AMERICAN CONCRETE INSTITUTE
(ACI PROCEEDINGS Vol. 42)
V o l. 17 November 1945 N o . 2
CONTENTS
Papers and Reports... 105-164
M ain te n a n ce and Repair o f Concrete Bridges on the O regon H ig h w a y System... G. S. P A X S O N 105
Should Portland Cement Be D is p e rs e d ? ...T. C. POWERS 117
A n Investigation o f the Strength o f W e ld e d Stirrups in Reinforced Concrete
Beams ORESTE M O RETTO 141
News Letter... 1-12 42nd A N N U A L A C I CO N V EN TIO N , New Yorl< City, Feb. 18-21,1946
N ew Mem bers • H o n o r Roll • W ho's W ho • Phaon H . Bates Retires
• Douglas E. Parsons
to provide a com radeship in finding the best ways to do concrete work o f a ll kinds and in spreading that knowledge
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 .
S7.50 by the year S1.50 per copy
Extra copImIo members S1.00
Discussion closes M a rch 1, 1 94 6
Sept. JU ’ 45 Concrete Construction in the N a tio n a l Forests— C liffo rd A . Betts
L a p p e d Bar Splices in Concrete Beams— Ralph W . Kluge and E dw ard C . Tuma Tests o f Prestressed Concrete Pipes C ontaining A Steel C ylinder— C ulbertson W . Ross Field Use o f Cement C ontaining V in s o l Resin— Charles E; W u e rp e l
N o v. J L '4 5 M ain te n a n ce and Repair o f Concrete Bridges on the O re g o n H ig h w a y System
— G . S. Paxson
Should P ortland Cement Be Dispersed?— T. C. Powers
A n Investigation o f the Strength o f W e ld e d Stirrups in Reinforced Concrete Beams—
O re s te M o re tto
V o l. 17— N o. 2 November 1945
r'Cr a 5 ivsiieuw
o f i t a * . >
J O U R N A L
o f the
AMERICAN CONCRETE INSTITUTE
P u b lis h e d b y th e A m e r ic a n C o n c r e te In s titu te . T h e In s titu te w a s fo u n d e d 1 9 0 5 ; in c o r p o r a te d in th e D is tric t o f C o lu m b ia in 1 9 0 6 as T h e N a tio n a l A s s o c ia tio n o f C e m e n t U se rs; th e n am e c h a n g e d 1 9 1 3 b y c h a r te r a m e n d m e n t; r e in c o r p o r a te d , w it h n e w sta te m e n t o f o b je c ts , A u g u s t 8 , 1 9 4 5 . T h e J o u r n a l is issued six times y e a r ly in th e m onths o f J a n u a ry , F e b ru a ry , A p r il , J u n e , S e p te m b e r a n d N o v e m b e r u n d e r th e a u th o r ity o f th e
BOARD OF DIRECTION, AMERICAN CONCRETE INSTITUTE
President D O U G L A S E PARSONS
V ic e Presidents
H A R R IS O N F. G O N N E R M A N , S T A N T O N WALKER Secretary-Treasurer
H A R V E Y WHIPPLE Regional Directors
P A U L W . N O R T O N FRANK H . J A C K S O N
M Y R O N A . S W A Y Z E CHARLES S. W H IT N E Y
ALEXAND ER FOSTER, JR. HERBERT J. GILKEY
Direcfors-at-Large
HARRY F. T H O M S O N ROBERT F. BLANKS HENRY L. KENNEDY
Past Presidents
RODERICK B. Y O U N G R A Y M O N D E. D A V IS
BEN MOREELL M O R T O N O . W ITHEY
RO Y W . CRUM
Papers a n d o th e r c o n trib u tio n s p re p a re d w it h a v ie w to J o u rn a l p u b lic a tio n sh ou ld b e s u b m itte d in t r ip lic a t e , a d d re sse d : S e c re ta ry , P u b lic a tio n s C o m m itte e , A m e r ica n C o n c r e te In s titu te , 7 4 0 0 S e co n d B o u le v a rd , D e tr o it 2 , M ic h ig a n . P u b lic a tio n o f a c o n trib u tio n d o e s n o t im ply th e a c q u ie s c e n c e o f c ritic s ( w h o s e a d v ic e is so ug ht b y th e C o m m itte e , p r io r to a c c e p ta n c e ) o r o f th e In s titu te in th e o p in io n s w h ic h it e xpresses n o r th e a p p ro v a l o f d a ta o r p ra c tic e w h ic h th e c o n tr i
b u tio n re c o rd s . In s titu te a u th o r ity a tta c h e s o n ly to S ta n d a rd s fo r m a lly a d o p te d as p r o v id e d in th e By- L a w s . A c o m m itte e r e p o r t im p lie s m e re ly th e jo in t c o n tr ib u tio n o f a n a p p o in te d g ro u p .
S u b s c rip tio n p ric e $ 7 .5 0 p e r y e a r p a y a b le in a d v a n c e . To m em bers, $ 7 .5 0 p e r y e a r , in c lu d e d in th e a n n u a l
d u e s . ( A s p e c ia l d ue s ra te o f $ 3 .0 0 p e r y e a r a p p lie s fo r “ a s tu d e n t in re s id e n c e a t a re c o g n iz e d te c h n ic a l o r e n g in e e r in g s c h o o l” a n d in c lu d e s J o u r n a l s u b s c rip tio n . B ou n d vo lu m e s 1 to 4 0 o f P R O C E E D IN G S O F T H E A M E R I C A N C O N C R E T E IN S T IT U T E (1 9 0 5 to 1 9 4 4 ) a r e fo r sa le as f a r as a v a ila b le , a t p ric e s to b e h a d on in q u ir y o f th e S e c re ta ry -T re a s u re r. S p e c ia l p ric e s a p p ly f o r mem bers o r d e r in g b o u n d volu m e s in a d d itio n to th e m o n th ly J o u r n a l.
P u b lic a tio n add re ss: 7 4 0 0 S e co n d B o u le v a rd , D e tr o it 2 , M ic h ig a n . C o p y r ig h t, 1 9 4 5 , A m e ric a n C o n c r e te In s titu te , P rin te d in U . S. A . E n te re d a t th e Post O f f ic e a t D e tr o it, M ic h ig a n , as m ail o f th e se co n d class u n d e r p ro v is io n s o f th e A c t o f M a r c h 3 ,1 8 7 9 .
d isto rtio n t h a t con crete m em bers will s ta n d w ith o u t serious in ju ry is su rp risin g in view of th e general opinion t h a t concrete is a stiff an d b r ittle m a te ria l. T h e cause of d am ag e of th is k in d can be d e term in ed easily, a n d th e re p airs can be expected to effect a p e rm a n e n t cure.
D e te rio ra tio n caused b y som e w eakness of th e m a te ria l itself is m ore difficult to tr e a t. T h e cause of th e tro u b le is obscure an d rem edial w ork is o fte n only a te m p o ra ry check to th e deterio ratio n .
T h e first alarm in g exam ple of th is second ty p e of d isin te g ra tio n to be n o ticed on th e O regon hig h w ay sy stem show ed u p on th e S nake R iv e r B ridge a t O n tario , Ore. T h is bridge consists of fo u r 200-ft. steel tru ss sp an s w ith concrete deck, cu rb s a n d piers, a n d w ith a sh o rt concrete v ia d u c t ap p ro a c h a t each end. A steel h an d ra il w as used on th e tru ss sp a n a n d a concrete h a n d ra il on th e a p p ro ach spans. T h e bridge was b u ilt in 1924 an d b y 1928 w ide cracks h a d opened u p in th e piers an d in th e la rg e r m em b ers of th e h a n d ra il. T hese cracks w ere p ecu liar in t h a t little , if an y , d isin te g ra tio n of th e concrete accom panied th e cracking.
