Durability of Concrete

Durability of concrete can be defined to mean ‘its resistance to deteriorating influences, which may reside inside the concrete itself, or which are present in the environment to which the concrete is exposed’.

Features of Durable Concrete

•A durable concrete is one that performs satisfactorily in the working environment during its anticipated exposure conditions during service.

•Durable concrete will retain its original form, quality and serviceability when exposed to its environment.

•It should not disintegrate or show sign of wearing under adverse conditions.

Significance of Durability

•In late 1930’s it was found that series of failures of concrete pavements have taken place due to frost attack.

•It is not always entirely true that the strong concrete is always a durable concrete.

•A structure which is very strong when it is constructed but can’t withstand for a long time the effect of the environment in which it has to exist is not really useful.

Causes of Lack of DurabilityIt is caused by external agents arising from the environment or by internal agents within the concrete; causes can be categorized as:

•Physical causes- action of frost (freezing and thawing), difference between the thermal properties of aggregates and of the cement paste.•Mechanical causes- they are associated mainly with abrasion (machinery and metal tyres).•Chemical causes- it is associates with chemical reaction in concrete (sulphate attack, acid attack by sea water, chloride attack, alkali aggregates reaction, carbonation).

Damage of concrete due to lack of Durability

• Fine to wide cracks developed in concrete.• Scaling (localized small patches) of concrete

can take place.• Spalling of concrete can happen. (It is a

result of water entering brick, concrete or natural stone and forcing the surface to peel, pop out or flake off)

• Disintegration of concrete takes place.• Deposits of salts can take place, which is

called efflorescence of concrete.• Complete structure failure can occur.

Environment, freezing and thawing, exposure to aggressive chemicals.

Cover to embedded steel. Type and quality of constituent

materials. Cement content and water-cement

ratio. Workmanship to obtain full

compaction and efficient curing, and Shape and size of member.

Curved members without adequate drainage and very thin members are more vulnerable to corrosion of reinforcement and cracking of concrete.

Adequate curing is essential to avoid the harmful effects of early loss of moisture.

Minimum cement content should be used, as increase in cement content increase the risk of cracking due to drying shrinkage and thermal cracking.

Type ofExposure

Environment DescriptionNominal

Coverin mm

MildConcrete surfaces protected against weather or aggressive conditions, expect those situated in coastal area.

20

Moderate

Concrete surface sheltered from rain or freezing Concrete permanently under waterConcrete in contact or buried non-aggressive soils.Concrete surface sheltered from saturated salt air in coastal area.

30

Severe

Concrete surface exposed to severe rain, alternate wetting and drying or occasional freezing.Concrete completely immersed in sea water. Concrete exposed to coastal environment.

45

Verysevere

Concrete surface exposed to sea water spray, corrosive fumes orsevere freezing conditionsConcrete in contact or buried aggressive soils.

50

ExtremeConcrete surface exposed to abrasive action like sea water carrying solids or chemicals (tidal zone).

75

For better understanding of durability following issues have been explained in detail: Permeability of concrete. Resistance of concrete to freezing and

thawing. Chloride and sulphate attack. Joints in concrete and cracks in concrete.

  Deteriorating Agencies Chemical – Sulphates, Chlorides, CO2, Natural waters, Mechanical – Abrasion, Erosion, Cavitation, Impact, Temperature.  

Nature of the Problem

Exposure Condition

Max. w/c ratio

Min. cement content, kg/m3

Min. grade of concrete

Min. cover, mm

Moderate 0.45 340 M25 40

Severe 0.45 360 M30 45

Very Severe 0.40 380 M40 50

Extreme 0.35 400 M45 75

This property is of interest in relation to the water-tightness of liquid-retaining structures and to chemical attack.( example - in sewage tanks, dams, pressure vessels in nuclear reactor, foundation in aggressive soils, etc.)

The attack by sulphates, acids, sea water, chlorides etc., which induce electro-chemical corrosion of steel reinforcement. Since this attack takes place within the concrete mass, the attacking agent must be able to penetrate throughout the concrete mass, which therefore has to be permeable.

Many aspects of concrete durability are improved by reducing the permeability of concrete.

The cement paste consists of C-S-H gel, Ca(OH)2 and water filled or empty capillary cavities. Although gel is porous to extend of 28%, the gel pores are so small that hardly any water can pass through normal conditions. The permeability of gel pores is estimated to be about 7 x 10-16 mls, i.e. approximately 1/100 of that of paste. Therefore, the gel pores do not contribute to the permeability of cement paste.

C-S-H Gel C6A2S 2H34 or {Ca6[Al(OH)6]2·24H2O}(SO4)2[Al(OH)4]2

The size and extend of capillary cavities depend on the wIc ratio. It is one of the main factors contributing to the permeability of paste.

At lower wIc ratio, not only extend of capillary is less but the diameter is also small. The capillary cavities resulting at low wIc ratio will get filled up within a few days by the hydration products of cement.

Only unduly large cavities resulting from higher w/c ratio (say more than 0.7) will not get filled up by the products of hydration, and will remain as unsegmented cavities, which is responsible for the permeability of paste.

Permeability of concrete is mainly dependent on the:

Water/cement ratio of concrete mix. The age of the concrete.

Apart from wIc ratio and age of concrete the other factors which are responsible for increasing permeability are as follows:Use of Porous aggregate - Permeability of concrete is generally of the same order when it is made with normal weight aggregates, which have permeability similar to that of the cement paste, but the use of a more porous aggregate will increase the permeability of concrete.

Curing of concrete - Interruption of moist curing for a period of drying will also cause an increase in permeability because of creation water passages by minute shrinkage cracks around aggregate particles, especially the large ones.

Steam Curing of concrete - Permeability of steam-cured concrete is higher than that of moist-cured concrete.

Gradation of aggregate - While a low W/C ratio is essential for the concrete to have a low permeability, it is not itself sufficient. The concrete must be dense and therefore a well-graded aggregate has to be used.

From durability point of view, it may be important to achieve low permeability as quickly as possible. This can be achieved by taking following measures :Use lower W/C mix - Adopt a mix with a low W/C ratio because the stage at which the capillaries become segmented is achieved after a shorter period of moist curing. Use pozzolanic materials - The use of pozzolanic materials like fly ash, blast furnace slag, silica fume, etc in optimum proportion reduces the permeability of concrete.

Do Proper Compaction - Proper compaction of concrete should be done, so that dense concrete is obtained.

Provide adequate cover - Adequate cover to the reinforcement is provided, so that cracks will not develop and attacking agents do not penetrate up to level of reinforcement easily.