ASTM C876-22b

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Designation: C876 − 22a C876 − 22b
Standard Test Method for
Corrosion Potentials of Uncoated Reinforcing Steel in
Concrete
This standard is issued under the fixed designation C876; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the estimation of the electrical corrosion potential of uncoated reinforcing steel in field and laboratory
concrete, for the purpose of determining the corrosion activity of the reinforcing steel.
1.2 This test method is limited by electrical circuitry. Concrete surface in building interiors and desert environments lose sufficient
moisture so that the concrete resistivity becomes so high that special testing techniques not covered in this test method may be
required (see 5.1.4.1). Concrete surfaces that are coated or treated with sealers may not provide an acceptable electrical circuit.
The basic configuration of the electrical circuit is shown in Fig. 1.
1.3 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for
information only and are not considered standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
G3 Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
G16 Guide for Applying Statistics to Analysis of Corrosion Data
G193 Terminology and Acronyms Relating to Corrosion
G16G215 Guide for Applying Statistics to Analysis of Corrosion DataElectrode Potential Measurement
3. Terminology
3.1 For definitions of terms used in this test method, refer to Terminology G193.
This test method is under the jurisdiction of ASTM Committee G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.14 on Corrosion of
Metals in Construction Materials.
Current edition approved Sept. 1, 2022Oct. 1, 2022. Published September 2022October 2022. Originally approved in 1977. Last previous edition approved in 2022 as
C876–22.–22a. DOI: 10.1520/C0876-22A.10.1520/C0876-22B.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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C876 − 22b
FIG. 1 Reference Electrode Circuitry
4. Significance and Use
4.1 This test method is suitable for in-service evaluation and for use in research and development work.
4.2 This test method is applicable to members regardless of their size or the depth of concrete cover over the reinforcing steel.
Concrete cover in excess of 75 mm (3 in.) can result in an averaging of adjacent reinforcement corrosion potentials that can result
in a loss of the ability to discriminate variation in relative corrosion activity.
4.3 This test method is not applicable to reinforced concrete structures with epoxy-coated reinforcement.
4.4 This test method is not applicable to reinforced concrete structures in which waterproofing membranes are located between
the reinforcement cage and the concrete surface as they can prevent the conduction of electricity and result in erroneous readings.
4.5 This test method may be used at any time during the life of a concrete member after the concrete has set, although it is
generally most useful for evaluating mature reinforced concrete that is suspected to be susceptible to corrosion.
4.6 The results obtained by the use of this test method shall not be considered as a means for estimating the structural properties
of the steel or of the reinforced concrete member.
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4.7 Temperature and humidity can impact potential readings. This is particularly important for periodic testing of the same test
location. An increase in the temperature leads to increasing ionic mobility, which in turn affects the reference electrode’s potential.
The temperature influence can be neglected if the measurements are taken within the range of 22.2 °C 6 5.5 °C (72 °F 6 10 °F).
Otherwise, the temperature-dependency of the measurements must be taken into account.
4.8 The potential measurements should be interpreted by engineers or technical specialists experienced in the fields of concrete
materials and corrosion testing. It is often necessary to use other complementary data such as chloride contents, depth of
carbonation, delamination survey, rate of corrosion, and environmental exposure conditions, in addition to corrosion potential
measurements, to formulate conclusions concerning corrosion activity of embedded steel and its probable effect on the service life
of a structure.
5. Apparatus
5.1 The testing apparatus consists of the following:
5.1.1 Reference Electrode:
5.1.1.1 The reference electrode selected shall provide a stable and reproducible potential for the measurement of the corrosion
potential of reinforcing steel embedded in concrete over the temperature range from 0 °C to 49 °C (32 °F to 120 °F).
5.1.1.2 For the purposes of this test method, corrosion potentials shall be based upon the half-cell reaction Cu→ Cu++ + 2e-
corresponding to the potential of the saturated copper-copper sulfate reference electrode as referenced to the hydrogen electrode
being −0.30 V at 22.2 °C (72 °F) (1). The copper-copper sulfate reference electrode has a temperature coefficient of approximately
0.0005 V more negative per °F for the temperature range from 0 °C to 49 °C (32 °F to 120 °F).
5.1.1.3 Additional information regarding measuring electrode potential can be found in Guide G215.
5.1.1.4 Other reference electrodes having similar measurement range, accuracy, and precision characteristics to the copper-copper
sulfate electrode may also be used. Calomel reference electrodes have been used in laboratory studies. For concrete submerged in
seawater, using silver-silver chloride reference electrodes avoids chloride contamination problems that may occur with
FIG. 2 Sectional View of a Copper-Copper Sulfate Reference Electrode
The boldface numbers in parentheses refer to the list of references at the end of this standard.
