Admixtures in Concrete

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Admixtures in Concrete

Ser THl N2lt2 no. 181 c . 2 -6!U\t

ri'gi'lir.--mry+

NATIONAL RESEARGH COUNCIL CANADA

DIVISION OF BUILDING RESEARCH

ADMIXTURES IN @NCRETE by

E . G . S w e n s o n

Paper presented to the Annual Meeting of the O n t a r i o R e a d y M i x e d C o n c r e t e A s s o c i a t i o n ,

held in Ottawa, March L964

A N A I Y Z E D

by

,lr E . G . S w e n s o n

The extent to which adrnixtures are used in concrete has g r e a t l y incrbased during the last l0 or I5 years. A g o o d e s t i r n a t e would be that well over ?5 per cent of the concrete placed in Canada ln the last year contalns an adrnixture, and in sorne cases rnore than o n e . A l t h o u g h rnost of these lnstances involve air-entraining a g e n t s , t h e r e h a s b e e n a n l n c r e a s i n g u s e o f w a t e r - r e d u c e r s , r e t a r d e r s ,

a c c e l e r a t o r s , d l s p e r s i n g a g e n t s , a n d o t h e r s .

T h e r e i s n o q u e s t l o n a s t o t h e e f f e c t l v e n e s s o f p r o p e r l y e n t r a l n e d air in improving t h e r e s i s t a n c e o f c o n c r e t e t o f r o s t a c t i o n and to de-icing salts. The general use of the rnany other adrnixture t y p e s h a s , h o w e v e r , b e e n t h e s u b j e c t o f c o n s i d e r a b l e c o n t r o v e r s y . There are those who belleve that adrnixtures should be handled like d r u g s o r p r e v e n t l v e r n e d i c i n e s , t o b e u s e d o n l y i n r a r e a n d s p e c i a l

sltuations, and then only when each case has been expertly diagnosed a4d prescribed for. On the other hand, there are those who rnaintaln that the introduction of adrnixtures represents a technlcal advance in t h e b a s l c c o r n p o s i t i o n o f g o o d o o n c r e t e , r n u c h t h e s a r n e a s s p e c i a l ingredients are incorporated ln rnetals to produce alloys whose properties constitute an irnprovernent over those of the orlglnal rnaterlal. * R e s e a r c h O f f i c e r , I n o r g a n i c M a t e r i a l s S e c t i o n , R e s e a r c h , N a t i o n a l R e s e a r c h G o u n c i l , O t t a w a , Division of Building Canada.

-z-P u b l i s h e d research literature o n the subject is extensive but, unfortunately, there are so many cornpllcatlng factors that the a v e r a g e p r o d . u c e r o r u s e r o f c o n c r e t e c a n b e c o r n e h i g h l y c o n J u s e d . W h a t appears to be urgently needed is practical g u l d a n c e l n d e c l d i n g whether an adrnixture is required, what the risks are, and what can be done about thern.

In this paper an atternpt ls rnade to clarify sorne of these g u e s t i o n s , a s a n a l y t t c a l l y a s p o s s l b l e , s o t h a t a u s e r o r p r o d u c e r o f concrete can rnake decisions with rninirnurn risk and cost. The following rernarks apply only to ordinary concrete and do not apply to the use of adrnixtures ln concrete block and brick rnanufacture and in lightweight concrete.

CONCRETE WITHOUT ADMIXTURES

Before considering the role of adrnbrtures, one rnlght well r e v l e w b r t e f l y the steps in rnaking concrete, i t s p r o p e r t i e s , h o w t h e s e a r e a c h i e v e d , and whether they are adequate.

S k n p l e 4 - i n g r e d l e n t c o n c r e t e i s r n a d e f r o r n c e r n e n t r w a t e r , s a n d , and coarse aggregate. T h e rnain steps leading to the flnal p r o d u c t are:

I . S e l e c t i o n o f g o o d p r i r n a r y i n g r e d i e n t s . Z . P r o c e s s i n g a n d s i z l n g o f a g g r e g a t e s . 3. Storing and handling of lngredients. 4 . D e t e r r n i n i n g p r o p e r rnix proportlons. 5. Mixing.

6 . T r a n s p o r t i n g a n d d i s c h a r g i n g . 7 . P l a c i n g and cornpacting. 8 . C u r i n g .

Q u a l i t y c o n t r o l o f e a c h o f t h e s e o p e r a t i o n s c a n b e achieved b y use of specifications, r n e t h o d s o f t e s t , and recornrnended practlces, a s p r o v i d e d by Canadlan Standards Association, A r n e r i c a n S o c i e t y for T e s t i n g and Materlals, a n d t h e A r n e r i c a n C o n c r e t e Institute.

T h e irnportant properties r e q u i r e d i n the plastlc state arel g o o d workability, l o w b l e e d i n g characteristics, l o w r a t e o f s t l f f e n l n g , a reasonable delay before hardening begins, low tendency to segregatlon, a n d g o o d finishing characteristics. F a c t o r s that influence these properties t o a greater or Iesser extent are: particle shape, grading, and absorptlon

c h a r a c t e r i s t i c s _{o f the aggregates; chernical cornposition and fineness}

of the cernent; air content; and the proportions and ternperature of the

rnixture.

T h e i r n p o r t a n t p r o p e r t i e s p e r t a i n i n g to the hardened state a r e : cornpressive, f l e x u r a l a n d b o n d strength; dirnenslonal stability, a b s o r p t i o n and perrneability, a n d d u r a b l l i t y t o f f o s t a c t i o n , d e - i c i n g s a l t s , and other chernicals. T h e s e are achieve,d in the requlred degree b y control of the sarne factors that influence the plastic properties, p l u s lnherent soundness of rnaterials, p r o p e r p l a c i n g and cornpactlon, a n d adequate curing and protection at early ages.

