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Cement and Concrete Research, 8, January 1, pp. 53-60, 1978-01-01

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Particulate admixture for enhanced freeze-thaw resistance of concrete

Litvan, G. G.; Sereda, P. J.

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TH1

' N21d

no.

748

c .

2

,

BLDG

NATIONAL

RESEARCH

COUNCIL

OF CANADA

I-- - -

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CONSEIL NATIONAL

DE RECHERCHES DU

CANADA

PARTICULATE ADMIXTURE FOR ENHANCED

FREEZE-THAW RESISTANCE OF CONCRETE

by G. G. itvan and P. J. Sereda

k

ANALYZED

Reprinted from

CEMENT AND CONCRETE RESEARCH

Val. 8, NO. 1, January 1978

P.

53

-

60

FEB 23

1978

NATIONAL RESOARCH COHClL

DBR P a ~ e r No. 748 Division of ~ u i l d i n ~ Research

Price 25 cents OlTAWA NRCC

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CEMENT and CONCRETE RESEARCH. Vol. 8, pp. 53-60, 1978. Pergamon Press, I n c . P r i n t e d i n t h e United States.

PARTICULATE ADMIXTURE FOR ENHANCED FREEZE-THAW RESISTANCE OF CONCRETE

by

G.G. L i t v a n and P . J . S e r e d a

Research O f f i c e r and Head, M a t e r i a l s S e c t i o n 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 Research Council of Canada, Ottawa Canada.

(Communicated by R. E. P h i l l e o ) (Received Oct. 11, 1977)

ABSTRACT

I n c o r p o r a t i o n of porous p a r t i c l e s w i t h a t l e a s t 30 p e r c e n t t o t a l p o r o s i t y and p o r e d i a m e t e r s , mainly between 0 . 3 a n d 2 microns, added

t o t h e p l a s t i c mix were found t o improve s i g n i f i c a n t l y t h e f r e e z e - thaw r e s i s t a n c e o f h y d r a t e d n e a t cement p a s t e and c o n c r e t e .

P a r t i c l e s o f c o m e r c i a l l y f i r e d c l a y b r i c k s , diatomaceous e a r t h , and

f l y a s h ( s i n t e r e d and agglomerated) were t e s t e d . The c o n c e n t r a t i o n

r e q u i r e d f o r t h e achievement o f a g i v e n l e v e l of f r o s t r e s i s t a n c e

depends on t h e p h y s i c a l c h a r a c t e r i s t i c s of t h e m a t e r i a l . T y p i c a l l y ,

g r a n u l e s of a p a r t i c u l a r b r i c k ( 0 . 5 -1 0 . 0 8 mm i n s i z e , 36 p e r c e n t

t o t a l p o r o s i t y ) i n 1 6 w t p e r c e n t c o n c e n t r a t i o n a l l o w e d 0 . 5 w/c cement p a s t e specimen t o e n d u r e 1260 freeze-thaw c y c l e s w i t h o u t

i n j u r y . I n c o r p o r a t i o n of porous p a r t i c l e s a s a d m i x t u r e s e l i m i n a t e s

t h e problems due t o t h e i n s t a b i l i t y and s t r e n g t h r e d u c i n g e f f e c t of c o n v e n t i o n a l l y e n t r a i n e d a i r b u b b l e s .

On a t r o u v g que des p a r t i c u l e s p o r e u s e s i n c o r p o r 6 e s B a u moins 30%

de p o r o s i t g t o t a l e e t de d i a m s t r e s d e s p o r e s , s u r t o u t e n t r e 0 . 3 e t 2 microns a j o u t c s a u mglange p l a s t i q u e , a m g l i o r e n t s i g n i f i c a t i v e m e n t l a r g s i s t a n c e a u g e l e t a u dEgel des piites de ciment e t de b 6 t o n

p u r s hy d r a t & . Les p a r t i c u l e s d e s b r i q u e s d' a r g i l e c u i t e s

commercialement, l a t e r r e 2 diatomacges, e t l a c e n d r e v o l a n t e

( c a l c i n g e e t agglomGr6e) o n t 6tG mises 2 l ' e s s a i . La c o n c e n t r a t i o n

n g c e s s a i r e pour l a r g a l i s a t i o n d'un n i v e a u donn6 de r 6 s i s t a n c e a u

