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NZ Construction News (Dec 10 / Jan 11) Concrete Floors Offer Peace of Mind Under Foot

01 Dec 2010

Concrete Floors Offer Peace of Mind Under Foot

Rob Gaimster – Cement & Concrete Association of New Zealand (CCANZ)

While the earthquake which hit Canterbury on September 4 thankfully claimed no lives and seriously injured only two people, its impact has been immense.  Damage to property and the fear of aftershocks have had an undeniably detrimental impact on the well-being of many people.

In such times of disruption the family home offers a great sense of security.  It appears from CCANZ’s initial review of the residential housing stock across the Canterbury region that properties with correctly designed concrete slabs performed well.

As a foundation for residential properties concrete slabs offer outstanding durability, along with low maintenance, fire resistance and an array of attractive surface finishes.

In addition, as designers and their clients become familiar with the principles of passive solar design and thermal mass, a concrete floor's ability to offer an efficient alternative to traditional methods of heating in winter and cooling in summer will contribute to New Zealand homes becoming more comfortable and energy efficient.

BRANZ research confirms that concrete slabs are the preferred flooring option for new homes built in New Zealand, with around 80% market share.

While there have been reports of concrete slab failure as a result of the Canterbury earthquake, this can be attributed to two reasons, neither of which detract from the advantages of an appropriately designed concrete slab.

Firstly, those concrete slabs that suffered damage were predominantly unreinforced or lightly reinforced.  Although unreinforced slabs are allowed under the New Zealand Building Code, a concrete slab should always be designed to suit site conditions.  For minimal cost a reinforced concrete slab offers excellent performance in response to ground movement as the combination of concrete and steel provides high levels of both strength and ductility. In particular, raft slab systems and slabs with concrete piles offer enhanced levels of performance.

Secondly, the concrete slabs that developed cracks were, in the majority of cases, located in areas previously identified by Environment Canterbury as at risk from liquefaction.  The decision to place an unreinforced concrete slab on land with geotechnical concerns was unwise.

Furthermore, during significant liquefaction most foundation and floor systems, including raised piled floors, will incur damage, requiring assessment and repair.  On properties where lateral spreading (the shifting or cracking of the ground beneath) occurred the entire house suffered damage, not just the foundation and floor.

In short, those concrete floor slabs with observed cracking following the earthquake were not optimally designed and were principally located in liquefied areas where damage occurred across the entire property, including foundation and floor systems regardless of type.

Concrete slabs have grown to become the favoured flooring option for New Zealand homes on merit. Anyone planning to build can be assured that a properly designed concrete floor will continue to offer great insulation, sound control and value for money, while also providing peace of mind under foot.

However, the key is, as with any building material and any structure, design and construction must be fit for purpose and fit for the site on which it is being used.