Codes referenced
50 mm disc, 60 V DC for 6 h, total charge in coulombs; Table 1 acceptance criteria.
Non-steady-state migration; measures penetration depth, derives D in 10⁻¹² m²/s.
Indian equivalent for water-side durability index; depth-of-penetration method.
ACI 222R — corrosion of metals in concrete; ACI 357R — offshore HPC guidance.
The RCPT cell — what is being measured
A 50 mm thick × 100 mm diameter concrete disc — cut from a cast cylinder or core, conditioned and vacuum-saturated — is clamped between two acrylic cells. One cell holds 3.0% NaCl solution (cathode side); the other holds 0.3 N NaOH (anode side). A 60 V DC potential is applied across the disc for 6 hours, with current logged every 30 minutes.
The total charge passed (Q, in coulombs) is the integral of the current-vs-time curve. ASTM C1202 explicitly notes this measures all ionic conductivity — not Cl⁻ specifically — and includes heating artefacts. The test is widely accepted as a comparative ranking of concrete permeability: lower Q means lower ionic transport, which empirically correlates to chloride resistance.
Charge calculation — ASTM C1202 §11.1
where I₀ is current (amps) at start, I₃₀ at 30 min, ..., I₃₆₀ at 360 min, and 900 is half the 30-minute reading interval in seconds (30 × 60 ÷ 2), the trapezoidal-rule factor. Read the current every 30 minutes through the 360-minute run (Cl 10.7) and integrate by the trapezoidal rule. Result expressed in coulombs (amp-seconds).
A typical OPC sample: the current opens at about 0.30 A and falls to about 0.10 A by 6 h, an average of roughly 0.20 A, so Q ≈ 21,600 × 0.20 = 4,300 C — the moderate band. A typical HPC with silica fume opens near 0.09 A and falls toward 0.02 A, an average of about 0.055 A, so Q ≈ 1,200 C. UHPC (130-180 MPa, w/c 0.18, 9% silica fume + steel fibres) integrates below 100 C.
ASTM C1202 Table X1.1 — chloride ion penetrability
| Charge passed (C) | Cl⁻ ion penetrability | Typical concrete |
|---|---|---|
| > 4000 | High | OPC, w/c 0.55-0.60, no SCM |
| 2000-4000 | Moderate | M30-M40 OPC, w/c 0.45-0.50 |
| 1000-2000 | Low | M40 OPC w/c 0.40, or 30% fly ash, or 50% GGBS |
| 100-1000 | Very low | HPC: w/c 0.35, 5-10% silica fume, or 70% GGBS |
| < 100 | Negligible | UHPC, polymer-impregnated, polymer concrete |
Typical specifications: marine bridges ≤ 2000 C; HPC piles in splash zone ≤ 1000 C; UHPC overlay / w/c 0.18 ≤ 100 C; parking deck ≤ 3500 C; sewer infrastructure ≤ 1500 C.
Limitations of RCPT
60 V across 50 mm drives significant current — sample temperature rises 5-8 °C during the 6-hour run. Higher temperature increases ion mobility, so the test slightly over-reads for low-permeability HPC.
Measures conductivity of all pore solution ions (Na⁺, K⁺, Ca²⁺, OH⁻, SO₄²⁻, Cl⁻). Admixture chemistry (lignosulphonate accelerator residues) can shift Q by ±15% without any change in chloride permeability.
Side surface of disc between cells must be sealed with epoxy; if any cell solution leaks past the seal, current goes around the disc — false-low Q if cells short-circuit, false-high if NaCl bypasses the disc.
ASTM C1202 acknowledges the test is comparative (rank-order), not predictive. Specifications should be set per project, not as blanket '≤ 1000 C' for every exposure class.
The shape of the curve matters more than the number
A charged-with-water OPC concrete has a slowly rising current (steady-state 30-50 mA throughout) — total Q of 3000-5000 C. A HPC with silica fume has a falling curve: 8-10 mA start, dropping to 3-5 mA — total Q 600-1200 C. UHPC has a flat curve: 1-2 mA throughout — total Q < 100 C. The curve shape itself tells you which mechanism dominates; do not be satisfied with one number. A HPC concrete with 800 C total but a flat profile (1-1.5 mA throughout) is fundamentally different from one with 800 C and a steep tail (initial low, sudden rise at 4 h) — the latter suggests cover-zone cracking or saturation defects.
Source: ASTM C1202 §11.1 current integration, §12 reporting requirements, Table X1.1 chloride ion penetrability.
Worked example — four concrete types, one RCPT day
| Concrete | w/c | Charge passed (C) | Rating |
|---|---|---|---|
| M30 OPC (lab control) | 0.50 | 3 500 | High |
| M30 OPC + 30% fly ash | 0.45 | 1 800 | Low |
| M60 HPC + 7% silica fume | 0.32 | 700 | Very low — HPC pile |
| UHPC + steel fibres | 0.18 | 80 | Negligible — UHPC overlay |
The same w/c reduction that delivers 3500 → 700 C is amplified by silica fume addition (pozzolanic pore refinement); the 30% fly ash sample takes 90 days to reach its low-charge steady state. Test at 28 days for OPC; 56-90 days for SCM mixes — early testing overstates permeability and rejects good concrete.
Common deviations seen on Indian sites
ASTM C1202 §9 conditioning: vacuum-saturate the disc for 18 h, then soak in water for 18 h. Skipping vacuum saturation leaves air in pores — current runs artificially low, false pass. Always keep the disc submerged between saturation and test.
