Corrosion of steel reinforcement bars in concrete
Corrosion of steel reinforcement bars in concrete
Reinforced concrete is a composite material comprising steel reinforcement bars embedded in a concrete mass. Reinforcement bars carries the bulk of the tensile load and imparts a degree of cracking resistance to the concrete which itself is compressively loaded. The corrosion of steel reinforcement bars in concrete is a big universal problem. The damage which happens from corrosion may due to a large extent reduce the serviceability and structural integrity of reinforced concrete.
Concrete provides stable long term corrosion protection to steel reinforcement bars because of passivation of the surface of steel by the highly alkaline property of the concrete. As long as this passivity is maintained, the corrosion of the reinforcement bars will not take place. For this the concrete should be sufficiently dense and impermeable so as not to allow transport of chlorides, carbon di oxide and oxygen. The passivation of steel reinforcement bars is deteriorates greatly due to chlorides even at high pH values. The carbon oxide in solution neutralizes the calcium hydro oxide saturated pore water (carbonation process) and lowers the pH value below the pH value at which the steel can remain passivated. Oxygen also plays a vital role in the passivation process.
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 greater than that of steel. The volume of the corrosion products vary by a factor which can be even more than 5 as indicated in Fig. 1. The expansive forces generated by the steel corrosion leads to tensile cracking and rust staining of the concrete. This in turn 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. Corrosion induced damage to the concrete structure necessitates an early repair or in some cases complete replacement of the concrete structure.
Fig 1 Specific volumes of the corrosion products of iron
Type of corrosion
There are two types of corrosion observed in the reinforcement bars embedded in concrete. These are given below.
- Crevice corrosion – It is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded area). Oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal. Oxygen content of the liquid at the mouth of the crevice which is exposed to air is greater. Hence a local cell is formed in which the anode (Area under attack) is the surface in contact with the oxygen depleted liquid.
- Pitting – Theories of passivity fall into two general categories, first one is based on adsorption and the other one is based on presence of a thin oxide film. Pitting in the first case arises as detrimental or activator species, such as Cl ion, compete with O2 or OH ion at specific surface sites. By the oxide film theory, detrimental species become incorporated into the passive film, leading to its local dissolution or to development of conductive paths. Once initiated, pits propagate auto-catalytically according to the generalized reaction, M?? + nH2O + nCl- = M(OH)? + nHCl, resulting in acidification of the active region and corrosion at an accelerated rate. (M?? and M are the ionic and metallic forms of the corroding metal).
Reasons for corrosion
The two main causes of the corrosion of the reinforcement bars 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 due to the reaction with atmospheric CO2. The following are the main factors affecting the corrosion of reinforcement bars in reinforced concrete.
- Loss of alkalinity due to carbonation – If steel surface is left unprotected in the atmosphere then a brown oxide rust forms and continue to grow till the scale flakes from the surface. This corrosion process can be prevented by surrounding the steel with an alkaline environment having a pH value in a range between 9.5 and 13. A passive film is formed on the steel at this pH value. This film reduces the rate of corrosion to a very low and harmless value. Thus the concrete cover provides chemical as well as physical protection to the steel. However the alkalinity can be lost as a result of i) Reaction with acidic gases (such as CO2) in the atmosphere and ii) Leaching by water from the surface. Concrete is permeable and it allows the slow ingress of the acidic gases from the atmosphere which react with the alkalis (usually Ca(OH)2, NaOH and KOH), neutralizing them by forming carbonates and sulphates, and at the same time reducing the pH value. If the carbonated front penetrates sufficiently deep into the concrete to intersect with the concrete reinforcement interface, protection is lost. The reinforcement bars start corroding due to the presence of both oxygen and moisture. The extent of the advance of the carbonation front depends, to a considerable extent, on the porosity and permeability of the concrete and on the conditions of the exposure. The carbonation reaction consumes alkalinity and reduces the pH value to a level of 8-9. This reaction is as follows
Ca(OH)2 + CO2 = CaCO3 + H2O
- Loss of alkalinity due to chlorides – The passivity provided in the concrete by the alkalinity is locally destroyed by the presence of chloride ions. The chloride ion can locally de-passivate the metal and can promote active metal dissolution. Chlorides react with calcium aluminate and calcium alumino-ferrite in the concrete to form insoluble calcium chloro-aluminates and calcium chloro-ferrites. In these compounds chloride is bound in non-active form. Generally this reaction is never complete and some active soluble chloride always remains in equilibrium in the aqueous phase in the concrete. It is this chloride in solution that is free to promote corrosion of the steel. At low levels of chloride in the aqueous phase, the rate of corrosion is very low, but higher concentration of chlorides increases the risk of corrosion.
