Morrison, A.

Smart, B. J.

The University of Sydney: Sydney eScholarship Journals online

14TH MARCH, 1918. PERMEABILITY OF CONCRETE BY WATER. By B. J . SMART, B.Sc., and A. MORRISON. j.. The wide app1i{Jation of concrete for construction pur-'-.!> poses has r endered it necessary to pay careful attention ~o the adopt ion of suita'ble means for rendering it water-tight . S9me t ime ago the present authors became inter-e ted in this matter, when asked to investigate the pro-perties of a material for which waterproofing qualit ies were claimed. A considerable number of pr eliminary tests had been made when the report of an extensive resear ch carr ied out by Wig and Bates at the Bureau of tandards came into our hands. The methods adopted in that work afforded a solution of many of the diffi-culties' which we had encountered, but in our opinion, although their work is of an exhaustive {Jharacter, it leaves impor tant matter of a practical nature still un-decided. We ther efore determined to go on with our own test , and apply the apparatus as devised by the Bureau of Standards. Having been asked t o bring forward a paper on this subj ect, however, it is necessary to make it perfectly clear that our own work has only reached a preliminary stage, and our remarks will therefore be based to a large extent on the results of Wig and Bates, at the same time indicating the direction in which our own investigations have proceeded. At the outset it may be as well to define clearly what is meant by the terms permeability and porosity, which we find are frequently confused with one another. Per-A .. 2 PBBMEABILI TY OF OONCRETE BY WATB K. meability i the quality of allowing water to pass through, while porosity is the property of containing pores or voids, which take up water when the mat erial is im-mersed. A por~us material may not be permeable owing to the fact that the pre sure required to dr ive water through the fine capillaries is so great as t o exceed all practical limits. Before coming to consider tests of permeability, it will be of advantage t o consider the question from first principles, and to endeavour to form a clear idea a to what we mean when we say that a concrete aggregate is yvatertight. In the first place, we may a ume ~hat prac-}.ically all the larger aggregate used will in itself be im-permeable to water. When such material f orms part of a concrete, the water which passes through, finds its way around the aggregate through the paces which are filled with mortar. B ence it folloW'S that the pro'blem of ren-dering a concrete watert ight really consists in sealing the void of stone by means of an impervious mortar . The success with which this is carr ied out will be af-fected by the following considerations :-1. The efficiency of the mortar as a watertight seal. 2. The quantity of voids to be sealed. 3. The thoroughness with which the mortar is placed in po ition. Thus the mortar may be absolutely impervious, but may not fill the voids, or adhere compl tely to the metal With a more or less permeable mortar, a much larger quantity of water will pass where the voids are large because of the greater area which has to be closed by the mortar. From the e considerations and the experience we have ained, we have formed the opinion that in order to deal with this subject effectively it is nece ary to confine on elf in the first place to a study of mortars, and the effect of varying conditions on their properti . l'BRl1lIABILITY OF OONORETE BY W ATIIB. The earlier work carried out on this subject wa prin-. . d pally due to Thompson, who has et forth the follow-ing conclusions :-1. The p rmeability is less a the proportion of cement is increased. 2. The permeability is less as the maximum size of the stone is greater. 3, Rounded aggregates like gray I require less ce-ment than sharp aggregates like broken stone. The same remark applies to sand. 4. As the concrete gets older its permeability de-creases. 5. With increased water pressur the permeability increase uniformly. 6. The thi<lker the layer of concrete the less th~ permeability. 7. Medium and wet consi tencies produce concrete much more watertight than dry consistencies, and slightly more watertight than very wet couAis-t encies. 