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Effect of applied stress on the helium inflow characteristics of hydrated
portland cement
Ser
TH1
N21d
National Research
Conseil national
1141
1
+
Council Canada
de recherches Canada
c .
2
BLDG
EFFECT OF APPLIED STRESS ON THE HELIUM INFLOW
CHARACTERISTICS OF HYDRATED PORTLAND CEMENT
by
R.F. Feldman and J.J. Beaudoin
Reprinted from
Cement and Concrete Research
Vol. 13,1983, p. 470
-
476
DBR Paper No. 1141
Division of Building Research
R i s u ~
Des Btudes du c i m e n t P o r t l a n d h y d r a t ' e poreux o n t 6 t B men'ees p o u r B v a l u e r q u a n t i t a t i v e m e n t c e r t a i n s a s p e c t s d e l a d g f o r m a t i o n d e l a m i c r o s t r u c t u r e d u e 1 u n e c o n t r a i n t e appliqu'ee. Une p l t e d e ciment P o r t l a n d hydrat'ee d u r c i e pendant s i x a n s 3 un r a p p o r t e a u l c i m e n t d e 0 , 6 a Bt'e tranch'ee e n d i s q u e s d e 1.25 mm d ' e p a i s s e u r . Ces d i s q u e s o n t 6 t 6 plac'es s u r u n s u p p o r t d ' 6 c h a n t i l l o n s p ' e c i a l e m e n t c o q u q u i a &t& u t i l i s ' e a v e c un a p p a r e i l d e mise e n c h a r g e p e r m e t t a n t d ' a p p l i q u e r aux e c h a n t i l l o n s d e s c o n t r a i n t e s d e compression u n i a x i a l e s d e 7,1,
1 4 , 8 e t 22,8 Mf'a. Des mesures a u pycnomZtre e t d e s mesures d ' a d m i s s i o n d1h61ium o n t BtB p r i s e s 1 d e s d i m i n u t i o n s d e t e n e u r e n e a u d e 0, 2,5, 4 , s . 7,O e t 9.0 pour c e n t 1 p a r t i r d'une humidit'e r e l a t i v e 3 11 pour c e n t . Les e f f e t s l e s p l u s i m p o r t a n t s s e s o n t p r o d u i t s a p r e s une d i m i n u t i o n d e 7,O pour c e n t d e l a t e n e u r e n eau; l ' a d m i s s i o n d'hglium a a l o r s Bt6 r e d u i t e d e 48 pour c e n t B 40 h. Les d e n s i t ' e s o n t dgalernent 6t'e r B d u i t e s p a r l a c o n t r a i n t e , e t on e n a c o n c l u q u e l e s o r i f i c e s d ' e n t r b 1 l ' e s p a c e i n t e r c o u c h e s r ' e t r ' e c i s s a i e n t pendant l ' a p p l i c a t i o n de l a c o n t r a i n t e .
CEMENT and CONCRETE RESEARCH. Vol. 1 3 , pp. 470-476, 1983. P r i n t e d i n t h e U S A . 0008-8846/83 $3.00
+
00. Copyright ( c ) 1983 Pergamon P r e s s , L t d .EFFECT OF APPLIED STRESS ON THE HELIUM INFLOW CHARACTERISTICS OF HYDRATED PORTLAND CEMENT
R.F. Feldman and J.J. Beaudoin Re s e a r c h O f f i c e r s D i v i s i o n of B u i l d i n g Research N a t i o n a l R e s e a r c h C o u n c i l Canada
Ottawa, O n t a r i o KIA OR6
(Communicated by F.H. Wittmann) (Received Nov. 1 0 , 1982)
ABSTRACT
S t u d i e s of porous h y d r a t e d p o r t l a n d cement have b e e n c a r r i e d o u t t o a s s e s s q u a n t i t a t i v e l y some a s p e c t s of m i c r o s t r u c t u r a l d e f o r m a t i o n due t o a p p l i e d s t r e s s . Hydrated p o r t l a n d cement p a s t e c u r e d f o r s i x y e a r s a t a waterlcement r a t i o of 0.6 was s l i c e d i n t o d i s c s 1.25 mm t h i c k . These were p o s i t i o n e d i n a s p e c i a l l y d e s i g n e d sample h o l d e r and l o a d i n g d e v i c e t h a t p e r m i t t e d t h e specimens t o be p l a c e d under u n i a x i a l
compressive s t r e s s a t 7.1, 14.8 and 22.8 MPa. H e l i u m i n t a k e and
pycnometric measurements were made a t 0 , 2.5, 4.5, 7.0 and 9.0 p e r c e n t m o i s t u r e l o s s w i t h r e s p e c t t o t h e 1 1 p e r c e n t RH c o n d i t i o n . Maximum e f f e c t s were observed a t 7.0 p e r c e n t m o i s t u r e l o s s , a t which helium i n t a k e was reduced by 48 p e r c e n t a t 40 h. D e n s i t i e s w e r e a l s o reduced by s t r e s s , and i t was concluded t h a t e n t r a n c e s t o i n t e r l a y e r s p a c e were c o n s t r i c t e d d u r i n g a p p l i c a t i o n of s t r e s s .
