ASTM C876 half-cell potential results: corrosion probability tables

What do ASTM C876 corrosion probability numbers mean?

A half-cell survey measures steel potential relative to a reference electrode, expressed as E in millivolts. More negative means numerically smaller: −400 mV is more negative than −250 mV.

The PM8500 manual, revision 4.0, October 2025, §2.2, attributes these screening bands to ASTM C876-22b Appendix X1, an informational appendix. FHWA’s bridge overview summarizes the older C876-09 edition.

How should boundary readings be classified?

This guide assigns exact boundary values to the uncertain band as its reporting convention; repeat near-boundary readings before reclassification. Instrument resolution and observed repeatability are separate from accuracy.

Neither 90% band says that 90% of a deck is damaged or sound. A point’s probability class, a mapped delamination and a structural-capacity decision are different records. Use the parking structure restoration guide for the wider assessment program and the concrete field guides for related repair methods.

Where does half-cell testing work, and where does it fail?

ASTM C876-22b addresses uncoated reinforcement and excludes epoxy-coated reinforcement and waterproofing membranes between the reinforcement cage and test surface. Steel-to-steel electrical continuity and electrode-to-concrete ionic contact are separate requirements: wetting cannot connect isolated steel, and a continuity check cannot prove usable surface contact.

Identify the steel and surface system before buying a survey package. Rebar corrosion protection distinguishes material systems, while concrete sealers and coatings explains surface protection.

What changes with galvanized steel, tendons or cathodic protection?

FHWA LTBP 2016 §3.3.6 identifies zinc coatings and galvanic anodes as causes of more negative potentials. Use the protection system’s measurement procedure and record current state. Switching off impressed current does not immediately remove steel polarization; an instant-off reading is not an unpolarized corrosion-probability reading. FHWA-RD-01-096 (2001) distinguishes CP potential and depolarization testing.

The PM8500 manual excludes assessment of reinforcement inside protective tubes by this potential-field method. Identify post-tensioned slab systems separately: an accessible conventional-rebar map does not establish the condition of an enclosed tendon.

Which reference electrode is used, and how do you convert its readings?

A half-cell potential uses a stable comparison electrode. SHE means standard hydrogen electrode, the reference zero for the constants below. CSE means saturated copper/copper sulfate; SCE means saturated calomel. Record the electrode chemistry, filling solution, temperature and instrument export settings before comparing surveys.

At 25°C, subtract 119 mV from a raw saturated-KCl Ag/AgCl reading to express it versus saturated copper/copper sulfate, using the +197 and +316 mV reference potentials in Proceq’s October 2025 PM8500 manual. Metrohm leaflet 8.109.8050EN, dated 2021-10-29 independently identifies +197 mV for saturated KCl and +207 mV for 3 mol/L KCl Ag/AgCl. The offsets below are Anvilfield calculations using the declared +316 mV CSE convention.

To convert, add the source reference potential and subtract the target reference potential: E_CSE = E_source + E_ref,source − E_ref,CSE. Thus −250 + 197 − 316 = −369 mV CSE for the stated saturated reference. Treating the raw −250 mV as CSE would instead place it in the uncertain band.

Why do published electrode offsets differ?

“Silver chloride” is not a complete specification. At a raw −235 mV, the saturated-KCl conversion gives −354 mV CSE, while 3 mol/L KCl gives −344 mV CSE. The electrolyte choice alone moves these two cases across the screening boundary.

Giatec’s legacy XCell documentation specifies a saturated-KCl Ag/AgCl electrode but a CSE-equivalent display. Do not convert that display twice. The legacy page states support through August 1, 2025; it is retained here to interpret existing instrument records.

Use the table only at its stated temperature. The PM8500 manual’s non-25°C calomel comparison table and printed temperature coefficients appear inconsistent in sign under its stated formula; obtain a model-specific correction before using that comparison away from 25°C.

How do you check the circuit and reinforcement continuity?

Connect steel to positive and the reference electrode to negative for the steel-versus-reference convention, following FHWA LTBP 2016 §6.4 and the instrument instructions. Verify the wiring physically; changing leads to obtain an expected negative number conceals a setup error.

Proceq’s October 2025 PM8500 manual checks reinforcement continuity by requiring the measured resistance between separated steel connections to be no more than 1 Ω above the resistance of the connecting cable. Record the numerical cable baseline and tap-to-tap measurement, not just a meter beep.

