A ”WHCD FOR DETERMINIRG ‘ICDI... TL-RE GRAD {BENT A CCfi-CRE 1E FAVE-MEN"? SLAB Tl‘csis for ’hé De cgree of B 1'3 K’E‘ICHTCAN STATE COLLEGE » 3 0 H. 2:. kacn 19.4 C? “HE Ills l.. v . u .l‘ \u , I v. I l . . \ . . : x . v. y . . .. . r .r . n. 10 £1»? ”.7. ”it! g'tSEw yarf...kd> inks... a) .up/ 5.. . FIE .- I . )ulrf. tarifunfill 3'": hi i If... . ... Mt E» ... .37! In?! fi.n.§;..r'mn. iii-in; 2...!!! ..‘B1 . 5324!; a . 'KVHDJE i bug‘s .. A. (“#6. .. r A Method For Determining The Moisture Gradient in a Concrete Pavement Slab A Thesis Submitted to The Faculty of MICHIGAN STATE COLLEGE of AGRICULTURE AND APPLIED SCIENCE by H: S? Wilson M. Candidate for the Degree of Bachelor of Science June 1940 INTRODUCTION: Field control in curing concrete has been developed to a point where future research will be directed toward an at- tempt to determine the internal phenomena which occurs within the slab during the curing period. It is a well known fact that as concrete cures there is a definite loss of moisture within the concrete. The extent of this moisture loss and the variation of the moisture content throughout the mass has long been a subject for investigation. Many studies have been made of the moisture gradient in concrete slabs, but only a few of these studies have yielded conclusive re- sults. As yet, all methods which render reliable results on this subject are not adapted to field use, and consequently are of little value for field control of concrete. The pur- pose of this project is to design an apparatus and develop a laboratory technique which will be practical in making field measurements of the moisture gradient in pavement slabs. By "moisture gradient," is meant the variation in mois- ture content from one point to another in the pavement slab. We are primarily interested in the variation at the surface of the slab and the first three or four inches below the sur- face. 138598 REVIEw CF RELATIVE RESEARCH: A review of previous investigations of this problem may serve to clarify the objectives and results presented herein. As far as could be found through reference work, the first. attempts to study moisture conditions within a large mass of concrete was done during the construction of Morris Dam in 1932. The tests run on this job were performed with a speci- ally constructed hair hygrometer. Difficulty was experienced in keeping good contact with the concrete and the results were not considered valuable. Several devices which measured the thermal diffusity of the concrete were tried and seemed to render reasonable accurate data provided the concrete was not alternately wetted and dried. However, if the concrete was alternately wetted and dried the data could not be dupli- cated, the reason being attributed to a reduced bond between the embedded contacts and the concrete. Electrical resistance measuring instruments have been used on soils,1 and to determine the relative efficiency of .curing materials for concrete. As yet, none of these devi- ces have been adapted to field use. The principle of resis- tivity measurements is the most practical theory used to date 1. Technical Bulletin 178, Micnigan State College Experimen- tal Station. because readings can be taken at any point in the concrete, electrodes are inexpensive, their abandonment in the con- crete is no material loss, and portable electrical equip— ment can be used which does not require an external source of power; The problem of calibrating an apparatus of this type is one of determining a moisture resistivity curve for every type of concrete tested.8 Any variation in the type of cement, the presence of dissolved salts in quantities, leaching of soluble materials, variation in size of aggregate and long periods of time between measurements in the concrete will cause a change in the resistivity curve. This curve, however, will assume the same general shape regardless of the type of concrete tested, and if a standard mix is used, a moisture resistivity curve can be worked out that will always apply to that particular mix. Several devices were experimented with in this problem. All were based on the Gish-Rooney Earth-resistivity Method. The reasons for adopting the principle worked out by Gish and Rooney are as follows: (1) he effect of currents within the concrete may be neutralized (2) The effect of natural currents due to electrolysis of bodies within the concrete can be eliminated 2. American Concrete Institute, Y9z45-61, Sept.