‘THE PERMEABILITY OF CONCRETE

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Gbe 3nstitution of Civi l EngineersSELECTED ENGINEERING PAPERS,

BEINGORIGINAL COMMUNICATIONSRDEKED BY THE COUNCILTO BE PUBLISHED WITHOUT DISCUSSION.

No. 20. PIHE PERMEABILITY O F

CONCEETE.BY

H A RO LD CHARLES TOY, M%., STUD.NST..E.

EDITED BY

H. H. JEFFCOTT, Sc.D., B.A.I., Assoc. M. INST. .E.,SECRETARY .

L O N D O N :lUu6lisfJeb bp Ehr fttstitution,

G R E A TG E O R G ES T R E E T ,W E S T M I N S T E R , S.W. 1.1924.

[Theright of Publ i ca t ion and of Translation is reserved.]


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ADVERTISE&IE-NT.

The Institution as a body is not, responsible either. for the statementsma.de, or for the opinions expressed, in. the ollowing pages.

LONDON : PRI NTED BY WILLIA81 CLOWES AKD SONS, LI MI TED,D U K E STREET, STANFORD STREET,S.E. l.,A ND GREAT WINDXILL STREET,W. 1.

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SELECTEDENGINEERINGPAPERS._I_

No. 20. The Permeability of Conorete.l

By HAROLDEARLES TOY,M.Sc., Stud. Inst. C.E.THISPaper describes research work which was carried out in 1923under Dr.F. C. Lea, M. Inst. C.E., at theUniversity of Birmingham.Flow of Water in Concyete.-It is important todistinguish clearlybetween permeability and porosity ; and the following theoreticalinvestigation of the flow of water through concrete clearly showsthe difference. An ana.logy s drawnbetween he flow of waterthrough a tube and that througha capillary vein in concrete. Twoforces act against bhe water-pressure to which the concrete blockis ubjected,namely, the frictional orcebetween the concretesurfaceand the water,. and the viscosity of the water itself. Asthe former is the greater, a film of water sticks to the walls of thepassage, inside which a minute stream of water flows. The volumeof water flowing through he ubeper second, V = r p a 4 / 8 L N ,where a denotes the radius of the tube ; p, the pressure-differencecausing flow ; L , the ength of the ube; and N the coefficientof viscosity.This ormula sapplied to he ca,se of aconcreteblock of thickness t , subjected to awater-pressure of p lbs.persquare nch,and he heoreticalassumption is made hat t ispierced by a number.of fine capillary veins, say 76, of a mean radiusa, nd a mean length L . The length of the vein is obviously somefunction of the thickness of the concrete block, therefore

L = f ( t ) = mt . . . . . . (1)Let S denote the area of surface in contact with water, and X thepercentage voids in set concrete. Then the volume of voids

Stx = a2Lk . , . . . . . (2)The volume of waterpassing throughone veinper econd is

Students Paper No. 784.B 2


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4 TOP OK THE PERMEAUTLITP OF CONCRETE.q a 4 / 8 L N , herefore the volumepassing through the whole mass= q a 4 k / 8 L N ,

Therefore the permeabilityfrom (2)

from (1)An example worked out for some practical values obtained bytheAuthorgave he following esult :--Concrete pipe, 6 inchesoutsidediameter, 3 inchesnsidediameter, and 9 inches high.Permeability 0-6 grams per minute. Mean radius of vein = 0.688

X 10-6centimetres.The difference between permeability and porosity is now clearlyevident :-

Porosity is proportional t o the percentage of voicis ;Permeability is proportional to (size of voids)' and alsoto the percentage of voids.That is to say, the size of voids smore important than theiractual volume in determining permeability. All the laws governingpermeabilitymaybeobtained from this ormula,and heyare

given below, togetherwith tatements basedon the researchescarried out.Pemeabi l i t y of Concrete.-(l) Permeability is directly proportionalto :-(i) Pressure. A pressure-permeability curve is a close approxi-mation oastraight ine,and seems to follow veryclosely the tensile stress-strain curve for concrete.(ii) Surface area of contact.(iii) Percentage of voids.(iv) (Mean radius of capillary veins)'.,(2) Permeability is inversely proportional to :-

(i) Visc,osity, whichis itself inversely proportional t o tempera-ture in the case of water, so that permeability increaseswith increase in temperature.(ii)thickness of Thealue of n will be foundt o vary with the size of the stone and sand. It has nofixed values, but it is certainly greater than unity. Forthis reason the permeability sverymuchmore thanhalved when the thickness is doubled.

