Investing in research and innovation has paid big dividends for Queensland’s Department of Transport and Main Roads, with millions of dollars saved in the wake of Cyclone Debbie through more resilient pavements.

The Department’s foamed bitumen pavements, which when constructed in the right environment with appropriate stabilisation are more resilient to flooding, have survived unscathed in some of the worst-hit parts of the state.

When 3 m floodwaters inundated Camp Cable Road on the Mt Lindsay Highway, district staff understandably feared the worst. When waters receded, however, the foamed bitumen pavement was found completely intact.

While some conventional thin asphalt/granular pavements, such as Rosewood-Karrabin Road in Ipswich, suffered catastrophic damage from flooding, foamed bitumen pavements in similar circumstances showed amazing resilience.

The Bruce Highway (Sandy Gully) near Bowen showed no evidence of damage despite heavy rainfall. Similarly, pavement construction in Warrill View (south of Ipswich) was able to continue without delay, after the unsealed foamed bitumen pavement was inundated. Yeppen floodway in Rockhampton also emerged from the deluge with a clean bill of health.

Transport and Main Roads Chief Engineer Julie Mitchell says these are just the latest encouraging examples of foamed bitumen’s resilience.

“We are already using this technology widely in coastal regions of Queensland, and seeing excellent results,” Mitchell says.

“By using foamed bitumen the department is not only saving on the cost of construction, but also on the cost of maintaining and rehabilitating roads after natural disasters like Debbie.”

But what is foamed bitumen, and how does it work?

Foamed bitumen is formed by injecting a small quantity of cold water into hot bitumen to produce an instantaneous expansion. In this foamed state, bitumen is highly efficient at coating the finer particles of the pavement material.

These coated fines act in two ways: first as ‘welding spots’, binding together mineral aggregates in a water- tight structure, and second as ‘rubber’, providing greater flexibility under loading. These combined effects deliver a pavement with improved fatigue properties and greater resilience to flooding.

This resilience to flooding is massively increased when the foamed bitumen stabilised base is combined with a subbase and subgrade stabilised with triple blend (a mix of lime, cement and fly ash).

Through project-linked training, and the development of new testing, design and construction procedures, Transport and Main Roads is delivering this more durable, cost-effective pavement in suitable areas throughout Queensland.

History of foamed bitumen in Queensland

Foamed bitumen started small in Queensland, as a trial on the Cunningham Highway in 1997. Since then, acceptance of the stabilisation technique has grown dramatically, with the process now used successfully on more than 1000 lane km of Queensland’s roads.

Performance monitoring shows that foamed bitumen stabilised pavements are not only strong and flexible, but also resilient to significant rain and flooding. Outstanding examples include 3 km of Jacobs Well Rd and 14 km of the Logan Motorway, which are still performing well after more than 10 years of traffic loading, and 17 km of New England Highway, which is now approaching 20 years of traffic loading. All of these sites have experienced significant rain and/or inundation.

A tale of two methods

Foamed bitumen stabilisation can be undertaken in two ways: the insitu method, which is completed onsite, or the pugmill/paver method, which requires an offsite plant.

Using the insitu method, the existing pavement material is milled, mixed with foamed bitumen and additives, and then laid (without having to leave the site). This method offers two main advantages: the cost is generally lower – owing to the elimination of haulage – and the subgrade spends less time exposed to potentially damaging weather.

The pugmill/paver method, however, has its own considerable advantages. Using this process, the milled material is transported to a central batch plant, where it is mixed with new granular material (if required) and additives. (If existing material is unsuitable, imported granular material can be used for the production of the plant mixed foamed bitumen.) The modified product is then hauled back to site for laying.

The pugmill/paver method offers the opportunity to:

• inspect the subgrade and rectify any weaknesses
• reject unsuitable material and/or add external materials
• place a geo-composite layer

The pugmill/paver approach has been shown to deliver a more consistent product, and with improved mobile plants now more readily available, the method has become more feasible and cost-competitive. Plant Mix foamed bitumen methodology can be adopted for both new construction and rehabilitated pavements.

This story was originally published in the July/Aug edition of inspire magazine.