w i t h p i p e d i a m e t e r in s u c h a w a y that it a l w a y s l i e s a b o v e the f u l l t h i c k n e s s of t h e pipe. C l e a r l y , t c is not p u r e l y a m a t e r i a l p r o p e r t y ; it a l s o d e p e n d s on t h e e f f e c t of d i a m e t e r on t h e c r a c k d r i v i n g f o r c e d e v e l o p e d in a p i p e of a g i v e n t h i c k n e s s ( p r o v i d i n g a f u r t h e r m o t i v e for c o m p u t i n g G). A l t h o u g h this d e p e n d e n c e k e e p s t h e d a n g e r of RCP at 0~
one s t e p f u r t h e r a w a y for e a c h i n c r e a s e in p i p e size, t h e e s t i m a t e d full t h i c k n e s s D B T T s e e m s to e d g e t o w a r d s 0~ T h i s t r e n d is in a g r e e m e n t w i t h t h e fact that, f o r a g i v e n resin, the t e m p e r a t u r e at w h i c h t h e $4 c r i t i c a l p r e s s u r e for RCP s w e e p s a b r u p t l y u p w a r d s (as in Fig. 5) is h i g h e r for l a r g e r d i a m e t e r s , a n d r e a f f i r m s t h e n e e d for c a u t i o n in u s i n g l a r g e r d i a m e t e r or t h i c k e r H D P E - 2 p i p e at h i g h e r s e r v i c e p r e s s u r e s .
T w o o t h e r i s s u e s h a v e b e e n e x p l o r e d u s i n g t h e s e m e t h o d s . F i r s t l y , as t h e p i p e is m a c h i n e d down, t h e r e s i d u a l s t r e s s p r o f i l e c h a n g e s . The p i p e w a l l is s t r e s s r e l i e v e d as m a t e r i a l c l o s e r to t h e o u t e r surface, a n d u n d e r h i g h c i r c u m f e r e n t i a l a n d a x i a l c o m p r e s s i o n , is r e m o v e d . T e s t s on a n n e a l e d a n d n o n - a n n e a l e d p i p e s h a v e d e m o n s t r a t e d tc l o c i t h a t c o n v e r g e at low t e m p e r a t u r e s w h e r e tc v a l u e s a r e low, a n d d i v e r g e at h i g h e r t e m p e r a t u r e s as t c a p p r o a c h e s the full t h i c k n e s s , for w h i c h the r e s i d u a l s t r e s s e s are h i g h e s t in n o n - a n n e a l e d p i p e (Fig. 8).
~3 0 .... 9 . . . . . . . . , . . . . . . . . . , . . . . . . . . . , . . . . . . . . .
E E HDPE-2,180 SDR 11 pipe
(n (~ 25
t-
, ( . )
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I- 20 Extrapolated critical thickness
at 0 ~ 18.2 mm
Full thickness ~ , ~
15 9 i" 1 ~ -""- t '
.Non-annea!ed.: propagation Criticalthickness [] Non-annealed: arrest ~
5 9 Annealed: propagation "1
O Annealed: arrest :1
-I
0 . . . . . . . . . I . . . . . . . . . I . . . . . . . . . I . .. . . l , , , I
-40 -30 -20 -10 0
Temperature (~
F I G . 8 - - E f f e c t o f r e s i d u a l s t r e s s o n t h e c r i t i c a l t h i c k n e s s t c o f H D P E - 2 180 m m S D R ii p i p e
S e c o n d l y , t h e r e is some e v i d e n c e that c r i t i c a l t h i c k n e s s is i n d e p e n d e n t of t h e v a r i a t i o n in c r y s t a l l i n i t y t h r o u g h the p i p e w a l l t h i c k n e s s . T e s t s on p i p e m a c h i n e d f r o m the i n s i d e ( r e m o v i n g m a t e r i a l of t h e h i g h e s t c r y s t a l l i n i t y ) a n d f r o m t h e o u t s i d e ( r e m o v i n g m a t e r i a l of t h e lowest) h a v e i n d i c a t e d s i m i l a r tc v a l u e s to w i t h i n the m a c h i n i n g t o l e r a n c e .
LEEVERS ET AL. ON RAPID CRACK PROPAGATION 145
U s e of C h a r D v t e s t s to l o c a t e the b r i t t l e - t o u a h t r a n s i t i o n
F o r t h e b e s t p a r t of a century, t h e m e t h o d of f i r s t r e c o u r s e for l o c a t i n g a D B T has b e e n the C h a r p y test. T h i s r e m a i n s t h e m o s t
a c c e s s i b l e q u a l i t y c o n t r o l m e t h o d for m o s t PE p i p e users, p r o v i d i n g a s t r o n g m o t i v e to d e v e l o p a C h a r p y - b a s e d p r o c e d u r e for l o c a t i n g the c r i t i c a l t h i c k n e s s / d i a m e t e r locus.
T h e e f f e c t of t h i c k n e s s on D B T T w a s m e a s u r e d u s i n g s h a r p - n o t c h e d i n s t r u m e n t e d C h a r p y t e s t s o n s p e c i m e n s 55 m m long, i0 m m d e e p a n d v a r y i n g in t h i c k n e s s B f r o m 4 to 16 mm. T h e span was 40 m m a n d the i m p a c t s p e e d 1.5 m s -1 , g i v i n g a s t r a i n r a t e of 56 s -1, in a l l tests.
