🧱 Chloride content in concrete

Construction PN-EN 14629

Overview

Chlorides in concrete are the main cause of steel reinforcement corrosion in reinforced concrete structures, leading to pitting corrosion and loss of load-bearing capacity. Sources of chlorides are: de-icing salts on roads and bridges, seawater (coastal structures), chlorides in aggregate or mixing water, and industrial environments.

Standard PN-EN 14629 describes methods for determining total acid-soluble chloride content in hardened concrete. Result is expressed as percentage of chloride mass relative to cement or concrete mass. Critical value initiating reinforcement corrosion is usually 0.2–0.4% Cl⁻ of cement mass (according to PN-EN 206 — max. 0.20–0.40% depending on exposure class).

The standard allows two titration techniques: potentiometric (with silver electrode as indicator) and Volhard (back titration of excess AgNO₃ with potassium thiocyanate with iron indicator). Both give comparable results with difference max. 0.005% Cl⁻. Potentiometric titration is preferred due to objectivity of endpoint detection.

The test is performed in structure diagnostics (bridges, parking lots, wharves), corrosion risk assessment, repair design and scientific research on concrete durability.

Method principle

Sample of powdered concrete (obtained by core drilling or milling) is dissolved in hot nitric acid, which releases bound and free chlorides. After filtration, solution is titrated: (1) potentiometrically — adding AgNO₃ solution and recording silver electrode potential vs. reference electrode — equivalence point corresponds to sudden potential change; (2) by Volhard method — adding excess AgNO₃ (chlorides precipitate as AgCl), then titrating excess AgNO₃ with KSCN solution with Fe³⁺ indicator (red color = endpoint). Chloride content is calculated from titrant consumption and converted to % cement or concrete mass.

Applications

• Corrosion diagnostics of bridges, viaducts and road flyovers
• Assessment of multi-level parking structures (de-icing salts)
• Testing of coastal and marine structures (seawater chlorides)
• Chloride profiling (concentration vs. depth) — prognosis of time to corrosion initiation
• Quality control of ready-mix concrete — Cl⁻ content in components
• Construction expert opinions — assessment of existing reinforced concrete structures
• Scientific research on chloride diffusion and corrosion inhibitors

Key parameters

Standard

Standard number

PN-EN 14629:2007

Title (PL)

Wyroby i systemy do ochrony i napraw konstrukcji betonowych — Metody badań — Oznaczanie zawartości chlorków w betonie

Title (EN)

Products and systems for the protection and repair of concrete structures — Test methods — Determination of chloride content in hardened concrete

Step-by-step procedure

1. Concrete sample collection

   Core drilling ∅20–50 mm or profile milling of layers every 5–10 mm. Collect concrete powder min. 20 g from each depth.

   ⏱ Time: 30–60 min

2. Drying and grinding

   Dry powder at 105°C / 2 h. Grind in mill to fraction <0.125 mm. Sieve through 0.125 mm sieve.

   ⏱ Time: 3 h • 🌡 Temperature: 105°C

3. Sample weighing

   Weigh 5.000±0.001 g powdered concrete on analytical balance. Transfer to 250 mL beaker.

   ⏱ Time: 5 min

4. Acid dissolution

   Add 25 mL deionized water and 10 mL HNO₃ (1:1). Heat to boiling, boil 3 min. Cool.

   ⏱ Time: 15 min • 🌡 Temperature: ~100°C

5. Filtration

   Filter through quantitative filter (Whatman 40/42 paper) into 250 mL flask. Wash residue with hot water. Fill to mark.

   ⏱ Time: 15 min

6. Potentiometric titration

   Take 50–100 mL aliquot of filtrate. Insert Ag and reference electrodes. Titrate with AgNO₃ 0.1 mol/L. Record E = f(V) curves. Read equivalence point.

   ⏱ Time: 10–15 min

7. Alternative: Volhard titration

   To aliquot add excess AgNO₃ (e.g. 25 mL). Precipitate AgCl, filter. Titrate filtrate with KSCN with Fe³⁺ until stable pink color.

   ⏱ Time: 20 min

8. Calculations

   Cl⁻ [%] = (V_AgNO₃ × c × 35.45 × 100) / (m × 1000 × aliquot/250). Convert to % cement mass (if cement content known).

   ⏱ Time: 5 min

9. Quality control

   Analysis of blank, duplicate, CRM sample with certified Cl⁻ content. Recovery 95–105%.

   ⏱ Time: 30 min

10. Interpretation and report

    Compare result with critical values PN-EN 206 (0.20–0.40% cement mass). Prepare chloride profile (concentration vs. depth).

    ⏱ Time: 30 min

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Nitric acid 65% — highly corrosive! Acid-resistant gloves, goggles, fume hood during dissolution
• Silver nitrate — stains skin (black spots), sensitizing, eye irritant
• Potassium thiocyanate — toxic when ingested, irritating
• Concrete dust — P2 mask when grinding and sieving (crystalline silica)
• Core drill — risk of vibration and noise, personal protective equipment

Frequently asked questions