T h e cracks w ere re la tiv e ly wide, an open w id th of as m u ch as th re e - e ig h th s in ch w as found, b u t th e concrete along th e edges of th e crack a p p e a re d sound. Fig. 1 shows a m assive h a n d ra il p o st w ith th e wide crack s show ing in th e to p . F ig. 2 shows a h ead w all of a c u lv e rt b u ilt te n y ears before th e p ic tu re w as ta k e n . T h e cracking occurred in th e first five y e a rs a fte r th e stru c tu re w as b u ilt, a n d h as show n little or no progress since t h a t tim e. T h e fa c t t h a t th ese cracks occur in m em bers n o t su b je c t to s tru c tu ra l stress in d icates a lack of a n y connection betw een stress a n d th is ty p e of cracking.
A n a tte m p t w as m ad e to seal th e cracks in th e S nake R iv e r B ridge p iers w ith a h e a v y m ix tu re of w hite lead a n d oil forced in u n d e r pressure.
T h e co n tin u ed opening of th e cracks broke th e seal, a n d th e p ier sh a fts a n d w ebs w ere encased w ith concrete in 1934. T h is new concrete w as w aterp ro o fed in 1937 a n d no fu rth e r tro u b le h a s occurred. T h e h a n d rails w ere n o t encased, a n d th e cracks h av e show n no increase in w id th fo r th e p a s t te n years. I t seem s t h a t th is ty p e of cracking ta k e s place d u rin g th e first few y ears a fte r th e concrete is placed, a n d if n o t com p lic a te d b y o th e r ty p e s of disin teg ra tio n , th e n ceases or a t least progresses v e ry slowly. T h ere are a few exam ples of th is ty p e of cracking in th e W illam ette R iv e r V alley w here only a few cycles of freezing a n d th aw in g occur each y ear. F ig. 3 show s a h a n d ra il p o st on th e W illam ette R iv e r B ridge a t A lb an y in 1937. N o re p a ir w ork has to be done, a n d th e re has been no a p p a re n t change in th e condition in th e la s t eig h t years.
A t th e tim e th ese stru c tu re s w ere b u ilt it w as n o t com m on p ractice to m ak e alkali d e te rm in a tio n s on th e cem ent used. I t so h ap p en ed , hm vever, t h a t m o st of th is k in d of tro u b le developed in concrete m ad e w ith c em en t m a n u fa c tu red d u rin g th e first tw o y ears o p e ratio n of one
Fig. 3— H a n d ra il post a t the W illa m e tte River
B r i d g e i n
A lb a n y .
cem en t p la n t. Sam ples w ere o b ta in e d fro m th e corners of a b in w hich p ro b a b ly re p re se n te d th e first c em en t sto red . T h ese sam ples show ed a n alk a li c o n te n t of 1.68 p er cen t, c a lc u la te d as Nc^O. I t is p ro b a b le t h a t th is ty p e of crack in g can be a ttr ib u te d to th e in te ra c tio n of a h ig h -alk ali ce m en t a n d a re a c tiv e ag g reg ate, as no n o tic e a b le c ra ck in g of th is ty p e h a s b e en fo u n d w hen th e se sam e ag g reg ate s a n d c em en ts ru n n in g fro m 0.6 to 0.8 p e rc e n t alk ali w ere used. S tru c tu re s b u ilt in th e sam e a re a w ith th e sam e ag g reg ates a n d w ith cem en t fro m th e sam e m ill a fte r a change in th e source of ra w m ate ria ls, h a v e show n no d is in te g ra tio n of th is ty p e .
W h en th e c rack in g h as b ee n serious en o u g h to n e c e ssita te re p a ir, th e su rfaces h a v e been ch ip p ed a w a y a n d th e m e m b ers en cased w ith co n crete m ad e w ith low -alkali c em en t a n d a g g re g ates k n o w n to b e n o n re a c tiv e . T h e p re p a ra tio n of th e surfaces a n d th e p lacin g of th e con
c rete is carefu lly done to in su re a good b o n d . W h ere a n e n c a se m e n t exceeding fo u r inches th ic k is placed, a n c h o r dow els a n d m esh rein fo rc e
m e n t are used.
T h e cause of th is d isin te g ra tio n , w hich m an ifests itself b y w ide crack s, is q u ite clearly re la te d to th e alk ali c o n te n t of th e c e m e n t a n d th e re
ac tiv e n ess of th e ag g reg ates. T h e fa c t t h a t it occurs in lo catio n s h a v in g m ild clim a te a n d few cycles of freezing a n d th a w in g as w ell as in th e colder clim ates in d ic a te s t h a t te m p e ra tu re ch an g e is n o t th e p rim a ry cause. A fte r th is c rac k in g h as occurred, m o istu re p e n e tra te s th e co n cre te m ore easily a n d d is in te g ra tio n d u e to freezing a n d th a w in g is ac c elerate d . T h is ty p e of d isin te g ra tio n , w hile serious in th e few s tru c
tu re s t h a t h a v e been affected, is n o t of g re a t concern in Oregon. T h e cause an d m ean s of p re v e n tio n are know n.
D u rin g th e la s t 15 y ears a n o th e r ty p e of d isin te g ra tio n h as developed th a t is m u ch m ore serious th a n th e ty p e described above. T h e first in d ic a tio n of tro u b le u su ally shows u p w ith in fo u r or five y e ars a fte r co n stru c tio n . A large n u m b e r of fine cracks appear. T hese are closely spaced a n d p arallel w ith th e edges of th e affected m em bers. T h e cracks are filled w ith a grey dep o sit an d are generally called “ D -lin e” cracks.
In so fa r as h as been observed, th is ty p e of d isin te g ra tio n h as been con
fined to th e e a ste rn section of th e s ta te a n d to th e hig h er p a rts of th e C ascade M o u n ta in s. I t a p p ears to be definitely a fu n ctio n of th e n u m b e r of cycles of freezing a n d th aw in g . I n th e w estern section of th e s ta te w here a few cycles only of freezing a n d th a w in g occur an n u ally , no tro u b le h as been experienced.
T h is d isin te g ra tio n , so far, has been confined to h an d rails, curbs, w ing w all to p s, a n d o th e r m em bers above th e ro ad w ay surface. M em b ers w hich are p ro te c te d b y th e ro ad w ay slab hav e n o t been affected. T hese o b serv atio n s p o in t to w a rd th e conclusion t h a t th e ten d en cy to disin
te g ra te , due p rim arily to a w eakness of th e concrete itself, can be over
com e, or a t least re ta rd e d , b y w aterproofing th e surfaces of m em bers exposed to ra in or snow.
One significant p o in t b ro u g h t o u t in th e analysis of d a ta from th e fre q u e n t inspections of th e bridges on th e h ighw ay sy stem is th e correla
tio n betw een th e d a te of co n stru ctio n an d th e presence or absence of th is
“ D ” line d isin te g ratio n . A ta b u la tio n of all concrete bridges on a 228- m ile section of th e Old Oregon T ra il H ighw ay (U. S. 30) betw een U m atilla a n d O n tario was m ade recen tly . T h ere are 61 stru c tu re s in th is section.
Of these 61 stru c tu re s, 37 were b u ilt p rio r to 1930 an d 24 were b u ilt from 1930 to 1937. Of th e 37 older stru c tu re s, only seven show evidences of
“ D -lin e” d isin teg ratio n , a n d five of th ese were b u ilt w ith th e high-alkali ce m e n t previously referred to . T h e d isin te g ratio n on th e o th e r tw o s tru c tu re s occurs a t th e end of a wing w all an d in a section of cu rb w here la ita n ce in d icate s im p ro p er placing. Of th e 24 stru c tu re s b u ilt from 1930 to 1937, 15 show “ D ” line d isin te g ra tio n in m ore or less degree, a n d fo u r of th e o th e r nine stru c tu re s were w aterp ro o fed w ith in tw o y ea rs a fte r th e ir con stru ctio n .
