[MUSIC] Like each one of us, bitumen behavior is affected by the amount that it has aged. Not only can a physical appearance change, but the ability to cope with stresses, resistance to extreme cold, and positive contribution to a working environment also diminishes with age. In fact, we're more like bitumen than you might imagine. In this unit, we shall consider what bitumen is. And what it takes, the way it behaves which will then progress to look at what we mean by aging in the context of bitumen and how we can try and predict this in a controlled laboratory environment. Finally, we will look at polymer modified binders and illustrate how the manufacturing protocol of these materials can positively impact on the aging of bitumen. Okay, so the first question we need to understand is what bitumen naturally is. Well, bitumen is a mixture of very complex organic molecules that are the result of millions of years of intense heat, radiation, and pressure. Acting on dead, decaying plant, animal, and fish life, this process breaks down and indeed reforms very complex molecules to form this mixture of hydrocarbons we call crude oil. The crude oil is then refined into its component parts, the heaviest and most complex molecules of which. Are called bitumen. As you might imagine, the bitumen molecules are very long, in excess of 70 carbon chain length, but also with the additional complicated side chains. A precise chemical analysis will be a nightmare to complete. And so the chemistry of the bitumen is often simplified into four generic groups often referred to as SARA, which help to define the behavior of the bitumen. These are as follows. Firstly, the saturates. These are colorless, non-polar oils. Secondly, the aromatics. These tend to be dark brown, non-polar viscous liquids. Thirdly, resins. These are dark brown polar semisolids or solids. Finally, the asphaltenes. These are dark brown or black very polar solids. The relative proportions of these SARA components dictate the physical behavior of the bitumen. For example Asphaltenes contribute significantly to the solid nature of the bitumen. The resins to the add exhibitive the bitumen and the aromatics will dictate how well dispersed the asphaltenes are and so will also influence the solid liquid balance of the bitumen. Therefore the balance of the SARA is fundamentally important in how the bitumen behaves in service. Having understood in very basic terms, what influences the behavior of the bitumen? It's now appropriate to consider what we mean when we refer to bitumen ageing. Essentially, we're talking about a changing chemistry of the bitumen. Resulting from the impact of exposure to air, heat, UV, etc. This process may occur in the storage tank, where very high temperatures are experienced, in the mixing chamber of asphalt plants, where it is exposed to both heat and air, or indeed, through its service life on the road, where it is exposed to UV and air for prolonged periods. The main impacts of these factors is oxidative reactions occurring to these complex bitumen molecules. The key changes that occur to the bitumen is a shift in SARA analysis, the most noticeable being an increase in asphaltene levels, decrease in aromatic levels and resin levels and an increase in the saturates. As the key impacts of the asphalt changes the contribution to stiffness of the bitumen, we see significant increase in its characteristic resulting in decrease penetration, increasing softening points and viscosity. In terms of contribution, the asphalt, there is low temperature performance influence in terms of increased brittleness and reduced flexibility. It's generally accepted that the processes manufactured through a asphalt plant results in a decrease in penetration of about 30 to 35%. In other words, in general terms, the bitumen will harden by approximately one grade. In reality of course, it is significantly invited by the temperature of the mix, the type of asphalt plant and the level of exposure of the bitumen to heat. This mixing stage account for the most rapid aging phase. Aging, or oxidation, continues thereafter through the service life of the road. To illustrate the extent of this oxidative change, we can look at the evidence from an extensive study that was carried out in Switzerland by the University of Abog. This study followed 16 test sections laid on the A9 through the mountains Over a period of more than 20 years. One additional aspect of this study was to follow the aging of a specific 8,100 bitumen over the life of the road. The line of best fit can be seen through this period. To simulate the impact of the mixing process at the asphalt plant, the Rolling Thin-Film Oven Test was developed. This method, EN 12607-1, involves placing a small amount of bitumen into a glass tube and placing it into a rotating carousel. This creates a thin film of bitumen around the internal face of the tube. Hot air, at 163 degrees C, Is then blown onto the surface of the bitumen as the carousel rotates for a period of 75 minutes. Ageing is measured by the amount of change occurring in the