Journal of the American Concrete Institute, Vol. 18, No. 4, Part 2

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J O U R N A L

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AMERICAN CONCRETE INSTITUTE

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Vol. 18 D E C E M B E R 1946 No. 4

C O N T E N T S

Title Page Contents

Closing Discussion Indexes

fo r

Vol. 42

AC I Proceedings , 1946

(JOURNAL V o l. 17)

fo p ro vid e a com radeship in finding the best w ays to do concrete work o f a ll kinds an d in sp read in g that know ledge

A D D R E S S : 7 4 0 0 S E C O N D B O U L E V A R D , D E T R O I T 2 , M I C H .

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PROCEEDINGS

AMERICAN CONCRETE INSTITUTE

V olu m e 42—1946

from J o u r n a l o f t h e A m e r i c a n C o n c r e t e I n s t i t u t e (Vol. 17) September, and November, 1945 and January, February,

April, and June, 1 9 4 6 (6 J o u r n a l issues) and P a rt 2 of December, 1946

P u b l i s h e d b y t h e A m e r i c a n C o n c r e t e I n s t i t u t e New Center Building

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a n d o p in io n s a d v a n c e d in its p u b lic a tio n s . P r in te d in U . S. A.

C O N T E N T S— Proceedings V o l. 42, 1946

v i (From September 1945 J O U R N A L )

C o ncrete Construction in the N a t io n a l Forests... 1 B y Cl i f f o r d A . Be t t s

L a p p e d B a r S p lic e s in C oncrete B e a m s ... 13 B y R a l p h W . K l u g e and E d w a r d C. T u m a

Tests of Prestressed C oncrete Pip e s C o n tain in g a S te e l C y lin d e r ... 37 B y Cu l b e r t s o n W . Ro s s

F ie ld U se of C em ent C o n tain in g V in s o l R e s in ... 4 9 B y Ch a r l e s E . Wu e r p e l

J o b Pro b lem s a n d P r a c t ic e ... 85 42-168 Non-skid Concrete Surfacing for W o o d e n Trestles at M u d M o u n tain D am —

H . H . Roberts... 85

42-169 Exposure of Concrete to H ig h Temperature 88

42-170 Derrick Stone and Cobbles in M ass Concrete 89

42-171 Effect of Brine on Concrete ... 90

Current R e v ie w s ... 93

(From November 1945 J O U R N A L )

M a in t e n a n c e an d R e p a ir of C oncrete B rid g e s on the O r e g o n H ig h w a y S y s te m 105 B y G . S . Pa x s o n

Sh o u ld P o rtla n d C em ent B e D is p e r s e d ? ... 117 B y T . C. P o w e r s

Discussion ( f r o m P a r t 2, December 1946 J o u r n a l ) ...140-1 t o 52

A n Investig atio n of the Strength of W e ld e d Stirrups in R e in fo rce d C o ncrete B e a m s ... 141 B y Or e s t e Mo r b t t o

Discussion (from P a rt 2, December 1946 J o u r n a l ) ...164-1 to 4

(From January 1946 J O U R N A L )

S h rin k a g e Stresses in C o ncrete— (P a r t 1 ) ... 165 B y Ge r a l d Pic k e t t

Flo a tin g B lo c k T h e o ry in Structural A n a ly s i s ... 205 B y St a n l e y U . Be n s c o t e r

I I I

By H o w a r d R. S t a l e y a n d D e a n P e a b o d y , J r .

Discussion (from P a rt 2, D ecember 1946 J o u r n a l ) 244-1 t o 4

P ro p o s e d M in im u m S ta n d a rd Requ irem en ts for P re cast C o n cre te F lo o r U n its... 245

R eport of A CI Com m ittee 711, Precast Floor Systems for Houses, F. N. M e n e f e e , Chairm an

P ro p o s e d R e co m m e n d e d P r a c tic e for the C onstruction of C o n cre te Farm S ilo s ... 261

R eport of ACI Com m ittee 7 1 4 , W i l l i a m W . G u r n e y , Chairm an

Current R e v ie w s ... ... 265

(From February 1946 J O U R N A L )

M a in t e n a n c e o f H e a v y C o n cre te Structures— M in n e s o ta P o w e r & L ig h t

C o m p a n y P r a c t ic e ... 277

By Clay C. B o s w e l l an d A l b e r t C. G i e s e c k e

Two S p e c ia l M e th o d s of R estorin g a n d Stren g thenin g M a s o n r y Structures... 2 8 9

By J. W. K e l l y a n d B. D. K e a t t s

L a b o r a to r y Stud ies of C o n cre te C o n ta in in g A ir - E n tr a in in g A d m ix tu r e s ... 305

B y C h a r l e s E. W u e r p e l

S h rin k a g e Stresses in C o n cre te — (P a r t 2 ) ... 361

By G e r a l d P i c k e t t

J o b Pro b lem s a n d P r a c t ic e ... 401 42-172 Influence of M ix in g W a te r “ H a rd n es s " on Air-Entrainm ent— C h a rle s E. W u e r p e l 401 42-173 Lo ca tin g Points A lo n g Beam A x is Corresponding to Known M o m en ts—

W . C . G o o d w i n ... 402

5th A n n u a l T e c h n ic a l Progress S e c t io n ... 405

(From A p ril 1946 J O U R N A L )

A n n o u n c e m e n t o f P ro p o s e d M a n u a l o f S ta n d a rd P ra c tic e for D e ta ilin g

R e in fo rce d C o n cre te S tr u c tu r e s ... 4 73

R eport by A CI Com m ittee 315

M a in t e n a n c e a n d R e p a ir o f P o rtla n d Cem ent C o n cre te P a v e m e n t ... 4 7 7 B y A . A . An d e r s o n

C uring C o n cre te with S e a lin g C o m p o u n d s... 493 B y R. F. B l a n k s , PI. S. M e i s s n e r a n d L. II. T u t h i l l

Discussion (from P a rt 2, December 1946 J o u r n a l ) 512-1 to 8

R a d ia n t H e a t in g b y R e in fo rce d C o n c r e te ... 513 B y J o h n R. N i c h o l s

Discussion (from P a rt 2, December 1946 J o u r n a l ) 516-1 to 12 IV

T h e E x p a n s i o n T e s t a s a M e a s u r e o f A l k a l i - A g g r e g a t e R e a c t i o n ... 5 1 7 B y R . F . Bl a n k s a n d IT. S . Me i s s n e r

C o n c r e t e a t A d v a n c e B a s e s ... 5 4 1 B y I . S . Ra s m u s s o n

