The main effect of chloride ions on reinforced concrete is to cause pitting corrosion of the steel reinforcement bars (rebar). It is a surreptitious and dangerous form of localized corrosion because the rebar sections can be decreased to the point that the steel reinforcement are no longer capable to withstand to the tensile efforts they are supposed to resist by design. When the rebar sections are too small or the rebar are locally broken, the reinforcements are lost, and concrete is no longer reinforced concrete.  
Chlorides, particularly calcium chloride, have been used to shorten the setting time of concrete.
However, calcium chloride and (to a lesser extent) sodium chloride have been shown to leach calcium hydroxide and cause chemical changes in Portland cement, leading to loss of strength, as well as attacking the steel reinforcement present in most concrete. The ten-storey Queen Elizabeth hospital in Kota Kinabalu contained a high percentage of chloride causing early failure.