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STUDIES ON DURABILITY OF REINFORCED CONCRETE

ANILKUMAR P M1 , Dr. J SUDHAKUMAR2 

Department of Civil Engineering

National Institute of Technology, Calicut

INTRODUCTION

Durability of a structure is its resistance to weathering action , abrasion , chemical attack, cracking , or any other process of destruction.

The structure is considered to be durable in the actual environment, as long as its function is acceptable.

Degradation in performance

Poor durability

Reduction in the useful life

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Durability of Reinforced Concrete

Durability of concrete is the resistance to deleterious influences arising from external or internal causes.

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• The external causes may be due to

Weathering

Extreme temperatures

Abrasion

Electrolytic action

Attack by natural or industrial liquids

and gases4

• The internal causes may be due to

Alkali-aggregate reaction

Volume changes due to difference in

thermal properties of aggregates and

cement paste

Permeability of the concrete

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• Some specific aspects are :

1. Water-cement Ratio

determines the permeability of concrete

Water-cement Ratio

0.55 for moderate exposure

0.45 for severe exposure

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2. Cement content

Increase in the cement content increases

durability, as it increases the alkalinity of the

concrete.

IS 456 gives the values of minimum cement

content for concrete exposed to mild,

moderate, severe, very severe and extreme

exposure conditions. These values are7

Reinforced Concrete Maximum w/c ratio as per IS 456

mild exposure – 0.55 moderate exp. – 0.50 severe exp. – 0.45 very severe exp. – 0.45 extreme exp. – 0.40

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Reinforced Concrete – Minimum grade

mild exposure – M 20 moderate exp. – M 25 severe exp. – M 30 very severe exp. – M 35 extreme exp. – M 40

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3. Type of Cement

It affects durability only when the concrete is subjected to chemical attack.

For chloride attack, cement having a moderate to high C3A content should be used.

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Certain admixtures such as pozzolanas, air-entraining agents and super plasticizers can be used for reducing the permeability of concrete.

In acidic environment, super- sulphated cement is preferred; since it can withstand acidic waters with PH value up to 3.5

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4. Permeability

The deterioration of concrete occurs when it is permeable to sulphate, chloride and acid attack.

Relatively low w/c ratios are essential for impermeable concrete.

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Certain admixtures such as pozzolanas, air-entraining agents and super plasticizers can be used for reducing the permeability of concrete.

In RCC, the ingress of moisture and air results in corrosion of steel, leading into cracking, spalling and ultimately to the deterioration of the concrete

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5. Corrosion of Reinforcement

It is defined as a gradual wearing away or alteration by a chemical or electrochemical oxidizing process

In many environments, most metals are not stable and they tend to revert to some more stable combinations.14

Mechanism of corrosion

Normal corrosion is an electrochemical phenomenon

In metals, corrosion is caused by a flow of electricity from one metal to another metal, or from one part to another part of the same metal

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Electrochemical corrosion

For this type of corrosion to occur, the following components must be present :

• A metal anode• A metal cathode• A metallic conductor between the anode

and cathode• An electrolyte in contact with the anode

and cathode16

The metal that ionizes more rapidly is called the anode

The metal at which the above reaction tends to reverse is called the cathode

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Consider the corrosion of iron or steel.

Following are the reactions that occur at the anode and cathode:

The metal Fe at the anodic area is ionized into ferrous ions, releasing the electrons

Fe Fe 2+ + 2 e-

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These electrons are consumed at the

cathodic area, where they combine with

oxygen and water to form hydroxyl

ions. O2 + 2 H2 O + 4 e- 4(OH)-

Fe++ + 4 (OH)- Fe (OH)2

4 Fe (OH)2 + O2 + 2H2 O 4Fe(OH)3

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Corrosion of steel in concrete

Steel gains protection from concrete If the alkalinity around the reinforcement falls

below a PH value of 11.5 , from the normal PH of 12.5 to 13, the protective film is impaired

This can be due to carbonation, chloride and acid attack

Other factors are presence of moisture, oxygen, electrolyte, high temperature and humidity.

The electrochemical potential which arises from the differences in the steel-concrete system is the main cause of corrosion

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Marine corrosion

Sea water is an electrolyte of high conductivity and it contains corrosive salts.

It causes corrosion, even when metals are exposed to atmospheric conditions in marine environment.

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Sea water contains chlorides and sulphates. Magnesium sulphate is the most harmful salt in sea water. Sea water has a PH value in the range of 7.8 to 8.3

Zones of corrosion can be atmospheric, splash, tidal, and full immersion.

Corrosion is most serious in the splash zone, due to the combined effects of oxygen, salt and water.

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The corrosion rate can be functionally represented as

dc/dt = f ( t, O2 , T, V, S, PH )

where

t = duration of exposure

O2 = oxygen content

T = temperature of sea water

V = velocity of sea water

S = salinity of sea water

PH = hydrogen ion concentration23

Measurement of rate of corrosion

The corrosion rate may be represented by the weight loss of metal in milligram per square decimeter of surface area per day ( mdd )

( Wo – Wf ) 1000 mdd =

---------------------------------

d l t24

where,

• Wo and Wf are the original and final weights in grams

• d is the diameter • l is the length of wire in decimeter• t is the age of the structure in days

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The corrosion rate can also be reported by metal penetration in cm’s of penetration per year (cpy) or by the percent loss of strength per year ( plspy )

( Wo – Wf ) 100 x 365

plspy = ---------------------- x ---------------------

Wo t

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Metals may be classified according to their corrosion rates as :

Good corrosion resistance material, having cpy < 0.013

Satisfactory, having cpy in the range of 0.013 and 0.13

Unsatisfactory, with cpy > 0.13

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Protective measures against reinforcement corrosion

1. Use of corrosion inhibiting admixtures in concrete. They can be organic and inorganic.

Organic inhibitors include sodium

benzoate, ethyl-aniline and mercapto benzo- thiazole28

Inorganic inhibitors are potassium dichromate, stannous chloride, zinc and lead chromates, calcium hypophosphate, sodium and calcium nitrate etc.