8. The surface as moulded is much more watertight than the bottom of a specimen, owing to the fine material which rises to the top. In addition to the above general conclusions, a certain amount of data had been published, 'but the fi ures are very largely disconnected, and in many cases insufficien iniormation is furnished to enable practical use to b made of the results. To have a practical value it is necessary that our know-ledge should be definite, and that we should be able to answer practical questions of an engineering character. Thus it hould be possible t o furnish -answers to the questions :-1. ~at quantity of water will flow through a con-crete of given composition, thickness and area under varying pressures f l'BR:HlIAlIILITY 0 ]/ OONORBTB :BY WAT BR. 2. How will the r ate fall off as the water continues: to flowf As g preliminary step t o planning a series of test S' with the object of gaining information on this subj ect , it was decided to set up an apparatus and carry out a series: of t ests on sandstone concrete. After t his work had been commenced the Report from the Bureau of Standards' came to hand, as a r esult of which we decided to alter-our methods, and instal an apparatus similar to that des-· cribed in their r epor t . . We will describe our preliminary tests as briefly as: possible, since they are of interest in showing some of tho difficultie of this work, an.d then pass on to consider the-improved apparatus, with the results obtained at the Bu-· reau of Standards, and our own work. Since the Repor t f rom the Bureau of Standards also deals with special in-gredients for wat erproofing which we have not yet ex-a ined ourselves, we propose. to treat that as a separat e l'Iubject, and deal with it at the end of our remarks. Preliminary Tests. The preliminary eries of tests was carried out 011 Sandstoll(, Concrete, which Waf> made to the following-composition :- 4 :2 :1 & 6:3:1 The metal used was broken to a maximum ize of 2 inches-graded to lh-inch, and the and was obtained from the epean River without special treatment. The blocks of concrete required for the permeability te ts were cast in 12in. arthenware pipes. These were first prepared by soaking in water, and were then well grouted with neat cement. ' They were placed on oiled iron plates, and concrete was filled in to a depth of 11 inches, the COD . tency 'being uch that it could readily be placed in po ition without 'ramming, working with a trowel being: sufficient. The layer of concrete is shown at H in fig. 1. PBBMlIABILITY 01' OONOBlITlI BY WATlIB. 6 The moulded blocks were kept covered with wet blankets for seven days, after which they w re allowed to dry out . .A. small quantity of pitch was then run round the edges of the surface of the concrete (G), where it came in contact with the pipe. This wa done to avoid the A ~f74----B ~~IZ!t=== ~ Ht<-t----- £ ~'_t_ilr_t----F ~~~~~~~~~~r----- c .Iiiiil'IIi+i"r-i----H .r. '4:.~t n .... .. ~ , ... -.~. 10 .... ... . , ~ .. . ... . t~*!?l{}J.~KJli;i~~~;!~i~ ~~~~~~~~~~~~~---- l -=.'-rt'.....,...+---- J Filr. 1 po sibility of any water finding its way between the con-crete and the walls of the pipe . .A. cover plate D, to which the water connections were attached, rested on a cylinder of galvanised iron E four inches deep, a water chamber F thus being formed above the concrete. 6 l'BRM1UlI ILITY 01' ONCRST1I BY WATBR. The COTer plate was luted down with a layer of bitu-men and pitch (). ince with the pressures used there was a tendency for the cover plate to r ise, it wa nece -sary to adopt means to hold it firmly in position, and for this purpose two pieces of curved steel pipe B were jammed tightly down the in ide of the earthenware pipe at right angles to each other. 2 :1 cement compo. wa filled in on top of the cover plate, completely surround-ing the pieces of steel piping. F rom previou experience it was found nece sary to intp.rpose a filter between the apparatus and the water main. ome trials were made with filter candles, but I.hey were found to choke up so rapidly that frequent renewal was essential. ccordingly in the tests here des-cri'bed a filter was con tructed con isting of a fine wire gauze filled with charcoal, at.d thi gave very satisfac-tory r suIt . Considerable trouble was, however, experienced on ac-count of variations in the water pressure, and for this. reason the wh~ apparatus was ubsequently dismantlerl and r constructed. The test blocks were connected up to the }Vater main under a pressure of 10lb . per square inch, and the quan-ttty of wat r passing throu h was measured periodically. It will be seen that at the beginning the blocks allowed water to pass freely throu h them, but that the flow rapidly decreased and finally fell to a more or less con· tant value. The results obtained are given below, and are shown in the curves in figure 2. It should be pointed out that m making the 6 :3:1 mixture it was nece ary to use a wetter consistency ow-ing to the lean mixture. The consistency of the 4:2:~ J i !. f r i J'BBMlIIABIt.ITY OP OONOBBTlil BY WATlIB. 