T h e co n stru c tio n of th e se 61 bridges covers a period of 20 years. In general ag gregates from th e sam e local sources w ere used. W e are n o t re a d y to a d m it t h a t th e la te r m eth o d s of pro p o rtio n in g , placing, an d cu rin g are inferior to th o se used 20 y ears ago. T h e rem ain in g in g re d ie n t is th e cem ent. A t a b o u t 1930, finer grinding of cem ent becam e general p ra c tic e to m eet th e d em a n d for higher e a rly -stre n g th concrete. T h e
s tre n g th w as o b tain ed , b u t i t seem s p ro b a b le t h a t i t w as a t a sacrifice of d u ra b ility .
B y 1937 th e e x te n t of “ D -lin e ” d isin te g ra tio n in th e h ig h w ay s tru c tu re w as causing serious concern. T h e d e te rm in a tio n of th e basic cause of th e tro u b le is to o com plex a p ro b lem for a single s ta te h ig h w ay o rg an izatio n , a n d b u t little progress along th is line h as been m ade. A m a in te n a n c e p ro ced u re h a s been developed, how ever, w hich h a s been q u ite successful in re p a irin g d am ag e t h a t h a s a lre a d y o ccu rred a n d in p re v e n tin g fu r th e r progress of th e d isin te g ra tio n . T h is p ro ced u re consists in th e rem o v al a n d rep la c e m en t of th e d isin te g ra te d con crete a n d w aterp ro o fin g of th e surfaces w ith a linseed oil a n d w h ite le a d p a in t co at. T h is p ro c ed u re follows, in general, th e suggestions m a d e b y F . R . M cM illan , D ire c to r of R esearch of th e P o r tla n d C em e n t A ssociation.
T h e follow ing is a n o u tlin e of th e p ro ced u re used b y th e O regon S ta te H ig h w ay D e p a rtm e n t in p a tc h in g d isin te g ra te d co n crete a n d w a te r
proofing con crete surfaces for th e p re v e n tio n of d isin te g ra tio n .
W h en d isin te g ra tio n of th e com m only called “ D -lin e ” ty p e h a s ta k e n place, i t u su a lly affects th e edges a n d corners of curbs, h a n d ra ils, w ing w alls, a n d o th e r exposed m em bers. T o p re v e n t fu rth e r progress i t is n ecessary to rem ove co m p letely all d isin te g ra te d m a te ria l, g re a t care bein g ta k e n to re ach so u n d co n crete b e y o n d th e e x te n t of th e d isin teg r a te d area. T h is ca n be accom plished b y th e use of h a m m e rs a n d chisels on sm all areas or b y p a v in g b re a k e rs o r ch ip p in g h a m m e rs on larg er areas. I f th e d isin te g ra tio n h a s reach e d a n a d v a n c e d stag e, th e co m p lete rem o v al a n d rep la c e m en t of th e affected p o rtio n of th e s tru c tu re m a y be n ecessary. T h e im p o rta n c e of th e rem o v al of all tra c e s of d isin te g ra te d m a te ria l c a n n o t be o verem phasized. E x p erien ce h a s show n t h a t o ften th e w o rk m a n w ill rem ove all visible affected m a te ria l, th e n place a p a tc h on w h a t w as, in his ju d g m e n t, so u n d concrete, o n ly to disco v er la te r t h a t th e m a te ria l a d ja c e n t to th e p a tc h co n tin u es to d isin te g ra te .
A fte r th e rem o v al of th e d isin te g ra te d co n crete a p a tc h is ap p lied , th e success of w hich d ep en d s u p o n secu rin g a b o n d to th e p a r e n t concrete, o vercom ing th e te n d e n c y of th e p a tc h to sh rin k a fte r p la c e m e n t, a n d p ro p e r curing. All places to be p a tc h e d should be c h ip p ed o u t to secure n o t less th a n th re e -fo u rth s-in ch th ick n ess for th e p a tc h . T h e edges of th e p a tc h sh o u ld be sq u a re a n d n o t fe a th e re d . All surfaces sh o u ld be clean a n d ro u g h so as to secure a good b o n d a n d sh o u ld be s a tu r a te d th o ro u g h ly b y sev eral ap p lic a tio n s of w a te r. T h e p re sh rin k a g e of m o rta r is re q u ire d for all p atch es. T h is is done b y m ixing th e m o r ta r well a h e a d of use a n d le ttin g i t sta n d . T h e tim e re q u ire d for p re sh rin k ag e of m o r ta r v arie s w ith th e d ifferen t b ra n d s of c e m en t a n d co n d itio n s of te m p e ra tu re a n d h u m id ity , a n d is b e s t d e te rm in e d b y e x p e rim e n t on th e
job. In general th e m o rta r th u s p re sh ru n k should be susceptible to use w ith o u t th e ad d itio n of w a te r before rew orking it for ap p licatio n . A fter th e p a tc h h a s been applied, p ro p er care in curing m u st be ta k e n b y keep ing th e p a tc h covered w ith w et b u rla p for six to te n hours, a fte r w hich it can be covered w ith d a m p sand or b u rla p u n til th e concrete h as th o ro u g h ly h ard en e d .
N ew concrete p a tc h e s should be allow ed a t le a st tw o w eeks to d ry o u t before a p p ly in g th e w aterp ro o fin g tre a tm e n t. N ew concrete should be
November 1945
given a n eu tralizin g w ash before th e ap p licatio n of th e linseed oil used in th e w aterproofing tre a tm e n t. A solution consisting of th re e p o u n d s of zinc su lp h a te cry stals to a gallon of w a te r is b ru sh ed over th e surface to be tr e a te d a n d allow ed to d ry for 48 hours. W hen th o ro u g h ly d ry , all c ry sta ls on th e surface are rem oved b y wire b rushing. T h is tre a tm e n t is n o t necessary on old concrete.
B efore th e w aterproofing tre a tm e n t is applied, it is necessary t h a t th e con crete surface be clean a n d d ry. D u s t an d loose m a te ria l can be rem o v ed w ith a w ire bru sh . R o a d oil or grease can be rem oved b y scru b b in g w ith gasoline or a so lv en t. Efflorescence can be rem oved b y sc ru b b in g w ith a te n p er cen t so lu tio n of hydrochloric acid. W hen w a te r is used in cleaning, am ple tim e m u st be allow ed to p e rm it th o ro u g h d ry in g of th e concrete surfaces before ap p ly in g th e w aterproofing.
A fter th e surface h as been p ro p erly p rep a red a n d is clean a n d d ry , tw o co a ts of h o t linseed oil are to be applied. T h e first co at consists of a m ix
tu r e of 50 p er c e n t raw linseed oil an d 50 p er cen t tu rp e n tin e h e a te d to 175 F . an d ap p lied w ith an o rd in ary p a in t b ru sh . B e tte r resu lts will be o b ta in e d if th e air te m p e ra tu re is above 65 F . T h e first coat is allow ed to set 24 ho u rs before a p p ly in g th e second coat. A fter th e first coat of th e linseed o il-tu rp e n tin e m ix tu re has set, sp o ts will u sually be n oticed w here th e concrete is m ore porous th a n th e rem ain d er of th e surface tre a te d . T h ese sp o ts should be sp o t tre a te d w ith th e h o t m ix tu re a n d allow ed to s e t before th e second coat of linseed oil is applied.
T h e second co at consists of u n d ilu te d raw linseed oil h e a te d to 175 F.
a n d ap p lied in th e sam e m a n n e r as th e first coat. W hen th is co at is th o ro u g h ly d ry , th e surface is re a d y for a p a in t coat.
T h e e n tire surface tre a te d w ith oil is given tw o coats of w hite lead a n d oil p a in t, tin te d to th e desired shade. A concrete color can be o b ta in e d b y th e ad d itio n of lam p b lack an d raw sienna ground in oil.
