Half Cell Potential Test
Astm C-876
1. Scope
1.1 This test method covers the estimation of the electrical
half-cell 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. A
concrete surface that has dried to the extent that it is a dielectric
and surfaces that are coated with a dielectric material will not
provide an acceptable electrical circuit. The basic configuration
of the electrical circuit is shown in Fig. 1.
1.3 The values stated in inch-pound units are to be regarded
as the standard.
1.4 This standard does not purport to address the safetyconcerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and
health practices and determine the applicability of regulatory
limitations prior to use.
Council.
2. Referenced Documents
2.1 ASTM Standards:
G 3 Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing>
3. Significance and Use
3.1 This test method is suitable for in-service evaluation and
for use in research and development work.
3.2 This test method is applicable to members regardless of
their size or the depth of concrete cover over the reinforcing
steel.
3.3 This test method may be used at any time during the life
of a concrete member.
3.4 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.
3.5 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 data such as chloride contents, depth of carbonation,
delamination survey findings, rate of corrosion results, and
environmental exposure conditions, in addition to half-cell
potential measurements, to formulate conclusions concerning
corrosion activity of embedded
4. Apparatus
4.1 The testing apparatus consists of the following:
4.1.1 Half Cell:
4.1.1.1 A copper-copper sulfate half cell (Note 1) is shown
in Fig. 2. It consists of a rigid tube or container composed of
a dielectric material that is nonreactive with copper or copper
sulfate, a porous wooden or plastic plug that remains wet by
capillary action, and a copper rod that is immersed within the
tube in a saturated solution of copper sulfate. The solution shall
be prepared with reagent grade copper sulfate crystals dissolved in distilled or deionized water. The solution may be
considered saturated when an excess of crystals (undissolved)
lies at the bottom of the solution.
4.1.1.2 The rigid tube or container shall have an inside
diameter of not less than 1 in. (25 mm); the diameter of the
porous plug shall not be less than 1⁄2 in. (13 mm); the diameter
of the immersed copper rod shall not be less than 1⁄4 in. (6 mm),
and the length shall not be less than 2 in. (50 mm).
4.1.1.3 Present criteria based upon the half-cell reaction of
Cu → Cu++ + 2e indicate that the potential of the saturated
copper-copper sulfate half cell as referenced to the hydrogen
electrode is −0.316 V at 72°F (22.2°C). The cell has a
temperature coefficient of about 0.0005 V more negative per°
F for the temperature range from 32 to 120°F (0 to 49°C).
NOTE 1—While this test method specifies only one type of half cell, that
is, the copper-copper sulfate half cell, others having similar measurement
range, accuracy, and precision characteristics may also be used. In
addition to copper-copper sulfate cells, calomel cells have been used in
laboratory studies. Potentials measured by other than copper-copper
sulfate half cells should be converted to the copper-copper sulfate
equivalent potential. The conversion technique can be found in Practice
G 3 and it is also described in most physical chemistry or half-cell
technology text books.
4.1.2 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 half cell. It
shall consist of a sponge or several sponges pre-wetted with a
5. Calibration and Standardization
5.1 Care of the Half Cell—The porous plug shall be covered when not in use for long periods to ensure that it does not become dried to the point that it becomes a dielectric (upon drying, pores may become occluded with crystalline copper sulfate). If cells do not produce the reproducibility or agreement between cells described in Section 11, cleaning the copper rod in the half cell may rectify the problem. The rod may be cleaned by wiping it with a dilute solution of hydrochloric acid. The copper sulfate solution shall be renewed either monthly or before each use, whichever is the longer period. At no time shall steel wool or any other contaminant be used to clean the copper rod or half-cell tube.
6. Procedure
6.1 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 2).
NOTE 2—A spacing of 4 ft (1.2 m) has been found satisfactory for
evaluation of 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 algebraic reading differences exceeding 150 mV (areas of high
corrosion activity). Minimum spacing generally should provide at least a
100-mV difference between readings.
6.2 Electrical Connection to the Steel: 6.2.1 Make a direct electrical connection to the reinforcing steel by means of a compression-type ground clamp, or by brazing or welding a protruding rod. To ensure a low electrical resistance connection, scrape the bar or brush the wire before connecting to the reinforcing steel. In certain cases, this technique may require removal of some concrete to expose the reinforcing steel. Electrically connect the reinforcing steel to the positive terminal of the voltmeter. 6.2.2 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, 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 deck. Where duplicate test measurements are continued over a long period of time, identical connection points should be used each time for a given measurement.
6.3 Electrical Connection to the Half Cell—Electrically
connect one end of the lead wire to the half cell and the other
the voltmeter.
6.4 Pre-Wetting of the Concrete Surface:
6.4.1 Under certain conditions, the concrete surface or an
overlaying material, or both, must be pre-wetted by either of
the two methods described in 6.4.3 or 6.4.4 with the solution
described in 4.1.3 to decrease the electrical resistance of the
circuit
.
6.4.2 A test to determine the need for pre-wetting may be
made as follows:
6.4.2.1 Place the half cell on the concrete surface and do not
move.
6.4.2.2 Observe the voltmeter for one of the following
conditions:
(a) The measured value of the half-cell potential does not
change or fluctuate with time.
(b) The measured value of the half-cell potential changes or
fluctuates with time.
6.4.2.3 If condition
(a) is observed, pre-wetting the concrete
surface is not necessary. However, if condition
(b) is observed,
pre-wetting is required for an amount of time such that the
voltage reading is stable (60.02 V) when observed for at least
5 min. If pre-wetting cannot obtain condition
( a), either the
electrical resistance of the circuit is too great to obtain valid
half-cell potential measurements of the steel, or stray current
from a nearby direct current traction system or other fluctuating
direct-current, such as arc welding, is affecting the readings. In
either case, the half-cell method should not be used.