T o achieve these properties, a n d t o o b t a l n a htgh quality p r o d u c t at a reasonable cost require a high degree of technical

rrknow-howrr _{and ot ganization.}

Two inherent problerns should be noted at this point:

( 1 ) unlike pre-fabricated p r o d u c t s , t h e r n a n u f a c t u r e of concrete r f i n sltutr is rnofe often than not split between the producer, e.g., r e a d y - r n i x s u p p l i e s , a n d the contractor w h o i s responslble for placing a n d curing; and (Zl a defective product is often difficult to detect untll

after placernent, _{and then its weakness rrray not show up for rnonths}

4

-P r o b l e r n s w i t h t h e p l a s t i c a n d t h e h a r d e n e d s t a t e s o f c o n c r e t e c a n u s u a l l y b e t r a c e d t o f a i l u r e t o r n a l n t a i n n o r r n a l g o o d p r a c t i c e , c o n t r o l , a n d o t g a n i z a t i o n a t s o r n e p o i n t i n t h e p r o c e s s i n g . A n exarnple is plastic shrinkage cracking which is generally agreed t o b e t h e r e s u l t o f r a p i d l o s s o f w a t e r b e f o r e t h e c u r i n g w a t e r c a n b e a d d e d . F a i l u r e to cornpact concrete adequately can accentuate this p r o b l e r n .

As with any other rnaterial, even good concrete for

c o n s t r u c t i o n p u r p o s e s rrray have sorne lirnitations l n r e s p e c t o f f i r e r e s i s t a n c e , t h e r r n a l c o n d u c t i v i t y , t e n s i l e a n d f l e x u r a l s t r e n g t h s , a c o u s t i c a l . p r o p e r t i e s , s h r i n k a g e , c r e e p , a b r a s i o n , e x t e n s l b i l i t y , w e i g h t , o r d u r a b l l i t y . E x c e p t for durability, a n d p o s s t b l y s t r e n g t h , t h e s e p r o p e r t i e s a r e n o t f u n p r o v e d s i g n i f i c a n t l y b y the use of

adrnixture s.

TYPES OF ADMIXTURES AI\D THEIR EFI.ECTS

C o n c r e t e adrnixtures can be divlded into types or classes, depending on the specific effects they are expected to contribute ln t h e h a n d l i n g c h a r a c t e r i s t i c s o f p l a s t i c c o n c r e t e o r t h e q u a l l t y o f t h e h a r d e n e d p r o d u c t . T h e r n a i n types arel I . a i r - e n t r a i n i n g a g e n t s . Z . a c c e l e r a t o r s 3 . r e t a r d e r s . 4 . w a t e r r e d u c e r s . 5 . d i s p e r s i n g o r w e t t l n g a g e n t s . 6. rnineral adrnixtures . 7 . i n h i b i t o r s o f alkali-aggregate r e a c t i o n . 8 . d a r n p - p r o o f i n g a n d w a t e r p r o o f i n g a g e n t s . O t h e r s include hardeners, g r o u t i n g a i d s , a n d g a s - f o r r n l n g a g e n t s .

Air -e.ntrainlng Agents

A f t e r sorrle 20 years of laboratory r e s e a r c h a n d f l e l d p e r f o r r n a n c e , i t h a s b e e n e s t a b l l s h e d w i t h o u t a n y d o u b t t h a t p r o p e r l y

e n t r a i n e d a i r w i l l g r e a t l y i r n p r o v e t h e r e s i s t a n c e o f concrete to frost a c t i o n . E v e r s the high-strength c o n c r e t e s a r e i r n p r o v e d . A l r - e n t r a l n e d c o n c r e t e l s especially effective for such severe casea as sidewalks e x p o s e d to freezing and thawing and de-icing ealts. A i r - e n t r a l n r n e n t

is usually accornpanied by sorne irnprovernent ln workablllty and

r e d u c t i o n i n w a t e r r e q u i r e r n e n t , a n d i n b l e e d i n g . I t s g r e a t w e a k n e s s

lies in the reduction in strength whlch is norrnally not cornpensated

f o r b y the reduced water requirernent. T h i s i s e s p e c l a l l y t r u e o f h t g h -s t r e n g t h r n i x e -s . O v e r - d o s a g e s c a n b e v e r y d e t r i m e n t a l .

A c c e l e r a t o r s

The only accelerator whlch is generally avallable ls calclurn

c h l o r i d e . I t accelerates t h e h y d r a t l o q o f c e r n e n t , t h e r e b y i n c r e a s i n g t h e r a t e o f h a r d e n i n g . I t i s u s e d I n s i t u a t l o n s w h e r e e a r l y u e e o f a

s t r u c t u r e i s r e q u i r e d , e a r l y s t r i p p i n g o f f o r r n s i s n e c e s s a r l r a n d l n

cool weather when hydration is norrnally slow. Sorne undesirable

s i d e e f f e c t s h a v e b e e n observed: incfease ln initlal setting or

stiffening, lower durabilityr and lower ultlrnate strengths. There

r n a y also be a considerable d i f f e r e n c e i n response with different

c e r n e n t s , and even srnall over-dosages c a n b e v e r y d e t r l r n e n t a l t o t h e c o n c r e t e . C a l c i u r n chlorlde in concrete can produce serlous corrosion o f p r e s t r e s s i n g s t e e l . T h e e f f e c t o n reinforcing s t e e l l s s t l l l a r n a t t e r o f sorne uncertainty. C a l c i u m c h l o r i d e should not be used with

embedded alurninurn conduits.

R e t a r d e r s , W a t e r - R e d u c e r s a n d D l s p e r s i n g A g e n t s

T h e s e a r e c o n s i d e r e d t o g e t h e r b e c a u s e a n a g e n t d e s l g n a t e d t o b e l o n g to one of these types usually has, or ls cla{rned to have, p r o p e r t i e s o f o n e o r r r r o r e o f t h e o t h e r s .

6

-R e t a r d e r s e x t e n d t h e p l a s t i c p e r i o d , t h a t i s , t h e y d e l a y t h e b e g l n n i n g o f h a r d e n i n g . T h i s p e r r n i t s a l o n g e r p e r i o d d u r i n g w h l c h t h e c o n c r e t e c a n b e v i b r a t e d . T h e y are used rnainly in hot weather w h e n there is risk of early hardening. T h e y are therefore used when l o n g h a u l s o r l o n g w a i t s f o r d i s c h a r g e r n a y b e e n c o u n t e r e d . T h e r a t e

of loss of slurnp is a separate phenornenon which ls not favourably a f f e c t e d to any significant extent by a retarder. A d v e r s e s i d e e f f e c t s sornetirnes encountered arel variability in effect with different cernents a n d r n i x c o r n p o s i t i o n , s e n s i t i v i t y t o t e r n p e r a t u r e , i n c r e a s e l n d r y i n g

s h r i n k a g e w i t h s o r n e a g e n t s , s p o n g y c o n s i s t e n c y w h i c h r n a k e s f i n i s h i n g d i f f i c u l t , a n d i n t r o d u c t i o n o f air by sorne agents. O v e r - d o s a g e s c a n b e v e r y d e t r i r n e n t a l .

