Long-term durability

The durability of bitumen is the focus for a ground-breaking collaboration between Nynas and the French road construction specialist Colas. Rheological testing of age induced cracking takes centre stage.

Three-year continuation project

During the next three years, Nynas and Colas will also be collaborating with Aston University, where researchers have expertise in pavement modelling and parameters that affect pavement ageing. The project will be supported by a PhD student.

During 2021, the project will be co-sponsored by the Asphalt Institute Foundation.

The purpose is threefold,” explains Hilde Soenen, Nynas Bitumen Research Manager. “We are evaluating which rheological parameters from the recovered binders relate to pavement performance, and we are making sure that the ageing we accomplish in the lab on fresh binders corresponds to what is actually happening in the field. The project will also tell us how the binder ageing evolves with pavement depth, and how this depends on the asphalt mixture properties.”

Asphalt core samples from a variety of different Colas pavement sites all over Europe provide examples of bitumen ageing in the field. In the Nynas specialist bitumen research laboratory, these are compared with bitumen aged in the lab. The comparison between the two offers the key to assess the long-term durability of bitumen.

“A detailed look at the asphalt cores we have received from Colas confirms that the ageing processes used in the lab introduce similar changes to the binder properties as ageing in the field. Although the extent of ageing in field sites is often greater than ageing in standard lab procedures, and for modified binders we also noted some differences between the two processes, our results are important as they show that we are on the right track towards establishing a reliable standard for durability assessments,” says Hilde Soenen.

Central to such a standard method is a parameter known as delta Tc that is calculated from rheological data easily obtained from a rheometer. Delta Tc is used as a measure of the ability of bitumen to relax stress in the asphalt, an ability that is lost over time. It’s a parameter hat can be calculated for standard and polymer-modified bitumen as well as for bitumen recovered from both fresh and used asphalt mixes.

“The project will also show us how this parameter changes over time for unmodified and modified binders,” adds Carl Robertus, Nynas Technical Director for Bitumen.

According to Carl Robertus there is considerable evidence that delta Tc correlates well with non-load-related cracking of pavements, which is why he argues that this suggests that delta Tc is a good measure of the contribution of bitumen to pavement durability.

Delta Tc could form part of a performance-related specification for binders in new asphalt mixes, including those containing recycled and aged binder materials. It could also be used to monitor the ageing of asphalt pavements, acting as an indicator for when cracks might start to appear. This, in turn, would provide valuable input to pavement management and maintenance strategies.

So far, the use of rheological parameters has been more widely tested in the United States. European standards are currently not based on rheological investigation.

“We need to make sure that proposed indicators are relevant also for asphalt pavements in Europe. Our collaboration with Nynas will help to ensure that the advice we give our clients is correct and will provide them with the best possible solutions,” says Xavier Carbonneau, Colas’ project manager.

In addition to the asphalt core samples, Colas also try to gather as much data as possible from the sites where the cores are taken. This data might include photographic evidence of cracking and other pavement defects as well as information regarding the number of cracks, the age of the pavement, traffic load, temperatures, and the general state of the road in question.

During the past few years, Colas and Nynas have worked on around a dozen pavement sites all over Europe. Moving on to a second phase, those involved in the project are identifying old pavement sites where they actually still have the original binder.

“We didn’t have the original binder of any of the pavements we studied earlier. But in this phase, we are able to look quantitatively rather than qualitatively at the damage on the pavement, and to combine these results with bitumen  properties from recovered cores and  fresh and lab aged original bitumen,”  says Carl Robertus.

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