Suction and its use as a measure of moisture contents and potentials in porous materials

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Suction and its use as a measure of moisture contents and potentials

in porous materials

Penner, E.

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29.

Suction and

I t s

Use

as

a Measure o f Moisture

Contents and Potentials in Porous Materials*

E . PENNER

Ncitional Research Council, Division of B7~ilrli?~g Research, Ottawa, Canada

ABSTRACT

illany practical problems that arise with the m e of ?naterials of a porous nature r r s ~ ~ l t from. their interacfiorl with water. There i s , therefore, a need to tlescribe the moisture status i n a more adequate 7uoy than on a weight or volume per- cenfnge basis.

T h e colzcept of suction provides a rational method for describing and controlling the moisture statzis of porous nzaterials, particularly . -

i?t

the high humidity region. I n m a n y materials, moisture conte?zts greatly increase with a t m ~ a l l change i n vapor pressure at relntive hunlidities i n excess of 0.95, and accurate measurem,ent and control of vapor pressure directly i s often so dificult ns to preclude its use.

T h e concept of s7~ction i s based on the Icelvin and height-of-capillary rise epzrations relating the reduction, i n the vapor pressure over a curved water nleniscus i n a c ~ p i l l a r y to the length of the zuater colzi?)~r1 this curved m r ~ ~ i ~ c u . s ~uills2rpport. On this scale, a colum?~ of zuater 10,000 c?n high is equivale?~t to a relative vapor pressure of 0.993. T h e log of this expanded ,scale based on suction i s SchoJield's p F . Its ease of determination and control makes i t most zrseful i n m a n y studies in

which moisture i n porous materials i s involverl. Reference i s made to some of the more importal~t work i n which i t has been employed.

The determination of moisture content of porous materials may bc rcquired for a variety

9 This paper is a contribution from thc Division of

Building Reucnrch, National Rcscnrcll Council, ant1 is published w ~ t h t h e approval of t h e Director of the Division.

of purposes. I n many instanccs the moisture content itsclf may bc of imrncdiate interest, in which case a method of incasurement is r c q ~ i r c d which rcsponds directly t o the moisturc prcscnt,. For many purposes, ho~vever, somc index t h a t is related to t h e potential a t which thc moist,ure is held in the material may bc much morc useful and more readily measurccl. One such index, commonly used, is relativc humidity. For many cases it is possible, having establislled empirically the equilibrium relation4lip t h a t exists between moisture content, temperature and relative humidity for a particular material, to predict moisture content from a measure of relative humidity. Thcrc is, holvever, another possible index, widcly used by soil scientists and commonly referred to as "suction," which can also be applicd in illany problems involving lnoisture in rigid porous systcms. It is not well Icllown in this connection, and it is the purpose of this papcr to discuss its usefulness and t o provide refcrcnces to some of t h e pertinent literature.

The general nature of the relationship between tl-re moisture content of many common materials and the relative humidity of the surronnding atmosphere a t the equilibrium condition is reasonably well Irno~vn. The relationship for wood, sllo~vn in Fig. 1, is a typical cxanlple for some natural organic materials. The inevitable hysteresis between wetting and drying curves is shown in Fig. 1.

Not shown is a secondary effect, t h a t of temperature which causcs a shift of the curves downward t o loxver moisture contcnt levels as the temperature is incrcascd. The effect of

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'I'CJllAT A N D M A TERIAL,V

0 10 2 0 5 0 4 0 5 0 60 70 8 0 9 0 1 0 0 R L L b T l V E H U M I O I T Y , 7.

FIG. 1. Equilibrium moisture content for spruce a t 20°C.

temperature is sufficiently small so t h a t a change of a few degrees may usually be dis- regarded without serious error. The relative lluinidity-moisture content relationships are a function of the material and its structure and must, therefore be determined experimentally.

