m o r t a r s p e c i m e n s w a s observed. W h e n the w a t e r - c e m e n t r a t i o increased, t h e A E t o t a l e v e n t s w e r e found to d e c r e a s e f o l l o w i n g a linear relation. (2) On the contrary, w h e n the c o m p r e s s i v e s t r e n g t h of t h e m o r t a r s p e c i m e n s increased, the A E t o t a l e v e n t s w e r e f o u n d to increase accordingly. A n a p p r o x i m a t e l y l i n e a r r e l a t i o n s h i p w a s also o b s e r v e d b e t w e e n the v a r i a t i o n of t h e s e two p a r a m e t e r s . (3) In addition, the v a r i a t i o n of the A E t o t a l e v e n t s w a s found to h a v e a c l o s e c o r r e l a t i o n w i t h the m e a s u r e d c o m p r e s s i v e strains. T h i s o b s e r v a t i o n shows t h a t the AE t e c h n o l o g y h a s g o o d p o t e n t i a l for m o n i t o r i n g the s t r a i n h i s t o r y of m o r t a r s p e c i m e n s .
652
(4)
(5)
(6)
(7)
C.C. Weng et al.
Vol. 22, No. 4
F r o m t h e c h a r a c t e r i s t i c s of the A E t o t a l events, the e n t i r e c o m p r e s s i o n p r o c e s s of a m o r t a r s p e c i m e n c a n be d i v i d e d into t h r e e s t a g e s w h i c h i n c l u d e the i n i t i a l , s t a b l e a n d u n s t a b l e stages. T h i s p h e n o m e n o n m a y c o n t a i n p o t e n t i a l i n f o r m a t i o n for j u d g i n g t h e s t a b i l i t y of a m o r t a r s p e c i m e n u n d e r c o m p r e s s i o n . In a d d i t i o n , the r e s u l t s of AE p e a k a m p l i t u d e d i s t r i b u t i o n showed t h a t in the i n i t i a l and u n s t a b l e stages, the p r o p o r t i o n s of the h i g h a m p l i t u d e A E e v e n t s (higher t h a n 75 dB) w e r e m u c h h i g h e r t h a n t h a t in the s t a b l e stage. F u r t h e r m o r e , it was found t h a t a l i n e a r r e l a t i o n s h i p e x i s t e d b e t w e e n the i n c r e a s e s of the w a t e r - c e m e n t r a t i o and the h i g h a m p l i t u d e A E e v e n t ratio. Also, for s p e c i m e n s t e s t e d at 28 days, t h e h i g h a m p l i t u d e e v e n t r a t i o was found to be s m a l l e r t h a n that of t h e 7 - d a y tests. T h e s t u d y of K a i s e r e f f e c t s h o w e d t h a t K a i s e r e f f e c t e x i s t e d in m o r t a r s p e c i m e n s w h e n the a p p l i e d load was w i t h i n a b o u t 80% of t h e u l t i m a t e s t r e n g t h of the specimen. In a d d i t i o n , the change of the l o a d i n g rate from 7.5 k N / m i n to 15 k N / m i n did not affect t h e e x i s t e n c e of the K a i s e r effect. References
i. R.G. Liptai, D.O. H a r r i s and C.A. Tatro, "An I n t r o d u c t i o n to A c o u s t i c E m i s s i o n , " A c o u s t i c Emission, A S T M STP 505, (1972). 2. N.N. Hsu, J.A. S i m m o n s and S.C. Hardy, "A A p p r o a c h to A c o u s t i c E m i s s i o n S i g n a l A n a l y s i s - T h e o r y and E x p e r i m e n t , " M a t e r i a l s E v a l u a t i o n , Vol. 35, No. ii, i00, (1977). 3. E.A. Hodgson, B u l l e t i n S e i s m o l o g i c a l S o c i e t y of America, Vol. 32, No. 249, (1942). 4. L. Obert, "Use of S u b a u d i b l e N o i s e s for P r e d i c t i o n of R o c k b u r s t s , " U.S. B u r e a u of Mines, R e p o r t I n v e s t i g a t i o n 3555, (1941). 5. J. Kaiser, " K n o w l e d g e a n d R e s e a r c h on N o i s e M e a s u r e m e n t s D u r i n g the T e n s i l e S t r e s s i n g of M a t e l s , " Arkiv. fur E i s e n h u t t e n w e s e n , Vol. 24, No. 43, (1953). 6. V.M. Malhotra, T e s t i n g H a r d e n e d Concrete: N o n - D e s t r u c t i v e Methods, Monogr. No. 9, A m e r i c a n C o n c r e t e Institute, Detroit, MI, (1976). 7. G.S. Robinson, " M e t h o d s of D e t e c t i n g the F o r m a t i o n and P r o p a g a t o n of M i c r o c r a c k s in C o n c r e t e , " Proc., Inter. Conf. on the S t r u c t u r e of C o n c r e t e a n d Its B e h a v i o r U n d e r Load, London, 131, (1965). 8. W.M. McCabe, R.M. K o e r n e r and A.E. Lord, " A c o u s t i c E m i s s i o n B e h a v i o r of C o n c r e t e L a b o r a t o r y S p e c i m e n s , " ACI Journal, 3 6 7 , ( 1 9 7 6 ) . 9. A. A l l i c h e and D. Francois, " F a t i g u e B e h a v i o r of H a r d e n e d C e m e n t P a s t e , " C e m e n t and C o n c r e t e Research, Vol. 16, 199, (1986). I0. M. Ohtsu, " A c o u s t i c E m i s s i o n C h a r a c t e r i s t i c s in C o n c r e t e and D i a g n o s t i c A p p l i c a t i o n s , " J o u r n a l of A c o u s t i c Emission, Vol. 6, No. 2, 99, (1987). ii. M. Ohtsu, " C r a c k P r o p a g a t i o n in Concrete: L i n e a r E l a s t i c F r a c t u r e M e c h a n i c s and B o u n d a r y E l e m e n t M e t h o d , " T h e o r e t i c a l and A p p l i e d F r a c t u r e M e c h a n i c s , Vol. 9, No. i, 55, (1988). 12. P. Rossi, J.L. Robert, J.P. G e r v a i s and D. Bruhat, " I d e n t i f i c a t i o n of P h y s i c a l M e c h a n i s m s u n d e r l y i n g A c o u s t i c E m i s s i o n d u r i n g the C r a c k i n g of C o n c r e t e , " M a t e r i a l s and S t r u c t u r e s , Vol. 22, No. 129, 194, (1989). 13. V.M. M a l h o t r a a n d N.J. Carino, CRC H a n d b o o k on N o n d e s t r u c t i v e T e s t i n g of C o n c r e t e , CRC Press, Inc., Boston, (1991). 14. A S T M CI09, " S t a n d a r d T e s t M e t h o d for C o m p r e s s i v e S t r e n g t h of H y d r a u l i c C e m e n t M o r t a r s , " ASTM, (1986).