T h e w h ite p a in t used in Oregon has th e following fo rm u la:
P aint Composition P er cent
Pigm ent not less th a n 70
Vehicle not more th a n 30
Pigment Composition
W hite lead carbonate 40.0 to 45.0
T itanium barium pigm ent 35.0 to 40.0
Zinc Oxide 15.0 to 20.0
T inting pigm ents, if required 0.0 to 5.0
T h e first co at is th in n e d b y th e ad d itio n of tw o q u a rts of tu rp e n tin e a n d tw o q u a rts of boiled linseed oil to th e gallon oa p a in t. T h e second co a t is th in n e d w ith a b o u t one q u a rt of boiled linseed oil to th e gallon of p a in t so as n o t to give a h e a v y p ig m en t co at t h a t will be susceptible to scaling, b u t w hich is still h ea v y enough to b ru sh o u t u niform ly an d ev en ly .
T h is w aterp ro o fin g tr e a tm e n t is n o t a cure for th e basic w eakness of th e concrete w hich m ak es i t su scep tib le to d isin te g ra tio n . I f p ro p e rly ap p lied a n d m a in ta in e d , it prom ises to p o stp o n e serious d isin te g ra tio n for m a n y years.
Fig. 4 show s a sidew alk w here th e edges h a v e d eveloped “ D -lin e ” d is
in te g ra tio n a n d th e con crete h as cru m b led aw ay. Fig. 5 show s th e sam e sidew alk a fte r p a tc h in g a n d w aterproofing. Fig. 6 show s a h a n d ra il w ith “ D -lin e” d isin te g ra tio n w ell a d v a n ced . Fig. 7 show s th e sam e h a n d ra il a fte r th e affected concrete h as been ch ipped o u t a n d th e m e m b e r b u ilt u p w ith new concrete, b u t before th e w aterp ro o fin g w as ap p lied . T h ese figures show t h a t con crete m em b ers w hich h a v e b een seriously affected can be rep aired a n d m ad e serviceable a n d p re sen tab le.
[
a v a ila b le from A C I a t 2 5 ce nts e a c h — q u a n tify q u o ta tio n s o n re q u e s t. Discussion o f th is p a p e r ( c o p ie s in t r ip lic a t e ) s h o u ld re a c h th e In s titu te n o t la te r th a n M a r c h 1 , 1 9 4 6 JTitle 4 2 -6 — a p art o f PROCEEDINGS, A M E R IC A N CONCRETE INSTITUTE V o l. 42
J O U R N A L
o f 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
(c o p y rig h te d )
V o l. 17 N o . 2 7 4 0 0 S E C O N D B O U L E V A R D , D ETR O IT 2 , M I C H I G A N Novem ber 1945
Should Portland Cement Be D ispersed?*
By T. C. POWERSt
M e m b e r A m e ric a n C o n c r e te In s titu te
S Y N O P S I S
A developm ent of definitions of wetting and dispersion is followed by a discussion of dispersion of portland cement.
F rom elem entary principles it appears th a t a w etting agent is unnecessary, for p ortland cem ent is highly hydrophilic.
T he dispersed state of portland cement in w ater is defined as th a t state in which interparticle attractio n in a fresh paste is absent or so weak th a t it has no appreciable effect on the physical properties of th e fresh paste.
E xperim ents and reasoning from general principles indicate th a t dis
persion would be undesirable because it would increase the rate and am ount of sedim entation and prom ote particle-size segregation in cement paste; it would destroy the plasticity of th e pastes and give them the properties of a fluid, a probably undesirable change; it would have no beneficial effect on rate of hydration during th e early stages through an increase in exposed surface area because the whole surface is norm ally exposed to w ater even when the particles are flocculated.
A reduction in interparticle attractio n short of actual dispersion should reduce th e w ater required for a given slump, b u t it would not improve w orkability except in unusually rich mixes. I t would increase bleeding.
Air entrainm ent requires an increase in paste content and reduction of w ater content to m aintain a given slump. I t reduces strength b u t improves frost resistance. I t im proves w orkability and reduces bleeding.
Air entrainm ent together w ith some reduction in interparticle a ttra c tion affects paste content and w ater requirem ent in th e same w ay as air entrainm ent alone, b u t the increase in paste content is smaller and the reduction in w ater content is greater th a n when there is no reduction in interparticle attractio n . Air entrainm ent offsets the undesirable effects of reducing interparticle attractio n on plasticity and reduces bleeding.
Some agents do n o t affect the chemical processes of hardening; their effects on strength can be predicted from the voids-cement ratio. Others tend to re ta rd hydration unless they contain an accelerating agent. Such agents have different effects w ith different cements.
* R eceiv ed b y th e I n s tit u te M a rc h 19, 1945.
f M a n a g e r of B asic R esea rc h , P o r tla n d C e m e n t A ssn. R e se a rc h L a b o ra to ry , C hicago.
(117)
IN T R O D U C T IO N
T h is discussion was p re p a re d in response to m a n y re q u e sts for in fo r
m a tio n on v a rio u s ad m ix tu re s sold as dispersing ag en ts. T h e q u estio n s a re u su a lly one of tw o ty p e s ; (1) W h a t is th e effect of A d m ix tu re X on concrete? a n d (2) W h a t is b a c k of th e id ea of dispersion of cem en t? T h is discussion will deal p rim a rily w ith th e q u estio n of w h a t is b a c k of th e id e a of dispersing cem en t— n o t w ith th e a c tu a l m e rits of p re p a ra tio n s t h a t h a v e been sold as dispersing agents.
Som e m a te ria ls sold as dispersing a g en ts m ay h a v e m e rit because of effects n o t re la te d to dispersion. H ow ever, th e y c a n n o t be endorsed on th e basis of som e of th e claim s m ad e for th e m . I n ex p lain in g re su lts o b ta in e d or h o p ed for th e assu m p tio n s h av e been m ade t h a t a n o rm al cem en t p a ste is com posed of in d iv id u a l floes (clusters) of cem en t grains such as m a y be seen w hen a sm all a m o u n t of cem en t is su sp en d ed in a large volum e of w a te r; also t h a t w a te r for h y d ra tio n does n o t p e n e tra te th e floes a n d hence t h a t th e cem en t c a n n o t h y d r a te as well as i t m ig h t;
t h a t th e w a te r t h a t is “ tr a p p e d ” in th e floes does n o t c o n trib u te to w o rk ab ility . I t is th e n fu r th e r assum ed t h a t th e floes of c em en t g rain s can be caused to d isin teg rate , t h a t is, t h a t th e grains in a floe can be dispersed, b y a d d in g a su ita b le a g e n t w hich causes th e g rain s to acq u ire e le c tro sta tic charges a n d th u s to becom e m u tu a lly re p e lle n t; a n d t h a t w hen th e cem en t grains are th u s dispersed th e y h y d r a te m o re ra p id ly a n d to a g re a te r e x te n t th a n w hen flocculated. M oreover, it is assum ed t h a t w hen th e p a rticle s a re b ro u g h t to a s ta te of m u tu a l repellency, th e a m o u n t of b leeding or “ s e ttle m e n t-sh rin k a g e ” is reduced. D isp e r
sion is said also to im p ro v e resistan ce to fro st actio n .
M o st of th ese assu m p tio n s seem plausible, b u t th e y are n o t co m p atib le w ith th e re su lts of re c e n t researches on th e n a tu r e of c em en t p a s te a n d on th e w hole are believed to be u n te n a b le on e ith e r a th e o re tic a l or em pirical basis. T h e basis for th is conclusion w ill be given briefly in th is discussion.
T h e q u e stio n to be discussed can easily becom e confused b y th e fa c t t h a t som e p re p a ra tio n s used as disp ersin g a g en ts are a c tu a lly m ix tu res co n ta in in g n o t only a disp ersin g a g e n t b u t also a n ac c e lera to r such as calcium chloride, a n d b y th e fa c t t h a t m ost, if n o t all, such m a te ria ls cause m ore or less a ir-e n tra in m e n t. Y e t all th e effects of such m ix tu re s h a v e been a ttrib u te d , b y som e, to th e su p p o sed dispersion of th e cem en t p articles. I t is th e refo re necessary to rem em b e r t h a t w h a t m a y be said in th e follow ing discussion a g a in st th e idea of dispersion does n o t necessarily im p u g n a n y p a rtic u la r p re p a ra tio n unless th e p re d o m in a n t effect of th e p re p a ra tio n is t h a t of dispersing th e cem ent.