C u r r e n t R e v i e w s ... 5 5 3

(From June 1946 J O U R N A L )

A s p h a l t i c O i l - L a t e x J o i n t - S e a l i n g C o m p o u n d ... 5 6 5 B y Br y a n t W . Po c o c k

D i s c u s s i o n ( f r o m P a r t 2 , D e c e m b e r 1 9 4 6 Jo u r n a l) ... 5 8 0 - 1 t o 8

P e t r o g r a p h y o f C o n c r e t e A g g r e g a t e ... 5 8 1 B y Ro g e r Rh o a d e s a n d R . C . Mi e l e n z

E n t r a i n e d A i r in C o n c r e t e : A S y m p o s iu m ... 6 0 1 Entrained A i r — A Factor in the Design of Concrete M ix e s ... 605

B y W . A. Cord on

Recent Experiences with A ir-Entraining Portland Cement Concrete in the Northeastern States... 621 B y L. E . An d r e w s

Experiences with A ir-En train in g Cement in Central-M ixed Concrete... 625 B y Al e x a n d e r Fo s t e r, Jr.

Studies of Concrete Containing Entrained A i r ... 629 B y St a n t o n Wa l k e r a n d Delm a r L. Bloem

H o m o g e n e ity of A ir-En train in g C o n crete ... 641 B y He n r y L . Ke n n e d y

M ethods of Entraining A i r in C o n cre te ... 645 B y E . W . Sc r ip t u r e, Jr.

Effect of A i r Entrainment on Stone San d C o n cre te ... 649 B y A. T . Go ld beck

A M e th o d for D irect M easurem ent of Entrained A i r in C o n crete ... 657 B y W . H . Kl e in a n d St a n t o n Wa l k e r

A u to m atic Dispensing Equipment for A ir-Entraining A g e n ts ... 669 B y R . R . Kau fm an

M e c h a n ic a l Dispensing D evices for Air-Entraining A g e n t s ... 673 B y E . M . Br ic k e t t

A Sim ple A c c u ra te M e th o d for Determining Entrained A i r in Fresh Concrete... 677 B y S. W . Benha m

Effect of Use of Blended and V in s o l Resin-Treated Cements on D u rab ility of Concrete... 681 B y W . F . Kell e r m a n n

Air-En train in g Concrete— Pen n sylvan ia Department of H ig h w a y s ... 689 B y W . H . Herm an

Portland-Rosendale Cement Blends G iv e H ig h Frost Resistan ce... 697 B y B. H . Wa it

D i s c u s s i o n ( f r o m P a r t 2 , D e c e m b e r 1 9 4 6 Jo u r n a l) 7 0 0 - 1 t o 1 2

T h e R e p a i r o f C o n c r e t e : A n I n t r o d u c t i o n ... 7 0 1 B y Ro d e r i c k B . Yo u n g

B e h a v i o r o f C o n c r e t e S tr u c t u r e s U n d e r A t o m i c B o m b i n g ... 7 0 9 B y E . H . Pr a e g e r

J o b P r o b l e m s a n d P r a c t i c e ... 7 2 1 42-174 W h a t kind of Cement S t u c c o ? ... 721 42-175 Setting H e a v y M a c h in e ry on Concrete Bases— R. R. Kaufm an... 721

(From Part 2, December 1946 J O U R N A L )

I n d e x P r o c e e d i n g s ... 7 3 3

I n d e x C u r r e n t R e v ie w s ... 7 4 1

V

A part of PROCEEDINGS O F THE A M ER IC A N CONCRETE INSTITUTE Vol. 42

J O U R N A L

of the

A M E R I C A N C O N C R E T E I N S T I T U T E

(cop yrig h ted )

V o l . 1 8 N o . 4 740 0 S E C O N D B O U L E V A R D , D ET R O IT 2, M I C H I G A N P a rt 2 D e c. 1 9 4 6

Discussion o f a p a p e r b y T. C. Pow ers:

Should Portland Cem ent Be D ispersed?*

By E. W . SCRIPTURE, JR., FRED M. ERNSBERGER and W ESLEY G. FRANCE, H O W A R D R. STALEY, C. A. G. W EYM O U T H , M. SPINDEL, EMIL SCHMID,

LOUIS R. FORBRICH, HENRY L. KENNEDY, and AUTH O R

By E. W. SCRIPTURE, JR .f

This paper attacks a problem of considerable interest in cement tech­

nology b u t is som ewhat confusing and in some instances contradictory.

In th e first, theoretical p a rt of th e paper, it appears to conclude on the basis of certain well-known principles of the behavior of dilute suspen­

sions, plus a num ber of assum ptions which are not supported by ex­

perim ental data, th a t dispersion of cement in cement pastes, i. e., in concentrated suspensions, would be undesirable because it would in­

crease bleeding, increase perm eability, decrease bond strength, weaken th e concrete and cause segregation in tra n sit and on standing.

The second p a rt of the paper, which includes a lim ited num ber of experim ental d a ta on three m aterials sold for addition to concrete mixes, seems to conclude, on the basis of th e preceding argum ent and the lim ited experim ental d ata th a t the m aterials which are stated to be cement dispersing agents by the m anufacturers do not in fact disperse cement in cement pastes; th a t one m aterial increases bleeding, lowers bond strength, produces fissures, weakens th e concrete, increases per­

m eability, and increases the tendency tow ard segregation; th a t a second m aterial, an air entraining agent b u t n o t a dispersing agent, has bene­

ficial effects w ith respect to w ater reduction, workability, bleeding and durability, although it reduces strength, and th a t a th ird m aterial which the m anufacturers call a dispersing agent and which also entrains some air, does no t disperse cement in cement pastes b u t has similar beneficial effects to the second m aterial b u t in a higher degree. No com m ent is m ade on the th ird m aterial w ith respect to its effect on strength.

* A C I Jo u r n a l, No v. 1 9 4 5 , P roceedings V. 4 2 , p . 1 1 7

f D ir e c to r of R esea rc h , T h e M a s te r B u ild e rs C o., C le v ela n d , Ohio (140 - 1)

The n et result of this ra th e r perplexing line of reasoning appears to be to conclude th a t cem ent dispersion does n o t exist in cem ent pastes, th a t if it did it would be undesirable, b u t th a t a m aterial which contains a cem ent dispersing agent (at least it disperses cem ent in dilute suspen­

sions) and also has some air entraining effect, has beneficial effects in concrete, greater th a n those of an air entrainin g agent per se. I t is difficult to reconcile the conclusion th a t cem ent dispersion (or reduc­

tion in interparticle attra ctio n ) plus air en train m en t has a m ore beneficial action th a n an equal or greater air en tra in m e n t alone w ith th e conclu­

sion th a t cem ent dispersion does n o t exist and th a t if it did th e effects would be harm ful.