A beep threshold is instrument-specific. Check separated taps across joints, repairs, precast connections and reinforcement layers; assign separate electrical-zone IDs wherever continuity is not demonstrated.

Inspect cables, the porous junction and the reference cell. A low-input-impedance meter can load the resistive circuit and distort the potential; use the device specifications in the table.

Locate reinforcement and tendons before opening concrete; use approved connection and reinstatement details from the cutting and coring scope. PTI DC80.2-10 treats openings in unbonded post-tensioned slabs as a structural-analysis and construction-procedure task. A test manual’s bar-drilling example does not authorize damage to reinforcement.

When using the Table 1 route for handheld or stand-mounted concrete drilling, OSHA FS 3630 (2017), explaining 29 CFR 1926.1153, specifies a compatible shroud and dust collector with manufacturer-specified airflow, at least 99% filter efficiency and filter cleaning; clean drilled holes with a HEPA-filtered vacuum.

How do moisture, temperature and cover change the reading?

Moisture changes both ionic contact and the electrochemical environment. The PM8500 October 2025 manual §§2.3 to 2.4 describes more negative surface potentials and broader gradients in wet, low-resistivity concrete, and more positive potentials with steeper gradients in dry, high-resistivity concrete. A coarse grid can miss those steep local gradients.

ASTM C876-22b warns that concrete cover greater than 75 mm (3 in.) can average adjacent reinforcement potentials and reduce the ability to distinguish local corrosion activity. That is a spatial-resolution limitation, not a universal maximum penetration depth. Record concrete cover at the steel surface and use the reinforcement cover guide to distinguish cover from bar-center depth.

ASTM C876-22b §4.7 permits neglecting temperature influence within 22.2 ± 5.5°C (72 ± 10°F); outside that interval, account for temperature dependence. This interval is separate from the meter’s operating-temperature rating. The PM8500 manual discourages testing below freezing because frozen pore water disrupts the ionic measurement.

FHWA LTBP 2016 calls for saturated-surface-dry conditions and water at test points. Giatec’s undated iCOR FAQ specifies thorough wetting followed by about 15 to 20 minutes draining to SSD. Use the named procedure and record actual conditioning rather than treating one product’s drainage time as universal.

Oxygen-starved marine concrete can be very negative with little ongoing corrosion. Carbonated concrete needs local carbonation depth, cover and moisture evidence; even fine-grid potential mapping can miss small corrosion cells. Compare like-season campaigns with recorded conditioning rather than applying a universal moisture or cover correction.

What grid spacing should a half-cell survey use?

Proceq’s October 2025 PM8500 manual suggests 0.25 to 0.50 m spacing for large horizontal areas and about 0.15 m for suspect or vertical areas; FHWA’s bridge overview uses a 2 ft by 2 ft example grid. Choose spacing with the member geometry, exposure and required local resolution.

Count intervals along each side, add one for each boundary-inclusive axis, then multiply: N = (L/s + 1)(W/s + 1) when dimensions divide evenly by spacing.

For nondivisible dimensions, round each length/spacing ratio upward, then add one node per axis. Keep regular spacing and add the far boundary; the last interval is shorter.

Establish the datum, accessible boundary, reinforcement layer and electrical zones before layout. Include cracks, joints, wet areas, patch edges and quiet controls. Keep refinement points tagged separately: pooling a dense hotspot grid with a coarse deck grid biases any percentage based on point counts.

How are stable readings collected and checked?

FHWA LTBP 2016 §3.3.5 requires monitoring each point for at least 3 seconds and stability within ±5 mV per minute. A three-second pause alone does not establish the minute-scale drift criterion; keep observing a drifting reading and resolve its cause before recording it as valid.

Use a wetted sponge and consistent contact pressure. Avoid exposed aggregate particles and insulating patches; control standing surface water between points using FHWA’s pavement procedure. Tag unstable readings with their cause and retain missing values as missing, never zero.

Anvilfield’s field-QA plan repeats designated control locations at the beginning and end of each electrical zone and after changing the tap, electrode or conditioning. Retain originals and repeats. Diagnose contact, continuity, reference drift and external-current effects before changing contours; set the repeatability action level in the project QA plan and instrument instructions.

