As per IS 456, an environmental exposure is divided into five parts, i.e. mild, moderate, severe, very severe and extreme. Structures constructed in different conditions have different effects and measures shall be taken to protect the structures under severe to extreme conditions.
Following table shows different environmental exposure conditions and their effect on structures.
|
Sl. No. |
Environment |
Exposure Conditions |
|
i |
Mild |
Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal areas |
|
ii |
Moderate |
Concrete surface 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 |
|
iii |
Severe |
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 |
|
iv |
Very Severe |
Concrete surfaces 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 |
|
v |
Extreme |
Surfaces of members in tidal zone; members in direct contact with liquid/solid aggressive chemicals |
Source: Table 3 of IS: 456 – 2000
Minimum cement contents, maximum water-cement ratio and minimum grade of concrete for different exposures with normal weight aggregates of 20 mm nominal maximum size
|
Exposure Conditions
|
Plain Concrete
|
Reinforced Concrete
|
||||
|
Minimum cement content kg/cum
|
Maximum free water-cement ratio
|
Minimum grade of concrete
|
Minimum cement content kg/cum
|
Maximum free water-cement ratio
|
Minimum grade of concrete
|
|
|
Mild |
220 |
0.60 |
- |
300 |
0.55 |
M 20 |
|
Moderate |
240 |
0.60 |
M 15 |
300 |
0.50 |
M 25 |
|
Severe |
250 |
0.50 |
M 20 |
320 |
0.45 |
M 30 |
|
Very Severe |
260 |
0.45 |
M 20 |
340 |
0.45 |
M 35 |
|
Extreme |
280 |
0.40 |
M 25 |
360 |
0.40 |
M 40 |
Source: Table 5 of IS: 456 – 2000
Note:
1) Cement content prescribed in this table is irrespective of the grades of cement and it is inclusive of additions mentioned in clause 5.2 of IS: 456.
The additions such as fly ash or ground granulated blast furnace slag may be taken into account in the concrete composition with respect to the cement content and w-c ratio if the suitability is established as long as the maximum amounts taken into account do not exceed the limit of pozzolana and slag specified in the IS: 1489 (Part I) and IS: 455, respectively.
Minimum grade for plain concrete under mild exposure condition is not specified.
Structures in Coastal Areas
Adjustment to minimum cement for aggregates other than 20 mm nominal maximum size
|
Sl. No. |
Nominal maximum aggregates size, mm in table above (kg/cum) |
Adjustment to minimum cement contents |
|
(i) |
10 |
+40 |
|
(ii) |
20 |
0 |
|
(iii) |
40 |
-30 |
Source: Table 6 of IS: 456 – 2000