The essence of th e pap er does n o t seem to be to show th a t m aterials which are sold as cem ent dispersing agents are n o t beneficial in concrete or are harm ful. On th e contrary, th e paper specifically sets fo rth th e beneficial effects of one such m aterial and even recom m ends its use where resistance to frost action is desired. T he p ap er does seem to a tte m p t to discredit th e explanation of any beneficial effects on the basis of cem ent dispersion and to a ttrib u te th em solely to air en tra in ­ m ent although no explanation is offered on th e b e tte r perform ance of A gent C, a cem ent dispersing agent plus air en train m en t com pared w ith Agent B, which is an air entraining agent only. T his a tte m p t to dis­

credit cem ent dispersion as a factor in concrete mixes is based on an a rb itra ry definition and a process of reasoning from well-known phe­

nom ena obtainable in dilute suspensions and assum ptions n o t supported by experim ental evidence.

In view of th e in terest which cem ent dispersion has aroused in some q uarters it m ay be worthwhile to a tte m p t to analyze in som ew hat greater detail th e reasoning and evidence in this paper. F o r th is p u r­

pose the paper m ay be considered in three parts, first th e enunciation of principles of colloid chem istry, th e definition of dispersion an d the assum ptions m ade, second th e conclusions reached w ith o u t experi­

m ental evidence on th e basis of th e preceding, and th ird th e d a ta and conclusions on three commercial m aterials. In m aking th is analysis the im p o rtan t statem ents (arabic num erals) in each p a rt of th e pap er are quoted, followed by a discussion.

1. P R I N C I P L E S A N D A S S U M P T I O N S a . W e ttin g

1. If the solid and the liquid show a strong m u tu al a ttra c tio n considerably greater th a n th e surface tension of th e liquid, th e liquid will spread over the solid surface w ithout outside aid.

2. The a ttra c tio n betw een cem ent an d w ater is so strong th a t each cem ent grain becomes completely surrounded b y w ater even though in dilute suspension the grains are clustered.

SH O ULD PORTLAND CEMENT BE DISPERSED 140 - 3

The first statem en t is undoubtedly a correct statem en t of a basic principle which defines the conditions under which a liquid (w ater) will spread on a solid surface (cement) when it is brought in contact w ith th a t solid surface and is free to spread w ithout obstacles. The second statem en t is an assum ption no t supported by experim ental evidence. If th e cem ent is norm ally flocculated, tending to hang to ­ gether in clusters (clumps), th en in order for th e w ater to break up these clusters and wet com pletely the entire surface area of all the particles it is necessary for th e a ttra c tio n between the cem ent and w ater to overcome no t only th e surface tension of the w ater b u t also th e force of a ttra c tio n betw een the cement particles. This is no t necessarily a condition which is fulfilled in th e case of cem ent and w ater, and anyone who has had experience w ith clays, also very hydrophilic solids, and the difficulty of m aking clay slips w ith w ater, will be led to the conclusion th a t complete w etting of cem ent by w ater is hardly probable.

b. D ispersion

1. Interparticle repulsion is n o t necessary for dispersion.

2. P o rtlan d cement cannot be caused to disperse spontaneously for cement particles are predom inantly microscopic, n o t colloidal.

3. Some of the phenomena pictured in connection w ith th e use of dispersing agents w ith portland cement actually can occur only among particles th a t are of truly colloidal dimensions.

The first statem en t is correct if applied to particles of colloidal dim en­

sions where th e force of B row nian m ovem ent is sufficient to disperse particles having no m utual attra ctio n (but no m u tual repulsion, or m utual a ttra ctio n of a very low order). I t is not, however, especially pertinent since the real problem is w hether a finely divided solid can be m aintained in a dispersed state in a liquid. A solid or a liquid can be dispersed mechanically b u t w hether it rem ains dispersed depends on the means used for stabilization. Interp article repulsion is n o t the only means of stabilizing the dispersion b u t it is one of the m ost effective. The second statem en t is entirely correct if it is stated, as seems to be in­

tended, th a t portland cem ent will no t disperse spontaneously b u t requires a dispersing agent for this purpose. The th ird statem en t is vague in th a t it does not specify w hat phenom ena are n o t applicable to portland cement. If it means th a t Brownian m ovem ent does no t occur w ith th e m ajor p a rt of the cem ent particles it is correct, although some of the cem ent particles are subject to this phenom enon when dispersed in w ater. O ther th a n th a t it is no t ap p aren t w hat this sta te ­ m ent means. T h a t portland cement particles, w hether of colloidal or microscopic size, can be dispersed in dilute suspensions by means of a suitable dispersing agent can readily be dem onstrated.

c. Inte rp a rticle attractio n a n d p aste p roperties

1. The greater the interparticle attraction, th e stiffer a paste will seem to be when it is stirred.

2. If the forces of repulsion predom inate (i. e., if the cem ent is dispersed) particles th a t would rem ain in contact when quiescent become separated as th e y fall through the liquid during sedim entation.

3. W hen th e attra c tiv e forces are very weak, and especially when th e particles are m utually repellent (dispersed), th e sedim ent th a t is formed tends to be non-uniform in composition, th e proportion of coarse particles increasing tow ard th e b o tto m of th e sediment.

The first statem en t is entirely correct and m erely confirms th a t cem ent dispersion gives greater fluidity of th e mix w ith a given w ater- cem ent ratio. The second statem en t is also correct b u t und uly lim ited since, when th e forces of repulsion predom inate th e particles become sep arated w hether th ey are falling thro u g h th e liquid during sedim enta­

tion or not. The th ird statem en t is quite correct also, if it is qualified b jr saying th a t this observation applies to dilute suspensions, n o t cem ent pastes. T he w riter’s work published some tw e n ty years ago on th e determ ination of particle size d istribution of clays b y sedim entation illustrates this phenom enon.9) L ater in the pap er it is correctly s ta te d :

“Also in dilute suspensions of particles, segregation of sizes takes place during sedim entation if th e in terp article a ttra c tio n is absen t o r weak and it does n o t tak e place if in terp article a ttra c tio n is stro ng .” W ith respect to cem ent pastes, which are highly concentrated suspension, the statem en t is an assum ption in support of which no d a ta are adduced, I t is confusing w hen included in a paragraph on p aste properties.

d. D e fin itio n o f dispersion a p p lic a b le to cem ent p aste

1. W hen interparticle attraction, in a fresh cem ent paste is so w eak th a t it has no appreciable effect on th e behavior an d physical properties of th e paste, th e particles in the p aste m ay be said to be dispersed.