g e l d6pend des c a r a c t g r i s t i q u e s physiques du m a t g r i a u . D'une

m a n i s r e t y p i q u e , l e s g r a n u l e s d'un b r i q u e p a r t i c u l i s r e (0.5 2 0.08 mm de g r o s s e u r , p o r o s i t g t o t a l e d e 36%) d ' u n e c o n c e n t r a t i o n du p o i d s de 16% a permis a un spgcimen de p P t e de c i m e n t a y a n t un t e n e u r e n e a u de 0 . 5 d ' e n d u r e r 1260 c y c l e s d e g e l e t de d g g e l s a n s a v a r i e . En i n c o r p o r a n t l e s p a r t i c u l e s p o r e u s e s e n t a n t qu' a d j u v a n t s on 6 l i m i n e l e s problsmes c a u s 6 s p a r 1 1 i n s t a b i l i t 5 e t l ' e f f e t de r s d u c t i o n de r g s i s t a n c e des b u l l e s d ' a i r e n t r a i n g e s d ' u n e manisre conven t i o n n e l l e

.

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-

54 Vol. 8,

No.

1

G. G. Litvan,

P.

J. Sereda

The method of air entrainment by the incorporation of small air bubbles in the plastic concrete mix has proven to be eminently successful in improving the freeze-thaw durability of concrete. The beneficial effect of the method on hardened paste depends on two factors: the total air content and the spacing between the air entrained voids. Controlling these two factors within narrow limits during the mixing, transportation, placement, and finishing operations is a most difficult task because the air content is affected by numerous factors such as mix composition, temperature, and the time taken for the mixing and finishing operations. The situation is made worse by the lack of a test for determining void spacing in the plastic mix. Only after hardening can the spacing be measured, and then by a cumbersome and lengthy procedure, at a time when corrective measures are no longer possible. Furthermore, incorporation of air weakens the concrete signifi- cantly, a serious drawback. In fact, the amount of air for frost protection is restricted usually to 6 per cent in order to avoid excessive reduction of mechanical properties.

All these difficulties can be overcome when the voids necessary for I durability are provided by the addition of porous particulate substances

I to the plastic mix. In this manner voids can be provided in the required

I number, size and distribution by adding an appropriate amount of a suitable material.

The possible use of this concept was tested and the results of preliminary experiments are reported in this communication.

Experimental

Cement mixes were prepared in accordance with ASTM Standard Method C- 305. Freezing and thawing test was performed in an automatically operated apparatus according to "Procedure B" of the Standard ASTM Method C-666. Performance was evaluated by monitoring the changes of the fundamental transverse frequency of concrete (ASTM C-215) or by measuring the residual length changes (1).

Porosity and pore size distribution were determined with an AMINCO 60,000 lb/in2 porosimeter. Compressive strength was assessed in conformity with ASTM C-39 Standard Method. Entrained air and spacing of voids were evaluated by the linear traverse method in conformity with ASTM C-457 Standard Method.

Materials

Type I cement, graded sand and limestone aggregate (0.6 to 1.2 cm) were used for the fabrication of test specimens. As porous additives, sized particles of commercially fired crushed bricks, designated A

Fr

B,

two types of diatomaceous earth, originating from California and B.C., and fly ash in spherically agglomerated and sintered forms were used. The air entraining agent was Darex.

Results

Series I. The freeze-thaw resistance, expressed as the number of cycles resulting in 0.2 per cent residual length change of 2.5- by 2.5- by 37.5-cm neat cement (w/c ratio 0.5) specimens containing crushed brick particles

(type B), is shown in Figure 1. Testing was performed after 28 days of curing. The results are plotted as a function of the average distance between the particle edges, henceforth referred to as spacing. Control sample containing no additive expanded 0.2 per cent after 13 cycles.

