An important question is raised during the recycling of asphalt pavement: what is the best way of calculating the effect on the new asphalt mixture of bitumen from the old? Nynas has been looking into ‘binder grade correction’ and ‘bitumen rejuvenation’.
Early approaches to recycling considered recycled asphalt pavement (RAP) as ‘black aggregate’ in which the binder made little or no contribution to the properties or volume of any new asphalt mixture to which it was added. But this is not actually true – the fact is that the new mixture contains two bitumen grades (fresh bitumen and that from around the RAP) and an increased proportion of binder.
And while the physical properties such as penetration and softening point of the fresh bitumen will be known, along with its volumetric content, those of the RAP may not be. In order that its overall influence on the asphalt mix can be determined, the blend and content of the recycled component must be calculated. Then ‘correction’ can be made to the RAP binder’s grade to meet the mix’s design requirements and also attention paid to its volumetric contribution.
So says Nynas UK Technical Manager Dr Ian Lancaster. Recycling asphalt is absolutely the right thing to do, he says; but everything to do with asphalt recycling requires thought and effort to do it properly. “Asphalt is 100% recyclable but care is essential to ensure the successful reuse of bituminous materials.”
Empirically, RAP binder will generally be expected to follow the normal blending rules for bitumen, making correction for grade a relatively simple process in principle. But formal ‘binder grade correction’ is a distinct move from empirical to a more mechanistic design and the process is not as easy as it looks, says Dr Lancaster. The penetration of the bitumen in the RAP will depend on its age, original asphalt type (open, dense; surface, binder course and so on) and geographical location.
Dr Lancaster has explored the practical applications of blending principles applicable to asphalt recycling and provided worked examples of binder corrections. One of these showed that RAP with a binder penetration of 15 could be corrected to 100/150 pen bitumen by blending with 190 pen bitumen in the ratio 17% RAP binder to 83%. Production of a 40/60 pen binder would require correction in a nominal 50:50 ratio. So far, so simple.
However when the binder volumetric content of the RAP and that of the asphalt mixture are taken into consideration, the calculation becomes significantly more difficult – “there is a lot of math behind it,” Dr Lancaster says – and considerable care is needed in the application of blending principles.
Working out the total effect of RAP binder on an asphalt mix containing recycled material involves an element of ‘back calculation’ to get exactly the right match of bitumen volumes to meet the required asphalt end result. “The volumetric contribution of the binder in the RAP is difficult to deal with.”
Certain ‘natural’ laws apply, apparently, for instance in some situations it is relatively easier to design harder mixes. “There are drivers for recycling to make use of all bitumen grades and to use as much RAP as you possibly can which – pushed to the extreme – could have you searching for 800 pen bitumen, which isn’t going to happen.”
Dr Lancaster goes on to say that: “It would be great if you could fully restore RAP bitumen back to its virgin state because then you wouldn’t have to deal with binder correction. The doors would be open to using very high quantities of RAP in a normal mix – surely the holy grail of asphalt recycling.”
To consider the practicalities of rejuvenation, Dr Lancaster says it is necessary to understand the ageing processes to which the binder in an asphalt mix is subjected. These can include the effects of oxygen, water, infrared and ultraviolet radiation, thermal stresses and mechanical and chemical attrition. An awful lot can happen to a binder during its in service life.
“As a result of these ageing processes, bitumen gets harder, stiffer, more brittle; its penetration goes down and its softening point up. While it is relatively easy to correct the penetration and softening point of bitumen around RAP – this can be done adding ‘rejuvenating oils’ or other such substances – this does not take into account molecular changes.”
He says that, if you compare the rheological profiles of unaged bitumen and one of equivalent penetration which has been generated using aged bitumen and a rejuvenator, they will not be the same. In effect this means that although the two bitumens have similar physical properties, they are fundamentally different.
To fully return a RAP bitumen to grade while restoring its chemical balance and reversing chemical oxidation is theoretically possible but a tough proposition. “Rejuvenation is very difficult,” said Dr Lancaster. It is perhaps more pragmatic to accept that while rejuvenation is possible, it is not actually feasible.
This report is based on the paper ‘Binder grade correction and rejuvenation’, delivered by Dr Ian M Lancaster to the IQ Conference 2011.
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