Proceq’s October 2025 manual recommends yearly calibration and a maximum ±10 mV error for its specified reference comparison. Giatec calls for XCell verification before a mapping project, while ODOT’s October 2014 evaluation checked meters and reference electrodes daily. These are distinct instrument and project practices, not one universal survey schedule.

What belongs on a corrosion potential map?

A usable corrosion map has three aligned views: potentials; continuity and conditioning; and corroborating findings. Show measured locations, electrical-zone boundaries, missing-data areas, reference, units, date, grid spacing and raw-versus-converted basis. Anvilfield uses 50 mV contours as a display choice, with numerical labels and raw points visible, not as an ASTM-required interval.

Do not interpolate across separate electrical zones or membrane-covered strips. State the interpolation method and clip it to valid measured coverage. Prioritize abrupt gradients that remain after checking reference, contact, continuity, moisture and cover; compare them with apparently quiet locations under similar exposure.

Report accessible and excluded area, valid primary nodes and separate refinement nodes. A fraction of sampled nodes is not automatically a fraction of surface area. For an area percentage, declare the cell-assignment or interpolation method and total the resulting clipped areas; a corrosion-probability area still is not a concrete-removal quantity.

Anvilfield’s register combines FHWA LTBP 2016 reporting fields with ASTM C876-22b complementary evidence. Its filled observation is from ODOT’s October 2014 report, page 57; link original data, conversions, photographs and laboratory results by location ID.

survey_id,location_id,element_face,x_m,y_m,timestamp,operator_id,instrument_id,grid_role,repeat_of,E_raw_mV,raw_reference,electrolyte,temperature_C,E_CSE_mV,conversion_basis,continuity_zone,tap_pair,lead_ohm,total_ohm,corrected_ohm,continuity_status,conditioning,cover_mm,resistivity_kohm_cm,corroboration_ids,data_quality,probability_class,represented_area_m2,excluded_reason,disposition

Empty fields mean missing, not zero. Keep air and surface temperatures in campaign metadata and electrode temperature with any conversion; the example header’s temperature_C is the electrode temperature. Define each electrical zone and its connected reinforcement layer.

import csv
base = {(x, y) for x in range(0, 10001, 500)
               for y in range(0, 10001, 500)}
fine = {(x, y) for x in range(0, 3001, 150)
               for y in range(0, 3001, 150)}
with open("survey-plan.csv", "w", newline="") as f:
    out = csv.writer(f)
    out.writerow(["location_id", "x_m", "y_m", "grid_role",
                  "E_raw_mV", "raw_reference", "E_CSE_mV"])
    for n, (x, y) in enumerate(sorted(base | fine), 1):
        role = "base+refinement" if (x, y) in base & fine else (
            "base" if (x, y) in base else "refinement")
        out.writerow([f"PLAN-{n:03d}", f"{x/1000:.2f}",
                      f"{y/1000:.2f}", role, "", "", ""])

Coordinate method: use integer millimeters, deduplicate coordinate pairs, then export meters. Both grids share the southwest origin.

Which tests should corroborate half-cell potential results?

Half-cell potential measures electrochemical potential; resistivity characterizes the concrete current path; sounding maps separations; and GPR locates reinforcement and maps electromagnetic response. None substitutes for measuring remaining bar section. ASTM C876-22b §4.8 identifies chemistry, delamination, rate and exposure evidence as complementary to potential measurements.

Compare carbonation depth with cover at the same location. ASTM C1152/C1152M-20 measures acid-soluble chloride, including some aggregate-bound chloride unavailable for corrosion. Record sample depth, method and concrete- or cement-mass denominator: NIST’s 2024 PGAA/C1152 study, for example, reports chloride by concrete mass. Compare laboratory results with a project criterion using the same method and denominator.

FHWA’s electrical-resistivity overview explains why moisture, chemistry, geometry and reinforcement affect resistivity. For corrosion-rate information, specify an actual polarization-based method with its area assumptions. Germann’s GalvaPulse documentation separates potential/resistance measurements from pulse-based rate estimation.

Compare suspect and quiet control locations. At repairs, include patch center, perimeter and adjacent original concrete, recording connected anodes. Corroborated deterioration and repair design establish removal scope.