S p e c i m e n s w e r e t e s t e d a c r o s s a r a n g e of t e m p e r a t u r e s f r o m -70 to +23~
C o n v e n t i o n a l i n s t r u m e n t e d i m p a c t t e s t r e a d i n g s p r o v i d e b o t h p e a k l o a d a n d t o t a l e n e r g y to f a i l u r e . C a l c u l a t i o n of an a p p a r e n t t o u g h n e s s b o t h
*legally' (from p e a k load) a n d 'illegally' (from t o t a l e n e r g y )
i d e n t i f i e s a c l e a r n i l - d u c t i l i t y t e m p e r a t u r e a b o v e w h i c h t h e y d i v e r g e (Fig. 9). T h i s a c c u r a t e l y r e f l e c t s t h e f r a c t u r e s u r f a c e D B T
t e m p e r a t u r e , m a k i n g s u b j e c t i v e ' p e r c e n t a g e d u c t i l i t y ' e v a l u a t i o n s u n n e c e s s a r y . N o t e h o w m u c h g r e a t e r the l o w e r b o u n d for Gc is t h a n G D p l o t t e d in Fig. 2; we h a v e as yet no w a y to d i s t i n g u i s h t h i c k n e s s e f f e c t s f r o m t h o s e of i n i t i a t i o n t r a n s i e n t s .
P l o t t i n g the D B T T v s . i n v e r s e t h i c k n e s s r e v e a l s a l i n e a r
r e l a t i o n s h i p (Fig. 10). The s u p e r i o r p e r f o r m a n c e of H D P E - 2 o v e r M D P E - 2 is i n d i c a t e d b y a l o w e r i n t e r c e p t on t h e t e m p e r a t u r e axis, r e p r e s e n t i n g a l o w e r ' i n f i n i t e t h i c k n e s s ' DBTT. H o w e v e r , t h e s e r e s u l t s s u g g e s t t h a t t h e D B T T of H D P E - 2 is m u c h m o r e s e n s i t i v e to t h i c k n e s s t h a n that of M D P E - 2 , o n c e a g a i n e m p h a s i s i n g the c a r e n e e d e d in u s i n g l a r g e d i a m e t e r H D P E - 2 p i p e s at h i g h p r e s s u r e s , s i n c e a t h i c k n e s s p r o m o t e d d u c t i l e - b r i t t l e t r a n s i t i o n m a y lift the D B T T a b o v e t h e m i n i m u m s e r v i c e t e m p e r a t u r e . However, t h e s e r e s u l t s s h o u l d be i n t e r p r e t e d w i t h care, s i n c e the D B T T is a l s o k n o w n to be v e r y s e n s i t i v e to s t r a i n rate. The e x t r a p o l a t e d ' i n f i n i t e t h i c k n e s s ' D B T T is t h e r e f o r e o n l y r e l e v a n t to a p a r t i c u l a r C h a r p y t e s t g e o m e t r y a n d s t r a i n rate, b u t m a y s t i l l b e u s e f u l for c o m p a r i n g d i f f e r e n t resins.
Fig. ii, for HDPE-2, also s h o w s t e m p e r a t u r e v s . i n v e r s e c r i t i c a l t h i c k n e s s f r o m $4 p i p e t e s t s - i.e. d a t a of the t y p e s h o w n in Fig. 6.
The l i n e s h a v e s i m i l a r s l o p e s , a l t h o u g h t h e t e m p e r a t u r e i n t e r c e p t s d i f f e r ( p r o b a b l y due to s t r a i n rate e f f e c t s ) . This s u g g e s t s that it m a y b e p o s s i b l e to u s e the C h a r p y test to p r o d u c e a m a s t e r c u r v e for e a c h resin, f r o m w h i c h a D B T T for e a c h p i p e s i z e c o u l d b e p r e d i c t e d u s i n g a s h i f t rule.
1 4 6 B U R I E D P L A S T I C PIPE T E C H N O L O G Y
3 0 . . . . , . . . . , . . . . , . . . . , . . . . , . . . . , . . . . , . . . . , . . . . , . . . . , . . . .
"= H D P E - 2 B
E
2 5
9 T o t a l E n e r g y I / I
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< : 10 i~ D B T T = - 4 0 C ]
0: I . . . . J, . . . , . . . . , . . . . , . . . . ' . . . . ' . . . . "
-80 -70 -60 -50 -40 -30 -20 -1"0 6 1 b 20 3 0
T e m p e r a t u r e , ~ F I G . 9 - - D e f i n i t i o n o f t h e D B T T f o r a n H D P E - 2 g r a d e f r o m m o d i f i e d C h a r p y
t e s t d a t a
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~ 2 0 D 10
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9 H D P E - 2
I I I , , . * I . . . . I , . . I | I I I
0 . 0 0 0 . 0 5 0 . 1 0 0 . 1 5 0 . 2 0 0 . 2 5
I n v e r s e t h i c k n e s s , 1 / ( B , m m ) F I G . 1 0 - - D B T T v s . i n v e r s e s p e c i m e n t h i c k n e s s f r o m C h a r p y d a t a f o r M D P E - 2
a n d H D P E - 2
LEEVERS ET AL. ON RAPID CRACK PROPAGATION 147
20
o o
o E -10
r
o -20
~- -30 --~ -40 -50 a -60
S4 critical thickness & Charpy tests
9 S4tests: 90SDR11 pipe
9 $4 tests: 180 SDR11 pipe (annealed) -70
-80 = I ~ , , I , , , a ! ~ | l r
0.00 0.05 0.10 0.15 0.20 0.25
Inverse thickness 1/(B, mm)
FIG. I I - - D B T T v s . i n v e r s e s p e c i m e n t h i c k n e s s f r o m C h a r p y data, a n d t e m p e r a t u r e v s . c r i t i c a l t h i c k n e s s f r o m $4 tests, for t h e s a m e H D P E - 2
b a t c h
C O N C L U S I O N S
RCP f a i l u r e of p r e s s u r i s e d PE p i p e l i n e s is v e r y rare, b u t it is t o e d a n g e r o u s to b e ignored.
The p l a n e s t r a i n d y n a m i c f r a c t u r e t o u g h n e s s of n e w ' t h i r d g e n e r a t i o n ' r e s i n s is p r o b a b l y no m o r e t h a n 50% h i g h e r t h a n t h a t of s t a n d a r d m e d i u m d e n s i t y grades, d e s p i t e t h e i r m a r k e d l y s u p e r i o r r e s i s t a n c e to R C P in f u l l - s c a l e b u r i e d p i p e t e s t s .
