SECTION 15 DURABILITY
15.1 General
A durable steel structures is one that performs satisfactorily the desired function in the
working environment under the anticipated exposure condition during its service life, without
deterioration of the cross sectional area and loss of the strength due to corrosion. The
material used, the detailing, fabrication, erection and surface protection measures should all
address the corrosion reduction and durability requirements.
15.2 Requirements for Durability
15.2.1 Shape, Size, Orientation of Members, Connections and Details
The design, fabrication and erection details of exposed structure should be such that good
drainage of water is ensured. Standing pool of water, moisture accumulation and rundown of
water for extended duration are to be avoided.
The details of connections should ensure that
a) All exposed surfaces are easily accessible for inspection and maintenance.
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 steel structure is
exposed during its working life, is classified into five levels of severity, as given in Table
15.2.
15.2.2.2 Abrasion – Specialist literature may be referred for durability of surfaces
exposed to abrasive action as in machinery, conveyor belts 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 per IS: 9077 and IS: 9172. The main corrosion
protection methods are given below:
a) Controlling the Electrode Potential
b) Inhibitors
c) Inorganic/Metal Coatings or Organic/Paint systems
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TABLE 15.2 ENVIRONMENTAL EXPOSURE CONDITIONS
(Section 15.2.2.1)
Environmental Classifications
Exposure conditions
Mild
Moderate
Surfaces normally protected against
exposure to weather or aggressive
condition as in interior of buildings, except
when located in coastal areas
Structures steel surfaces exposed to
i) condensation and rain
ii) continuously under water
iii) non-aggressive soil/groundwater
iv) sheltered from saturated salt air in
coastal areas
Structural steel surfaces exposed to
i) severe frequent rain
ii) alternate wetting and drying
iii) severe condensation
iv) completely immersed in sea water
v) exposed to saturated salt air in
coastal area
Severe
Very severe
Extreme
Structural steel surface exposed to
i) sea water spray
ii) corrosive fumes
iii) aggressive sub soil or ground water
Structural steel surfaces exposed to
i) tidal zones and splash zones in the
sea
ii) aggressive liquid or solid chemicals
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, as per Specialist literature.
15.2.4.2 Steel surfaces shall be provided with atleast on coat of primer immediately
after its surface preparations, such as sand blasting to remove all mill scale and rust to expose
steel.
15.2.4.3 Steel without protective coating shall not be stored for long duration in out
door environment.
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15.2.4.4 Surfaces to transfer forces by friction as in HSFG connections shall not be
painted. However it shall be ensured that moisture cannot be trapped on such surfaces after
pretensioning of bolts by proper protective measures.
15.2.4.5 Member to be assembled by welding shall not be prepainted at the 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 Prepainted members shall be protected against abrasion of the coating during
transportation, handling and erection.
15.2.5 Protective Coating Requirements – Specialist literature may be referred to for
appropriate surface preparation and type and extent of protective coatings for the different
types of exposure condition. Table 15.3 gives guidance to a protection for steelwork for
different desired protection lives.
15.2.6 Special Steels – Steels with special alloying elements and production process to
obtain better corrosion resistance may be used as per Specialist literature.
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TABLE 15.3 PROTECTION GUIDE FOR STEEL WORK APPLICATION
(Section 15.2.5)
(a) Coating System Desired Life in Different Environments (In Years)
Atmospheric Condition
Coating
System 1
Coating
System 2
Coating
System 3
Normal Inland (Rural and
Urban areas)
12 years
18 years
20 years
*Polluted Inland (High
airborne sulphur dioxide)
10 years
15 years
12 years
Normal Coastal (As normal
inland plus high airborne
salt levels)
10 years
12 years
20 years
8 years
10 years
10 years
Polluted
Coastal
(As
polluted Inland plus high
airborne salt levels)
Coating
System 4
Coating
System 5
Coating
system 6
About
About
Above
20 years
20 years
20 years
About
18 years
15-20 years
20 years
About
About
Above
20 years
20 years
20 years
About
15 - 20
years
Above
15 years
*For desired life of 18years in polluted inland environment coating system 4 to be selected and so on
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Above
20 years
TABLE 15.3 (Continued…)
(b) Specification for Different Coating System
(i) Shop Applied Treatments
Coating System
1
2
Surface
Preparation
Blast Clean
Blast Clean
Pre-fabrication
primer
Zinc Phosphate
Epoxy 20 m
2 pack Zincrich Epoxy 20
m
Post fabrication
primer
High-build Zinc
Phosphate
modified Alkyd
60 m
2 pack Zincrich Epoxy
20 m
Intermediate coat
Top coat
3
Blast Clean

Hot tip
Galvanise
85 m

High-build Zinc
Phosphate 25
m




155
4
Blast Clean
2 pack Zinc-rich
Epoxy 20 m
2 pack Zinc-rich
Epoxy 25 m
5
Girt Blast

Sprayed Zinc or
Sprayed
Aluminium
6
Blast Clean
Ethyl Zinc
Silicate 20
m
Ethyl Zinc
Silicate 60
m
2 pack Epoxy
Micaceous Iron
oxide
Sealer
Chlorinated
Rubber
Alkyd 35
m
2 pack Epoxy
Micaceous Iron
Oxide 85 m
Sealer

TABLE 15.3 (Continued…)
(b) Specification for Different Coating System
(ii) Site Applied Treatments
Coating System
Surface Preparation
Primer
Intermediate Coat
Top Coat
1
As necessary
Touch in

High-build
Alkyd Finish
60m
2
As necessary
Touch in
Modified
Alkyd
Micaceous
Iron Oxide
50 m
Modified
Alkyd
Micaceous
Iron Oxide
50 m
3
No site
treatment



156
4
As necessary

5
No site
treatment

6
As necessary
Touch in
Touch In

High-build
Micaceous Iron
Oxide Chlorinated
Rubber Micaceous
75 m
High-build
Chlorinated
Rubber

High-build Iron
Oxide Chlorinated
Rubber75 m