the nature of concrete by powers.pdf

Research and Development Laboratories

of the

Portland Cement Association

RESEARCH DEPARTMENT

Bulletin 196

The Nature of Concrete

Authorized Reprint from the Copyrighted

Significance of Tests and Properties of

CONCRETEAND CONCRETE-MAKINGMATERIALS

STP No. 169-A, 61-72 (1966)

Published by

American Society for Testing and Materials

By

T. C. Powers

THE NATURE OF CONCRETE

By

T. C. Powers

PORTLAND CEMENT ASSOCIATION

RESEARCH AND DEVELOPMENT LABORATORIES

5420 Old Orchard Road

Skokie, Illinois 60076

THE NATURE OF CONCRETE

T. C. POWERS1

The term concrete can be construed toinclude a considerable variety of productsmade from portland cement or othercementing media, but in this publicationthe term concrete usually refers to amaterial which was at first a plastic mix-ture (or mixture that became plastic as aresult of manipulation, especially vibra-tion) of portland cement, water, air, andmineral aggregate. Therefore, this discus-sion of the nature of concrete will havethe scope indicated by that description,

A writer’s concept of the nature ofconcrete can hardly be revealed in a fewwords, but his treatment of certain topicsand his definitions are indicative. Hereare some examples: in 1878 Trautwine,in the 1lth edition of his Pocket Book forCivil Engineers, said “Cement concrete,or beton, is , . . cement mortar mixedwith gravel or broken stone, brick, oystershells, etc., or with all together.” Hedescribed mortar as sand containing avolume of cement equal to the volume ofvoids in the sand.z In 1907, L. C. Sabinin a book on concrete said, “Concrete issimply a class of masonry in which thestones are small and of irregular shape.The strength of concrete depends largelyon the strength of the mortar; in fact,this dependence will be much closer than

1Research counselor, Research Dept., Port-land Cement Assn., Skokie, Ill., (retired).

2He said also, “Nearly all the scientificprirmiples which constitute the foundation ofcivil engineering are susceptible of complete andsatisfactory explanation to any person whoreallv possesses only so much elementary knowl-edge of arithmetic and natural philosophy as issupposed to be taught to boys of 12 or 14 in ourpublic schools.”

in the case of other classes of masonry,since it may be stated as a general rule,the larger and more perfectly cut are thestone, the less will the strength of themasonry depend on the strength of themortar.” Feret in 1896 considered waterand air to be definite components of mor-tar (and presumably also of concrete),but it is not clear that he thought ofcement paste as an entity, Zielenski, oncehead of the Hungarian Association forTesting Materials, in 1910 called con-crete a conglomerate body; he consideredthe conglomerate to be composed ofmortar and coarse aggregate, and themortar to be composed of paste and sand,with or without air voids. Taylor andThompson, authors of perhaps the bestof early books on concrete, in the 1912edition said, “Concrete is an artificialstone made by mixing cement, or somesimilar material which after mixing withwater will set or harden so as to adhere toinert material, and an aggregate com-posed of hard, inert, materials of varyingsize, such as a combination of sand orbroken stone screenings, with gravel,broken stone, cinders, broken brick, orother coarse material.” D. A. Abrams, inthe first bulletin from the StructuralMaterials Research Laboratory, LewisInst., Chicago, Ill. (1918) emphasizedthe significance of the ratio of water tocement in concrete, and he abandonedthe notion that concrete is a mixture ofmortar and coarse aggregate, pointingout that the whole aggregate, fine andcoarse combined, should be considered asone, even though fine and coarse aggre-

61

62 SIGNIFICANCE OF THE PROPERTIES OF CONCRETE

gates are proportioned separately. F. R,McMillan in his book, Basic Principlesof Concrete Making (1929) said, “Ex-pressed in the simplest terms, concrete isamass of aggregates held together by ahardened paste of portland cement andwater . . . the paste is the active ele-ment.” In their textbook on concrete,Troxell and Davis (1956) wrote, “Con-crete is a composite material which con-sists essentially of a binding mediumwithin which are embedded particles orfragments of a relatively inert mineralfiller. In portland cement concrete thebinder or matrix, either inthe plastic orin the hardened state, is a combinationof portland cement and water.” Thisdefinition was adopted by the AmericanConcrete Inst. (ACI) Committee l160nNomenclature in 1964. The EncyclopediaBritannica, 1963edition, says, “Concreteis a building material consisting of a mix-ture in which a paste of portland cementand water binds inert aggregates into arock-like mass as the paste hardensthrough chemical reaction of cement withwater.”

Although it is possible to discern anevolution of concepts in the above defini-tion and descriptions, the early conceptof concrete as a mixture of mortar andcoarse aggregate tends to persist, despiteAbrams’ contention that the total aggre-gate functions as a unit, The idea lingerswith us, perhaps, because it hassimplicityand plausibility and partly because it isnot altogether unrealistic, especiallywhen there is a gap between the largestsize in the sand and the smallest size inthe coarse aggregate. However, gap grad-ings are not common, and in any case thecorrectness of Abrams’ conclusion canhardly be questioned,

Even the latest definitions provide buta superficial idea of the nature of con-crete. They give no hint as to how andwhy the originally plastic mass becomeshard and strong and, indeed, say nothing

as to how and why the mixture hadplasticity in the first place. Moreover,they give no adequate basis for under-standing such aspects of concrete asvolume change characteristics, andstress-strain-time phenomena. We must,therefore, go far beyond a superficialdefinition or description to attain today’sunderstanding of the nature of concrete.To do the subject full justice would be abook-length project; I cannot do morethan touch on a few fundamental topics.

GROSS STRUCTURE OF CONCRETE

When we inquire into the nature ofconcrete we find it necessary to regardconcrete not as an entity—a substance-but as a structure having componentparts, as has already been indicated. Thepredominant component of concrete is anaggregation of mineral particles, calledthe aggregate, and this aggregation re-quires a certain minimum of space perunit weight of material. The volume ofspace occupied by a properly compactedfresh concrete mixture is slightly greaterthan would be the compacted volume ofthe aggregate it contains, and the differ-ence is significant: it shows that the in-dividual rocks particles in concrete neednot be in contact with each other. Thereis clear evidence that the rock particlesin concrete are, in fact, not in contactwith each other: freshly mixed concretecould not be plastic if the solid particleswere not dispersed to some degree; in-spection of broken sections of hardenedconcrete show that not only are the rockparticles in a dispersed state while themixture is fresh, but also they remaindispersed, although generally not exactlyto the same degree that prevails immedi-ately after mixing, owing to settlementunder the force of gravity before settingoccurs.

8Throughout this discussion, the term rockrefers to the particulate mineral matter thatmakes up the aggregate, regardless of the size orshape of the particlee.

POWERS ON NATURE OF CONCRETE 63

Rock particles in plastic concrete aredispersed in a matrix composed of pasteand air bubbles;4 the paste is composedof portland cement and water. The de-gree of dispersion actually depends uponthe consistency of the paste and thevolume of air; the stiffer the consistencyand the higher the air content the greaterthe mean clear distance between aggre-gate particles. In practical terms, thismeans that the degree of dispersion ofrock particles is greater the lower thewater-cement ratio of the paste, and thehigher the air content; in any case, thevolume of concrete seldom if ever exceedsthe compacted volume of the aggregateby more than 10 per cent, and usually itdoes not exceed it by more than 3 percent when no air-entraining agent is used.

Without an air-entraining agent, con-crete placed by a standardized procedurecontains a characteristic amount of airin the form of bubbles, the amount beinga function of certain variable factors.The main factors are the consistency ofthe paste and the gradation of the aggre-gate. At a given paste consistency, andwith a standardized mixing procedure,air content depends mainly on thosefeatures of aggregate grading that con-trol the mean size of the voids in theaggregate, voids being here defined as thespace occupied by paste and air. Themean size and the size range of the airbubbles in concrete are also significantstructural features, particularly withrespect to the ability of concrete to with-stand the effects of freezing. These char-acteristics also are subject to systematicvariation as will be discussed later.