physical characteristics of the bitumen. For example, changing penetration often expressed in percentage retained penetration. Or it may be changing softening point, or indeed, a ratio of complex modulus before and after the RTFOT process. This process is often referred to as the short-term aging test. To look at the longer term aging that may occur in the field, the pressure aging vessel was developed. So called en 14769. Now this method describes the protocol for the longer term editing. Having completed the rolling thin film oven process the residual bitumen is placed on specified thin layers on trays and placed in a pressure vessel that elevated pressure and elevated temperature. The pressure is elevated to 2.1 mega pascals and the temperature can be anywhere between 8 and 215 degree C depending on which protocol. It is impossible to define how this correlates to real life on the road as the actual energy in situn will depend on many variables such as ambient temperature, amount of UV exposure, indeed how open the asphalt structure is, and so on. Therefore, the PAV protocol is a method solely to benchmark one binder against another in standard conditions. Once again, the comparison is a change in performance from verging to each state. To provide a context to the extent of the changing between properties on the real site situation, it's useful to review the figures emanating from the reboxolayde. We see the bitumen with an original penetration of 77 dropping to 13 after 14 years. Likewise softening point increases from an original 41.4 degrees C to a final 71.70 degrees C after 14 years in service. The fras brittle point moved from lower than minus 15 degrees C in the original buy to a final figure at 14 years of -1 degrees C. In order to enhance the overall performance of bitumens in the field, polymers of different types can be added to the bitumen. The most common type currently utilized are elastimerit polymers of the SBS, that is, styrene, butadiene, styrene class. SBS modified bitumens impart beneficial properties to the bitumen to enhance in situ performance. However, it's important to understand that all polymer modified bitumens are different, and this relates to the way they perform in the short and the long term. And so, the specific bitumen polymer combination has to be understood. One manufacturing protocol for SBS modified bitumens includes a cross linking process, whereby the individual polymer strands are chemically linked together, forming a network of interlinked polymer units. It is important to differentiate these materials as each have a significant impact upon the ageing process. Ageing of polymer modified bitumen is a little more complex than standard bitumens as we have to take account of what is happening to the polymer units themselves. Not only does the bitumen date of ageing, increasing the asphaulting levels, but also the polymer units undergo oxidation and can begin to fracture. As an example of this imagine an elastic band that is left on the window sill for several weeks. What was formerly very elastic, showing a great ability to expand and recover back to its original shape. Is not brittle and non-elastic. This change is due to fracturing of the polymerase, creating much smaller, less elastic polymer units. Cross linking of the SPF molecules, not only creates a much more elastic network, but the cross linking appears to protect the vulnerable sites of the polymer molecules from oxidation. This means that the aging or fracturing of the polymers is slowed down. The network of polymer also appears to protect the bitumen itself from ageing quite rapidly. The result of this is that cross linked SBS polymer bitumens age much more slowly penetration grade bitumens. And they're therefore more durable in service. Before I mention the vox study, including sections of asphalt manufactured with crosslinked SBS polymers, that is Styrelf 13/80, review the ageing graphs compared to the standard 8100 pen bitumen illustrate this point very well. Whichever parameter is studied, whether it be penetration, softening point, viscosity, cold-temperature behavior, whichever. A change in performance over time is much less for the styrelf than for the penetration-grade bitumen. To further support the view that the polymer fracturing in this cross-linked type of product is reduced. We only have to look at the elastic recovery of the bitumen. At time zero, the recovery is showing 80%. After 19 years in service on the road, the bitumen is still showing around 70% recovery. So in this module, we've looked at bitumen aging. We've seen that, like us humans, bitumen is subjected to an aging process. That reduce his performance. Chemical analysis illustrates that this is due to oxidative reactions that increase the proportion for ash faulting and saturates and reduces the amount of aromatics and resins. Therefore, bitumens become stiffer, less flexible, and less fatigue resistant. Cross league bnb's can reduce or at least slow down this aging process significantly, and therefore increase durability on the road. [MUSIC]
