15.2.1 Shape, Size, Orientation of Members, Connections and Details
The design, fabrication and erection details of exposed
structures should be such that good drainage of water is ensured. Standing pool of water, moisture accumulation and rundown of water for extended duration shall be avoided.
The details of connections should ensure that:
- a) All exposed surfaces are easily accessible for inspection and maintenance; and
- b) All surfaces, not so easily accessible are completely sealed against ingress of moisture.
15.2.2 Exposure Condition
15.2.2.1 General environment
The general environment, to which a steel structure is exposed during its working life is classified into five levels of severity, as given in Table 28.
| Sl No. | Environmental Classifications | Exposure Conditions |
|---|---|---|
| (1) | (2) | (3) |
| i) | Mild | Surfaces normally protected against exposure to weather or aggressive condition as in interior of buildings, except when located in coastal areas |
| ii) | Moderate | Structural steel surfaces: a) exposed to condensation and rain b) continuously under water c) exposed to non-aggressive soil/groundwater d) sheltered from saturated salt air in coastal areas |
| iii) | Severe | Structural steel surfaces: a) exposed to severe frequent rain b) exposed to alternate wetting and drying c) severe condensation d) completely immersed in sea water e) exposed to saturated salt air in coastal area |
| iv) | Very severe | Structural steel surface exposed to: a) sea water spray b) corrosive fumes c) aggressive sub soil or ground water |
| v) | Extreme | Structural steel surfaces exposed to: a) tidal zones and splash zones in the sea b) aggressive liquid or solid chemicals |
15.2.2.2 Abrasion
Specialist literature may be referred for durability of surfaces exposed to abrasive action as in machinery, conveyor belt support system, storage bins for grains or aggregates.
15.2.2.3 Exposure to sulphate attack
Appropriate coatings may be used when surfaces of structural steel are exposed to concentration of sulphates (SO3) in soil, ground water, etc.
When exposed to very high sulphate concentrations of more than 2 percent in soil and 5 percent in water, some form of lining such as polyethylene, polychloroprene sheet or surface coating based on asphalt, chlorinated rubber, epoxy or polymethane material should be used to completely avoid access of the solution to the steel surface.
15.2.3 Corrosion Protection Methods
The methods of corrosion protection are governed by actual environmental conditions as specified in IS 9077
and IS 9172. The main corrosion protection method are given below:
- a) Controlling the electrode potentia
- b) Inhibitors, and
- c) Inorganic/metal coatings or organic/paint systems.
15.2.4 Surface Protection
15.2.4.1
In the case of mild exposure, a coat of primer after removal of any loose mill scale may be adequate. As the exposure condition becomes more critical, more elaborate surface preparations and coatings become necessary. In case of extreme environmental classification, protection shall be as per specialist literature. Table 29 gives guidance to protection of steelwork for different desired lives.
Table 29 (a) Protection Guide for Steel Work Application — Desired Life of Coating
System in Different Environments
| SI No. | Atmospheric Condition/ Environmental Classification | Coating System | |||||
|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | ||
| (1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) |
| i) | Normal inland (rural and urban areas), mild | 12 years | 18 years | 20 years | About 20 years | About 20 years | Above 20 years |
| ii) | Polluted inland (high airborne sulphur dioxide), moderate | 10 years | 15 years | 12 years | About 18 years | 15-20 years | Above 20 years |
| iii) | Normal coastal (as normal inland plus high airborne salt levels), severe | 10 years | 12 years | 20 years | About 20 years | About 20 years | Above 20 years |
| iv) | Polluted coastal (as polluted inland plus high airborne salt levels), very severe or extreme | 8 years | 10 years | 10 years | About 15 years | 15-20 years | Above 20 years |
Table 29 (b) (i) Protection Guide for Steel Work Application — Specification for Different Coating System (Shop Applied Treatments)
(Clause 15.2.4.1)
| SI No. | Protection | Coating System | |||||
|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | ||
| (2) | (3) | (4) | (5) | (6) | (7) | (8) | |
| i) | Surface preparation | Blast clean | Blast clean | Blast clean | Blast clean | Girt blast | Blast clean |
| ii) | Pre-fabrication primer | Zinc phosphate epoxy, 20 µm | 2 pack zinc-rich epoxy, 20 µm | — | 2 pack zinc-rich epoxy, 20 µm | — | Ethyl zinc silicate, 20 µm |
| iii) | Post-fabrication primer | High-build zinc phosphate modified alkyd, 60 µm | 2 pack zinc-rich epoxy, 20 µm | Hot dip galvanized, 85 µm | 2 pack zinc-rich epoxy, 25 µm | Sprayed zinc or sprayed aluminium | Ethyl zinc silicate, 60 µm |
| iv) | Intermediate coat | — | High-build zinc phosphate, 25 µm | — | 2 pack epoxy micaceous iron oxide | Sealer | Chlorinated rubber alkyd, 35 µm |
| v) | Top coat | — | — | — | 2 pack epoxy micaceous iron oxide, 85 µm | Sealer | — |
Table 29 (b) (ii) Protection Guide for Steel Work Application — Specification for Different Coating System (Site Applied Treatments)
(Clause 15.2.4.1)
| SI No. | Protection | Coating System | |||||
|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | ||
| (1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) |
| i) | Surface preparation | As necessary | As necessary | No site treatment | As necessary | No site treatment | As necessary |
| ii) | Primer | Touch in | Touch in | — | — | — | Touch in |
| iii) | Intermediate coat | — | Modified Alkyd Micaceous iron oxide, 50 μm | — | Touch In | — | High-build micaceous iron oxide Chlorinated rubber Micaceous, 75 μm |
| iv) | Top coat | High-build Alkyd finish, 60μm | Modified Alkyd Micaceous iron oxide, 50 μm | — | High-build chlorinated rubber | — | High-build iron oxide Chlorinated rubber, 75 μm |
15.2.4.2
Steel surfaces shall be provided with at least one coat of primer immediately after its surface preparation such as by sand blasting to remove all mill scale and rust and to expose the steel.
15.2.4.3
Steel without protective coating shall not be stored for long duration in out door environment.
15.2.4.4
Surfaces to transfer forces by friction as in HSGF connections shall not be painted. However it shall be ensured that moisture is not trapped on such surfaces after pretensioning of bolts by proper protective measures.
15.2.4.5
Members to be assembled by welding shall not be pre-painted at regions adjacent to the location of such welds. However, after welding, appropriate protective coatings shall be applied in the region, as required by the exposure conditions. If the contact surfaces cannot be properly protected against ingress of moisture by surface coating, they may be completely sealed by appropriate welds.
15.2.4.6
Pre-painted members shall be protected against abrasion of the coating during transportation handling and erection.
15.2.5 Special Steels
Steels with special alloying elements and production process to obtain better corrosion resistance may be used as per specialist literature.