2. I t should be noted th a t the definition does n o t rest on th e presence or absence of particle clusters.

3. The reader is asked to avoid applying the final conclusions concerning dispersion as defined her, to dispersion defined in some other way.

A n arbitrary definition of dispersion is here established which has abso­

lutely no meaning and is not consistent with scientific and in du strial use of the term. Any definition which includes th e term “ appreciable” is q u an titativ ely useless. In order to apply the definition it is necessary to know w hether interparticle a ttra c tio n appreciably affects th e be­

havior and physical properties of th e paste. No criteria are given for telling w hether the behavior and physical properties of th e p aste are or are n o t affected by the existing in terp article attra c tio n in any

i E d w a r d S c h ra m m a n d E . W . S c rip tu re , J r .: T h e P a r tic le A n aly sis of C la y s b y S e d im e n ta tio n , J . A m . Ceram. Soc., V. 8, p. 243-252.

T h e P a r tic le Size D is tr ib u tio n of T y p ic a l F e ld s p a rs a n d F lin ts , J . A m . Ceram. Soc., V. 10, (A p ril 1927).

SHO ULD PORTLAND CEMENT BE DISPERSED 140 - 5

given case. Hence it is impossible to apply this definition and im­

possible to determ ine in any given case w hether the cem ent is or is not dispersed in accordance w ith this definition. I t is suggested th a t when interparticle a ttra c tio n is absent or negligible (dispersed) a suspension th a t is n o t too concentrated flows like a tru e liquid, b u t when in ter­

particle a ttra ctio n is no t negligible, th e suspension acquires the proper­

ties of a plastic solid to some degree. The fallacy in this as a criterion for dispersion is th a t at some concentration the suspension will flow like a liquid w hether it is dispersed or flocculated and th a t a t some other concentration it will have th e properties of a plastic solid to some degree w hether dispersed or flocculated. The commonly accepted scientific and technical meanings of the term s flocculated and deflocculated are, in th e first instance th a t the particles form clusters and in th e second th a t they do not, y e t the second statem ent given seems to m ake a virtue of discarding all previous concepts of dispersion and flocculation. The th ird statem en t sim ply emphasizes th a t, since all accepted definitions of dispersion have been abandoned, anything which m ay be said or concluded is no t applicable to m aterials which defiocculate or disperse cem ent in accordance w ith accepted definitions of this phenomenon.

e. D ispersion of p o rfla n d cem ent

1. There is no question b u t th a t cem ent particles in a norm al paste are flocculated.

2. The electrolytes (apparently th e hydroxyl ions) bring about flocculation of the cement particles.

3. D uring this period (the first five m inutes after addition of water) a coating of hydrates forms on th e cement grains.

The first statem en t simply confirms w hat is well-known, th a t th e norm al state of portland cement in w ater is the flocculated state. The second statem en t m ay be correct b u t no evidence is adduced to show th a t it is. A later statem en t “Electrolytes in solution or certain types of organic molecules tend to m ake th e particles electrostatically re­

pellent” seems to contradict it. Experim ental evidence® w ith clays, flints, feldspars, and similar m aterials which show th a t hydroxyl ions exert a deflocculating effect also indicates th a t this explanation of the flocculation of cement is no t correct. I t seems m uch more probable th a t the cement is n aturally flocculated in the original state and be­

comes slightly less so during the first five m inutes. The th ird statem en t is probably correct, although unsupported, b u t is ra th e r curious in th a t it seems to negative th e statem en t previously m ade th a t the entire cem ent surface was w etted and free to hydrate. E ven if it is assumed, and th e assum ption is no t supported by evidence, th a t all the cement

2 E . W . S c rip tu re , J r ., a n d E . S ch ram m : T h e D eflo cc u latio n of C la y S lips a n d R e la te d P ro p e rtie s , J . A m . Ceram Soc., V. 9. p. 175-178 (A p ril 1926).

E . W . S crip tu re , J r .: T h e B e h a v io r of F e ld s p a rs a n d F lin ts w ith A cids a n d B ases, J . A m . Ceram.

Soc., V. 10, p. 238-242 (A p ril 1927).

grains are com pletely w etted when w ater is added, it ap pears highly probable th a t, in the flocculated condition, these h y d ra te coatings would be in contact w ith one anoth er and would preclude or a t least im pede fu rth e r hyd ratio n a t th e points of contact.

2. C O N C L U S I O N S a . E ffe ct o f flo ccu la tio n on am o u n t o f settlem ent

1. The d a ta dem onstrate th a t th e forces of interparticle a ttra c tio n in cem ent-w ater paste are not as high as they m ight be and th a t if a change in th e force of flocculation is desired, it could be either an increase or a decrease according to choice.

2. Used in concrete or pastes in proportions recom m ended for field use, th e y (various dispersing agents for portland cement) do n o t cause m uch dispersion; th e pastes clearly show th e effects of interparticle attraction.

This first statem en t is probably correct b u t is certainly n o t derivable from the d a ta given. All these relate to dilute suspensions, m ainly sus­

pensions of solids other th a n p ortland cem ent in w ater. Such d a ta cannot justify any conclusions regarding cem ent pastes which are con­

cen trated suspensions of p o rtlan d cem ent in w ater w ith respect to th e second statem ent. I t has been shown b y E rnsberger an d F rance® th a t calcium lignosulfonate is adsorbed by p o rtlan d cem ent and endows th e particles w ith electrostatic charges m aking th em m u tu ally repellent in dilute suspensions. This is clearly dem onstrable b y microscopic observation and th e increase in fluidity of concretes and m o rtars for a given w ater-cem ent ratio w ith such an agent, even when no additional air is incorporated in the mix, shows th a t this dispersion effect is carried over from dilute suspensions to pastes to a v ery appreciable extent.

W hether th e cem ent particles in a paste (concentrated suspension) can be said to be dispersed is largely a m a tte r of definition since th ey obviously cannot be widely separated although th ey m ay still be m u tu ally repellent.

b. E ffe ct o f flo ccu la tio n on rate o f sed im en tatio n

1. So far as the effects of bleeding are concerned th e results obtained w hen th e particles are subject to the force of flocculation are clearly preferable to w h at th ey are when th e particles are free from th a t force.