C876 − 22b
copper-copper sulfate electrodes. Silver-silver chloride/potassium chloride reference electrodes are also applied to atmospherically
exposed concrete. Potentials measured by reference electrodes other than saturated copper-copper sulfate should be converted to
the copper-copper sulfate equivalent potential. The conversion technique can be found in Practice G3 and Reference Electrodes
Theory and Practice by Ives and Janz (2).
5.1.2 Electrical Junction Device—An electrical junction device shall be used to provide a low electrical resistance liquid bridge
between the surface of the concrete and the reference electrode. It shall consist of a sponge or several sponges pre-wetted with a
low electrical resistance contact solution. The sponge may be folded around and attached to the tip of the reference electrode so
that it provides electrical continuity between the porous plug and the concrete member. The minimum contact area of the
electrochemical junction device shall be the area equivalent of a circle with 3× the nominal diameter of the concrete coarse
2 2
aggregate to a maximum of 0.01 m (16 in. ).
5.1.3 Electrical Contact Solution—In order to standardize the potential drop through the concrete portion of the circuit, an
electrical contact solution shall be used to wet the electrical junction device. One such solution is composed of a mixture of 95 mL
of wetting agent (commercially available wetting agent) or a liquid household detergent thoroughly mixed with 19 L (5 gal) of
potable water. Under working temperatures of less than about 10 °C (50 °F), approximately 15 % by volume of either isopropyl
or denatured alcohol must be added to prevent the clouding of the electrical contact solution, since clouding may inhibit penetration
of water into the concrete to be tested. Conductive gels may be employed to reduce drift in the measured corrosion potential that
can derive from dynamic liquid junction potentials. On large horizontal reinforced concrete structures, such as bridge decks,
preliminary cleaning of the concrete surface with “street sweepers” has proven successful.
5.1.4 Voltmeter—The voltmeter shall allow DC voltage readings, have the capacity to be battery operated, and provide adequate
input impedance and AC rejection capability for the environment where this test method is applied.
5.1.4.1 Prior to commencing testing, a digital voltmeter with a variable input impedance ranging from 10 MΩ to 200 MΩ may
be used to determine the input impedance required to obtain precision readings. The use of a meter with variable input impedance
avoids meter loading errors from high concrete resistivity. An initial reading is taken in the 10 MΩ position, and then switching
to successively higher impedances while watching the meter display until the reading remains constant through two successive
increases. Then decrease the impedance on setting to reduce noise and provide the most precise readings. If the voltmeter does not
display a constant reading through 200 MΩ, then the use of a galvanometer with an input impedance of 1 GΩ or 2 GΩ should be
considered. Logging voltmeters may also be used.
5.1.4.2 Electromagnetic interference or induction resulting from nearby AC power lines or radio frequency transmitters can
produce an error. When in the proximity of such interference sources, the readings may fluctuate. An oscilloscope can be used to
define the extent of the problem and be coupled with the DC voltmeter manufacturer’s specification for AC rejection capability
to determine the resolution of induced AC interference with successful application of this test method.
5.1.5 Electrical Lead Wires—The electrical lead wire shall be of such dimension that its electrical resistance for the length used
will not disturb the electrical circuit by more than 0.0001 V. This has been accomplished by using no more than a total of 150 m
(500 linear ft) of at least AWG No. 24 wire. The wire shall be coated with a suitable insulation such as direct burial type of
insulation.
5.1.6 In addition to single reference electrodes connected to a voltmeter, multiple electrode arrays, reference electrodes with a
wheel junction device and logging voltmeters that record distance and potential may also be used.
6. Calibration and Standardization
6.1 Care of the Reference Electrode—Follow the manufacturer’s instructions for storage, calibration, and maintenance. Electrodes
should not be allowed to dry out or become contaminated. The porous plug shall be covered when not in use for long periods to
ensure that it does not become dry to the point that it becomes a dielectric (upon drying, pores may become occluded with
crystalline filling solution).