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Vol. 8, No. 1

5 5

POROUS PARTICULATE, FREEZE-THAW RESISTANCE, CONCRETE

Spacing can b e r e a d i l y c a l - c u l a t e d from a knowledge o f t h e s i z e and c o n c e n t r a t i o n o f t h e added p a r t i c l e s . The mesh s i z e s u t i l i z e d and t h e c o r r e s - ponding average p a r t i c l e d i a - meters a r e given i n Table l . The s p a c i n g f o r v a r i o u s concen- t r a t i o n s , assuming s p h e r i c a l shape, a r e shown i n F i g u r e 2 . I t i s e v i d e n t t h a t t h e s p a c i n g can b e reduced by i n c r e a s i n g t h e c o n c e n t r a t i o n o r by r e d u c i n g p a r t i c l e s i z e because a t a given w e i g h t p e r c e n t t h e number o f p a r t i c l e s i n c r e a s e s w i t h d e c r e a s i n g s i z e . S e r i e s 1 1 . The o b j e c t i v e o f t h e second s e r i e s o f

experiments was t o o b t a i n con- f i r m a t i o n o f t h e r e s u l t s o f t h e f i r s t s e r i e s and i n v e s t i - g a t e t h e e f f e c t o f a n o t h e r t y p e o f b r i c k p a r t i c l e and diatomaceous e a r t h . TABLE 1 Average P a r t i c l e Diameter i n t h e U t i l i z e d F r a c t i o n s F i g u r e 2 . Spacing ( d i s t a n c e between t h e edges of p a r t i c l e s ) a s f u n c t i o n of p a r t i c l e d i a m e t e r f o r v a r i o u s c o n c e n t r a t i o n s . S P A C I N G , m m F i g u r e 1. Freeze- thaw r e s i s t a n c e , e x p r e s s e d a s number o f c y c l e s r e q u i r e d t o produce 0 . 2 % r e s i d u a l expansion, a s f u n c t i o n o f s p a c i n g , of n e a t cement samples. O I A , m r n

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Vol. 8, No. 1

G .

G.

L i t v a n , P.

J.

Sereda

TABLE 2

Number of Freeze-Thaw Cycles Endured by Neat Cement Specimens, C o n t a i n i n g Porous

Particles, Before Breakdown. C a l c u l a t e d S p a c i n g s , i n mm, a r e g i v e n i n P a r e n t h e s i s The w/c = 0 . 5 . I ADDITIVE

+-

I

~ o n ~ . SIZE mm

1

Typc . % by w t . .15 - .30 .30 - .42 .42 - .59 . 5 9 - .83 .83 - 1.17 9 (0.42) 95, 400 (0.60) 54, 100 (0.86) 130, 130 (1.24) 127, 475

,

"A"

I;

16 (0.18)>1260 (0.29) 650, 1250 (0.42)>1260,>1260 (0.58)>1260,>1260 (0.84)>1260,>1260 28

i

j

(0.26)>1260,>1260 (0.36)>1000 B 9 (0.60) 75, 111 (0.86) 35, 39 I<

,

"0" 16 [ 0 . 1 2 ) 4 1 , 3 3 ( 0 . 2 4 ) 1 1 5 , 67 (0.42) 131, 145 (0.58) 323, 399 (0.84) 48, 59 C I( 28 (0.26)>1260,>1260 (0.36) 115,>1260 .- I) E 2 . 6 650,>!26n I ~ S , 1 I!) I A A R 3 . 6 26. 37 T 7 . H

I---

1

S

t

A N 16 2 1 28, 44 n

j

NEAT CEMENT PASTE 48. 53

I

I n c o n t r a s t t o t h e f i r s t s e r i e s where t h e 0 . 2 p e r c e n t expansion was taken a s c r i t e r i o n f o r t h e t e s t , i n t h e second s e r i e s t h e a c t u a l number of c y c l e s needed t o d e t e r i o r a t e t h e specimens t o such an e x t e n t t h a t t h e y had t o b e removed from t h e automatic f r e e z e - thaw machine, were recorded and a r e g i v e n i n Table 2. A d i f f e r e n t b a s i s o f comparison was n e c e s s a r y because i n a c o n s i d e r -

TABLE 3

Number o f Freeze-Thaw Cycles Endured by Neat Cement Samples, C o n t a i n i n g Fly Ash P a r t i c l e s , Before Breakdown

FLY ASH w / C Conc. % SIZE mm r a t i o Type by w t .I49

-

.30 .30

-

.42 .42

-

.59 .59

-

.84 ' I .05 agglom. 9 25 16 4 2 2 3 s i n t e r e d 9 18 30 30 16 26 26,77 p l a i n 0 25, 26 .07 agglom. 9 1 3 20 1 3

