Water, permeability and durability; corrosion remedy

Water, permeability and durability

Water is the vehicle for almost all deterioration processes: it carries aggressive ions (chlorides, sulphates), enables carbonation and corrosion, and freezes in pores. The mixing water decides the pore system: water in excess of that needed for hydration (w/c above about 0.38) is left as capillary pores, which become interconnected, so permeability increases rapidly with w/c (e.g. between w/c 0.4 and 0.7 it may rise by several hundred times). Higher permeability means faster ingress and less durable concrete. Hence IS 456 (Table 5) limits w/c to 0.40 to 0.55 by exposure, and requires full compaction and curing.

Corrosion of steel

Steel is protected by a passive film in the alkaline concrete (pH above 12.5). Chlorides or carbonation break this film down. With moisture and oxygen, an electrochemical cell forms: anode $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$, cathode $\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$. The product is rust of 2 to 6 times the volume of the steel, which cracks and spalls the cover and reduces the bar area.

Remedial measures

• Prevent: low w/c, enough cement, adequate cover (Table 16), limited chloride, PPC, coatings, epoxy-coated or galvanised bars, inhibitors, and crack control.
• Repair: remove the cracked cover, clean the bars to bright metal, apply an anti-corrosive primer, patch with polymer-modified mortar or micro-concrete, and apply a protective coating, or use cathodic protection.