Why does this test exist?

Steel reinforcement in concrete is protected by the high alkalinity (pH of about 12.5 or more) of the cement paste, which forms a thin passive film on the bar. Chloride ions, coming from sea water, deicing salts, contaminated aggregates or groundwater, can break down this film. Once enough chloride reaches the bar, the steel corrodes, the rust expands, and the concrete cracks and spalls.

So the real question for durability is: how quickly can chloride travel through this concrete to reach the steel? The traditional answer is a ponding test (AASHTO T259), where salt solution sits on a slab for 90 days or more. That is accurate but slow. RCPT gives a quick indication in about one day of testing (plus specimen preparation).

The key idea in one line RCPT does not measure chloride penetration directly. It measures how much electrical charge (in coulombs) passes through a concrete disc in 6 hours. More charge means the pore network is more open and connected, so ions move easily and the concrete is more penetrable.

The theory behind the test

Concrete is a porous material

Hardened cement paste contains capillary pores and gel pores. When the concrete is saturated, these pores are filled with a salt-rich water called pore solution. This pore solution contains dissolved ions such as Na⁺, K⁺, OH⁻ and Ca²⁺. A dense concrete (low w/c ratio, good compaction, supplementary cementitious materials, proper curing) has fewer, smaller and less connected pores. A poor concrete has a wide-open, well-connected network.

Ions can carry electric current

Concrete is not a metal. It does not carry current with electrons. Current passes through the saturated concrete only because ions move through the pore solution when a voltage is applied. Think of the pore network as a bundle of tiny wet pathways:

• Open, connected pores mean easy ion movement, high current, high charge passed.
• Fine, disconnected pores mean difficult ion movement, low current, low charge passed.

What the setup does

The concrete disc is placed between two chambers. Both faces are in contact with solution, and 60 V DC is applied across it:

Side	Solution	Connected to
Top surface of specimen	3.0% NaCl (sodium chloride)	Negative terminal (−)
Bottom surface of specimen	0.3 N NaOH (sodium hydroxide)	Positive terminal (+)

Negatively charged chloride ions are pushed by the electric field from the NaCl side toward the positive electrode, so they are driven through the concrete. A dense concrete resists this migration and lets little current through. The applied voltage speeds up the movement that would take months in natural ponding, which is why the test is rapid.

Why charge (coulombs) is the result

Current is the rate of flow of charge. If you add up the current over time you get the total charge:

Q = ∫ I dt     (coulombs = ampere × seconds)

Q is the area under the current-versus-time curve over the 6 hours. This single number represents the overall electrical conductance of the concrete during the test, and it is the value used to rate the concrete.

An important catch: current is carried by all ions

The meter cannot tell which ions are carrying the current. It measures all of them: chloride, but also hydroxide, sodium, potassium and others already in the pore solution. So RCPT is really a test of the electrical conductivity of the concrete. It correlates well with chloride penetration for many ordinary concretes, but not for all (see Section 8).

Heating matters A current of 0.1 A at 60 V is about 6 W of heat inside a small specimen. Heat raises the temperature of the pore solution, which increases conductivity, which raises the current further. This is why the test must start at 20 to 25 °C and why the solution temperature must never exceed 90 °C. Very porous concrete can heat up quickly.

Test at a glance

Item	Requirement (ASTM C1202-22)
Specimen	100 mm nominal diameter (95 to 100 mm), 50 ± 3 mm thick
Number of specimens	At least 2 for quality control or acceptance
Applied voltage	60.0 ± 0.1 V DC
Duration	6 hours
Current readings	At least every 30 minutes (and initial reading at start)
Start temperature	Specimen, cell and solutions at 20 to 25 °C
Negative side (top face)	3.0% NaCl by mass
Positive side	0.3 N NaOH
Result	Total charge passed in coulombs, corrected to 95 mm diameter and 50 mm length

Apparatus and materials

• Vacuum saturation set-up: vacuum desiccator, vacuum pump or aspirator able to hold less than 50 mm Hg absolute (6650 Pa), vacuum gauge, separatory funnel (500 mL or larger), beaker (1000 mL or larger). Protect the pump with a water trap or change the pump oil after each use.
• Applied voltage cell: two symmetric poly(methyl methacrylate) (Plexiglas) chambers, each with a brass mesh (850 µm, No. 20) and connectors. Dimensions are fixed by Fig. 1 of the standard.
• Power supply and readout: 0 to 80 V DC constant voltage supply that holds 60 ± 0.1 V at all currents; voltmeter and a shunt resistor (or a device that reads current to ± 1 mA).
• Sealants and coating: rapid-setting, electrically non-conductive coating for the curved side surface; silicone or rubber sealant (or a rubber gasket 100 mm OD × 75 mm ID × 6 mm thick) to seal specimen to cell.
• Cutting and measuring: water-cooled diamond saw or silicon carbide saw, belt sander for burrs, jaw caliper.
• Optional: thermocouple (0 to 120 °C) to monitor solution temperature.