Ow ing to th e m a n n e r in w hich v a rio u s ty p e s of m a te ria ls of th is class h a v e been in tro d u c e d to th e con crete in d u s try , th e re is som e confusion
SHOULD PORTLAND CEMENT BE DISPERSED?
w ith resp ect to th e te rm s “ w e ttin g a g e n t,” “ dispersing a g e n t,” a n d “ air- e n tra in in g a g e n t.” T herefore, a seco n d ary aim of th is discussion is to clear a w a y som e of th e confusion w ith resp ect to term inology. I t is especially im p o rta n t to grasp th e fa c t t h a t ce rta in difficulties su rro u n d th e use of th e te rm “ d isp ersio n ” as applied to p o rtla n d cem ent. T o b rin g o u t th e n a tu re of th ese difficulties, it is necessary to p re se n t a brief discussion of fu n d a m e n ta ls. T h is discussion leads to a definition of dispersion of p o rtla n d cem en t p a ste on w hich th e final p a r t of th e p a p e r is b ased . T h e rea d e r is asked to av o id a p p ly in g th e final con
clusions concerning dispersion as defined here, to dispersion defined in som e o th e r w ay.
T h e discussion of fu n d a m e n ta ls will be found r a th e r sk etch y . F o r th o se seeking fuller in fo rm a tio n a guide to th e lite ra tu re is provided.
THE P H E N O M E N O N O F W E T T IN G *
W h en a d ro p of liquid is placed on a clean surface of a solid, it m ay e ith e r sp re a d on th e surface or rem ain as a m ore or less fla tten ed drop, show ing a definite angle of c o n ta c t w ith th e surface. W hen th e liquid sp read s sp o n tan eo u sly , w e ttin g is said to be com plete (co n ta ct angle = zero) and, if th e liq u id is w ater, th e solid is called hydrophilic. If w ater does n o t sp read b u t establishes a finite c o n ta c t angle, w e ttin g is incom plete or p a rtia l a n d th e solid is reg ard ed as h y d ro p h o b ic .16 P a rtia l w et
tin g , t h a t is, th e fo rm atio n of a c o n ta c t angle, denotes t h a t th e force of adhesion b etw een th e solid a n d th e liquid (which te n d s to cause th e liq u id to sp re a d over th e surface) is less th a n th e surface tension of th e liquid, w hich te n d s to cause th e liquid to g a th e r itself in to a drop. C on
versely, w e ttin g is alw ays com plete w hen th e force of adhesion exceeds th e surface tension of th e liquid.
A w e ttin g a g e n t is u su a lly a m a te ria l com posed of elongated organic m olecules, th e m olecules h a v in g an affinity for th e solid a t one end a n d affinity for th e liq u id a t th e other. Such a n ag en t form s, a lay er of m olecules on th e solid, th e m olecules being so oriented as to p resen t a w e tta b le surface to th e liquid.
W hen th e force of adhesion betw een a pow dered solid an d a liq u id is less th a n th e surface ten sio n of th e liquid, th e pow der a n d th e liquid are difficult to m ix unless a su ita b le w e ttin g a g en t is used. Or if th e a d hesion ten sio n exceeds th e surface ten sio n only slightly, a w e ttin g a g en t will be n o ticeab ly helpful. On th e o th e r h a n d , if th e solid a n d liquid show a stro n g m u tu a l a ttra c tio n , th e liquid will sp read over th e solid surface w ith o u t outside aid. F o r a n y given case th e surface ten sio n an d adhesion ten sio n m a y be m easured an d th e need for a w e ttin g ag en t
*See R ef. 1 a t e n d of p ap e r.
ju d g e d from th e results. I t is sim pler, how ever, m erely to observe d ire c tly w h e th e r w e ttin g is com plete or n o t. F o r exam ple, if th e specific g ra v ity of th e solid exceeds t h a t of th e liquid, sm all p a rtic le s of th e solid, w hen s c a tte re d on th e surface of th e liq u id , will sin k re a d ily if th e con
ta c t angle is v e ry sm all or zero, or th e y will re m ain on th e su rface if th e c o n ta c t angle is sufficiently large, as does an oiled needle on w ater.
I n th e case of p o rtla n d c em en t a n d w a te r it is still sim p ler m erely to observe th e a b so rp tio n w hen w a te r is p laced in a c ra te r of d ry ce m e n t;
cap illary ab so rp tio n is re ad ily a p p a re n t unless th e cem en t co n ta in s a
“ w a te r-re p elle n t” sub stan ce.
A n y such c rite ria ap p lied to p o rtla n d cem en t w ould show t h a t th e w e ttin g of p o rtla n d cem en t b y w a te r could h a rd ly be im p ro v e d b y a w e ttin g ag en t, unless th e c em en t h a s a cq u ired a “ w a te r-re p e lle n t”
coating. T h e degree of so lu b ility of th e c o n stitu e n ts of c em en t a n d , in fact, th e ra p id fo rm atio n of h y d ra te s t h a t occurs im m e d ia te ly on c o n ta c t w ith w a te r show t h a t p o rtla n d cem en t h as a stro n g affin ity for w ater.
T h e a ttra c tio n is so stro n g t h a t each c em en t grain becom es co m p letely su rro u n d e d b y w a te r even th o u g h in a d ilu te suspension th e g rain s are clustered. T h is is show n b y th e fa c t t h a t d u rin g s e d im e n ta tio n of a con
c e n tra te d suspension, i.e., d u rin g bleeding, th e whole su rface a re a of th e c em en t is effective in re g u la tin g th e ra te of flow of w a te r.2 So fa r as th e w rite r know s, no one h as seriously c o n te n d ed t h a t cem en t n eeds a w e ttin g agent.
W h e n a pow der is rea d ily w e tte d , so t h a t each g ra in becom es s u r
ro u n d ed w ith w ater, th e g rain s are n ecessarily s e p a ra te d , a t le a st b y a th in film of w ater. B ecause of th is, th e w e ttin g of a p o w d er b y im m e r
sion in a liq u id is som etim es referred to as dispersion. H ow ever, it is a d v a n ta g e o u s a n d in line w ith m o d e rn te x tb o o k te rm in o lo g y to d ra w a d istin c tio n b etw een w e ttin g a n d dispersion. As described ab o v e, th e te rm wetting p e rta in s to th e sp o n ta n e o u s sp read in g of a liq u id over a surface. I t applies to a n y sh ap e of surface, n o t to p a rtic le s alone.
D ispersion, on th e o th e r h a n d , p e rta in s only to p articles.* I t is th e opposite of flocculation, ag g lo m e ratio n , or co ag u latio n . W e tte d p a rtic le s m a y be in e ith e r a flocculated or a dispersed s ta te . F lo c c u la te d p a rtic le s in a liq u id m a y be h eld to g e th e r b y forces a c tin g across se p a ra tin g films of liq u id ; hence, a flocculated s ta te is n o t necessarily one re q u irin g p a rtic le -to -p artic le co n ta c t.
T h ese re m a rk s give a general id ea of w h a t is m e a n t b y dispersion, flocculation, a n d w e ttin g , b u t a definition of dispersion w holly a d e q u a te for th e p re se n t discussion c a n n o t be given w ith o u t considering som e a d d itio n a l d e ta ils of th e phenom enon.
* B u t n o t n e c essarily to solid p a rtic le s o n ly .
SHOULD PORTLAND CEMENT BE DISPERSED?
INTERPARTICLE FORCES
A ccording to p re se n t-d a y theories, th e in te rp a rtic le force in an aggre
g a tio n of p article s in a fluid m edium is m ade u p p rin cip ally of th e follow ing co m p o n en ts:
(1) A n e v er-p resen t force of a ttra c tio n (van d er W a a l’s forces) w hich causes a d ja c e n t p articles to ad h e re ; an d
(2) A n e le c tro sta tic force of repulsion t h a t opposes th e force of a ttra c tio n . T h is repulsion is stro n g ly d e p e n d e n t on th e en v iro n m e n t of th e p articles a n d is th erefo re su b ject to control.