-Oct., 1937 (3) The effect of polarization or galvanic action on the electrodes can be stepped.3 All other schemes for measuring resistivities have failed in one or more of these three ways if an attempt was made to get contact between electrodes and the concrete. Gish and Rooney discovered that by reversing the current on the elec- trodes the above mentioned difficulties could be eliminated and there was no material effect on the results. For this reason the direct current circuit must be set up so that the direction of the current flow may be reversed at frequent intervals. The theory upon which a multielectrode unit is based was worked out by Dr. L. V. King and is called Wanner's Formula. The proof is as follows: Let V be the potential at any point due to the current flow. The electrodes Cl and Cg are current electrodes. P1 and P3 are potential electrodes. V must satisfy Vfiho at an infinite distance in a homogeneous medium. At any point P at distances r1 and r2 from electrode Cl and 02 (the distance the electrodes penetrate the concrete be- ing negligible) a solution of VzV=O is: (l) V: A/r14B/r3. A and B. are constants. Then: If ,0 is the specific resistance, ia-l/P all/1n. Assuming the electrode has a spherical field, the outflow cur- rent from A is I: l/FJBV/an dS. 3. Applied Geophysics, p. 88 .i z - l w -«--~-h—~ C“ I ‘ 'H g ) M/AA/AMMé race PO Vii/VT/O/WEFER“ 1,. M in- ,/~,/ rm ,1 w/ 7:? f) ,. 2 2i O E?! @ ‘I/',/ /_;f -‘g.4‘{"'r/_.—R : BUCK/NC; C/RCy/T—"T O W u i 1 r4 C .: A’R £N r . ‘ D f I " /‘: .I a .' ;\ q) "Irv r (J- M/ I. ,7 ( /. ,‘ I f” r L P.97Z7t‘7/AL Afflfl3 m,, ”r H'M'MW C. 1". 1‘ .5. 53 . Fflflq /// 6// R557.-. .5 If [“6“ , 5 K) A F6? U/ /‘ ‘ ’. 4’ T x 9) 5* MIT/75' “WA/’3: L... _. JL J— //'(,Cv TFVV / //;‘ UR Of; A/VKJ (THERE/V T //\/ THF :51. A5 “" f i «Q 2'A/‘l’d247fi'5 SC‘HE/Vf/i T/C va’//‘€’//VG fl/AG/Q’A/W (.1 o o F" o o o "7 f" rm” ' '0' O O O O :o' o Q ,0] L ._ _L I .' I ' ‘ i I I I I l g I l V o' o o o o -d- -‘- I“; O L.J O O O O LOJ O Q Li] L_.__ \ ‘J I “(‘OPPt—R ELECTRODE s-..\_________ /C/15’ER 50.4.9.5? \———//OL£_5 TO ADC/US7— SPAC/NG 0F ELECfRODES C, P, Pg 6‘7. P fl [1 “ ‘B/fl/D/NG #057 I v c I? v w T‘ f'. u . v , ‘T .: v? ’ L I 11l 1' ii if 11 Li L! H i‘ it :1, q: LI ,i' j 1| II H ll :1 H :,| L' U L - " _ . — g - I A A . '- ’ J I \ i I . Q ~ I I q ‘ / ‘ A ‘ / . ‘ " , , -' QA A d/ x . . ' ' K I h ' A ' I _ f. A ' A A - ’ ‘ _. ., " o . I. , ‘ d- I . . - / , . . . l ‘ \ \ ‘ . . ’ \ ’ . ‘ I , , 'A. <2 b A, I A A . A . ., _ ' J I \ 1 \ t I / ‘ ‘ v A. ' " . _’ C .A \ ' 1 . — ‘ . ' l .. I \ ‘ a A ' ‘ u A b ’ ‘ . . <3 ’ " ’ q A] ' \ . \ ' \ \\ - . ‘ . g“? - MID; 7/52-;5‘6‘7‘RODF U/V/7~ /"//7/"/V72’.D ON A CO/VC/fi’th‘zfi' 5406/!” F/G ~j_ Neglect B/r2 and allow ds-rgdug wherecu is a solid angle in the sphere. Then: (2) I: -l/pfa/ar(A/r) rgdw(from equation (1)) I: A/p 277. Asz/ZTI and by symmetry B: -pI/2,TI, V=IOI/2T7(1/r, - l/rz). If the electrodes are on a straight line, rlzrgsA. .‘. Vplzffl/217(l/A - l/BA). vp2= pI/ZTI (l/ZA - l/A). vpl - Vp2=pI/271(l/A) V :pI/Zfll/A and/a: BANE/I. 3pequals the Specific re- sistance. LABORATORY STUDY: The first multielectrode unit used in an attempt to measure resistivities at different points in a concrete slab consisted of four COpper electrodes mounted on a strip of fiber board. (Fig. 1.) The fiber board was so constructed that the electrodes could be Spaced at intervals of from one to four inches. This theoretically facilitated the measure- ment of resistivities at any depth that the electrodes happened to be spaced. If good contact could have been obtained between the electrodes and the concrete, it would have been possible to read the resistivity of the concrete at points one to four 3. Applied GeOphysics, p. 241. inches from_the surface. Various methods of loading the unit and adjustment of the length of the electrodes to con- form to the irregularities of the concrete were tried in or— der to bring the electrodes into intimate contact with the concrete. All of these alterations changed the nature of our readings to the extent that they could not be depended upon. There were several important observations made while ex— perimenting with this unit which influenced the design of the equipment used in later work. The first of these observations was that a constant and adequate contact had to be made between the electrode and concrete if the measured resistivities were to be relied upon. Second, the contact could be improved by weighting the unit but no definite limit for the weights could be reached because upon the slightest movement of the center of mass of the weights the potentioueter readings changed. The third observation was that a multielectrode unit must be placed on a slab of sufficient size to prevent the field set up around the electrodes from passing through any medium other than con- crete. The block of concrete used for test readings on this first unit was not wide enough to allow the field set up around the electrodes, Spaced at four inches, to remain totally in- side the concrete. The result was that part of the field had to