ACOUSTIC EMISSION CHARACTERISTICS OF MORTAR UNDER COMPRESSION
C.C. Weng*, M.T. Tam** and G.C. Lin** (*) A s s o c i a t e Professor, Department of civil Engineering, National chiao Tung university, Hsinchu, 300, Taiwan, R.O.C. (**) Graduate Research Assistants, Department of civil Engineering, National Chiao Tung University, Hsinchu, 300, Taiwan, R.O.C. (Ref=~d) (R~iv~d Aug.8.1991) ABSTRACT This research is devoted to the study of the characteristics of acoustic emission (AE) signals of mortar under uniaxial compression. A total of 40 specimens were tested, in which the w a t e r - c e m e n t ratio, age, compressive strength and strain were the major parameters of mortar related to the AE signals. In addition, different compressive modes were used in the tests in order to investigate the AE Kaiser effect in mortar. From the test results, good correlation between the AE signals and the major parameters of mortar was observed. The experimental findings may provide useful information for further understanding of the AE characteristics of concrete and reinforced concrete. Introduction In recent years, there has been an increasing need to develop new NDT (nondestructive testing) methods for m o n i t o r i n g the integrity of concrete structures. Among the NDT methods available today, acoustic e m i s s i o n (AE) technology is considered a potential new m o n i t o r i n g tool. The AE technology has the advantage of highly sensible detect a b i l i t y of active microscopic events such as p l a s t i c d e f o r m a t i o n or c r a c k i n g within materials. Acoustic emission signals can be described as t r a n s i e n t elastic stress waves generated by the rapid release of internal strain energy (i). Although the AE t e c h n o l o g y belongs to the c a t e g o r y of NDT methods, it differs from the other methods in that the signals which are picked up are emitted by the m a t e r i a l under stress, i.e., the AE technology is a dynamic detecting m e t h o d (2). Thus, the AE t e c h n o l o g y can be used to detect a growing crack inside a material. In order to utilize the AE technique as a d i a g n o s t i c method in concrete, it is felt that an AE investigation focusing on the basic component of the concrete material, mortar, may provide useful information for future studies. This research is d e v o t e d to the study of AE characteristics of mortar under uniaxial compression. The c o r r e l a t i o n between the variations of the major p a r a m e t e r s of mortar (e.g. w a t e r - c e m e n t ratio, compressive strength, age, strain, etc.) and 641
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the d e t e c t e d A E s i g n a l s are i n v e s t i g a t e d in t h i s study. In addition, s i n c e the K a i s e r e f f e c t is an i m p o r t a n t c h a r a c t e r of a c o u s t i c e m i s s i o n , t h e A E c h a r a c t e r i s t i c s of m o r t a r s u b j e c t e d to t h e u n l o a d and r e l o a d p r o c e s s are also studied.