T h e co n tro l of in te rp a rtic le force m ay involve c o ntrolling th e k in d a n d co n c e n tra tio n of electrolytes, or th e use of ce rta in kinds of organic m olecules or colloids.
B ecause of th e n a tu re of th e relatio n sh ip s betw een d istan ce of se p ara
tio n a n d in te n s ity of repulsion an d in te n s ity of a ttra c tio n , tw o p a rtic le s m a y h av e m in im u m p o te n tia l energy w hen th e y are se p a ra te d b y a sm all b u t definite d ista n c e .3-4 H ence, even in th e flocculated s ta te , suspended p articles m a y te n d to rem ain slig h tly sep arated .*
DISPERSION Spontaneous dispersion
D ispersion h as been p ic tu re d as a spo n tan eo u s process w hereby p articles b earin g like e lec tro static charges “ spring a p a r t an d s ta y a p a r t”
b y d istan ces easily observed w ith th e m icroscope. H ow ever, even u n d e r th e m o st fav o rab le conditions, th e forces of repulsion in a suspension are effective over only v e ry sh o rt distances. Indeed, if th e particles w ere se p a ra te d only b y th e effective d istan c e of repulsion, th e y w ould a p p e a r to be in c o n ta c t u n d e r o rd in a ry m agnification. N evertheless, if th e p articles are v e ry sm all, th e y m a y be capable of dispersing th e m selves b y th e ir B row nian m otion, fa r bey o n d th e ran g e of in te rp a rtic le forces. B row nian m o tio n is a h a p h a z a rd m ovem ent, or v ib ra tio n , caused b y u n b a la n ced im p a c ts of th e m olecules of th e su rro u n d in g m edium . If a p a rtic le is sm all enough, th e forces of th e im p acts received sim u ltan eo u sly from different directions do n o t balance an d th e p article th u s acquires m otion.
P a rtic le s h av in g B row nian m otion w an d er a t ran d o m a n d te n d to b ounce aw ay from each o th e r w hen th e y collide. If th e forces te n d in g to keep th e p article s in m o tio n exceed th e force of a ttra c tio n betw een th e p articles, th e n a s ta te of dispersion will sp o n tan eo u sly be m ain tain e d . On th e o th e r h a n d , if th e forces of a ttra c tio n betw een th e particles exceed th e forces te n d in g to keep th e particles in m otion, th e n a s ta te of flocculation, or agglom eration, will persist. N o te t h a t in te rp a rtic le
* F o r a th o ro u g h d iscu ssio n of in te r p a r tic le forces, see R ef. 5.
rep u lsio n is n o t necessary for disp ersio n ; b u t, of course, th e m o re th e in te rp a rtic le a ttr a c tio n is cancelled b y in te rp a rtic le repulsion, th e sm aller th e a m o u n t of k in e tic energ y re q u ire d to keep th e p a rtic le s dispersed.
B ro w n ia n m o tio n can occur to a significant degree only a m o n g v e ry sm all p a rtic le s— colloidal p articles. H ence, p o rtla n d c e m en t c a n n o t be caused to disperse sp o n ta n eo u sly , for cem en t p a rtic le s are p red o m i
n a n tly m icroscopic, n o t colloidal.6
M ech a n ica l dispersion
E v e n w hen sp o n tan eo u s dispersion is n o t possible, disp ersio n can be effected m echanically. S tirrin g a su spension te n d s to se p a ra te th e p a r ti
cles, p a rtic u la rly if th e stirrin g is v io len t. T h e degree to w hich th e p a rtic le s will becom e se p a ra te d on stirrin g d ep en d s larg e ly on th e in te n s ity of th e in te rp a rtic le a ttra c tio n . Of course, m u tu a lly re p ellen t p a rtic le s sho u ld be m ore easily dispersed th a n th o se w hich h a v e som e te n d e n c y to stic k to g eth er.
* * *
T h e discussion im m e d ia te ly ab o v e w as p re se n te d to show t h a t som e of th e p h en o m e n a p ic tu re d in connection w ith th e use of disp ersin g a g e n ts w ith p o rtla n d cem en t a c tu a lly can occur only a m o n g p a rtic le s t h a t are of tr u ly colloidal dim ensions. I t m u s t be a d d e d t h a t w h e th e r or n o t sp o n tan eo u s dispersion ta k e s p lace is of little p ra c tic a l conse
quence. The im portant question is whether it is necessary or desirable to cancel or reduce the forces of interparticle attraction that norm ally pre
dominate over the forces of repulsion in a suspension of cement particles in water.
INTERPARTICLE A T T R A C T IO N A N D PASTE PROPERTIES
A lth o u g h th e influence of in te rp a rtic le a ttr a c tio n on th e p h y sic al p ro p e rtie s of p a ste s will b e discussed m ore fu lly in th e follow ing section, tw o im p o rta n t effects m a y be m e n tio n e d a t th is p o in t. T h e first is t h a t th e g re a te r th e in te rp a rtic le a ttra c tio n , th e stiffer w ill a p a ste seem to b e w hen i t is stirred . In c o m p a ra tiv e ly c o n c e n tra te d suspensions su c h as cem en t p astes, w here in te rp a rtic le a ttr a c tio n p re d o m in a te s o v er re pulsion, th e suspension b e h av es m ore like a solid th a n a fluid.711' n * T h e o th e r effect a p p e a rs w hen suspensions are allow ed to settle. I f th e forces of a ttr a c tio n p re d o m in a te , th e large a n d sm all p a rticles se ttle to g e th e r.
If th e forces of rep u lsio n p re d o m in a te or if th e n e t force of a ttr a c tio n is v ery w eak, p a rticles t h a t w ould rem ain in c o n ta c t w hen q u iescen t becom e se p a ra te d as th e y fall th ro u g h th e liquid d u rin g sed im en tatio n .
T h e cause of th is m a y be seen b y considering tw o p a rticle s of different size a d h e rin g to one a n o th e r a t th e beginning of th e ir fall th ro u g h th e
* I t b e h a v e s lik e a so lid in t h a t i t is c a p a b le of w ith s ta n d in g s m a ll s h e a iin g forces. S e e p a g e 129, E ffe c t of F lo c c u la tio n o n P la s tic ity .”
liquid. T h e d rag of th e liq u id on th e sm all p a rtic le will be g re a te r p er u n it m ass th a n t h a t on th e large o n e ; therefore, a force te n d in g to se p a ra te th e m w ill develop. If th is force, w hich depends u p o n th e difference in th e size of th e p artic les, exceeds th e force of a ttra c tio n , th e p articles w ill se p a ra te , th e larg e r p a rtic les falling m ore rap id ly . W hen th e a ttr a c tiv e forces are v e ry w eak, a n d especially w hen th e p articles are m u tu a lly rep ellen t, th e sed im en t t h a t is form ed te n d s to be n o n -u n ifo rm in com position, th e p ro p o rtio n of coarse p artic les increasing to w a rd th e b o tto m of th e sedim ent. M oreover, th e sed im en t form ed is co m p act a n d difficult to redisperse, w hereas a flocculated sed im en t is b u lk y , soft, a n d easily re sto re d to its original s ta te in th e suspension. T h is fe a tu re of th e b e h a v io r of th e dispersed suspension is v ery significant w ith respect to th e questio n of dispersing cem ent, as will be developed below.