I n t r o d u c t i o n
The mechanical p r o p e r t i e s of c e m e n t i t i o u s m a t e r i a l s s u c h a s cement p a s t e , m o r t a r s and c o n c r e t e a r e dependent o n p o r o s i t y and p o r e s i z e d i s t r i b u t i o n , p r o p e r t i e s t h a t a r e v e r y c r i t i c a l w i t h r e g a r d t o p e r m e a b i l i t y and r e s i s t a n c e t o c h e m i c a l s ( 1 , 2 ) . The p o r o s i t y of t h e s e m a t e r i a l s i s d i f f i c u l t t o d e f i n e and measure; i t h a s t o be done i n a s p e c i f i c way because a l a r g e p a r t of t h e s p a c e v a c a t e d by w a t e r i s i n t e r l a y e r space. I n c o n t r a s t w i t h o t h e r p o r e s p a c e , i n t e r l a y e r s p a c e h a s s p e c i a l p r o p e r t i e s , f o r example, i t i s p o s s i b l e t o i n c r e a s e s u b s t a n t i a l l y t h e modulus of e l a s t i c i t y a t a g i v e n p o r o s i t y i f complete r e - e n t r a n c e of i n t e r l a y e r w a t e r o c c u r s a f t e r i t h a s d r i e d
i n i t i a l l y ( 3 ) . I n a d d i t i o n , i n t t r l a y e r s p a c e d o e s n o t r e t a i n i t s s i z e and shape when w a t e r i s removed, and t h i s r e s u l t s i n changes i n mechanical p r o p e r t i e s ( 4 ) .
Many s t u d i e s have b e e n made of t h e e f f e c t of s t r e s s o n cement p a s t e o r c o n c r e t e
(5).
It h a s always been assumed t h a t d e f o r m a t i o n o f t h e specimen f o l l o w i n g a p p l i e d s t r e s s i s n o t p r i m a r i l y a result o f change i n t h e geometry o fVol. 1 3 , No. 4 4 7 1 HELIUM INFLOW, STRESS APPLICATION, MOISTURE LOSS, DENSITY
t h e p o r e s b u t of a change i n a b a s i c p r o p e r t y of t h e s o l i d m a t e r i a l of t h e specimen. Such s t u d i e s have n o t i n c l u d e d any q u a n t i t a t i v e a s s e s s m e n t s of m i c r o s t r u c t u r a l d e f o r m a t i o n due t o a p p l i e d s t r e s s .
The helium i n f l o w t e c h n i q u e h a s b e e n used i n s t u d y i n g h y d r a t e d t r i c a l c i u m s i l i c a t e and p o r t l a n d cement s y s t e m s , p e r m i t t i n g o b s e r v a t i o n of changes i n s o l i d volume, d e n s i t y and volume of i n t e r l a y e r s p a c e t h a t o c c u r d u r i n g removal and r e - e n t r y of w a t e r ( 4 ) . Helium i n t a k e v e r s u s time c u r v e s f o r a sample a t d i f f e r e n t m o i s t u r e c o n t e n t l e v e l s p r o v i d e d t h e d a t a and u l t i m a t e b a s i s f o r a n assessment of t h e s e changes. The changes i n p r o p e r t i e s and composite terms c a l c u l a t e d from t h e s e measurements were compared and p r o v i d e d a means of d e t e r m i n i n g t h e e x t e n t t o which t h e s p a c e v a c a t e d by t h e w a t e r was a f f e c t e d .