Literature
Review
T h e i n i t i a l s t u d i e s of a c o u s t i c e m i s s i o n s w e r e r e p o r t e d by Obert in 1941 (3) and by H o d g s o n in 1942 (4). T h e y w e r e i n t e r e s t e d in p r e d i c t i n g r o c k b u r s t in mines. In the e a r l y 1950's, G e r m a n s c i e n t i s t K a i s e r (5) u s e d e l e c t r o n i c i n s t r u m e n t s to d e t e c t t h e s o u n d e m i t t e d f r o m t h e d e f o r m e d metals. He d i s c o v e r e d the i r r e v e r s i b i l i t y p h e n o m e n o n of AE s i g n a l s k n o w n as K a i s e r effect. A E s t u d i e s in c o n c r e t e w e r e found in e a r l y 1960's. A t that time, a c o u s t i c w a v e s w e r e o b s e r v e d from the f r a c t u r e p r o c e s s of c o n c r e t e m a t e r i a l s by e m p l o y i n g r e l a t i v e l y c r u d e d e v i c e s (6). In 1965, R o b i n s o n (7) i n v e s t i g a t e d the AE c h a r a c t e r i s t i c s of m o r t a r a n d c o n c r e t e specimens. He found that in c o m p a r i s o n to the o t h e r N D T methods, the e a r l y a n d t i n y c h a n g e s of c o n c r e t e m a t e r i a l s c o u l d be d e t e c t e d more e a s i l y by u s i n g A E method. In the 1970's, s e v e r a l A E r e s e a r c h e s were s t a r t e d to m o n i t o r m i c r o c r a c k s in concrete. In 1976, M c C a b e et al. (8) i n v e s t i g a t e d h o w the s p e c i m e n size and the c o m p r e s s i v e m o d e affect t h e A E s i g n a l s e m i t t e d from c y l i n d r i c a l c o n c r e t e s p e c i m e n s . They f o u n d t h a t the A E total e v e n t s i n c r e a s e d as t h e s i z e of t h e s p e c i m e n decreased. In a d d i t i o n , t h e y a l s o s t u d i e d the K a i s e r e f f e c t in p l a i n c o n c r e t e a n d o b s e r v e d t h a t the K a i s e r e f f e c t e x i s t e d w h e n t h e a p p l i e d load d i d not e x c e e d 80% to 85% of its u l t i m a t e s t r e n g t h . M o r e r e c e n t l y , A l l i c h e and F r a n c o i s (9) u s e d t h e A E m e t h o d to i n v e s t i g a t e t h e f a t i g u e b e h a v i o r of mortar. T h e y r e p o r t e d t h a t the c h a r a c t e r i s t i c s of A E s i g n a l s w e r e c l o s e l y r e l a t e d to t h e l o n g i t u d i n a l d e f o r m a t i o n of t h e specimen. In 1987, O h t s u (i0) i n v e s t i g a t e d the AE c h a r a c t e r i s t i c s of R C beams and f o u n d t h a t t h e A E c o u n t r a t e i n c r e a s e d e x p o n e n t i a l l y for t h e b e n d i n g - m o d e - f a i l u r e . O h t s u (ii) a l s o u s e d the A E t e c h n i q u e t o g e t h e r w i t h the f r a c t u r e m e c h a n i c s a n d t h e b o u n d a r y e l e m e n t m e t h o d to i n v e s t i g a t e the c r a c k p r o p a g a t i o n in m o r t a r and c o n c r e t e n o t c h e d beams. In 1989, R o s s i et al. (12) s t u d i e d t h e p h y s i c a l m e c h a n i s m s d u r i n g c r a c k i n g of m o r t a r a n d c o n c r e t e by u s i n g the AE technique. T h e i r r e s u l t s s h o w e d t h a t it is p o s s i b l e to utilize t h e A E t e c h n o l o g y to d i s t i n g u i s h c r a c k s at t h e m a t r i x - i n c l u s i o n interface a n d c r a c k s p r o p a g a t i n g in the matrix. In a d d i t i o n to the b r i e f r e v i e w p r e s e n t e d above, a m o r e d e t a i l e d l i t e r a t u r e s u r v e y can be f o u n d from the l a t e s t e d i t i o n (1991) of CRC h a n d b o o k on N D T of c o n c r e t e (13). F r o m t h e e x i s t i n g l i t e r a t u r e s , it is f o u n d t h a t m o r e r e s e a r c h e s on the A E c h a r a c t e r i s t i c s of m o r t a r are s t i l l needed. T h e A E r e s e a r c h of m o r t a r m a y p r o v i d e u s e f u l i n f o r m a t i o n for f u r t h e r u n d e r s t a n d i n g of the AE c h a r a c t e r i s t i c s of c o n c r e t e and reinforced concrete. Materials
and T e s t S p e c i m e n s
In t h i s study, the m o r t a r s p e c i m e n s w e r e m a d e by u s i n g the Type I P o r t l a n d c e m e n t and the g r a d e d s t a n d a r d s a n d for c o m p r e s s i v e s t r e n g t h t e s t a c c o r d i n g to A S T M Ci09 (14). The c e m e n t to s a n d r a t i o was 1:2.75. T h e s p e c i m e n s are cubes of d i m e n s i o n s of 5 0 x 5 0 x 5 0 mm. A t o t a l of 40 s p e c i m e n s w e r e tested. T a b l e 1 g i v e s t h e n u m b e r of s p e c i m e n s t e s t e d for d i f f e r e n t c o m b i n a t i o n s of t h e m a j o r p a r a m e t e r s of
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mortar. It is s e e n from the table t h a t four s p e c i m e n s w e r e t e s t e d for e a c h c o m b i n a t i o n of w a t e r - c e m e n t r a t i o and t h e a g e of specimen. The w a t e r - c e m e n t r a t i o of the s p e c i m e n s v a r i e d f r o m 45% to 70%. The s p e c i m e n s w i t h two d i f f e r e n t ages (7 and 28 days) w e r e tested. For t h e s t u d y of the K a i s e r effect, two s p e c i m e n s w e r e t e s t e d for each g r o u p of c o m b i n a t i o n to observe the r e p r o d u c i b i l i t y of the experiment. TABLE 1 Specimens for AE study of Cc~pression Test of Mortar