(3) Permeabilitydecreasesmore apidly than cement-contentincreases, because both tot81 volume and size of voids are affected.


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TOT ON THE PEIt;\IEABILITY OF COSCKETE. 5With the sameproportion of sandandstone, but wi th differentproportions of cement, ,hepermeability s nverselyproportionalto the tensile strength of the mortar.(4)Permeability decreases appreciably with the time of testing.The percentage decrease is proportional to the cement-content.( 5 ) The tensile strength f concrete bears a closer inverse relabion-ship to permeability than does its compressive strength.(6) A fine sand results in large permeability and a small percentagedecrease with time, unless excess cement is used.(7 ) Stone of uniform size is better than badly-graded stone withmixed sand, and probably about as good as graded stone.(8) The composition of the sand is the determining factor in th e

permeability of a mixture, and it is to this, rather than to the gradingof the stone, that attention should be paid.(9 ) A stone mixture may be obtained similaro a graded aggregateby the admixturef two or more aggregates, with great improvementin impermeability.(10) The mechanical-analysis curves, with the maximum-densitycurve, serve as a better guide for proportioning a mixture than dothe voids in stone and sand.Methods of rendering concrete watertight may be divided fairly

distinctly into two classes, each of which has its adherents, namely,internal or natural methods, in which the materials forming th econcreteareregarded and desirableproportionsareascertained ;andexternal or artificialmethods,consisting of the addition offoreign materia,ls to the cement, or water, or the application of animpermeable coating t o th e concrete.The apparatus sed by the ut,hor for determininghe permeabilityof various mixtures was arranged so th at th e water flowing into theconcrete was measured witha calibrated gauge, read by a telescope,and the leakage through the concrete pipe was collected merely asa check. The apparatus w7a.s used fo r testing both concrete pipes6 inches outside dia,meter, inches inside diameter, and inches high,and 4k-inch diameter slabs,up t o pressures of 40 lbs. per square inch.Water-tight ointsbetweenconcreteand ronplates were madeafter some trouble with rubber rings of round section. The results,all from concrete pipes, have been obtained from the qua,ntities ofwater computed from hourly readings of the gauge, and are for amean pressure of 22 lbs. per square inch.

Ferrocretecement wasused in allconcretemixtures. It isknown that free lime s always present n concrete, due o thehemicalreactions that occur in the cement, and it has been observed thatthis is washed out on the sides of the pipe during a test lastingP O 1


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6 T O P O F T H E PEKMERGILITT OF CONCRETE.a week. It is said tha t thi s loss renders the concrete weaker andmore liable to decay. A s a remedy the addition of a small quan-tity of trass or puzzolan to the cement has been suggested: thiscombineswith the free ime to formanaturalcement, which, itis claimed, ncreases thestrengthand reduces the permeabilityappreciably.Th e Effec t of the Cement-content on Permeability.-Given a certainsand and stonewhose ratio one to th eot'heris fixed, there s a criticalproportion of cement which will give rconomica.lly the best result

Pig. 1.

8 10 12 14 la 15 20 2:C E M E N T - C O N T E N T : PER C E N T00025 0 0 0 3 O W 3 5 0 004 00045 0005 00055 0006

MDrLor TensSe JtrentnEFFECT1.' ~ Y E X T - C O N T E K TN PICIIMEABILITTSD T E N B I L YTREXCTH

O F M O R T A R .as regards impermeability of the resultant concrete. This is shownin Fig . 2 , where permeability, in grams of water passing per minutethrough he concretepipe, is plotted aga.inst the percent'age ofcement by weight. It is readily seen that the permeability valuesdecrease from the value fora 1 : 2 : 4 mixturc by volume much morerapidly th an th e percentage of cement increases, until a peicentageof 15.8 in this case is reached. From this point the graph, whichhas been a steep straight line, fallsoff in a curve ; and it is obvious


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TOY OX T H E PERMEABILITY OF CONCRETE. 7that the addition of more cement than hhis critical proportion doesnot result in nearly as great an improvement in watertightness.Mortar briquettes were made up in half-dozens, of cement andsand in the same ratios as in the concrete mixtures. Consistencywas kept uniform by means of a standard slump test. The tensilestrengths, at 28 days, showed thesamecriticalpoint,and heirreciprocals, .e., ensile strength, plotted against the permeabilityvalues for the corresponding concrete mixtures, givea straight line.This procedure affords an efficient and handy means of finding thecorrect proportion of cement for any particular sand. It does nothold good, however, or comparing possible permeability values withdifferent sands, a.s will be seen later.