D E F IN IT IO N O F DISPERSION A P P L IC A B L E T O CEM EN T PASTE
T h e foregoing discussions serve to show t h a t a criterio n for dispersion ap p licab le to colloidal suspensions is n o t alto g e th e r applicable to te m p o ra ry suspensions of non-colloidal particles; dispersion ca n n o t m ean ex a c tly th e sam e th in g for b o th ty p e s of suspension. Y e t circum stances seem to require using th e sam e te rm for b o th cases; indeed, th e re is consider
ab le ju stificatio n for it. T h e dilem m a is avoided b y th in k in g in te rm s of in te rp a rtic le a ttra c tio n in ste a d of th e repulsion t h a t is im plied b y th e w ord “ dispersion.” A su itab le definition of dispersion can be set u p in te rm s of th e influence t h a t in te rp a rtic le a ttra c tio n has on certain p ro p erties of suspensions, p a rtic u la rly , th e force-flow relatio n sh ip . W hen in te rp a rtic le a ttra c tio n is a b se n t or negligible, a suspension t h a t is n o t to o c o n c e n tra ted flows like a tru e flu id ; b u t w hen in te rp a rtic le a ttra c tio n is n o t negligible, th e suspension acquires th e p ro p erties of a p lastic solid to som e degree. Also, in a dilute suspension of particles, segregation of sizes ta k e s place d u rin g se d im en tatio n if th e in te rp a rtic le a ttra c tio n is a b se n t or w eak an d it does n o t ta k e place if in te rp a rtic le a ttra c tio n is stro n g . I n view of such o bservations as th ese th e following definition of dispersion is used in th e discussion of cem ent p aste t h a t follow s:
When interparticle attraction in a fresh cement paste is so weak that it has no appreciable effect on the behavior and physical properties of the paste, the particles in the paste m ay be said to be dispersed.
B y th is definition we w ould call a n y suspension dispersed in w hich th e in te rp a rtic le a ttra c tio n is zero or negative. T h is w ould n o t disagree w ith o th e r definitions ap p lied to colloidal suspensions. B u t we w ould also call a suspension dispersed if th e in te rp a rtic le force w as positive, b u t to o w eak to h av e an appreciable effect on th e physical p ro p erties of
th e suspension. I t should be n o te d t h a t th e defin itio n does n o t re s t on th e presence or absence of p a rtic le -c lu s te rs ; n e ith e r does i t im p ly t h a t dispersion is a sp o n tan e o u s process.
T h is definition a d m its th e po ssib ility of v ario u s degrees of in te rp a rtic le a ttr a c tio n am ong th e p a rtic le s in flocculated suspensions, t h a t is, su s
pensions in w hich th e a ttr a c tio n h a s a n effect on flow -properties, etc.
C o n seq u e n tly , we can discuss tw o q u estio n s, one p e rta in in g to th e d e sira b ility of p ro d u c in g dispersion, a n d one p e rta in in g to th e d e sira b ility of changing th e in te n s ity of in te rp a rtic le a ttr a c tio n in a flo ccu lated p aste.
DISPERSION O F P O R T L A N D C E M E N T Flocculated state o f normal po rtla n d cement paste
As im plied a t v ario u s p o in ts in th e foregoing discussion, th e re is no q u estio n b u t t h a t cem en t p a rticles in a n o rm a l p a s te a re flocculated.
T h is m a y be seen b y m icroscopic e x a m in a tio n of c e m e n t-w a te r m ix tu re s sufficiently d ilu te to tra n s m it lig h t or b y o b se rv a tio n , a t low m agnifica
tio n , of th e te x tu re of p a ste s th ro u g h th e w all of a glass c o n ta in er.
T h e process b y w hich cem en t becom es flocculated m a y be v isu a lized as follow s: U n d e r th e a c tio n of a m e ch an ical m ixer, ce m e n t p a rtic le s p ro b a b ly te n d to be dispersed d u rin g th e first few seconds of c o n ta c t w ith th e w ater. C hem ical reactio n s begin im m e d ia te ly a n d co n tin u e a t a re la tiv e ly h ig h ra te for a perio d of n o t o v er five m in u te s— p ro b a b ly less th a n tw o m in u te s— d u rin g w hich tim e th e e le c tro ly te c o n c e n tra tio n in th e m ixing w a te r increases ra p id ly . T h e ele c tro ly te s (a p p a re n tly th e h y d ro x y l ions) b rin g p b o u t flocculation of th e c e m e n t p articles.
D u rin g th e sam e perio d a c o a tin g of h y d ra te s form s on th e c em en t grains. Once th is c o atin g h a s form ed, a n d th e e le c tro ly te so lu tio n h as reach ed full stre n g th , th e ra te of re a c tio n becom es v e ry low a n d th u s th e cem en t rem a in s c o m p a ra tiv e ly d o r m a n t chem ically. T h is s ta te la sts for a considerable period, u su a lly a b o u t a n h o u r. T h e p a s te rem ain s p lastic d u rin g th is period, a n d , if left u n d is tu rb e d , u n d erg o es s e d im e n ta tio n (“ b leed in g ” ).
EFFECT O F F L O C C U L A T IO N O N A M O U N T O F SETTLEM ENT
E x c e p t for c e rta in fe a tu re s t h a t n eed n o t be discussed h ere, th e sedi
m e n ta tio n of cem en t p a ste s h a s been show n to be essen tially like t h a t of o th e r c o n c e n tra te d suspensions of flocculated m in eral pow ders. V arious ex p e rim e n ts h a v e been c a rrie d o u t t h a t rev eal th e effect of flocculation on th is process. S te in o u r26 o b serv ed th e se d im e n ta tio n of em e ry p articles b o th flocculated a n d n o t flocculated. T y p ic al re su lts are giv en in T ab le 1:
T A B L E 1
M aterial D iam eter, microns
Fluid C ontent, %
A m ount of F inal Settlem ent,
% of original height D ispersed Flocculated
Em ery A 12.2 65 36 17
A 12.2 80 63 44
B 9 .6 75 55 35
As show n in th e la s t tw o colum ns of th e ta b le , flocculation reduced th e a m o u n t of se ttle m e n t a t all co n cen tratio n s.
T h e a m o u n t of s e ttle m e n t is influenced b y th e co n c e n tra tio n of th e flocculating a g en t. T h is is deduced from d a ta such as th e following o b ta in e d b y th e w rite r.8
T A B L E 2— S E D IM E N T A T IO N O F P U LV E R IZ E D S IL IC A IN L IM E -W A T E R (60 ml of solution per 100g of silica)
Initial C oncentration of Ca(O H )2 in Mixing W ater
R ate of Settlem ent cm /sec x 106
F inal Settlem ent per U nit of Origi
nal Height, %
14 satu rate d solution 58 10
satu rate d solution 53 5
satu rate d solution 53 • 1
S te in o u r9 in te s ts on a n o th e r silica-pow der observed sim ilar b u t less p rono u n ced effects, th e se ttle m e n t being 12 an d 19 p er cen t for s a tu ra te d a n d 1 /6 -s a tu ra te d solutions, respectively, w ith th e in itia l fluid c o n te n t a t 60 p e r cent.
R e su lts of som e experim ents w ith p o rtla n d cem en t are given in Fig. 1.
T h is show s th e volum es of sed im en t form ed from d ilu te suspensions (in itial flu id -c o n ten t 86 p er ce n t b y volum e) of cem ent in v ario u s m ix
tu re s of eth y l-alco h o l (d en a tu red ) an d w ater, ran g in g from all w a te r to all alcohol. One p o in t rep re se n ts th e resu lts o b tain ed in p u re toluene.
T h e ad d itio n of alcohol u p to a b o u t 50 p er cen t b y volum e increased th e sed im e n tatio n -v o lu m e;* hig h er co n c en tratio n s of alcohol reduced th e volum e, finally to a p o in t below t h a t for w a te r alone. All suspensions w ere flocculated to th e e x te n t t h a t no se p a ra tio n of p article sizes could be observed, except th e one in s tra ig h t alcohol. I n th e la tte r, m o st of th e cem en t s e ttle d o u t quickly, th e larg e st p article s co n c e n tra tin g to w ard th e b o tto m of th e sed im en t while th e v e ry finest flour rem ain ed in sus
pension, even a fte r 24 hours, p ro b a b ly because of B row nian m otion.