7 mixture would be described as "wet," but it is possible that the lower permeability of the 6:3 :1 mixture is due to the use of a larger quantity of water. f.(, I 1-4 I·~ 1'0 ·s ~. .'" ·6 .4-) 1', .2. ~~~ 0' #', oA ~-11'\--- __ Hours from Commencement. Commencement 2 4 24 48 7t '16 95 117 216 S" ...... r.;::. ~S....c . Nol 16 " \C. 6 •. ~C" . 37Slbs -+1" No 2. . 1~ . 12- . ~75 • & ~3 :1 --60 60 100 Il.O /40 Fig. 2 Rate of Flow-Gallons per sq. foot per hour 1: 2 : l. 6 : 3 : l. 1'52 "376 '450 'S8 '057 '1\ 7 '032 '047 --'Ot7 --'018 • '012 _. '022 *Readings missed owing to a leak. '. 8 PBHMBABILITY oj OONOBETB BY W ATEX. Standard Apparatus for Measuring Penneability. The improved apparatu as constructed from the des-cription given in the report of the Bureau of Standards is shown in the photograph (fig. 5), and in deta,il in figs . ::! and 4. The methods of preparing the test pieces al'e those adopted in our own work, and differ lightly from that adopted by Wig and Bates. G Fig. a o lLJ " 10 PBRKRAlIILITY O:r OOl!lORBT B lIY WATBR. In carrying out tests of mortars, the test block A (fig. 3) is placed between rubber ring B, held in position be-tween the cover plate C and the annular bedplate D by means of bolts with wing nuts E. Above the test piece is a space F, the air in which can be displaced with water by means of the inlet pipe G and the pet cock H. .. PBRlIIBABI LI'rY OF OONORIITE BY W ATER. It The water supply is arranged as in figure 4 in such a manner as to d l iver filtered water at constant pressure, connection being made to the pipes G-G. A charcoal filter is placed in the cyiinder C, which is connected to the main 'by means of a 'blow-off valve W ater is fitst allowed to flow through the whole ap-paratus until it reaches a certain level in the drum E de-t ermined by trial. Compressed air is then forced into E through the pipe D until the requisite pressure is ob-tained. This seryes to form a cushion, and to prevent sudden changes of pressure due to outside causes. Cylindrical test blocks are moulded in short lengths of 6-inch steel casing, which have previously been thor-oughly washed and grouted with neat cement on the in-side. The blocks may be made in any required thick-nes , the sizes adopted being mainly 1 inch and 2 inches_ When moulding, the rings of casing are placed on steel plate which have been slightly greased, the top surface of the compo being levelled off with a trowel. The block are allowed to remain in the moulds until tested, and may be stored dry or wet as required. Blocks made to be tored dry are kept moist between blankets for seven day , whilst tho e stored in water are first main-tained between wet blankets for 24 hours. In some. cases the. outside skin was removed from the top and bottom surfaces 24 hours after the blocks were moulded. For test on concrete the same apparatus may be used, larger mould being provided so the area under test may be sufficiently great com,pared with the size of the stone. The test blocks, which may be conveniently 18in. in dia-_ meter, are attached to the water supply in a similar man-ner to that above described. I II 12 PlIBHlIABILITY 01' OONORlITE BY WATBR. Results of Tests Made at the Bureau of Standards. Before coming to con ider the actu~il results of the tests made at the Bureau of tandards, it is necessary to make a few observations regarding the conditions under which the tests were carried out. In proportioning the mortar and concret e the weight <If 1 cub. ft. of cement is taken as l OOlbs. Although this is quite correct, it is our experience that in actual prac-tice it is customary to take 3 bags of cement as 4 cub. ft. For this reason we prefer to make all our te t on the ba is of 93.71h . per cub. ft., so as to avoid getting mix-ture richer than tho e in ordinary use. The t st blocks were made by hand of two consi tencies, "" damp" and "quaking." A mortar of "damp " consist-ency is described as one containing only sufficient water to hold the particle together. when made into a ball in the hands, and the ball held between thumb and finger . .A mortar of quaking" con isten{lY contained all th water it would hold, and permitted of moulding a ball in the hand and holding between thumb and finger. The re-sults show a great difference between uch consistencies, .and it may here be remarked . that the practical value of tests of mortars made to a "damp" consistency is mall. In praetice the tenden<lY will often be to make use of mixture wetter than that described as "quaking,' ow-ing to ' the advantage of placing such mortar in po ition. The test blocks were kept in a moist place for one day, then placed on shelves in a damp room and prayed with water three time every 24 hours. Thi practice is in our opinion very open to criticism, since the intermittent spraying and dryina will calli e changes in the surface of the blocks by which pores will be clo ed and the urface ' will become less permea:ble. J ust prior to testing, the blocks w 1'e brushed with a stiff wire br h in an attempt to remove any "skin" of (lement drawn to the surface in moulding. This was ac-" P BBllIU.BILITY OF OOllOBBT8 BY WATER. 13 complished only with difficulty in some cases. In our view it is de irable that this skin should be removed, and we-have treated the test blocks with the wire brush imme-diately set ting is complete, our experience being that if defer red until a lat er st age hardening has proceeded too-far to allow this to be done with success. The work carried out 'by the Bureau is of a very volum-inous character, and we fin d it a little difficult to follow t he details in places. It is not possible to present the-mass of curves and tables in this paper, but we shall be content with a r eview of the principal points to be noted. When mortars and concrete are exposed to water un-der pressure, the quantity of wat er which passes through may not 'be measurable, and from some points of view the mortar may be regarded a impermeable. At the same time it may be wet, and thus from other points of view it may 'be decidedly objectionable. In the remarks which follow we shall discriminate between these con-ditions 'by referring to the surfaces as "wet" and "dry.'~ From the tables given by Wig and Bates we have sum-mariseQ the following conclusions:-1. The test blocks made to a "damp" consistency allowed large quantities of water to pass under a pressure of 20lbs., the best showing as high as. 18 gallons per square foot per hour. For this reason we shall only refer to "quaking" consis-tencies in the paragraphs which follow. 2. Test blocks 2 inches thick from 4:1 mixture at an age of 1 month do not allow measurable quan-tities of water to pass through them under 20lbs. pressure. 6:1 mortar reaches this stage in twO' months; whilst 8:1 mortars cannot be relied upon even after 6 months' storage. 3. imilar test blocks of mortar 1 :1 remain dry under Olbs. at an age of one week. Mortar of 2:1 pro-14 PBRMBAllILlTY 011 OON08BTB BY W ATKR. portion takes 12 week to reach this condition; whilst)n 24 weeks 4 :1 , mortar was dry, whil~t 6:1 mortar became moist. 4. Well graded sand containing .a considerable quan-tity of graded fine material assists in rendering the mortars more impermeable. 5. The permeability is diminished by trowelling the urface of the mortars. 6. Test blocKs of concrete 6 inches thick made with a graded lime tone (2iin.), of which 20 per cent. pas ed the tin. ieve, all showed marked perme-ability under 20lbs. jJres ure, with the exception of a mixture 5t :! :1, which when made with one variety of sand (30 per cent. through 50 mesh sieve) withstood a pres ure of 60l'bs. '9.fter 13 weeks. The m chanical analy i of the lime tone and sand in this mixture w.as as follows :-Siev~ 100 80 60 40 30 20 10 tit 1 I t Ii 1* 2 21 SlI.nd 1.50 10.0 91.6 46.5 62.0 72.0 86.0 100 L'stn. 1.9 9.4 4.94 4.9 5 .36 7.0 10.0 19.0 90.5 57.0 69.0 81.0 89.0 96.6 9~.0 100 7. It appears from the above that the grading of the st one is very important as regard the pro-portion of fine . Of all the other concret ex-amined, a mixture 4:2 :1 (stone graded to l iin. with a omewh~t coal' er sand than the above) came neare t to being impermeable. Geneml Remarks. Taking the re ults as a whole, it would appeal' from a practical point of view that, in order to ecure concrete impermeable to water, it is nece ary, firstly, to pay par-t icular attention to the thorough mixing of and and , ement, and to use enough water to ensure that the whole mixture bas reached at lea t a "quaking ' consistency. Rich mortar are not necessary, but with lean mortars there is a greater chance that, on account of irr~aular mixing, patches may occur which are not watertight. 

Permeability of concrete by water

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Permeability of concrete by water

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