1

16 17 1 3 13,22 s i n t e r e d 9 20 1 3 16 18 1 3 13,15 p l a i n 0 13

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Vol. 8, No. 1

57

POROUS PARTICULATE, FREEZE-THAW RESISTANCE, CONCRETE a b l e number o f c a s e s t h e sought v a l u e could n o t b e a s c e r t a i n e d owing t o t h e l a c k o f continuous l e n g t h change d a t a . I t must be n o t e d t h a t i n every c a s e where t h e samples endured more than 1260 c y c l e s , no r e s i d u a l l e n g t h change

g r e a t e r than 0 . 0 1 p e r c e n t was o b s e r v e d .

Porous f l y ash p a r t i c l e s made by s p h e r i c a l agglomeration and s i n t e r i n g were a l s o used a s a d d i t i v e i n n e a t cement specimens and t h e number o f f r e e z e -

thaw c y c l e s endured by them b e f o r e breakdown a r e given i n Table 3 .

For experiments w i t h c o n c r e t e , b a r s ( 7 . 5 by 7.5 by 30 cm) were f a b r i c a t e d w i t h c r u s h e d b r i c k ( t y p e A) o r diatomaceous e a r t h a s a d d i t i v e . The b a s i c c o n c r e t e mix had a cement:aggregate:sand r a t i o o f 1 : 2.75:2.25 and a w a t e r : cement r a t i o o f 0.58. I n T a b l e 4 t h e performance o f c o n c r e t e b a r s c o n t a i n i n g diatomaceous e a r t h p a r t i c l e s .84 t o .30 mm i n s i z e and approximately 4 . 4 p e r c e n t by weight i n c o n c e n t r a t i o n i s compared t o t h a t o f b a r s made o f t h e same mix b u t w i t h o u t t h e a d d i t i v e . F r o s t r e s i s t a n c e was e v a l u a t e d on t h e b a s i s of r e s i d u a l l e n g t h changes, At.12, and d e c r e a s e o f t h e r e l a t i v e modulus o f

e l a s t i c i t y , E/Eo, d u r i n g r e p e a t e d f r e e z i n g and thawing. _f^"

The r e s i d u a l l e n g t h changes o f c o n c r e t e b a r s c o n t a i n i n g b r i c k p a r t i c l e s (Type A , 0 . 3 t o 0.8 mm) i n v a r i o u s c o n c e n t r a t i o n s a r e given i n Table 5 and compared w i t h t h o s e o f a i r - e n t r a i n e d c o n c r e t e ( 1 mR/1520 g cement). A d i s c r e p a n c y a p p e a r s t o e x i s t inasmuch a s specimens c o n t a i n i n g b r i c k p a r t i c l e s i n 10 p e r c e n t c o n c e n t r a t i o n showed l e s s f r o s t r e s i s t a n c e t h a n t h e sample cont'aining 5 p e r c e n t b r i c k p a r t i c l e s . A s d u r a b i l i t y i s d i r e c t l y p r o p o r t i o n a l t o t h e q u a l i t y o f e n t r a i n e d a i r , and t h u s t o p a r t i c l e c o n c e n t r a t i o n , t h e sample c o n t a i n i n g 5 p e r c e n t b r i c k i s expected t o be t h e most s u s c e p t i b l e t o f r e e z e - thaw damage. Values o f Table 6 a r e c o n s i s t e n t w i t h t h e s t a t e d c o n c e n t r a t i o n s and, t h e r e f o r e , t h e r e l a t i v e l y poor performance o f t h e specimen w i t h 10 p e r c e n t b r i c k h a s t o be a s c r i b e d t o e x p e r i m e n t a l e r r o r i n f a b r i c a t i o n . T h i s view i s supported by t h e v a r i a t i o n o f slump which was 3.5 i n . i n t h e c a s e o f t h e

TABLE 4

Residual F r a c t i o n a l Length Changes,

A R I R ,

and R e l a t i v e Dynamic Modulus of E l a s t i c i t y , E/Eo of Concrete Specimens With and Without Diatomaceous

Earth A d d i t i v e s , During Repeated Freezing and Thawing Cycles.

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1

G . G .

L i t v a n , P .

J .