For aggregates a loca,l fine red sand obtained from the Tame andRea Drainage Board was used, whilst the large aggregate consistedof Rowley rag-stone, a dolerite rock from the Rowley Regis district,or Nuneaton quartzite (Hartshill stone).Methods of Proportioning.-Methods based on voids are unsound,because the methods of finding the percentage voids in the sandand stonedonot give accurate esults. One reason for this stha.t inactual work,smalla.ggregate will, inaddition to fillingvoids, become wedged between larger stones and incidentally causemore voids.The granulometric-analysis curves for the aggregates used (foundby sieving into a number of sizes and weighing residues) are givenin Fig. 2. Possible improvements in the grading of the large a.ndsmall aggregate may very often be judged from inspection of thesecurves, and effected by headdition of the one size of stone orsand required, or, more easily and practically, by the additidn ofanother aggregate in a calculated proportion.nlechnnicaZ-AlzaZysis Curve.-Messrs. W. B. Fuller and S. E.Thompson 1 carried out some very full tests on certain a.ggregates,from which they evolved a mechanical-analysis curve representinga mixture of the stone,sand,andcement of maximumdensity,i.e.,minimum voids. The quations of thismaximum-densitycume, which was a combination of an ellipse and a straight line, hadonly one unknown -the maximum size of stone.A curve of this nature was drawn out for the local aggregateswhich were then combined, withoutegrading, in proportionscalculated to give resultant mechanical-analysis curves lying moreor less,close to Fuller'scurve (F ig . 2 ) . It will benoticed thatcurve GRQ ,represents a mixture of twoaggregateswithsand

Trans. Am . Soc. C.E., vol. l ix , p. 67 (1907).


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8 TOYN THE PENXEBBILITY O F CONCItETE.andcement.The equiredproportion of cement wascalculatedfrom the determination of voids in the mixture of sand and stone.Medium consistency was maintained,nd a pipe6 inches and3 inchesin outside and inside diameter respectively, andinches high, and twocubes, were made from each mixture. Agraded mixture ( G R 3 ) wasalso made up by actually obtaining theeights of each size requiredfromhemaximum-density urve.The esults of permeabilityand crushing tests, together with density figures (obtained from thequantities of concrete usecl to fill the pipe-mould)are shown inTable I. Theyseem to indicate that GRQ, amixturecontainingbothRowley agandNuneatonquartzite, whose curve fits the

DIAMETERS O F PART ICLES : I N C H . ..S ~ E T H O DOF PROPORTIONINGITH ONE- AND TWO-STO~EQQRECATES.

maximum-density curve best of all, is least permeable, though ithas less cement than the other mixtures. I n fact, it is better thanth e graded mixtureGR3, but that s due to a slightly greater cement-content. This shows the help obtained by the use of Fullers curvein proportioning. It isatheremarkable that flow decreasesaccording to he height at which t.he maximum-density urveand he esultantcurve cross-in other words, as he at io ofsand to stone approaches 1 : 1. A possible explanation is that thisis due to Rowley rag being deficient in small material. From thenature of the maximum-density curve i t is seen thatr th e cementis included in t he sand, and, as far as density is concerned, servesthesame purpose as fine sand. In he threeconcretemixtures


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T OYN THE PRIXIEABILITY OF COSCRETE. 9

Pipe K O .

G R 1Q R 2G R 3G R 4G R Q

G R l BC R l CG R l D

Size ofStone.

Inch.B RB Ri3R!QB RB R

TABLE~.-PROPORTIONED IXTURES.

_ _ _ _ P _ ~Per cent, c 'Grams per Grams Lhs. per

1 :2 .67 :2 .62:1 .89 :2 .284 .9 ~ 1 4 . 5 '0 . 5 9 1 0.855 1 , 9 9 4Winnte. per cc . ' Sq. Inch.1 :1 .56 :2 .94:1 .1 :2 ,56 17.05 17 .1 1 . 6 5. 8 2 9, 6 6 8

(1 :3 .1 :2 .56 )i :2 .2 :2 .23 )4 .24 .7'964 0.853 2 , 6 7 01 :2 . 0 4 :2 . 9 6 1 :1 . 4 4 :2 . 5 i 1 5 . 7 0 1 4 . 3 ' ! ' 902 1 0.857 ' 2 , 3 7 81 :1 . 8 1 :3 . 1 8:1 . 2 8 :2 . 7 25 . 6 8 16.2 0'523 ' 0.838 2 , 4 7 91 :2 .46 :2 .56 ' 1 3 1 ' 7 4 3 2 . 2 25 . 7 4 15.8 1 '3 0 . 8 4 2, 9 3 01:3 .85:3 .58:2 .72:3 .121.03 18.0 1.36 0 .830 1,118

IGradedaterials.

