Concrete Carbonation: Understanding the Phenomenon and Preventing It Long-Term with RD-Elastoflex

Posted on: 28/10/2025

Concrete carbonation is a natural process that occurs gradually when concrete is exposed to air.

Our RD Academy experts present a clear and structured explanation:

1. Principle of Carbonation

  • Concrete is naturally alkaline (pH ~ 12.5–13) due to calcium hydroxide [Ca(OH)₂] formed during cement hydration.
  • This high alkalinity protects the steel reinforcement by forming a passive, adherent oxide layer.
  • When carbon dioxide (CO₂) from the air penetrates the concrete (through pores and microcracks), it reacts with calcium hydroxide: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
  • This reaction consumes lime and produces calcium carbonate (CaCO₃), which lowers the pH of the concrete to around 8–9.

      2. Effects on Reinforcement

      • The passive protection of steel disappears once the pH drops below ~9.
      • If moisture and oxygen are also present, corrosion starts.
      • Corrosion leads to:
        • Formation of rust (iron oxides) occupying 2–6 times the volume of the original steel.
        • Cracking of concrete due to internal pressure.
        • Spalling of the concrete cover.
        • Reduction in steel cross-section → decreased mechanical strength and durability.

          3. Factors Influencing Carbonation

          • Permeability of concrete (linked to density and water/cement ratio).
          • Thickness of concrete cover over reinforcement.
          • Environmental conditions: CO₂ concentration, relative humidity (~50–70 % promotes carbonation), temperature.
          • Presence of cracks.

          4. Prevention Measures

          • Use dense concrete (low W/C ratio, proper curing).
          • Provide adequate cover thickness.
          • Use mineral additives (pozzolans, fly ash, etc.) to improve compactness.
          • Protect exposed surfaces (coatings, hydrophobic treatments).

            🎯 How RD-Elastoflex Prevents Carbonation

            RD-Elastoflex is a flagship coating from RD Coatings’ technical range, developed through many years of research and testing.

            Since its launch in 2006, it has been applied on millions of m² of concrete façades and structures, including bridges.

            “Concrete will always crack somewhere – the question is just where and when!”

            RD-Elastoflex forms an elastic barrier that is waterproof and impermeable to CO₂ while remaining vapour-permeable.

            Key Property

            Function

            Result

            Very low CO₂ permeability

            Blocks CO₂ penetration

            Prevents pH drop → stops carbonation

            Waterproof

            Prevents water and salt migration

            Protects concrete and reinforcement

            Vapour-permeable (breathable film)

            Allows internal moisture to escape

            Prevents blistering, peeling, and mould growth

            Outstanding elasticity (> 600 %)

            Bridges microcracks and movement

            Stops water ingress through cracks

            Water-based – solvent-free

            Safe, low-odour application

            Ideal for interior and exterior works

            UV- and weather-resistant

            Excellent long-term stability

            Maintains appearance and performance

            ⚙️ Key Technical Performances

            • Complies with EN 1504-2 (Carbonation protection – principles 1.3 & 2.2)
            • CO₂ diffusion coefficient Sd > 50 m (EN 1062-6) → excellent carbonation barrier
            • Crack-bridging ability (EN 1062-7):
              • Cyclic crack movement 0.3 mm ± 0.1 mm
              • Crack widening > 0.5 mm – Class A3
            • Capillary absorption S ≤ 0.008 [kg/(m²·h⁰˙⁵)] (EN 1062-3)
            • Water vapour permeability Sd < 4 m → breathable film (DIN 53 122-1)
            • Frost/thaw and thermal shock resistance (EN 13687-1 & -2): no visible degradation

            🧩 Typical System for Carbonated or Cracked Concrete

            1. Surface preparation: cleaning and repair of spalled areas (use EN 1504-3 mortar if required)
            2. Primer (if porous): RD-Unifix
            3. Two coats of RD-Elastoflex (approx. 0.4–0.6 kg/m² per coat)

            RD-Elastoflex is available in many colours and can be custom-tinted on request.

            💡 Summary

            RD-Elastoflex is a breathable, elastic protective membrane designed to prevent concrete carbonation and extend the lifespan of structures.

            Thanks to its CO₂ barrier, water resistance, exceptional elasticity (> 600 %) and EN 1504-2 compliance, it offers a complete, durable and environmentally friendly solution for concrete protection and renovation.