Stalactites and corrosion of reinforcement in concrete

For those of you brave enough to have ventured into caves, particularly the limestone rock variety, you may have observed stalactites forming on the underside of the roof to the cave. But should we really be expecting to see these curious phenomena forming on the underside of exposed concrete balconies, elevated walkways, and car park decks?

Stalactites are formed in such concrete structures from rain water leaching out the hydrated lime from the Portland cement which then reacts with the cardon dioxide in the air.

The exposed concrete structures should therefore be designed to keep rainwater out. Relatively small structures can be designed without the need to provide a damp proof membrane, as long as the concrete is of the required grade to resist significant damp ingress.

However, large open concrete structures should be protected from rainfall by a membrane placed beneath the finished surface. Without such a membrane, the finished surface above the structure may retain rainwater and the resulting hydrostatic pressure would force the water into the concrete.

Water droplets seeping down the stalactites indicates a current leak and may be combined with rust stains on the surfaces below. Rust marks may be caused by impurities within the concrete but if the damp ingress is extensive then this is probably caused by corrosion to the steel reinforcement. This has serious connotations for the structural integrity of the concrete structure and an investigation is required using an electrical potential half-cell test; this will indicate the probability of reinforcement corrosion having taken place.

For more information on such tests, click here

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