pass through a layer of air around the outside of the block and the measured resistivities were far out of preportion. It is known that air has a much higher resistance than wet concrete, and since the resistivities ran exceedingly high, the only thing that could have happened was that the field of the elec- trodes was passing outside the concrete block. A second multielectrode set up was tried which operated on the same principles as the first and Wenner's formula was applied. Instead of using electrodes which had to be weighted for contact, we used four pieces of tool steel sharpened at the end and with large enough diameter to allow them to be driven into the concrete a short distance. By driving the points into the concrete excellent contact was obtained, and the readings on small spacings were nearly duplicated when several readings were taken at a time. However, the difficulty of the field pass- ing outside the concrete when the electrode spacing exceeded two inches, was encountered again. The observations made with this apparatus were: (1) that it is possible to take resis- tivity measurements with a system of electrodes, and (2) that the results can be duplicated by taking several measurements in a small interval of time. Also it was observed that the read- ings were not affected by the pressure applied to the elec- trodes provided a good contact was secured when the electrodes were driven into the concrete. The principal fault in the use of the point electrode was that the concrete chipped out when 779/5 556 770W *0 55 0550 FOR 3 ” 5/?54/1’M/6 0.07- JAMPZ. 1:13 %” - COP/3153‘? ,9 CREE/v ,4 5 NEAR SURFACE/45 / // \7" P055 /B.L E / / /// // / // // I / ”'1 /// / / / // // // // // // // // / I 4 / é ,/ // / // // // // é // // C 1459/ $724» \ / '- /0CAKJQ 7: / 4 // // Fa»? o c satiety «V / 4’ // // _____ ’7 / — / // / // // // /' / / // / // / // I // // ,/ / / I // // / _._.._..._._..___.._.. /_._—.—————--—-—-———————————-1—-———£————¢——/ ——ij{—— 11—— ____________ 0, ' // // / , AV / // // // / —I_ //t // // / AM. 2 (9 // // ' W // /yl : / ' y // / ,', I 1'— ' ///'l l -<;, - 6 ~~ 1335-4 :1 / Y " r i ' \ \ ,, P /2” -<-—----— \ x ————— 30 . lcwvmcr poor __\. \_5/_/45 55719 (W _ FOR Z/NC FAA 7Z— Z/NC‘ FLA TE 044 gram we“ 7‘55 T 5.4/25 F/G~2 it became dry and the electrode could not be held in the con- crete by driving it in. The use of these two multielectrode units just described gave us sufficient preliminary data on design and technical difficulties to set up a test apparatus that Would measure re- sistivities which did not need correction for mechanical er- rors. The apparatus was constructed in the following manner. (Fig. 2) A wooden form was built for the slab, which measured 30" x 24" x 7", and set on a zinc plate. In a plane twenty— four inches from one end of the box and perpendicular to the face of the slab, three 3/16" brass rods were installed to serve as electrodes. The rods were spaced 1", 3", and 5“ from the face of the slab and fastened in place by running them through holes drilled in the side of the form. Before the top electrode could be placed, the form had to be filled with con- crete. The concrete was a standard mix used by the Mic igan State Highway Department for regular pavement work. The Opera- tion of pouring and puddling the concrete in the form was per- formed in nearly the same manner as that used in pouring pave- ment slabs. When the form was completely filled and troweled off, the top electrode was placed on the surface of the concrete. This electrode was a piece of copper screen 24" x 30", laid as near to the surface as possible. In order to secure a good contact, the screen was pushed down into the concrete as much as possible and then sprinkled with a thin layer of cement. The cement formed a mortar when it united with the water com- ing to the top of the concrete. Wires were fastened to the screen and zinc plate, and the brass rods were allowed to pro- ject far enough past the form to facilitate a connection for an electrical circuit. This arrangement of electrodes provi- ded a means of passing current through the upper inch of the slab (between the screen and the first brass rod), through a section betwe n one inch and three inches below the surface (between the first and second brass rods), through a section between three and five inches below the surface (between the second and third brass rods), and through a section between five inches below the surface and the bottom of the slab (be— tween the third brass rod and the zinc plate.) (Fig. 2). The electrical circuit, (Fig. 1.) consisted of a source of current from an ediscn cell, a potentiometer for measuring the voltage drOp between electrodes, an ammeter for measuring the induced current, and a bucking circuit