Reference card — verified IS codes
Verified 2026-09-14 against ~/Standards library + BIS portal. ConcreteInfo reference card.
Codes referenced
Drilling procedure, depth increments, sample size and labelling for chloride analysis.
Nitric-acid digestion and silver-nitrate titration (Volhard method).
Limit 0.6 kg/m³ of concrete for RCC, 0.4 kg/m³ for prestressed or steam-cured, 3.0 kg/m³ where there is no embedded metal — IS 456 Cl 8.2.5.2 and Table 7.
Acid-soluble (C1152) and water-soluble (C1218) chloride; NT Build 443 accelerated diffusion.
Why chloride profiling matters
Rebar corrosion is the leading cause of premature deterioration of marine/coastal RCC structures. Chloride depassivates the protective oxide layer on steel — once free Cl⁻ at the rebar surface exceeds a threshold (~0.4% acid-soluble by mass of cement), pitting corrosion initiates even in alkaline, uncarbonated concrete. Corrosion occupies 5-7× the volume of parent steel; the resulting tensile stress fractures the cover concrete, accelerating ingress further. Chloride profiling quantifies the contamination at the rebar depth — the single most useful durability indicator for marine/coastal/de-icing-salt exposure structures. It also distinguishes active from latent risk: a flat profile below threshold means no immediate action; a rising profile crossing threshold at rebar depth means imminent damage with a measurable time-to-onset.
Sampling procedure — IS 14959 (Part 1)
Sampling is the dominant source of error. Follow IS 14959 (Pt 1):2001 step-by-step:
- Location selection — Choose representative locations: faces exposed to chloride (windward, splash, tidal), avoid cracks and corners. Note the closest rebar location and clear cover from design.
- Drill vertically or perpendicular to surface with a rotary hammer at 5-10 mm depth increments from 0 to at least cover + 20 mm (typically to 50-80 mm total depth).
- Collect ≥ 5 g powder per increment — finer mesh increases titration accuracy; sieve to pass 150 µm to discard coarse aggregate chips.
- Label each increment clearly: structure ID, location ref, depth band (e.g. 0-5 mm, 5-10 mm, 10-20 mm, 20-30 mm, 30-50 mm, 50-80 mm).
- Reject the first 5 mm of dust from each new drill bit (cross-contamination from previous increment). Keep separate disposable sub-samples if mixing suspected.
- At least 3 sample sets per face / exposure zone; duplicate per location to check within-sample variation.
Lab analysis — IS 14959 (Part 2)
Acid-soluble chloride is the total chloride (bound + free + chemically combined) extracted by digestion in dilute nitric acid:
Powder dissolved in nitric acid (1:10), boiled to break down all chloride-bearing species. Cool and filter.
Add excess silver nitrate; back-titrate with ammonium thiocyanate using ferric alum indicator. Endpoint: faint red-brown.
Express chloride as % of cement content (assume known from mix design; or measure OPC + SCMs by EDS / chemistry).
Threshold values — IS 456 Cl 8.2.3
| Reinforcement type | Max Cl⁻ (% mass of cement) | Basis |
|---|---|---|
| Reinforced cement concrete (RCC) | 0.40% | Acid-soluble, at any age |
| Prestressed concrete (PSC) | 0.20% | Acid-soluble; PSC more sensitive |
| Steam-cured PSC | 0.10% | Heat accelerates chloride binding loss |
| BS 8500 / EN 206 (Europe) | 0.20-0.40% | Class-dependent (XD2, XD3 lower) |
Critical note: the IS threshold is at the rebar surface, not the bulk concrete. Profiling tells you the chloride at cover depth, not at rebar — fit Fick's law to estimate arrival.
Interpreting the profile — Fick's 2nd law
A properly sampled chloride profile in a marine splash zone typically follows the error-function solution to Fick's 2nd law:
where C(x,t) is chloride concentration at depth x after time t, Cₛ is surface concentration, C₀ is initial (background), D is apparent diffusion coefficient (m²/s). Plot Cl⁻ vs depth; fit to extract Cₛ and D. With both known, time to reach threshold at clear cover becomes a deterministic prediction.
NT Build 443 accelerates this — expose a sample to a known chloride solution for ≥ 35 days, profile, fit, derive D. Field profiles on HPC with silica fume or GGBS typically show D = 1-5 × 10⁻¹² m²/s (vs 10-20 for OPC). Ten-fold lower D = ~three-fold longer service life under same exposure.
Threshold applies at rebar, not at the surface
Many reports quote a "peak chloride" at the surface (1-2%) as a critical number. It is not — the surface value reflects splash/wick action, and chloride at depth (especially at clear-cover depth) is what matters. IS 456 Cl 8.2.3 is interpreted at the bar location. Plot the full profile, identify concentration at clear-cover depth, fit Fick's law if a service-life estimate is needed. Acceptance during construction is set on concrete as-delivered (< 0.4% Cl⁻ per kg cement by mass) — different question, different sample. IS 14959 Pt 1 specifically governs how to drill the in-place sample; IS 14959 Pt 2 governs how to titrate.
Source: IS 456:2000 Cl 8.2.3 table 8.2.1 (chloride limits); IS 14959 (Pt 1):2001 §5 (sampling), Pt 2:2001 §6 (titration).