Safety NaOH can cause very severe burns and eye injury. Use a full-face shield, rubber apron and NaOH-resistant gloves (check gloves for pinholes). Dissolving NaOH also generates heat, so allow the solution to return to room temperature before use.

Specimen preparation

Where specimens come from

• For evaluating a mix: 100 mm cast cylinders (made as per ASTM C192 in the lab or C31 in the field), or cores from test slabs.
• For evaluating an actual structure: 100 mm cores drilled with a diamond-dressed core bit as per ASTM C42.
• The standard notes that repeatability is satisfactory for aggregate up to 25 mm nominal maximum size.

Cutting the test slice

Cut a 50 ± 3 mm slice from the top of the core or cylinder, with the cut parallel to the top face. Remove burrs with a belt sander. If the surface has a sealer, curing compound or texture and you do not want to include its effect, remove that portion and use the adjacent 50 ± 3 mm slice. Measure two diameters at right angles at mid-height and average them to the nearest 1 mm. Measure two lengths at right angles and average them to the nearest 1 mm.

Curing and age of testing

Age and curing strongly affect the result, because concrete becomes less permeable as hydration continues. When testing cast specimens for quality control or acceptance, use one of these methods (and the same method when comparing mixes):

Curing method	Duration	When to use
Moist curing	28 days	Concrete with portland cement only
Extended moist curing	56 days	Concrete with supplementary cementitious materials (fly ash, slag, silica fume), because they hydrate slowly
Accelerated moist curing	7 days moist, then 21 days in lime-saturated water at 38 ± 2 °C	When an earlier indication is needed and it is specified; may not give the same results as 56-day curing

Concrete with supplementary cementitious materials can keep improving beyond 56 days, so testing at later ages such as three months may be appropriate.

Test procedure step by step

Step A: Coat and vacuum-saturate the specimen

The aim is to make sure every pore is completely filled with water. Air-filled pores would not conduct and would give falsely low charge.

1. Boil about a litre of tap water, cap the container tightly, and let it cool. This is the de-aerated water.
2. Let the specimen surface-dry in air for at least 1 hour.
3. Brush rapid-setting coating onto the side (curved) surface only. Let it cure until it is not sticky, fill any pinholes, and allow extra curing time. Both end faces must stay bare.
4. Place the specimen in the desiccator, seal it, and run the vacuum pump. Absolute pressure must be below 50 mm Hg (6650 Pa). Maintain for 3 hours.
5. With the pump still running, open the water stopcock and drain enough de-aerated water into the beaker to cover the specimen. Do not let air enter.
6. Close the water stopcock and let the pump run for 1 more hour.
7. Close the vacuum line stopcock, switch off the pump, then open the line to let air back in.
8. Leave the specimen soaking in the same water for 18 ± 2 hours.

Step B: Mount the specimen in the cell

1. Remove the specimen, blot off excess water and keep it in a sealed container at 95% relative humidity or higher until mounting.
2. Seal the specimen between the two cell halves, either with silicone or rubber sealant around the edge (taking care that it does not block the mesh) or with the rubber gasket clamped between the halves. The conducting solutions must touch the entire end faces.
3. If using sealant, cover the exposed face with rubber or plastic sheeting, insert a rubber stopper in the filling hole to limit moisture loss, and let the sealant cure as the manufacturer specifies.

Step C: Fill, connect and run

1. Fill the side containing the top surface of the specimen with 3.0% NaCl. This side connects to the negative terminal.
2. Fill the other side with 0.3 N NaOH. This side connects to the positive terminal.
3. Connect lead wires to the banana posts and wire the power supply and readout.
4. Check that the specimen, cell and solutions are at 20 to 25 °C. Switch on, set 60.0 ± 0.1 V, and record the initial current.
5. Record current at least every 30 minutes for 6 hours. Keep both chambers full throughout. Keep the surrounding air at 20 to 25 °C.
6. At 6 hours, stop the test. Remove the specimen, rinse the cell in tap water and discard the old sealant.

If the cell gets too hot If the solution temperature approaches 90 °C (only likely for very porous concrete), stop the test early. Report the time of termination and rate the concrete as having very high chloride ion penetrability.

Calculating and reporting the result

Total charge by the trapezoidal rule

Plot current (A) against time (s) and find the area under the curve. With readings every 30 min (1800 s), the trapezoidal rule simplifies to:

Q = 900 (I₀ + 2I₃₀ + 2I₆₀ + … + 2I₃₃₀ + I₃₆₀)

Here Q is in coulombs and each I is in amperes (not milliamperes). The factor 900 comes from 1800 s ÷ 2 for the trapezoid. The first and last readings are counted once and all the middle ones twice.