' W a t e r

r e d u c e r s a r e u s e d p r i r n a r i l y t o o b t a i n h i g h e r s t r e n g t h s through reduction in water-c€rn€nt ratio and consequently are often u s e d to save cernent. T h e s e agents are frequently r e t a r d e r s a s w e l l and are sornetirnes cornpounded with accelerators to elirninate

r e t a r d i n g e f f e c t s , a n d i n s o r n e c a s e s t o p r o d u c e w a t e r - r e d u c i n g

a c c e l e r a t o r s . T h e i r s i d e e f f e c t s a r e t h e r e f o r e s i r n i l a r t o r e t a r d e r s .

D i s p e r s i n g a g e n t s a n d wetting agents are used to irnprove w o r k a b i l i t y " T h e i r l i r n i t a t i o n s a r e s i r n i l a r t o t h o s e o f r e t a r d e r s a n d w a t e r r e d u c e r s

-M o s t of these agents are clairned to rninirnize segregation a n d i r n p r o v e workability. U n l e s s they entrain alr, or are used with a n air-entraining a g e n t , t h e s e a d r n i x t u r e s c a n actually be conducive t o h a r s h n e s s and segregation, a n d d e f i n l t e l y p r o r n o t e b l e e d i n g .

Mineral Adrnixtures

T h e s e include fly ashes and various a r e u s e d in partial replacernent of cernent, or generally have sorne cernentitious value, with I a t e r a g e s . T h e y a r e u s e d t o r e d u c e h e a t r i s e

pozzolanic powder s which s a n d a n d c e r n e n t . T h e y s t r e n g t h a d v a n t a g e s a t

reduce expansion due to alkali aggregate reaction, and to save on cernent. Their rnain lirnitation lies in extra bulk storage and handling. T h e s e adrnixtures are inorganic in nature and are therefore less risky t o u s e t h a n t h e o r g a n l c t y p e s . T h e i r u s e r n a y i n c r e a s e w a t e r r e q u i r e -r n e n t s . I n such cases water-reducers a r e s o r n e t l r n e s u s e d .

D a r n p - p r o o f e r s and l&rt6rproofers

T h e s e a r e u s u a l l y s o a p s o r s o a p d e r l v a t i v e s , a s p h a l t e r n u l s i o n s , o r o i l s . T h e y are clairned to decrease perrneabllity a n d abso rption. Calciurn chloride solutions sold under proprletary narnes a r e a l s o u s e d f o r t h i s p u r p o s e .

Considering the risks involved and the questlonable benefits derlved, there is very little doubt that these adrnixturea should not b e used fur norrnal concreting operations, e x c e p t p o s s i b l y w h e r e f u l l g u a r a n t e e l s r n a d e .

F r o r n this brief outllne of the propertles a s s o c i a t e d with the rrrore cornrnon types of adrnixtures, it is evident that their use in

c o n c r e t e lntroduces extra problerns of control. O n e r n u s t b e c o n c e r n e d not only wlth the property for which a given adrnixture is used, but a l s o w i t h the various side effects.

GENERAL PROBLEMS ASSOCIATED WITH ADMIXTURES

T h e r e h a v e b e e n periodic outbreaks of violent controversy o v e r the increasing use of adrnixtures in concrete. T h e rnost recent w a s caused by

" p a p . r ' F in which the author criticized a d v e r s e l y the h i g h - p r e s s u r e s a l e s rnethods of rnany adrnixture rnanufacturers, t h e i r e x a g g e r a t e d clairns, their rnisrepresentations, a n d t h e i r e x p l o l t a t i o n o f architects a n d engineers. A s was to be expected, there was equally

M e r c e r , L . B o y d . W a t c h Those Adrnixtures. A u s t r a l i a n C l v i l E n g i n e e r l n g a n d C o n s t r u c t i o n . V o l . 4 , 5 M a r c h 1 9 6 3 , p . ? O - 2 9 .

8

-sharp reaction frorn sorne adrnixture rnanufacturers and this was p u b l i s h e d . i n t h e s a r n e j o u r n a l a n d e l s e w h e r e .

It is not intended here to discuss all the clairns and counter-c t r a i r n s r n a d e o n t h i s s u b j e counter-c t . l t i s n e c e s s a r y , h o w e v e r , t o o u t l i n e t h e r n a n y g e n e r a l f a c t o r s , i n a d d i t i o n t o t h e s p e c i f l c s i d e e f f e c t s a l r e a d y rnentioned, that are responsi'ble for the confusion and rnisunderstanding t h a t e x i s t s , s o t h a t s o r n e s o r t o f p r a c t i c a l p h t l o s o p h y c a n b e d e v e l o p e d in dealing with the use of adrnixtures.

B r i e f l y t h e s e g e n e r a l p r o b l e r n s a n d i s s u e s a r e a s f o l l o w s :

l . I t h a s b e e n n o t e d t h a t t h e r e a r e s o r n e d o z e n d i f f e r e n t c l a s s e s o f adrnixtures. W i t h i n e a c h c l a s s t h e r e a r e l i t e r a l l y d o z e n s o f proprietary rnaterials. one would guess that in canada there are

sorne 300 proprietary adrnixtures on the rnarket. Selecting a suitable adrnixture is therefore not an easy rnatter.

z. Many of thesg proprietary adrnixtures clairn ralore than one f u n c t i o n , e . g . w a t e r - r e d u c i n g a n d r e t a r d i n g p r o p e r t i e s ' I n sorne cases rnodified adrnixtures are cornpounded frorn rnore than one functional ingredient. Thus one has in effect an adrrlxture wlthin an adrnixture.

3. The great rnajorlty of corrrnercial adrnixtures derive frorn i n d u s t r i a l w a s t e p r o d u c t s o r b y - p r o d u c t s w h i c h h a v e b e e n s u b j e c t e d to varying degrees of refinernent, treatrnent or rnodification. They a r e t h e r e f o r e u s u a l l y v e r y c o r n p l e x i n n a t u r e a n d s u b j e c t t o c o n s i d e r -a b l e v -a r i -a t l o n i n c o r n p o s i t i o n . T h e p r e c i s e f o r r n u l a t i o n o r c o r n P o s i t i o n i s u s u a l l y a corrlpany secret. A n a l y s i s b y a c h e r n i s t i s a l r n o s t

i r n p o s s i b l e i n r n o s t c a s e s b e c a u s e o f t h i s c o r n p l e x i t y , a n d o f t e n i s u s e l e s s i n any case because of frequent change in forrnula.

