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Precast poles: when surface spalling doesn’t impact structural integrity

 are suitable for use in all bushfire-prone areas.

With the Australian bushfire season fast approaching, it’s imperative for local and state governments to ensure the country’s infrastructure is bushfire ready. An often overlooked yet undoubtedly vital piece of infrastructure is the ubiquitous utility pole.

Most commonly facilitating overhead power lines, but also telephone lines, utility poles are a lifeline to rural communities, both day-to-day and during a bushfire.

The material properties of precast concrete make it an excellent material choice for utility poles that are deployed to bushfire prone areas. As well as being inherently non-flammable, non-combustible and ember resistant, precast poles ensure a fast construction turnaround, low per-unit cost and high degree of design customisability.

National Precast’s Master Precaster Rocla has taken great pride in protecting electrical and telecommunications services for rural communities for the last 45 years with the supply and installation of their tried and tested, bushfire-safe precast concrete poles.

Maintaining structural strength during a bushfire is important as high winds (creating high loads on poles) are common in bushfires. Poles can collapse due to the complete loss of structural strength or from wind loads acting on poles that have partially lost structural strength during the bushfire.

With Rocla concrete poles maintaining structural strength, electricity supply during and after a bushfire can be maintained.

Core unimpeded by surface defects

With temperatures exceeding 660 degrees Celsius – the melting point of aluminium –Australian bushfires experience rare, short-lasting heat fluxes. While prolonged exposure to the high heat loads of a bushfire can occasionally see precast concrete poles exhibit cosmetic, non-structural surface defects known as pop-outs, Rocla’s poles boast little to no structural degradation under these conditions.

It is in the most extreme heat fluxes when the surface layer of precast concrete poles may spall, as the outermost aggregate layers degrade and expose the concrete core. In such rare cases where inner concrete layers have been exposed, the core which retains its original grey off-form colour, indicates that the inner, structural layers have not lapsed 300 degrees Celsius, and thus the pole’s structural integrity is not compromised.

Pole walls that exceed industry standards

As an industry leader in the manufacture and testing of precast poles, Rocla’s bushfire testing – which is in accordance with AS3600 ­– has proven precast concrete as an ideal material to withstand both the heat loads and wind speeds that far surpass that of a typical Australian bushfire.

The company has designed a wall thickness of 60 millimetres – double that of the recommended fire rating standard. Poles have been recorded to endure fire for six hours at 250 degrees Celsius and 20 minutes at 840 degrees Celsius. Both tests pass the typical specification of 115 per cent bending strength.

In the highest risk scenarios, it is not uncommon for precast concrete poles to be coated with additional fire-retardant materials such as vermiculite or cementitious grout. The external joint lug surface is not sensitive to losing the zinc coating and could be repaired if damaged.

Pole joints near ground level are most susceptible to fire. Joints higher up the pole are less likely to be exposed to the heat intensity and duration found near the ground. If the surface of a concrete pole is damaged, as described above, the surface can be readily repaired. A cementitious, non-shrink mortar can be applied to the surface after any loose or damaged concrete is removed. The rough, exposed surface created by high temperatures provides an excellent surface for bonding the repair mortar to the pole concrete.

As precast concrete elements are manufactured offsite in a factory-controlled environment, the cost, consistency and quality are unmatched. With Master Precasters such as Rocla at the forefront of the industry with their innovative, fire-resistant products, Australian communities will increasingly be assured of a safer bushfire season.

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