The foregoing statement as to theeffect of aggregate grading on air contentis applicable principally to the leanertypes of mixtures. In richer mixtures,water-cement ratio less than about 0.5 byweight or 5* gal per sack, aggregate char-

4Voids due to incomplete filling of mold orform are not considered here, only the voidsthat me normal components of the mixture.

acteristics have little effect. Throughoutthe range of the most frequently usedmixtures, air content and void character-istics are strongly influenced by water-cement ratio, the air content and averageair void size increasing with increase ofwater-cement ratio under given condi-tions,

When a suitable air-entraining agent isused, the air content can be raised toalmost any desired level. At an air con-tent higher than that normally presentin a given mixture, the mean size of airbubbles is controlled by the characteris-tics of the air-entraining agent used and,in the leaner range of mixtures, by thesame factors that control the normal airvoids. Specifically, if the normal air con-tent of a given mixture is relatively highand the mean size of the voids relativelylarge (the two usually go together), rais-ing the air content by means of a givenapproved air-entraining agent will resultin a system of voids having a relativelylarge mean size, as compared with themean size when the amount of air nor-mally present is smaller, and the originalmean size also smaller, This means thatwith a given air-entraining agent used indifferent mixtures a wide range in aver-age void size may be observed.

In short, the gross structure of concreteappears to be that of an aggregation ofrock particles slightly dispersed in amatrix of paste and air bubbles, theproportion and size characteristics of airbubbles being subject to systematicvariation just as is the proportion ofaggregate.

A conclusion arising directly from con-sideration of the gross structure of con-crete is that the firmness or mechanicalstability of concrete cannot be attributedto mechanical stability of the aggrega-tion of rock particles; it is clearly due tothe mechanical stability of the matrixand to the mechanical stability of indi-vidual particles of rock. Also, it is clear


that the stability of thematrix is due tothat of hardened cement paste.

INTERPARTICLE FORCES IN FRESHLY

MIXED CONCRETE

We have already seen that plasticityof freshly mixed concrete is possible be-cause the rock particles of the aggregateare slightly separated from one anotherby matrix material; otherwise, any de-formation would necessarily be dilatantrather than plastic. The matrix, itself, isplastic because the cement particles andair bubbles are dispersed in water andespecially because the interparticle forcestend to hold particles together while atthe same time preventing actual point topoint contact. Such astateisdue to thecoexistence of forces of attraction andrepulsion between cement particles. At-traction is due to relatively long rangeintermolecular forces known as van derWaals forces, about which more later;the forces of repulsion are, in this case,due to electrostatic repulsion and to a“disjoining pressure” (Deryagin)5 main-tained by adsorbed water moleculescovering the surfaces of the grains. Elec-trostatic repulsion is due to what iscalled a Gouy diffuse layer of ions, in thiscase negative ions, the negative ionsbeing held near each cement particle bypositive ions selectively adsorbed fromthe surrounding aqueous solution.

Owing to the existence of opposinginterparticle forces, a pair of cementgrains has a minimum of potential energywith respect to those forces when theparticles are separated from each other

sIn previous publications I have used termsSuch as film pressure, swelling pre8sure, orspreading pressure, to indicate the force devel-oped by adsorbed films of water in spaces toonarrow to accommodate the normal thickness ofthe films. Several years ago Deryagin, a surfacephysicist of the Soviet Union, introduced theterm disjoining pressure and another, disjoiningaction; when thinking of the tendency of waterto disperse a coherent system of particles,Deryagin’s term seems most apt.

by a certain small distance, the distanceamounting to perhaps ten water moleculediameters, more or less; hence, cementparticles tend to assume positions withrespect to each other corresponding tominimum potential energy with respect tobalance of internal forces, and this is anessential condition for the plastic state.

When particles are in positions ofminimum potential energy with respectto the forces acting between them, theyare said to be in “potential troughs,” andany mechanical displacement of particleswith respect to each other requires a cer-tain amount of work to “lift” the partic-les out of their potential troughs, Thus,we see that interparticle forces givefreshly mixed paste in the quies-cent state, a structure having a lowdegree of firmness or shearing strength.When shearing stress exceeding shearingstrength is applied and maintained, apaste is caused to flow continuously if itssolid content is considerably smaller thanthat at normal consistency, which is thecase for pastes used in concrete. Theshearing strength (yield value) and theresistance to continuous shearing stress(the mobility or structural viscosity) isoften used as a measure of paste consist-ency, but such a measure pertains only topaste in the fluid state maintained by asufficiently high shearing stress.

Consistency of freshly mixed pastecan be made softer by diminishing thedepth of the potential troughs, and thiscan be done by using an appropriatesurface-active material able to increaseinterparticle repulsion; indeed, repulsioncan be raised to such a degree as to de-stroy plasticity, changing paste from aplastic to a fluid material. In some con-crete mixtures, the water content of thepastes is so high that the paste has verylittle plasticity to begin with; eventhough the interparticle forces discussedabove exist, the particle concentration isso low that interparticle forces are rela-


tively ineffective. The paste in a properlyconstituted freshly mixed concrete hasan optimum consistency, neither too softnor too firm (stiff); under no circum-stances should it be completely fluid,

CHEMICAL NATURE OF

HYDRATED CEMENT

The chemical compounds found inhydrated cement are complex; most ofthem are impure in the sense that theycontain elements not ordinarily given intheir formulas, and they do not haveexactly the same composition whenformed under different conditions, espe-cially with respect to temperature andoriginal cement concentration. For ourpresent purpose it will suffice to mentiona few outstanding characteristics.

All the components of hydrated ce-ment are basic; the hydroxyl-ion concen-tration is always at least as high as thatof a saturated solution of calcium hy-droxide, and it is usually considerablyhigher because of the presence of alkalihydroxides. Any of the compounds canbe decomposed by carbonic acid and,therefore, by ordinary rain water. Thatconcrete is not generally destroyed thisway is explainable mostly in terms ofphysical factors: under ordinary condi-tions of exposure the quantity of acid incontact with concrete during a given timeis small relative to the quantities of basicmaterial available, and the permeabilityof concrete to water is so low that theaction of weak acids is only superficial.Even contact with soft-water streamsusually causes decomposition at a negli-gible rate. In cities where rain fallsthrough industry-polluted air and be-comes distinctly acid, acid action isevidenced by surface etching. But, when-ever a continuous supply of strong acid isencountered, concrete must be protectedor it will be destroyed.

Hydrated cement is able to react alsowith carbon dioxide gas in the presence

of water vapor, but the effect is not de-structive; actually such reaction mayincrease chemical stability.

The reactions between the anhydrouscomponents of portland. cement andwater are remarkable in that they involvea doubling of the volume of space re-quired by solid material while the appar-ent volume of the system remains con-stant. There is one exception: theformation of calcium sulfo-aluminate byreactions involving gypsum and trical-cium aluminate tends to cause volumeexpansion, and when the sulfate ion con-centration is too high, expansion can bedestructive. Normally, the amount ofgypsum needed to control the setting ofcement gives only tolerable expansion,but when an unlimited supply of sulfateion is present in the environment, con-crete may be destroyed by it. Practically,such destruction is avoidable by using acement of low tricalcium aluminatecontent.

Chemical Aspects of Mineral Aggregates:

The petrographers and mineralogistscontributing to this publication havemuch to say on this subject; I shall men-tion only some chemical characteristicsthat are of special interest because of thestructural and chemical characteristicsof the paste component of concrete.