4 . l m p a r t l a l r e s e a r c h a n d t e s t i n g a g e n c i e s h a v e n o t b e e n a b l e t o undertake evaluation of all adrnixtures on the rnarket for reasons of n u r n b e r , c o r n p l e x i t y , f o r r n u l a c h a n g e s , c o s t s , a n d t h e f r e q u e n t a p p e a r a n c e o f n e w p r o d u c t s . T h e i r test inforrnation i s l i r n i t e d t o o n e o r two specific cernent and aggregate cornbinations or types, and rrsua.lly designate the admixtures by code nurnber.

The rnanufacturerrs technlcal data are naturally, if not always a c c u r a t e l y , s u s p e c t e d o f b i a s . E v e n sorrre of the research papers in t h e l l t e r a t u r e a r e s u s p e c t b e c a u s e t h e a u t h o r s h a p p e n t o w o r k f o r adrnixture rnanufactur e r s.

5 . P u b l i s h e d case histories o f l a r g e - s c a l e u s e and good perforrnance of adrnixtures can be sornewhat rnisleading or questionable. fulany

o f these are wrltten up by persons associated with adrnixture

r n a n u f a c t u r e r s . T h e r e i s a l s o a n u n d e r s t a n d a b l e u n w i l l i n g n e s s t o p u b l i c i z e unsuccessful cases, both on the part of the adrnixture s u p p l i e r and the concrete producer or user- F r e q u e n t l y s u c h r t b a d c a s e s r r are not due to a faulty adrniiture b u t are due to irnproper use. 6 . M o s t a d r n i x t u r e d o s a g e s a r e r e l a t i v e l y s r n a l l a n d a r e t h e r e f o r e d i f f i c u l t t o d e t e c t i n t h e h a r d e n e d c o n c r e t e . I n rnost cases, lt would b e p r a c t i c a l l y i r n p o s s i b l e to distinguish between a norrnal dose and a d o u b l e d o s e .

7. The evaluation of an adrnixture for any given job or plant is necessarlly a rnatter of econorny as well as irnprovernent in quality o f c o n c r e t e . C o r r i p r e h e n s i v e testing to adapt a given adrnixture to a given situation involves initial costs which rnay be prohibitive for t h e s r n a l l e r p r o d u c e r o r u s e r o f c o n c r e t e

-8 . T h e c o n s i d e r a b l e e x t r a c o n t r o l r e q u i r e d w h e n a n a d r n i x t u r e i s used is frequently not achieved. T h e p r o p e r t i e s w h i c h adrnixtures c o n f e r on concrete can vary rnarkedly with wetness of rnix, cernent

1 0

-content, aggregate grading, type of cernent, and length of rnixing. 9 . T h e e s s e n t i a l l y c h e r n i c a l n a t u r e o f t h e e f f e c t s a n d v a r i a t i o n s a s s o c i a t e d w i t h a d r n i x t u r e s i n c o n c r e t e h a s n o t b e e n f u l l y r e c o g n i z e d i n p r a c t i c e . B a c k g r o u n d l n c o n c r e t e t e c h n o l o g y ' w i t h r e s p e c t t o r n a t e r i a l s r a t h e r t h a n d e s i g n , a n d a k n o w l e d g e o f t h e p h y s i c a l - c h e r n i c a l r e l a t i o n s h i p s , a r e e s s e n t i a l . t O b o t h d i a g n o s i s a n d p r e s c r l p t i o n . 1 0 . R e l i a b l e adrnixture rnanufacturer6 d o , h o w e v e r ' h a v e c o n c r e t e experts on staff who can and will advise cornpetently on request.

SPECIFICATIONS AND TESTS FOR ADMIXIU RES

B e f o r e the developrnent of cornprehensive s p e c i f i c a t i o n s a n d rnethods of test for adrnlxtures, the selection of an adrnixture was based rnainly on lirnited field experience on sorne job or ln sorne plant. l f i t p r o v e d satisfactory, i t w a s thereafter d e s i g n a t e d i n t h e s p e c i f i -c a t i o n b y i t s p r o p r i e t a r y n a t n e . T h i s i s still true of rrrany speciflcations i n use today.

Unfortunately, this practice has led to discrirnination and c e r t a i n abuses. S o r n e r n a n u f a c t u r e r s w h o s e p r o d u c t s w o n e a r l y

a c c e p t a n c e a n d w e r e n a r n e d i n s p e c i f i c a t i o n s f o r r n a j o r p r o j e c t s h a v e been accused of exploiting this advantage unfairly by rnaintaintng high p r i c e s for their products and by using their reputation to expand

aggresively in the rnarketing of other adrnixtures they have developed. It has been clairried that this practice has rnade it difficult for newer a n d i r n p r o v e d p r o d u c t s t o w i n a c c e p t a n c e .

O v e r the years a rash of single or lirnited tests have a p p e a r e d w h i c h o f t e n w e r e d e v e l o p e d i n t h e f i r s t i n s t a n c e b y a

rnanufacturer to show his product in a favourable light. It should be also stated that the evolution of specifications and rnethods of test to t h e p r e s e n t rnore satisfactory s t a t e h a s b e e n t o a c o n s i d e r a b l e e x t e n t

the result of research and support by responeible adrnixture manufacturers.

ASTM Cornmittee G-9 on Goncrete and Concrete Aggregatee developed lts first speclfication for alr-entratrirg agents about 25 years ago. Although it was designed as a general speclflcp,tion, for several yeats it included a footnote narning three or four proprietary producte as the only ones which had at that tlrre been fufly subjected to tests and found to conforrn.

Since then ASTM C-260, a tentatlve specificatlon for Air-entraining Adrni:<tures for Goncrete, has refralned frorn rnentloning narnes, It now stlpulates that concrete made with an air-entralnlng agent rrust rneet certain regulrernents for bleedlng, cornpresslve etrength, fle:nrral strength, resistance to freezing and thawlng, bond strength, and volume change of concrete. The methods of test for each of these properties are covered by ASTM C-233, a tentative rnethod of testfurg air-entraitrhg adrnixtures for concrete.

