Moisture In Concrete

Moisture In Concrete: Understanding its Effects on Coatings

Moisture, pH, and Alkalies:

Freshly placed Portland Cement has a pH level of 12.5 (highly alkaline); it will provide passivation of embedded steel.

Carbon Dioxide and ambient air react, meaning that:

• Over time, concrete pH will reach 8.5
• Carbonizing starts from the surface and moves down at a rate of 0.04″ per year
• Cabonization proceeds most quickly at 50% RH and ceases if concrete is submerged

Effects of Alkalies and Moisture:

• Alkalies can be a problem for coatings if:
  • Surface pH is too high
  • Moisture traveling up through concrete brings alkalies to the surface

Sources of Moisture:

• Weather
  • Precipitation before, during, and after concrete placement
    • Destabilize prepared sub-base
    • Saturated blotter layer can increase concrete drying time
    • Erosion of top surface of newly placed concrete
    • Creates high moisture content in concrete
• Capillary Rise
  • High water tables and small enough soil granule size can result in capillary rise of water
• Hydrostatic Pressure
  • High outside water table
• Osmosis
  • Soluble salts at the concrete surface and the presence of moisture create osmotic blistering in coatings
• Sub-slab Vapor
  • Sub-grade soil RH at 100%
  • State of equilibrium is achieved between low and high RH
• Ambient RH
  • Moisture in the air and the top surface of the concrete reach equilibrium
• Dew Point
  • Concrete surface temperature is lower than the dew point of the ambient air

Further discussion can be found in the full PDF download.

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