Relationships between equilibrium moisture and humidity have been dcvcloped. and are used most comn~only for those materials in which the "natural" moisture contents more or less in equilibri~lm with the surrounding atmosphere are of particular interest. This applies particularly to plant products includ- ing wood, foods and fibers. Similar relationships can be established for the whole range of porous materials, both inorganic and organic, incll~ding building materials such as soil, stone and briclr. Equilibrium moisture curves for many of these common materials are not widely used, a t least outside the laboratory, because the levels of their moisture content in use are usually much closer t o saturation and thus well above those corresponding t o t h e usual ambient relative humidity levels. A t high degrees of saturation, relative humidity normally ceases t o be a usefill index because its variation with increasing moisture contents closc t o saturation becomcs disproportionately small, and it becomcs cxtrcmely dificlllt t o measure with useful accuracy. Recently a psychrometric method has bcen dcviscd which apparently measures relative vapor pressures

close. to .qat,l~ration w i t h grcat accumcy.l

It has b e c ~ ~ common practicc in thc cave of wood, for cxnmplc, to tcrminatc the curves of e q ~ ~ i l i h r i u ~ n ~noisturc a t thc vnluc of moisture

content

Irno~vti as t,he ,fiber s n t ~ i m f i o n point wllicll corrc.sponds t o a rclativc humitlity very close to 100 pcr cent (Fig. 1). 't'his has bcen largely a practical mattcr in wood tcclinology sinc,e frhis is tllc valuc of moisture content below which significant ~illrinltage takcs place. ' r h e remainder of the curve t o satmation is

usilally omitted, partly hccausc i t is of l c ~ s concern and partly bccausc i t cannot uscfully be displayed on the same rclative humidity scale.

When thc moisture contents of interest are in the range representing t h e progressive filling of large pores in the material a s saturation is approached, it is clearly necessary t o find some more convenient index and new instru- mental tcchniqnes. The suction concept provides such a n approach.

THE SUCTION CONCEPT

I n sinlplest terms, suction is a measure of thc affinity between water and a porous hydro- pllilic solid. Relative humidity is also such a m c a s ~ ~ r c though an invcrse one, sincc i t reficcts thc forccs by which water is retained through the depression of its vapor pressure below t h a t of free water.

Suction can be expressed simply in terms of tlie theoretical capillary rise corresponding t o thc forces by which water is llelcl in the porous structure of a material. I n tlie case of a n idealized capillary, these are surface tension forces which are rcfiected both in the height of capillary risc and in the curvature of t h e meniscus. Tlle relationship is:

where

g = gravitational constant

1~ = capillary rise

y = interfacial tension between water a n d air

d = density of water

r = radius of curvature of t h e meniscus. Thc vapor pressure, p , over the meniscus i s related t o the curvature of t h e surface, a n d

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29. SUCTJON .IN T'IIE 11IEAXUJ~R OP' JiOlS7'URlil COrV7'RNi11X 247

FIG. 2. Itolntionship botwoon p F a n d rolativo humidity nt 20°C.

differs from the sat~iration vapor pressure po for a flat ~ v a t c r sm'facc by a n amount equivalent t o the xvc~ght of the vapor column having a height cqual t o tllc capillary rise, h. Sincc

7~

and r are rclatcd, t h e vapor prcssurc over a curved surface call also be related t o radius of curvature. This is given by the ICclvin e ~ i i a t i o n

M = molecular wcigllt of water

R = gas constant

T = absolute temperature.

Co~nbinirig Ilkl.q. ( I ) nnd (2) cstal~lisl~cfi the rclntionsl~il> l~c~t~\vccn c:ll)ill:~ry risc,

IL,

the C I I ~ \ ~ : L L I I ~ C of tJlr ~ n ~ n i s c ~ ~ s , and the relative vlrpor pressure, as follows:

This ccluation is truc whether thc col~lmn of watcr of hcight

IL

is rcal or imaginary, and it rclatcs capillary risc t o thc radius of curvature of the ~ v a t c r snrfnccs occurring in the pores of thc material. Thc value calculated from Eq. (3)

is colnmonly rcferrcd t o as thc suction S for r a c l ~ corresportdillg vapor pressure.

Thc suction, usually expressed in ccnti- meters, is a n extremely scnsitive indicator of very s ~ n a l l changcs in relative humidity near saturation. It will havc Iligll values for relative vapor prcssurcs in thc normal range. As a con~rcnicncc, log,, S can be used rather than S , yiclcling numl~ors bctwccn 0 and 7 for most cascs of intcrcst. Units ofsuction or of negative potcntial csprcssecl in this way arc called p F , aftcr Scliofir1d.l Tlnis a suction of 100 cm of water beco~nrs pV 2 . A plot of p F us p/po is givcn in Fig. 2. '1'11~ cspa~ldccl scalc provided above relativc vapor pressures of 0.99 is evi- dent. Tlir rcla tionsli ips betwccil rclativc vapor Iwcssure, radius of curvaturr, of the meniscus cquivnlcnt llciglit of colulnn

IL,

and Schofield's p1' arc givcll in'rablc 1.