This ls eSsentlally a perforrneurce specification and is not concerned with the source, nature, or chernlcal cornpositlon of the adtnixhrre. It is fuatended to ensure that an alr-entrahfutg agent will do specifi cally what ls clairned for it and, of equal irnpctance, that other propertles of concrete will not be adversely affected.

Two other speclfications in use today are ASTM C-350, a tentative specification for Fly Ash for Use as zur Adrnlxture in

Portland Gement Goncrete, and ASTM C-402, a tentdtive specificatlon for Raw or Galclned Natur aL Pozzolans for Use ae Admlxtures in Portland Gement Concrete. These are similar in intent to C-260 but contaln certain chernical as well b,s physical requirernents.

I Z

-T h e m o s t r e c e n t d o c u r n e n t i s A S -T M C - 4 9 4 , a t e n t a t i v e

s p e c i f i c a t i o n f o r G h e r n l c a l A d r n l x t u r e s f o r C o n c r e t e . I t covers water-r e d u c e water-r s , r e t a r d e r s , a c c e l e r a t o r s , a n d c o r n b i n a t l o n s o f w a t e r

-reducing and retarding agents, and water-reducing and accelerating

agents. The specifications and rnethods of test are patterned after those of. G-?60. A tlrnit is placed on the specific effect clairned for a given type of adrnixture and, at the salne tirne, it requlres that it does not a d v e r s e l y a f f e c t o t h e r p r o p e r t i e s o f t h e c o n c r e t e s u c h a s w a t e r

requirernent, tirne of set, cornpressive and flexural strengths, bond strength, volurne change, durability to freezing and thawing, and bleeding, A footnote cautions that the tests should, if possible, be rnade using the cernent, aggregates, adrnixture, and rnix to be ernployed on the job.

These speciflcatlons, coverfurg the rnost cornrnon types of adrnixtures, represent a great stride forward ln providlng a fair evaluation of cornrnercial products. They ensure that an adrnixture will perfornr according to clairn without other harrnful effects, thus providing full protection to the user; they effectively show up useless, inferior or harmful products; and they provide equal opportunity for all products regardless of nature, source, and chernical cornposltlon, and thus greatly sirnplify the problern for the user. Perhaps their lone dlsadvantage is the cost of carrying out the cornplete series of t e s t s .

The Ganadian Standards Association has not yet developed sirnilar specifications for admixtures.

A S T M , h a s s o f a r n o t d e v e l o p e d c o r r e s p o n d i n g s p e c l f i c a t i o n s for certain other adrnixture types, such as waterprooflng and damp-proofing agents, hardeners, pigrnents, expanding agents and wetting agents. Some of these types are of questionable value and a rnajor

difficulty is the developrnent of tests that will yield significant inforrnation.

APPLICATION TO INDIYIDUAL PLANTS OR IVIATERIALS

T h e f o r e g o i n g g e n e r a l s p e c i f i c a t i o n s p r o v l d e a s a t i s f a c t o r y

means of elirninating uselesd or harmful adrnlxtures and for developing

p e r f o r r n a n c e r e c o r d s f o r i n d l v i d u a l u s e r s o r p r o d u c e r s . T h e r e a r e s t i l l p r o b l e r n s i n v o l v e d w i t h t h e a c h i e v e r n e n t o f a c c e p t a b l e t e s t r e c o r d s , c o s t s of testing, and the provision of adequate plant and job facllitles a n d c o n t r o l .

It would seern logical that an adrnixture rnanufacturer should

p r o v i d e adeguate test inforrnation on his product, based on a general s p e c i f i c a t i o n s u c h a s n o w p r o v i d e d b y A S T M f o r the more cornrnon

adrnixture types, and covering in part sorne of the rrrore irnportant

v a r i a b l e s , f o r exarnple, high and low water cernent ratios, h i g h and

low cernent contents, and over-dosage. Although sorne adrnixture

rnanufacturers have their own excellent testing facilities and cornpetent

c o n c r e t e technologists, i t i s o b v i o u s t h a t s u c h i n f o r r n a t i o n s h o u l d b e

provided by a reliable independent testing laboratory at the exPense

of the adrnixture rnanufacturer. The availability of such certified

inforrration would reflect the confidence of the adrnixture supplier in

h i s p r o d u c t .

T h e concrete user or producer would still be faced with

applying the sarne tests on his own particular rnaterials and with hls

o w n p a r t i c u l a r o p e r a t i o n s . H e rnust know what dosages to use for b e s t results and what facillties a n d c o n t r o l s h e r n u s t p r o v i d e . A l t h o u g h

s o m e l a r g e o p e r a t o r s h a v e t h e i r o w n c o r n p e t e n t t e s t f a c i l i t i e s a n d

can tolerate the cost involved, sorne adrnixture rnanufacturers are

1 4

-s u r p r l -s i n g t h a t i n s o r n e c o - o p e r a t i v e e v a l u a t i o n p r o g r a r n s o f t h i s n a t u r e , t h e c o s t t o t h e c o n c r e t e p r o d u c e r h a s b e e n r n o r e t h a n c o r n p e n s a t e d b y t h e d i s c o v e r y a n d c o r r e c t i o n o f w e a k n e s s e s l n h i s o r i g i n a l o p e r a t i o n s . A t l e a s t t h i s r n u c h c a n b e s a l d f o r t h e a d r n i x t u r e manufacturer who is willtng to provide help in individual cases' It m u s t a l s o b e r e r n e r n b e r e d t h a t t h i s a d d i t i o n a l i n f o r r n a t i o n c a n b e u s e d to advantage by the adrnixture rnanufacturer in dealing with srnaller p l a n t s o r j o b s w h e r e c o s t s o f c o r n p r e h e n s i v e t e s t i n g w o u l d b e p r o h i b i t i v e .