Sereda

TABLE 5

Residual Length Changes o f Concrete Specimens C o n t a i n i n g Type "A" Brick P a r t i c l e s . C o n c e n t r a t i o n s a r e Given i n Weight Per Cent of Cement Content

a a / a ,

% Sample 50 c y c l e s 160 c y c l e s 360 c y c l e s P l a i n 0.117, 0.103 0.702, 0.478 1.908, 1.202 5% b r i c k added 0.009, 0.006 0.015, 0.018 0.026, 0.025 10% b r i c k added 0.011, 0.010 0.108, 0.034 0.218, 0.158 20% b r i c k added 0.003, 0.005 0.012, 0.015 0.022, 0.022 a i r e n t r a i n e d 0.007, 0.005 0.010, 0.009 0.025, 0.019 TABLE 6

The A i r and Brick C o n t e n t s , and Spacing F a c t o r s o f Specimens C o n t a i n i n g Type

"A1' Brick P a r t i c l e s . Brick C o n c e n t r a t i o n s a r e Given i n Weight Per Cent o f Cement Content and t h e Spacing F a c t o r s i n Inches

A i r Content Brick Content

Sample

Vol % Spacing F a c t o r % Spacing F a c t o r

P 1 a i n 2.25 0.046 0.00 o3 A i r e n t r a i n e d 5 . 3 4 0.013 0.00 rn 5% b r i c k 1.92 0.033 1.16 0.030 10% b r i c k 1.62 0.029 1.91 0.022 20% b r i c k 2.49 0.033 3.69 0.012 TABLE 7

Compressive S t r e n g t h o f P l a i n and A i r E n t r a i n e d Concrete Compared w i t h t h a t o f Concrete C o n t a i n i n g Type "A" Brick P a r t i c l e s . C o n c e n t r a t i o n s a r e Given i n Weight P e r Cent

o f Cement Content

Sample Compressive s t r e n g t h kPa ( l b / s q i n . )

P l a i n 24,408 (3540) 25,925 (3760)

5% b r i c k added 26,821 (3890) 27,235 (3950)

10% b r i c k added 27,304 (3960) 27,717 (4020)

20% b r i c k added 28,269 (4100) 27,304 (3960)

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Vol. 8, No. 1

I POROUS PARTICULATE, FREEZE-THAW RESISTANCE, CONCRETE

10 p e r c e n t b r i c k sample and 2.5 i n . i n t h e o t h e r s . Because o f t h e small q u a n t i t i e s a v a i l a b l e , t h e r e s u l t s o f t h e slump t e s t a r e s u s p e c t , too, b u t i f c o r r e c t t h e h i g h e r slump c o u l d e x p l a i n t h e i r r e g u l a r b e h a v i o u r . The

compressive s t r e n g t h o f c y l i n d e r s made from t h e same mixes a r e l i s t e d i n Table 7 . The pore s i z e d i s t r i b u t i o n and t o t a l p o r o s i t y o f t h e v a r i o u s a d d i t i v e s i s summa- r i z e d i n t h e histograms of Figure 3 . D i s c u s s i o n The r e s u l t s o f Tables 2 and 4 a t t e s t t o t h e e f f e c t i v e - n e s s o f t h e porous p a r t i c l e s i n enhancing t h e f r e e z e - thaw d u r a b i l i t y o f cement p a s t e ; e=

while t h e mortar p a s t e was u

E d e s t r o y e d a f t e r 21 f r e e z e - =J A thaw c y c l e s , cement c o n t a i n - o > i n g 16 p e r c e n t b r i c k p a r t i - c l e s endured 1260 c y c l e s w i t h o u t i n j u r y . I n p r a c t i c e , t h e obvious aim i s t o provide adequate d u r a b i l i t y w i t h l e a s t amount o f b r i c k . A t a given b r i c k c o n c e n t r a t i o n f r e e z e - thaw F i g u r e 3 Pore s i z e d i s t r i -

I

b u t i o n o f a d d i t i v e s used i n

!

experiments F L Y A S H " P O W D E R " TOTAL P O R O S I T Y 50% F L Y A S H " S P H E R E S " TOTAL P O R O S I T Y 40% B R I C K "A" TOTAL P O R O S I T Y 36%

I

B R I C K " B "