Thc validity of the suction concept is n o t dcpcndcnt on knowing tllc watcr-rctaining nlccllanislus t h a t arc a t work. Tllese may be adsorption, capillary or surface tcnsion forccs and, in tllc casr, of sxvelling clays, osmot,ic prcssurcs wllicli arise from the inter- action of diffusc laycrs of exchangeable ions TAULE 1. TJIZ RELATIONSHIPS BIST\VEI~N RICLATIVB VAPOR PRESSURE,

R A D I U ~ 01. CURVATUKE O F ~IENISCUS, EQUIVALENT COLU&IN O F WATER

h, A N D pF Radius of Curvature (PI Hoight of Column of h (psi) Schofield's XVatcr, PF IL (cm)

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TECHNIQUES OF MEASURING SUCTION S n c t i o l ~ n l ~ d its 11sr n s n I T I C ~ L ~ I I ~ O of ~ ~ o t r n t t a l

is nl~l)l~cnl)lc fro111 tllr ovr11-tlric.tl st:lt,c to ~n t urn t,ioll n l t l ~ o ~ ~ g l ~ it is norm:~lly c o ~ l v r n i ~ n t t o ~ I S C scvrr~11 t ( ~ c I l n i q ~ ~ e s of r n ~ n s l ~ r c l n e n t t o

covcr tile colnplctc rang(.. Itclnlivc vapor

~ ~ ~ S S I I ~ C nlet,hods nrc st,ill l ~ c ~ t , l~ci,n,ccn p11' 4

nnd 7, I)lltr wit11 a I I C W : I ~ ~ I ~ ~ : I ~ I I S d(\sigrlcd 11y

Croncy et nl,:' tllc 0rigir1:~ prcssllrc m r ~ n l ~ r n ~ ~ c m c t l ~ o d of Itichnrds%c:bn I)r rxtentlctl t o pT? 6.2 (1600 atnls). 'I'l~e s:~.nie 111oist11rc content vs potential rclatrionsI~ips oht;~inctl from rclnt ive vapor prcssllrc methods arc ohtaincd with Croney's new metl~otl, tlius suhstnnt intring t h e theory on which t,he suction concrpt is based. Thc various t,ccluniclurs t h a t 11avr. been developed a n d their nscfiil ranges arc given in Table 2. Most of thesc arc drscril>cd in a paper on suction trcllniql1c.s by Cronry et al."

Most of the methods usptl t o drtcrminc suction [lo n o t rcquirc clnborntc. nppnratlls :mtl arc simple t o carry ant. Tllc npl)amtus in Fig. 3, known ns n cornhinc~tl q~ietion plate a n d pressure memhmnc, is typical of this. Moisture p ~ t c i l t ~ i a l s in the n e t porous ]date arc cstal)-

Direct

Snct<ion p1nt.o

'T'cw~iorliotor

( h r l b1.i f11go

I'~(T~IIT.o rnomhrnno

Corisolirlr~t.iotl (for sn,t,urnt.c!cl clnya only)

3borl.tl rnst~nrcli now prossuro rnonlhrano

1not1~1,tl

I ~ l d i ~ e c t

Vacuum r1osiccn.tor

Sorpt,ion hnln.nco

I<loctl.itnl rcsistnncr? gnugos (cloponding

on pore goornotry)

* J3ascd o n a tnblo by Cronoy cl a1.4

lishctl by increasing the air pressure in t h e chn1n1)cr nhovc atmospheric pressure. I-Tcnce the pr('ssI1re (lrop across the wet porous plate tlrtcrn~incs thc lcvcl of potential or suction. Any o f htlr porous nlatcrial in contact with t h e porous plate inside thc c h n ~ n b c r will also IIrconlo conditionccl t o the same potential. I t is

desirable t o have a good contact so t h a t t h e w a t w films will coalcscc and facilitate a rapid cxchnngc of ~ n o i s t ~ l r e . I f tllc porous sample is a t a higher s l ~ c t i o n , water xvill be withdrawn from t h e I'orous plate ant1 t h e i n r u i s c ~ ~ s in t h c pipet

xi ill rcccde. If thc suction is lower in t h e sample

lgrtr. 3. I'rcssuro tnoltlbrr~~io nppnrnkus for moisture block cal1bratior1.