- Cracks in the concrete because of mechanical loading – Cracks in concrete are formed because of tensile loading, shrinkage or other factors. These cracks can also allow the ingress of the atmosphere and can provide a front for the development of the carbonation reaction. If the crack penetrates to the interface of steel then the protection is lost. The tensile loading causes debonding of steel and concrete to some extent on each side of the crack. It removes the alkaline environment and destroys the protection in the vicinity of the debonding.
- Stray currents – Stray currents can arise from railway traction, cathodic protection systems, or high voltage power lines etc. Stray currents induce corrosion on buried metal structures leading to severe localized attack. They may find a low resistance path by flowing through metallic structures buried in the soil (pipelines, tanks, industrial and marine structures). A cathodic reaction (oxygen reduction or hydrogen evolution) may take place where the current enters the buried structure, while an anodic reaction (metal dissolution) occurs where the current returns to the original path through the soil. Metal loss results at the anodic points, where the current leaves the structure; usually, the attack is extremely localized and can have dramatic consequences especially on the steel in buried concrete.
- Corrosion of reinforcement steel due to atmospheric pollution – During the transportation and during storage at the construction site before their installation in the concrete structures, reinforcement steels are exposed to the atmosphere. At any of these stages, reinforcement steel can be contaminated by chloride ions. This fact leads to the formation of corrosion products on their surface.
- Moisture pathways – If the surface of the concrete is subject to long term wetting due to water accumulation then the water may reach the level of steel reinforcement either through diffusion through the porous structure of the concrete or by traveling along cracks in the concrete. Concrete roofs are meant to be protected from moisture. However, the presence of moisture on roofing systems can be result of failure of the roofing membrane, poor detailing of drainage facilities or lack of maintenance of drainage facilities.
- Water-Cement Ratio – High water- cement ratio can be a reason for corrosion of reinforcement steels. Concrete placed with a high water-cement ratio is more porous due to the presence of excess water in the plastic concrete. The porosity increases the rate of diffusion of water and electrolytes through the concrete and makes the concrete more susceptible to cracking.
- Low tensile strength of concrete – Concrete with low tensile strength facilitates corrosion damage of the reinforcement steel in two steps. In the first step the concrete develops tension or shrinkage cracks more easily and thus admits moisture and oxygen, and in some cases chlorides to the level of the reinforcement. In the second step the concrete becomes more susceptible due to the developing cracks at the point with the result that the reinforcement begins to corrode.
- Electrical contact with dissimilar metals – Dissimilar metals in contact initiate a flow of electrons that promotes the corrosion of one or the other, by a process known as galvanic corrosion. When two dissimilar metals are in contact with each other the more active metal (lower on the list) will induce corrosion on the less active. Such corrosion may induce cracking and damage in the concrete.
10. Corrosion due to difference in environments – Corrosion occurs when two different metals, or metals in different environments, are electrically connected in a moist or damp concrete. This will occur when i) steel reinforcement is in contact with an aluminum conduit ii) concrete pore water composition varies between adjacent or along reinforcing bars iii) where there is a variation in alloy composition between or along reinforcing bars and iv) where there is a variation in residual/applied stress along or between reinforcing bars
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. The choice is normally made based on the cost. Coatings employed on the reinforcement bars 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 electrochemically penetrated so that the base steel is again vulnerable to the usual corrosion process. Steel reinforcement bars of special composition to resist corrosion have also been tried. Several steel plants in India and abroad have experimented with various compositions of the weathering steels. In fact some producers in India have tried to market such reinforcement steel bars with very aggressive marketing. However after extensive in-house and independent tests, it was found that there is consistently poor performance of weathering steels when buried. Hence all the reputed steel makers abroad abandoned the production of such reinforcement steel bars. However some producers in India ignored such evidence and continue to market reinforcement bars made of weathering steels without adequately warning prospective users about the risks involved with the use of these reinforcement steel bars.