Understanding & Preventing Corrosion in RC Structures

Corrosion of concrete is a major issue and many concrete structures on adverse environment have experienced unacceptable losses in terms of serviceability, ultimately requiring replacement, rehabilitation and strengthening of reinforcement.

Pranav Desai, Vice President- R&D, Head -CDIC and
Samidha Pathak, R&D Manager, CDIC, Nuvoco Vistas Corp. Ltd

The durability of concrete is an important parameter for the long service life of structures. Durable concrete is one which is designed properly and produced with good quality control; however, concrete may be vulnerable to adverse conditions such as chemical attack, corrosion, permeability and porosity. The intensity and nature in each of these properties varies considerably based on the environmental conditions.

Corrosion is one of the single most leading causes for deterioration of concrete structures as it compromises the serviceability of structure. In India, the direct cost of corrosion for the construction industry was approximately ₹300 crores in 1985 as per a research report. Corrosion of steel embedded in concrete produces hydrated iron oxide, often known as rust. Corrosion in concrete occurs due to the destruction of the reinforcement by the electrochemical reactions that occur within the environment.

The ingress of chlorides and carbonates from the external agencies lead to corrosion. CO2 and chlorides penetrate concrete and get dissolved in the pore solution to form carbonic acid. This acid reacts with the alkali in the cement to form carbonates and to lower the pH level of the concrete. Once the threshold is reached, the concrete cover is compromised and the pH of the concrete surrounding the rebar allows corrosion. When the alkalinity begins to drop from 12-13 to about a value of 9, the embedded steel becomes de-passivated. In the presence of water and oxygen, corrosion is initiated. Rust formed on the steel expands in volume three to six times that of the original steel. This increase in volume change increases the stresses in the concrete resulting in cracks and de-lamination.

Chloride pitting weakens the steel and the hydrated iron oxide being expansive in nature, builds up internal pressure in concrete structures and causes spalling. Corrosion is accelerated at relative humidity levels ranging between 70 - 80%. A corrosion protection strategy to minimise the repair and maintenance costs is a must. It is therefore advisable that the concrete mix proportions should be used in lieu of design target parameters of concrete.

Preventive Measures
Usually, certain preventive measures are taken to prevent the reinforcement from being corroded. Galvanization, epoxy coating, metal coating and cathodic protection are a few to name. However, reinforcement coatings have proven to have bond issues with concrete due to poor chemical adhesion and techniques like cathodic protection are way expensive. Galvanization is not very effective in RC structures subjected to chloride rich corrosive environment as galvanized coating dissolves very quickly in high chloride concentration. If concrete is pumped a few feet above rebar, the impact of aggregates is liable to damage the epoxy coating. On an average, 40 such defects are created per metre of casting. Corrosion inhibitors do not prevent the ingress of chloride ions or external harmful environmental agencies. The addition of corrosion inhibiting admixtures alone does not ensure prevention of corrosion, since the inhibitors work through formation of passive membrane on the reinforcement surface.

Hence, it is desirable to make concrete denser, so that the ingress of chlorides and carbonates to the reinforcement surface is prevented. Incorporation of supplementary cementitious materials and proper gradation of aggregates can make concrete denser and helps to improve the durability of concrete thereby delaying the onset of corrosion. These materials produce a denser concrete, reduce its permeability, constrain the flow of ions, increase electrical resistivity, and slow down the corrosion current. In order to make the structure corrosion resistant, it is very important to define the exposure condition of the location of structure. Indian Standard classifies the environmental conditions into five exposure zones – mild, moderate, severe, extreme and very extreme. The grade of concrete and mix requirements needs to be selected according to the zonal location of the structure.

Corrosafe by Nuvoco
Nuvoco has developed corrosion resistant concrete under the brand name – Corrosafe. This innovative product is designed with a denser packing at the micro structural level and is admixed with corrosion inhibitors. The dense packing restricts the ingress of external agencies like chlorides and carbonates and the inhibitors will diffuse and form a passive layer, thus coating the reinforcement. This dual mechanism delays the de-passivation of reinforcement by strengthening the passive film. A lower w/b ratio, incorporation of supplementary cementitious materials, well graded particle distribution ensures a corrosion resistant concrete structure. Corrosafe has lower permeability, lower chloride penetration depth, lesser values of chloride diffusion, better resistance to abrasion and very low probabilities of carbonation and chloride attacks.

Nuvoco offers the most durable concrete mix, along with Service Life Prediction Report. Life 365 simulation report governed by Fick’s law of diffusion and Crank Nicolson Finite Difference equation is used for service life prediction. Nuvoco has designed and provided concrete with service life prediction of 125 years.

Life 365 software displaying Chloride ConcentrationFig: Screenshot from Life 365 software displaying Chloride Concentration versus duration required for chloride ions to penetrate the cover

Life 365 software displaying the estimated service life of structureFig: Screenshot from Life 365 software displaying the estimated service life of structure 125.6 years

Service life is a broad concept, which is defined in various terms by various researchers. However, in a nutshell, service life is the time required for the initiation of corrosion and the time required for the corrosion to propagate throughout the entire structure.

Table 1: Durability parameters for concrete compliant to various codes (actual site data tested at third party where Corrosafe was used for concreting)
Sr. No. Durability Parameter Value Codal Compliance
1. Drying Shrinkage (%) 0.011 IS 1199:1999
2. Moisture Movement (%) 0.013 IS 1199:1999
3. Water Permeability (mm) 7.7 DIN 1048 (Pt-5)-1991
4. RCPT (Coulombs) 805 ASTM C1202
5. Chloride Migration Coefficient 0.81*10-12 m2/s NT Build 492
6. Water Absorption (%) 1.6 BS 1881 – (Pt - 122)

Dense micro packing model of concrete achieved with addition of supplementary cementitious materials and consideration of particle size distribution reduces the diffusion co-efficient of concrete. Corrosafe finds applications in wide variety of structures located near the coastal areas, marine structures, sewage treatment plants, retaining walls, sub-structures, jetties, and dolphin structures. Corrosafe allows the concrete cover to remain intact for longer duration. It makes the structure durable and minimises the repair and maintenance cost by increasing the service life of structures. It also eliminates the need of pre-coating the steel surfaces. Thus, with specialized mix design technology, Nuvoco Corrosafe offers remarkable corrosion resistance and durable structures.
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