It was t h e p u r p o s e of t h e p r e s e n t s t u d y t o a s s e s s t h e e f f e c t of a p p l i e d s t r e s s on t h e r a t e of f l o w of helium i n t o h y d r a t e d p o r t l a n d cement. Study o f t h e changes i n q u a n t i t i e s measured by t h e helium i n f l o w t e c h n i q u e t h a t r e s u l t e d from changes i n a p p l i e d s t r e s s w i l l a l l o w a s s e s s m e n t of t h e e f f e c t of s t r e s s o n t h e p o r e s and s h o u l d p r o v i d e some i n s i g h t i n t o t h e n a t u r e of t h e pores. It should a l s o p r o v i d e some u n d e r s t a n d i n g of t h e b a s i c phenomena r e s p o n s i b l e f o r c r e e p of c o n c r e t e under a p p l i e d s t r e s s .
E x p e r i m e n t a l M a t e r i a l s
A t y p e I p o r t l a n d cement was h y d r a t e d a t a waterlcement r a t i o of 0.6 f o r a p p r o x i m a t e l y s i x y e a r s . M s c s 1.25 mm t h i c k and 32 mm i n d i a m e t e r were c u t from c y l i n d e r s and used a s samples f o r helium i n t a k e measurements. Each d i s c had a 7.94 mm h o l e a t i t s c e n t r e and r a d i a l g r o o v e s e v e r y 20 deg.
Helium Comparison Pycnometry and Helium I n f l o w
The a p p a r a t u s and p r o c e d u r e f o r helium i n f l o w measurements have b e e n d e s c r i b e d ( 4 ) . The a p p a r a t u s d i f f e r e d , however, i n t h a t t h e sample-holding d e v i c e , which m a i n t a i n s t h e specimens i n a s t r e s s e d s t a t e , was p l a c e d i n t h e pycnometer sample c y l i n d e r w i t h t h e h y d r a t e d p o r t l a n d cement d i s c s i n s t a l l e d . Helium i n t a k e i n t o t h e sample was measured by t h e pycnometer f o r 4 0 h , t h e n t h e d e n s i t y of t h e m a t e r i a l was measured. The t e c h n i q u e u s e s t h e g a s laws and t h e i d e a l g a s a s s u m p t i o n f o r c a l c u l a t i n g s o l i d volume of t h e sample. I n t e r l a y e r s p a c e s o r v e r y s m a l l and p a r t i a l l y e n t r a p p e d s p a c e s t h a t might d e l a y f l o w o f helium i n t o t h e sample were c o n s i d e r e d a s p a r t of t h e s o l i d o r a s p o r e s i n c a l c u l a t i n g d e n s i t y .
Apparatus f o r S t r e s s i n g Samples
The a p p a r a t u s f o r h o l d i n g t h e specimen i n p l a c e and t h e s t r e s s i n g equipment a r e shown i n F i g . 1. The sample i s composed of a b o u t 1 5 d i s c s r e s t i n g on t o p of each o t h e r on a hardened s t e e l b a s e . A t h r e a d e d p r e s t r e s s i n g w i r e i s a t - t a c h e d t o t h e b a s e and p a s s e s t h r o u g h t h e c e n t r a l h o l e i n each d i s c . A s u f f i c - i e n t number of d i s c s a r e mounted s o a s t o o v e r l a p , w i t h t h e t h r e a d e d p o r t i o n a t t h e t o p of t h e w i r e . A t h r e a d e d c a p i s screwed i n p l a c e on t o p of t h e l a s t d i s c .