Rocks used for concrete aggregatesare generally materials that have sur-vived geologic ages and are thus thosethat have demonstrated some degree ofchemical and physical stability. Some arechemically basic; some are acidic. Lime-stone is, of course, basic and vulnerableto acid attack, but when used as concreteaggregate it is less vulnerable than thehydrated cement paste which envelopesit. There are at least two other kinds ofundesirable chemical attack on certainminerals that seem to be the result ofconditions peculiar to the interior of con-


crete; one kind involves siliceous rocks;the other, certain dolomites.

Some kinds of siliceous rocks, opalbeing an outstanding example, may bedecomposed by the caustic solutionsfound in concrete. Under some conditionsthis reaction is accompanied bydestruc-tive expansion, and under other condi-tions, no expansion. The conditions men-tioned involve chemico-physical factorstoo complex to be described here, as maybe ascertained by referring to the litera-ture on thealkali-silica reactionsin con-crete. Aprincipal factor determining thephysical effect of such reactions is thespecial and selective hindrance to thediffusion of various ions through thestructure of cement paste; another is thequantity and specific surface area of thereactive form of silica.

Also, some dolomites react with thecaustic aqueous solution in concrete andexpand destructively; the reaction in-volves decomposition of the dolomite andthe formation of magnesium hydroxide(brucite); it is called dedolomitization.

Structureof Hardened Cement Paste:

We have already seen that freshlymixed cement paste is a dispersion ofcement particles in water and that it hasa certain structure owing to the forces ofattraction and repulsion among thecement particles. This structure is thestarting pattern of the structure that sub-sequently develops from the materialsproduced by reactions between the com-ponents of cement and water. Thesereaction products are collectively thatwhich we have already called hydratedcement; now we shall stress the physicalaspects of hydrated cement.

Although hardened cement paste looksthe way we might expect an amorphouscontinuum to look, we know that itactually comprises a hierarchy of aggre-gations of matter. Moreover, we knowthat cement paste contains submicro-

scopic voids, its void content usuallybeing upwards of 40 per cent, although alower void content is possible. Since, ingeneral, we have learned to think ofmatter as intrinsically granular, we areinclined to regard the pore space ashaving the character of interstices in agranular aggregation. By dispersing thestructure and examining the fragmentsby electron microscopy, we have seenparticles and have been impressed bytheir smallness and irregularity of shape.These particles, not all of the same kindchemically, may be regarded as theprimary particles of paste structure, eventhough atoms and molecules are theprimary and secondary aggregations ofmatter in general.

The term gel particle refers to particleshaving dimensions in the submicroscopicrange of sizes called colloidal; in thisparticular case colloidal bodies can bedefined as molecular or ionic aggrega-tions having a very high specific surfacearea, such as is possible only in the sub-microscopic range. The colloids observedin cement paste are mostly quasicrystal-line, lack of normal crystallinity beingdue to the extremely small size and im-perfect atomic or molecular organizationof the solid material. The high specificsurface area is due mostly to the thinnessof the particles, one of the three dimen-sions, and perhaps two, being greaterthan the limit usually stipulated for thecolloidal state; these particles are usuallyonly three or four molecules thick.

Along with the colloidal material inhardened cement paste is crystallinecalcium hydroxide having relatively lowspecific surface area. The amount of cal-cium hydroxide is different for cementshaving different chemical compositions,but it is usually between 20 and 30 percent of the weight of the dry hydratedcement. Calcium hydroxide crystals areusually surrounded by and intergrownwith colloidal material, and thus they


constitute an integral part of the solidstructure.

The most abundant colloidal constitu-ent of hydrated cement is an impure cal-cium silicate hydrate of somewhatindefinite stoichiometry. It has character-istics resembling those of a naturalmineral called tobermorite and has thuscome to be called tobermorite gel. (Theterm gel designates a rigid aggregation ofcolloidal material.) There are alsoamounts of calcium aluminate hydrateand calcium alumino ferrite hydrate; thephysical states of these materials are notknown exactly, but it appears that theyare colloidal but with a lower order ofspecific surface area than those of tober-morite gel.

An outstanding characteristic of thecolloidal matter in hydrated cementpaste is that its specific surface area isvirtually the same in all pastes made ofthe same cement regardless of differencesin paste density, and among pastes madewith different portland cements it is notmuch influenced by differences in chemi-cal composition, This observation is oneof the cornerstones of our concept ofpaste structure.

The colloidal matter together with cal-cium hydroxide appear as a continuoussolid structure apparently occupyingthe whole volume of any specimen ofhardened cement paste. As already indi-cated the structure is porous; normallythe solid matter occupies 45 to 60 percent of the apparent volume, and thehighest possible solid content (exclusiveof unhydrated cement if any) has beenfound to be about 72 per cent or perhapsa little more of the total volume occupiedby hydrated material. In other words, aspecimen of hydrated cement paste mayhave a porosity of not less than about28 per cent, and it usually has a porositybetween 40 and 55 per cent; the porositywill have a higher range if the paste isnot fully mature, which means if com-

plete curing has been deliberately orinadvertently omitted. The apparentvolume of a specimen of paste, expressedas a volume per unit quantity of cement,is determined by the net volume of mix-ing water per unit volume of cement, Thenet volume of mixing water is that whichremains within the specimen at the endof the period of settlement (bleeding)which is normally from 1 to 2 hr aftermixing.

The solid content of any freshly mixedpaste is a little over 62 per cent of whatthe solid content will be after chemicalreactions have converted all the cementto the hydration products describedabove, As already mentioned, the in-crease of solid volume takes place with-out appreciable change of over-ail vol-ume,Gregardless of how high the cementcontent may be or how little water perunit of cement. One cubic centimeter ofcement, solid volume, produces about 1.6cc of hydrated cement. On this basis wemight expect that the hydration productswill require 0.6 cc of space in addition tothe space originally occupied by 1 cc ofcement, or 0.19 cc of space per gram ofcement; this amounts to saying that ifthe water-cement ratio is 0.19 by weight,there will be ample room within the speci-men for all hydration products that canbe derived from one gram of cement, andthe material would become a voidlesssolid. But experimentally it was foundthat void space cannot be eliminated.This experimental observation is.anothercornerstone on which ‘our concept ofpaste structure rests.

As just indicated, it was found thatunder no circumstance can completelymature paste be made entirely solid; thehighest possible solid content is about 72per cent, and the rest of the unit volumeremains full of water or is void if all the

sThe microscopic changes that may occurlater as the result of drying and wetting aceproperly ignored here.


water in such space is caused to evapo-rate. The densest possible hydratedcement paste contains a continuous sys-tem of pores, as evidenced by its permea-bility to water. Because of the intrinsicporosity of the structure, which limits thesolid content to 72 per cent of the appar-ent volume, it follows that instead of1.6 cc/cc of cement, the volume of pastemust be at least 1.6/0.72 = 2.2 cc/cc ofcement to provide enough space for allthe hydration products that can be de-rived from 1 cc of cement, and this meansthat the water-cement ratio by weightmust be at least 0.38. In any paste con-taining less than this amount of water-filled space, some of the cement remainsanhydrous regardless of the duration ofcuring, and a residue of the original ce-ment remains a permanent feature ofthe structure of the paste, the residueappearing in cross sections as scatteredremnants of the largest grains.

Of course, when the volume of cementpaste is greater than 2.2 cc/cc of cement,as it usually is, there is more than enoughspace to accommodate the hydrationproducts and thus less than 72 per centof the space can become filled with solidmatter. When there is more space availa-ble than the minimum required by thehydration products, the extra space is afeature of the structure of paste.

