Modified to make the grade

22/10/2018

The fundamental premise of FM Conway self-delivery model is to maintain control of raw materials, while reducing input costs and the overall carbon footprint of the business. One such component addressed is the supply of bitumen.

The push towards further urbanisation and changes in bitumen manufacturing across the world have seen the global bitumen market become volatile. This unpredictable nature saw the commissioning of Imperial Wharf Bitumen Terminal in the south east of England (pictured below).

The Thames side facility has a storage capacity of over 8,000 tonnes of bitumen and an average daily supply of 500 tonnes.

The next step to further FM Conway’s self-delivery ethos has seen the development of a PMB plant, at the terminal. The increase in demand for PMB has been proportional to the increased demand for penetration grade bitumen, due to the growing pressure for higher-quality road surfaces.

The incorporation of PMB in surface course asphalt mixes provides benefits such as;

• Reduced temperature susceptibility

• Improved resistance to rutting and cracking

• Improved cohesion and adhesion

The development of the PMB plant makes FM Conway the only asphalt manufacturer in the UK with the capability to manufacture its own PMB. Moreover, coupling this with the research and development facilities at FM Conway’s Innovation Centre, allows bespoke PMB to be tailored to suit the project at hand. Therefore, the project being undertaken dictates the PMB being used, rather than the PMB available off the shelf dictating which projects FM Conway are able to undertake.

Although the art of polymer modification has a long history, the design considerations for manufacturing still require careful consideration and planning. Bitumen is a highly complex colloidal system and a polymer is defined as a large molecule made up of smaller monomers. The properties of the polymer are influenced by the type and quantity of each monomer. The characteristics of the bitumen, the polymer, the polymer content and the manufacturing process determine the final properties of the PMB.

Achieving homogeneity in polymer modified mixtures can be a major hurdle during manufacturing, and is governed by the heat and entropy of mixing. Furthermore, increasing the polymer content can lead to a phase inversion within the bituminous system, where the polymer becomes the dominant phase. The control of the phase morphology allows the optimum polymer content to be selected in order to achieve two interlocked continuous phases. An effective polymer modification results in a thermodynamically unstable but kinetically stable system.

FM Conway’s SureFlex range of PMBs are designed using high quality polymers and base bitumen. The properties and morphology of these products have been determined and characterised using empirical test methods, rheology and fluorescence microscopy. Each PMB in the SureFlex range has been manufactured to provide superior performance in situations where challenges have been identified. Furthermore, these solutions, in the form of PMBs, have been determined and optimised by assessing the relevant mechanical properties of hot mix asphalt.

This unique setup has provided a pivotal role in various FM Conway projects, possibly none more significant than the collaboration between FM Conway and Transport for London to resurface the A40 trunk road in London by making use of an asphalt concrete (AC) surface course containing 50% recycled aggregates.

Implementing this quantity of recycled material in base and binder courses is not unusual, however, addition to a surface course is restricted to 10%. This is due to concerns that high recycled content cannot provide adequate skid resistance and durability. The concerns around the long-term durability of the AC surface course stem from the fact that the aged binder, coating the reclaimed asphalt, will reduce pavement life due to the risk of early onset of thermal and fatigue cracking. Conventionally, this issue is compensated by making use of a softer grade of virgin penetration grade bitumen during the manufacturing of the new material. However, rheological test data shows that this method, although alleviating the distress of fatigue cracking, aggravates another major distress factor, namely rutting. Moreover, the use of a well-designed PMB with tailored rheological characteristics can address both distress factors simultaneously.

The specially formulated PMB was designed to compensate for the ageing of the binder from the reclaimed asphalt, as well as meeting the target properties required for optimum binder performance. A range of polymers and additives were trialled at various quantities to assess their compatibility and modified binder properties, through use of empirical and rheological tests. The rheological evaluation for this project was carried out by measuring the linear visco-elastic properties of the binder, through use of dynamic tests.

The target properties were met by more than one type of PMB, after which, the shortlisted binders were assessed for their long-term durability and resilience. The PMBs, by themselves, and blended with bitumen from reclaimed asphalt, were short-term and long-term aged using laboratory simulation tests. The aged binders were then subjected to rheological tests to estimate the damage tolerance of the binder and assessed against functional models, which predict the onset of damage and significant cracking in pavements. The final selection was made on a PMB which provided compatibility, binder performance, durability and cost-effectiveness.

Recycling can be carried out using high performance PMB materials without adverse impact on material performance. Projects addressing other durability related issues, such as extension of PMB to all layers of asphalt and understanding long-term PMB ageing, are already underway at FM Conway’s Innovation Centre.

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