for neutralizing the effect of any self-induced potential in the concrete it- self. The object of the bucking circuit was to "buck out" any self-induced potential in the slab due to polarization and electrolytic action of the wet concrete on the electrodes. This self-induced EMF presented a serious problem while the readings were being taken, and another piece of equipment was needed to correct the error. When the current passed through the electrodes in one direction, polarization took place in the same direction. When the current flowed in the opposite direction, the polarization was reversed. For this reason a reversing switch (Fig. 2.) was introduced into the circuit, and after each successive reading the current was reversed. By reversing the current in this manner, the polarization was kept neutralized, and we were not measuring the resistances of the gasses collecting on the electrodes along with the re- sistances of the concrete. With this arrangement of electrodes, the resistance could easily be calculated from Ohms law Rs E/I. The proof of this is as follows: Assume that the bar electrode passes through a unit vol- ume of concrete. Then integrating i: ANAO , we have I=V/R or R-E/I which is Ohms law. The reciprocal of the resistance equals conductance (0), measured in Mhos. If the resistance is divided by the area of concrete, the result gives the re- sistance measured in inch ohms. The following data was taken over a period of one hundred and twenty-three hours. During the first eighty—seven hours the concrete was allowed to cure in the Open air at a tempera- ture of approximately 75°F. After the first eighty-seven hours, the surface of the concrete was kept moist with wet bur— lap while the remainder of the readings were taken. The sym- bols used on the data sheets are as follows: -10- Vp equals the potentiometer reading before the current was turned on. Vr equals the potentiometer reading while the current was turned on. Vp - Vr equals the potential drop between the electrodes which were receiving current. Vo-l equals the potential drop between the screen and first brass rod. Vl-Z equals the potential drop between the first and sec- ond brass rod. Vz-S equals the potential drOp between the second and third brass rod. V3-4 equals the potential drop between the third brass rod and the zinc plate. All V's are measured in millivolts. C equals the conductance. R equals the resistance AW equals the increment of time between readings. The current is measured in milliamperes. 7— /ME my YE V21 16-1 103. V213 V334- C 5/35/91 100 .57 7 .3070 " -/J‘0 . 46.5 .3225 -/5.5 .432 .3587 A v = .3267 .670 ./77 /.035 £53 /.42/ ./40 105‘ .000 ./0.5' /. 5/3 .600 ./0.9 /.030 .04/ A420 J50 ./.97 .000 ./.97 A522 . Av: /. 46.9 .5295 .738 /.6'/Z " .260 .33! A5252 .6‘00 . 747 £686 .290 .370 [567 A Y.= /.6'04' . /20 .0/0' /. 035' .220 /. 0.90 .340 .939 .333 .606 /-/.22. ./0‘0 .8/5’ [/00 .2 85 /-/22. Av = /. / ll W49”! .240 .04/ .079 . 033 .375 ~ -- ./.90 .050 .6152! .605 .3/4 ./75 .500 . 3.50 J50 .5615 .265 ‘ ” .2025“ .930 .274 . " .240 /. I40 .210 ., 4y -.- . 298 .360 /. 020 . 706 .245 . 700 . 700 .350 /. 0 /0 .693 .260 . 736 . 706 " Av: . 70/“ .2 /0 . 4/5 /. 0/2 .390 .7/5’ /-090 .. . .250 .525 . 9.52 2340‘ .705 .97 IME -omm/ V13 W 1/04 1/12 1/23 3-4- C “7/49/74 AV: /.000 -- -- .060 .564 .833 .263 .446 .050 .6/0 .053 .245 .408 .060 070 .348 .278 .43/ .045 .666 .874 .208 .433 Av.- .429 5 H434. .2/5 .524 .409 -- " ./63 .396 .4// .2/0 ~53)“ .392 ./70 .383 443 Aw .4/4 .037 10.9 0'39 .040 ./45 .56/ .007 ./09 .339 .040 J46 .5025 Av: .348 u .040 .096 .829 .052 .128 .812 " .44/ ./00 .320 .067 ./27 .8// " Av: .8/8 .0/9 .400 .505 ./05 .005 .026 .6/0' .756 ./4/ .36/ . 0.32. .579 -.?.5‘0 ./69 .384 .026 .49/ .632 ./4/ .36/ AV: .368 \5//4‘/ZF/ff .0470 ./0/.5* .290) .. " .0302 .//05 .3451 .0483 ./.5'00 .3220 . 0384 .//03 .348! ' H ” AV: .3265 Dag/{0.00 17.; vs 10.. 10—2 172'... 175-4. (3 1 .5774 -/2I.’_ .0307 .7000 .4090 .. .. .0400 .2727 .450J .0300: .7000 .4 703 .0400 .2009 . 4526 AK: .460/ .0400 .7300 .6063 .0300 .7003 . 7300 . 0 470 .7300 . 70 7/ .. .0330 .7007 7303b Av.= .7//.9 .0300 .4920 .777 .279 .3070 .0493 .0070 .222 .279 .3034 .0300 .4970 .093 .20/ .3000 .0470 .4900 .740 nausea»: 300 2047 Al" 3633L 000-7472 .0200 .0940 28/9 .. -- .0342 .7300 246.9 .0200 .7000 2000 .0347 .72 00 2064 ~ Av.= 2050 .0333 . 7000 40 72 .0200 .7343 3900 .0340 . 7000 .4036 .02 69- ./340 40/0 A v: .4012 " 'v .0265 .0001 .0007 ' ~ .0343 4000* .6533 " .0260 .08/6' .6576 . 034/ . /042 .0045 ‘ " A v: .0000 u .. .0240 .0000 .2030 .2370 .2025 .1 .0790 .0000 .0470 .7470 20441 .0240 .0000 .2000 .2000 .7920 .079 000010070 - 4070,2039 [ME VP W? 16-7 Vt; We 36—4- C 0775-7774 74%- .2207 5/75-3074. .02 90 .7040 .7 700 " ” .0230' , 7070 .270fi .0293? .7007 .7046 .0230 . 