Worked example: why −41 mV did not clear a Coos Bay location

ODOT’s October 2014 Coos Bay evaluation recorded −41 mV CSE at a location with mapped delamination and resistivity of 9.4 kΩ·cm, showing why a favorable potential reading must be interpreted with corroborating evidence. The published location is Column 4A, Zone 5, Span 9, on the south face of Beam 4 at its east side; see §4.4.1.1 and Figure 4.14, printed page 57.

The mapped delamination is physical damage, and the report’s combined evidence suggests early corrosion. Across the Coos Bay observations, the authors propose shallow cover or permeable concrete, with easier oxygen transport, as explanations for unusually positive potentials. They leave the cause of neighboring exposed-bar corrosion unresolved.

The report records offsetting the Gecor probe for a 5 cm counter-electrode-to-rebar distance because cover was only one inch. Its printed rate unit is μA/cm rather than current per unit area; use the documented damage and resistivity here, with a properly reported rate test for further assessment.

source_id,report_date,source_page,location_id,element_face,E_reported_mV,reported_reference,E_raw_mV,raw_reference,E_CSE_mV,conversion_basis,x_m,y_m,continuity_status,conditioning,temperature_C,cover_mm,resistivity_kohm_cm,corroboration,data_quality,probability_class,disposition
ODOT-2014-Fig4.14,2014-10,57,CoosBay-Span9-Zone5-Column4A,Beam 4 south face east side at Column 4A,-41,CSE,,,-41,reported CSE,,,not reported,not reported,,25.4,9.4,delamination mapped in source figure,published extract with incomplete local QA,,investigate combined evidence

What should a survey scope cost and include?

Anvilfield estimates $2,800 for a 100 m² accessible deck survey package using 441 grid points, a two-person field day and a documented interpretation allowance, before laboratory work, traffic control or access equipment. Changing field time from six to ten hours gives $2,300 to $3,300 with the other inputs fixed. These November 5, 2026 budget inputs are estimating assumptions, not market averages.

Method: multiply the assumed labor quantities by their rates, then add equipment. The observation floors use FHWA LTBP 2016’s minimum dwell time; movement, longer stabilization, conditioning and QA make the field day longer. Laboratory turnaround is scheduled separately.

Specify access, taps and reinstatement, electrical zones, grid/refinement, conditioning, raw data, corroboration and report review. Price lifts, scaffolds, traffic control, permits, travel, taxes and additional closures separately; replace assumed rates with local quotes.

Public-building survey packages should state deck closures, laboratory allowances and the evidence required before repair quantities are priced. Restoration estimators tracking public-building rehabilitation use [PorchRocket's Bid Control](/bidcontrol/general) to identify early signs of that work in board meetings, budgets and capital plans before bids post.

The corrosion specialist interprets electrochemical evidence; the structural engineer sets capacity-related repairs using deterioration, section loss, chemistry, exposure and structural needs. Carry those decisions into concrete repair and restoration, the parking structure restoration program or bridge deck rehabilitation.

How often should surveys and instrument checks be repeated?

Anvilfield estimates one same-season repeat at 12 months as the first budgeting allowance for a stable baseline selected for monitoring; new leakage, worsening damage or a repair intervention triggers earlier assessment. This annual budget-cycle allowance supports a like-season comparison. It is not a measured deterioration interval or an ASTM C876 schedule.

Set the actual interval from exposure, corroborated deterioration and the decision the repeat supports. Give known defects their own action date rather than waiting for the monitoring cycle.

Record repairs, anodes, coatings and system-state changes before comparing campaigns. Reuse reference basis, point locations and conditioning wherever possible.

Field checklist and report handover

Deliver raw and normalized data, QA records, maps, exclusions, photographs, laboratory results and an action register. Dispositions are: repeat invalid measurements; investigate conflicting evidence; or scope repairs from corroborated deterioration and design needs.

Assign each action to a responsible party and date. Keep surface- and steel-system limitations beside the affected electrical zone.