Subjects of Study
AE Sig~al~ vs. Water-Cement Ratio, Age, Compressive Strength and Strain
Age (~a~) Water-Cement Rat{o (%)
Kaiser Effect in Mortar
28
28
45
50
60
70
45
50
60
70
45
50
60
70
4
4
4
4
4
4
4
4
2
2
2
2
Nmt~r of
AE M o n i t o r i n g
System
T h e A E T m o d e l 5500 a c o u s t i c e m i s s i o n d e t e c t i n g s y s t e m of the A c o u s t i c E m i s s i o n T e c h n o l o g y C o r p o r a t i o n w a s u s e d in t h i s study. A p i e z o e l e c t r i c t r a n s d u c e r (ACI75L A E sensor) w i t h r e s o n a n t f r e q u e n c y at 175 k H z w a s u s e d to d e t e c t the A E signals. T h e g a i n s of t h e p r e a m p l i f i e r and t h e m a i n a m p l i f i e r w e r e set to b e 60 a n d 30 dB, respectively. A b a n d p a s s filter w i t h f r e q u e n c y r a n g e b e t w e e n 125 to 250 k H z w a s used. T h e t h r e s h o l d level for A E d e t e c t i o n w a s set to 0.i volt. D u r i n g the tests, the AE s e n s o r was c a r e f u l l y a t t a c h e d on each s p e c i m e n at the s a m e l o c a t i o n to m a i n t a i n the s a m e e x p e r i m e n t a l c o n d i t i o n s all t h r o u g h the tests. Experiments F i g u r e 1 shows a s c h e m a t i c b l o c k d i a g r a m of t h e t e s t s e t u p used in t h i s study. T h e c o m p r e s s i o n test was c a r r i e d o u t w i t h a u n i v e r s a l t e s t i n g machine, a n d the s t r a i n r e a d i n g s w e r e r e c o r d e d by u s i n g a d i g i t a l s t r a i n indicator. To study the r e l a t i o n s h i p s b e t w e e n the A E s i g n a l s a n d the s p e c i m e n w a t e r - c e m e n t ratio, age, c o m p r e s s i v e s t r e n g t h a n d strain, t h e f o l l o w i n g e x p e r i m e n t a l p r o c e d u r e w a s used: (i) M o r t a r s p e c i m e n s w e r e c a r e f u l l y p r e p a r e d a c c o r d i n g to the r e q u i r e m e n t s of A S T M Ci09. (2) A f t e r 7 or 28 days, t h e s p e c i m e n s w e r e t a k e n o u t f r o m the m o i s t r o o m a n d n a t u r a l l y d r i e d in air. Then, a s t r a i n g a g e (TML PL-10) w a s g l u e d on t h e specimen. (3) T h e A E s e n s o r w a s a t t a c h e d to t h e s p e c i m e n w i t h a c o u s t i c a l i m p e d a n c e m a t c h i n g couplant. (4) T h e A E m o n i t o r i n g s y s t e m a n d t h e s t r a i n i n d i c a t o r w e r e prepared. (5) T h e l o a d i n g r a t e of t h e u n i v e r s a l t e s t i n g m a c h i n e at 7.5 k N / m i n w a s set. Then, t h e c o m p r e s s i o n t e s t and t h e r e c o r d i n g of the AE s i g n a l s and s t r a i n r e a d i n g s w e r e b e g a n u n t i l t h e s p e c i m e n failed.
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loading
Strain Gage
Preamplifier FIG. 1 AE Setup for Compression Test of Mortar
Sensor
[
Mainframe
Strain Indictator
l_,oadi_ng
T o s t u d y the K a i s e r e f f e c t in m o r t a r s p e c i m e n s , t h e s a m e p r o c e d u r e s t a t e d a b o v e w a s u s e d e x c e p t that: (I) T w o d i f f e r e n t l o a d i n g r a t e s of 7.5 k N / m i n a n d 15 k N / m i n w e r e used. F r o m t h e s e tests, t h e i n f l u e n c e of the l o a d i n g r a t e on K a i s e r e f f e c t c a n be o b s e r v e d . (2) T h e l o a d w a s f i r s t i n c r e a s e d to 20 kN and w a s h e l d at t h i s load level for a s h o r t p e r i o d of time. Then, u n l o a d e d to 5 kN. (3) R e l o a d e d to 40 kN, h e l d t h e load, t h e n u n l o a d e d to 5 kN. (4) R e p e a t e d the r e l o a d a n d u n l o a d p r o c e s s to h i g h e r l o a d s of 60, 80, or i00 k N u n t i l t h e f a i l u r e of the specimen. Results Correlation Compressive
and Discussion
Between AE Total Events S t r e n g t h of M o r t a r
and Water-Cement
Ratio
and
F i g u r e 2 s h o w s t h e c o r r e l a t i o n b e t w e e n the v a r i a t i o n of the A E t o t a l e v e n t s a n d t h e w a t e r - c e m e n t r a t i o (W/C) of the m o r t a r s p e c i m e n s (28 days). F r o m t h i s figure, it is s i g n i f i c a n t to o b s e r v e t h a t as 5000
4000
•
E 3o00
~
Age = 28 days
to
A
t
t~J
FIG. 2 Correlation Between AE Total Events and Water-Cement Ratios of Mortar (28 days)
:
•
o
2000 A
1000
o 4O
'
5O
'
6O
Water-Cement
7
Ratio ( ~ )
'0
80
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the w a t e r - c e m e n t r a t i o increased, t h e total n u m b e r of AE e v e n t s d e c r e a s e d . A n a p p r o x i m a t e l y linear r e l a t i o n s h i p e x i s t s b e t w e e n these two parameters. F i g u r e 3 shows t h e c o r r e l a t i o n b e t w e e n the c o m p r e s s i v e s t r e n g t h of m o r t a r s p e c i m e n s (28 days) a n d the AE total events. It can be o b s e r v e d f r o m t h i s f i g u r e t h a t the AE total e v e n t s i n c r e a s e d as the c o m p r e s s i v e s t r e n g t h increased, s i m i l a r to the p h e n o m e n o n o b s e r v e d f r o m Fig. 2, an a p p r o x i m a t e l y linear r e l a t i o n s h i p a l s o e x i s t s b e t w e e n the AE t o t a l e v e n t s a n d the c o m p r e s s i v e s t r e n g t h of mortar. 5000
4000
Age = 28 days &
FIG.