Adapting an adrnixture to specific rnaterial,s in a plant should not be lirnited to laboratory tests but should lnclude field trials

f o l l o w e d b y s u i t a b l e t e s t s c a r e f u l l y c a r r i e d o u t . o n e o f t h e r n o s t irnportant requirernents is uniforrnity of propertles of both the plastic and hardened concrete. An adrnixture which is difficult to control in t h i s r e s p e c t c a n b e a s o u r c e o f c o n t i n u i n g t r o u b l e t o t h e c o n c r e t e p r o d u c e r o r u s e r . O t h e r s p e c i f i c e f f e c t s n o t n o r r n a l l y c o v e r e d b y s p e c i f i c a t i o n s s h o u l d b e n o t e d , s u c h a s , f o r e x a r n p l e , f i n i s h i n g p r o p e r t i e s . S o r n e a d r n i x t u r e s a r e s u p p l i e d i n s o l i d f o r r n , o t h e r s a s l i q u i d s . P o w d e r s , f l a k e s , a n d p a s t e s a r e o b v l o u s l y d l f f i c u l t t o d i s t r i b u t e h o r n o g e n e o u s l y i n a i n i x . I t i s p r e f e r a b l e t o p r e - m i x t h e w a t e r - s o l u b l e t y p e , s o l i d o r l i q u i d , w i t h t h e g a u g i n g w a t e r . s o r n e , even in the liquid state, forrn colloidal suspensions with water which appear to be stable but which may result in coagulation or settling in a t a n k . I n s u c h c a s e s t h e s u s p e n s i o n s h o u l d b e c o n t i n u o u s l y s t i r r e d '

D i s p e n s e r s o f a d r n i x t u r e s i n a p l a n t r n u s t b e f r e q u e n t l y

c a l i b r a t e d a n d r n u s t b e f o o l p r o o f . O v e r - d o s a g e s c a n l e a d t o s e r i o u s f i e l d p r o b l e r n s . T h i s i s p a r t l c u l a r l y c r i t i c a l w i t h r e t a r d e r s w h e r e delay in hardening in cool weather, for example, has been knovn to

e x t e n d t o r n o r e t h a n a w e e k .

Another problern, not so well known but sornetirne s of critical irnportance, is the effect of one adrnixture upon another under c e r t a i n c o n d i t i o n s , e . g . , air-entraining a g e n t p l u s r e t a r d e r . I f t h e s e are introduced into the rnix at the sarne tirne, or are pre-rnixed

t o g e t h e r i n . t h e r n i x i n g w a t e r , t h e y s o r n e t i r n e s r e a c t o r c o : p r e c i p l t a t e , t h e r e b y p r o d u c i n g u n u s u a l a n d v a r i a b l e e f f e c t s . T h i s c a n b e a v o i d e d by introducing the two adrnixtures at different tirnes in the sequence of batching and. rnixing. lf possible, however, such cornbfuratlons should b e avoided.

In the case of alr-entrainrnent it should be rernernbered that a satisfactory arnount of alr as deterrnined by an air rneter does not necessarily mean that good durability will naturally follow.

Certain natural sands and sorne lignosulphonate adrrixtures wlll e n t r a i n c o n s i d e r a b l e a r n o u n t s o f a l r . ' T h e c o n c r e t e p r o d u c e r m a y b e d e l i g h t e d t o f i n d t h a t t h e d o s a g e o f a i r - e n t r a i n i n g a g e n t r e q u i r e d t o give the specified arnount of alr is srnall. He rnay be rnisled; the entrained air he is getting rnay not be conducive to good durability. T h e r e a l t e s t s h o u l d b e r n a d e o n t h e h a r d e n e d c o n c r e t e b y t h e s o - c a I l e d linear-traverse method in whlch the size and spacing of alr bubbles i s d e t e r m i n e d m i c r o s c o p i c a l l y .

s o r n e a d r n i x t u r e s a r e c l a i r n e d t o p o s s e s s b u l l t - i n s a f e t y f a c t o r s t h a t p e r r s r i t c o n s i d e r a b l e o v e r - d o s a g e w i t h . n o l l l e f f e c t s . T h t s

s h o u l d n o t b e t a k e n f o r g r a n t e d .

The user of an adrnixture should set up a rigid speclfication for the product itbelf, just as he ncnrnally does for any other ingredient o f c o n c r e t e w h i c h h e p u r c h a s e s f o r h i s o p e r a t i o n s . E a c h shiprnent should be tested for uniforrnity. The adrnixture suppller should be required to give notice of rnodifications or forrnula changes and should, i n Euch instances, assurne responsibility f o r r e t e s t i n g .

l 6

-T h e concrete producer should lnforrn his custorner w h e t h e r a n a d r n i x t u r e i s p r e s e n t i n t h e c o n c r e t e . I t i s t r a d l t i o n a l t h a t t h e r n a n u f a c t u r e r o f a finished product can, if he wishes, keep h l s f o r r n u l a s e c r e t . P l a s t l c concrete is not a flnlshed product, however, and subsequent operations rnay involve difficulties if this inforrnation is not available.

The rnost irnportant consideration in the adaptatlon of an adrni-:<ture to plant or job operations is to obtain the advice and supervision of a cornpetent and irnpartial expert on concrete

technology. Norrnally it pays to have a good doctor to diagnose your i l l s and prescribe a r e r r r e d y l f o n e i s n e e d e d . A t h o r o u g h c h e c k - u p , rnedical or technical, rnay reveal a situation where adjustrnent of o n e l s p r e s e n t habits will be all that is necessary. I t s h o u l d n o t b e forgotten that an adrnlxture introduces a fifth ingredient into concrete, and, no rnatter how effective lt is, the problerns of control are

i n c r e a s e d .

THE OUESTION OF ECONOMY

D . L. Bloern of the National Ready Mixed Concrete

A s s o c i a t i o n , w h o h a s w r i t t e n s e v e r a l excellent papers on adrnixtures, d i v i d e s these rnaterials into two categories: the rrcurative typesrr which are used to irnprove the quality of concrete in the plastic state, the hardened state, or both; and the rreconorny typesrr which are usually u s e d to save cernent. H e r e f e r s t o t h e l a t t e r a s l r c e r n e n t s t r e t c h e r s . r l

W a t e r - r e d u c i n g a d r n i x t u r e s a r e f r e q u e n t l y u s e d t o p e r r n l t a reduction in rnixing water and, consequently, in cernent content, to a c h i e v e a glven strength. F l y ash or pozzolans are sornetirnes used l n rnass concretes, i n some cases ln conjunction with water-reducing a g e n t s . L a r g e j o b s with,precision c o n t r o l l e d b a t c h i n g o n t h e j o b c a n

effect substantial savings in this way where resPonsibility for quality c o n t r o l i s n o t d i v i d e d . T h e g e n e r a l u s e o f s u c h a d r n i x t u r e s i n r e a d y -r n i x o p e -r a t i o n s i s o b v i o u s l y s u b j e c t t o e x t -r a p -r o b l e -r n s a n d -r i s k s .

