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2020 / 2021 Edition

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Bitumen Durability

First published in the June 2019 issue of Quarry Management 

Carl Robertus, technical director with Nynas, provides an insight into a potential new way of assessing the long-term durability of bitumen using a parameter known as ΔTc. 

In recent years the supply landscape for bitumen in Europe has changed, thanks to a combination of refinery changes, closures and rationalizations, and new players entering the market. Today, bitumen is produced in around 50 refineries in Europe, meaning that many customers are getting their bitumen from further afield than they did previously, or from a variety of different suppliers. As a result, it can be difficult to ensure they are getting consistency in quality.

In Europe, bitumen quality is regulated by the Construction Products Regulation, while bitumen for roads has to conform to standards produced by the European Committee for Standardization (CEN). When it comes to specifying road bitumen, most specifications are performance-based, albeit using properties derived from traditional, empirical test methods.

However, new materials are coming to market all the time, and customers keep demanding better performance from their road materials, so it is worth looking at whether materials that comply with standards based on empirical tests are suitable for all of today’s applications and circumstances.

‘Since the introduction of harmonized European bitumen standards, new methods are being developed and new ways of evaluating bitumen performance are continuously researched all over the world,’ explained Nynas’ technical director for bitumen, Carl Robertus. ‘This has helped deepen our understanding of the bitumen products we use to build our roads.’

Increasingly, customers are focusing on the long-term performance of road materials; they need their roads to be durable and to last as long as possible, while also meeting the higher performance demanded by today’s usages and traffic levels. But how can they know which bitumen products are going to give them the best long-term durability?

Recently, researchers in both Europe and North America have been looking closely at the premature surface cracking of asphalt pavements that can be attributed to ageing, rather than to only load- or temperature-related stresses. This has provided interesting insights into new measures that could be used to assess a bitumen’s long-term durability, particularly a parameter known as ΔTc.

ΔTc is calculated from rheological data that can be easily measured using a rheometer – nowadays a standard piece of equipment in most bitumen producers’ laboratories and in many customers’ and highway agencies’ R&D facilities. ΔTc is used as a measure of the ability of bitumen to relax stress in the asphalt, and can be applied to both standard and polymer-modified binders, and to bitumen recovered from fresh asphalt mixes or asphalt that has been in situ for some time.

Asphalt binders lose their ability to relax stresses at low temperatures as they age. As a result, ΔTc of an aged binder reduces and becomes more negative than that of a fresh, unaged binder, and there is also likely to be some cracking.

‘There is considerable evidence that ΔTc correlates well with non-load-related cracking of pavements, suggesting that it is a good measure of the contribution of bitumen to pavement durability,’ said Mr Robertus.

Research carried out so far suggests there are critical levels for ΔTc values. ‘As a very general rule of thumb, if ΔTc is less than –3oC there is a potential risk of pavement cracking; and when ΔTc is less than –5oC there is a very probable risk of pavement cracking,’ explained Mr Robertus. 

Current European bitumen standards do not specify ‘long-term ageing’ properties. However, a wealth of rheological data on fresh and aged European bitumen has been published, allowing the industry to understand bitumen properties beyond those defined in the standards. Analysis of the rheological data from around 200 different bitumens available in the market has given some very interesting results. While most bitumens meet the requirement for short-term ageing – a key part of the current European bitumen standard – the ΔTc values after long-term ageing range from +4oC to –17oC.

‘If ΔTc is indeed indicative of pavement surface cracking or durability, as the current research suggests, the wide range of values found may result in large variations of actual pavement performance across Europe, depending on the origin or choice of bitumen used,’ cautioned Mr Robertus. ‘Not every asphalt type, application or pavement construction is equally sensitive to ageing, but this example clearly demonstrates that choosing the right bitumen for the right application is all the more important.’

He added: ‘There are many factors that will influence pavement performance ultimately, including asphalt mix design, binder content and pavement design. But I’m convinced that ΔTc is a measure that characterizes the contribution of bitumen when it comes to long-term ageing and durability. Consequently, if I had to choose a bitumen in an application where the binder is ultimately exposed to the elements, I would prefer to choose a bitumen with a positive ΔTc and which doesn’t change rapidly over time, to avoid suffering early pavement failures.’

Nynas are currently working with some of their customers to confirm the relationship between actual pavement performance and both ΔTc and the Glower-Rowe (G-R) parameter, which is a measure of bitumen stiffness under certain standardized conditions. ‘We want to verify that the experience elsewhere is relevant for European conditions such as climate, pavement/asphalt types and bitumen,’ explained Mr Robertus. ‘We’re examining pavement core samples and determining the state of the bitumen in terms of ΔTc and G-R.

‘It’s very interesting,’ he added. ‘We’ve seen, for example – as others also have reported – that when you carefully extract bitumen from a pavement core in layers, you often see the top layer showing poorest ΔTc, while lower down in the same layer ΔTc is better. Of course, the top of the pavement is more exposed to UV, oxygen and weathering than deeper down, but it gives some confidence that ΔTc is a valid parameter for long-term durability.’

In Europe, ΔTc is still being evaluated and is not yet well enough accepted to be included in product specifications. However, it is also already being used to help characterize asphalt mixes that contain recycled asphalt pavement (RAP) materials, due to the proportion of older – and, therefore, highly aged – bitumen in the recycled material. ‘It is accepted, but not yet at the product specification level,’ said Mr Robertus.

If ΔTc is accepted as a measure of the loss of relaxation properties of asphalt binders, it has many potential benefits. As Carl Robertus suggests, it could be used as part of a performance-related specification for binders in new asphalt mixes. Equally, roads managers could monitor the rate at which ΔTc changes over time to give an indication of the ageing behaviour of the binder, to help determine when intervention is needed before cracks start to appear. It also has obvious potential in evaluating binders from mixes containing recycled materials and identifying how best to reactivate the aged bitumen from RAP.

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