MOISTURE BLOCK PRESSURE CHAMBER

-

BURET

-

c '

-

SEALING COMPOUND S P I R A L GROOVE

POROUS PLATE HOLDER LEGEND:

0 ) WATER OUTLET b) AIR I N L E T

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rxrccclcd. S11cl1 men11)rnnos sl1o11ltl not, 1)c sod

- l)elow 1111' 3 I)OC:L~ISC of t11c low moi,rtlirc:

- _t,mr~s"fcr_{m t r s n t l o w prcssllrcs.} -

- -

SUCTION CURVE FOR SPRUCE WOOD AND CLAY SOIL

180 - -

Suction c1lrvc.s for two conlmon materials,

-

spruce ancl clay, arc givcn in Figs. 4 and 5

160 --

-

rcspcctivcly. Tllc possi1)lc value of such

W

- _{inform:~tion a t m o i s t ~ ~ r c}_{contcnts up to satura-}

140 -- - tion becomes apparent, from a comparison of i%

r-' - - the appropriate portlions of the curve in Pigs.

1 wit11 4. The llystercsis effect betwcen wetting and drying for both materials is clearly

-

demonstrated. The theories advanced to

-

explain llystercsis arc discussed in detail by

-

Carmanfl and arc beyond tlie scope of this

-

_paper._It_{must be emphasized, however, t h a t} - _{failure t o recognize and talre into account} - hysteresis effects has complicated many attempts t o measure and predict moisture content and moisture movement. Some measure of potential such as t h a t provided by tlie suction concept is essential t o any study of hysteresis effects a t moisture contents above those corresponding t o p F 4, i.e., from relative humidity of 99.3 per cent t o complete satura-

0 I 2 3 ' 4 5 6 7

P F tion.

Fro. 4. Suction-water content curve for spruce APPLICATIONS OF THE SUCTION

s t 20°C. _CONCEPT

when it is placed in contact with the porous plate, water will be transferred to tlie plate and t h e meniscus in the pipet will advance.

Air pressure is applied in successive steps t o the chamber, and the volume of water displaeecl is measured on the pipet between the various stages. Starting with a fully saturated sample and increasing the suction gives the drying curve. The wetting curve is obtained by reducing the pressure a n d noting the water intake.

The p F limits of this apparatus are determined by the largest pore in the porous plate, which in turn deternlines the air pressure a t which the porous plate will no longer remain saturated. To continue to higher pF's a membrane such as Vislring sausage skin is placcd on top of the porous plate from c to c' (Fig. 3). These membranes will usually remain saturated and not pass air until a p F of 5 is

Indirect Moisture Content and Potential Measurement with Suction Meters Two wet materials placed in contact for a sufficient length of time will establish equal moisture potentials by water transfer. The equilibrium condition develops very slowly by vapor transfer but occurs very rapidly, as pointed out previously, when there is intimate contact and the water filins coalesce. Moisture meters are based on this principle although a s first used by Bouyoucos and Mick7 only moisture contents were obtained; their use in mcasllrii~g moisture potential was a later d e ~ e l o p m e n t . ~ These meters consist of plaster of paris bloclrs, nylon, glass fiber7, 9 9

or other porous dielectrics in which the electrical resistance between two electrodes in the mass (or somc otlicr function highly sensitive t o moisture ehangcs sue11 as capacit- ance or hcat conduetion) is measured a s a

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I ~ \ ~ ~ ' / ~ ~ / ? , ~ l ~ ~ 7 ' I O W O F I ~ ~ O I A S Y ' ~ J / ( I C A N / ) / V ~ i ' l ' f i % l ~ I A T ~ ~ ~ '

LOG ELECTRICAL R F S I S T A N C E

M O I S T U R E C O N T E N T % D R Y WT.

