January. 1935
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I,ITEHATUS~E CITED i.l .l Bartow, E., Blaak, A,, and Sanabury, W. IW. Ex". (:am.. 25.901 (1933). (2) Black, A., Itiee, 0.. and Bartow, E.. ibid., 25. 811 (lYd.%: (3) Mattson, S..J . Am. Soc. Ayron., 18,458(1026). (4) \Iattson,S.,J.Phz,s. Chrn.,32, 1532 (1928). (5) Msttaon, S.. Soil Sci.. 25,289 (1028). ,.,
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( 8 ) IlA.,32.343(1931). (9) Miller. L. B., Pub. Hcaith Ileyls., 40,361 (1925). (10) Petorson, B., and Hartow, E.,IND,ENQ.Caax.. 20,5l (1I)W.
Bleeding of Cements lielationship between Bleeding Tendency of Various ;\Jornial arid Poly, Purpose Cements and Water Gain under the Aggregate in Concretes Made from Them LEVI S. RHOWN, Massachusetts Institute of Technology, Cambridge, Mass. HE tendericy of water to rise to the top of concrete soon
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after it has been placed, here spoken of as "bleeding," is well recognized. Also, various investigators have noted that the bond between the cement paste of a concrete and the aggregate in the concrete, as a rule, is much tighter over the upper surface of the individual pieces of aggregate. Those who have studied these phenomena regard them both as effectsof gravitational settlement or compaction wbile the concrete mass is still fluid. Undoubtedly such compaction tends to express or squeeze out excess water, and it is apparent that in concrete some of the water, in rising toward the top surface of the mass, is entrapped under the bottom surface of the pieces of aggregate in such a way that a lateral flow around the pieces of aggregate is necessary for continued unward mieration. .. tfnder such condit i o n s a f i l m of water tcnds to lie along the hottom surface of the piece
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: 1. S ~ i n n s nPauu~aFOR
PASTE
The examinatiori qf sezieral cornni,ereiulnormal and TDA Poly-Purpose cements by "bleeding" tests, and the petrographical cxaminatiorr of the hardened concrete showed that the separation of uater from poured concrete and the presence of chanrlels un&r the aggregate in the hardened concrete are different manifestations of the same phmonienon. Bleeding and water gain under the aggregate, while controlled lo some degree by finer grinding or by a reduelion of the water-cement ratio, are reduced to negligible amounts by use of cenienls ground in the presence of a dispersing r\, aynu,, n1 ,UN of aggregate. This fihii uf water is spoken of as "water gain under the aggregate." As the concrete sets and hardens, this water is slowly reabsorbed during the later stages of the
MIXING Concrete pillails reach to bed ruck.
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1mrih:i~ingpnlaess and is later replaced by air or otlier water r l r i i w i in froirk the outside. It, has lieen recognined that bhe use of large aggregate tends t,u aceerrtuatc the developrnerrt of the water-gain filrrr. In coltsequence, even though the bleeding tetidency of concrete c::mbe rneasrtralily controlled hy a perfection in the design of tiic irrix a i d the its(: of hnv water ratios,' many engineers lirefer tn limit the size of tlie large aggregates used in inass i:oncrete. Rut even in tibe case of very perf e c t l y designed c o n c r e t e mixes, such as are used in t l i e Ihulder lliini, there is some evid e u c e of w a t e r gain. Tiiere is on d i s p l a y in t h e 1)cnverofiiceof the United States Bureau of Reelamatiun u verticirl slab cut Srom a 2 X 5 foot prism of Boulder Dam inass concrete. I3elom the larger pieces of coarse aggregate tire water-gain vnids that tire visible to the unaided eye atid ~ n be n detected easily with a feelergage or tlie finger. It SCEIIIS reasoilable to expect that these channels facilitate the triuisverse passage of water tlvougli dams and retaining walls. It is (:oiiitiioii knnwledge that there is a differrnce in the aitmunt of hleeding from neat cement p s t e m~& with different eeinients and that the bleediiig tciiileimy of :my cenient is, within c e r t n i n limits, il fiiuction OS its finetiess. Ceinents of d,iionnally low surface area exhibit an excessive teodeircy to bleed. It is HISO well known that a t last a qutLlit,ativerelntiiinship csists betiveen the bleedirrg OS sue11 n e a t c e m e n t pastes and iif concretes iiiade with these cctrmits, and it has seemed logical to assume tliat the iileedirig cliarxcter~ioornmtat t u e istics nS i:einent refiect thernselves in the water gain under the a g g r e g a t e in t h e c o n c r e t e inado froin tliein. H o Wever, 1 a c k i n g actual proof, some have questinned t,his deduction and the esistence of a relationship between the hleeding tendency of a cemeut and the magnitude of water gain under the sinitller particles of aggregate (i. e., 0.75 to 1.5 inches) that are found in concretes often used in hydraulic structures. T l i e principal purpose of this study was to deterniine whether or not sirch a relationship exists. lteaogniaiiiij that bleeding atid water gain under the aggregttte nre scrious disadvantages accompanying the use of stand-
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Mc>liilzr~,Ir K.."Beaic Principles oi Coosrete Uesign," I,. 0. Now Yori. A I ~ G ~ L T Y I3ook - I ~ ~ ICurnimny. I 1029.
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iird Portland cetnerit, modern practice, with reasonaiily fii~ely ground ccnients, has taken advantage of t,wo methods nf mntrolling these evils. The first is the use of lower water-cement ratios. This rnethod decreases the bleeding effect hut is limited by the lowered workability and placeability of the drier mixes. The orga.nic chemist, by his fundamental discovery that agglon~crationof cement particlei in a cement-in-water suqznsion may be prevented or dest,royed liy the incorporation of coiiiplex organic dispersing agents and that these dispersed cement pastes have little tendenry to settle, lias led to tlie second nietlrod now employed to cmitnll blreding and water gain under tlie aggregate. Tliis lattcr method employs the usual cement-concrete teclmir, using, liowever, a cement which has been prepared by grinding a standard f'ort,land cement clinker to n soperior degree of tiiieiicss in the presence of a sinall aniount of a c o m p l ~ sorgtmic ilispersing agetit. A specific example of such a dispersing agcnt is TI)& a mixture comprising salts of polyniera of condensed naplrtlialene being TI)\ sulfonic acids, the treated finely ground reii~ent,~ Poly-Porposo cements. Consergiently, two further objecbives uf this research were, first, to study the effect OS varying typical m t e r ratios upon bleeding and water gain uridcr bhe aggregate, and seenndly, to deteririine if the sanie relationship existed in Piily-l'itrpose cements between bleeding of neat i:ement and water gain under the aggregat,e in coricrete nuidc froni it, 2,s in the untreated cements. As &ti aid in accoiriplisliirig this ~ ~ u r g o a ethe , water gain under tlie aggregate of conaretot made wit11 cenieiits showing wiilcly differelit Lendoiicir.; to bleed were ,?tudied with the microscope Iiy rncans of thin sections . I )eseriptiotis fdlow of the llletliods used: ( I ) in the dcteririiiiatioii of tlie bleeding tendency OS the point ,"It pIY"'"p ccmonts, ( 2 ) for the preparation of tlie tliiii sections, atid (:$ for the esniriiiratirin of the t,iiin wct i o n s , ~ l l ~ ~ Y l ~03' l OI(LI?EI>N TYQ TI:NIIENUY
T I E inetliod used for tiic detenninatii>n