F o r t h i s a n d o t h e r reasons, the p r e d i c t i o n of c r i t i c a l p r e s s u r e f r o m p l a n e s t r a i n f r a c t u r e t o u g h n e s s , u s i n g f r a c t u r e m e c h a n i c s a n a l y s i s , m a y be u n r e l i a b l e -- e s p e c i a l l y for gas r a t h e r t h a n w a t e r p r e s s u r i s e d p i p e l i n e s .
The m a t e r i a l d e p e n d e n c e of the c r i t i c a l p r e s s u r e a b o v e w h i c h RCP w i l l self a r r e s t is p r i m a r i l y r e s t r i c t e d , at l e a s t for p o l y e t h y l e n e , to t h e t e m p e r a t u r e at w h i c h t h e r e is an a b r u p t t r a n s i t i o n to v e r y h i g h f r a c t u r e r e s i s t a n c e .
T h i r d g e n e r a t i o n r e s i n s s c o r e t h e i r RCP a d v a n t a g e o v e r m e d i u m d e n s i t y g r a d e s b y h a v i n g t r a n s i t i o n t e m p e r a t u r e s b e l o w r a t h e r t h a n a b o v e t h e i r m i n i m u m s e r v i c e t e m p e r a t u r e (usually 0~
The t r a n s i t i o n t e m p e r a t u r e is s t r o n g l y t h i c k n e s s d e p e n d e n t , e s p e c i a l l y in the t o u g h e r m a t e r i a l s . The u s e of t h e s e m a t e r i a l s for t h i c k e r p i p e of l a r g e r d i a m e t e r s r u n n i n g at h i g h e r p r e s s u r e s m u s t
148 BURIED PLASTIC PIPE TECHNOLOGY
t h e r e f o r e be b a c k e d up b y a s e c u r e p r o c e d u r e for e v a l u a t i n g t h e t r a n s i t i o n t e m p e r a t u r e a n d its t h i c k n e s s d e p e n d e n c e .
I n s t r u m e n t e d C h a r p y t e s t s on s h a r p - n o t c h e d s p e c i m e n s a c r o s s a r a n g e of t e m p e r a t u r e s m a y have an i m p o r t a n t role to p l a y in s u c h a p r o c e d u r e .
A C K N O W L E D G F / M E N T S
The a u t h o r s w i s h to e x p r e s s t h e i r a p p r e c i a t i o n of a S p e c i a l R e s e a r c h G r a n t for t h i s p r o j e c t f r o m t h e U K S c i e n c e a n d E n g i n e e r i n g R e s e a r c h C o u n c i l . R o b e r t M o r g a n w i s h e s to a c k n o w l e d g e a d d i t i o n a l s u p p o r t f r o m U p o n o r A l d y l ( U K ) a n d f r o m N e s t e P o l y e t e n (Sweden). The t e s t s on w a t e r p r e s s u r i s e d p i p e w e r e c a r r i e d out b y C h r i s t o p h e r G r e e n s h i e l d s , who is s u p p o r t e d b y the U K W a t e r R e s e a r c h C e n t r e . R E F E R E N C E S
[I] Greig, J. M., L e e v e r s , P. S. a n d Yayla, P., " R a p i d C r a c k
P r o p a g a t i o n in P r e s s u r i s e d P l a s t i c Pipe. I: F u l l S c a l e a n d S m a l l S c a l e RCP T e s t i n g " , E n g i n e e r i n g F r a c t u r e M e c h a n i c s Vol. 42, No. 4, 1992, p p 663-673.
[2] Greig, J. M., " R a p i d C r a c k P r o p a g a t i o n in H y d r o s t a t i c a l l y
P r e s s u r i s e d 250 m m P o l y e t h y l e n e Pipe", P r o c e e d i n a s of the S e v e n t h I n t e r n a t i o n a l C o n f e r e n c e on P l a s t i c P i p e s (Bath, E n g l a n d , 19-22 Sept. 1988). P l a s t i c s a n d R u b b e r Institute, London, 1988.
[3] Yayla, P. a n d Leevers, P. S., " R a p i d C r a c k P r o p a g a t i o n in
P r e s s u r i s e d P l a s t i c Pipe. II: C r i t i c a l P r e s s u r e s for P o l y e t h y l e n e Pipe", E n m i n e e r i n ~ F r a c t u r e M e c h a n i c s Vol. 42, No. 4, 1992, p p 675- 682.
[4] Wheel, M. A. a n d Leevers, P. S., " H i g h S p e e d D o u b l e T o r s i o n T e s t s on T o u g h P o l y m e r s . I: L i n e a r E l a s t i c S t e a d y S t a t e a n d D y n a m i c A n a l y s i s " . To be p u b l i s h e d in I n t e r n a t i Q n a l J o u r n a l of F r a c t u r e , 1993.
[5] Wheel, M. A. a n d Leevers, P. S., " H i g h S p e e d D o u b l e T o r s i o n t e s t s on t o u g h p o l y m e r s . II: N o n l i n e a r e l a s t i c d y n a m i c analysis['. To be p u b l i s h e d in I n t e r n a t i Q n a l J o u r n a l of F r a c t u r e , 1993.
[6] L e e v e r s , P. S. a n d Yayla, P., " T h i c k n e s s E f f e c t s on R a p i d C r a c k P r o p a g a t i o n in P o l y e t h y l e n e Pipe". P r o c e e d i n a s of the T w e l f t h P l a s t i c F u e l Gas P i o e S v m m o s i u m (Boston, M a s s a c h u s e t t s , S e p t e m b e r 2 4 - 2 6 1991). A m e r i c a n Gas A s s o c i a t i o n , A r l i n g t o n VA, 1991.