The h d y r a t e d cement samples c o n d i t i o n e d a t 11 p e r c e n t RH weighed 15.5g. Fig- u r e 1 B shows how t h e sample-holding d e v i c e was p l a c e d on a r e a c t i o n frame. The exposed, t h r e a d e d end was screwed t o a t e n s i o n rod and t h i s , i n t u r n , was f a s t - ened t o a b e a r i n g n u t . Tension t o t h e p r e s t r e s s i n g w i r e was a p p l i e d by a hy- d r a u l i c j a c k t h r o u g h t h e b e a r i n g u n i t . A f t e r t h e r e q u i r e d t e n s i o n was a t t a i n e d , a s determined from t h e c a l i b r a t e d gauge on t h e j a c k , t h e t h r e a d e d cap was t i g h t - ened s e c u r e l y t o t h e specimen and t h e p r e s s u r e i n t h e j a c k was r e l e a s e d s o t h a t
472
R.F. Feldman, J . J . Beaudoin
THREADED CAP THREADED
PRESTRESSING WIR
TYPICAL DISC SAMPLE
I I
WITH CENTRAL HOLEVol. 1 3 , No. 4,
FIG. 1
STEEL Schematic of sample-holding d e v i c e and s t r e s s i n g a p p a r a t u s A . S A M P L E - H O L D I N G D E V I C E
-
S C H E M A T I CSAMPLE-HOLDING s t r e s s (compressive) was t r a n s -
TIGHTENING N f e r r e d t o t h e samples. ( I t was
assumed t h a t t h e f i n a l compressive l o a d was e q u a l t o t h e i n i t i a l l y REACT'0N FRAME a p p l i e d t e n s i l e l o a d . Loss of t e n -
s i o n i n t h e w i r e , d e t e c t e d a t t h e TENSION ROD
l o a d a t which t h e i n i t i a l l y t h r e a d - ed cap became l o o s e on u n l o a d i n g , i s s m a l l and c o r r e c t i o n s were n o t
made f o r t h i s . ) The assembly was
t h e n p l a c e d i n t h e sample c y l i n d e r
of t h e pycnometer. P r e s s u r e s used
SUPPORT STAND i n t h i s work were 7.1, 1 4 . 8 and
22.8 m a . It was e s t i m a t e d t h a t t h e t e n s i o n i n t h e p r e s t r e s s i n g 0 . S A M P L E S T R E S S I N G A P P A R A T U S - S C H E M A T I C w i r e a t t a i n e d 445 m a .
G e n e r a l P r o c e d u r e
A f t e r f a b r i c a t i o n , samples w e r e c o n d i t i o n e d t o 11 p e r c e n t h u m i d i t y and l o a d e d i n t h e h o l d e r . The whole assembly was t h e n t r a n s f e r r e d t o t h e
pycnometer sample c y l i n d e r p l a c e d i n t h e gloved box. D e n s i t y and helium i n t a k e
measurements were determined. P r i o r t o i n c r e a s i n g s t r e s s o n t h e specimens t o
t h e n e x t l e v e l , t h e samples were degassed under c o n d i t i o n s s i m i l a r t o t h o s e n e c e s s a r y t o b r i n g them t o t h e m o i s t u r e l e v e l a t which measurements were b e i n g made. M o i s t u r e l o s s e s r e l a t i v e t o 11 p e r c e n t RH, a t which specimens were t a k e n , were 2.5, 4.5, 7 and 9 p e r c e n t . I n o r d e r t o change m o i s t u r e c o n t e n t t h e s t r e s s on t h e samples was reduced t o z e r o and t h o s e s t i l l i n t h e sample h o l d e r were p l a c e d i n a s e p a r a t e vacuum v e s s e l . The s a m p l e s were e v a c u a t e d o r h e a t e d and e v a c u a t e d ; t h e l a s t two m o i s t u r e l e v e l s were a t t a i n e d by h e a t i n g t o 75OC i n t h e l a t t e r s t a g e s of e v a c u a t i o n . S t r e s s was removed o r i n c r e a s e d by r e p l a c i n g t h e sample on t h e r e a c t i o n frame and c o n n e c t i n g i t t o t h e t e n s i o n r o d ; t h e p r e s s u r e i n t h e h y d r a u l i c j a c k was i n c r e a s e d t o t h e l e v e l t o which i t had been r a i s e d p r e v i o u s l y . The e f f e c t i v e s t r e s s on t h e samples themselves was t h e r e f o r e z e r o and t h e t h r e a d e d c a p c o u l d b e l o o s e n e d i n p r e p a r a t i o n f o r
d e g a s s i n g t r e a t m e n t .
R e s u l t s and D i s c u s s i o n Helium I n t a k e a s a F u n c t i o n of Time
The helium i n t a k e v e r s u s time c u r v e a t a w e i g h t l o s s of 7.0 p e r c e n t r e l a t i v e t o t h e 1 1 p e r c e n t c o n d i t i o n i s p r e s e n t e d i n Fig. 2. R e s u l t s a r e shown f o r samples under compressive s t r e s s e s of 0 , 7.1, 14.8 and 22.8 MPa.