* D efin ed a s th e r a tio of b u lk v o lu m e of th e se d im e n t to th e so lid v o lu m e of th e p articles.
Fig. 1— Sedim entation v o l
umes o f cement in a lc o h o l- water solutions and in pure toluene. — Period o f settle
ment 24 hours, except as noted.
C em ent 1 5 7 5 4
I t is reaso n ed t h a t in se ttlin g from d ilu te suspensions th e p a rtic le - flocs, in m ak in g c o n ta c t w ith th e sed im en t, te n d to fo rm arch es or b rid g es enclosing re la tiv e ly larg e spaces w hich c o n trib u te to th e b u lk i
ness of th e sed im e n t.10’11 T h e g re a te r th e e x te n t to w hich such arch es are able to w ith s ta n d th e pull of g ra v ity , th e g re a te r w ill be th e final v o lu m e of th e sed im en t. I t seem s reaso n ab le to assum e t h a t th e g re a te r th e in te rp a rtic le a ttr a c tio n th e g re a te r th e s tre n g th of th e arch es a n d th e b u lk of th e sed im en t. T herefore, th e fa c to rs t h a t c o n tro l in te rp a rtic le a ttra c tio n should co n tro l se d im e n ta tio n volum e.
T h e rev ersal in th e slope of th e cu rv e of Fig. 1 can be a c c o u n te d for as follow s:* T h e in te rp a rtic le force in a n y given suspension is d e te rm in e d b y th e k in d a n d c o n c e n tra tio n of su b stan ces in so lu tio n a n d b y th e force of ad h esio n b e tw e en th e p a rticles a n d th e liq u id m ed iu m . E le c tro ly te s in so lu tio n or c e rta in ty p e s of organic m olecules te n d to m ak e th e p a rtic le s e le c tro sta tic ally rep elle n t. (U n d e r m o st circu m stan ces th e ele c tro s ta tic rep u lsio n is a t a m ax im u m a t v e ry low e le c tro ly te co n c e n tra tio n s.) Also, th e adh esio n of th e liq u id to th e surface a n d th e a d so rp tio n of ions o r m olecules fro m th e so lu tio n b y th e surface te n d to cancel som e of th e p a rtic le s’ surface energy, th e surface en erg y bein g th e source of th e force of a ttr a c tio n b etw een th e p articles. W h en alcohol is a d d ed , it a p p a r e n tly reduces th e force of adhesion f b etw een th e w a te r a n d th e c em en t p a rticles
* T h is e x p la n a tio n is n ec essarily sp e c u la tiv e , sin ce specific d a t a o n in te r p a r tic le forces in th is s y ste m a re la c k in g . O th e r e x p la n a tio n s ca n b e d ev ised .
tT h is is d ire c tly in d ic a te d b y th e f a c t t h a t th e h ig h e r th e alco h o l c o n te n t, th e lo w er th e d ie lectric c o n s ta n t of th e s o lu tio n . See R ef. 4.
SHOULD PORTLAND CEMENT BE DISPERSED?
a n d th u s increases th e n e t force of a ttra c tio n . A t th e sam e tim e, th e a d d itio n of alcohol decreases th e so lu b ility of th e cem en t c o n stitu e n ts a n d th u s favors a n increase in e le ctro sta tic repulsion, a change te n d in g to offset th e effect of th e decrease in th e force of adhesion betw een liquid a n d solid. E v id e n tly , from Fig. 1, one effect is p re d o m in a n t a t low alcohol co n c e n tra tio n an d th e o th e r effect is p re d o m in a n t a t high con
c e n tra tio n .
T hese d a ta d e m o n stra te t h a t th e forces of in te rp a rtic le a ttra c tio n in ce m e n t-w a ter p a ste are n o t as high as th e y m ig h t be a n d t h a t if a change in th e force of flocculation is desired, it could be e ith e r an increase or a decrease, according to choice.
T e sts w ith v ario u s dispersing a g en ts for p o rtla n d cem en t show t h a t th e y pro d u ce a co n d itio n sim ilar to t h a t found w ith cem en t alone in p u re alcohol, b u t only in v e ry d ilu te suspensions. U sed in concretes or p astes in p ro p o rtio n s recom m ended for field use, th e y do n o t cause such dis
p ersio n ; th e p a ste s clearly show th e effects of in te rp a rtic le a ttra c tio n . T h e re is evidence, how ever, t h a t th e se ag en ts w eaken th e forces of a ttra c tio n .
Effect o f flo ccu la tio n on rate of sedimentation
T h e effect of flocculation on th e r a te of se d im e n ta tio n is illu stra te d in F ig. 2. T h e curve illu stra te s th e follow ing general rule:
I n dilute suspensions, flocculation increases th e r a te of sedim en
ta tio n over th e av erag e for th e dispersed m a te ria l; in concentrated suspensions flocculation decreases th e ra te .
P o rtla n d cem en t p a ste s as used in p rac tic e m a y be classed as con
c e n tra te d suspensions; accordingly, they settle (bleed) more slowly than they would i f the 'particles were dispersed.
A t h ig h d ilu tio n , flocculation increases th e ra te of sed im e n ta tio n b e
cause th e d isplaced w a te r is able to flow m o stly a ro u n d th e floes, each floe a c tin g m u ch like a single large p article . B u t a t som e sufficiently h ig h p a rtic le -c o n c e n tra tio n th e p articles form a flo c-stru ctu re so con
tin u o u s t h a t th e d isplaced w a te r can flow only th ro u g h th e flo c-stru ctu re itself. I n an in te rm e d ia te range, th e flow is p a rtly a ro u n d an d p a rtly th ro u g h th e floes, a co n d itio n giving rise to “ channeled b leed in g .” 13
T h e a c tu a l co n d itio n s in a n o rm a l cem en t p a ste seem to be a b o u t as follow s: F ro m consid eratio n s a lre ad y discussed, i t seem s t h a t before or d u rin g th e process of flocculation th e cem en t p articles becom e co ated w ith h y d ra te s a n d th e co atin g acquires a lay er of ad so rb ed w a te r an d ions. T h e p a rticles a re so" c o n c e n tra ted t h a t w hen flocculation occurs they do not draw together into discrete groups b u t th e y form a co n tin u o u s netw o rk . T h e bo n d s of th e n etw o rk are, it w ould be im agined, a t p o in ts
Fig. 2 - Effect o f flo ccu la tio n on rate o f settlement as influenced b y concentra
tion o f solids.
(F ro m R e f. 2 b )
on a d ja c e n t p a rtic le s t h a t w ould be in c o n ta c t w ere i t n o t for th e stro n g ly a tta c h e d a d so rb e d layer.
T h is conclusion is re q u ire d b y a t le a st th re e co n d itio n s:
(1) T h e n a tu r e of th e re la tio n sh ip b etw een changes in w a te r c o n te n t a n d corresponding changes in th e ra te of bleed in g is such as to show t h a t a change in w a te r c o n te n t causes a ch ange in th e spacin g of th e n e tw o rk ; t h a t is, th e effect is not t h a t of ch an g in g th e spacin g b etw een clu sters of p articles.
(2) S te in o u r’s ex p erim en ts show ed t h a t th e viscous re sistan ce to se d im e n ta tio n involves th e sam e a m o u n t of p a rtic le su rface w hen th e p a rtic le s are flocculated as w hen th e y are n o t flocculated.
(3) M e a su re m e n ts of h y d ro s ta tic p ressu re in c em e n t p a ste s show ed t h a t th e p a rtic les in a fresh p a ste are s u p p o rte d e n tire ly b y th e liq u id ; t h a t is, none of th e ir w eig h t is tra n s m itte d to th e b o tto m of th e vessel th ro u g h p o in t-to -p o in t co n tacts.
P o in ts (1) a n d (2) show t h a t th e particle's c a n n o t exist as se p a ra te floes. P o in t (3), s u p p o rte d b y (1), show s t h a t th e p a rtic le s are n o rm a lly su rro u n d e d w ith w ater.