[7] O ' D o n o g h u e , P. E., Green, S. T., K a n n i n e n M. F., a n d Bowles, P. K.,
" T h e D e v e l o p m e n t of a F l u i d / S t r u c t u r e I n t e r a c t i o n M o d e l for F l a w e d F l u i d C o n t a i n m e n t B o u n d a r i e s w i t h A p p l i c a t i o n s to Gas T r a n s m i s s i o n a n d D i s t r i b u t i o n P i p i n g " , C o m m u t e r s a n d S t r u c t u r e s Vol. 38 No. 5/6, 1991, p p 501-513.
[8] K a n n i n e n , M. F., Leung, C. P. a n d O ' D o n a g h u e , P. E., " P r o c e d u r e s to P r e v e n t R a p i d C r a c k P r o p a g a t i o n in PE Gas Pipes", P r o c e e d i n a s of t h e E i g h t h I n t e r n a t i o n a l C o n f e r e n c e on P l a s t i c P i o e s (Koningshof, N e t h e r l a n d s , 2 1 - 2 4 S e p t e m b e r 1992). P l a s t i c s a n d R u b b e r I n s t i t u t e , London, 1992.
Phillip A. Sharff' and Steven J. DelloRusso'
EFFECTS OF ACID ENVIRONMENT AND CONSTANT DEFLECTION ON PVC SEWER PIPE
REFERENCE: Sharff, P.A. and DelloRusso, S . J . , "Effects of Acid Environ- ment and Constant Deflection on PVC Sewer Pipe," Buried Plastic Pipe Technoloqv: 2nd Volume, ASTM STP 1222, Dave Eckstein, Ed., American Society for Testing and Materials, Philadelphia, 1994.
ABSTRACT: A test study was undertaken t o determine the effects of a sustained exposure t o sulphuric acid solution on pipe stiffness of PVC sewer pipes held at fixed 5% deflection. This study was part of a multi-participant round-robin program t o evaluate and develop a test method for long-term effects of deflection in various environments on plastic sewer pipe. Sections of 6 in. (152.4 mm) diameter SDR 35 PVC sewer pipe ASTM D 3034 - Specification for Type PSM Poly (Vinyl Chloride) (PVC) Sewer Pipe and Fittings 6 in. (152.4 mm) long were tested for pipe stiffness by the parallel plate method ASTM D 2412 -
Test Method for Determination of External Loading Characteristics of Plastic Pipe by Parallel-Plate Loading, as received from three
suppliers. Pipe specimens were then immersed in a 1.ON sulphuric acid solution for 1 week and retested for pipe stiffness. The pipe sections were then held at 5% deflection in special test fixtures and
continuously immersed in a 1.ON solution of sulphuric acid at room temperature. The pipe stiffness of each specimen was measured at
various time intervals over a period of two years, without removing them from the acid bath. Results were analyzed t o evaluate the effects of the acid immersion, coupled with the fixed deflection. One control pipe specimen was fixed at 5% deflection in an air-only environment and tested for pipe stiffness at the same time intervals as the acid- immersed specimens.
Test data indicates a minimal effect of fixed deflection and acid
immersion over time on short-term pipe stiffness. Analysis of load data illustrates the significant relaxation response of PVC pipe under constant deflection, i.e. the decrease in wall stress over time. Two year data exhibit good correlations to the design-oriented viscoelastic predictions derived from Findley's power law. The influence of filler content on stiffness behavior is examined but no conclusive relationship is apparent. The influence of the test fixture on interpretation of results is discussed and recommendations for improvements are presented.
KEY WORDS: Poly (vinyl chloride) (PVC) pipe, sewer pipe, sewer acid, pipe stiffness, stress relaxation, viscoelastic behavior.
' Staff Engineer and Senior Engineer, respectively, Simpson Gumpertz E, Heger Inc., 297 Broadway, Arlington, M A 02174
149
150 BURIED PLASTIC PIPE TECHNOLOGY
I N T R O D U C T I O N
T h e u s e of t h e r m o p l a s t i c p i p e for b u r i e d s e w e r s a n d o t h e r g r a v i t y - f l o w a p p l i c a t i o n s c o n t i n u e s to expand. A m o n g o t h e r a t t r i b u t e s , t h e r m o p l a s t i c p i p e is r e c o g n i z e d as h a v i n g g o o d r e s i s t a n c e t o a g g r e s s i v e e n v i r o n m e n t s ; h o w e v e r , i n d u s t r y c u r r e n t l y has no s t a n d a r d t e s t m e t h o d for m e a s u r i n g t h e r e l a t i v e l o n g - t e r m e f f e c t s of e n v i r o n m e n t a l a g e n t s c o m b i n e d w i t h s u s t a i n e d s t r e s s (or strain) on its s t i f f n e s s p r o p e r t i e s . B u r i e d s e w e r p i p e is f r e q u e n t l y e x p e c t e d t o r e m a i n in s e r v i c e 50 y e a r s or longer, s o m e t i m e s s u b j e c t e d t o a g g r e s s i v e e n v i r o n m e n t s s u c h as s e w e r acids;
thus, l o n g - t e r m p r o p e r t i e s u n d e r s e r v i c e c o n d i t i o n s a r e a c r i t i c a l d e s i g n c o n s i d e r a t i o n .
