The Long Read: Closing the testing gap

08/03/2018

To mark National Pothole Day, David Smith, development director at FM Conway, looks at how material testing that simulates real-world conditions will support the sector to deliver a road network that’s fit for the 21st century.

As we design and develop new materials to combat climate change and respond to shifting resource availability, we need to guarantee material performance on the road, not just in the lab.

It’s a pivotal time for the highways sector. The industry is facing significant challenges that are transforming the way we design, construct and maintain our road network – most notably in terms of the materials we use.

On the one hand, the ongoing need to find a more sustainable way of managing the finite resources of aggregate and bitumen that make up our roads is pushing the industry towards ever greater use of recycled material.  In particular, the sector is having to think differently about how it sources bitumen, as technological advances in the oil industry cut supplies in favour of extracting more valuable fuels.  That means either exploring greater use of recycled bitumen or using new additives to enhance lower grade stocks.

At the same time, the industry is grappling with the impacts of climate change.  Wetter winters and warmer, drier summers in the UK are shortening the lifespan of our highways, calling for teams to find new ways to design in resistance to our roads and build long-term resilience to manage the impact of these weather events.

While we can design and test materials under controlled conditions, the critical part of the process is understanding how modifying highway materials affects real-world performance on the road. Research into the properties of recycled material, for example, has positively demonstrated the ability of RAP to meet the specification expected of virgin products, but there is still further work to be done in this field.  In the case of tackling climate change, it’s less a case of evaluating how adding different materials will affect performance, than actively selecting those components that will deliver the desired results on the highway.

With the sector in such a state of flux, we need to ensure that the processes we use to inform material specification and selection are still effective and appropriate to modern conditions.  This means establishing a baseline for research that both validates our current approach and informs future innovation.

That’s why FM Conway has been working with the University of New Hampshire and the University of Nottingham to put these practices under scrutiny. It’s the vital first step to closing the gap between theoretical and real-world performance and giving us certainty that the roads we create will stand up to the challenges of 21st century usage.

Predicting performance from lab to lane

The path to material specification starts in the laboratory, with mixtures developed and evaluated at a smaller scale before the move to full-scale plant operations.  Production of asphalt on an industrial scale alters the properties of the material and so we need to ensure that the intended outcomes targeted in the lab are the ones delivered on the road.

As plant operations continue to advance and we trial new materials, we need to constantly evaluate and test existing protocols for achieving this consistency of performance from lab to lane, to ensure that we can accurately predict and design for the changes that a material experiences at an industrial level.

When an asphalt mixture is produced in a hot mix plant, the bitumen is heated to reduce its viscosity so that it can properly coat the aggregate particles when mixed together.  The high temperatures that the bitumen experiences causes ageing, thereby producing a stiffer material.  For the purposes of anticipating performance on the road, this has two key effects.  On the one hand, greater stiffness means increased loadbearing capacity and therefore resistance to permanent deformation; yet it also reduces the pavement’s flexibility and relaxation capacity, hence making it more susceptible to cracking.

If we’re trying to create a mix for the purposes of resisting temperature fluctuations caused by climate change, it’s vital then that the effects of this heating process in the plant are taken into account in the lab. The established process for doing this is to short-term oven age a mixture over a period of four hours.

Working with the University of New Hampshire and the University of Nottingham, we set up a research study to test this approach to act as a baseline for modelling increasingly bespoke and complex materials in the future.  We selected a 20mm heavy-duty, high modulus (HDM) asphalt concrete binder course – as an example of a standard mix produced in the industry – and subjected it to four, five and five-and-a-half hour oven ageing, comparing the results of these tests to those for a mix produced at FM Conway’s Heathrow asphalt plant, which uses the latest technology available in the UK.

Of the three ageing tests, our study showed that four-hour oven ageing at 135 degrees Celsius provides a match to the rheological and fatigue characteristics of this particular mix when produced in the plant, thereby establishing an effective baseline against which to predict the performance of future mixes.

Looking forward

The next step is to repeat the process across other scenarios, using different plant operations and different mixes to reflect the fact that ageing is affected not only by mix composition, but also by the specific conditions experienced by a material in a plant’s mixer box.  As we continue to push the boundaries of materials research, this foundation for testing ageing processes must be revisited in each case.  This is crucial to giving the sector and its customers certainty that any research conclusions are based on real-world conditions; providing peace of mind that what’s produced in the lab can be replicated in the plant and ultimately on the road.

Our research partnership is now looking to do just this.  The next stage of our study is to trial different ageing processes for different plants and other sets of mixtures, with varying levels of recyclate and additives such as polymer-modified bitumen.

What we’re looking to establish is the baseline for all of these different variations so that we can start to put them to best use on the road, whether tackling resource challenges or climate change.  The first phase of the research is already enabling more innovative work to take place.  Having established the foundations for accurate lab production, our partners at the University of New Hampshire now have the confidence that the lab tests they perform on the standard 20mm asphalt concrete 20 HDM mix will apply equally to industrial-scale production.  They are now incorporating climate inputs into fatigue modelling for the mix, alongside the more standard process of measuring the effects of material properties, structure and traffic.  This approach marks a major step forward in the way that we predict long-term asphalt performance.

It’s an exciting time to be working in the highways sector.  There are challenges on the horizon but opportunities too, with major investment planned for the strategic network through Highways England’s second Road Investment Strategy and local authorities continuing to tackle long-term underfunding from central government and investing in our local roads.  Maximising this capital resource, however, relies on having a sound base for future innovation.  By starting from a point of knowledge, we have the confidence needed to push the boundaries of research and deliver better results for both operators and the travelling public.

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