7000 .2766 ' A v: ./.909 .0435 .2523! . 3045 .0540 .3240 .3359 .0440 .2070 .3499 . .. .0543 .3/50 .3448 “ A v .3538 .. .03 75 .730/ .0760 ~ ' .0305 .0970 . 6288 .0305 . 72 9.5“ -594 .0305 .7050 .5809 ' ” Av: .0950 ' ~ .0744 .4002- .0202 .7400 .2007 " .0700 .4900 .3730 .7035 .7902 ~ .0744 .4000 .0940 .7430 .2007 " 0/83 .4950 .3770 7040 -/_989 Av = . 2004fl Lfi/AF'lZfi‘fi .028/ .2700 ./040 ~ " .0232 .7779 .7304 .029! . 2 750 . 7022 .02 30 . 7 749 .7375 74 V - // 7o ' v .0370 .2730 .297! .0395 .2085 .2942 .0377 .2720 29331 . 0390 . 2 7/3 . 2934 A v .2930 7M5 -mr V73 M5. 1/04 V72 1/23 W4 C 5/75/2794. .03 70 .7577 .4903 .. " .03/51 .7272 .5798 .0390 .7000 .0200 .. .03/0 .72/7 .57/9 .. " Av: .5725 .0/90 .4920 .0525’ .7000 .237 ~ .0239 .4960 .2280 .2680 .7784 .0793 .4 740 .5452 .7772 ‘. 2255 .0230 .4950 .2307 .2049 . 7797 Av: .2001! 500—0371. .0279 .5950 .036 7 -~ " .07001 .3020 .0596 .02 /5 .547/ .0393 . 0 702 .2 00.5 .0637 747.: .0498 ' .. .0225 .7032 . 2 45 6 .0290' .24/ 7 .23991 .0229 .705 7 .24 06 ~ .0295 . 2467 . 2392 AV.- .2428 . 02 75 . /288 . 42 70 . 0235 . /022 .4598 .. 02781 7328 .4/87 .. .0232 . 7027 .4546 H. AV= .4400 .. .. .0225 .2770 .2006 .. . .0280 .3033 .7547 .. .. .0230 .2700 .2756 n H ‘0270 .343/ ./55/ 7a .. 74v: .7963 . 22mg FM W V2 3/017 16—2 143 V34 C .0/70-4. 020:] .3275 ..0306 -° " .027 .5909 .0350 " .02/0 .5970 .0356 .0230 .8065 .0206 .. 747.: .0324 . 0200i .2039 .2077 . 0266 .259/ .2053 .0273 .2030 .20904 -' .0248 .2400 .2066 74v: .2055 .0242 .726/ .3038 .0272 .7040 .40 77 . 0235 . 7249 .376 .0206 - .7005 .409 ~ 747-— .394 .. .0200 .4709 .6002 .2037 .7963 ~ .0234 .4025 737.57 .3450 .7356 ~ ~ ".0202 .4600 .6000 .2200 .7036 .. 7. .0235 .4900 .7495 .3400 .7300 .. " 74v: .7034 0777-8074 .0727 7.0400 .0770 -- " .0707 .7707 .0742 .0/2/ /-0825 . 077/ -. .0700 .7704- .0740 74V= .07274 .0262 .3065 . 735.4 . 0.750 . .5309 . 72 70 .0258 .3 770 .73 90 .0340 .5391 .72 7o .. .. Av .732! .0337 .2000 .3370? .02 77 .7779 .3046 .035 .2250 .3702 .0230: .7770 .3755 77 ME 7722' Vk MR 164 W—z 1&3 V34 C 3777—0074 747- .3768 -- .0790 .4740 '.6902. .2707 .7003 .0259 .4050 .7000 .3050 .7345 .0203 .4608 .7052 .2444 .7667 .0249 .4829 7007 .3022 .7302 Av: .7528 07775.”!J .0225 .2022 .0000 ~ ~ .0702 .7007 .7736 .0226 . 79 75 .7739 .0705 .7335 . 7305 " ' . Aw .7729 .0707 .0447 .0090 . 0234 .0050 . 7745 .. .0706 .0570 .6526- .0225 . 0600 . 0070 ~ “ - 747 - .7747 ~ . 022 7 .0030 . 5320 -- - .0709 .0605 .0570 .0220 .0060 . 5302 .. . .0790 .0700 .5307 ' '- 747: .5353 ' “ .0702 .4609 .6563 .7954 .7062 .0220 .4702 .7057 .0057 .0000 ~0/03 .4400 .6539 .2/39 ./7// -- .0707 .4400 .640/ .2007 .7797 .0227 .4750 .7345 .3405 .7290 .. A7: 1868 3/7020! .0220: .2920 .0752 -- .0704 .7402 .7372 .0220 .2725 . 70 35 . 0703 .740 7 . 725/ 747- .7000 [ME mi V73 Viz 1/04 V712. 16—3 15.4— C 0770-2771 .0702 .052 . 0 94/ " " . 0230 .0659 .6985 " . 0703 . 0543 .00 72 " " . 022! .0662 . 6’6 7/ ” " A V - . 68/7 u H .02/7‘ .0758 .5 7/7 H N .0/8 / .0643 .5627 .. ~ .0223 .0072 .5496 -- .0/8/ . 0622 .582/ " Av- .5660” ~ .0700 .4475 0465’ .2050 .7 757 " .0223 . 472/ 7467 .3260 ./369 ' ~ .0700 .4250 .6427 .2777 .7599 .02/7 . 4700 . H90 .3670 ./238 ' " A V: ./4 9/ 97/04/01. .0206 .046] .0007 -- .0700 . 7.3 7/ .7372 .0222 . 7 775 , 72 90 .. .0700 .7407 .7275 . 022.5“ - IJSJ . #270 Ay : . /468 .0220 . 0000 . 652/ . 0/3/ . 0.5.30 . 68.36 .0226 .06‘ 7/ .6735 " .0700 . 0529 . 6809J " A V- . 6'726‘ . 0700 .0 00% . 59/2 .0229 .06/9 , 5020 .0/80 .08 I4 .5640 .0/80 .0599 .6005 A v .5046H 77745 0770 7077 5770- 0671 W W 16-7 MZ-z. _V2'—3 3—4 C .02.:l . 4300 .6422 . 2042 . .7 752 .0 . 450/ . 7220 .3367 . 7309 .0707 .4750 .6307 .2237 . 7020 . 0227 .4522 .7233 .3289 . 7.3 4/ 747 .7506 . 0270 . 2952 . 0 729 .0779 .7479 .7277 . 0227 .2770 .70 40 . 0700 . 7339 .7347 A y- .7004 .0779 .0504I .6770 .0220 . 0070 . 6079 . 0703 . 0557 .6 74+ .0226 - 0607 . 6572 A V: .6 777 . 0220* . 0 7 72 . 5700 . 0703 . 0623i .50 75 . 0223 . 0 700 .5727 . 0702 . 062.51’ .5023; A v . 5700 . 07 79 . 436/ . 6495 .2044 . 7 75a . 0277 .4350 .7000 . 3 350 7294 .078/ .4775 . 6035 .7060 ./946 . 0277 .4525 .7003 .357 7 . 7235 74 v = ./307 SUMMARY OF DA TA CONDU " TLV/J‘l 5 [AL MHOd TIME] A 7.- 1/0-7 v7-2 V2-3 V34 0 H00 0 770003 .3267 7. 4690 7. 0040 7. 7770 /6‘ ' 76 .2902 . 7073 /. 0000 . 42.96 23 " 7 .4743 .5407 .0700 .3604 3/ " 0 .32 65 .460/ . 7779 .3033 .38 ' 7 " . 2 650 . 40/2 . 6555 .226 7 46‘ 0 .7909 .3530 .5952 .2004 55 " .9 " .7770 .2930 .5725 .2053 63 8 .0490 .2428 .4400 .7963 77 . 8 ~ .0324 .2055 .3944 .7534 07 '- 76‘ " .0727 .7327 .3700 .7520 .96 ~ .9 " .7729 .7747 .5353 .7000 705 ' .9 .7000 .6077 .5665 .7497 776 77 " .7460 .0720 .5046 .7506 . 