Presentation by: Niraj Thapa

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Service Life Prediction of RC Structure

With Respect to Corrosion of Steel

Introduction

Damage caused by corrosion

Corrosion mechanism

Carbonation induced corrosion

Chloride induced corrosion

Corrosion control

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What is Corrosion?

Concrete corrosion is the chemical, colloidal or

physicochemical deterioration and disintegration of

solid concrete components and structures, due to

attack by reactive liquids and gases.

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ASTM terminology defines corrosion as “the

chemical or electrochemical reaction between a

material, usually a metal, and its environment that

produces a deterioration of the material and its

properties.”

Corrosion of steel reinforcements represents the

major cause of degradation of reinforced concrete

structures.

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The corrosion of steel can only be delayed or slowed

down but can never be stopped.

A passive oxide film on the surface of the steel

protects the reinforcing steel in concrete from

corrosion. But with time cover concretes cannot

provide enough resistance due to the degradation of

concrete and the ingress of corrosive species from

environment.

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Corrosion leads to several coupled effects:

longitudinal cracking of concrete cover due to

expansive corrosion products.

steel cross-section reduction.

the degradation of steel–concrete bond.

Thus the service life and the load-bearing capacity of

reinforced concrete elements are considerably

reduced.

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Steel in sound concrete is protected by passivation .

Corrosion can take place when the passive film is

removed or is locally damaged. This may take place

due to carbonation of concrete or to chloride

penetration.

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Carbonation penetrates the concrete cover and

corrosion takes place.

Corrosion then initiates when the carbonation front

reaches the steel reinforcement. With time, corrosion

products cause cracking, spalling and delamination

of the concrete cover, which may compromise the

serviceability and the stability of the structure.

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For corrosion of steel,

Service Life = Initiation time + Propagation time

The initiation period is the time required for the

carbonation depth to equal the concrete cover

thickness. The propagation period begins when the

steel is depassivated, and finishes when a given limit

state is reached.

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The presence of chloride ions in the pore solution of

concrete may induce a form of localized corrosion onthe embedded steel called pitting corrosion.

Corrosion of steel in concrete is an electrochemical

process. When there is a difference in electrical

potential along the steel reinforcement in concrete, an

electrochemical cell is set up. In the steel, one part

becomes anode and other part becomes cathode

connected by electrolyte in the form of pore water inthe hardened cement paste.

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At cathode:

Fe Fe++ + 2e–

Fe++ + 2(OH)–Fe(OH)2 (Ferrous hydroxide)

4 Fe(OH)2 + 2H2O + O2 4Fe(OH)3 (Ferric oxide)

At anode:

4e– + O2 + H2O 4(OH)–

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It has been reported that 40% of failure of structures

is on account of corrosion of embedded steel

reinforcement in concrete. Therefore corrosion

control of steel reinforcement is of paramount

importance.

To prevent corrosion:

Use lowest possible water/cement ratio with regard to

workability.

Superplasticizers should be used to cut down w/c

ratio.

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Adopt proper mix design, use of right quality and

quantity of cement for different exposure conditions.

Use of supplementary cementitious materials such as

fly ash, ground granulated blastfurnace slag, silica

fume etc. are required

In short, if we make good concrete with low

permeability and improved microstructure, it will be

durable by itself and also it can take care of the

reinforcement contained in it to a great extent.

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For other serious problems, following methods are

used:

Metallurgical methods

Corrosion inhibitors

Coatings to reinforcement

Cathodic protection

Coatings to concrete

Design and detailing

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Metallurgical methods:

Steel can be made more corrosion resistant by

altering its structure through metallurgical processes:

quenching of the hot bars by series of water jets.

keeping the hot steel bars for a short time in a water

bath.

Corrosion inhibitors:

by using certain corrosion inhibiting chemicals such

as nitrites, phosphates, benzoates etc.

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Coatings to reinforcement:

Simple cement slurry coating is a cheap method for

temporary protection against rusting of reinforcement

in storage.

The steps suggested by CECRI are:

Derusting

Phosphating

Cement coating

Sealing

Fusion Bonded Epoxy Coating

Galvanised reinforcement

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Cathodic Protection:

Most effective, well-known and extensively used

method.

High cost and long term monitoring required.

In this method current is applied to an electrode laid

on the concrete above steel reinforcement. This

electrode serves as anode and the steel reinforcement

which is connected to the negative terminal of a DC

source acts as a cathode. Here the external anode is

subjected to corrode and the cathodic reinforcement

is protected against corrosion and hence the name

“Cathodic protection”.

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Coatings to Concrete:

In addition to the coating of reinforcement byappropriate material, a surface coating to the concretemember is given to increase the durability further.

Design and Detailing:

Precautions should be taken in designing anddetailing with respect to:

spacing between bars for the concrete to flowbetween reinforcements

to facilitate vibration of concrete.

to give proper cover to the steel reinforcements.

to restrict the crack width.

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References:

Bertolini, L., Steel corrosion and service life of reinforced

concrete structures.

Liang, M.T., Wang, K.L., Liang, C.H., Service life prediction ofreinforced concrete structures.

Vidal, T., Castel, A., Francois, R., Corrosion process and structuralperformance of a 17 year old reinforced concrete beam stored inchloride environment.

Guangling, S., Ahmad, S., Corrosion of steel in concrete: causes,detection and prediction

Shetty M.S.(1982), Concrete Technology Theory and Practice, S.Chand & Company Ltd., pp 402-410

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Thank you!

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