2. I t is plain th a t any claim th a t dispersion is a m eans of reducing bleeding or

“shrinkage before hardening" is based on knowledge of the effect of dispersion on the settlem ent of dilute suspensions an d n o t of th e effect on pastes.

3. Also, any deductions based on th e assum ption th a t th e cem ent particles in a norm al paste exist in discrete floes from w ithin which w ater for h y d ratio n is excluded are bound to lead to erroneous conclusions for the evidence is overw helm ing th a t no such condition exists.

4. Dispersion would n o t only increase this effect (weakening of bpnd w ith th e u n d er­

surfaces of aggregate particles) b u t also would ten d to destroy th e uniform ity of th e hardened paste by prom oting stratification.

3 F r e d M . E rn s b e rg e r a n d W e sley G . F ra n c e : P o r tla n d C e m e n t D is p e rs io n b y A d so rp tio n of C a lc iu m L ig n o su lp h o n a te , I n d . a n d E n g . Chem ., V. 37, p . 598-600 (J u n e 1945).

SH O ULD PORTLAND CEM ENT BE DISPERSED 140 - 7 5. If cement pastes were not norm ally flocculated, it would seem advisable to add a flocculating agent.

F or these conclusions there is very little evidence. In addition to the previous statem ents of principles and assum ptions the only experi­

m ental evidence is some d a ta on em ery suspensions of 12.2 m u and 9.6 mu particle sizes which do no t bear any necessary relation to portland cement pastes. I t seems desirable to tak e each conclusion up in dividually:

I t is by no means clear th a t, for concentrated suspensions such as pastes, the flocculated sta te is preferable to th e dispersed sta te w ith respect to bleeding. A ctually “ bleeding” , as th e term is commonly used in concrete technology, is n o t a phenom enon which occurs in dilute suspensions b u t this conclusion is based on reasoning from observation on dilute suspensions. I t has been dem o nstrated th a t bleeding is greatly reduced and in m any cases practically elim inated w ith a cement dispersing agent. The au th o r m ight, of course, a t ­ trib u te this to air entrainm ent, as appears later in th e paper, b u t th is is n o t consistent w ith the fact th a t the dispersing agent exhibits a greater reduction in bleeding th a n do air entraining agents which do not have a dispersing effect on cem ent b u t do en train m uch m ore air.

Some d a ta on this point are shown in Table A.

T A B L E A

Percent bleeding Concrete Mix—6.0 sks. cu. yd.

Addition Cement I Cem ent I I Added air—percent

N one... 100 100 0.0

Air entraining agent A ... 42 34 3.4

Air entraining agent B ... 49 49 3.3

Dispersing a g e n t... 25 20 2.9

I t is obvious th a t a reduction in bleeding of 773^ percent w ith the dispersing agent cannot be solely attrib u tab le to air entrainm ent since th e same mixes w ith higher air contents give only 62 percent and 51 percent reductions. I t is a logical conclusion th a t the additional reduction in bleeding is a ttrib u ta b le to cement dispersion.

M uch the same rem arks apply here as were m ade in connection w ith the first statem ents. T h a t claims for cem ent dispersion are based only on a knowledge of th e behavior of cem ent in dilute sus­

pensions is an unw arranted statem en t; th e w riter’s work on cement dispersion has included dilute suspensions b u t studies of bleeding and “ shrinkage before hardening” have been extensive and necessarily conducted on pastes, m ortars, and concretes. The w riter has, m ore­

over, carried on extended investigations of dispersion and r e l a t e d

phenom ena w ith b o th dilute and concentrated suspensions of clays and o ther ceramic m aterials which behave in m an y respects in a sim ­ ilar m anner to cem ent. Any claims for reduction in bleeding and

“shrinkage before hardening” would be based on a knowledge of behavior of concentrated not dilute suspensions. T he behavior of dilute suspensions would lead to th e conclusion sta te d in th e paper, th a t bleeding would be increased, b u t th is does n o t im ply th a t i t would be in concentrated suspensions. In draw ing this conclusion th e au th o r does ju st w h at he says should n o t be done, in applying reason­

ing from observations on dilute suspensions to cem ent pastes.

C ertainly discrete floes exist in dilute suspensions as can be shown by microscopic observation. If there is overw helm ing evidence th a t such floes disappear when th e suspension is co n cen trated it is n o t given. T h a t these floes coalesce as th e suspension becomes m ore concentrated u n til in a paste there is a continuous netw ork of floes does n o t alter th e fund am ental condition th a t th e p aste is basically m ade up of discrete floes. As an illustratio n, sand d istrib u te d sparsely on the floor exists in th e form of discrete particles; th a t these grains of sand are all in contact w ith one an other w hen th is sand is gathered together in a box does not alter th e fact 'th at th e sand is m ade up of discrete particles. T h a t w ater for hyd ratio n is rigidly excluded from th e floes is very im probab le; it does appear logical t h a t h y d ra tio n would be im peded where th e particles m aking u p th e floes are in contact an d th a t w ater trap p e d w ithin th e floes would n o t contribute to th e fluidity of the paste.

This is borne o u t by the well-established increase in fluidity of a m o rtar or concrete when a dispersing agent is added, even w ith ou t ad ditional air. If this explanation is incorrect, th e a u th o r fails to offer any other for this phenom enon which he adm its exists.

The fou rth conclusion is unsupported b y evidence. T he au th o r again comm its th e error in reasoning which he deplores, th e draw ing of conclusions w ith respect to cem ent pastes from th e beh avio r of dilute suspensions. As previously noted, a dispersing agent reduced bleeding to a greater extent th a n is secured b y air en train m en t, and observation of countless m ortars and concretes over a period of years has failed to disclose an y indication of stratification.

The fifth is again a to tally u nsupp orted conclusion based on assum p­

tions. I t is fu rth e r contradicted b y the a u th o r him self later in th e paper when he says “ on th e whole it appears th a t w ith respect to the properties of fresh concrete, agents like B and C (a dispersing agent) are beneficial, C being som ew hat more so, for w ith an agent of this type, concrete of a given slum p and air conten t can be obtained

SHO ULD PORTLAND CEMENT BE DISPERSED 140 - 9

w ith less w ater in the concrete.” W here th e au th o r has m ade any actual tests he finds cem ent dispersion beneficial; where he departs into the realm s of theoretical reasoning and assum ptions he finds it harm ful.

c. E ffect o f flo ccu latio n on p la s tic ity

1. Flocculation is essential to th e plasticity of granular suspensions.

2. I t seems self-evident th a t th e cohesiveness or stickiness of the paste arises largely from these interparticle forces th a t give th e paste its rigidity.