Fm. 6 . S~ict~ion-~noist,uro ror~tnr~t. rrln.t.ionsl~ip for snrnplo oF air-driorl

Leda c1n.y nr~tl a J301iyouco.s ~rroist.uro mr:t,cr cnlibrn.tion curve.

function of moistr~rc corltrnt;. 'l'hcy m a y be ~ a l i b r a t ~ c d rls n fi~nct,ion of rnoist,~~rc potc:ntinl and such n cltrvc. is show^^ in 17ig. 5 for I,otll t,llc \crt.t.ing :i.li(I tlryi~ig c:ontlit.ion. S11ct1 cnlihmt,ctl

mctrrs inst.n.ll(~(l in t,Ilt: soil give n nlonsllrc of

j~ot.r~lt,ial vrlirri ~noi.qtr~rt~cc~~~ilil,ri~~~~l is rc.:lcllc,tl n.it.li t.li(: s ~ ~ r r o ~ l n d i l i g ~ i i r c l i l l ~ ~ l . I ilfi>rrn:~t.ion on

tlir nioist.~irc: c o ~ r f ( ~ n t ~ of t.lir: Inass, c.g., soil, can

also br ol)t,:~inrd if a pot,cnti:ll m o i ~ t ~ ~ i r c c o ~ i t c r ~ t c ~ l r v o is Irnonw. In f:l.ct,, t,hc c~st.al)lisli- mcnt of sncll 1:urves scpnr~~t,c:ly n1:l.y I)r :I. n111ch

morc sntisfnct,ory way of ca.lit)r:~ting t,ha.n t o consitlcr moist~lre coiltent tlirc:ctlly. 'l'l~c t,\ro curvcs togcellcr (moist,~irr potcnl,in.l-c:l(:o- trirnl resistnncc for t h e bloclr and ~noist.urc potantinl-moisture content for t811r soil)

['rovido a w:ty n o t only of cst,irn:~t.i~~g t,lic,

potantinl of t h e soil ~noist~rrro h t it.8 : ~ c t ~ ~ n . l rnoist,~lre c o ~ l t ~ c n t (Pig. 5). Altl~orlgh this is sour~cl in p r i ~ ~ c i p l c such mct,crs a,rc only nscful for n ~ c n a ~ ~ r i n g moistuvc? kr.e?id.~ in. pv((.clice

bccnrise I~ystcrcsis c:fYcct,s a,rc unnvoidnl)ly prcscnt nntl since, wllc.11 selcct.i~ig t,hc appro- prin.t,c cnlik)re.t,ion v:~lrlcs for co11vc:rsion to rrloist,urc co~lt,c~l(,, i t , is sc,ltlom I<l~on.n prccisc:ly whctl~c:r t,ho soil :~ntl t.11(, ~nc?tt,r arc, t l r ~ r i ~ ~ g or wcttting. Tllis co~n~)licat.ion is r~ot, introtl~lcctl b y tllc ~rlctioil COIIC(:~I~., which SCL'VCR it1 this

cnsr to show tlie li~nitrtl prrformnncc of R I ~ C I I

mc,trrs.

Cont,rol of Wat,er for Plant CIrowt,h

M o ~ s t ~ i r r nlctcrs for moisture potential ~ r l c ~ a s ~ ~ r r n i c n t s hnvr 1)ccn n s~icccssful aid t o c~conom~rnl irr~gatlon 1)rnctircs. I t is 1rnon.n that tllc, c.aw of n7atrr rxtr:~ct,ion b y plants from so11 tlrpc~l~tls on t h c srlct,~on lcvcl and i t thcraforc, pl:~ys n major role in controlling crop pro(l~rrt,~on 'l'ctlsiomctcrs lo a n d moisture

IN(-trrs arct both b r ~ n g r~setl cxtensivc~ly a s rlr>rir\.; ~ ~ ~ t l r c n t o r s for a11 typcs of irrlgatcd rrop9. 'I'c~nsiornctcrs arc l ~ r n ~ t c d in practice t o al)orlt 850 ctn (wntcr) suction b n t moisture nicXtcrs c:~ri I)(: 11sctl to cxtclnd this range well l~cyontl tli(\ w ~ l t ~ n g point of most growing c r o p b " 1 ~ 1 1 t11(\ 1)1? ~ : L I I ~ C for tlie bcst pro-

(111rt1o1i lins lwcn rstabl~slierl tllcse mctrrs can be usctl t o great nt1vant:~ge.

Moistlire 1t.eclistribution in Soils

'I'hc actual ~noist~rlrc potential ~ l n t l e r field ~ontlit~ions \vhicll nlust tnlre into account difft~rrnocs i r ~ c~lcvation can be ~tscfiilly c ~ s l ) r o s s ( ~ l in t,t:rn~s of suction. l h r c y ' s Inm for moist~iro flow :~l)l)licls cq~inlly t o t h e 11nsatur- :1t(~1 mugr: b u t the coefficient Ic varies with

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sect i o ~ ~ n l nrc:l il~\-olr'd in l i ( l ~ ~ i t l flo~v.