co
3
Correlation Between AE Total Events and Compressive Strengths of Mortar (28 days)
•
3000
kJ
0
2000
1000
o
i
i
Compressive Strength (MPo)
(i) (2)
T h e p h e n o m e n a o b s e r v e d from Figs. 2 and 3 are p o s s i b l e because: S p e c i m e n s w i t h h i g h e r w a t e r - c e m e n t r a t i o are g e n e r a l l y w e a k e r in c o m p r e s s i v e strength, w h i c h m a y c a u s e e a r l i e r f a i l u r e of the s p e c i m e n s and r e s u l t in a d e c r e a s e of the A E t o t a l events. H i g h e r w a t e r - c e m e n t ratio u s u a l l y m e a n s h i g h e r p e r c e n t a g e of p o r o s i t y w i t h i n the specimen, w h i c h m a y f a c i l i t a t e the g r o w t h of c r a c k s a n d r e s u l t in e a r l i e r f a i l u r e of the specimen. Thus, f e w e r A E events w e r e detected.
AE Signals
vs.
Compressive
Strains
F i g u r e 4 shows a t y p i c a l c o r r e l a t i o n b e t w e e n the AE total events a n d t h e m e a s u r e d c o m p r e s s i v e s t r a i n s as a f u n c t i o n of time. The test r e s u l t s i n d i c a t e d t h a t the AE total e v e n t s h a d a g o o d c o r r e s p o n d e n c e to t h e m e a s u r e d strains. It is seen from the f i g u r e t h a t d u r i n g the f i r s t t w o - t h i r d s of the c o m p r e s s i o n process, b o t h of the AE total e v e n t s a n d the c o m p r e s s i v e strains i n c r e a s e d l i n e a r l y and m a i n t a i n e d at a r e l a t i v e l y low level. However, w h e n the s p e c i m e n was a p p r o a c h i n g f r a c t u r e , the m e a s u r e d strains w e r e found to i n c r e a s e r a p i d l y and n o n l i n e a r l y , and so did the AE total events. T h e g o o d c o r r e l a t i o n s t a t e d above a l s o e x i s t e d b e t w e e n the v a r i a t i o n s of the AE c o u n t r a t e and the m e a s u r e d c o m p r e s s i v e strains, as s h o w n in Fig. 5. B a s e d on t h e s e e x p e r i m e n t a l findings, it is b e l i e v e d t h a t the AE t e c h n o l o g y has g r e a t p o t e n t i a l for m o n i t o r i n g t h e s t r a i n h i s t o r y of m o r t a r u n d e r u n i a x i a l c o m p r e s s i o n .
that
In a d d i t i o n to the o b s e r v a t i o n s m a d e above, it w a s a l s o found for t h e s p e c i m e n s t e s t e d in this study, t h e r e e x i s t s a c o m m o n
646
C.C. Weng et al.
Vol. 22, No. 4
t r e n d of the v a r i a t i o n of the AE total e v e n t s as a f u n c t i o n of time. Fig. 6 shows a t y p i c a l v a r i a t i o n of the AE total e v e n t s a l o n g the e n t i r e c o m p r e s s i o n process. T h e c u r v e s h o w n in t h e f i g u r e c a n be d i v i d e d into t h r e e s t a g e s w h i c h include: (a) Stage 1 (initial stage): A t the b e g i n n i n g of t h e test, large n u m b e r of A E e v e n t s a p p e a r e d w i t h i n a s h o r t p e r i o d of time. When t h e h e a d of u n i v e r s a l t e s t i n g m a c h i n e c a m e i n t o c o n t a c t w i t h the specimen, cracks propagated rapidly between the weak boundaries w i t h i n t h e specimen, w h i c h r e s u l t e d in l a r g e n u m b e r of A E signals. (b) S t a g e 2 (stable stage): As the c o m p r e s s i o n p r o c e s s continued, the c r a c k s w i t h i n the s p e c i m e n g r e w steadily, and the AE e v e n t s i n c r e a s e d gradually. (c) S t a g e 3 (unstable stage): W h e n the s p e c i m e n w a s a p p r o a c h i n g its u l t i m a t e strength, the g r o w t h of the c r a c k s b e c a m e unstable. Correspondingly, the AE e v e n t s i n c r e a s e d a b r u p t l y , as shown in Fig. 6. F r o m the d e s c r i p t i o n s m a d e above, it is n o t e d t h a t the rapid i n c r e a s e of A E e v e n t s in stage 3 m a y c o n t a i n p o t e n t i a l i n f o r m a t i o n for j u d g i n g t h e s t a b i l i t y of a m o r t a r s p e c i m e n u n d e r u n i a x i a l compression. T h i s p h e n o m e n o n m a y be t r e a t e d as a c r i t e r i o n t h a t t h e s p e c i m e n will s o o n failed. 5000
5000
/ 4000
-
- -
- Strain Total Events
-4000
," ,' i
4--"
/
i
~sooo-
3000 o
b3
. -" "" -jz
02000
o
/
-2000
1000
1000 FIG. 4 A Typical Correlation B e t w e e n
AE Total Events and Compressive Strains of M o r t a r (W/C = 60%,
0 0
i;o
~;0
4~o
3;o
Time 3000
s~o ....600 ~-
- '700
8~o
. . . . . . . . . . . . . . . . . .