0 7 .57 0 RES/6 T7 1/ 7 I75 1 N [N H 077/13 T/ME A t Vo-7 VL—z Afiz-J V3—4- 07700725 07700720 .004240 000944 0000653 .007249 76 76 . .004554 .007979 .00/376 .003230 23 ' 7 " 003350 .0026'29 .00/696 .003 767 3/ 8 ~ 004257 .003076 007949003020 .70 7 " .005237 .003459 .00277 7. .000/22 46 8 ~ .006970 .003923 .00233/ .000926 55 9 " .07706 .004737 .002700 .006920 63 8 .02704 .005776 .003754 .007070 77 8 .04270 .000 754 .003579 .000494 07 76 .70920 .070500 .00 4353 009003 .96 .9 '° .07729 .007942 002092 .007430 705 " 9 .07275 .002036‘ .002450 .009309 770 ' /7 .00945 .002063 .002374 .009270 72.3 " _7 " 07200 002066 J ;‘07067/ -11- COMPUTATICNS: Let us use the readings taken at 4 P.M. on May 14. V3-4 = .505-.4OO : .105 I = .0192 " s .756-.615 : .141 I : .0255 " = .5199.350 - .159 I : .0325 " = .632-.491 = .141 I = .0255 Since the spacing of all the electrodes, except Vo-l, was two inches, the distance the current had to travel in pass- ing from one electrode to the other was two inches. In order to correct our readings for the one inch spacing of the tOp set of electrodes the current was multiplied by two for all readings except those for Vo-l. C = Conductance = I/E 2x.0192/.105 - .3657 2x.0255/.141 . .3617 2x.0325/.169 - .3846 Ex .0255/.141= &g§;z Average: .3684 : Conductance on mhos. R in inch ohms = I/CxA A s 24" x 30” = 720 Square inches. R in inch ohms = 1/.3684x730 = .003767 inch ohms. 0. 0 574 AU h, \i\\\ Q AU AU 177 \..\I\U\a\.lN1 \(\ N 0. Mb IIU AIM [at >\ ‘IK h) \ .m. 1V9: 011' Hod/*5 . '7'" l - . - 1., 1 ., 21--- A 1 R _ T h/IU A. 1 11. 1'. Ar #111 .- 7.11 t 11111.2 .1 .11? A 2% n 4 r... .IIIILF" A 1111. 1% 1. LT. 11111411 1'14. .III .. M . _ ., 1.. 1 1-- 1.1. 1 , i _ _ m __ 2 A if T1 l\ .1 1 Y .' 1111 211111 ll! 1., 1111., u, 1 ..| - 11 111 I11 1 ... 1, _ A [fl “ _ A _ T _+ R7 .1. *1'14-|11 .2. 11 1111*-» 1 t 1 2 111.11 W111 41111 1 l“. - 111111.. 11 1 -2111 Ar 111.411.11.11 1 A 1 11. A. - - - 1:11. VVATVIIIIJTI11+1111 1.1-111-.- $11.. n..| l A Y 11,1211 1-11 A11..- '1 I 'II 'I AYIlilIV *1, III 111V.‘ 1'.I .l. I‘IIDIIILvI _ ILT" LY . .‘1' l1. .I1I’Ii Aw I. .040 .038 . 036‘ .034 .032 . 030 . 028 m u. 0 «75K. 6 I _ 14 _ IQ {PX—RN. IN. .V 4 m w 0 u? ‘12. 006' .004 002 000 750 00 .90 /00 //0 /20 A70 /40 70 77777: I717 #0005 2.0 30 40 50 6'0 70 0 .038 .036 .034- .032 A17— MIT (11? VI M-‘ Q Q m .006 .004- .002 .000 0 \ X \ /0 20 J0 40 J0 6'0 70 30 77M: IN HOURS 90 /00 //o /20 /30 / *M ’31) _040 __a....4 .. -..——-—41 .038 W . -_ --. . . . .. ____- . W ..... -..-.---..--..- 1%36 “.h_-___-_~. .___.-¥. ——-~— V«--A-—-— —v.~—-—1r——-—«——-—A————J> -—— . 034 . 032. —-—-——<> —— ———-—- 1b- —- _-?A_._._ - .030 ; f5.” rg/TQ; Cu 11: V -‘L_.__ -—-—-—-—— - «1 .028 W._.__.- --_.._.____.._... .1 .mW -. .026 .___. .r.-.____ 0. (0.024 _, 1L-.‘-.-‘ {022 i i 1 Q . 020 0......ng NCI‘! Q/8 ‘5’0/6‘ 1L-__ .. -. L. __._._-1}.__. ._..-—.—-4 __._1.._. ._-_.1_ -__ .ngs/srANc Q a 008 I - -__ .004 '1»— 1 _ .4». \\ T ‘ 1 002 14 ’ 3 Lfi 1 // 1 ‘ 1 1 I I .000 K I. 1 L 0 /0 20 30 40 .50 6‘0 70 80 .90 /00 //0 /20 A30 /40 A50 T/ME IN HOURS .040 1___. _~_ __ - . . _ _ __ -____ _._. -- _ “1----.. _ .000 _. --._....-_._-_..-_-_. --__.._,_-_ ___.__-_,____ ___.-- __ - .036 “1---- .-..-_ _+ -« F—— - —— -<‘——~—~-——‘—Y———- — _... — ——-———» - -— 1V --__.4.___ .026 +-_—.T—__+_-._. % A, 1 ”. 024 i __t _ § . @022 _- 1----..1— - - . _1 W/Qt 5.020 . *f 0' z . 1 | 1‘10/8 1 i L. _; HQ/é -~ 1- —- --—- -~~— -— 1._____.1,-- ----~11—--—v»—~— -—-. 4». _——_ 1P ___- . ___”— —.——__ ,_ .- ___ _. __ __. __1y___ ._ ._ _4}.__ .- - .. _ ___1..______ -.....11_ I700 . - .. -1 [6'00 — -»-- .—— ~+~— 1—~ —-4 —~ «~ 1 1— /3ao\ Ca 01/ fax TY C‘u VE 8/112 1 I '“11‘ _. — — “-11—~ ———-1«———-—T— 1 1 ’— - 200 - -1. ___.r___ .... _ _ \q .-.. -——-— 41-— . /00 .000 1 1 0 /0 20 30 460 .50 60 70 80 .90 /00 //0 /20 A30 /40 A50 77045 IN HOURS /'7m ~11 -— -—---.>~-—- —» w~———--~11— - ___—__11___-_ "—41 —--— -~-- na—H— 41—— - [600 [$0 x400 ./J00 X Cfigyflj/ *‘ 71 (1)11 6 RV Q3 new \ 1 31/00 .1 f t \ r t / 00 +. + ¢ a 1 ’ -900 _, .___._-_11.__.1 ._.. 1 :1 a 2500 ‘1. 1. \ b 700 x b \ Q 2 .600 \ V‘ G) \\ 1/1 \ 600 ~ K # 1——— — 1—7 1 \ - 1 .400 + i 300 . .0. M. _..._ » 1 -- 1.- .- . _w 1--. - -...._ 1,” ”w“ 1 ___...-—-.1»~—.-— — v«r~___ 1 E 1 . f 1 - -200 -- -_..-.‘1_ . -.. ___ i ___-.. r_|L.. ~_.-4 __ -..T.._____;__—......4‘p_. ' ' 1 1 1 I 1 1 1 .0100 1 1 1 1 . L .1 1 0 /0 2’0 JO 40 6'0 0‘0 70 80 70 /00 //0 A20 /.30 /40 /J"0 ”MEI/V HOURS 4......“ .1 Y. m 1:03-10: 1‘ 1 1 1 1 160/! 110 mu n fr - j111-Av L. . - 1 1.0 In :1 1 111 1&- -I-1.--1r111. _ 1 _ _ _ .. 1 -.- - -1. :1 1111;. 1* a _ fi 00 .90 /00 //0 /20 IJO /40 A! 0 HouR-s TIME IN 0% 1\\.\ /. 700 /. 