Lab temperature drift during the 6-hour test shifts ion mobility. RCPT is rated for 20-25 °C ambient. > 30 °C summer labs in India can inflate Q by 10-20%; air-conditioned testing required.
Poor power supplies sag from 60 V to 55 V during the test → current drops proportionally. Log voltage alongside current; reject any sample where voltage fell below 58 V at any point.
A core taken from a cold-joint zone, repair patch, or reinforcing congestion has artificially high Q — not representative of the bulk concrete. Document core location with photos; flag any anomalies.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how a coulomb reading and the shape of the current curve behind it resolve into a decision on whether the concrete can go into a marine structure.
Questionnaire — ask before the durability sample is cast and sent to the lab
- What charge limit applies to this element, and is it on the test request? Marine bridges want ≤ 2000 C, HPC piles in the splash zone ≤ 1000 C, a UHPC overlay ≤ 100 C, parking deck ≤ 3500 C, sewer infrastructure ≤ 1500 C.
- At what age is the specimen being tested? OPC at 28 days. An SCM mix needs 56–90 days, because early testing overstates permeability and rejects good concrete.
- Has the disc been vacuum-saturated for 18 hours and then soaked for 18 hours per ASTM C1202 §6, and will it stay submerged right up to the test?
- Is the lab running at 20–25 °C? Above 30 °C in a summer lab, Q inflates by 10–20%.
- Where in the structure was the disc taken from? A core from a cold-joint zone, a repair patch or a congested section will read artificially high.
- Is RCPT the right test for this exposure, or is chloride the only ingress mechanism we care about, in which case NT Build 492 gives us the migration coefficient directly?
- Are we getting the current-versus-time log, not just the final coulomb figure? The shape of that curve is the diagnostic, not the total.
Inspector's checklist — what to verify on site
- Specification line on the test request: the charge limit for this element and exposure, in coulombs, written down before the sample leaves site.
- Test age on the request: 28 days for OPC, 56–90 days for SCM mixes.
- Conditioning record: 18 h vacuum saturation, 18 h soak, disc submerged between saturation and test.
- Lab environment log: ambient held at 20–25 °C, with the 5–8 °C sample temperature rise during the 6-hour run noted.
- Voltage log: recorded alongside the current. Any run where the supply sagged below 58 V is rejected, not adjusted for.
- Current-versus-time log retained: the readings from I₀ through I₃₆₀, so the reported charge can be checked against the ASTM C1202 §9 integration and the curve shape reviewed.
- Core provenance: photograph and note the location, and confirm it is not a cold joint, a repair patch or a heavily congested section.
- Admixture chemistry flagged: lignosulphonate accelerator residue can shift Q by ±15% with no change in chloride permeability, so the mix admixtures go on the request too.
What happens if… three worked examples
What happens. 3 500 C lands in the 2,000–4,000 C moderate band and is 1 500 C over the ≤ 2000 C limit the bridge element was specified to. The curve is the OPC shape as well: a slowly rising current, 30–50 mA throughout, which is what a w/c 0.50 mix is expected to produce.
Likely outcome. The concrete does not meet the specified durability, and no amount of argument about the test will change that number. You go back to the mix: the same w/c reduction that took the HPC sample from 3 500 C to 700 C is what is missing here, so the fix is a lower w/c with silica fume or GGBS, and the new mix needs 56–90 days of cure before it can be tested to prove the point.
Preventive correction. Test the mix, do not argue the concrete. Charge against the specification before you commission the test, and pick the test age to match the mix.
What happens. 700 C sits in the 100–1,000 C very low band and clears the ≤ 1000 C limit with 300 C to spare. On the number, the pile is fine.
Likely outcome. You accept it, but only after the conditioning record is checked. A disc that was not vacuum-saturated leaves air in the pores, current runs low, and you get a false pass with the same 700 C on the certificate. That is exactly the failure the ASTM C1202 §6 conditioning step exists to prevent, and it is invisible in the number. Review the curve as well: 700 C with a falling profile (8–10 mA start down to 3–5 mA) is the expected HPC behaviour, while a flat 1–1.5 mA throughout means something else is going on and needs a second look.
Preventive correction. Keep the conditioning record with the result. A low charge is only meaningful if the disc was saturated.
What happens. The 30% fly ash sample takes 90 days to reach its low-charge steady state, and at 1 800 C it is still 300 C over the ≤ 1500 C limit the sewer structure was specified to. Testing that mix at 28 days instead of 56–90 days makes the reading worse, not better, so there is no testing strategy that rescues the number.
Likely outcome. The mix does not meet the specified charge, so the spec and the mix have to be reconciled before the next pour, not after the cubes fail. If the project can accept 1 800 C, that is a written change with the designer attached to it. If it cannot, you need more SCM or a lower w/c, and the new mix has to be aged 56–90 days before the charge is quoted as proof.
Preventive correction. Quote the charge the mix reaches at its steady state, at the test age the mix needs, and never at 28 days for the sake of a quicker answer.
Frequently Asked Questions
What is the practical interpretation of coulombs in RCPT?
How does RCPT differ from NT Build 492?
Does RCPT measure chloride permeability directly?
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Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in concrete technology, HPC mix design, durability testing, and quality audit across India. For RCPT witness, HPC mix qualification, or durability specification review on a marine/coastal project, contact amit@concreteinfo.in.
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