25ooj ....
d3
Strain Count
g00o 28 5000
b 4000 Rote
,I/
~2000-
d~ i -5000 0 x
c 1500co
-2000 o
days)
(sec)
c 0
1000-
~f
1000
m 500o o 0
0
16o
260
s~o
460
5;o
Time
(sec)
o;o
7;0
8;0
0 9o0
FIG.
5
A Typical Correlation Between AE Count Rates and Compressive Strains of M o r t a r (W/C = 60%, 2 8 days)
Vol. 22, No. 4
ACOUSTIC EMISSION, MORTAR, COMPRESSION
6000
-
-
647
-
5000
4000
-
FIG. 6 Typical
Variation
t
of
AE T o t a l
Events During Three Stages of Compression Process
c
> 3000 % ~- 2000-
1000"
Stage
1
/
' ~6o
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I
Stage 2
I
260
l
s6o
4oo~-- 560
II
600
Stage 5
~o
800
900
Time (sec)
Variation
of A E A m p l i t u d e
During
Compression
Process
B a s e d on the t h r e e stages of the c o m p r e s s i o n p r o c e s s s t a t e d above, it is felt t h a t a study on the v a r i a t i o n s of the AE p e a k a m p l i t u d e for each of the t h r e e s t a g e s m a y p r o v i d e f u r t h e r insight t h e AE r e s p o n s e a l o n g the e n t i r e l o a d i n g history.
to
In o r d e r to e s t a b l i s h the AE p e a k a m p l i t u d e d i s t r i b u t i o n curve for e a c h stage, a n e w t e r m c a l l e d "event ratio" is introduced. The event r a t i o is d e f i n e d as the n u m b e r of AE events for c e r t a i n peak a m p l i t u d e d i v i d e d by the total n u m b e r of AE events w i t h i n a c e r t a i n stage. By c a l c u l a t i n g the e v e n t r a t i o for each d i f f e r e n t p e a k amplitude, three c u r v e s for the t h r e e stages of c o m p r e s s i o n p r o c e s s were established. Figs. 7(a) and (b) s h o w two t y p i c a l d i a g r a m s of event r a t i o vs. p e a k a m p l i t u d e for m o r t a r s p e c i m e n s w i t h d i f f e r e n t w a t e r - c e m e n t r a t i o s and ages. F r o m f i g u r e s 7(a) and (b), an i m p o r t a n t p h e n o m e n o n w a s found w h i c h s h o w e d t h a t the p r o p o r t i o n s of the h i g h p e a k a m p l i t u d e AE events (higher t h a n 75 dB) in the initial and u n s t a b l e s t a g e s (stage 1 and 3) w e r e m u c h h i g h e r t h a n that in the s t a b l e stage (stage 2). This p h e n o m e n o n e x i s t e d in all test r e s u l t s of this s t u d y no m a t t e r what t h e w a t e r - c e m e n t r a t i o or age of the s p e c i m e n was. In addition, it w a s f o u n d t h a t the p r o p o r t i o n of the lower p e a k a m p l i t u d e AE events (50 to 60 dB) in the stable stage (stage 2) was g e n e r a l l y h i g h e r than t h a t in t h e u n s t a b l e stage (stage 3). T h e c h a r a c t e r i s t i c s of the AE p e a k a m p l i t u d e d i s t r i b u t i o n p r e s e n t e d a b o v e s h o w that there exists a good c o r r e l a t i o n b e t w e e n the h i g h a m p l i t u d e A E s i g n a l s (higher t h a n 75 dB) and the u n s t a b l e growing of c r a c k s in the m o r t a r specimens.
Correlation of M o r t a r
Between
Hiqh Amplitude
AE S i g n a l s
and W a t e r - C e m e n t
Ratio
F r o m t h e a b o v e discussion, it is k n o w n t h a t the h i g h a m p l i t u d e AE s i g n a l s h a v e a g o o d c o r r e l a t i o n with the g r o w t h of u n s t a b l e cracks in m o r t a r specimens. In this section, a study is m a d e to c o r r e l a t e the v a r i a t i o n s of the h i g h a m p l i t u d e AE events and the w a t e r - c e m e n t ratios of t h e specimens.
648
C.C. Weng et al.
12
.
.
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.
.
.
.
.
.
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.
.
