Concrete balconies - killers in waiting

Source: ACRA column in Australian Concrete Construction

Many cantilever balconies typical on home unit buildings in seaside suburbs are at risk of collapse.

Micro Fracture of Balcony surface
Damaged Reinforcement Due to Corrosion

A typical cantilever balcony has its main reinforcement at the top. Further, with normal deflection of the balcony concrete, it is not unusual for a crack to develop in the concrete at a point just outside the wall of the building. This crack allows salt from sea air to penetrate to the main top steel. The resulting corrosion reduces the steel cross-section over time, thereby eroding the capacity of the balcony to carry its design load.

No warning

Compounding this, the concrete used in many of these buildings has typically been of very poor quality, so that the corrosion product tends to be absorbed into concrete pores. This can cause serious deterioration of the reo long before the eventual spalling of the concrete cover. In other words, expect no good early warning signs that a balcony has degraded to danger point!

A near miss

A recent example of this has come to light in which the concrete was suffering from what appeared to be typical spalling due to reinforcement corrosion. Fortunately, the building owner had engaged a skilled specialist consultant (an ACRA member) to review the condition of the building before any repair work was begun. This condition assessment revealed the true state of the concrete, prompting a suitable program of reconstruction.

But had it simply been assumed that normal concrete repair was sufficient, the building might well have been returned to duty apparently repaired but in fact with a potentially lethal defect.

So why isn’t it raining balconies? 

Just plain lucky

Luck, most likely. Fortunately, buildings are designed for the largest load envisaged for that type of structure, and a home unit balcony is unlikely to be loaded to its design load very often.

But this is not to say it never happens (eg, ask yourself how many parties you've seen massing on balconies on warm summer evenings!), and it is only a matter of time before a balcony collapse finally occurs. 
Solving this is not easy, as even quite responsible building owners are unlikely to call for professional help until substantial deterioration becomes apparent. But even then, the signs may only be obvious to those with experience and training in the highly specialised field of concrete deterioration and repair.

The Balcony Bill?

It might be time to legislate for such buildings to be reviewed periodically by a qualified expert once they reach a certain age. This would ensure that the buildings and their balconies don't deteriorate to the point where they put lives at risk.

ACRA believes that it has an obligation to the community at the very least to raise this issue and is currently considering discussing it with legislators. Stay tuned.

By ACRA President, David Mahaffey,
Australian Concrete Construction, Dec 2000

Note: Please click here to learn more about MJ Civil Engineering's recent work with defective concrete balconies.