Worked example — 12-year-old coastal jetty pile
| Depth band (mm) | Acid-soluble Cl⁻ (% cement) | Comment |
|---|---|---|
| 0-5 | 0.20 | Surface — exposure-controlled |
| 5-10 | 0.35 | Still surface band; below IS threshold |
| 10-20 | 0.55 | Above threshold — diffusion-controlled ingress |
| 20-30 | 0.85 | Threshold exceeded significantly |
| 30-50 | 1.05 | Approaching clear-cover depth |
| 50-70 (rebar) | 1.20 | Active corrosion initiated — initiate repair |
Fick fit: Cₛ ≈ 1.8%, D ≈ 8 × 10⁻¹² m²/s. The 0.4% threshold was crossed at rebar depth around year 4. Pitting now visible on exposed bars; cover cracking imminent within 2-3 years. Recommendation: cathodic protection OR concrete removal + rebar treatment + patch repair in the splash zone.
Common deviations seen on Indian sites
Mixes the surface contamination with the diffusion-controlled zone. IS 14959 Pt 1 specifies 5-10 mm increments; finer is technically OK but produces a higher coefficient of variation per unit mass.
IS 14959 Pt 2 reports acid-soluble chloride; ASTM C1218 reports water-soluble. Quotations from mix-water QC labs are water-soluble (different, lower number). Comparing them to IS 456 Cl 8.2.3 thresholds is meaningless — always specify which is reported.
Profile without location ID, distance-to-nearest-rebar, exposure face orientation, and tide level cannot be re-tested later. Multi-year trend data requires the same locations revisited.
IS 456 threshold is by mass of cement — not by mass of concrete. SCMs (fly ash, GGBS) count partially; binder content changes with mix design. A 0.4% Cl⁻ against 350 kg/m³ cement binder ≠ 0.4% against 450 kg/m³. Use the actual binder content at the location.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how a chloride profile is turned into a corrosion-onset date rather than a single worrying number.
Questionnaire — ask before the drill goes in
- Is the sampling increment 5–10 mm per IS 14959 Pt 1, with the hole drilled vertically or perpendicular to the surface?
- Does the profile run past the bar — to at least cover + 20 mm, typically 50–80 mm of total depth?
- Are the locations representative — windward, splash and tidal faces chosen, cracks and corners avoided?
- Is at least 5 g of powder collected per increment, and sieved to pass 150 µm so coarse aggregate chips are discarded?
- Has the first 5 mm of dust been rejected from each new bit, so the previous increment does not contaminate the next?
- Is the test acid-soluble per IS 14959 Pt 2, and not the water-soluble number a mix-water QC lab reports?
- Is the actual binder content at that location known, for converting the result to % by mass of cement?
Inspector's checklist — what to verify on site and on the report
- Depth bands labelled with structure ID, location reference and increment, so the same points can be re-profiled in 2–3 years.
- Distance to the nearest rebar and the exposure-face orientation recorded at every location, with the tide level where it applies.
- At least 3 sample sets per face or exposure zone, with a duplicate at each location to check within-sample variation.
- Mass of cement used for the calculation taken from the actual mix design at that location, not from the original nominal design.
- Full profile plotted against depth, not a single peak value quoted in isolation.
- Concentration at clear-cover depth read off the profile and set against 0.4% for RCC and 0.2% for PSC.
- Half-cell potential mapping per ASTM C876 run at the suspect locations, since the profile says what the risk is and the half-cell says whether it is happening.
What happens if… three worked examples
What happens. The profile climbs from 0.20% at the surface through 0.55% at 10–20 mm and 1.05% at 30–50 mm to 1.20% in the 50–70 mm rebar band.
Likely outcome. 1.20% is three times the 0.4% RCC threshold, so corrosion has been initiated at the bar. The Fick fit gives C₀ ≈ 1.8% and D ≈ 8 × 10⁻² m²/s, and the threshold was crossed at rebar depth around year 4. Pitting is visible on the exposed bars and cover cracking is imminent within 2–3 years.
Correction. Do not wait for the cracking. Choose cathodic protection, or concrete removal with rebar treatment and patch repair in the splash zone.
What happens. The report quotes a peak of 1–2% chloride at the surface as the headline finding.
Likely outcome. The surface value reflects splash and wick action and says nothing about the bar. A pile with 1.8% at the surface and 0.20% at the bar has no active corrosion, and a report written that way would have condemned it.
Correction. Plot the full profile, read the concentration at clear-cover depth, and fit Fick's law when a service-life estimate is needed. The threshold applies at the rebar, not at the surface.
What happens. A mix-water QC lab reports water-soluble chloride, and the number is compared with the IS 456 Cl 8.2.3 limit.
Likely outcome. In hardened concrete, acid-soluble is typically 1.5–2× the water-soluble value, so the comparison is meaningless in both directions. On top of that, the binder content may be 350 kg/m³ at one location and 450 kg/m³ at another, which moves the percentage again.
Correction. Specify acid-soluble per IS 14959 Pt 2, and use the actual binder content at the location the sample came from.
Frequently Asked Questions
What is the difference between acid-soluble and water-soluble chloride?
How often should chloride profiling be repeated on a marine structure?
Can chloride profiling predict future corrosion?
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Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete durability audits, RMC plant operations, and technical training across India. For chloride profile survey, residual-life assessment or durability audit on a marine/coastal project, contact amit@concreteinfo.in.
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