~ l

~

1:1.87:2.12:1 '32:1.85 18.7 ~ 1 6 . 7 0 ' 1 6 5

UNIFORX-SIZEDIXTURESF O R PURPOSRSF COMPARISON).

1:1.33:2.661:1.33:2.661:1.33:2.66

1:1.5:31:2:41:2:4

Per ccnt. Grams perMinute. ' S q . Inch.Lbs. per, 1 6 . 5 2 0.87

!Il 2 , 3 9 0~

~ 2 , 1 6 7Q deno tes Nunea ton Quartzite stone. R denotes Rowley Ragstone.

GRlB, GRlC, and GRlD, the same proportions of sand and stonewere used, but in the case of GRlC, a certain reduction was madein thequantity of cement,comparedwith GRlB, andanequalvolume of finest sand was added. With G R l D the cement-contentwas increased, he excess cement replacingan equal weightof ordinarysand. Both changes resulted in hardly any alteration in density, aswas expected,bu t the ermeability was affected, no doubtdue to thedifference in fineness of the fine sand and cement. Of the fine sand

c201


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10 TOY ON TH E PEKMEABILITY OF CONCRETE.2.8 per cent. was less than inch, while 88 per cent. of the cementwas less than . inch. Nevertheless, a remarkably low permeabilityvalue was obtained for GRIC, considering the low percentage ofcement, which proved the feasibility of the method where strength isnot required.Un$orrn-sized Mixtures.-It has already been mentioned that itis possible, on tamping concrete, for the small stone to act notonlyas a void-filler, but also as an extra oid-former by becoming wedgedbetween the large stones. It may be questioned whether a stoneof uniform size would not be better, as regards producing a densemixture,hannaturallygraded tone, or even an artificially 'gradedaggregate. Messrs. Pullerand Thompson, in heir ests a tJeromePark,carriedouta ittle work on hese ines,and heyfound ha t a oarse ggregate of uniform izegavea lightlydenser concrete than did a graded aggregate. With both mixturesa graded sand was used.The question how far this may be applied t o the sand has notyet been solved. I ts function in the concrete is different from thatof the stone, and, being relatively much smaller in size, the effectof water on it,, for instance,s very different. A scheme was preparedwhereby it was hoped to find how finely the stone and sand shouldbegraded, if at all, for maximumdensityand mpermeability.The two extreme cases are uniform sizes of stone and sand mixedtogether,andacombination of gradedaggregatesconforming t osome predetermined curve. The degree of grading depends first onthe number of sieves used. One size of stone mixed with one sizeof sand may be ermed a broadly-graded mixture. The materialsused were washed before use and sieved into their different sizes :-Rowley rag, g, g, 4 inch ; h'uneaton quartzite, 4, , 4, & inch ;and Drainage-Board and, 3, Go-, & -;c inch.

A number of uniform-sizemixtureswereprepared from thesegrades.That is tosay, one size of stoneand one size of sand,in he atio of' 2 : 1, weremixedwithdifferentproportionsofcement. Only the sizes &, &, and -$G- inch of sandwereused,except in the case where the natural sand was used with a uniformsize of stone. In allcasesamediumconsistency, udged by eye,was used for all he mixtures, and uniformityof conditions of mixing,ramming,andcuring wasobtainedasnearlyas possible. Fromeach mixture a pipe was made, 6 inches outside and 3 inches insidediameter, and 9 incheshigh, and these pipeswereair-cured in aroom for 1 month before testing.The permeability values, given in grams per minute, are thoseobtained after 24 hours of testing and for a pressure of 22 Ibs. per


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TOY ON THE PERMEABILITY OF CONCRETE. 11TABLE T.-UNIFORM-SIZEDMIXTURES.

EFFECTO F SIZE OF S A N D NA . Permeability (values gicen in grams per minute).