3. I t has not been dem onstrated th a t concrete made w ith a fluid paste is more workable th a n one m ade w ith a plastic paste.

4. I t is plain th a t during any delay in transportation or placing a dispersed paste would exhibit very undesirable characteristics.

M ost of these conclusions are unsupported b y experim ental evidence and are based on observations of the behavior of dilute suspensions, a fine of reasoning which the au th o r has already vehem ently decried.

The first is a sweeping generalization which is m anifestly incorrect.

D ilute suspensions will no t be plastic w hether the suspended granular solid is flocculated or dispersed. C oncentrated suspensions m ay have plastic properties even though th e particles are m u tually repellent, i. e., dispersed; provided th a t they have suitable shape, surface char­

acteristics, and w ater adsorption properties. This is tru e of cem ent as is readily dem onstrable. I t is equally tru e and even more readily dem onstrable w ith clays. Some clays are natu rally flocculated; others exist natu rally in the dispersed s ta te ; b o th form plastic pastes w ith w ater. T h a t dispersion alters th e plastic characteristics of cement and other pastes is self-evident b u t th a t it destroys the plasticity is not a tenable proposition. I t is of interest to note th a t Lewis, Squires, and B roughton® state th a t “ Dispersion of th e clays is essential to plasticity ” . The second conclusion appears fairly reasonable in itself b u t neglects the considerations applicable to concentrated suspensions such as pastes. A lthough the particles m ay be electrostatically charged and m utually repellent, in concentrated suspensions interparticle forces still play a p a rt and also the forces of adsorption which tend to produce a condensed layer of w ater around each particle are factors which m ust be given consideration. The th ird conclusion is to tally meaningless.

I t m ight equally well be said, w ith as m uch foundation, th a t it has not been dem onstrated th a t concrete m ade w ith a stiff paste is more workable th a n one w ith a fluid paste. E ith er statem en t involves a definition of w orkability which is an indefinite term varying in m eaning depending on the application. W orkability m erely means th a t the m aterial has suitable fluid a n d /o r plastic properties for the use to

4 L ew is, S q u ires, a n d B r o u g h to n : I n d u s tr ia l C h e m is try of C o llo id al a n d A m o rp h o u s M a te ria ls , T h e M a c M illa n C o. 1942, p . 455.

which it is to be put. N othing could be less plain th a n th e last u n ­ supported conclusion. The experience of readym ix com panies, as well as o th er users of dispersing agents in concrete over a nu m b er of years, shows conclusively th a t, w ith some dispersing agents a t least, concrete in which a dispersing agent has been used exhibits highly desirable ch ar­

acteristics during delays in tra n sp o rta tio n and placing ®.

d. E ffe ct o f w e a k e n in g in te rp article attractio n

1. None of these m aterials when used in concrete in recom m ended quantities produced dispersion as defined above. T h a t is, th e y all left th e pastes in th e flocculated state, b u t some of them seemed to reduce the in ten sity of interparticle attractio n .

2. The statem ents as to interparticle a ttra c tio n are m ade te n ta tiv e because the evi­

dence concerning interparticle a ttra c tio n is not quantitive and is somewhat indirect.

These two statem ents ju s t ab o u t answ er each other. T he a u th o r is n o t w arranted in his conclusion th a t th e m aterials of which he is w riting do n o t produce dispersion simply because, as he says in his next sentence, he has neither q u an titativ e nor direct evidence on the interparticle forces existing in th e pastes. F u rth er, he has used as a criterion of the existence or non-existence of dispersion a definition of dispersion which he has invented for himself and which does n o t accord w ith th e com m only accepted m eaning of the term .

I t is, very difficult to discuss th e results of th e tests given in th e paper and the conclusions draw n therefrom since th e a u th o r does n o t nam e the agents used or even give th eir chemical com positions. T he reader is a t a distinct disadvantage in trying to app ly th e a u th o r’s conclusions in a p articu lar case or to cem ent dispersion generally. F ro m th e general descriptions given an a tte m p t is m ade to exam ine th e a u th o r’s con­

clusions as th ey relate to th e w riter’s experience w ith certain dispersing agents, especially those of th e lignosulphonic acid type.

3. E F F E C T O F T H R E E A G E N T S O N C E M E N T S а . A g e n t A

1. F rom the d a ta it m ay be deduced th a t when using th is p articu lar agent w ith these m aterials a given w ater-cem ent ratio can be obtained w ith ab o u t 5 percent less cem ent th a n th e am ount required when n o t using th e agent.

2. The n et effect of using the agent was to replace a given paste w ith a sm aller q u an tity of softer paste.

3. The lack of benefit from weakening the force of flocculation is probably due to th e fact th a t the cem ent particles are norm ally n o t very strongly flocculated. T he value of fu rth er reduction in interparticle a ttra c tio n is debatable.

4. T he effect is to increase th e am ount of settlem ent bleeding.

5. T his m ight be advntageous were it n o t for the fact th a t any increase in th e am o u n t of paste settlem ent is accom panied by an increase in th e d ep th of th e under-aggregate fissures, which fissures weaken th e concrete a n d m ake it more perm eable.

б. To be considered also is th e possibility th a t weakening the force of interparticle a ttra c tio n increases th e tendency tow ard channeled bleeding w ith its a tte n d a n t undesir-

6 R . E . H ick so n , “ P la c in g a H e a v y C o n c re te T e r m in a l o n th e S o u th J e t t y of th e C o lu m b ia R iv e r ,”

P acific B u ild e r a n d E ngineer, p . 41-44 (A p ril 1944).

SHO ULD PORTLAND CEM ENT BE DISPERSED 140 - 11 able “sand boils” and it decreases the ability of the paste to hold th e cements in a plastic state while the concrete is standing or in transit.

The au th o r does no t appear to be sure w hether th is agent is or is not a dispersing agent b u t states in a footnote th a t “ This m aterial is not sold as a dispersing agent, nevertheless tests show th a t when used in sufficient am ount it reduces interparticle a ttra c tio n ” . From d a ta on a product which m ight or m ight no t be a dispersing agent, and prob­

ably is not, it seems hardly reasonable to draw an y general conclusions regarding the effect of dispersion of th e cem ent on concrete. W hat is said under Agent A m ay be correct w ith respect to Agent A b u t it is desired to point out th a t these conclusions are not applicable to cem ent dispersing agents other th an Agent A, even if it is assum ed th a t Agent A is a dispersing agent.

Increased Bleeding— Bleeding is reduced w ith a dispersing agent.

