Carbonation is another factor – fresh concrete is contains calcium hydroxide and is therefore an alkaline environment, which has the effect of protecting the re

Carbonation is another factor – fresh concrete is contains calcium hydroxide and is therefore an alkaline environment, which has the effect of protecting the rebar from corroding. Over time however, carbon dioxide from the atmosphere slowly seeps into the concrete (anything that's only intermittently wet fares the worst here), turning the calcium hydroxide into calcium carbonate. While this has the beneficial side effect of actually increasing the strength of the concrete itself, it also lowers its pH value, which means the rebar is no longer protected against corrosion once the carbonation process reaches the rebar.