Retarders of the llgnosulphonate and hydroxylated carboxylic

a c i d t y p e s u s u a l l y r e d u c e w a t e r r e q u i r e r n e n t s a n d i n c r e a s e s t r e n g t h s f o r a given cernent content. T h e r i s k o f u s i n g t h e s e a s r r c e r n e n t

s t r e t c h e r s r r i s a s s o c i a t e d r n a i n l y w i t h t h e v a r i a b l l i t y i n t h e i r e f f e c t s

with varlations in rnix properties. In the leaner rrixes it is particularly

knportant to have enough cernent to produce a dense, irnperrneable

r n a s s .

A i r - e n t r a i n i n g a g e n t s g e n e r a l l y r e q u i r e a r n i x p r o p o r t i o n u s i n g I e s s s a n d . S o r n e e c o n o r n y i s e f f e c t e d l n s u c h c a s e s . A d r n i x t u r e s

that irnprove workability rnay facilitate placernent of concrete thus

reducing an irnportant cost itern.

A factor often overlooked is the effect of an admixture on

yield. The difference rnay be quite large and here the cost would be

d i r e c t l y a f f e c t e d . A sirnple test can be used to check this effect.

Using an adrnixture prirnarily to save cernent ls not

recornrnended practice as there are Inany risks involved. At best

such savings in cost will constitute only a srnall part of the total

cost of the adrni:<ture ltself plus the extra costs of handling, testing,

and batching.

T h e p r o d u c e r o f c o n c r e t e c a n p r o b a b l y a c h i e v e r n u c h

greater econorny by sirnply irnproving the quality of his rnaterlals and

the efficiency of his operations. Refinernents in quarrying, crushing,

sizing, stockpiling and handling of. aggregates, _{in rnix ProPoitioning,}

in quality control and uniforrnity, go hand in hand wlth rnore

econornical production. Greater efficiency in the dispatching office

1 8

-Ready-rnix producers rnight well consider classiflcatlon o f c o n c r e t e , f o r p r i c e p u r p o s e e , i n t o t h r e e r n a i n c a t e g o r i e s ! ( a ) c o r n r n g n c o n c r e t e s - f o r o r d l n a r y u n e x P o s e d s t r u c t u r e s ;

( b ) p r e s c r i p t i o n c o n c r e t e s - f o r s p e c i a l p u r p o s e s b u t h a v l n g s t a n d a r d p l a n t rnixes, e.g. alr-entralned c o n c r e t e , r e t a r d e d c o n c r e t e , ( s u c h concretes should justlfy a ssrall prerniurn); atta (c) spectal forrnula c o n c r e t e s - w h e r e t h e c u s t o l n e r r e q u l r e s h i s o w n r n i x d e s i g n t o b e u s e d ( s u c h c o n c r e t e s s h o u l d a l s o j u s t t f y a p r e r n l u r n ) . T h t s s y s t e r n ,

or sorne rnodificatlon depending on the cornpetitive situatlon, should enable a producer to rnake reasonable proflts without resortlng to means which may irnpalr the quality of his product.

Wtth the rrethods now avallable for ratlng rnost adrnixture types, it is possible to deterrnlne the least expensive product whl'ch will rneet requirements.

In general, where adrnixtures are used, they should be e r n p l o y e d f o r s o - c a l l e d r r c u r a t l v e r l P u r p o s e s o n l y , e x c e p t p o s s l b l y on large jobs where optlrnurr and single control can be exerclsed through all operations frorn the selection of rnaterials to the final job curing. Onty in such casea can the use of rrcenrent-stretchersrl b e j u s t i f i e d . T h e d e g r e e o f p r e c i s i o n c o n t r o l r e q u l r e d i s n o r r n a l l y not obtainable in ready-rnix operations.

SITUATIONS ASSOCIATED WITH THE USE OF ADMIXTURES AND THE ALTERNATIVES

Assurning that sorne situations exist or occur where adrnixtures can have sorne benefit, and assurning that such benefits have been proved by tests for rnaterials and operatlons for a given plant or job, there is still the question of recognizing such situations and belng fully aware of alternatlves which rnay be equally effective,

l e s s c o s t l y , a n d l e s s l i k e 1 y t o i n t r o d u c e c o n t r o l p r o b l e r n s .

In the following an atternpt is rnade to classify the conditions and situations which call for special rneasures and where adrnixtures o r alternative r n e a s u r e s a r e r e q u i r e d .

l . C l i m a t i c C o n d i t i o n s

I n rnost parts of Canada concrete is subjected to cycles of f r e e z i n g and thawing. A i r - e n t r a l n e d c o n c r e t e h a s p r o v e d t o b e r n u c h r n o r e r e s i s t a n t t o s u c h e x p o s u r e t h a n n o n - a l r - e n t r a i n e d c o n c r e t e , a n d s h o u l d t h e r e f o r e j u s t l f y t h e g e n e r a l u s e o f a i r - e n t r a i n r n e n t f o r e x p o s e d e l e r n e n t s . P o s s i b t e exceptlons are high-strength, d e n s e c o n c r e t e s above ground which are not exposed to wettirig.

Sidewalks and pavenaents subjected to de-iclng salts derive

p a r t l c u l a r b e n e f i t f r o r n t h e u s e o f e n t r a i n e d a i r . I n s u c h s e v e r e c a s e s t h e r e l s r n o r : n t i n g e v i d e n c e t h a t a b o u t 7 p e r c e n t a i r i s n e c e s s a r y

r a t h e r than the norrnally s p e c i f i e d 5 p e r c e n t . T h e r e i s e v i d e n c e a l s o t h a t , following curing, a drying perlod before exposure to frost and d e - i c i n g salts further increases resistance to scaling. A d e q u a t e a g i n g b e f o r e exposure is also beneficial.

T o the authorls knowledge, there is no alternative t o a i r -e n t r a i n r n -e n t u n d -e r s u c h c l k n a t i c e x p o a u r e s . T h e u s e o f a i r

-entraining agents solely for workabtlity purposes in unexposed cernents,

s u c h a s f l o o r s l a b s , i s n o t n o r r n a l l y j u s t i f i a b l e o r d e s i r a b l e .

? . S e a s o n a l C o n d i t i o n s

During cold weather concreting the rate of hydration of

cernent and, consequently, the rate of strength gain rnay be unduly s l o w . A c c e l e r a t o r s s u c h a s c a l c i u r n c h l o r i d e a r e o f t e n u s e d i n s u c h s i t u a t i o n s . A l t e r n a t i v e s w h i c h c a n b e e q u a l l y e f f e c t i v e , a n d u s u a l l y

e 0

-safer, are ad.equate protection by cover and./ot heatingr and the u s e o f h i g h - e a r l y - s t r e n g t h c e r n e n t .