In 1 9 9 0 a t e s t p r o g r a m w a s i n i t i a t e d in w h i c h s p e c i m e n s of 6 in. (152.4 mm) d i a m e t e r p o l y (vinyl c h l o r i d e ) (PVC) p i p e w e r e c l a m p e d at a f i x e d 5%
d e f l e c t i o n ( d e c r e a s e in v e r t i c a l d i a m e t e r ) a n d i m m e r s e d in a 1 . 0 N s o l u t i o n of s u l p h u r i c a c i d at r o o m t e m p e r a t u r e . A c o n t r o l s p e c i m e n w a s c l a m p e d at t h e s a m e d e f l e c t i o n in a i r only. S p e c i m e n s w e r e m o n i t o r e d for s t i f f n e s s p r o p e r t i e s at p e r i o d i c i n t e r v a l s . T h e s e t e s t s w e r e p a r t of a l a r g e r r o u n d - r o b i n t e s t p r o g r a m to i n v e s t i g a t e a s t a n d a r d m e t h o d for d e t e r m i n i n g t h e e f f e c t s o n t h e s t i f f n e s s p r o p e r t i e s of t h e r m o p l a s t i c p i p e h e l d at c o n s t a n t s t r a i n and e x p o s e d to v a r i o u s e n v i r o n m e n t s for l o n g p e r i o d s of time. S i m i l a r t e s t s c o n d u c t e d e a r l i e r at U t a h S t a t e U n i v e r s i t y are r e p o r t e d b y B i s h o p [i]. R e q u i r e m e n t s of t h e r o u n d - r o b i n t e s t i n c l u d e d r e s u l t s on s h o r t - t e r m p i p e s t i f f n e s s p r o p e r t i e s u p to o n e y e a r of e x p o s u r e . T h e r e s u l t s r e p o r t e d h e r e i n e x t e n d b e y o n d t h e r o u n d - r o b i n t e s t r e q u i r e m e n t s a n d i n c l u d e e x p o s u r e p e r i o d s u p t o t w o y e a r s and a n a l y s i s of d a t a for l o n g - t e r m b e h a v i o r of p i p e s p e c i m e n s .
A v e r y b e n e f i c i a l a s p e c t of e x t e n d e d s t u d i e s s u c h as c o n d u c t e d h e r e is t h e q u a n t i f i a b l e i n f o r m a t i o n o b t a i n e d on t h e l o n g - t e r m s t i f f n e s s
p r o p e r t i e s of t h e p i p e m a t e r i a l . It is e s s e n t i a l t h a t t h e p i p e d e s i g n e r c o n s i d e r t h e t i m e - d e p e n d e n t b e h a v i o r of b u r i e d p l a s t i c p i p e so t h a t r e a s o n a b l e p r e d i c t i o n s can b e m a d e of s t r e s s e s , strains, a n d s t a b i l i t y u n d e r s u s t a i n e d l o a d i n g c o n d i t i o n s . In o r d e r t o a c c o m p l i s h this, a p p r o p r i a t e m e a n s for m o d e l i n g t h e v i s c o e l a s t i c r e s p o n s e of t h e m a t e r i a l m u s t b e a v a i l a b l e (see Ref. [2] for a t h o r o u g h d i s c u s s i o n of
v i s c o e l a s t i c i t y ) . V a r i o u s d e s i g n a p p r o a c h e s r e p o r t e d in t h e l i t e r a t u r e [2,3,4,5] t a k e into a c c o u n t t h e v i s c o e l a s t i c b e h a v i o r of t h e p l a s t i c p i p e m a t e r i a l . T h e A m e r i c a n A s s o c i a t i o n of S t a t e H i g h w a y a n d
T r a n s p o r t a t i o n O f f i c i a l s (AASHTO) S t a n d a r d S p e c i f i c a t i o n s for H i g h w a y B r i d g e s [6] r e q u i r e s t h e u s e of an e f f e c t i v e l o n g - t e r m e l a s t i c m o d u l u s in b u c k l i n g e q u a t i o n s to a c c o u n t for t h e v i s c o e l a s t i c b e h a v i o r of t h e r m o p l a s t i c pipe. U n f o r t u n a t e l y , s p a r s e d a t a is a v a i l a b l e o n long- t e r m s t i f f n e s s p r o p e r t i e s of t h e m a n y c o m m e r c i a l l y a v a i l a b l e
t h e r m o p l a s t i c p i p e m a t e r i a l s . T h e t e s t m e t h o d a n d a n a l y s e s u s e d in this s t u d y p r o v i d e an e f f i c i e n t m e a n s of o b t a i n i n g s u c h p r o p e r t i e s .
T h i s p a p e r d i s c u s s e s t h e s i g n i f i c a n c e of t e s t f i n d i n g s r e l a t i v e to t h e f o l l o w i n g :
E f f e c t s of s u l p h u r i c a c i d e x p o s u r e a n d f i x e d d e f l e c t i o n o n p i p e s t i f f n e s s
T i m e - d e p e n d e n t r e l a x a t i o n r e s p o n s e V i s c o e l a s t i c p r o p e r t i e s for d e s i g n
I n f l u e n c e of t e s t c o n f i g u r a t i o n on i n t e r p r e t a t i o n of r e s u l t s
SHARFF AND DELLORUSSO ON PVC SEWER PIPE 151
T E S T P R O G R A M
T h e t e s t p r o c e d u r e s u s e d in t h i s s t u d y w e r e e s t a b l i s h e d as p a r t of an o v e r a l l r o u n d - r o b i n p r o g r a m i n i t i a t e d b y an A S T M T a s k G r o u p u n d e r t h e a u s p i c e s of A S T M C o m m i t t e e F - 1 7 o n P l a s t i c P i p i n g S y s t e m s . T h e r o u n d - r o b i n t e s t p r o c e d u r e s are p r o v i d e d in Ref. [I]. D e t a i l s of t h i s study, i n c l u d i n g w o r k e x t e n d e d b e y o n d t h e r e q u i r e m e n t s of t h e r o u n d - r o b i n p r o g r a m are g i v e n b e l o w .
S p e c i m e n s
A t o t a l of 13 p i p e s p e c i m e n s w e r e t e s t e d in t h i s s t u d y - 12 a c i d - i m m e r s i o n s p e c i m e n s a n d o n e a i r - o n l y c o n t r o l s p e c i m e n . N i n e of t h e a c i d - i m m e r s i o n s p e c i m e n s w e r e cut f r o m p i p e p r o v i d e d , as p a r t of t h e r o u n d - r o b i n study, f r o m t w o m a n u f a c t u r e r s ( M a n u f a c t u r e r I a n d II).