6.2 Calibration of the Reference Electrode—Reference electrodes shall be calibrated against an approved standard traceable to a
national standard at regular intervals as specified by the manufacturer or when the solution is changed. If cells do not produce the
reproducibility or agreement between cells described in Section 12, cleaning may rectify the problem. If reproducible and stable
readings are not achieved, the reference electrode should be replaced.
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6.3 Calibration of the Voltmeter—The voltmeter shall be calibrated against an approved standard traceable to a national standard
at regular intervals specified by the manufacturer.
7. Procedure
7.1 Spacing Between Measurements—While there is no pre-defined minimum spacing between measurements on the surface of
the concrete member, it is of little value to take two measurements from virtually the same point. Conversely, measurements taken
with very wide spacing may neither detect corrosion activity that is present nor result in the appropriate accumulation of data for
evaluation. The spacing shall therefore be consistent with the member being investigated and the intended end use of the
measurements (Note 1).
NOTE 1—A spacing of 1.2 m (4 ft) has been found satisfactory for rapid evaluation of structures with large horizontal surfaces like bridge decks. Generally,
larger spacings increase the probability that localized corrosion areas will not be detected. Measurements may be taken in either a grid or a random pattern.
Spacing between measurements should generally be reduced where adjacent readings exhibit reading differences exceeding 50 mV (areas of high
corrosion activity). Cracks, cold joints, and areas with dynamic structural activity can produce areas of localized corrosion activity where the corrosion
potential can change several hundred millivolts in less than 300 mm (1 ft). Therefore, care must be given that relatively large spacing between readings
does not miss areas of localized corrosion activity. For small, lightly reinforced members, it may be advantageous to map the reinforcement locations
with a cover meter and place the reference electrode over the bars on a suitable grid.
7.2 Electrical Connection to the Steel:
7.2.1 The type of connection used will depend on whether a temporary or permanent connection is required. Make a direct
electrical connection to the reinforcing steel by means of a compression-type ground clamp, by brazing or welding a protruding
rod, or by using a self-tapping screw in a hole drilled in the bar. To ensure a low electrical resistance connection, scrape the bar
or brush the wire before connecting to the reinforcing steel to ensure a bright metal to bright metal contact. In certain cases, this
technique may require the removal of some concrete to expose the reinforcing steel. Electrically connect the reinforcing steel to
the positive terminal of the voltmeter. Special care should be exercised with prestressing steels to avoid serious injury, and only
mechanical connections should be made. Where welding is employed to make connections to conventional reinforcing steel,
preheating will be necessary to avoid forming a brittle area in the rebar adjacent to the weld. Such welding should be performed
by certified welders.
7.2.2 Attachment must be made directly to the reinforcing steel except in cases where it can be documented that an exposed steel
member is directly attached to the reinforcing steel. Certain members, such as expansion dams, date plates, lift works, scuppers,
drains, and parapet rails, may not be attached directly to the reinforcing steel and, therefore, may yield invalid readings. Electrical
continuity of steel components with the reinforcing steel can be established by measuring the resistance between widely separated
steel components on the test section. The resistance values should be in the range below 1 Ω to ensure electrical continuity. Where
duplicate test measurements are continued over a long period of time, identical connection points should be used each time for a
given measurement.
7.2.3 Care should be taken that the whole area of the reinforcing mat being measured is electrically continuous by checking
electrical continuity between diagonally opposite ends of the area surveyed.
7.3 Electrical Connection to the Reference Electrode—Electrically connect one end of the lead wire to the reference electrode and
the other end of this same lead wire to the negative (ground) terminal of the voltmeter.
7.4 Pre-Wetting of the Concrete Surface:
7.4.1 Under most conditions, the concrete surface or an overlaying material, or both, must be pre-wetted by either of the two
methods described in 7.4.3 or 7.4.4 with the solution described in 5.1.3 to decrease the electrical resistance of the circuit.
7.4.2 A test to determine the need for pre-wetting shall be made as follows:
7.4.2.1 Place the reference electrode on the concrete surface and do not move.
7.4.2.2 Observe the voltmeter for one of the following conditions:
(1) The measured value of the corrosion potential does not change or fluctuate with time.
(2) The measured value of the corrosion potential changes or fluctuates with time.
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7.4.2.3 If condition (1) is observed, pre-wetting the concrete surface is not necessary. H
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