Helium i n t a k e v a l u e s a r e a c c u r a t e t o k0.05 m1/100 g. A t 14.8 MPa t h e volume of
helium i n t a k e a f t e r 40 h i s reduced by a l m o s t 48 p e r c e n t compared t o t h a t f o r t h e sample a t no load. F u r t h e r i n c r e a s e of s t r e s s t o 22.8 MPa i s a c t u a l l y
Vol. 1 3 , No. 4 473 HELIUM INFLOW, STRESS APPLICATION, MOISTURE LOSS, DENSITY
TIME, h
FIG. 2
Helium i n t a k e v e r s u s time a s f u n c t i o n of a p p l i e d s t r e s s a t 7.0% weight l o s s
r e v e r s a l t a k e s p l a c e a s w e l l a t t h e 11 p e r c e n t RH c o n d i t i o n , b u t a t 2.5, 4.5 and 9.0 p e r c e n t weight l o s s l e v e l s t h e helium i n t a k e d e c r e a s e s w i t h a p p l i e d s t r e s s . These c u r v e s a r e t y p i c a l f o r a l l w e i g h t l o s s l e v e l s and g e n e r a l l y f l a t t e n o u t more a b r u p t l y a t t h e h i g h e r s t r e s s e s .
The change i n t h e t o t a l volume of helium i n t a k e a t 40 h a s a f u n c t i o n of m o i s t u r e c o n t e n t i s shown i n Fig. 3 a t t h e f o u r s t r e s s l e v e l s . The v a l u e f o r z e r o s t r e s s l e v e l i s s i m i l a r t o t h a t p u b l i s h e d p r e v i o u s l y f o r a waterlcement r a t i o of 0.6 p a s t e ( 4 ) . The r e s u l t s show t h a t s t r e s s h a s a g r e a t e r e f f e c t i n d e c r e a s i n g helium i n t a k e a t h i g h e r m o i s t u r e l o s s e s ( u p t o a l o s s of 7 p e r c e n t ) . The e f f e c t i s g e n e r a l l y g r e a t e r w i t h i n c r e a s i n g s t r e s s , a l t h o u g h t h e r e i s some o v e r l a p . The d e c r e a s e i n t o t a l helium i n t a k e a f t e r 40 h , r e l a t i v e t o u n s t r e s s e d samples, i s p l o t t e d a s a f u n c t i o n of m o i s t u r e l o s s i n Fig. 4. The r e s u l t s a r e shown f o r t h e t h r e e s t r e s s l e v e l s used: 7.1, 14.8 and 22.8 MPa. They a r e compatible w i t h an i n t e r l a y e r model f o r t h e p o r e s and c a n be s u p p o r t e d i n t h i s view a s f o l l o w s : a t t h e 1 1 p e r c e n t c o n d i t i o n ( z e r o m o i s t u r e l o s s ) t h e r e i s l i t t l e change i n helium i n t a k e under a p p l i e d s t r e s s s i n c e t h e s p a c e s a r e l a r g e l y o c c u p i e d by water. There i s , however, a s l i g h t i n c r e a s e i n i n t a k e a t 22.8 MPa. A s w a t e r i s removed from t h e i n t e r l a y e r s p a c e w i t h o u t major c o l l a p s e o f t h e l a y e r e d s t r u c t u r e t h e v a c a t e d s p a c e c a n b e f i l l e d w i t h helium. Applied s t r e s s c a n r e d u c e t h e s i z e of t h e e n t r a n c e s t o t h e water-vacated s p a c e s and p o s s i b l y l e n g t h e n them by b r i n g i n g s u r f a c e s a t t h e e n t r a n c e s c l o s e r t o g e t h e r ( s e e Fig. 5B), t h e r e b y r e d u c i n g t h e r a t e of e n t r y of helium. A s t h e v a c a t e d s p a c e i n c r e a s e s , t h e helium i n t a k e a f t e r 40 h (due t o c o n s t r i c t e d e n t r a n c e s ) d e c r e a s e s . T h i s e f f e c t r e a c h e s i t s maximum a t around 7 p e r c e n t weight l o s s , where t h e e f f e c t i s g r e a t e s t n o t a t t h e a p p l i e d s t r e s s of 22.8 MPa b u t a t
14.8 MPa. T h i s l a t t e r e f f e c t may be t h e r e s u l t of b r i n g i n g s u f f i c i e n t s u r f a c e s i n t o c l o s e enough p r o x i m i t y a t 22.8 MPa t o form new s p a c e i n t o which helium flows. A t m o i s t u r e l o s s e s g r e a t e r t h a n 7 p e r c e n t t h e d i f f e r e n c e between t h e
Vol. 1 3 , No. 4 R.F. Feldman, J.J. Beaudoin W - J k!E A W I W . % FIG. 3 FIG. 4 Helium i n t a k e v e r s u s m o i s t u r e l o s s a t d i f f e r e n t s t r e s s l e v e l s . Change i n helium i n t a k e v e r s u s m o i s t u r e l o s s a t v a r i o u s s t r e s s l e v e l s
s t r e s s e d and u n s t r e s s e d samples d e c r e a s e s a b r u p t l y . T h i s i s due t o t h e f a c t t h a t i n t h e u n s t r e s s e d s t a t e i n t e r l a y e r s p a c e a l s o c ~ l l a p s e s a b r u p t l y i n t h i s r e g i o n (Fig. 3). Removal of w a t e r a l o n e r e d u c e s t h e e n t r a n c e s i z e of t h e i n t e r l a y e r space. A t t h i s m o i s t u r e l e v e l , however, some f u r t h e r c o n s t r i c t i o n o f t h e e n t r a n c e s s t i l l seems t o t a k e p l a c e o n a p p l i c a t i o n of s t r e s s .
