EXPOSURE CONDITIONS as per IS 456 on concrete pdf

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What is Exposure Conditions on Concrete?

Exposure condition means Concrete surface that exposed to environment or to its surrounding nature, that cause some effect over the Concrete to which concrete should be designed. As per IS 456 the Exposure Condition on Concrete divvied into five category.

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General environment : 

The general environment to which the concrete will be exposed during its working life is classified into five levels of severity, that is mild. moderate, severe, very severe and extreme.

S.NoEnvironment Exposure Conditions
1 Mild Concrete surfaces protected against weather or aggressive conditions; except those situated coastal area.
2Moderate Concrete surfaces sheltered from severe rain or freezing whilst wet .
Concrete exposed to condensation and rain 
Concrete continuously under water 
Concrete in contact or buried under non-aggressive soil/ground water 
Concrete surfaces sheltered from saturated salt air in coastal area 
3Severe Concrete surfaces exposed to severe rain
alternate wetting and drying or occasional freezing whilst wet or severe condensation. 
Concrete completely immersed in sea water 
Concrete exposed to coastal environment Concrete surfaces
4Very severe exposed to sea water spray; corrosive fumes or severe freezing conditions whilst wet 
Concrete in contact with or buried under aggressive sub-soil/ground water 
5ExtermeSurface of members in tidal zone 
Members in direct contact with liquid/solid aggressive chemicals Extreme

Abrasive: 

Specialist literatures may be referred to for durability requirements of concrete surfaces exposed to abrasive action for example in case of machinery and metal typres. 

Freezing and thawing : 

Where freezing and thawing actions under wet conditions exit, enhanced durability can be obtained by the use of suitable air entraining admixtures. When concrete lower than grade M 50 is used under these conditions, the mean total air content by volume of the fresh concrete at the time of delivery into the construction should be

 Nominal Maximum Size Aggregate (mm)Entrained Air Percentage
204 to 6
403 to 5

Since air entrainment reduces the strength, suitable adjustments may be made in the mix design for achieving required strength.

Concrete Mix Proportions 

The free water-cement ratio is an important factor in governing the durability of concrete and should always be the lowest value. Appropriate values for minimum cement content and the maximum free water-cement ratio are given for different exposure conditions. The minimum cement content and maximum water-cement ratio apply to 20 mm nominal maximum size aggregate. 

MINIMUM CEMENT CONTENT. MAXIMUM WATER-CEMENT RATIO AND MINIMUM GRADE OF CONCRETE FOR DIFFERENT EAPOSURES WITH NORMAL WEIGHT AGGREGATES OF 20 MM NOMINAL MAXIMUM SIZE
MINIMUM CEMENT CONTENT. MAXIMUM WATER-CEMENT RATIO AND MINIMUM GRADE OF CONCRETE FOR DIFFERENT EAPOSURES WITH NORMAL WEIGHT AGGREGATES OF 20 MM NOMINAL MAXIMUM SIZE
Adjustment of minimum cement content for aggregate other than 20 mm nominal maximum size
Adjustment of minimum cement content for aggregate other than 20 mm nominal maximum size
Adjustment of minimum cement content for aggregate other than 20 mm nominal maximum size

NOTES 

  1. Cement content prescribed in this table is irrespective of the grade of cement and it is inclusive of additions. The additions such as fly ash or ground granulated blast furnace slag may be taken into the concrete composition with respect to the cement content and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolana and slag specified in IS 1489 (Part 1) and IS 455 respectively. 
  2. Minimum grade for plain concrete under mild exposure condition is not specified. 

Maximum cement content : 

Cement content not including fly ash and ground granulated blast furnace slag in excess of 450 kg/m' should not be used unless special consideration has been given in design to the increased risk of cracking due to drying shrinkage in thin sections, or to early thermal cracking and to the increased risk of damage due to alkali silica reactions.

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