Iftekhar Waseem, Structural Engineer for ComFlor® at Tata Steel Europe, discusses issues on the durability aspects of composite metal decking.

Zinc performance background

As evident from history, the performance of galvanized steel decking within protected environments in dry applications has proven excellent. The predicted design life of the standard Z275 coating, based on the measured loss of zinc from the strip steel is over 200 years, provided that the decking is properly maintained. The evidence of this conclusion is based on measurement of zinc loss in various applications and locations.

In the UK, the standard coating is Z275 (i.e. 275 g/m2). This equates to a coating thickness of about 20 microns average on each side. The Z275 grade was formerly used in BS 2989 and has now been incorporated within BS EN 10346. The coatings are thinner than those formerly specified (in BS 729) because research shows that the corrosion resistance is satisfactory for most internal building applications.

Although the hot-dip galvanized product has not fundamentally changed over the last 30 years, the expected product lifetime in external atmospheres has almost doubled in the UK as a consequence of improved air quality. This has enabled hot-dip galvanized coatings to protect steel for longer periods, and newly manufactured components have a much longer life expectancy than would have been predicted 30 years ago, while old coatings are expected to exceed the original predicted life expectancy.

The approximate performance of zinc coatings in different environments is shown in the table below, which is taken from EN ISO 14713. The lifetime of zinc coatings has improved, and research work suggests that these figures are conservative.

Structural integrity and design life – All applications

The design life of a galvanized metal decking comprises the life of the protection system plus that of the underlying metal sheet. The design life of the protection system could be defined as the time period to the first maintenance of the coating, when recoating or some other treatment is required to restore the total effectiveness of the protection. If there is no maintenance at this time, the coating would continue to deteriorate and the underlying metal deck starts to corrode.

The design life does not represent structural failure of the metal decking, and there may be a considerable margin between the design life of the protective system and potential structural failure.

However, when the metal decking is utilized in exposed applications as highlights in Categories C3, C4, & C5, the decking would be highly susceptible to the environment and would start to corrode aggressively with time, jeopardizing the structural integrity of the composite slab eventually.

Recommendations

There may be various recommendations in the industry to overcome the corrosion issues but the options below could be considered as prime especially in aggressive environments like swimming pools, chemical plants and coastal applications.

• Post-coating/ treatment with adequate protective paint to suit desired life-span
• Regular maintenance depending upon the complexity of slab construction in terms of time, costing, and location
• Utilize the metal deck as a permanent formwork and design the slab in RC with adequate reinforcement to cope with the permanent loading condition

Multi-Storey Car Park applications

Car park applications can be quite an aggressive environment, as it would not just involve the risk of deck corrosion from underside but also from the top due to frosting moisture, de-icing salt etc. that would lead to significant moisture penetration through cracks into the slab leading to reinforcement and deck corrosion.

As per an IStructE theory published in one of their manual ‘Design recommendations for Multi-Storey Car parks’ – fourth edition, metal decking can only be used as a permanent formwork in car park applications and not as a composite slab. However, past experience through various projects has proven this is surely not the case. As per the MCRMA flooring group, the theory seems to prove unreasonable, as the moisture permeation issue would also apply to RC slabs in terms of reinforcement corrosion.

Recommendations

• Top surface – Seal the top slab surface adequately to avoid moisture permeation
• Underside – Apply protective paint OR adopt pre-finished steel decking such as ComFlor® with Colorcoat  FD®
• Regular maintenance

For more information, contact the Technical department at Tata Steel: comfloruk@tatasteel.com