D e n s i t y Measurements
D e n s i t y v a l u e s may b e c a l c u l a t e d i n two ways: (1) u s i n g t h e s o l i d volume c a l c u l a t e d from d a t a o b t a i n e d by compressing t h e helium g a s s u r r o u n d i n g t h e
c sample from one t o two atmospheres
-
( t h i s method, i n which helium i n t a k e h a s n o t o c c u r r e d , i s u s u a l l y used f o r d e n s i t y measurements); ( 2 ) r e c a l c u l a t i n g t h e v a l u e s f o r s o l i d volume o b t a i n e d i n (1) by s u b t r a c t i n g t h e volume of helium i n t a k e a t 40 h ( F i g . 6) ( 6 ) . R e s u l t s of d e n s i t y c a l c u l a t i o n s from ( 1 ) v a r y l i t t l e w i t h a p p l i e d s t r e s s . A t 7.0 p e r c e n t weight FIG. 5 S t r a i n a t s p e c i f i c r e g i o n s due t o s t r e s s and change i n m o i s t u r e c o n t e n t
Vol. 1 3 , No. 4 475 HELIUM INFLOW, STRESS APPLICATION, MOISTURE LOSS, DENSITY
2.40 l o s s , where maximum e f f e c t s I I o c c u r r e d w i t h r e g a r d t o d e c r e a s e W E I G H T L O S S i n helium i n t a k e , t h e d e n s i t y o v e r A 2 . 5 0 t h e whole s t r e s s r a n g e v a r i e d from 6 4 . 5 2.19 t o 2.20 20.01 x
lo3
kg/m3, w i t h l i t t l e o r no a p p a r e n t 2.35-
dependency on s t r e s s . It must be concluded t h a t t h e d e c r e a s e i n r a t e of helium i n t a k e i s l a r g e l y due t o c o n s t r i c t i o n of t h e ICI e n t r a n c e s t o v a c a t e d i n t e r l a y e r s p a c e s and n o t t o e l i m i n a t i o n of t h e s e s p a c e s . Z Y The r e s u l t s i n Fig. 6 a r e a l s o a p l o t t e d a s a f u n c t i o n of a p p l i e d s t r e s s . It was found t h a t up t o about 6.0 p e r c e n t m o i s t u r e l o s s ( w i t h o u t a p p l i e d s t r e s s ) helium e n t e r s a l l t h e s p a c e s p r e v i o u s l y v a c a t e d by w a t e r a t 40 h ( 4 ) . The more complete t h e e n t r y of h e l i u m 2 . 2 0 t h e lower w i l l be t h e c a l c u l a t e d o 500 ( 7 . 1 ) 1 1 a . g ) ( 2 2 . 8 ) s o l i d volume and t h e h i g h e r t h e G A U G E P R E S S U R E , p s i I M P a ) d e n s i t y . A t z e r o a p p l i e d s t r e s s d e n s i t y i n c r e a s e s w i t h m o i s t u r e l o s s up t o 7 p e r c e n t , t h e n d e c r e a s e s a t 9 p e r c e n t l o s s . The FIG. 6 e f f e c t of s t r e s s a t z e r o m o i s t u r e l o s s is s l i g h t , b u t from 2.5 t o 7 Change i n d e n s i t y w i t h p r e s s u r e a t p e r c e n t m o i s t u r e l o s s i t d i f f e r e n t w a t e r c o n t e n t s i n c r e a s e s p r o g r e s s i v e l y . D e n s i t y mainly d e c r e a s e s w i t h i n c r e a s i n g a p p l i e d s t r e s s , p r o b a b l y a s a r e s u l t of c o n s t r i c t i o n a t t h e e n t r a n c e s and t h e r e s u l t a n t i n a b i l i t y of helium t o e n t e r a l l t h e s p a c e w i t h i n 40 h o u r s . A t 2.5 p e r c e n t weight l o s s t h e d e n s i t y d e c r e a s e s from 2.31 t o 2.26 5 . 0 1 xl o 3
kg/m3; a t 4.5 p e r c e n t , from 2.34 t o 2.26 5 . 0 1 x 103 kg/&; a t 7.0 p e r c e n t , from 2.40 t o 2.31 +O. 0 1 xl o 3
kg/m3; and a t 9 p e r c e n t weight l o s s , from 2.34 t o 2.31+