Various observations lead to the con-cept that in every paste the hydrationproducts tend to become locally concen-trated to the maximum degree possible,even when excess space is available, butat the same time they form a continuousstructure having an over-all volumeequal to the apparent volume of thepaste. One line of evidence supportingthis view develops from consideration ofthe structure of freshly mixed paste. Al-though cement particles in freshly mixedpaste are individually dispersed through-out the volume of mixing water, they

can not be uniformly spaced because ofthe interparticle forces, already dis-cussed, that hold the particles practically(but not exactly) in point to point con-tact. This being true, it seems that if wecould subdivide a freshly mixed specimenof paste having a certain water-cementratio into a large number of cubical cellseach having an edge length of say 100 Y(about the same as the mean diameter ofthe largest cement particles but about100 times as large as the spherical equiv-alent of a “particle” of tobermorite gel),we would not find the same volumes ofcement and water in each cell; we wouldfind some cells almost filled with a singlegrain, some would contain many smallgrains, and some might contain few ifany grains. In other words, we wouldfind that the over-all water-cement ratiois an average of many different localwater-cement ratios some higher andsome lower than the average. Since someof the cells must be nearly full of solidmaterial to start with, considering thesize of many of the cement grains, andsince interparticle attraction tends tohold particles close together, it seems al-most certain that the 72 per cent limit ofthe content of solid hydrated materialwill be achieved in many cells after allthe cement has become hydrated, eventhough some of the cells cannot becomefilled to this extent. In other words, it isreasoned that if the hydrated material ina specimen of paste can reach an averagedensity of 72 per cent but no higher, atany sufficiently low water-cement ratio,that same degree of density can beand is produced locally at various placesthroughout any paste, however high thewater-cement ratio. If any of the imag-ined cells contain excess cement, whichpresumably is the case wherever the localwater-cement ratio is lower than about0.38 by weight, such cells can get rid oftheir excess material by diffusion of mate-


rial into adjacent cells lacking cement, sothat eventually all the cement can be-come hydrated if the average water-cement ratio is high enough.T

I have used the term cement gel todesignate hydrated cement paste in itsdensest form. It should be noted that bythis definition cement gel is not synony-mous with gel as used above, for example,tobermorite gel. The term cement gel isconvenient for designating the predomi-nantly colloidal material found in hy-drated cement paste, but it must be keptin mind that the term includes noncol-loidal calcium hydroxide and othernoncolloidal material, if any, and there-fore it does not conform exactly to theterm gel, properly defined as a solidcomposed of colloidal material.

The concept of paste structure de-scribed above, which requires us tovisualize an uneven distribution of ce-ment gel, entails a corresponding unevendistribution of the sizes of interstitialspaces; surface to surface distances rangefrom zero at chemically bonded pointsto a maximum distance that is probablygreater the greater the capillary porosityof the paste, but which in any case is notknown exactly. The order of mean poresize is indicated by the quotient of thevolume of pore space by the boundaryarea of that space, which quotient inhydraulic engineering is called the hy-draulic radius. For a porosity of 28 percent and for solid matter having a specificsurface area of 5.2 X 106cm2/cc of solid

7Theoretically, if a 2-in. cube, for example,of fresh paste containing excess cement is joinedto one with a deficiency of cement, and if afterhardening both are kept saturated with water,the excess cement in one cube could, after thedensity of hydrated material had reached 72per cent, diffuse into the excess space in theother; but this conclusion would be hard toprove because the process would probably re-quire geological ages for completion. However,diffusion for a distance of a few microns canoccur in a normal curing period, as has beenverified by microscopic observations.

matter, which is about the specific sur-face area of cement gel, the quotient is7.5 X 10-8 cm or 7.5 A,bThe correspond-ing average distance from solid surface tosolid surface is between two and fourtimes the hydraulic radius, dependingupon the shape of the interstitial spaces;in the present case where many of thespaces are believed to be slit-like, about18 A seem to be a reasonable estimate.When the porosity exceeds 28 per cent,which is to say when the paste containsspaces other than gel pores, the meanpore size is, of course, greater than themean size of gel pores.

We can now describe hardened pasteas a solid composed of cement gel, rem-nants of cement grains, if any, and spacenot filled with cement gel, if any. Anyspace not filled with cement gel or grainremnants is regarded as interstitial spacesamong masses of cement gel and is calledcapillary space, capillaries, or capillarycavities. The latter term is applied inpastes so dense that the capillaries arediscontinuous, the original interstitialspace among the cement grains havingbecome segmented into isolated cavitiesby the growth of cement gel, The poreswithin cement gel are called gel pores.

Rock Structure:

The structure of concrete is character-ized not only by the structure of cementpaste but also by the structure of individ-ual pieces of rock making up the ag-gregate. Some pieces, particularly thesmallest, may consist of practically void-less crystals or fragments of crystals; butin most concrete aggregates most of theparticles have granular structure and are

8To acquire an idea of the magnitude of anangstrom unit, try the following: if one couldplace two marks one angstrom apart on a pieceof superrubber 1 in. long, and then if one couldstretch the rubber until it could encompass theearth, the two marks would have become a littleover 6 in. apart.


porous, and most are permeable to fluids.The pores in permeable rock are usuallylarger than those in cement paste; alsorocks are less porous than paste, as arule. A rock having a porosity of say oneper cent, may have a coefficient of per-meability to water equal to that of aspecimen of hardened paste having aporosity of 50 per cent; thus, we knowthat the pores in rocks are usually largerthan those in hydrated cement paste; inother words, rocks have a relativelycoarse texture. A few kinds of rock dohave fine texture, and unless they arealso practically nonporous they do notmake satisfactory concrete aggregate;certain argillaceous limestones are exam-ples of unsuitable fine textured rocks;certain cherts also.

Some rocks contain pores that reducetheir apparent specific gravity but donot affect their permeability. Amongthese are vesicular rocks and artificiallyexpanded shales in which isolated voidswere formed by expansion of trapped gas.Such rocks are useful when concrete oflow unit weight is desired.

States oj Water in Concrete:

The solid matter in mature concretemay contain water molecules or ionsderived from water, as indicated by theformulas for the compounds in hydratedcement, and by those for some mineralsfound in concrete aggregate. Of principalinterest, however, is the water that re-mains chemically free, and which isfound in gel pores, or in capillary spacesin paste or rock. We shall confine thediscussion mostly to chemically freewater found in cement paste; it influencesthe properties of concrete to a very im-portant degree.

It is commonly known that water in avessel open to the atmosphere will even-tually evaporate unless the atmosphereis saturated with water vapor. Cementpaste can be regarded as a vessel open to

the atmosphere so far as contained wateris concerned, but not all of the water it iscapable of holding can evaporate unlessthe surrounding atmosphere is practicallyempty of water vapor; if the cementpaste was originally saturated, some ofthe water can evaporate, but a definitefraction of it will be retained, the amountretained being a larger fraction of thetotal evaporable water the higher thedegree of saturation of the ambient at-mosphere, that is, the higher the humid-ity. At low ambient humidities, watermolecules are restrained from evaporat-ing by the van der Waals forces of attrac-tion between them and the surfaces ofthe gel particles, which attraction holdsa condensed film of water molecules; inother words, the restrained moleculesare held by van der Waals forces.

Because of cement paste the internalsurface area is very extensive, a largefraction of the total evaporable watercan exist as a thin film spread over solidsurface. As has already been pointed out,opposite surfaces are necessarily quiteclose together. In spaces up to about 18Awide (see above) adsorption of two mo-lecular layers on each surface is sufficientto fill most of such space, and such willbe the condition when the ambient hu-midity is about 50 per cent. At higherambient humidities, the pore space maybe almost completely full, adsorptionbeing aided by hydrostatic tension main-tained by curved meniscuses of the waterin capillary spaces. A state of mechanicaltensile stress between water molecules inthe condensed state restrains evapora-tion in about the same way that van derWaals attraction restrains evaporationfrom a thin layer on a solid surface.