2. Another protective measure is coatings on the reinforcement. Examples are epoxy chlorinated rubber, zinc, nickel, copper etc.

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3. Another method is by coatings on the surface of concrete. They include oil based coatings, varnishes, lacquers, bitumen and tar-based coatings etc.

4. An important method of protection is known as cathodic protection

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Cathodic protection is the most versatile method of

corrosion control. There are two ways in which it can

be achieved.

1. Sacrificial anode method

2. Impressed current method

In the first method, an electrode made of a metal or

alloy, more negative than the structure to be protected

is used.

In the second method, a favorable electrochemical

circuit is established by introducing electrical current

from an external source.

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Sulphate attackSodium, magnesium and calcium sulphates can cause distress in concrete.

Effects include expansion, cracking, loss of strength and stiffness, disintegration etc.

This is caused by the expansive forces created by the reaction of soluble sulphates with the C3A content in cement.

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A low C3A content cement such as sulphate-resisting cement with less than 5% content, is ideally suited for concrete subjected to sulphate attack.

Chloride attackcorrosion of reinforcement can take place even in highly alkaline conditions, if sufficient chloride ions are present in the concrete.

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If C3A content is low, the amount of free chloride in the pore water increases and steel corrosion becomes more likely.

IS 456 gives limiting value of 0.15% by mass of cement in fresh concrete.

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Experimental Investigations

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Laboratory tests

1. Alternate heating and cooling test-simulate the field conditions of structures partially immersed in corrosive liquids such as sea water, and subjected to alternate heating and cooling.36

Specimens are kept in a trough containing sea water, in a semi-immersed condition

Heating can be done by using infra-red lamps at 600C

One cycle of 12 hours heating and 12 hours cooling

Total of 90 cycles

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2. Alternate soaking and drying test:

Specimens are fully immersed in sea water for 12 hours and then they are dried in an oven at 650C.

This test simulates the actual conditions of structures exposed to aggressive solutions and subjected to cycles of full immersion and drying.38

3. Salt spray test :

Specimens are suspended vertically inside the salt spray chamber and spray is done for 12 hours per day at room temperature.

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Field exposure tests

It is done for a longer duration-one year or more

Specimens are kept in the following zones

1. Atmospheric zone2. Splash zone3. Immersion zone40

Tests on specimens after exposure

1. Tensile strength test2. Chemical analysis : Chloride content Sulphate content

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Experimental flexural beam tests

Full scale beams can be tested under third point loading until failure for both concrete grades.

Full scale beams can be tested under third point loading until failure for both concrete grades.

The beams can be loaded up to failure and the curvature can track through material strains using electrical gauges and transducers.42

Column ductility investigation

A square column 400x400 mm with eight 16 or 30 mm bars is may studied using grade 25 normal and lightweight concretes.

Lightweight concrete ductility is better than that of normal concrete.

The advantage of LWC is more pronounced with higher steel ratio and increased compression forces.43

Suggestions to improve ductility

Confinement using highly stiff material use of rubber wastes,polymeric admixtures Partial substitution of cement with micro or

Nano- sized pozzolonic materials in presence of suitable super plasticizer may also increase the ductile property.

Material like steel fibers enhances the dutilitic properties.

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CONCLUSIONS Ductile detailing is provided in structures so as

to give them adequate toughness and ductility to resist severe earthquake shocks without collapse.

The different tests discussed in this paper can be used as a guideline.

Accurate prediction of the performance of structures made with reinforced concrete can be done, by increasing the duration of the laboratory studies as well as field exposure studies.

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REFERENCES Visvesvaraya (1988), 'Corrosion and Durability', Ferrocement -

Applications and Progress, Proceedings of the 3rd International Conference on Ferrocement, India, pp. xxx - xxxii.

Pranesh and Sudarsan (1981), 'Mathematical Model for the Corrosion Rate in Marine environment', Behaviour of Materials under Marine Environment, First Indian Conference on Ocean Engineering, IIT Madras, February 1981.

Trikha, Sharma, Kaushik and Tiwari (1984), 'Corrosion Studies in Ferrocement Structures', Journal of Ferrocement, Vol.14, No.3, July 1984, pp. 221-233.

Chalisgaonkar, (1987), 'Corrosion of Steel in Concrete and Ferrocement', Ferrocement Corrosion, Proceedings of the International Correspondence Symposium.46

REFERENCES 

Rangaswamy, Srinivasan and Mohan (1987), 'Evaluation of Protective Coatings for Reinforced Concrete', Ferrocement Corrosion, Proceedings of the International Correspondence Symposium, Bangkok, Thailand.

  Aslam, Srivastava and Minocha (1987), 'Durability of Concrete in

Sulphorous Atmosphere', Indian Concrete Journal, Vol.61, May 1987, pp. 135 - 138. 

ACI Committee 222 (1985), 'Corrosion of Metals in Concrete', ACI Journal, Jan-Feb 1985, pp. 3-32. 

Cusens (1985), 'Corrosion of Reinforcement - A Review', Journal of Ferrocement, Vol.15, No.4, Oct. 1985, pp. 365 - 370.

Abdelhamid Charif,M. Jamal Shannag,Saleh Dghaither (2014) , 'Ductility of reinforced lightweight concrete beams and columns', Latin American Journal of Solids and Structures On-line version ISSN 1679-7825

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Contact: anilkumar_bce11@nitc.ac.in skumar@nitc.ac.in

Department of Civil Engineering. National Institute of Technology, Calicut

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