T h r e e a d d i t i o n a l a c i d - i m m e r s i o n s p e c i m e n s a n d t h e o n e a i r - o n l y s p e c i m e n w e r e cut f r o m p i p e f r o m a t h i r d m a n u f a c t u r e r ( M a n u f a c t u r e r III). T h i s p i p e w a s p u r c h a s e d l o c a l l y a n d w a s n o t p a r t of t h e r o u n d - r o b i n study.
A l l p i p e w e r e A S T M D 3034 S D R 35 P V C s e w e r pipe, 6 - i n c h (152.4 mm) n o m i n a l d i a m e t e r .
P i p e s p e c i m e n s w e r e cut to n o m i n a l 6 - i n c h (152.4 mm) l e n g t h s . S p e c i m e n length, w a l l t h i c k n e s s a n d o u t s i d e d i a m e t e r (OD) d i m e n s i o n s w e r e m e a s u r e d a n d r e c o r d e d as d e s c r i b e d in A S T M D 2122 - M e t h o d for
D e t e r m i n i n g D i m e n s i o n s of T h e r m o p l a s t i c P i p e a n d F i t t i n g s . T h e a v e r a g e i n s i d e d i a m e t e r (ID) w a s d e t e r m i n e d b y s u b t r a c t i n g t w i c e t h e a v e r a g e w a l l t h i c k n e s s f r o m t h e a v e r a g e OD. A l l c o m p u t e d p e r c e n t d e f l e c t i o n s a r e b a s e d on t h i s v a l u e of ID.
T a b l e 1 p r o v i d e s i n i t i a l d a t a on t e s t s p e c i m e n s .
T a b l e 1 - I n i t i a l D a t a o n T e s t S p e c i m e n s
M a n u f a c t u r e r I II III
No. o f S p e c i m e n s 6 3 4
C e l l C l a s s A S T M D 1784 N o t g i v e n N o t 1 2 3 6 4 C g i v e n
S p e c i f i e d M o d u l u s o f 4 0 0 , 0 0 0
E l a s t i c i t y p s i (kPa) - ( 2 , 8 0 0 , 0 0 0 )
A S T M D 1784
A v e r a g e W a l l T h i c k n e s s , 0 . 2 0 1 0 . 1 8 8 0 . 1 9 6
in. (mm) (5.10) (4.78) (4.98)
A v e r a g e I n s i d e D i a m e t e r , 5 . 8 7 7 5 . 8 8 2 5 . 8 7 1
in. (mm) (149.3) (149.4) (149.1)
A s h C o n t e n t (%) 3.9 4.5 11.7
A S T M D 2 2 9
T e s t F i x t u r e s
F i x e d - d e f l e c t i o n t e s t f i x t u r e s for a c i d - i m m e r s i o n s p e c i m e n s w e r e f a b r i - c a t e d f r o m s t a i n l e s s s t e e l b e c a u s e of t h e c o r r o s i v e t e s t e n v i r o n m e n t . D i m e n s i o n s of f i x t u r e p a r t s s p e c i f i e d for t h e s t u d y are p r o v i d e d in F i g u r e i. A n a s s e m b l e d t e s t f i x t u r e is s h o w n in F i g u r e 2. T h i s s e t - u p
152 BURIED PLASTIC PIPE TECHNOLOGY
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i SMALL HOLES
3 . 0 5 5 / 1 6 " CIA
i ! i STANOARO / 4 "
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q 4 . 0 0 I'---
~- 5"OOTjF5"OO SMALL HGLES
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F i g u r e 1 - D i m e n s i o n s o f F i x e d D e f l e c t i o n T e s t F i x t u r e f o r A c i d I m m e r s i o n S p e c i m e n s . (1 i n c h = 2 5 . 4 mm)
F i g u r e 2 - F i x e d D e f l e c t i o n T e s t F i x t u r e f o r A c i d I m m e r s i o n S p e c i m e n .
SHARFF AND DELLORUSSO ON PVC SEWER PIPE 153
p e r m i t t e d a d d i t i o n a l i n c r e m e n t a l d e f l e c t i o n of t h e p i p e b y i n s e r t i n g t h e l o a d h e a d t h r o u g h t h e t o p p l a t e a n d b e a r i n g o n t h e p u s h plate. T h e a i r - o n l y s p e c i m e n w a s c l a m p e d at f i x e d d e f l e c t i o n u s i n g s t e e l c h a n n e l s , l O x 2 - 1 / 2 x l / 4 in. (254 x 6 3 . 5 x 6.4 nun) for t o p and b o t t o m p l a t e s a n d 6 x 2 x 3 / 1 6 in. (152.4 5 0 . 8 x 4.8 mm) for t h e p u s h p l a t e .
A s p e c i m e n c o n t a i n m e n t s y s t e m w a s s e l e c t e d t h a t p e r m i t t e d c o n t i n u a l a c i d i m m e r s i o n e v e n d u r i n g l o a d t e s t i n g . E a c h t e s t s p e c i m e n , e x c e p t for t h e c o n t r o l s p e c i m e n in air, w a s i m m e r s e d in an a c i d e n v i r o n m e n t in s e p a r a t e p l a s t i c c o n t a i n m e n t v e s s e l s , w h i c h c o u l d be m o u n t e d in t h e Z w i c k 1474 U n i v e r s a l T e s t i n g m a c h i n e , t h u s e l i m i n a t i n g t h e n e c e s s i t y of r e m o v i n g t h e s p e c i m e n f r o m t h e t e s t e n v i r o n m e n t t o m e a s u r e p i p e s t i f f n e s s (Figure 3).