• Identify structure, element, face, steel type, coatings, membranes, anodes and CP state; select applicable method before laying out points.
• Locate steel and tendons; approve access and reinstatement details before opening concrete.
• Establish datum, dimensions, spacing, separate electrical zones and refinement areas; mark inaccessible areas.
• Record instrument, electrode chemistry and electrolyte, serial IDs, displayed reference basis and verification results; confirm input impedance and export settings.
• Check polarity against the instrument instructions and the steel-positive convention.
• Measure cable resistance and steel-tap resistance numerically; retain locations, corrected result and continuity decision.
• Record weather, surface temperature, prior wetting, current conditioning and contact method.
• Confirm stable readings with the named procedure; repeat designated controls after tap, electrode or conditioning changes; keep exclusion reasons and missing values.
• Save raw and normalized data separately; verify one conversion by hand and check that already-normalized exports are not converted twice.
• Log cover, delamination, cracks, joints, patches and wet zones against the same coordinate system.
• Obtain corroboration in suspect, uncertain and apparently quiet comparison areas; retain lab methods and units.
• Plot measured points, exclude invalid zones and report interpolation and contour settings.
• Define percentage denominators and area-assignment method; deduplicate overlapping grids and separate corrosion-probability mapping from repair quantities.
• Reinstate connections, export the evidence register and assign each location's next action and follow-up date.

Sources checked and review date: 2026-11-05. Update log: 2026-11-05, article review corrected source references, clarified measurement limits and added reproducible grid export and drilling controls.

How to cite this page: Anvilfield, ASTM C876 half-cell potential results: corrosion probability tables, 2026, https://anvilfield.com/field-guides/concrete/half-cell-potential-survey-interpretation/.

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Sources checked

• S01: ASTM C876-22b, active publisher page, updated October 11, 2022 Public publisher scope and significance/use text reopened with curl 2026-11-05; licensed full text not accessed.
• S02: FHWA InfoTechnology, Bridge: Half-Cell Potential, undated, cites C876-09 Reopened with curl 2026-11-05; technical claims checked in the public page or document.
• S03: FHWA-HRT-16-007, LTBP Protocol FLD-DC-NDE-003, Version 1, January 2016 Reopened with curl 2026-11-05; technical claims checked in the public page or document.
• S04: Proceq / Screening Eagle PM8500 User Manual, revision 4.0, footer October 22, 2025 Reopened with curl 2026-11-05; technical claims checked in the public page or document. Revision 4.0; document-information date says August 20, 2025, revision history says October 15, 2025 and page footers say October 22, 2025. Article identifies the October-footer document.
• S05: Giatec XCell product specifications and FAQ, undated legacy page Curl attempted 2026-11-05 returned HTTP 403; page text reopened and verified through web access the same day.
• S06: Giatec NDT & Lab Testing FAQs, undated Curl attempted 2026-11-05 returned HTTP 403; page text reopened and verified through web access the same day.
• S07: FHWA InfoTechnology, Pavements: Half-Cell Potential, undated, cites C876-15 Reopened with curl 2026-11-05; technical claims checked in the public page or document.
• S08: FHWA-RD-01-096, Long-Term Effectiveness of Cathodic Protection Systems on Highway Structures, April 2001 Reopened with curl 2026-11-05; technical claims checked in the public page or document.
• S09: Oregon DOT, Cathodic Protection Evaluation, contract PSK28885, October 2014 Reopened with curl 2026-11-05; technical claims checked in the public page or document.
• S10: FHWA InfoTechnology, Bridge: Electrical Resistivity, undated Reopened with curl 2026-11-05; technical claims checked in the public page or document.
• S11: ASTM C1152/C1152M-20, active public page, updated July 27, 2020 Public publisher scope and significance/use text reopened with curl 2026-11-05; licensed full text not accessed.
• S12: Germann Instruments, GalvaPulse product technical page, undated Reopened with curl 2026-11-05; technical claims checked in the public page or document.
• S13: NIST, Measurement of Chloride in Concrete by Prompt Gamma Neutron Activation, 2024, ACI Materials Journal 121(5) Reopened with curl 2026-11-05; technical claims checked in the public page or document.
• S15: Metrohm, Reference electrodes, leaflet 8.109.8050EN, 2021-10-29, Table 1 p. 2 Reopened with curl 2026-11-05; technical claims checked in the public page or document.
• S16: OSHA FS 3630, Handheld and Stand-Mounted Drills, 2017; 29 CFR 1926.1153 Table 1 Curl attempted 2026-11-05 returned HTTP 403; PDF and current regulation text reopened through web access the same day.
• S17: PTI DC80.2-10, Guide for Creating Openings and Penetrations in Existing Slabs with Unbonded Post-Tensioning, 2010 Public publisher abstract reopened with curl 2026-11-05; full guide not accessed.