A review into the performance of steel reinforced concrete exposed to both carbon dioxide and chloride environments

A large majority of the durability studies focus on a single mode of deterioration mechanism, i.e., chloride ingress and subsequent corrosion of rebar or carbonation. Many structures in our otherwise moderate exposure environment suffer from both modes of deterioration. For example, an abutment of a bridge deck will be affected by de-icing salt in the form of water run-off from the road above or spray due to vehicular movement. The same region, due to vehicular movement may also be affected by a higher concentration of CO2 and therefore will carbonate. The micro and macro climate around the exposed area will decide if the two deterioration events are successive or seasonal. The reference to micro and macro climate can be elaborated as: salt spray into a dry concrete will lead to higher absorption of chlorides and if sun light dries the region it may be subjected to subsequent carbonation or further chloride ingress. For example, de-icing salts are used mostly in wet winter periods where concrete remains at a higher relative humidity and carbonation in a very wet concrete is nominal. It is known that carbonation will densify the microstructure and therefore will reduce the rate of ingress of chlorides (or any ions) at the same time carbonation can release some of the bound chlorides (Friedel's Salts) therefore accelerate the onset and rate of corrosion. It is also known that further carbonation can induce cracking in the microstructure which acts to accelerate the rate of ingress. This paper will present the data from studies that have focused on both chloride ingress and carbonation so as to outline the physical and chemical changes that occur in such concretes. This will identify if the current practice of specifying concrete, performance testing and service life modelling based on a single deterioration mechanism is appropriate for structures subjected to carbonation and chloride ingress.