T e s t P r o c e d u r e s
A n i n i t i a l p i p e s t i f f n e s s t e s t t o a b o u t 10% d e f l e c t i o n w a s c o n d u c t e d for e a c h s p e c i m e n p r i o r to i m m e r s i o n u s i n g t h e p a r a l l e l p l a t e m e t h o d (ASTM D 2412). T h i s e n a b l e d d e t e r m i n a t i o n of i n i t i a l s h o r t - t e r m p i p e s t i f f n e s s b e t w e e n 5% a n d 7.5% d e f l e c t i o n (PS0) as s h o w n in F i g u r e 4a. A l l t e s t s p e c i m e n s , e x c e p t for t h e a i r - o n l y c o n t r o l s p e c i m e n , w e r e t h e n i m m e r s e d , u n d e f l e c t e d , in a 1 . 0 N s u l p h u r i c a c i d (H~SO4) s o l u t i o n for o n e w e e k . A f t e r t h e o n e w e e k i m m e r s i o n , a s e c o n d p z p e s t i f f n e s s t e s t t o 10%
d e f l e c t i o n w a s c o n d u c t e d for e a c h of t h e i m m e r s e d s p e c i m e n s . E a c h s p e c i m e n w a s t h e n p l a c e d in t h e f i x e d d e f l e c t i o n t e s t f i x t u r e and c l a m p e d at 5% d e f l e c t i o n and, e x c e p t for t h e a i r - o n l y s p e c i m e n , i m m e r s e d in t h e a c i d bath. P e r i o d i c t i t r a t i o n t e s t s a n d c o r r e c t i o n s w e r e m a d e t o m a i n t a i n a c i d s o l u t i o n n o r m a l i t y .
A s h o r t - t e r m p i p e s t i f f n e s s t e s t f r o m t h e d e f l e c t e d 5% c o n d i t i o n t o a b o u t 10% d e f l e c t i o n w a s c o n d u c t e d for e a c h i m m e r s i o n s p e c i m e n a n d t h e a i r - o n l y s p e c i m e n a f t e r t h e f o l l o w i n g i m m e r s i o n t i m e i n t e r v a l s (as r e q u i r e d b y t h e i n i t i a l r o u n d - r o b i n study): 24 hours, l, 2, a n d 4 w e e k s a n d at 1 m o n t h i n c r e m e n t s t h e r e a f t e r for 1 year. T h i s e n a b l e d d e t e r m i - n a t i o n of s h o r t - t e r m p i p e s t i f f n e s s b e t w e e n 5% a n d 7.5% d e f l e c t i o n at e a c h t i m e i n t e r v a l (PSt) as s h o w n in F i g u r e 4b. A d d i t i o n a l p i p e s t i f f - n e s s t e s t s w e r e c o n d u c t e d at 18 m o n t h s and 27 m o n t h s . All t e s t i n g w a s c o n d u c t e d at r o o m t e m p e r a t u r e , 73~ (23~
R E S U L T S A N D D I S C U S S I O N S h o r t - T e r m P i p e S t i f f n e s s
F i g u r e s 5 a n d 6 i l l u s t r a t e t h e c h a n g e in i n c r e m e n t a l s h o r t - t e r m p i p e s t i f f n e s s of t e s t s p e c i m e n s w h e n d e f l e c t i o n is i n c r e a s e d u n d e r a s h o r t - t e r m l o a d f r o m t h e f i x e d 5% d e f l e c t i o n to a n e w d e f l e c t i o n of 7.5% at p e r i o d i c t e s t i n t e r v a l s u p t o 27 m o n t h s . F i g u r e 5 s h o w s m e a n r e s u l t s for a c i d - i m m e r s e d s p e c i m e n s for M a n u f a c t u r e r s I, II, III and F i g u r e 6 c o m p a r e s m e a n r e s u l t s of t h e a c i d - i m m e r s e d s p e c i m e n s f r o m M a n u f a c t u r e r III w i t h t h e a i r - o n l y s p e c i m e n f r o m M a n u f a c t u r e r III. S c h e m a t i c r e p r e s e n t a t i o n of t e r m s is p r o v i d e d in F i g u r e 4.
It is a p p a r e n t f r o m F i g u r e 5 t h a t f i x e d d e f l e c t i o n a n d e x t e n d e d i m m e r - s i o n in 1 . 0 N s u l p h u r i c a c i d has not s i g n i f i c a n t l y a f f e c t e d t h e s h o r t - t e r m p i p e s t i f f n e s s of s p e c i m e n s h e l d at 5% d e f l e c t i o n . V a r i a t i o n s are on t h e o r d e r of 15% or less of t h e i n i t i a l p i p e s t i f f n e s s a n d s h o w no r e l a t i o n s h i p t o t i m e of e x p o s u r e . M a n u f a c t u r e r II a n d Ill s p e c i m e n s s h o w an a l m o s t i m m e d i a t e i n c r e a s e in s h o r t - t e r m p i p e s t i f f n e s s of a b o u t i0 t o 15%, and t h e n l a t e r f l u c t u a t i o n s d o w n a n d u p w i t h i n t h e 15% range.
154 B U R I E D P L A S T I C P I P E T E C H N O L O G Y
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Figure 5 - Short Term Pipe Stiffness (5% to 7.5% Deflection) of PVC Pipe Held at 5% Deflection and Immersed in 1N ~ l p h u ~ Acid
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F~gure 6 - Short Term Pipe Stiffness (5% to 7.5% Deflection) of PVC Pipe Held at 5%
Deflection: Air vs. Immersed in 1N Sulphuric Acid
156 BURIED PLASTIC PIPE TECHNOLOGY
M a n u f a c t u r e r I s p e c i m e n s s h o w e s s e n t i a l l y no c h a n g e in s h o r t - t e r m p i p e s t i f f n e s s o v e r t h e t e s t period.
E x a m i n a t i o n of F i g u r e 6 i n d i c a t e s no m a r k e d d i s t i n c t i o n in t h e s h o r t - t e r m s t i f f n e s s r e s p o n s e of t h e a c i d - i m m e r s e d p i p e s p e c i m e n s c o m p a r e d to t h a t of t h e c o n t r o l s p e c i m e n e x p o s e d t o air only. T h e s e r e s u l t s a r e s i m i l a r t o t h o s e o b t a i n e d b y B i s h o p [i].