Of the total capacity of mature cementpaste for evaporable water, from aboutone third to two thirds of it will be fullwhen the ambient humidity is only 50per cent. In rocks having porosities up-wards of one per cent the relatively large


pores can retain very little water at thathumidity, but at humidities upwards of90 per cent, a considerable fraction of thetotal capacity of rock for evaporablewater maybe retained. Of course, differ-ent rocks differ considerably in thisrespect; the fine textured rocks usuallyretain a comparatively large amount ofwater at an intermediate humidity.

The failure of water in concrete toevaporate as it normally does shows thatit is altered to some degree by the mate-rial with which it is in contact. Some ofthe alteration may be due to dissolvedmaterial, particularly the alkalies, butremoval of solutes only modifies thesituation. The principal forces acting onwater and preventing it from evaporatinginto an unsaturated atmosphere have al-ready been identified as van der Waalsforces of attraction and capillary-inducedtension. Naturally, such forces are bal-anced by counterforces, the counterforcescorresponding to elastic strains of onekind or another in the solid structurewithin which the water is held. Suchstrains correspond to the reversible partof volume changes due to drying andwetting of concrete.

There is good evidence that some ofthe evaporable water normally held inconcrete contributes to its strength, thatis, van der Waals attraction betweenwater molecules and between solid mate-rial and water molecules seems to con-tribute to the total cohesive force. How-ever, there is a somewhat greaterweakening effect due to swelling.

Strength:

In the above discussion dealing withcohesion and adhesion we necessarilyspoke of strength. But there are otheraspects of the strength of concrete de-pending on the nature of its structure,and some of these will be discussed in thefollowing paragraphs.

It would be expected that with a given

aggregate the more cement gel per unitvolume of paste and the more paste perunit volume of matrix, which means thelower the air content of the matrix, thegreater the strength of the cpncrete.Strength tests of many kinds of concreteshow that such is indeed the case; it was.the fact underlying the cement-spaceratio law for strength given by Feret(1897), and the less general water-cementratio law given by Abrams (1918), andearlier by Zielinski (1908). For aggm- .gates composed of strong particles, theupper limit of concrete strength tends tobe established by the upper limit of thedensity of the matrix, that limit usually ,being established by the means at handfor compacting the mixture, On the otherhand, with aggregates composed of rela-tively weak particles, the upper limittends to be established by th$ strengthsof the aggregate particles. This does notmean, however, that concrete cannot bestronger than the strength of individualaggregate particles, as is evident whenone considers air bubbles as aggregate.

The strength of a given kind of con-crete is not a single valued property ofthe material. Compressive strength, inparticular, is a function of the rate atwhich stressis applied, the function beingsuch that strength appears higher thehigher the rate of loading. If a stress lessthan the strength indicated by an ordi-nary test procedure, and yet greater thanabout two thirds of that stress, is main-tained without increase, the specimenwill eventually fail. There are good rea-sons to believe that failure is essentiallya random (stochastic) process, and there-fore the stress existing at time of failureis intrinsically a variable number; inother words, two absolutely identicalspecimens would not be likely to fail atexactly the same stress, or, under a sus-tained high stress less than the mean“instantaneous” strength, they are notlikely to fail at the same elapsed time


after loading. Thus, it seems that evenwith a perfect testing machine and withtest specimens exactly alike, we wouldstill find variation of test results aboutthe mean value from a large number ofidentical specimens.

When a concrete cylinder is subjectedto a steadily increasing axial stress, asinthe ordinary compression test, the ob-served increase in diameter at low stressis a certain constant fraction (Poisson’sratio) of the longitudinal shortening(compressive strain), There is no tensilestress associated with the lateral strainjust mentioned, but after the compressivestress becomes about two thirds of thestress at failure, further increase in com-pressive stress causes lateral dilation toincrease more than can be accounted forby Poisson’s ratio, and this extra straindoes denote the development of tensilestress. When such stress begins to exceedtensile strength, vibrations due to inter-nal splitting can be detected with suitableinstruments. At failure, fractures appearas uneven surfaces having an inclinationto the axis somewhat less than 45 deg,or fractures appear as cracks parallel tothe axis, or both kinds of fractures ap-pear. Such experimental evidence indi-cates that the failure of concrete undercompression, or when subjected to shearstress, is essentially failure in tension. Iffailure under compressive stress werelimited only by strength in pure shear,the principal strains preceding failurewould involve only the sliding of onesmooth surface over another, and thusno increase in volume other than thataccounted for by Poisson’s ratio wouldbe necessary; hence, the observed extradilation with simultaneous internalcracking shows that tensile stress de-velops across the incipient fracture sur-faces and causes separations at suchsurfaces.

Current theory of fracture indicates

PCA.R&D.Ser.1111-l

that during a compression test tensilestress necessarily develops around holes,cracks, or flawsin the material, and thustensile stress would develop even in acontinuum, structurelessexcept for flaws,Without questioning this deduction wecan at the same time suggest that tensilestress would arise anyway, simply be-cause of the granular nature of the mate-rial. Since the individual particles ofwhich paste or rock are composed aremuch stronger than the structure, it isimpossible for the surface of a fracturein paste to be smooth. Specimens of neatcement show evidence of tension failurein a compression test; they have a strongtendency to split. In concrete made ofstrong aggregate material and weakpaste, the nominal shearing surfacesare likely to follow the contours of rockparticles, in which case the cause of ten-sile stress is obvious; in concrete madewith strong pastes, fracture surfacesusually pass through rock particles, buteven so the fracture surface is notsmooth, and again it is not difficult toaccount for the development of tensilestress as one rough surface is forced tomove away from the other as it tends toslide. These considerations amount toclassifying cement paste or concrete asan intrinsically dilatant system, just ascompact, uncemented, granular systemsare intrinsically dilatant,

CONCLUSION

While discussing the structure of con-crete and the internal forces that give itstability, various properties were dis-cussed. It would be possible to elaboratethese brief discussions and to add othertopics; however , other papers in thispublication are concerned with suchreviews, and no doubt interpretations interms of structure will be found in them.Therefore, this paper is concluded with-out further development of the subject.

Bulletins Published by theResearch Department

Research and Development Laboratoriesof the

Portland Cement Association

100. “List of Published Bulletins and Papers of the Research Department,”May, 1959 (Also lists earlie~ research papers of the Portland CementAssociation).

101. “Determination of the Apparent Density of Hydraulic Cement in WaterUsing a Vacuum Pycnometer,” by C. L. FORD.

Reprinted from ASTM Bulletin, No. 231, 81-84 (July, 1958).

102. “Long.Time Study of Cement Performance in Concrete—Chapter 11.Report on Condition of Three Test Pavements After 15 Years of Serv-ice,’) by FR.ANKH, JACKSON.

Reprinted from Jou?’?zat of the American Concrete Institute (June, 1958); Pro-ceedings, 54, 1017-1032 (1957-1958).

103. “Effect of Mixing and Curing Temperature on Concrete Strength,” byPAUL KLIEGER.

Reprinted from Journal of the American Concrete Institute (June, 1958); Pro-ceedings, 54, 1063-1081(1957-1958).

104. “The Successive Determination of Manganese, Sodium and PotassiumOxide in Cement by Flame Photometry,” by C. L. FORD.

Reprinted from ASTM Bulletin, No. 233, 57-63 (October, 1958).

105. “The Surface Energy of Tobermorite,” by STEPHEN BRUNAUER, D. L.KANTRO and C. H. WEISE.

Reprinted from Canadian .70UT?2U1 of Chemistry, 37, 714-724 (April, 1959).

106. “The Flow of Water in Hardened Portland Cement Paste,” by T. C.POWERS, H. M. MANN and L. E. COPELAND.

Reprinted from ffighwau Research f50ard Special Repo~t 40, 308-323 (1958).

107. “The Ball-Mill Hydration of Tricalcium Silicate at Room Temperature,”by D. L, KANTRO, STEPHEN BRUNAUER and C. H, WEISE.