T A B L E B

Percent bleeding Concrete mix— 6.0 sks./cu. yd.

Addition Cement I Cement II

N o n e ... 100 100 Dispersing a g e n t... 25 20

T A B L E C

Concrete Percent reduction in bleeding sks./cu. yd. w ith dispersing agent

4 ]Ą 60

6 55

Weakened Concrete— Compressive and flexural streng ths are increased w ith a dispersing agent. One example® from a large mass of available d a ta on compressive and flexural strengths is shown in Fig. A.

T A B L E D

A ddition Perm eability*

N o n e ... 11.3 Admix B (corresponds to Agent C ) ... 7.6 Admix C (corresponds to Agent A ) ... 6.5

♦ L iq u id inflow in g als, p e r 1000 sq. ft. p e r 24 h rs.

8 R e p o r t of T e sts, S m ith -E m e ry C o m p a n y , L os A ngeles, C al. — C o n so lid a te d V u lte e A irc ra ft — L in d ­ b erg h F ie ld P ro je c t, 1944.

Increased Perm eability— C oncrete is m ade less perm eable w ith a dis­

persing agent. Reference is m ade to one published paper® and to a p u b ­ lished study® on th e p enetration of kerosene th ro u g h concrete. In th is

7 W . M . D u n a g a n : M e th o d s for M e a s u rin g th e P a s sa g e of W a te r T h ro u g h C o n c re te , P ro c . A S T M

866-880 .

8 F . B . H o m ib ro o k : T h e effectiv en ess of V ario u s T r e a tm e n ts a n d C o a tin g s fo r C o n c re te in R e d u c in g t h e P e n e tr a tio n of K ero sen e, J .A .C .I. 41, 13-20 (S e p te m b e r 1944).

A G E

Fig . A — C o m p a ra tiv e com pressive strengths A — 5.5 sks. per cu. yd . Dispersing ag en t ad d ed B— 4.75 sks. per cu. y d . Dispersing ag en t ad d ed C— 5.5 sks. per cu. yd . N o dispersing ag en t D— 4.75 sks. per cu. yd . N o dispersing agent

A G E O E C O /V C /P E rr-/M /5

Fig . B — P lo t o f p e r m e a b ility test d a ta (sp ecim ens under 6 0 ft. pressure h e a d o f w a ter 2 4 hr. p er d a y for e a c h d a y sh ow n)

A — Series " A " . 6 sks. per cu. yd . N o dispersing agent B — Series “ D ” . 5 sks. per cu. yd . Dispersing ag en t ad d ed C— Series “ E " . 6 sks. per cu. yd . Dispersing ag en t a d d e d

SH O ULD PORTLAND CEM ENT BE DISPERSED 140 -13

study tw o agents were used which ap pear to correspond to A gents A and C. The perm eability results are shown in Table D.

A report® of some tests of w atertightness is shown in Fig. B.

Quite contrary to th e a u th o r’s reasoning, dispersion or “reduction of interparticle a ttra c tio n ” appears to reduce perm eability. This is entirely logical and quite to be expected if th e au th o r’s description of flocculation “in settling from dilute suspension th e particle-flocs, in m aking con tact w ith the sedim ent, ten d to form arches or bridges, enclosing relatively large spaces which contribute to th e bulkiness of th e sedim ent” , is accepted. While this is the a u th o r’s description of th e sedim ent from a dilute suspension, it is reasonable to suppose th a t a similar, though m ore condensed, stru ctu re would exist in a con­

centrated suspension. If so, channelled bleeding an d higher perm eability would be expected in th e flocculated paste, n o t in th e dispersed paste

(or more com pact paste w ith lower “interparticle a ttra c tio n ” ).

Decreased P lasticity on Standing or in Transit—A dispersing agent m aintains concrete mixes in tra n sit or on standing in a m ore plastic sta te and reduces segregation. This is a phenom enon clearly shown by field experience w ith concrete mixes and is especially evident and im ­ p o rta n t in th e ready-m ix and tra n s it mix fields. I t is hardly a question th a t can be readily resolved by laboratory d a ta b u t years of experience in th e field have am ply dem onstrated th e beneficial effects of one dis­

persing agent in these respects. Reference is m ade to one report, of te sts(10) and a published article® from am ong other experim ental evidence on this point.

b. A g e n t B

1. The increase in air content was accompanied by a decrease in water content. But since the decrease in water was not as great as the increase in air. the paste content also increased in direct proportion to the increase in air content.

2. It follows that when the water content is not reduced, the increase in slump caused by entraining air in a given mix is due to increase in paste content and not to softening of the paste.

3. The data show conclusively that entrained air has a stiffening effect even though it is more fluid (i. e., it has a lower viscosity) than the water it displaces.

4. So far as workability and bleeding characteristics are concerned the effect of entrained air may be regarded as highly beneficial.

5. Entrained air reduces strength but greatly increases resistance to frost action.

These conclusions regarding the behavior of entrained air in concrete are supported in p a rt b y th e experim ental evidence presented, as well as m uch other evidence in th e literature. The only phenom enon pro­

duced b y this agent is air entrainm ent and cem ent dispersion is n o t involved. I t should be noted th a t th e au th o r has confined himself

* R e p o r t o f T e sts, R u s h E n g in e e rin g C o m p a n y , C h icag o , Illin o is. D e c e m b e r 21, 1938.

10 R e p o r t o f T e sts, T o le d o T e s tin g L a b o ra to rie s, T o led o , O ., A p ril 25, 1945.

to one m echanism of air en tra in m e n t and one ty p e of air en training agent, nam ely, a surface active chemical com pound which d rastically reduces th e surface tension of w ater, i. e., a w etting or foam ing agent.

T here are o ther m echanism s for bringing ab o u t air en tra in m e n t, for example, th e use of alum inum or hydrogen-peroxide in th e mix (strictly, of course, this is n o t air en tra in m e n t b u t en tra in m e n t of hydrogen or oxy­

gen, b u t it serves th e same purpose). C em ent dispersing agents, th a t is chemical compounds, which are preferentially absorbed b y cem ent and im p a rt an electrostatic charge to th e particles, a t least in dilute suspensions, also effect a certain degree of air en train m en t. These reagents are no t w etting or foam ing agents and have only a sm all effect in reducing th e surface tension of w ater. O bviously th e m echanism s of air entrain m en t differ in each of these three cases. I t is quite probable th a t there are o ther m eans and m echanism s of en trainin g air in concrete, such as, possibly, th e use of protective colloids which n eith er affect th e surface tension of w ater nor are preferentially adsorbed b y cement.