,

TOTAL P O R O S I T Y 15 3 6 1.1 .U .OU I I 3.1 .86 -31 .m6 7 . 2 2.5 .R .a .026 P O R E S I Z E . D I A . pm r e s i s t a n c e can be enhanced by u s i n g s m a l l e r p a r t i c l e s , t h u s a c h i e v i n g s m a l l e r spacing. P a r t i c l e s i z e cannot be reduced, however, beyond t h e s t a g e a t which t h e i n t e r n a l p o r e volume o f t h e p a r t i c l e becomes i n s u f f i c i e n t l y l a r g e t o p r o t e c t t h e a r e a surrounding i t . The l i m i t i n g p a r t i c l e s i z e i n c a s e o f b r i c k s a p p e a r s t o be between 0.4 and 0.8 mm.

The-two t y p e s o f b r i c k p r o t e c t t h e c o n c r e t e t o a d i f f e r e n t d e g r e e , t y p e

"A1' being more e f f e c t i v e t h a n t y p e "Bfl. Apparently t h e l a r g e r p o r o s i t y of b r i c k A (36 p e r c e n t v s 16 p e r c e n t ) and i t s p a r t i c u l a r pore s i z e d i s t r i b u t i o n

( F i g u r e 3) i s advantageous.

None o f t h e f l y ash conglomerates proved t o be b e n e f i c i a l , presumably because o f unfavourable p o r e s i z e d i s t r i b u t i o n ( F i g u r e 3 ) . The r e s u l t s of t h e p r e s e n t experiments, however, must n o t be i n t e r p r e t e d a s proof o f t h e i n a b i l i t y of f l y ash based p a r t i c l e s t o a c t a s a p r o t e c t i v e a g e n t a g a i n s t freeze-thaw damage. I t may be assumed t h a t i f p a r t i c l e s w i t h t h e d e s i r e d 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 can be f a b r i c a t e d , t h e p r o d u c t w i l l b e an e f f e c t i v e admixture.

(10)

60 Vol. 8, No. 1

G.

G.

L i t v a n , P. J .

Sereda

Diatomaceous e a r t h h a s t h e r e q u i r e d t o t a l 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 and when mixed i n c o n c r e t e o r cement p a s t e c a u s e s a s i g n i f i c a n t improve- ment i n t h e f r e e z e - t h a w r e s i s t a n c e . I t h a s t o be n o t e d , however, t h a t

diatomaceous e a r t h p a r t i c l e s o r i g i n a t i n g from c e r t a i n d e p o s i t s , w h i l e e f f e c t i v e from a f r o s t r e s i s t a n c e p o i n t o f view, c a u s e "map c r a c k i n g , " s u s p e c t e d t o be caused by a l k a l i - s i l i c a r e a c t i o n .

The compressive s t r e n g t h v a l u e s o f Table 7 r e v e a l t h a t t h e i n c o r p o r a t i o n of b r i c k p a r t i c l e s , i n c o n t r a s t t o a i r e n t r a i n m e n t , i n c r e a s e s compressive s t r e n g t h , an important c o n s i d e r a t i o n f o r c e r t a i n a p p l i c a t i o n s .

The d e s c r i b e d method of r e n d e r i n g cement and c o n c r e t e f r o s t r e s i s t a n t i s

d e s c r i b e d i n Canadian P a t e n t A p p l i c a t i o n 241,032. Conclusion

I n c o r p o r a t i o n o f s u i t a b l e porous p a r t i c l e s i n t h e p l a s t i c mix i n c r e a s e s t h e freeze-thaw r e s i s t a n c e of hardened c o n c r e t e w i t h o u t t h e s e r i o u s s h o r t - comings o f t h e c o n v e n t i o n a l a i r e n t r a i n m e n t method ( i n s t a b i l i t y o f bubbles and s t r e n g t h r e d u c t i o n ) . Acknowledgements D r . I r a Puddington's h e l p f u l a s s i s t a n c e i n agglomeration of f l y a s h i s g r a t e f u l l y acknowledged. We a r e i n d e b t e d t o Herman S c h u l t z f o r c a r r y i n g o u t t h e e x p e r i m e n t a l work w i t h g r e a t competence. 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 o f B u i l d i n g Research, N a t i o n a l Research Council o f Canada and 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 .

Reference

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