13:lsctl on ~uoistu~.c, l ) o l c ~ ~ t i a l s (not ~noist~urc co~ltcnts) thc t l i r c c t i o ~ ~ of m o i s t ~ ~ ~ . r flow can cnsily l,c ~)rctlictctl. 'L'llc rcdist r i l ~ u t ~ i o ~ i of u ntcr a b o r c a \\.atcxr table, n,hcn (,v:~por:~tio11 is c u t ofT, (,all also bc l~rcclictcd by laliing into n c r o ~ ~ l ~ t t l ~ o ovcrb~~r'tlcn prc~ssurc, tlic coinprcs- sibility of t l ~ c soil n ~ i d tlic l ~ c i g h t of thc water tnblc. Croncy ct aL3 claim considcrnblc

success 11 itJll n spccial incthod dcviscd by tllcnl

for tliis purposc as wcll a s for predicting

moisture potcntial ancl moisture contcnt clinngcs follon~ing a chnngc in the overburden p ~ s s u r c in roacl cinbanlcinent construction. This is a n important coilsicleration in m a n y soil engineering projects.

Use of the p F Concept in Common Porous Materials

T h e bchavior of moisture in t h e performance of porous building inatcrials can often be rcaclily stuclicd in terins of this simple conccpt.

It has already been used in studics of the setting of plastcrll and may possibly be extendccl to studics of effects of plaster bascs and plaster aggregates.

Thc rate of wntcr absorptiori b y briclrs apparently intluenccs the bond strength t h a t develops bctnrcen the briclr ancl mortar of masonry nralls.13 R a t c of wntcr absorption is kno\\7n to bc a functioi~ of thc driving force or potential and t h e conductivity. It is usl~ally measured in briclts b y a simple t c s t l ~ v h i c h unfortunately gives no information on the variations in its separatc components. Suction techniques could be uscd t o advantagc in studics ~vllen thc pore structure of the bricks is of interest. Similarly, tllc fiber saturation point in wood which docs not y e t appcar to have been defincd s a t i s f a c t o r i l y l ~ o u l d be readily studied and, if clcsircd, established on the basis of suction potential.

Although the suction conccpt has bccn used for years in soil scicncc its application t o moisturc movcmcnt during frost heaving

is a recent t l c v c l o y ~ n e n t . ~ ~ Studies of t h e strcngth propcrtics of soils have shown t h a t t h e dcgrce of suction in clays influcnccs the shear strength by adding to thc cffcctivc strcss.

s n c t . i o ~ ~ in t,llc por'c, Jvatcr for tl~c. A ~ ~ s t r a l i a n on\~ir.o~~rnc~~ltt : L ~ J ) ( ' : L ~ s t o I)(\ a, major strcngth

t l c t c r ~ n i ~ ~ n n t . for col~csivc soils.

'I'hcrc arc 1u:Ln.y insl;rnccs in dcnling with co1nmorI mntcrinls w l ~ c ~ i 11sc is madc of ~noisturc contcnt mcnsurcmcnts ol~tained nftcr conditioning in a s~~pposcclly snturatcd atmos- phrrc. When t l ~ c nnturc of relationships

bct1wccn relative humitlity, stictioil and moisturc contcnt arc nypreciatcd, i t may bc conclud- cd t h a t snch conclit,ioning in order t o establish a r c p r o d ~ ~ c i b l c moisture contcnt will often be futile. I t is scldom t h a t t h c dcgrce of tempcr- aturc control providcd will permit thc estab- lishment of a precisely maintained relative hnmidity a n d t l ~ c cxchange of moisture in t h e vapor s t a t c is extremely slow. Conditioning in contact with suction and pressure plates t h a t provide ready control of i o t c n t i a l a n d also relatively rapid exchange of moisture b y capillarity can be used to good effect when linown e ~ n i l i b r i u m conditions a t moisture conditions approaching saturation m u s t be established.

SUMMARY

The suction concept provides a simple approach t o t h e cstablisliment and controlof moisture potentials in the region of high relative humidity. Suction, unlike relative h u ~ n i d i t y , can be used conveniently a s a mcasurc of potcntial involved in water migra- tion. T l ~ c r c are also useful techniques of nlcasureinent which follow logically from t h e suction coriccpt.

Acltnomledgmont. Tho author wisllos to oxpress his approriation to 111.. N. B. liutrhcon for his assistanco in tho preparation of this paper.

