Effects of carbonation and permeability on durability

Effect of carbonation on durability

Carbonation is the reaction of atmospheric CO$_2$ with the calcium hydroxide of the cement paste:

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

• 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 reduces alkalinity, so the steel passive layer breaks down and corrosion begins; rust expansion then cracks and spalls the cover.
• Surface shrinkage cracks may form, giving access to more CO$_2$, water and chlorides.
• It slightly increases surface hardness, but this does not offset the corrosion risk.

Effect of permeability on durability

• Permeability controls how quickly water, oxygen, CO$_2$, chloride and sulphate ions enter the concrete.
• Low permeability (low w/c, good curing and compaction, supplementary cementitious materials) delays carbonation, corrosion, sulphate attack, freeze-thaw damage and leaching, and so extends service life.
• High permeability makes the concrete deteriorate quickly.

Link between them

Carbonation depth increases with the permeability of the concrete to CO$_2$: dense concrete of w/c 0.45 carbonates a few mm in 50 years, while w/c 0.7 concrete may carbonate to 30 mm or more. Both effects are therefore controlled by the same measures: w/c not more than 0.45 to 0.55, minimum cement content, full compaction, curing for 7 to 10 days and adequate cover (IS 456 Tables 4, 5 and 16).