0‘00 / 400 [300 £200 A5220 _ 0000 coma wwwwmww0mm H:>\V\LKU\VQ>\00V 40 .50 60 70 20 30 /0 r) -12- The above data was plotted as curves for each set of electrodes and presents a graphical representation of what happened as the moisture gradient of the concrete varied. A study of these curves allows us to predict how the moisture content of the slab varies at different sections during the curing period, and how adapted this apparatus is to determin— ing the moisture gradient in a concrete slab. Both the conductivity and resistivity curves interpret the moisture variation in the slab, but since the resistivity curves are in smaller units (inch ohms), any change in moisture content gives a more pronounced lepe to the resistivity curve. This fact makes it easier for us to draw our conclusions about the success of this test from the resistivity curves. Examining the resistivity curve for the upper inch of the slab we find that as the slab cured and lost moisture, the re- sistance increased progressively. There is one discrepancy in this curve at the twenty-three hour reading which is diffi- cult to explain. During the first sixteen hours of curing the resistance increased, but at twenty-three hours the resistance had drOpped off. From twenty-three hours on, the resistance increased until water was applied with wet burlap at eighty- seven hours. As soon as water was applied, the concrete ab-. sorbed some moisture and the resistance drOpped down to about two hundred percent of the resistance it offered at the time -13- it was poured. There is one possible explanation for this un- usual drOp at twenty-three hours. The fact that the top elec- trode was covered with a cement paste in order to secure bond may have caused a chemical reaction betwean the copper screen and cement which affected our readings. This possibility should be investigated in future research. The resistivity curves for all points in the slab below one inch have the same general form, and show an increase in resistance as the slab cured without application of water. As soon as the wet burlap was applied to the surface, there is evidence that the moisture affected the resistivity throughout the slab. The curves, however, show that there was a decreas- ing influence on the resistance of the slab from the surface downward. In the upper inch of the slab, the resistance dropped about .09 inch ohms as compared to .002 inch ohms in the lower two inches of the slab when the wet burlap was ap- plied. At the intermediate points there was a gradual de- crease in resistance from the surface downward, and in the bot— tom two inches the resistance remained nearly constant as shown by resistivity curve V 3-4. Curves V 1-2 and V 2—3 show that the resistance dropped abruptly to some value and then remained nearly constant while the burlap was kept saturated. Curve v 0-1 also dropped abruptly but was eratic during 100 applica- tion of moisture. The reason this occurred is because the bur- lap was not kept saturated to its ultimate capacity at all /8 070A Cm 0!] TH 1:! [URL/Z” 010/ (1C *tlyl‘ /60 V04 0‘? I00 19/ D g/xv 7:9 50.01 a A'TA 01 V£7V Mt l500 g § Q 1 //00 I0 M0000 g 1-0 .15 [0142 ‘0 Q Q § d o \\\ Comp/10 T/V/Ty ‘& Q 101/8 - 400 .300 - 200 -/00 / /r .000 / Z. I/vc/ygs 554014 SURF/{Cf 0.: 54040 3 4 5 -14- times, but was watered at about six hour intervals. The re- isistivity varied somewhat depending on how soon readings were taken after the burlap was wet down. The conductivity curves showing the conductivity at vari- ous depths in the slab at a given time represents the variance in moisture content throughout the depth of the slab. From these curves it is shown that at the time the concrete was poured, a section about four inches below he surface had the highest conductivity. Examination of the data shows that this section continued to have the highest conductivity until the surface was covered with wet burlap. After the wet burlap was applied, a section about two inches below the surface gave the highest conductivity readings. This indicates that the mois- ture penetrated'the slab and caused a change in the moisture gradient. It is also shown that the tOp and bottom of the slab dried out much faster than the middle section because both ends of the conductivity curves are below the intermedi- ate points on the curves. At no time, however, did the con— ductivity of the bottom of the slab become lower than the top. This is explained by the fact that the bottom was covered by a zinc plate and was not subjected to the drying conditions the top had. The value of these curves is that by plotting curves for several successive sets of readings, we are able to deter- mine how much the conductivity of the concrete changed during the time interval betwe n readings. -15- CONCLUSIONS: (1) In general, it may be said that this experiment pro- duced evidence of the practicability of resistivity measure- ment for determining the moisture gradient in concrete slabs. (2) The resistivity curves indicate that the apparatus used was extremely