Enhancing Corrosion Resistance of Steel Bars in Reinforced Concrete Structures
Reinforced concrete is a composite material which is made using concrete and steel bars. Concrete takes the compressive forces and steel bar takes tensile forces. Concrete around the steel bar protects it from corrosion by providing an alkaline environment around it. So long as concrete remains alkaline, the reinforced concrete structures perform satisfactorily.
Corrosion of steel bar is amongst the most important causes of deterioration of reinforced concrete structures (Fig.1) Corrosion occurs due to the presence of chlorides in concrete / environment and / or ingress of carbon-dioxide into the concrete from the environment (i.e. carbonation of concrete).
The initiation of corrosion of steel bar is largely attributed to the breakdown of the passive film surrounding the steel bar. Once this film breaks down, corrosion propagates due to the formation of the corrosion product, which consists of Iron oxides. Formation of iron oxide decreases the effective cross-sectional area of the steel bar, thereby increasing corrosion and decreasing the capacity of taking tensile forces by steel bars.
Further, the conversion of iron into iron oxide increases the volume of the area around the bar by 2.2–6.4 times, depending on the type of oxide formed. This increase in volume is enough to initiate internal cracks in a concrete structure. The volume of iron oxide goes on increasing over time, and cracks appear on the concrete surfaces. Once cracks appear, water, oxygen, and other harmful materials find easy ingress into the concrete and can penetrate the steel bars and cause significant damage.
Therefore, enhancing the corrosion resistance of steel bar in concrete is required from structural as well durability point of view.
Corrosion due to Chlorides
All constituents of concrete may contain chlorides or concrete may be contaminated by penetration of chlorides from the external environment. Chloride penetration in concrete is mainly due to diffusion process (Fig.2) which depends on the geographical locations of the structure, ambient temperature, ingredient of concrete, etc.
Corrosion due to chlorides starts when chloride contents in the concrete exceeds the threshold value. To minimize the chances of deterioration of concrete from Chlorides, the levels of Chlorides in concrete coming from concrete materials (cement, aggregates, water and admixtures) as well as by diffusion from the environment should be limited.
Corrosion due to Carbonation of Concrete
Concrete has micro-pores that are filled with liquid, and pH-value as high as 12.5, which makes concrete alkaline in nature. Due to this alkaline nature of concrete, steel bars present in the concrete do not corrode. With the passage of time, Calcium Hydro-oxide [Ca(OH)2] present in the concrete reacts with Carbon dioxide [CO2] present in the atmosphere and forms Calcium Carbonates which lowers the alkalinity of the concrete (up to pH-value of about 8.3). Due to this reduction in alkalinity, the ability of the concrete to protect the steel bar from corrosion is also reduced. The outer zone of concrete is affected first, but with the passage of time, carbonation penetrates deeper into the mass of the concrete. If the depth of penetration becomes equal to the cover of concrete, steel bars are then prone to corrosion.
Rate of carbonation depends on environmental conditions such as structure exposed to rain or not; situated near sea or far away from sea, dry-wet cycles, relative humidity, temperature, CO2 concentration, etc.
Methods for Enhancing Corrosion Resistance
Galvanised Steel Bars: Galvanization is the process of applying a protective zinc coating to steel or iron to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are coated by submerging them in a bath of hot molten zinc (Fig.3). Zinc oxide coating is highly stable and adheres tightly to the metal substrate, is very durable, and does not flake off easily.
Galvanised steel bars are used in reinforced concrete structures where corrosion due to chlorides is suspected. Galvanised steel bars can withstand exposure to chloride ion concentrations several times higher than the chloride level that causes corrosion in them.
Corrosion Inhibiting Admixtures: These are chemicals that are added into the concrete in small amounts to delay the time for corrosion to occur in reinforced concrete structures. The admixtures increase the passivation of steel bars. They are added in the concrete during mixing of ingredients of the concrete. Some of the most popular corrosion inhibiting admixtures are Amine Carboxylate, Amine-ester Organic Emulsion, Calcium Nitrite, Organic Alkenyl Dicarboxylic Acid Salt, etc. By using corrosion inhibiting admixtures, maintenance costs of reinforced concrete structures get significantly reduced.
Cathodic protection: This is one of the most effective methods for preventing corrosion on a metal surface and is used all over the world to protect metals. In this corrosion protection method, steel bars are converted from anode to cathode of an electrochemical cell. The desired current for cathodic protection is either provided through a power source or by connecting the steel bars to an anodic material (galvanic cathodic protection) such as Magnesium. Galvanic cathodic protection method uses a sacrificial anode that corrodes before the steel bars.