Reprinted from Jou?’na~ of CoWirf Science, 14, 363-376 (1959).

108. “Quantitative Determination of the Four Major Phases of PortlandCement by Combined X-Ray and Chemical Analysis,” by L. E. CoPE-LAND, STEPHEN BRUNAUER, D. L. KANTRO, EDITH G. SCHULZ and C. H. WEISE.

Reprinted from Analytical Cite?nistw, 31, 1521-1530 (September, 1959).

109. “Function of New PCA Fire Research Laboratory, ” by C. C. CARLSON.Reprinted from the Journal of the PCA Research and Development .Laboru-tories, 1, No. 2, 2-13 (May, 1959).

110. “Capillary Continuity or Discontinuity in Cement Pastes,” by T. C.POWERS, L. E. COPELAND and H. M, MANN.

Reprinted from the Journal of the PCA Research and Development Laborat-ories, 1, No. 2, 3848 (May, 1959).

111. “Petrography of Cement and Concrete,” by L. S. BROWN.Reprinted from the Journal of the PCA Research and Development Labora-tories, 1, No. 3, 23-34 (September, 1959).

112. “The Gravimetric Determination of Strontium Oxide in PortlandCement,” by C, L. FORD.

Reprinted from ASTM Bulletin, No. 245, 71-75 (April, 1960).

113, “Quantitative Determination of the Four Major Phases in PortlandCement by X. Ray Analysis,” by STEPHEN BRUNAUER, L. E. COPELAND,D. L. KANTRO, C. H. WEISE and EDITH G, SCHULZ.

Reprinted from Proceedings of the AmeTican Society for Testing Materials, 59,1091-1100 (1959).

114. “Long-Time Study of Cement Performance in Concrete—Chapter 12.Concrete Exposed to Sea Water and Fresh Water,” by I. L. TYLER.

Reprinted from Journal of the American Concrete Institute (March, 1960);Proceedings, 56, 825-636 (1960),

115. “A Gravimetric Method for the Determination of Barium Oxide in Port.land Cement,” by C. L, FORD.

Reprinted from ASTM BuUetin, No. 247, 77-60 (July, 1960).

116. “The Thermodynamic Functions for the Solution of Calcium Hydroxidein Water,” by S. A. GREENBERG and L, E. COPELAND.

Reprinted from Journal of Ph@cat Che?nk’try, 64, 105’7.1059 (August, 1960).

117. “Investigation of Colloidal Hydrated Silicates. I. Volubility Products,”by S. A. GREENBERC, T. N. CHANG and ELAINE ANDERSON.

Reprinted from Journal of Ph@cat Chemistrg, 64, 1151-1156 (September, 1660).

118. “Some Aspects of Durability and Volume Change of Concrete for Pre-stressing,” by PAUL KLIEGER.

Reprinted from the Jou?mat of the PCA ReseaTch and Development Labora-tories, 2, No, 3, 2-12 (September, 1960).

119. “Concrete Mix Water—How Impure Can It Be?” by HAROLD H. STEINOUR.Reprinted from the Journal of the PC’A Research and Development Labora.tories, 2, No. 3, 32-50 (September, 1960).

120. “Corrosion of Prestressed Wire in Concrete,” by G. E. MONFORE andG. J. VERBECK,

Reprinted from JOU?VWof the American Concrete institute (November, 1960);Proceedings, 57, 491-515 (1960).

121. “Freezing and Thawing Tests of Lightweight Aggregate Concrete,” byPAUL KLIEGER and J. A. HANSON.

Reprinted from Joumat of the American Concrete Institute (January, 1961):Proceedings, 57, 779-796 (1961).

122. “A Cement-Aggregate Reaction That Occurs With Certain Sand-GravelAggregates,” by WILLIAM LERCH.

Reprinted from the Jou?mal of the PCA Research and Development Labo~ato-~ies, 1, No. 3, 42-50 (September, 1959).

123, “Volume Changes of Concrete Affected by Aggregate Type,” byHAROLD ROPER.

Reprinted from the Journal of the PCA Resea~ch and Development ‘La bora-tories, Z, No. 3, 13-19 (September, 1960).

124. “A Short Method for the Flame Photometric Determination of Magne.sium, Manganic, Sodium, and Potassium Oxides in Portland Cement, ”by C. L. FORD.

Reprinted from ASTM BuUetin, No. 250, 25-29, (December, 1960).

125. “Some Physical Aspects of the Hydration of Portland Cement,” byT. C. POWERS,

Reprinted from the Jowna~ of the PCA Research and Development Labora-tories, 3, No. 1, 47-56 (January, 1961).

126. “Influence of Physical Characteristics of Aggregates on Frost Re.sistance of Concrete,” by GEORGE VERBECK and ROBERT LANDGREN.

Reprinted from Proceedings of the American SocietV for Testing Materials, 60,1063-1079(1960).

127. “Determination of the Free Calcium Hydroxide Contents. of HydratedPortland Cements and Calcium Silicates,” by E. E. PRESSLER, STEPHENBRUNAUER, D. L. KANTRO, and C. H. WEISE.

Reprinted from A?aaWkaI Che?nistw, 3!3, No. 7, 877-682 (June, 1881).

128. “An X-ray Diffraction Investigation of Hydrated Portland CementPastes,” by D. L. KANTRO, L. E. COPELAND, and ELAINE R. ANDERSON,

Reprinted from P?’oceecIhws of the American Soctetv for Testing Mate?+ds, 80,1020-1035(1880).

129. “Dimensional Changes of Hardened Portland Cement Pastes Causedby Temperature Changes,” by R. A. HELMUTH.

Reprinted from Htghwav Research 130ard Proceedings, 40, 315-336 (1981).

130. “Progress in the Chemistry of Portland Cement, 1887-1960,” by HAROLD H.STEINWJR.

Reprinted from the Journal of the PCA Research and Denvelopment Labora-tories, 3, No. 2, 2-11 (May, 1961).

131. “Research on Fire Resistance of Prestressed Concrete,” by HUBERTWOODS, including discussion by V. PASCHKIS, and author’s closure.

Reprinted from Jo~rnal of the Structural Division, Proceedings of the Ameri-can Soczetu of Civd Engineers, Proc. Paper 2640, S6, ST 11, 53-64 (November,1960); Discussion, 87, ST 2, 58-60 (February, 1961); Cloeure, 87, ST 5, 81 (June,1961).

132. “Centralized Control of Test Furnaces in the PCA Fire Research Labo-ratory,” by PHIL J. TATMAN.

Reprinted from the Journal of the PCA Research and Development Laborat-ories, 3, No, 2, 22-26 (May, 1961).

133. “A Proposed Simple Test Method for Determining the Permeability ofConcrete,” by I. L. TYLER and BERNARD ERLIN.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 3, No. 3, 2-7 (September, 1961).

134, “The Behavior at High Temperature of Steel Strand for PrestressedConcrete,” by M. S, ABRAMS and C. R. CRUZ.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 3, No. 3, 8-19 (September, 1961),

135. “Electron Optical Investigation of the Hydration Products of CalciumSilicates and Portland Cement,” by L. E. COPELAND and EDITH G. SCHULZ.

Reprinted from the JournaC of the PCA Research and Development Labora-tories, 4, No. 1, 2-12 (January, 1962).

136. “Soil-Cement Technology—A Resume,” by MILES D. CATTON.Reprinted from the Journat of the PCA Research and Development Labora-tories 4, No. 1, 13-21 (January, 1962),

137. “Surface Temperature Measurements With Felted Asbestos Pads,” byM. S. ABRAMS.

Reprinted from the Journal of the PCA Research and Development Labora-to~ies, 4, No. 1, 22-30 (January, 1962).