sensitive to slight changes in moisture con- tent of the slab, and that it may be develOped to measure the resistivity at any point in the slab. (5) The electrical difficulties heretofore encountered have been eliminated by the use of a bucking circuit and a re- versinglswitch. (4) Subsequent addition of water to the concrete after it has partially cured will be detected readily by this ap- paratus and the effect on any point in the slab may be deter- mined regardless of the point of application of the water. (5) The method of securing bond between the top elec- trode and the concrete must be in such a manner that all of the electrodes are embedded in a homogeneous material. (8) This apparatus is portable and can be adapted for field use. The electrodes are inexpensive and easily cash in the concrete. Due to the time limitations a numerical evaluation of the relationship between the resistivity and moisture gradient in II 3 5 R003 [Lfd‘ TRODE OR I YE N //V “ficgfigipr/oygq 6R0 U/VD 200 300 FEET FRO/’1 SLAB ,. - P b ‘7 I . CURR£NT 1.1571031 r 6) C2. SECT/O/V OF H FAVE'Mf/VT \S‘AAB SHOW/N6 x4 Tf/VATIVE' LAYKJUT 12F THE EAEC TRCDffi AS THEY WILL .55 Ué'é‘D //v fl/A K/Nci AE'S/S‘T/ wry MEASUREME/VTS IN THE FIELD F/G~3 the slab was not possible. This correlation is very easily made by taking measurements on concrete samples of known moisture content and plotting a curve of resistivity against percent moisture. A separate curve will be necessary for any particular type of concrete and should not be made up until the mix is decided upon. In setting up this aaaaratus in the field some changes will be necessary in arranging the electrodes. The tOp elec- trode can be a 3/18" brass rod laid flush with the surface of the slab. The other current electrode should be a brass rod driven in the ground two hundred to three hundred feet from the slab. The potential electrodes can be 3/16 inch brass rods spaced to give readings wherever they are desired. The length of the electrodes will have to be worked out experimen- tally and depends on what lengths give the best results. (Fig. 3.) Ohms law will not apply when one of the electrodes is at infinity. However, the line electrode formula, based on Wen- ner's formula, can be worked out easily and applied to an apparatus using an electrode at infinity. Aside from this change in electrode arrangement, the field procedure will be exactly like that carried out in the labora— tory. The resistivities will be calculated by the line electrode formula and the results should be comparable to the laboratory results calculated by Ohms law. -17- ACKNOWLEDGMENTS: The results of this investigation were made possible through the copperation and interest shown by those who gener- ously donated their services in the conduct of the tests. The writer gratefully acknowledges this, and his special indebted- ness to E. A. Phinney for his advice and assistance in procur- ing equipment, and to W. G. Keck for aid in the design and con— struction of testing apparatus. Mr. Keck's technical advice in solving mechanical difficulties throughout the tests is also I greatly appreciated. 9. Bibliography An Electrical Resistance Method for the Continuous Measure— ment of Soil Moisture Under Field Conditions, by G. J. Bou- youcos and A. H. Mick. Technical Bulletin 172, hichigan State College Experiment Station. Measurement of the Moisture Content of Concrete, by R. W. Spencer. Journal of the American Concrete Institute, V9:45-81, Sept.-Oct., 1937. Applied Geophysics, by Eve and Keys. Schlumberger Well LoggingL by Schlumberger Well Surveying Corporation. Plain Concrete; by Edward Bauer., p. 140 Concrete Durability and the Water-Cement Ratio by L. H. Tuthill, Journal of the American Concrete Institute, VlO:588-90, June, 1939. Electric Curing of Concrete and Mortar by A. G. Fleming. Concrete 44:11-12, December, 1938 Studying the Durability of Concrete, by C. H. Scholex., Journal of the American Concrete_;nstitutelV73593-807, May, 1936 Tests of Concrete Curing Materials, by F. H. Jackson and W. F. Kellerman, Public Roads, V20:67-75, June, 1939 Aug27'4s .§. .\l A t .. 1.: u... .. o‘— t . . .1. .. O J 5:: «A9. . 1....' v. x... f q. . . , 4.. . _. 1 . in”! . v . . a}, I .. . I _ 1‘s). . ‘\ . I, II' .8 W a. a‘ g . , ,. 1 . _ . . , . '1 I I» r I'm . 1.. g . .I I. g 4. a r. , .. vi». \ a M. v . . ”Iv-Ix. «I, v ..-v _ .... .C \ .. . film . s. u. . . ‘1 __ zwx .I‘J f? I. u. .1. l 1.. . 1 .1 4 . 5.. l. y I .... .. v.1. u I 1.. a r . .’.\\v 7} .‘ 1.! . w. . a. . v M; W} r ‘ -. . ‘ . . I . .- in”... ‘ I a. .1. 4'1, ,hca 3. 15‘ . . i. ., > I .v .V (I 1 I L .. a I J n i 11... Y . v . ; it. i... . .r 1 L A —.i al.—.. .e. x D . .4 1. gr *5. a. c H .. ,_.... . . n . ___ ,2... , .L 4/ f. .(w ,x, . 5f ‘ 4, .. ./ ‘04... .M A'Zu‘u‘? ‘