SMD, Structural Metal Decks, have supplied and installed millions of square metres of steel decking, across all sectors, from office/mixed-use and its typically non-aggressive environment to leisure, where a considerably more aggressive environment may exist (i.e. adjacent Swimming Pool areas). 

A key consideration when specifying a composite steel deck floor is durability and more specifically, the life to first maintenance (LTFM). 

Here we provide some guidance on the subject. It does not provide a definitive answer, as one does not exist due to the numerous factors involved (i.e. corrosivity of environment, project location, ventilation etc.). 

SMD Profiles – Standard Coating 

Unless otherwise specified, SMD profiles are manufactured from hot-dip galvanised steel strip designated: BS EN 10346: S350GD+Z275-N-A-C (S450GD+Z275-N-A-C for 450N/mm² grade products). 

These products have a coating mass of 275g/m² which equates to a thickness of approximately 0.02mm (20μm) per face. 

Hot-Dip Galvanised Coating 

Although the galvanising provides a protective coating, it does weather, albeit at approximately one tenth of the rate of steel (depending upon the prevailing conditions and thickness of coating). 

Factors which may affect the speed at which the galvanising deteriorates and subsequently exposes the steel substrate to produce rust as a result include: 

• Length of time the installed deck is exposed to the elements before the building envelope is complete

• Localised damage to the deck during installation (deck sheets are typically dragged across the supporting structure which could scuff the finish and remove some of the coating)

• Failure to provide adequate ventilation when allowing buildings to dry out leading to condensation on the deck soffit

• High humidity levels/poor ventilation when the building is in use

• Presence of chemical agents

• Presence of other liquid based materials that are detrimental to the zinc coating

• Airborne pollutants

 

PLEASE NOTE – Both the 275g/m  and 310g/m2 coatings options for SMD products are adequate for the relatively short period of exposure between rolling of the deck profile and enclosure within the building fabric.

Corrosivity Category

BS EN ISO 9223 documents corrosivity categories C1 to CX; with C1 for Interior: dry (low corrosivity) to CX Extreme Humidity (high corrosivity). 

This subject which is covered in more detail on TGN Online, Section 7.4 and 7.5.

Aggressive Environments 

Where the environment is deemed to be aggressive (i.e. Categories C3 – CX), additional corrosion protection measures should be considered by the responsible party, considering both aesthetics and structural implications. 

SMD offer a High Durability (HD) Coating which has the designation: BS EN 10346: S350GD+ZMA310. These products have a coating mass of 310g/m2 which equates to a thickness of approximately 0.025mm (25μm) per face. 

Another option that may be considered for extending life to first maintenance is the addition of a suitable paint finish, see subsequent section. 

General Considerations / Conclusions 

The general advice for use of steel decking is; 

For a typical dry internal location, no further corrosion protection is required. 

Steel deck composite slabs can be used in aggressive environments 

Corrosive environments may require additional protection (SMD HD coating or paint finish) 

NOTE: The guidance outlined in this document is based on the most economical solution with the assumption that the decking is providing tensile reinforcement to the slab (design in accordance with BS5940-4 or Eurocode 4). 

An alternative in aggressive environments where HD coating is not deemed appropriate is to utilise the steel decking as permanent formwork only with no contribution to the final slab design. In this scenario, the engineer will need to design the slab as a standard in-situ reinforced concrete slab with appropriate quantities of in-slab re-bar (this will typically result in greater quantities of reinforcement than when the decking is considered as tensile reinforcement). Since the deck is only required to carry load during the wet concrete construction stage, any deterioration of the galvanising and steel substrate over time does not affect the structural. 

Further Guidance 

In addition to the guidance and references provided in this document, further general guidance on corrosion protection of steel as a material can be found at the BCSA steel construction information portal