Correcting for specimen size

A thicker specimen offers more resistance, and a wider one offers more area for current. To compare results fairly, convert the charge to that of a standard 95 mm diameter, 50 mm long specimen:

Qs = Qxy × (95 / x)² × (y / 50)

where x is the actual diameter (mm) and y is the actual length (mm). Note that the standard’s printed definition of the standard length says 50 µm, but the equation uses 50 mm.

Worked example

A 100 mm diameter specimen, 48 mm long, gave the following current readings (these are illustrative numbers):

Time (min)	Current (mA)	Multiplier in formula
0	100	1
30	105	2
60	110	2
90	112	2
120	114	2
150	115	2
180	116	2
210	116	2
240	115	2
270	114	2
300	113	2
330	112	2
360	111	1

Sum = I₀ + I₃₆₀ + 2 × (all middle readings) = 2.695 A

Q = 900 × 2.695 = 2426 coulombs

Qs = 2426 × (95/100)² × (48/50) = 2101 coulombs

Result: about 2100 coulombs, so chloride ion penetrability is rated Moderate (see the table below).

What to include in the report

• Source and identification of the core or cylinder, and where the specimen sat within it.
• Type of concrete: binder type, water-cement ratio and other data supplied.
• Description of the specimen: reinforcement, overlay or surface treatment, if present.
• Curing history (moist, extended moist or accelerated) and any unusual preparation.
• Average diameter and average length to the nearest 1 mm.
• Total charge passed (corrected as per 7.2) for each specimen, and the average.

Interpreting the result

Charge passed (coulombs)	Chloride ion penetrability	Practical meaning
Greater than 4,000	High	Open pore system, typically high w/c ratio, poor concrete
2,000 to 4,000	Moderate	Typical of ordinary concrete
1,000 to 2,000	Low	Good quality, low w/c concrete
100 to 1,000	Very low	Dense concrete, often with SCMs or silica fume
Less than 100	Negligible	Very dense, such as polymer-modified concrete

The ratings come from the standard’s Table X1.1, which is guidance and not mandatory. The last column is general engineering context to help you picture each class. Acceptance limits for a project should be set in the project specification, considering the exposure condition, the test age and the variability of the test.

Limitations

• Calcium nitrite admixture: makes the concrete conduct more, so RCPT shows high coulombs, although long-term ponding tests show it resists chloride at least as well as the control. Other admixtures may do the same. If an admixture effect is suspected, confirm with ponding tests.
• Reinforcing steel: steel placed lengthwise creates a continuous electrical path between the two faces, so the test is not valid. Steel elsewhere in the specimen can also influence results.
• Surface treatments (sealers): RCPT can show low resistance while 90-day ponding tests show higher resistance. Remove the treated layer if you want to test the concrete itself.
• Field concrete already exposed to chlorides: extra ions in the pores change conductivity and skew results. The standard says not to use RCPT on such concrete for assessment.
• Age and curing: results fall as concrete matures, so always state the age and compare like with like.
• Many factors influence the result: w/cm ratio, type and amount of SCMs, polymeric or ionic admixtures, air-void system, aggregate type, compaction and curing.

Precision of the test

Type	Coefficient of variation	Two results should not differ by more than
Single operator	12.3%	34%
Multilaboratory	18.0%	51%

The average of three specimens tested in two different laboratories should not differ by more than 42%. The method has no bias because resistance to chloride penetration is defined by the test method itself. Because of this variability, testing replicate specimens is strongly advisable.

Common mistakes to avoid

• Using milliamperes in the formula instead of amperes, which gives a result 1000 times too large.
• Starting the test with warm solutions or specimen, which inflates the current.
• Incomplete vacuum saturation or air leaks in the desiccator, leaving air in the pores.
• Coating leaking onto the end faces, or sealant covering part of the mesh, which reduces the effective area.
• Letting a chamber run low on solution during the test.
• Comparing specimens of different age or curing method as if they were equivalent.
• Forgetting the size correction when the diameter or length differs from the standard.

Key takeaways

• RCPT measures electrical charge passed in 6 hours at 60 V, not chloride movement itself.
• Less charge means a denser, less connected pore system and better protection for reinforcement.
• Specimen: 50 mm thick slice of 100 mm core or cylinder, vacuum-saturated, NaCl on the negative side and NaOH on the positive side.
• Result: Q = 900 (I₀ + 2I₃₀ + … + I₃₆₀), corrected to a 95 mm × 50 mm standard specimen.
• Rating: below 100 negligible, up to 1,000 very low, up to 2,000 low, up to 4,000 moderate, above 4,000 high.
• It is a quick comparative screening tool. Know its limits, especially with admixtures, steel, sealers and chloride-contaminated concrete.

Source: ASTM C1202-22, Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.