0 . 0 1 x 153 kg/m3.It is c l e a r t h a t m o i s t u r e c o n t e n t h a s a s i g n i f i c a n t i n f l u e n c e on t h e observed e f f e c t of a p p l i e d s t r e s s . A s water i s removed from t h e 11 per cent RH c o n d i t i o n , t h e i n t e r l a y e r s p a c e s a r e vacated. Thus t h e p o t e n t i a l f o r helium i n t a k e i s g r e a t e r and, a t t h e same time, under a p p l i e d s t r e s s t h e c o n s t r i c t i o n t h a t may occur a t t h e e n t r a n c e s may a l s o be g r e a t e r . A t 9 p e r c e n t weight l o s s t h i s e f f e c t of a p p l i e d s t r e s s diminishes, probably because c o l l a p s e of both spaces and e n t r a n c e s o c c u r s l a r g e l y by p r i o r removal of water,
It i s suggested t h a t p o r t i o n s of t h e spaces vacated by water, i . e . , t h e
entrances and p o s s i b l y regions n e a r t h e e n t r a n c e s , may be s i g n i f i c a n t l y reduced i n c r o s s - s e c t i o n a l a r e a and t h a t water from t h e s p a c e s may have a s i g n i f i c a n t r o l e i n r e s i s t i n g deformation due t o s t r e s s . A few experimental p o i n t s i n Fig. 6 d e v i a t e from t h e g e n e r a l t r e n d ; f o r example, t h e p o i n t f o r 4.5 p e r c e n t moisture l o s s a t 14.8 MPa s t r e s s . Whether t h i s i s due t o experimental e r r o r o r t o t h e i r r e v e r s i b l e e f f e c t s of previously a p p l i e d s t r e s s i s not known.
T h a t "fixed-dimension" p o r e s p o s s i b l y o c c u r r i n g i n t h e cement p a s t e a r e n o t r e s p o n s i b l e f o r t h e d i f f e r e n c e s i n helium i n t a k e can be e x p l a i n e d : Consider a p r i s m a t i c specimen h a v i n g modulus of e l a s t i c i t y , E , u n d e r compressive s t r e s s ,
Vol. 1 3 , No. 4 R.F. Feldman, J. J. Beaudoin
a , r e s u l t i n g i n a n o v e r - a l l a v e r a g e s t r a i n , E . I f t h i s specimen c o n t a i n s
simple c y l i n d r i c a l p o r e s l i k e t h a t shown i n Fig. 5A, and i f i t i s assumed t h a t t h e o v e r - a l l E, i n c l u d i n g t h e p o r e , i s 6900 MPa, t h e r e w i l l be a s t r a i n E of 0.1
p e r c e n t under a compressive l o a d of 6.90 MPa. I f , however, one assumes no o v e r l a p p i n g of s t r e s s f i e l d s from one p o r e t o a n o t h e r b u t c o n s i d e r s t h e u s e of E,, ( t h e z e r o p o r o s i t y modulus and s t r e s s i n t e n s i f i c a t i o n of approximately 3 i n t h e v i c i n i t y of t h e p o r e i t s e l f ) , i t w i l l s t i l l be found t h a t s t r a i n i n t h e v i c i n i t y of t h e p o r e i s of t h e o r d e r of 0.1 p e r c e n t . The maximum s t r e s s i n t h e v i c i n i t y of t h e pore i s a p p r o x i m a t e l y t h r e e t i m e s t h e a v e r a g e s t r e s s ( 7 ) . This h a r d l y a c c o u n t s f o r t h e 48 p e r c e n t r e d u c t i o n i n helium i n t a k e . But i f l o c a l s t r a i n s a t t h e ends of l a y e r s a r e c o n s i d e r e d t o be d e r i v e d from t h e p a r t i a l c o l l a p s e of water-vacated i n t e r l a y e r s p a c e , a s shown i n Fig. 5B, where i n t e r l a y e r s h e e t s a r e p a r t i a l l y drawn t o g e t h e r by d r y i n g a l o n e , i t w i l l be u n d e r s t o o d how a p p l i e d s t r e s s can cause c o n s t r i c t i o n of t h e e n t r a n c e s and r e d u c t i o n i n helium i n t a k e of t h e magnitude observed i n t h e s e experiments.