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FIG. 7(a) AE Peak Amplitude Distribution Patterns for the Three Stages of Compress Process ( W / C = 70%, 7 days)
,,,
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Vol. 22, No. 4
85'oo
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Amplitude
(dB)
12
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Stage Stoge Stoge
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:,~ FIG. 7 (b) AE Peak Amplitude Distribution Patterns for the Three Stages of Compress Process ( W / C = 45%, 28 days)
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F i g u r e s 8(a) and (b) show the c o r r e l a t i o n b e t w e e n the variations of t h e w a t e r - c e m e n t r a t i o and the "high a m p l i t u d e e v e n t ratio". The h i g h a m p l i t u d e e v e n t ratio is d e f i n e d as the n u m b e r of h i g h peak amplitude A E e v e n t s (higher than 75 dB) d i v i d e d by the total number of A E events. F r o m these figures, it is o b s e r v e d t h a t the v a r i a t i o n of t h e w a t e r - c e m e n t ratio was l i n e a r l y p r o p o r t i o n a l to the v a r i a t i o n of t h e h i g h a m p l i t u d e A E event ratio. This o b s e r v a t i o n is applicable to s p e c i m e n s t e s t e d at 7 and 28 days. F u r t h e r m o r e , Fig. 8(c) shows a c o m p a r i s o n b e t w e e n the h i g h a m p l i t u d e A E e v e n t r a t i o s o b t a i n e d from s p e c i m e n s t e s t e d at 7 and 28 days. It w a s o b s e r v e d that the h i g h a m p l i t u d e e v e n t r a t i o s for the 2 8 - d a y s m o r t a r s p e c i m e n s were less than t h a t of t h e 7-days. This p h e n o m e n o n is p o s s i b l e b e c a u s e that a l o n g w i t h the p r o c e s s of h y d r a t i o n , t h e p o r o s i t y w i t h i n the m o r t a r s p e c i m e n d e c r e a s e d g r a d u a l l y w i t h time. Therefore, the h i g h a m p l i t u d e event r a t i o b e c a m e fewer for t h o s e s p e c i m e n s t e s t e d at longer age.
Kaiser
Effect
During
in M o r t a r
the u n l o a d
Specimens and r e l o a d
process,
AE s i g n a l s
are not
likely
Vol. 22, No. 4
ACOUSTIC EMISSION, MORTAR, COMPRESSION
649
27
A A
f~ ~25o 5D o ct~ 23-
Age = 7 days /
c > b321
J
D F I G . 8 (a) Correlation Between High Amplitude AE Event Ratio and Water-Cement Ratios (specimens age = 7 days)
~ ~j]7-
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7b
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F I G . 8 (b) Correlation Between High Amplitude AE Event Ratio and Water-Cement Ratios (specimens age = 28 days)
Age = 28 days
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is
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650
C.C. Weng et al.
5000
.
.
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.
.
.
.
.
.
Vol. 22, No. 4
.
120
%-. 2500
100
Load Count
.....
Rate
L
~2000-
,
2 ~
80
i i
i •
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j
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/
(D
/ '
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-20
500
1000
15O0
2000
2500
3000
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sooo -[ . . . . . . . . . !
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FIG. 9 A Typical AE Result During the Unload-Reload Process for the study of Kaiser Effect in Mortar
2boo
3500
~
toad Count Rate
4000
4500
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120
1
O0
I
"
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t
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i
1000
~ o
FIG. 10(a) AE Result During the Unload ° Reload Process Using Strain Rate of 7.5 kN/min for the Study of Kaiser Effect in Mortar ( W / C = 7 0 % , 28 d a y s )
-40
Of /
D o
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o
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.
.
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m
/
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i /
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r
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Time (sec)
1500
--'0 2000
FIG. 10(b) AE Result During the UnloadReload Process Using Strain Rate of 15 kN/min for the Study of Kaiser Effect in Mortar ( W / C = 7 0 % , 28 d a y s )
Vol. 22, No. 4
to be o b s e r v e d m a x i m u m load.
ACOUSTIC EMISSION, MORTAR, COMPRESSION
u n t i l the newly This phenomenon
651
a p p l i e d load e x c e e d s t h e p r e v i o u s is k n o w n as K a i s e r effect.
F i g u r e 9 shows a typical AE r e s u l t o b t a i n e d f r o m t h e u n l o a d and r e l o a d process. In this figure, the d o t t e d lines r e p r e s e n t the l o a d i n g history. It is seen that at the b e g i n n i n g of t h e test, very h i g h A E c o u n t rates appeared. Then, w h e n the l o a d w a s h e l d at the c o n s t a n t level of 20 kN for a short p e r i o d of time, the AE signals w e r e f o u n d to d e c r e a s e rapidly. W h e n the AE s i g n a l s a l m o s t d i s a p p e a r e d w h i c h i n d i c a t e d t h a t c r a c k s w i t h i n the s p e c i m e n s h a d s t o p growing, the u n l o a d p r o c e s s began. Then, the s p e c i m e n w a s r e l o a d e d to a h i g h e r level, 40 kN. F r o m the AE r e s u l t s s h o w n in Fig. 9, it is seen that AE s i g n a l s w e r e rare w h e n the n e w l y a p p l i e