Size of Sand.

ability. I Voids ~ abilit)Perm6__Penne-thility.~

0 . 4 3

Voids.__cent.Pe r

1 7 . 6

Q deno te3 Nunea ton Quar tz i te s tone . R deno tes Rowley Itagstone.B. T e n s i l e Strength of Mortar (16s. per s p a r e inch).

square nch.Table I1 shows the differencesbetween the flowsthroughmixturesontainingnd $,-inch size sand,houghthe percentage voids of the different sizes are practically the same.It has been shown that the tensile strength of the mortar was auseful guide in finding the right proportion of cement for a certainmixture of sand and stone, but the results here indicate that it isno help where different sands are concerned. The sand of ;&chsize seems to give the densest and least permeable mixtufe in eachcase, yet ts mortar strength is considerably less than th at with,',-inch sand, both fo r 1 : l and 1 : 2 mixtures.It is nteresting to note th at a mixture of ordinary stone andordinaryandasermeability 1.7, and voids9.2 ; &inchstoneandordinarysandhaspermeability 0.43, and voids17.6 ;+-inch stoneand &-inch sand has permeability .24, and voids 19.6 ;mixed stone and &-inch sand has permeability2.67, and voids 19.3.


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1 2 TOY O X THE PERMEARI1,ITI ' O F COSCRETE.I n other words, starting with a mixture of &-inch stone and


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T O P ON THE PERXEABILITY OF COKCRETE. 13The following appear to be reasonable deductions from the resultsobtained :--

l . Reinforcement of a slab seems to increase its permeability,rather than decrease it, as would be expectfed. Possiblythe ramming was not doneso efficiently.2 . Permeability is very much less with sea water than with tapwater. Fig 3.

EFFECTF TINEOF TESTINGN PBRYEABILITY.3. All the time-permeabilitycurves are more or less of the sametype, The bend is sharper in some than in others, bu tthey all finally pproach zero leakage, and a rapiddecrease occurs with them all when testing first starts.Tests on all the concrete pipes were continued as longas possible,and for at least 2 days.

From the readings the following values were evolved for thepercentage decrease n permeability in 24 hours on the value obtainedafter the pipe was wet all over :-

Mixture. Average Percolation Decrease.1 :1 : 2 . . . . . . . 57.6 perent. (average of six).1 : 2 : 4 . . . . . . . 36 perent.average of eight).

CZOI


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14 TOY ON THE PEBAfEABILITY OF CONCRETE.These results point to the percentage decrease in permeability withtime being proportiona,l to the cement-content ; a richer mixturenot only hasa smaller flow of water through itat the start, butlsocauses that flow to decrease more rapidly han does a poorermixture.It appears simply to be a case of the finer filter stopping more siltand its pores blocking up more quickly than those of the coarserfilter. This conclusion is supported by the curves in ig . 3, obtainedfor GRlB, GRlC, and GRlD. Though GRlC had very much th esame permeability as G R l B at the start, being the poorer mixture,its reducingactionpracticallystoppedafter the second day oftesting, whilst GRlB went on decreasing.

Fig. .

LONG-PERIODESTSNOS. 2 AND 3.The ong-periodcurvesobtained were verymuch of t,hesamecharacter as the others shown, the point of change approximatingto 48 hoursafter hecommencement of the est.Toattemptto discover whether urthercrystallization of the cement ookplace, two solid cylinders of concrete, 73 inches thick and having

11 square inches surface area of contact, were tested under a steadypressure of 26.3 Ibs. per squareinch until thelow ceased. They wereboth 1 :2 : 4mixtures by volume, butne was a mediumwet mixturewith 7.23 per cent. of water, whilst the other had only .50 per cent.It was expected that, if further chemical reaction took place in the


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T O T ON TIIE PERMEABILITY OF CONCRETE. 15dry mixture, the flow would be larger a t first, but would decreasemore rapidly than tha,t through the wetter mixture. However, i ttook twice as long for the flow to be reduced to zero ( F ig . 4 ) . Itwill be seen thak a remarkable jump occurred in one of these curvesfor no apparent reason. If no further, or very little, chemical actiontakes place, the other alternative is the deposition of fine materia1in the pores of the concrete, and in connection with this effect i twould be nteresting to find out if the rate of reduction of flowcould be increased by forcing watercontaining fine matter insuspension through the concrete instead of a filtered water.

The Paper is accompanied byfourdiagrams, from which thefigures in the text have een prepared.


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