Perfecting the process

The Queensland Department of Transport and Main Roads (TMR) is advancing leaps and bounds in the development of foamed bitumen stabilisation (FBS) in Australia. Its latest efforts have seen it receive a top industry award.The use of foamed bitumen stabilisation (FBS) has nearly 20 years’ of history in Queensland, with TMR at the forefront of its development and subsequent technical specification.

Since 1997, the state road authority’s (SRA) Engineering and Technology branch has continually developed a FBS technique to provide a more flexible and fatigue-resistant stabilisation treatment specific to Queensland conditions.

It is a technique used by TMR state wide with the intention of avoiding fatigue and transverse cracking – a result of stiff pavement layers.

“Traditional pavement stabilisation typically involves using high cement additive contents, resulting in the development of a fully bound cement-treated base (CTB) layer,” says TMR’s Director Pavements Rehabilitation Jothi Ramanujam.

Due to the stiffness of the CTB layer, transverse shrinkage cracks develop along the length of the pavement.

“Although the SRA has changed from general purpose cement to more slow setting additives, it still aims for higher unconfined compressive strengths,” says Mr. Ramanujam.

“This will always lead to higher stiffness and associated cracking problems.”

Because of the relatively thin nature of granular pavements in Queensland, TMR’s Pavement Rehabilitation Unit has developed several treatments for modifying, rather than stiffening, the base layer. This involves using FBS treatments that increase material strength and reduce permeability in order to produce a layer with improved properties compared to the original granular materials.

Early projects (constructed during late 1997) that utilised FBS included a 1.6-kilometre section of Cunningham Highway in Inglewood and a 17-kilometre stretch of the New England Highway in Allora.

The organisation’s Pavement Rehabilitation Unit has been proactive in sharing the experiences and knowledge gained from these past projects with other road builders over the past two decades.

“This includes participating in pre-start construction meetings, in-house and project linked training,” says Mr. Ramanujam. “Training has helped provide technical advice to both client and contractor. This technical advice has led to significant improvements in production, quality and efficiency, and these learnings allow the client and contractor to identify risks and improve quality of construction.

“Monitoring early projects has provided vital knowledge on how to improve materials and mix design testing, and construction processes,” Mr. Ramanujam adds.

These monitoring processes include site inspections, fatigue and rut resistance testing, deflection testing, back analysis of deflection and coring of samples from the field.

As FBS has become more accepted with greater volumes of materials recycled, the design, construction and maintenance techniques have been developed even further. As a result, the department has also developed a technical specification based on those real-world experiences.

“Industry and AustStab have played a major role in assisting TMR to develop a robust specification.

This included several rounds of discussions and reviews to arrive at the best specification covering FBS in Australia,” says Mr. Ramanujam.

The continual advancement of TMR’s FBS technique has seen the practice employed on larger and more significant projects within the state.

FBS-based projects, designed to take heavy traffic loading and provide flood resilience, were carried out on sections of major routes, such as the Warrego, Gore and Bruce Highways between 2012 and 2015.

TMR has also used different combinations utilising FBS on a number of projects. This includes the use of a foamed bitumen base and lime-stabilised subgrade, designed to take medium traffic loading, provide resilience to flooding, improve subgrades and encourage recycling of existing granular or cement stabilised base pavement materials.

It also employed foamed bitumen base and lime/cement/fly ash stabilised sub grade, designed to take medium to heavy traffic loading, provide resilience to flooding, improve subgrade and subbase (through triple blend additives) and encourage recycling of existing granular or cement-stabilised base pavement materials.

More than 2000 kilometres of pavement have now been completed in Queensland using the SRA’s FBS technique.

Mr. Ramanujam says the advantage of employing FBS base in combination with lime-stabilised subgrade is best explained through the example of the 14-kilometre project on the Flinders Highway.

The project, located on the highway between Julia Creek and Richmond, was notable for its remote location and challenges with local marginal materials.

“The project incorporated the two technologies, resulting in a 600-millimetre-deep stabilised pavement with significant cost savings due to the utilisation of all existing materials,” Mr. Ramanujam says.

Due to its relative flexibility and resistance to saturation induced strength loss, FBS was identified as having potential as the best solution during the project design stage. Asphalt was not an option due to cost and the isolated location.

“Cementous blends were discounted due to brittleness and the desire to avoid managing extensive cracking as experienced over many years of cement stabilising maintenance works on this road,” says Mr. Ramanujam. “The challenge was to achieve a FBS layer with significant improvements in production, quality and cost efficiency.”

Notable outcomes from the project included significant savings through continuously testing samples of stabilised field pavement and establishing optimum bitumen content and re-use of existing fatigued cement-treated bases, using a profiler and cross-blending techniques. A reduction of bitumen content from the design value of 3 per cent to 2.5 per cent led to major savings without affecting quality. Further to that, there were significant project savings through unconfined compression strength testing of subgrade samples and establishing optimum lime content.

Given the successful employment of FBS on a range of projects, and the massive amount of construction and specification development work in Queensland, AustStab acknowledged TMR’s lead in harmonising foamed bitumen technical specification with other SRAs.

“It was AustStab that acknowledged the harmonisation process was ready to start after formal meetings with all SRAs in 2014,” says Mr. Ramanujam.

AustStab proposed to facilitate and support the process, and TMR offered to lead the project.

The fruits of TMR’s labours came when it held its most recent forum on the subject, which marked a major milestone in developing FBS for use not just in Queensland, but around the country too.

The most recent FBS technical specification was discussed at the forum, held over two days at the end of February 2016.

Mr. Ramanujam and Senior Technical Officer Damian Volker used the event to explain the philosophy of the TMR FBS specification.

Mr. Ramanujam talked about the history of FBS in the state, and its progress over the past two decades, pointing out the close work between AustStab members and TMR to develop sound procedures.

Mr. Volker talked about the durability of the FBS used on the Oakey Pittsworth Road, where floodwaters had damaged the road, bar the newly laid FBS layer and its lime stabilised subgrade. He also touched on the various clauses used in the specification, including accurate depth control, complete mixing of evenly distributed binders, moisture control and compaction.

“The forum was considered highly informative and encouraged discussion from road authorities and ARRB,” says Mr. Ramanujam. “As a result, all SRAs agreed to work towards incorporating and/or harmonising the important aspects of TMR’s foamed bitumen technical specification.”

Through this harmonisation process, Mr. Ramanujam asserts that industry will benefit from improved design, construction and performance monitoring procedures, and that sharing this knowledge and expertise in FBS will help deliver more suitable pavements around Australia.

“The harmonisation project will also facilitate consultation between SRAs and TMR to expand their use of FBS, provide a platform to work on national foamed bitumen test procedures and encourage all SRAs to further research FBS,” he says.

Off the back of its continual research into FBS and the positive outcomes of the forum, TMR was awarded the prestigious Excellence in Research or Education award at the 2016 AustStab Awards of Excellence held this past July.

As for the future development of a harmonised technical specification for FBS, TMR, ARRB and AustStab are now working closely to develop national foamed bitumen test methods, which may also lead the way for other specification in the sector.

“The success of the harmonisation of foamed bitumen stabilisation technical specifications and testing methods will encourage similar harmonisation in the areas of lime and cementitious stabilisation,” concludes Mr. Ramanujam.

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