ASTM_G_57_-_06_2012.pdf

Designation:G5706(Reapproved 2012)Standard Test Method forField Measurement of Soil Resistivity Using the WennerFour-Electrode Method1This standard is issued under the fixed designation G57;the number immediately following the designation indicates the year of originaladoption or,in the case of revision,the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This test method covers the equipment and proceduresfor the field measurement of soil resistivity,both in situ and forsamples removed from the ground,for use in the control ofcorrosion of buried structures.1.2 To convert cm(metric unit)to metre(SI unit),divide by100.1.3 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2.Terminology2.1 Definitions:2.1.1 resistivitythe electrical resistance between oppositefaces of a unit cube of material;the reciprocal of conductivity.Resistivity is used in preference to conductivity as an expres-sion of the electrical character of soils(and waters)since it isexpressed in whole numbers.2.1.1.1 DiscussionResistivity measurements indicate therelative ability of a medium to carry electrical currents.Whena metallic structure is immersed in a conductive medium,theability of the medium to carry current will influence themagnitude of galvanic currents and cathodic protection cur-rents.The degree of electrode polarization will also affect thesize of such currents.3.Summary of Test Method3.1 The Wenner four-electrode method requires that fourmetal electrodes be placed with equal separation in a straightline in the surface of the soil to a depth not exceeding 5%ofthe minimum separation of the electrodes.The electrodeseparation should be selected with consideration of the soilstrata of interest.The resulting resistivity measurement repre-sents the average resistivity of a hemisphere of soil of a radiusequal to the electrode separation.3.2 A voltage is impressed between the outer electrodes,causing current to flow,and the voltage drop between the innerelectrodesismeasuredusingasensitivevoltmeter.Alternatively,the resistance can be measured directly.Theresistivity,r,is then:r,Vcm5 2p aRa in cm!(1)5191.5 aRa in ft!where:a=electrode separation,andR=resistance,V.Using dimensional analysis,the correct unit for resistivity isohm-centimetre.3.3 If the current-carrying(outside)electrodes are notspaced at the same interval as the potential-measuring(inside)electrodes,the resistivity,r,is:r,Vcm5 95.76 b R/S1 2bb1aD(2)where:b=outer electrode spacing,ft,a=inner electrode spacing,ft,andR=resistance,V.or:r,Vcm5 pb R/S1 2bb1aD(3)where:b=outer electrode spacing,cm,a=inner electrode spacing,cm,andR=resistance,V.3.4 For soil contained in a soil box similar to the one shownin Fig.1,the resistivity,r,is:r,Vcm5 R A/a(4)where:R=resistance,V,1This test method is under the jurisdiction of ASTM Committee G01 onCorrosion of Metals and is the direct responsibility of Subcommittee G01.10 onCorrosion in Soils.Current edition approved May 1,2012.Published June 2012.Originallyapproved in 1978.Last previous edition approved in 2006 as G5706.DOI:10.1520/G0057-06R12.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 A=cross sectional area of the container perpendicular to thecurrent flow,cm2,anda=inner electrode spacing,cm.NOTE1The spacing between the inner electrodes should be measuredfrom the inner edges of the electrode pins,and not from the center of theelectrodes.4.Significance and Use4.1 Measurement of soil resistivity is used for the control ofcorrosion of buried structures.Soil resistivity is used both forthe estimation of expected corrosion rates and for the design ofcathodic protection systems.As an essential design parameterfor cathodic protection systems,it is important to take as manymeasurements as necessary so as to get a sufficiently represen-tative characterization of the soil environment that the entireburied structure will experience.5.Apparatus5.1 At-Grade Measurements in situ:5.1.1 The equipment required for field resistivity measure-ments to be taken at grade consists of a current source,asuitable voltmeter,ammeter,or galvanometer,four metalelectrodes,and the necessary wiring to make the connectionsshown in Fig.2.5.1.2 Current SourceAn ac source,usually 97 Hz,ispreferred since the use of dc will cause polarization of mostmetal electrodes,resulting in error.The current can be providedby either a cranked ac generator or a vibrator-equipped dcsource.An unaltered dc source can be used if the electrodes areabraded to bright metal before immersion,polarity is regularlyreversed during measurement,and measurements are averagedfor each polari