Reinforced concrete (RC) comprises of steel reinforcement bars (rebars) embedded in a concrete mass. Rebars carries the bulk of the tensile load and imparts a degree of cracking resistance to the concrete which itself is compressively loaded. Steel in concrete is normally in a non-corroding, passive condition. Non-corroding, passive conditions are not always achieved due to which corrosion of rebars takes place. Corrosion of rebars has physical consequences such as decreased ultimate strength and serviceability of the concrete structures. The damage which happens from corrosion can to a large extent reduce the serviceability and structural integrity of the RC
Whenever the reinforcement bar embedded in steel corrodes, the corrosion products increase its volume. All forms of iron oxide and hydro-oxide have specific volumes which is greater than that of the steel. The expansive forces generated by the steel corrosion leads to tensile cracking and rust staining of the concrete which causes reduction in the serviceability and structural integrity of concrete besides affecting its aesthetics. Once the corrosion starts, it is only a matter of time before a cumulative amount of damage occurs to the concrete structure and it fails well before its design life.
The two main causes of the corrosion of the rebars in concrete are (i) localized failure of the passive film on the steel by the chloride ions and (ii) general failure of the passivity by neutralization of the concrete alkalinity due to the reaction with atmospheric Carbon dioxide
The main factors affecting the corrosion of rebars in RC are (i) loss of alkalinity due to carbonation, (ii) loss of alkalinity due to chlorides, (iii) cracks in the concrete because of mechanical loading, (iv) stray currents, (v) atmospheric pollution, (vi) moisture pathways, (vii) water-cement ratio, (viii) low tensile strength of the concrete, (ix) electrical contact with dissimilar metals, and (x) corrosion due to difference in environments.
Corrosion Mechanism
There are numerous reasons for the corrosion enhancing non-uniformity like it can be honeycombed, porous, and unevenly wet and dry. Cracking causes differences in steel stress, differential aeration, and depositions of salt. There are always inherent non-uniformities in the rebar itself due to initial locked-in residual stresses and the manufacturing processes. As a result, regions of lower potential become anodic and regions of higher potential become cathodic. Moist concrete acts as the electrolyte, the action of which is further accelerated if salt ions exist. Corrosion in reinforced concrete normally falls under two general groups namely (i) cracked concrete, and (ii) uncracked concrete.
In case of fresh uncracked concrete normally there is ample resistance to corrosive attack. The concrete cover over the rebar is very effective in inhibiting the penetration of corrosive agents to the level of the steel. It is obvious that the thicker and denser the concrete cover the more effective it becomes in resisting corrosion. The pH number is an index of the acidity or alkalinity of a medium. Fresh concrete has a high Calcium Hydroxide content which gives it a pH of around 13.
However, as time passes the above conditions tend to alter. Water, Salt, Oxygen, Carbon Dioxide, and industrial gases (if present) slowly begin penetrating the concrete, the rate of which depends on the permeability of the concrete cover. Carbon Dioxide, which penetrates into concrete through pores and cracks, reacts with Calcium Hydroxide and produces Calcium Carbonate. Thus, both the pH value and the protective quality of concrete are reduced.
Preventive Measures
To reduce and prevent the corrosion of reinforcement steel bars in concrete several methods are employed. Some are related to the making of concrete while the others are related to the quality, composition and coating of steel used in the making of reinforcement of bars. Coatings employed on the rebars are (i) hot dip galvanizing, (ii) fusion bonded epoxy coating, and (iii) stainless steel cladding. Reinforcement bars of stainless steels are also being used. Coatings suffer from the disadvantage since coatings can be physically damaged or electro-chemically penetrated so that the base steel is again vulnerable to the usual corrosion process. Steel rebars of special composition to resist corrosion have also been tried. Several steel plants have experimented with various compositions of the weathering steels. However after extensive testing, it has been found that there is consistently poor performance of weathering steels when buried. Hence, the production of steel rebars has been abandoned by most of the producers.
