The spacing factor in entrained air-bubbles. Has it any significance?

CEMENT and CONCRETERESEARCH. Vol. 14, PP. 757-758, 1984. Printed in the USA 0008-8846/84 %3.00+00. Copyright (c) 1984 Pergamon Press, Ltd.

THE

S P A C I N G F A C T O R IN E N T R A I N E D A I R - B U B B L E S . HAS IT ANY S I G N I F I C A N C E ?

S. C h a t t e r j i Teknologisk Institut 2630 T a a s t r u p . D e n m a r k

(Communicated by D.~I. Roy) (Received Feb. l , 1984)

It has n o w g e n e r a l l y been a c c e p t e d that the p e r f o r m a n c e of a concrete sample in a f r e e z e - t h a w e n v i r o n m e n t improves, if it contains a fine a i r - b u b b l e system. Soon after the o r i g i n a l e m p e r i cal o b s e r v a t i o n , P o w e r s p r o p o s e d a s a t u r a t e d flow h y d r a u l i c p r e s sure h y p o t h e s i s to e x p l a i n the o b s e r v e d i m p r o v e m e n t (i) . A c c o r d ing to this h y p o t h e s i s w h e n a s a t u r a t e d c o n c r e t e s a m p l e is in a f r e e z i n g e n v i r o n m e n t , ice c r y s t a l s start to form in w a t e r - f i l l e d c a v i t i e s of the paste. This ice f o r m a t i o n forces some w a t e r out of the c a v i t i e s , t h r o u g h n a r r o w c a p i l l a r i e s , to the a i r - b u b b l e s , and it also g e n e r a t e s the h i g h h y d r a u l i c p r e s s u r e w h i c h is n e c e s sary for the w a t e r - f l o w . If the h y d r a u l i c p r e s s u r e is h i g h e r than the t e n s i l e s t r e n g t h of the paste, a r a p t u r e occurs. An e n t r a i n m e n t of a fine a i r - b u b b l e s y s t e m in c o n c r e t e , by r e d u c i n g the p a t h l e n g t h of w a t e r - f l o w , limits the p o s s i b i l i t y of g e n e r a t i o n of a h i g h h y d r a u l i c p r e s s u r e . Thus in the c o n t e x t of the s a t u r a ted flow h y d r a u l i c p r e s s u r e h y p o t h e s i s , the a v e r a g e i n t e r - b u b b l e d i s t a n c e has a d e f i n i t e f u n d a m e n t a l s i g n i f i c a n c e . B a s e d upon a s s u m p t i o n s that all a i r - b u b b l e s in a sample are e q u a l - s z z e d s p h e r e s a r r a n g e d in a s i m p l e cubic l a t t i c e t h r o u g h o u t the paste, P o w e r s c a l c u l a t e d a c h a r a c t e r i s t i c l e n g t h c a l l e d "the s p a c i n g factor" (2), w h i c h is r e l a t e d to the a v e r a g e i n t e r - b u b b l e distance. The s p a c i n g factor is u s e d e x t e n s i v e l y to d e f i n e the perf o r m a n c e of c o n c r e t e s a m p l e s both in r e s e a r c h and in p r a c t i c e (3, 4). In some c o u n t r i e s , a m a x i m u m v a l u e of the s p a c i n g factor is r e q u i r e d for all c o n c r e t e s t r u c t u r e s that are to be e x p o s e d to f r e e z e - t h a w e n v i r o n m e n t s . Powers' a s s u m p t i o n s of e q u a l - s i z e d a i r - b u b b l e s and their a r r a n g e m e n t in a s i m p l e cubic l a t t i c e are s e l d o m s a t i s f i e d in p r a c t i c e . A m o r e r e a l i s t i c v a l u e of the s p a c i n g f a c t o r may be o b t a i n e d by an a u t o - c o r r e l a t i o n a n a l y s i s of the data c o l l e c t e d to o b t a i n the air c o n t e n t by the m o d i f i e d p o i n t - c o u n t m e t h o d (5). The a b o v e u n c e r t a i n t y is of m i n o r t a i n t y a b o u t the " s p a c i n g factor"

757

i m p o r t a n c e . The m a j o r u n c e r a r i s e s from the r e f u t a t i o n of

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the s a t u r a t e d f l o w h y d r a u l i c p r e s s u r e h y p o t h e s i s itself. Powers and H e l m u t h h a v e s h o w n t h a t the s a t u r a t e d f l o w h y d r a u l i c p r e s s u r e m e c h a n i s m p l a y s a m i n o r r o l e in b r e a k d o w n of c o n c r e t e s a m p l e s in freezing environments (6). H e l m u t h has f u r t h e r c o n c l u d e d t h a t the w a t e r - f l o w to the a i r - b u b b l e s is a l w a y s u n s a t u r a t e d (7). In the c a s e of u n s a t u r a t e d flow, the h y d r a u l i c p r e s s u r e is i n d e p e n d e n t of the p a t h l e n g t h of w a t e r - f l o w . In t h i s c a s e the s p a c i n g f a c t o r l o s e s all its p h y s i c a l a n d s c i e n t i f i c r e l e v a n c e . If a b o v e is c o r r e c t , t h e n the p r e s e n t - d a y e m p h a s i s on the s p a c i n g f a c t o r is h a m p e r i n g the d e v e l o p m e n t of o t h e r m o r e m e a n i n g f u l m e a s u r e s by d r a w i n g a t t e n t i o n a w a y f r o m t h e r e a l n e e d s . I w o n d e r if o t h e r w o r k e r s h a v e a n y v i e w s on t h i s s u b j e c t . It s h o u l d be u n d e r s t o o d that I am not t r y i n g to c a s t d o u b t on the b e n e f i c i a l e f f e c t of air e n t r a i n m e n t ; I a c c e p t t h a t as p r o v e n b e y o n d d o u b t . T h e o n l y p u r p o s e of t h i s d i s c u s s i o n is to a s c e r t a i n if the s p a c i n g f a c t o r is s t i l l a r e l e v a n t m e a s u r e . References !.

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H e l m u t h - P r o c . 4th 829-833. Washington,

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