In hot surnlrrer weather settlng and hardenlng are accelerated. Problerns rnay occur with long hauls and delays in discharge, with the alrnost inevitable result that extra water is added to the rnix. Retarders are now being used extensively, and in rnany i n s t a n c e s i n a l l c o n c r e t e s , t o d e l a y s e t t i n g a n d e a r l y h a r d e n i n g . A l t e r n a t i v e s a r e p r o t e c t i o n o f a g g r e g a t e s a g a i n s t e x c e s s i v e t e r n p e r a -t u r e s , a v o i d i n g u s e o f t r h o t c e r n e n t r r , p r e - c o o l i n g , a n d i r n p r o v e d dispatch tirning. 3 . S p e c i a l J o b R e q u i r e r n e n t s E x c e s s i v e h e a t r i s e i s a s p e c i a l p r o b l e r n w l t h r n a s s

c o n c r e t e s . F l y ash or pozzolan adrnixtures are often used to reduce the cernent content. Reduced ternperatures can be achieved usfurg a l o w - h e a t c e r n e n t , o r p o s s i b l y b y p r e - c o o l i n g t h e c o n c r e t e . M o d i f t e d c e r n e n t s a r e u s u a l l y a v a i l a b l e . f o r c e r t a i n o t h e r s p e c i a l j o b s i t u a t l o n s , such as sulphate-resistant cernent in locations where the soils contain e x c e s s i v e sulphate salts.

Where water problerns exist the perrneablllty of the concrete i s an irnportant factor. T h e u s e o f w a t e r p r o o f e r s , s u c h a s b u t y l

s t e a r a t e , i s s o r n e t i r n e s followed. ' W e I I d e s l g n e d a n d p r o p e r l y p l a c e d concrete is itself quite irnperrneable and, as rnost leakage problerns o c c u r as the result o f c r a c k s o r h o n e y - c o r n b i n g , I t c a n b e s u g g e s t e d that adrnixtures can have no significant value in such cases'

T h e u s e o f aids to irnprove workability a n d r e d u c e t h e a r n o u n t o f w a t e r r e q u i r e d c a n s o r r r e t i r n e s b e j u s t i f i e d i n v e r y s p e c i a l o p e r a t i o n s such as purnpcreting. S u c h i n s t a n c e s , h o w e v e r , r e q u i r e

ordin4ry operatlone.

In most epecial cases, of thle eort, lt is often poseible to achieve good concrete without reeort to the extra lngredlent. Location of certain structures rrr41rr for econornic reaeone, require the uee of inferior or borderline aggregates in the concrete. Thie problem should not concern the ready-mix operator who can be preeurned to have aasured himeeU of a continuous Bource of good rnaterlale before he began hls operations.

4. Inherent Materlal Problems

Goncrete rnade with artificial sand sornetirnes poses problerne of workability, fintehing, or early etiffening. Ttlater reducera,

retarders, _{or workability aide are often ueed ln euch caaes. Adequate} irnprovernents can ueually be effected by reflnernente fur aggregate grading and rnodlflcatlon ln rnethods of finishlng.

The problerne produced by a false eetting cernent cannot be corrected by the addition of an adrnlxture to the concrete.

'Vtlhere

the uee of alkali-reactive aggregates le unavoidable, pozzola"ns or chemical tnhibltore euch ag llthiurn chlorlde have been found to be effective in reduclng expansion. A slrnple alternatlve le, of course, a low alkall cernent.

Concrete rnade with aggregates of lnferlor or borderline physical _{quality cannot be irnproved by the use of admixtures, with} the possible exception of air-entrainrnent. Bad concretlng practlce c a n never be cornpensated for by such extra ingredlents. I n h e r e n t problerns _{in operation, or problems with quality of rnaterials, call} f o r expert diagnosls and prescription, a n d this can be done only by a c o r n p e t e n t technologist.

-zz-RESEARCH NEEDED

It is evident frorn this general survey of the problerns

associated with the properties and use of adrni-:rtures in concrete that t h e r e are rnany areas in which further s t u d i e s a r e r e q u i r e d . T h e many variables involved and the cornplex nature of rnost of the c o m n r e r c i a l p r o d u c t s rnake it almost irnpossible to discover the particular rnechanisrn by which each one operates in concrete under different conditlons. Publlshed research is useful but is lirnited in walue in that it applies in each case to a partlcular adrnixture, a particular combination of rnaterials, or a particular conditlon of t e s t .

The critical need ls for an understanding of the fundarnental p r o c e s s e s that occur and the specific effects produced. T h i s suggests basic studies fue colloid and surface chernlstry in relation to the

p h y s i c a l structure of cernent paste. T h e D i v i s i o n o f B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l , h a s b e g u n i n v e s t i g a t i o n s along this line. Studies to date have been rnainly concerned wtth air -entrainrnent.

SUMI\4ARY

Good concrete can be rrade without the use of adrnixtures, E n t r a i n e d air ls necessary, h o w e v e r , i n e x p o s e d c o n c r e t e s i n c l i r n a t e s where freezlng and thawing cycles occur. C e r t a i n o t h e r adrnixture t11pes rnay provide sorne quality benefits in special

situations but rnany such cases can be equally aided by better practlce and control. Except in very special situations the use of an adrnixture

s p e c i f i c a l l y t o r e d u c e c e r n e n t r e q u i r e r n e n t s h a s questionable rnerit. Methods of test and perforrrrance speclflcatlons now rnake i t p o s s l b l e to distinguish clearly between useful and useless or

harmful adrnixtures. The practical application of theee ln lndlvidual c a s e s p o s e s c e r t a i n p r o b l e r n s , n o t t h e l e a s t o f w h i c h i s c o s t .

Deterrnination of the adaptability of an adrnixture to a

particular plant operation or combination of rnaterlals, and

deterrnination of the conditions under which it should be used, can

be done correctly only by a concrete technologlst who is fully cornpetent

t o d i a g n o s e a n d p r e s c r i b e .

Admixtures have becorne a part of general concrete

t e c h n o l o g y a n d a r e h e r e t o s t a y , U s e d as rlcure-allsll t o c o r n p e n s a t e

for bad practice or poor rnaterials, they can be a serious ltability.

Used with intelligent discrlrnination, they rnay perforrn useful