56 BUILD 123 April/May 2011

COATINGS FOR STEELWORKIf you can estimate the corrosion rate of a building’s steelwork and specify an appropriate and cost-effective protective coating system, you can maximise the performance of the building.By Raed El Sarraf, Structural Engineer, Heavy Engineering Research Association

In 2005, the Heavy Engineering Research Association (HERA) published a report entitled New Zealand Steelwork Corrosion Coatings Guide. It was originally intended to cover the coating selection for steel bridges, but it was expanded to include guidance for all types

of steelwork. The report provides a process to select and specify an appropriate and

cost-effective coating system for structural steelwork. Engineers have used the coatings guide to estimate the corrosion rate of steelwork and specify protective coating systems with known (present and future) costs. This has helped reduce costs and uncertainties associated with specifying the wrong product. The coatings guide also provides general information that can help ensure the proper use and application of coating systems.

Revisions and additionsIn 2011, the coatings guide was revised, and a renamed New Zealand Steelwork Corrosion and Coatings Guide was released with several improvements, including: ❚ the latest advances on protective coating systems, their appearance and costs

❚ an overview of corrosion protection from NZS 3404.1:2009 Steel structures Standard – Part 1: Materials, fabrication, and construction

❚ simplified corrosion maps and an up-to-date prevailing wind diagram, also from NZS 3404.1:2009

❚ guidance for determining the corrosion performance of galvanised steel decking in car parking buildings

❚ guidance for determining the life cycle costing of a coating system, including the future maintenance cost for bridges and other steelwork

❚ an introduction to the New Zealand Coatings Industry Checklist

❚ new design examples.Some of the new additions are summarised here.

Corrosion mapsCorrosion maps from the 2005 edition of the coatings guide and NZS 3404.1 are included in the 2011 edition. These maps are a world first, and they provide the most comprehensive macro-climate corrosion rates of mild steel in different atmospheric corrosivity categories around New Zealand. An example of the difference between versions is shown in Figure 1.

Galvanised steel decking in car parking buildingsGalvanised coil products, such as steel deck slabs, are increasingly being used in external applications, such as exposed car parking buildings. Although New Zealand has several 25-year-old car parking buildings, there is little guidance to help determine the time to first maintenance of the galvanising for these decking systems.

Site visits were undertaken in Auckland and Wellington to study the performance of the galvanising in steel decking in exposed car parking buildings. Based on these observations, revised guidance is presented in the

MATERIALS PERFORMANCE

Figure 1: Detailed (left) and simplified (right) corrosion rates of carbon steel in the North Island.

BUILD 123 April/May 2011 57

2011 edition of the coatings guide. This includes an introduction to the ‘shelter reduction factor’ for use in a number of atmospheric corrosivity categories. The section includes guidance on repairing the galvanising of the steel decking and how to extend its durability by applying an additional protective coating system.

Determining the life cycle costing of coatingsThe life cycle costing of a corrosion protection system can be used to determine whether a bridge should be constructed using structural steel.

Designers can apply this research to other steelwork as well, and with

advances in coating systems for severe environments, the coatings guide and the development of the life cycle costing model, designers now have the tools to determine the life cycle costing of coatings more confidently. Guidance on determining the life cycle costing of the specified protective coating system is outlined in the 2011 edition of the coatings guide.

An estimate of an inorganic zinc silicate coating over a 40-year period in an atmospheric corrosivity category C is shown in the example shown in Table 1.

The New Zealand Steelwork Corrosion and Coatings Guide can be ordered from the HERA Library on (09) 262 2885.

Figure 2: Example of water-induced corrosion at the edge of galvanised steel decking after 20 years service.

Table 1: Example life cycle costings for an inorganic zinc silicate protective coating on a New Zealand Transport Agency bridge.

Year System designation Area maintained (%)

Current costa ($ per m²)

Net future valueb ($ per m²)

Net present valuec ($ per m²)

0 washdown, initial coat, coating inspection 100 44.00 44.00 44.00

0 2% erection touch-up, coating inspection 2 1.78 1.78 1.78

3 2% or 5% touch-up repair, coating inspection 2 1.78 1.78 1.50

Initial cost per m² 47.56 47.28

30 Surface inspectiond 100 0.00 0.00 0.00

30 2% or 5% touch-up repair, coating inspection 5 4.46 4.46 0.78

40 Surface inspectiond 100 0.00 0.00 0.00

40 100% recoat, coating inspection 100 57.00 57.00 5.54

43 2% or 5% touch-up repair, coating inspection 2 1.78 1.78 0.15

70 Surface inspectiond 100 0.00 0.00 0.00

70 2% or 5% touch-up repair, coating inspection 5 4.46 4.46 0.08

80 Surface inspectiond 100 0.00 0.00 0.00

80 100% recoat, coating inspection 100 57.00 57.00 0.54

Maintenance cost per m² 124.70 7.09

TOTAL 172.27 54.37

a Current cost equals the area maintained multiplied by the system cost.b Net future value is the current cost plus inflation. In this example, the recommended inflation rate for New Zealand Transport Agency is 0%.c Net present value is the net future value adjusted for the discount rate. In this example, New Zealand Transport Agency’s discount rate is 10% for 25 years.d Inspection costs are covered by New Zealand Transport Agency.