138, “Tobermorite Gel—The Heart of’ Concrete,” by STEPHEN BRUNAUER.Reprinted from the American Scientist, 50, No. 1, 210-229 (March, 1962).

139. “Alkali Reactivity of Carbonate Rocks—Expansion and Dedolomitiza-tion,” by DAVID W, HADLEY.

Reprinted from Hightuav Research 130ard Proceedings, 40, 462-474 (1961).

140. “Development of Surface in the Hydration of Calcium Silicates ,“ byD, L. KANTRO, STEPHEN BRUNAUER, and C, H. WEISE.

Reprinted from Solid Surfaces and tire Gas.Solfd Interface, Advances inChemistry Series 33, 199.219 (1961).

141. “Thermodynamic Theory of Adsorption,” by L. E. COPELAND and T. F.YOUNG.

Reprinted from Solid Surfaces and the Gas.Solid Interface, Advances inChemistry Series 33, 348-356 (1961),

and

“Thermodynamics of Adsorption. Barium Sulphate—Water System,” byY. C. Wu and L. E. COPELAND.

Reprfnted from Solid Surfaces and the Gas. Solid Interface, Advances inChemistry Series 33, 357-368 (1961).

142, “The New Beam Furnace at PCA and Some Experience Gained fromIts Use,” by C. C. CARLSON and PHIL J, TATMAN.

Re rinted from Symposium on Fire Test Methods. ASTM Special TechnicalEPu Ucation No, 301, 41-59 (1961),

143. “New Techniques for Temperature and Humidity Control in X-Ray Dif-fractometry,” by PAUL SELICiMANN and N. R. GREENING.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 4, No. 2, 2-9 (May, 1962).

144. “An Optical Method for Determining the Elastic Constants of Concrete,”by C. R. CRUZ.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 4, No. 2, 2432 (May, 1962).

145. “Physical Properties of Concrete at Very Low Temperatures,” by G. E.MONFORE and A, E. LENTZ.


146. “A Hypothesis on Carbonation Shrinkage,” by T. C. POWERS,


147. “Fire Resistance of Prestressed Concrete Beams, Study A — Influenceof Thickness of Concrete Covering Over Prestressing Steel Strand,” byC. C. CARLSON.

Published by Portland Cement Association, Research and Development Labora-tories, Skokie, Illinois, (July, 1962).

148. “Prevention of Frost Damage to Green Concrete,” by T. C, POWERS.

Reprinted from Ffdunio?s International des Laborutoires d’llssats et de Re-cherches sur les Mat&iaux et les Constructions, RILEM Bulletin 14, 120-124(March, 1982).

149. “Air Content of Hardened Concrete by a High-Pressure Method,” byBERNARD ERLIN.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 4, No. 3, 2429 (September, 1962).

150. “A Direct Current Strain Bridge,” and “A Biaxial Strain Apparatus forSmall Cylinders,” by G. E. MONFORE.

Reprinted from the Journal of the PCA Research and Development Labora.tories, 4, No. 3, 2-9 (September, 1962).

151. “Development of Surface in the Hydration of Calcium Silicates. II. Ex-tension of Investigations to Earlier and Later Stages of Hydration, ”by D. L. KANTRO, STEPHEN BRUNAUER, and C. H. WEISE.

~@&inted from The Jou?mat of Piz@cat Chemistry, 66, No. 10, 1804-9 (October,

152. “The Hydration of Tricalcium Silicate and &D~lcalcium Silicate atRoom Temperature,” by STEPHZN BRUNAUER and S, A. GREENBERG.

Reprinted from Chemistry of Cement, Proceedings of the Fourth InternationalSvmpostum, Washington, D. C., 1960, held at the National Bureau of Stendards(U.S. Department of Commerce), Monograph 43, Vol. I, Session III, PaperIII-1, 135-165.

153. “Chemistry of Hydration of Portland Cement,” by L. E. COPELAND, D. L.KANTRO, and GEORGE VERBECIC.

Reprinted from Chemistry of Cement, Proceedings of the Fourth InternationalSymposium, Washington, D. C., 1960, held at the National Bureau of Standards(U.S. Department of Commerce), Monograph 43, Vol. I, Session IV, PaperIV.3, 429.465.

154. “Physical Properties of Cement Paste,” by T. C. POWERS.

Reprinted from Chemistry of Cement,Proceedingsof the Fourth InternationalSvmposium, Washington, D. C., 1960, held at the National Bureau of Standards(U.S. Department of Commerce), Monograph 43, VO1. IL Session v, PaPerV-1, 577-609.

155. “The Rheology of Fresh Portland Cement Pastes, ” by MOSHEISH-SHALOMand S. A. GREENBERG.

Reprinted from Chemistry of Cement, Proceedings of the Fourth InternationalSUmposium, Washington, D. C,, 1960, held at the National Bureau of Standarde(U.S. Department of Commerce), Monograph 43, Vol. 11, Session V, paperV-S4, 731-744.

156. “Capillary Size Restrictions on Ice Formation in Hardened PortlandCement Pastes,” by R. A, HELMUTH.

Reprinted from Chemistryof Cement,Proceedings of the Fourth internationalSymposium, Washington, D. C., 1960, held at the National Bureau of Standards(U.S. Department of Commerce), Monograph 43, Vol. II, Session VI, PaperVI-S2, 855-869.

157. “Extensions to the Long-Time Study of Cement Performance in Con.crete, ” by PAUL KLIEGER.

Reprinted from the JousvraCof the PCA Research and Development Labora-to~ies, 5, No. 1, 2-14 (January, 1963).

158. “Durability Studies of Exposed Aggregate Panels,” by A. W. ISBERNER.Reprinted from the JOUTW21 of the PCA Research and Development Labora-tories, 5, No. 2, 14-22 (May, 1963).

159. “Corrodibility of Prestressing Wire in Concretes Made With Type I andType IS Cements,” by 130RJE OST and G, E. MONFORE.

Reprinted from the Jour?tat of the PCA Research and Development Labora-tories, 5, No. 2, 23-26 (May, 1963).

160. “A Small Probe-Type Gage for Measuring Relative Humidity,” by G. E.MONFORE.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 5, No, 2, 41-47 (May, 1963).

161. “Abnormal Cracking in Highway Structures in Georgia and Alabama,”by CALVINC, OLESON.

Reprinted from Journal of the American Concrete Institute (March, 1963);Proceedings, 60, 329-353 (1963).

162, “Rheology of Fresh Portland Cement Pastes: Influence of Calcium Sul-fates,” by S. A. GREENBERG and L. M. MEYER.

Reprinted from IJightoau Research Record, Number 3, 9-29 (1963).

163. “A Sonic Method to Determine Pavement Thickness,” by RICHARDMUENOW.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 5, No. 3, 8-21 (September, 1963).

164. “Effect of Restraint on Fire Resistance of Prestressed Concrete, ” byS. L, SELVAGGIOand C. C. CARLSON.

Reprfnted from symPosinm on Fire Test Methods, ASTM Special Tec~nicQIPublication No. 344, 1-25 (1962).

165. “Effect of Surface Grinding and Joint Sawing on the Durability of Pav-ing Concrete, ” b Y WILLIAM PERENCHIO.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 6, No. 1, 16-19 (January, 1964).

166.

167.

168.

169.

170,

171.

172.

173.

174.

175.

176.

177.

“Quantitative Determination of the Major Phases in Portland Cementsby X-Ray Diffraction Methods,” by D. L. KANTRO, L. E. COPELAND, C. H.WEISE, and STEPHEN BRUNAUER.

Reprinted from the JouraaL of the PCA Research and Deoetop?nent Labora-tories, 6, No. 1,20-40 (January, 1964).

~~pore Structures and Surface Areas of Hardened Portland CementPastes by Nitrogen Adsorption, ” by R. SH. MIKHAIL,L. E, COPELAND,and STEPHENBRUNAUER.

