Whole life costing: Steel corrosion protection

Peter Mayer

European structural design codes will introduce
the concept of ‘design working life’ to British
engineering. Peter Mayer of Building LifePlans 
explains exactly what that means.

The Eurocodes are a series of standards that will provide a common approach to structural design across the EU. As conditions affecting structures vary across the EU, each country can vary certain parameters. These will be introduced as annexes to the standards in mid 2008 with conflicting standards withdrawn by 2010.

When the Eurocodes for designing steel structures are fully adopted, one of the biggest changes for UK engineers will be the concept of “design working life” (see table).

While British Standards consider durability for a range of environments, Eurocodes take a more detailed approach, focusing on:
• The material deterioration processes
• Design options to resist degradation and actions on the structure
• Maintenance strategies to ensure functional performance.
These provide the basis for life-cycle costing analysis.

 

Design working life


Design working life is the period of intended use, with maintenance but no major repair anticipated, for a structure. Most buildings fall into one of two categories. Category 4 has an indicative working life of 50 years (building structures), while category 5 has a life of 120 years (monumental building structures and infrastructure construction).

The Eurocodes set out how materials, design, details, protective measures, workmanship and quality controls can be managed to achieve a required life.

 

Achieving the design working life


Putting these concepts into practice may involve standards needing to be cross-referenced to arrive at an answer. In certain cases, advisory organisations have published guidance as a shortcut.

With steel, for example, to determine a solution for the design working life, the EN 1993-1.1 refers to the EN 1090, which outlines the process for achieving the design working life for steel structures.

Once the solution has been determined, the risk of corrosion needs to be managed by some form of surface protective treatment. Issues to take into account include:
• The durability required
• The atmospheric-corrosivity category or exposure classification
• Metal spraying, galvanising, painting, subsequent decorative coatings, choices of colour and bimetallic interfaces.

 

Relevant standards


A suitable protective system can be specified by cross-referencing relevant standards for a given design working life, corrosivity category and maintenance strategy.

• Classification of corrosivity atmospheres: ISO 9223
• Preparation of surfaces: BS EN ISO 8501
• Protection of steel structures, zinc and aluminium coatings: BS EN ISO 14713: 1999
• Sprayed zinc or aluminium coatings: BS EN ISO 2063
• Paints and varnishes: BS EN ISO 12944
• Hot dip galvanized coatings: BS EN ISO 1461:1999
• Post application testing: National structural steelwork specification

 

Calculating the steel design working life


For atmospheric corrosion, refer to the Galvanizers Association Millennium Map of average zinc corrosion rates. About 50% of England and Wales has a rate of under 1µm/year. Steel with a 85?m galvanised zinc coating, in an environment with a corrosion rate of 1µm/year would have an expected life of 85 years. This falls within the 50 year category. For 120 years, thicker coatings of zinc galvanising is an option.

Internal steel structures where the internal relative humidity does not exceed 80% do not require corrosion protection. Information on steel corrosion is provided by Corus, the Galvanizers Association, National Physical Laboratory and Steel Construction Institute.

 

 

Specification options

 

Steelwork exposed to atmosphere Capital cost
£/m2
Life Cycle Cost for 50 or 120 years
£/m2
Period to first major maintenance
Years
Category 4: 50 years      
Hot dip galvanized to BS EN ISO 1461 mean coating thickness 85μm 35 42 80
Epoxy micaceous iron oxide (MIO) finish: 100?m; Zinc phosphate epoxy primer: 80μm. Steel: blast cleaned to Sa 2.5 22 61 20
High solids polyurethane finish:100μm. Zinc phosphate epoxy primer: 100μm. Steel: blast cleaned to Sa 2.5 17 48 20
Category 5: 120 years      
Hot dip galvanized to BS EN ISO 1461 mean coating thickness 85μm 35 43 80
Hot dip galvanized mean coating thickness 140μm includes shotblast preparation 50 58 120
Epoxy micaceous iron oxide (MIO) finish: 100μm; Zinc phosphate epoxy primer: 80μm. Steel: blast cleaned to Sa 2.5 22 73 20
High solids polyurethane finish:100μm. Zinc phosphate epoxy primer: 100μm. Steel: blast cleaned to Sa 2.5 17 57 20

 


Table notes

• External environment category C3 to ISO 9223, Average annual zinc corrosion rate: 1?m/yr. First protective system factory applied. Subsequent coating assumed at 8 – 10 year intervals including allowance for treating corrosion and quinquennial inspections. Dimensions refer to dry film thicknesses.

• Discount rates of 3.0% (50 years) and 2.5% (120 years) are used to calculate net present values.

• There are a huge range of alternative coating systems; costs and options are indicative. Use specialist advice for suitable options and a cost analysis based on project specific information for a realistic best value appraisal.

First published in Building 2007

 

Further information


BLP provides latent defect warranties for buildings www.blpinsurance.com

Further information contact peter.mayer@blpinsurance.com or telephone: 020 7204 2450

 

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