In considering the a u th o r’s tre a tm e n t of A gent C it should be borne in m ind th a t th e m echanism w hereby A gent C en train s air is probably n o t th a t which he has described under A gent B.

c. A g e n t C

1. Agent C (air entraining agent an d dispersing agent) reduced the w ater requirem ents more th a n did A gent B (air entraining ag en t only).

2. I t is clear th a t th e n e t effect was a stiffening of th e paste.

3. The paste content required for a given slump increased in direct proportion to the increased air content.

4. W hatever undesirable effects on plasticity an d bleeding characteristics th e reduction in interparticle a ttractio n m ight have, th e y are off-set b y th e en train ed air

5. On the whole, it appears th a t with respect to the properties of fresh concrete, agents like B an d C are beneficial, C being som ewhat more so.

6. W hether or not th e properties of th e hardeued concrete are benefitted equally, or a t all, by agents of this type, is a question th a t cannot be dealt w ith in general term s.

7. W ith respect to durability, entrained air is decidedly beneficial, an d therefore the use of agents like B an d C can be recom m ended when special protection against frost action is necessary.

8. I t is difficult to see how either dispersion, per se, or a w eakening of th e forces of flocculation in th e absence of air entrainm ent, could have m uch influence on frost resistance.

Since this agent is tak en as typical of those which disperse cem ent and also entrain some air, th e a u th o r’s conclusions m ay be considered in the light of the behavior of a dispersing agent w ith which th e w riter has worked.

T he first conclusion is entirely correct and consistent w ith extensive d a ta secured by the w riter on w ater reduction w ith a dispersing agent.

SHO ULD PORTLAND CEM ENT BE DISPERSED 140 - 15

I t is by no m eans clear th a t the n et effect is a stiffening of th e paste.

On the contrary num erous d a ta which th e w riter has accum ulated show th a t th e sum of the absolute volumes of the cem ent, w ater, and air in the mix w ith a dispersing agent com pared w ith the correspond­

ing plain mix of the same slum p is less, th a t is, there is less“ p aste”

in the dispersing agent mix. According to th e a u th o r’s line of reason­

ing, this proves th a t the paste is more fluid. Some typical d a ta on this subject ta k e n a t random from concrete mixes prepared in the investigation of o th er phenom ena are shown in Tables E and F.

T A B L E E — C E M E N T F A C T O R — L B . P E R C U . Y D . 4 95

Absolute volum e per

cu. yd. in cu. ft. N o addition Dispersing agent W a te r... 4.46

0.38 (1.4 percent) 4.84

2.52 7.36

3.82

0.81 (3.0 percent) 4.63

2.52 7.15 A ir...

W ater plus a i r ...

C em en t...

W ater plus air plus cem ent

T A B L E F— C E M E N T F A C T O R — L B . P E R C U . Y D . 5 1 0

A bsolute volume per

cu. yd. in cu. ft. N o addition Dispersing agent W a te r... 4.82

0.27 (1 percent) 5.09

2.59 7.68

4.28

0.76 (2.8 percent) 5.04

2.59 7.63

1 A ir...

W ater plus a ir ...

C em en t...

W ater plus air plus cem ent.

Some other d a ta tak en from a series of tests m ade in an independent laboratory are shown in Table G.

T A B L E G

Cement factor sks./cu. yd.

Slump in.

Air content -—percent

Vol. cem ent + w ater + air

cu. ft./y d .

Addition

5.48* 4M 1.7 7.76 None

4.45** 4M 2.6 7.19 Dispersing agent

5.37** 5 4.6 7.72 Dispersing agent

♦A v erag e of e ig h t m ixes. ♦♦ A verage of th r e e m ixes.

The d a ta im m ediately preceding show th a t, although it m ay be tru e, and probably is, th a t an increase in air co n ten t necessitates an increase in paste content for a given slum p due to th e stiffening effect of air, w ith one cem ent dispersing agent the n e t effect of reduction in interparticle a ttra c tio n and air en tra in m e n t is to require th e sam e or sm aller volum e of cem ent paste com pared w ith sim ilar m ixes w ithout th e dispersing agent for a given slump. T his is d irectly con­

tra r y to th e inference to be draw n from th e a u th o r’s Fig. 6. This reduction of the to ta l am ount of paste required w ith a dispersing agent holds over the norm al range of concrete mixes and th e range of air content usually secured w ith this dispersing agent, which is fairly constant. T h at, as the air co n ten t was increased over 5 percent, the paste requirem ent m ight increase is en tirely possible, b u t such air contents, as those of th e u pper lim its of Fig. 6 w hich seem to reach over 10 percent, would n o t be secured w ith th e dispersing agent except in some very abnorm al mix. In o th er words, if th e air content w ith the dispersing agent in question is k ep t betw een 3.0 percent and 4.0 percent or a t m ost betw een 2.5 percent an d 4.5 p ercen t as would auto m atically be th e case in norm al mixes, th e n e t effect is a reduction in paste volume for a given slum p and by the a u th o r’s line of reason­

ing a softening of the paste.

I t is no t ap p a ren t th a t dispersion or “ reduction of in terparticle a ttra c tio n ” has undesirable effects on p lasticity and bleeding. On the contrary, d a ta previously given show t h a t a dispersing agent increases the effect of en trained air in reducing bleeding. T he a u th o r himself shows th a t cem ent dispersion increases th e beneficial effect on plasticity when he says th a t: “A gent C red u ced th e w a te r require­

m ent more th a n did A gent B ” .

T he fifth conclusion requires no com m ent unless it is to p o in t o u t t h a t in reaching it the au th o r has contradicted his whole preceding argum ent. A fter arguing th a t dispersion is undesirable he concludes th a t air entrain m en t is desirable b u t th a t air en tra in m e n t and dis­

persion are more desirable.

T he sixth conclusion is non-com m ittal.

I t is well-established th a t entrain ed air, provided no o th er dele­

terious effects are introduced, im proves d u rab ility . Again, a fte r condemning cem ent dispersion, th e a u th o r recom m ends A gent C which includes a dispersing agent in preference to A gent B, which is merely an air-entraining agent w ith o u t a dispersing agent.

I t is no t a t all difficult to see how dispersion, w ith o u t air e n tra in ­ m ent, can influence frost resistance. M c M i l l a n / 11^ has shown th a t

11 F . R . M c M illa n : B asic P rin cip les of C o n c re te M a k in g , M c G ra w -H ill B o o k C o ., In c . 1929, p p . 38-41.