T h e s h o r t - t e r m p i p e s t i f f n e s s is u s e f u l in c h a r a c t e r i z i n g t h e d e f o r m a - t i o n r e s p o n s e of t h e b u r i e d p i p e s u b j e c t e d t o l o a d s w h e r e t h e s h o r t - t e r m r e s p o n s e is of p r i m e i n t e r e s t . T h i s w o u l d b e t h e case, for e x a m p l e , u n d e r q u a s i - i n s t a n t a n e o u s l o a d i n g s s u c h as t r a f f i c l i v e l o a d s or d e a d w e i g h t s u r c h a r g e s a f t e r o r i g i n a l s o i l / p i p e i n s t a l l a t i o n has s t a b i l i z e d . U n d e r s u c h l o a d i n g s , t h e d e f o r m a t i o n b e h a v i o r of t h e p i p e can be r e a s o n a b l y d e t e r m i n e d u s i n g i n i t i a l s t i f f n e s s p r o p e r t i e s as d e t e r m i n e d b y t h e p a r a l l e l p l a t e m e t h o d .
T h e s l i g h t v a r i a t i o n s in r e s p o n s e s h o w n in F i g u r e s 5 and 6 i n d i c a t e t h a t f i l l e r q u a n t i t i e s , as i n d i c a t e d b y a s h c o n t e n t (Table I) m a y p l a y s o m e r o l e in t h e r e t e n t i o n of s h o r t - t e r m p i p e s t i f f n e s s . P i p e s p e c i m e n s w i t h a h i g h e r a s h c o n t e n t ( M a n u f a c t u r e r II at 4.5% a n d M a n u f a c t u r e r III at 11%) s h o w a s l i g h t i n c r e a s e in s h o r t - t e r m p i p e s t i f f n e s s , w h e r e a s s p e c i m e n s w i t h l o w e r a s h ( M a n u f a c t u r e r I at 3.9%) s h o w a l m o s t no c h a n g e in s h o r t - t e r m s t i f f n e s s o v e r t h e t e s t period. T h e m e c h a n i s m of i n f l u - e n c e of f i l l e r c o n t e n t on v a r i a t i o n s in p i p e s t i f f n e s s o v e r t i m e is not w e l l e s t a b l i s h e d and f u r t h e r c o n t r o l l e d s t u d y a p p e a r s w a r r a n t e d .
T i m e - D e p e n d e n t Stress R e l a x a t i o n
F i g u r e 7 t r a c k s t h e d e c a y in load r e q u i r e d to s u s t a i n t h e 5% f i x e d d e f l e c t i o n for all a c i d - i m m e r s e d s p e c i m e n s . T h e d e c a y in load o v e r t i m e is a m e a s u r e of t h e t i m e - d e p e n d e n t s t r e s s r e l a x a t i o n in t h e p i p e
m a t e r i a l . R e l a x a t i o n is d e f i n e d as t h e d e c r e a s e in s t r e s s w i t h t i m e u n d e r c o n s t a n t strain. G r e a t e r s t r e s s r e l a x a t i o n is e x h i b i t e d for p i p e s p e c i m e n s f r o m m a n u f a c t u r e r I a n d III t h a n for m a n u f a c t u r e r II s p e c i - m e n s . A f t e r 27 m o n t h s (19,500 hrs) of t e s t i n g M a n u f a c t u r e r I and III s p e c i m e n s h a v e r e l a x e d to a b o u t 45% of i n i t i a l stress, w h e r e a s M a n u f a c - t u r e r II s p e c i m e n s h a v e r e l a x e d to a b o u t 60% of i n i t i a l stress.
C o m p a r i s o n of t h e r e l a x a t i o n r e s p o n s e of a c i d - i m m e r s e d s p e c i m e n s t o the a i r - o n l y s p e c i m e n f r o m t h e same p i p e (III) is s h o w n in F i g u r e 8. A l m o s t f r o m t h e b e g i n n i n g , a c i d - i m m e r s e d s p e c i m e n s e x h i b i t a b o u t 10% less load r e t e n t i o n (i.e. g r e a t e r r e l a x a t i o n ) at 5% d e f l e c t i o n ; t h i s d i f f e r e n c e r e m a i n s a b o u t c o n s t a n t o v e r t h e t w o y e a r s of t e s t i n g . T h e d i f f e r e n c e s in d e f l e c t i o n f i x t u r e s for t h e a i r - o n l y and t h e a c i d - i m m e r s e d s p e c i m e n s m a y h a v e h a d s o m e i n f l u e n c e on t h e d i f f e r e n t r e s p o n s e s .
N o t e t h a t for all t e s t s p e c i m e n s , s t r e s s r e l a x a t i o n is m o s t p r o n o u n c e d w i t h i n t h e f i r s t m o n t h (700 hrs) of t e s t i n g . A f t e r a b o u t I0 m o n t h s
(7000 hrs), r e l a x a t i o n b e g i n s t o l e v e l out, b u t c o n t i n u e s at a s l o w rate e v e n at 27 m o n t h s ( 1 9 , 5 0 0 hrs).
T h e t i m e - d e p e n d e n t r e l a x a t i o n b e h a v i o r e x h i b i t e d in t e s t s p e c i m e n s s h o u l d a p p r o x i m a t e t h a t of a p i p e in t h e b u r i e d c o n d i t i o n if one a s s u m e s t h a t t h e b u r i e d p i p e is in a s t a t e of f i x e d d e f l e c t i o n ( c o n s t a n t
s t r a i n ) . S o m e i n v e s t i g a t o r s a d o p t t h e c o n c e p t of f i x e d d e f l e c t i o n in t h e a n a l y s i s of l o n g - t e r m b e h a v i o r of b u r i e d t h e r m o p l a s t i c pipe. It s h o u l d b e r e c o g n i z e d , h o w e v e r , t h a t e a r l y in t h e l i f e of t h e s o i l / p i p e system, p i p e d e f l e c t i o n s can and d o i n c r e a s e , l a r g e l y d u e to t i m e - d e p e n d e n t d e f o r m a t i o n of t h e s u r r o u n d i n g soil. In t h i s c o n d i t i o n t h e
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