F i g u r e 5C shows how t h e model f o r a d i s o r d e r e d l a y e r e d s t r u c t u r e c a n p r o v i d e many s i t e s where c o n s t r i c t i o n of e n t r a n c e s can c a u s e d e c r e a s e d helium i n t a k e . A model s u c h a s t h i s h a s a l r e a d y been u s e d t o d e s c r i b e i r r e v e r s i b l e c r e e p ( 8 ) , and t h e p o s s i b i l i t y t h a t t h e s e s i t e s may be c o n s t r i c t e d by t h e a p p l i c a t i o n of s t r e s s h a s s i g n i f i c a n t r a m i f i c a t i o n s w i t h r e g a r d t o t h i s mechanism. More s t u d y w i l l be n e c e s s a r y t o f u r t h e r t h e u n d e r s t a n d i n g of t h e e f f e c t of r e p e t i t i v e l o a d i n g and u n l o a d i n g , and of changing m o i s t u r e c o n t e n t on a p r e v i o u s l y l o a d e d sample.
C o n c l u s i o n s
1. Helium i n t a k e i s d e c r e a s e d a l m o s t 50 p e r c e n t by a p p l y i n g 14.8 MPa compressive s t r e s s t o a sample of h y d r a t e d p o r t l a n d cement p a s t e .
2. L i t t l e r e d u c t i o n i n helium i n t a k e o c c u r s a t 11 p e r c e n t RH when s t r e s s i s a p p l i e d t o t h e systems.
3 . The r e d u c t i o n of helium i n t a k e due t o a p p l i e d s t r e s s i s a f u n c t i o n of m o i s t u r e c o n t e n t , t h e maximum e f f e c t o c c u r r i n g a t 7.0 p e r c e n t m o i s t u r e l o s s
( w i t h r e s p e c t t o t h e 11 p e r c e n t RH c o n d i t i o n ) .
4. Applied s t r e s s h a s no s i g n i f i c a n t e f f e c t on s o l i d d e n s i t y v a l u e s and e l i m i n a t i o n of i n t e r l a y e r s p a c e i s u n l i k e l y .
5. Reduction of helium i n t a k e i s probably due mainly t o c o n s t r i c t i o n of narrow i n t e r l a y e r e n t r a n c e p o s i t i o n s v a c a t e d b y i n t e r l a y e r w a t e r t h a t c o n t r i b u t e d t o t h e s t i f f n e s s of t h e m a t e r i a l .
5. Helium i n t a k e r e s u l t s a r e c o n s i s t e n t w i t h t h e h y p o t h e s i s of a d i s o r d e r e d , l a y e r e d s h e e t model f o r h y d r a t e d calcium s i l i c a t e .
Acknowledgement
The a u t h o r s w i s h t o acknowledge w i t h t h a n k s t h e f i n e work of A. L i u and A. Charron i n performing most of t h e experiments. T h i s p a p e r i s a c o n t r i b u t i o n from t h e D i v i s i o n of B u i l d i n g Research, N a t i o n a l R e s e a r c h C o u n c i l of Canada, a n d i s p u b l i s h e d w i t h t h e a p p r o v a l of t h e D i r e c t o r of t h e D i v i s i o n .
R e f e r e n c e s
J . J . Beaudoin and R.F. Feldman, Cem. and Conc. Res.
5,
103-118 (1975). R.A. Helmuth and D.M. Turk, HRB S p e c i a l Report 90, p. 135 (1966).P . J . S e r e d a , R.F. Feldman and E.G. Swenson, HRB S p e c i a l Report 90, p. 58-73 (1966).
R.F. Feldman, Cem. and Conc. Res. 1, 285-300 (1971).
A.M. N e v i l l e , P r o p e r t i e s of c o n c r e t e , 2nd Ed., London, Pitman, 686 pp, 1973. R.F. Feldman, Cem. Technol.
2,
5-14 (1972).S. Timoshenko and J . N . Goodier, Theory of E l a s t i c i t y , 2nd e d . , New York, McGraw-Hill, p. 506 (1951).