d load d i d n o t e x c e e d the p r e v i o u s m a x i m u m load. However, m a n y A E s i g n a l s a p p e a r e d again w h e n t h e n e w l y a p p l i e d l o a d i n g p a s s e d the p r e v i o u s m a x i m u m . T h i s p h e n o m e n o n c o n t i n u e d u n t i l the load was i n c r e a s e d to 60 kN. T h e above o b s e r v a t i o n i n d i c a t e d t h a t the K a i s e r e f f e c t e x i s t s in m o r t a r s p e c i m e n up to the load level of 60 kN ( a p p r o x i m a t e l y 80% of its u l t i m a t e strength). F r o m Fig. 9, it was also n o t e d t h a t w h e n the a p p l i e d load a p p r o a c h e d the u l t i m a t e s t r e n g t h of the specimen, A E s i g n a l s w e r e f o u n d to i n c r e a s e c o n t i n u o u s l y even w h e n the load w a s h e l d at the c o n s t a n t level of 80 kN. This m e a n s t h a t the c r a c k s w i t h i n the s p e c i m e n s w e r e growing, i.e., the s p e c i m e n w a s in an u n s t a b l e condition. As the last u n l o a d and r e l o a d p r o c e s s s h o w n in Fig. 9, m a n y A E s i g n a l s a p p e a r e d b e f o r e the p r e v i o u s m a x i m u m load was e x c e e d e d , w h i c h i n d i c a t e d that the K ~ i s e r e f f e c t d i d not exist a n y more. In a d d i t i o n to the o b s e r v a t i o n s m a d e above, Figs. 10(a) and (b) c o m p a r e the K a i s e r e f f e c t for two s p e c i m e n s t e s t e d w i t h d i f f e r e n t l o a d i n g rates, 7.5 k N / m i n and 15 kN/min, r e s p e c t i v e l y . T h e loading p a t t e r n u s e d in t h e s e tests w e r e also d i f f e r e n t f r o m t h a t shown in Fig. 9. F r o m Figs. 10(a) and (b), it was o b s e r v e d that: (i) In spite of the d i f f e r e n c e of the l o a d i n g r a t e u s e d in the tests, K a i s e r e f f e c t e x i s t e d in b o t h s p e c i m e n s w h e n the a p p l i e d load w a s w i t h i n about 80% of the u l t i m a t e s t r e n g t h of the specimen. (2) As c o m p a r e d to the l o a d i n g p a t t e r n u s e d in Fig. 9 w h e r e the load w a s h e l d at a c o n s t a n t level for a s h o r t p e r i o d of t i m e before unloading, the t e s t r e s u l t s h o w e d t h a t the K a i s e r e f f e c t still e x i s t e d r e g a r d l e s s the c h a n g e of the l o a d i n g p a t t e r n .
Summary
and C o n c l u s i o n s
B a s e d on the r e s u l t s of this study, the f o l l o w i n g s u m m a r y and c o n c l u s i o n s are made: (i) A n a p p r o x i m a t e l y linear r e l a t i o n s h i p b e t w e e n t h e v a r i a t i o n of t h e AE total e v e n t s and the w a t e r - c e m e n t r a t i o of m o r t a r s p e c i m e n s w a s observed. W h e n the w a t e r - c e m e n t r a t i o increased, t h e A E t o t a l e v e n t s w e r e found to d e c r e a s e f o l l o w i n g a linear relation. (2) On the contrary, w h e n the c o m p r e s s i v e s t r e n g t h of t h e m o r t a r s p e c i m e n s increased, the A E t o t a l e v e n t s w e r e f o u n d to increase accordingly. A n a p p r o x i m a t e l y l i n e a r r e l a t i o n s h i p w a s also o b s e r v e d b e t w e e n the v a r i a t i o n of t h e s e two p a r a m e t e r s . (3) In addition, the v a r i a t i o n of the A E t o t a l e v e n t s w a s found to h a v e a c l o s e c o r r e l a t i o n w i t h the m e a s u r e d c o m p r e s s i v e strains. T h i s o b s e r v a t i o n shows t h a t the AE t e c h n o l o g y h a s g o o d p o t e n t i a l for m o n i t o r i n g the s t r a i n h i s t o r y of m o r t a r s p e c i m e n s .
652
(4)
(5)
(6)
(7)
C.C. Weng et al.
Vol. 22, No. 4
F r o m t h e c h a r a c t e r i s t i c s of the A E t o t a l events, the e n t i r e c o m p r e s s i o n p r o c e s s of a m o r t a r s p e c i m e n c a n be d i v i d e d into t h r e e s t a g e s w h i c h i n c l u d e the i n i t i a l , s t a b l e a n d u n s t a b l e stages. T h i s p h e n o m e n o n m a y c o n t a i n p o t e n t i a l i n f o r m a t i o n for j u d g i n g t h e s t a b i l i t y of a m o r t a r s p e c i m e n u n d e r c o m p r e s s i o n . In a d d i t i o n , the r e s u l t s of AE p e a k a m p l i t u d e d i s t r i b u t i o n showed t h a t in the i n i t i a l and u n s t a b l e stages, the p r o p o r t i o n s of the h i g h a m p l i t u d e A E e v e n t s (higher t h a n 75 dB) w e r e m u c h h i g h e r t h a n t h a t in the s t a b l e stage. F u r t h e r m o r e , it was found t h a t a l i n e a r r e l a t i o n s h i p e x i s t e d b e t w e e n the i n c r e a s e s of the w a t e r - c e m e n t r a t i o and the h i g h a m p l i t u d e A E e v e n t ratio. Also, for s p e c i m e n s t e s t e d at 28 days, t h e h i g h a m p l i t u d e e v e n t r a t i o was found to be s m a l l e r t h a n that of t h e 7 - d a y tests. T h e s t u d y of K a i s e r e f f e c t s h o w e d t h a t K a i s e r e f f e c t e x i s t e d in m o r t a r s p e c i m e n s w h e n the a p p l i e d load was w i t h i n a b o u t 80% of t h e u l t i m a t e s t r e n g t h of the specimen. In a d d i t i o n , the change of the l o a d i n g rate from 7.5 k N / m i n to 15 k N / m i n did not affect t h e e x i s t e n c e of the K a i s e r effect. References
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