1. Richards, I,. A., "A Thcrmocouplo Psychromotor for Jtoasuring tho Iiolativo Vapor Pressure of \J'at,er in 1,icluids or Porous hIatcrials," in "1-Iumidity and hIoisturo," Vol. I V , p. 13,

Now York, ltoir~llolcl Publislling Corp., 1905.

2. Schoficld, It. I<., "Tho p P of Water i n tho Soil,"

l'vrrl13. I?tfevn. Co?ly?. Soil S c i . 3rd, 2, 37-48 (1935).

3. Cror\c:y, D., Colc~nan, J. D., ant1 13lacli, W. P., "Jlovemont and Distribution of \Vator ill Soil

(11)

in Rolntion t,o FIigl~wny J)ouign nnd I'orform- nnro," in "t4'ntc.r nnd 71s ('o~l(Jt~ct,ion in Soilq," S l t 10, p. 226-2158, IIighnfny ltosonrrh 13oartl. 1068.

4. Rirhnrds, L. A., "A Prnssnro Mo~nbrnno ISxtrno- tion A p p n r n l u ~ for Soil SoluLion,"SotlSci , 51, 377-386 (1941).

5. Cronoy, D., C o l o ~ n ~ n , J. D., n t ~ d Br~tlgo, I'ntnoln.

JI., "Tllo s u c t ~ o n of Moist,r~ro IToltl In So11 a n d Other l'orous nlntorlnlu," Doport~nont of Scientific and Tntlu~trinl Itoqonrch, ltoacl ltoscnrch Laboratory. Road Res. Tech. Paper, 24 (1952).

0. Carmnn, P. C., "Properties of Capillnry 1Iolcl Liquids," J. Plcys. Chena., 67, No. 1, 56-64

(1063).

7. Bouyoucos, G . J., and "ick, A. H., "An Eloctrlcnl Resistance Mothod for t h e Continuous Measurement of Soil Moisture untler Field Conditions," Mich. State Univ. Agr. E z p t . Sta.,

Tech. Bull., 172 (April 1940).

8. Bouyoucos, G. J., "Nylon Eloctricnl Resistance U n i t for t h e Continuous Moasuremont of Soil Moisture i n t h e Field," Sozl Sci., 67, 319-330 (1949).

9. Bouyoucos, O. J., ant1 Mick, A. H., "A Fabric Adsorption Unit for Continuous Moasuroment of Soil Moisture in the Field," SoilSci., 66, No. 3, 217-232 (1948).

10. Colmnn, TC. A., and IIonrlrix, T . M., "Tho F ~ b r o - glnss IClor~tr~crrl So11 R.101qturo InnLrllnlont~,"

So?/ S c i . , 55, 219-223 (1043)

"11. Richnrtlq, I,. I\., ant1 C!nrtlnor, W., "Tonqiomotors for DIonsurlng tho Cap~llnry 'I'ms~on of So11 Wntor,".J. Am.Soc. Aqron ,2H, 352-368 (1930). 12. O'I<nlly, U. M., L'I'hys~cnl Chnngcq In S o t t ~ n g

G y p s o ~ n Plaster," ASZ'M Bull , 237, T P 75-81 (Ap11l 1959).

13. Ritohie, T., nnd Moincko, 13. It., "Cnpillnry Atlsorpt~on of Somo Canadinn Uuilcl~ng Brlcks," Nntlonul Rosoarch Council, D i v l s ~ o n of 13uildlng Hosearch, 12 pp., NRC No. 2066, 1053.

14. Amorlcnn S o c ~ o t y for Tostlng &fatorinls, Standard Methocls of Sampling a n d Testing Briclc, ASTM Designation C 67-50: 1960, Supplement to Boolc of ASTM Standards, P a r t 3.

15. Porotn, E., "Determ~nation of Fiber Saturation Polnt of Wootl by Centrifuging," J. Forest

Products Res. Soc., 10, No. 2, 77-84 (April

1054).

16. Ponner, E., "The Mechanism of Frost Heaving in Soils," Highway Res. Board Bull., 226, p. 1-22 (1969).

17. Aitchison, G. D., "The strength of Quaai- saturated a n d Unsaturated Soils in Relation t,o t h e Pressure Deficiency In the Pore Water,"

Proc. Intern. Conf. Sot1 Mech. Found. Eng. 4th,