Porosity, permeability, corrosion, carbonation

Porosity

Porosity is the ratio of the volume of voids (gel pores, capillary pores, air voids) to the total volume of the material, expressed in percent. It affects strength and the amount of water that can be absorbed. Pore sizes: gel pores (1 to 3 nm), capillary pores (10 nm to 10 µm), air voids (above 50 µm).

Permeability

Permeability is the ease of fluid flow through the material under pressure ($q/A = K\,\Delta h/L$). It depends on the connection and size of the pores, not only on their volume.

 same porosity            different permeability
 +--------------+         +--------------+
 | o  o   o  o  |  low    | o=o=o   o=o  |  high
 |  o   o   o   |  perm.  |  \\ ||  //   |  perm.
 | o   o   o  o |         | o=o=o=o==o   |
 +--------------+         +--------------+
 isolated pores            connected pores

Corrosion of steel

Steel in sound concrete is protected by a thin passive oxide film, formed in the highly alkaline pore solution (pH 12.5 to 13.5). If the film is destroyed (by carbonation reducing pH below about 9, or by chlorides above a threshold of about 0.4% by weight of cement), the steel can corrode when oxygen and moisture are available. The bar surface behaves like a small battery:

   concrete (electrolyte, pore water)
     anode               cathode
   Fe -> Fe2+ + 2e-    O2 + 2H2O + 4e- -> 4OH-
 ===[ steel bar ]==============[ steel bar ]===
         \-- e- flow in the steel -->--/
        Fe2+ ------- ion flow ------- OH-

1. Anode (corroding area): $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$
2. Cathode (oxygen-rich area): $\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$
3. The ions react: $\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}_2$, which oxidises to $\text{Fe(OH)}_3$ and finally to hydrated oxide $\text{Fe}_2\text{O}_3\cdot n\text{H}_2\text{O}$ (red rust).

The rust has 2 to 6 times the volume of the steel it replaces. Moisture, oxygen and a conductive path are needed; hence corrosion is high in wetting-drying and coastal zones.

Carbonation

Carbonation is the reaction of atmospheric carbon dioxide with the hydrated cement compounds, mainly calcium hydroxide, in the presence of moisture:

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$$

• The reaction begins at the surface and moves inwards; the depth follows $d = K\sqrt{t}$ ($t$ in years).
• The pH of the pore solution falls from about 12.5 to 13.5 to about 8 to 9, which destroys the passive film on the steel and starts corrosion once the carbonation front reaches the bar.
• The rate is highest at a relative humidity of 50 to 70% (dry concrete has no water; saturated concrete blocks CO$_2$ entry). It increases with high w/c, low cement content, poor curing and cracks, and high CO$_2$ concentration (cities).
• It is detected with a phenolphthalein spray: the uncarbonated concrete (pH above 9.2) turns pink, while carbonated concrete stays colourless.
• It causes some shrinkage (carbonation shrinkage), and slightly raises surface hardness and strength by filling pores.
• Prevent it by low w/c, enough cement, proper curing, adequate cover (IS 456 Table 16), and protective coatings.

Effect on durability: carbonation and corrosion together shorten the service life. The life of the member is the initiation time (until the carbonation front reaches the bar) plus the propagation time (rusting, then cracking and spalling of the cover and loss of steel section).