Reprinted from Canadian Journal of Chemtstru, 42, No. 2, 426-438 (February,1964).

i~rnfluence of the Cement on the Corrosion Behavior of Steel in COn-

crete,” by HAROLDH. STEINOUR.Publiehed by Portland Cement Association, Research and Development Labora-tories, Skokie, Illinois, May, 1964.

“Silicone Influence on Concrete Resistance to Freeze-Thaw and De.Icer Damage,” by PAULKLIEGERand WILLIAMPEREIJCHIO.

Reprinted from Highwag Research Record, Number 16, 3347 (1963).

“Properties of Expansive Cement Made With Portland Cement, Gypsum,and Calcium Aluminate Cement,” by G. E. MONFORE.

Reprinted from the Journal of the PCA Research and Development Labo~a-tories, 6, No. 2, 2-9 (May, 1964).

“Fire Resistance of Prestressed Concrete Beams. Study B. Ifluence ofAggregate and Load Intensity, ” by S. L. SELVAGGIOand C. C. CARLSON.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 6, No. 1, 41-64 (January, 1964), and 6, No. 2, 10-25 (May, 1964),

“Petrographic Studies on Concrete Containing Shrinking Aggregate,” byHAROLDROPER,J. E. Cox and BERNARDERLIN.

Reprinted from the .10t4TT2d of the PCA Research and Devetop?nent Labora-tories, 6, No. 3, 2-18 (September, 1964).

“Corrosion of Aluminum Conduit in Concrete,” by G. E. MONFORE andBORJE OST.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 7, No. 1, 10-22 (January, 1965).

“Topics in Concrete Technology — 1. Geometric Properties of Particlesand Aggregates, 2. Analysis of Plastic Concrete Mixtures, 3. MixturesContaining Intentionally Entrained Air, and 4, Characteristics of Air-Void Systems,” by T. C. POWERS.

Reprintedfrom the Journal of the PCA Resea~ch and Development LUbOTQ-tories, 8, No. 1, 2-15 (January, 1964) 6, No. 2, 46-64 (May, 1964), 6, No. 3, 19.42(September, 1964), and 7, No. 1, 23-41 (January, 1965),

t~TwentYYear ReportontheLong-Time Study of Cement performance

in Concrete,” by Advisory Committee, Long-Time Study of Cement Per-formance in Concrete, W. C. HANSEN, Chairman.

Published by Portland Cement Association, Research and Development Labora-tories, Skokie, Illinois, May, 1965.

“Alkali Reactivity of Dolomitic Carbonate Rocks,” by DAVIDW. HADLEY.Reprintedfrom ~@lwav Research Record, Number 45, 1-19 (1964).

~~Alkali.ReaCtiVe Carbonate Rocks in Indiana—A Pilot Regional ~vesti-gation,” by DAVIDW, HADLEY.

ReprintedfromIfigiwau Research Record, Number 45, 196-221 (1964).

178. “Water-Vapor Adsorption-Desorption Characteristics of Selected Light-weight Concrete Aggregates,” by ROEERTLANDGREN.

Reprinted from Proceedings of the American Socfetu for Testing and Materials,64, 830-845 (1964).

179. “Determining the Water Absorption of Coarse Lightweight Aggregatesfor Concrete,” by ROBERTLANDGREN.

Reprinted from Proceedings of the American Society for Testing and Materials,64, 846-865 (1964).

180. “Investigation of Colloidal Hydrated Calcium Silicates. II. VolubilityRelationships in the Calcium Oxide-Silica-Water System at 25° ,“ byS. A. GREENBERGand T. N. CHANG.

ReprintedfromJow?salof PhwtcalC7te?nist?’v,69, No. 1, 162-166(January, 1965).

181. “Concrete Drying Methods and Their Effect on Fire Resistance,” byM. S. ABRAMSand D. L. ORALS.

Reprinted from Moisture of Materials tn Relation to Fire Teets, ASTM SpecialTechnical publication No. 385, 52-73 (1965),

182, “Thermal Conductivity of Concrete at Very Low Temperatures,” byA. E. LENTZ and G. E. MONFORE,

Reprinted from the Journal of the PCA Research and Development Labor-atories, 7, No, 2, 39-46 (May 1965).

183. “An Improved Procedure for Proportioning Mixes of Structural Light-weight Concrete,” by R. LANDGREN,J. A. HANSON,and D. W. PFEIFER.

Reprinted from the Journal of the PCA Research and Development Labor-atories, 7, No. 2, 47-85 (May 1965).

184. “Particle Size Distribution of Portland Cement from Wagner Turbidi-meter Data, ” by W. G. HIME and E. G. LABONDE.

Reprinted from the Journal of the PCA Research and Development Labor-stories, 7, No. 2, 66-75 (May 1965).

185. “Studies of Early Hydration Reactions of Portland Cement by X-RayDiffraction,” by PAULSELIGMANNand NATHANR. GREENING.

Reprintedfrom HtgitwavResearcltRecord, Number62, 80-105(1964).

186. “Application of a Small Probe-Type Relative Humidity Gage to Re-search on Fire Resistance of Concrete, ” by M. S. ABRAIWSand G. E.MONFORE.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 7, No. 3, 2-12 (September 1965).

187, “A Rapid Method for Studying Corrosion Inhibition of Steel in Con-crete,” by VENICEK. GOUDAand G, E. MONFORE.

Reprintedfrom the Journalof the PCA Research and Development Labora-to~ies, 7, No. 3, 24-31 (September 1965).

188. “Oxygen Enrichment of Combustion Air in Rotary Kilns,” by ROBERTA.GAYDOS.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 7, No. 3, 49-56 (September 1965).

189. “Cement Hydration Reactions at Early Ages,” by GEORGEVERBECK.Reprintedfrom the Journalof the PCA Research and Development Labora-tories, 7, No. 3, 57-63 (September 1965).

190. “Expansive Concrete—Laboratory Tests of Freeze-Thaw and SurfaceScaling Resistance,” by A. H. GUSTAFERRO,N. GREENING,and P. KLIEGER.

Reprintedfrom the JoumaCof the PCA Resea~cit and Development Labora.tories, 8, No. 1, 10.36 (January 1966).

191. “Elastic Properties of Concrete at High Temperatures,” by CmLos R.CRUZ.

Reprintedfrom the Journa& of the PCA Research and Development Labora.tories, 8, No. 1, 37-45 (January 1966).

192, “Penetration of Chloride into Concrete,” by BORJEOST and G. E. MON-FORE.

Reprinted from the Journal of the PCA Research and Development Labora-tories, 8, No. 1, 46-52 (January 1966).

193. “Methods Used in Petrographic Studies of Concrete, ” by BERNARDERLIN.Reprintedfrom AnalyticalTechniquesfor HydraulicCementand Concrete,ASTMS~ecia~TechnicalPublicationNo. 395,3-17(1966).

194. “Use of Infrared Spectrophotometry for the Detection and Identificationof Organic Additions in Cement and Admixtures in Hardened Concrete,”by W. G. HIME, W. F. MIVEI.AZ,and J. D. CONNOLLY,

Reprintedfrom AnalyticalTechniquesfor Hydratic Cementand Concrete,ASTM Special Technica~ Publication No. 395, 18.29 (1966).

195. “Improved Method of Testing Tensile Bond Strength of MasonryMortars,” by W. H. KUENNING.

Reprintedfrom ASTM Jormna2 of Materkzk, 1, No. 1, 180-202 (March 1966).

196. “The Nature of Concrete,” by T. C. POWERS.Reprinted from Significance of Tests and Propertied of CONCRETEANDCONCRET&MAKING MATERIALS, ASTM Special Tec%nicat Publication No. 169-A, 61-72 (1966).

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