Coatings on Steel Bars
Electrocoating: In this process, electrically charged particles are deposited out of a water suspension to coat a conductive part. During the electrocoat process, paint of a certain thickness is applied to a conductive part. The electrically charged paint will attach or deposit on any surface in the bath that is electrically opposite in charge. Electrocoating immersion process provides coverage of complex parts and a uniform thickness without runs or drips.
Metallic coatings: These are used in cases where the substrate is coated with a more noble metal, such as copper on steel. Metallic coatings can be applied by using a sprayer, electrochemically, chemically, or mechanically. This type of protective coating is effective only when the coating is free from pores or damages. Metallic coatings are generally applied on steel surfaces using one of following methods, namely, Anodizing, Hot-dip galvanizing, Thermal spraying, Sherardizing, Electroplating, etc.
Organic coating: This is a type of coating formed by carbon-based polymeric chains derived from natural material (vegetable, animal) or synthetic material. Solid, adhesive, and cohesive organic coatings can be found in the form of paints, varnishes, lacquers. These coatings can be water-based or reduced solvent, compared to traditional coatings with a higher rating of volatile organic compounds. The coatings act as a barrier and prevent or retard the transfer of electrochemical charge from the corrosive solution to the steel bars.
Powder coating: This is sprayed on the surfaces of the steel by powder spraying equipment. Powder coating is applied electrostatically. The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish which is tougher than conventional paint.
Coatings on Exposed Surfaces of Concrete
Coatings applied to exposed concrete surfaces provide protection against ingress of harmful agents, aggressive liquids and gasses from the immediate environment, thereby increasing their durability. The coatings also enhance the aesthetic appearance of the structure.
There are many parameters that influence the deterioration process of coatings, such as chemical agents, temperature, solar radiation, pressure, abrasion, cyclic temperature-moisture changes etc. All these can occur simultaneously, or they can be complementary to one another. The coatings may be subjected to continuous exposure or intermittent contact occasioned by splash, spray, or accidental wetting with aggressive substances.
Degradation and de-bonding of coatings is the major problem, leading to their cracking and delamination. Basic requirements for coatings of concrete structures are a good bonding to concrete; resistivity to chemical/physical actions; low permeability for ingress of water, solutions and gases; sufficient flexibility to avoid cracking caused by thermal or mechanical movements; similar physical properties of the coating and concrete; bridging of fine cracks in concrete; etc.
The design of an appropriate protective system for structures is a complex process which involves identification of service environment of the particular structure in the original design; identification and assessment of the condition, state, and deterioration of the existing structure; selection of an appropriate protection system; definition of coating parameters (type of binder, formulation, coverage/thickness); anticipated time between periodic re-coating, etc.
Study on Various Treatments to Enhance Corrosion Resistance
Ministry of Roads Transport and Highways (MoRTH) has sponsored a research project to CSIR-CRRI under which studies on corrosion susceptibility of steel bar bars protected with anti-corrosive coatings / special treatments and embedded in ordinary and high-performance concretes as well as concretes treated with surface coatings under different environmental exposure conditions are being carried out.
Table-1 presents the various types of treated steel bars of Fe500 grade, being used to cast steel bar embedded concrete specimen or RCC specimen for different tests.
|Table-1: Various Types of Treated Steel Bars
|Types of Treated Steel Bars
|Coatings on Reference bars
|Hot Dip Galvanised
|Zinc-Aluminium coated bar
|Fusion Bonded Epoxy Coated
|Cement Polymer Composite Coated
|Special types of steel
|Corrosion Resistant Steel
Concrete is a porous material, leading to deterioration of reinforced concrete structures due to ingress of gases, water, etc. through the pores. Enhancing the corrosion resistance of steel bar in concrete is required from structural as well durability point of view. The protection of concrete should actually begin at the conceptual stage itself and meticulous strategies are adopted for protecting the concrete.
- Abdulrahman Fahad Al Fuhaid and Akbar Niaz (2022), “Carbonation and Corrosion Problems in Reinforced Concrete Structures”, https://www.mdpi.com/2075-5309/12/5/586
- M. N. Ramesh (2017), “Concrete Protection Coatings for Reinforced Concrete Structures”, New Building Materials and Construction World, June.