Page 4 - Hub 4 Magazine issue 53

Total Bitumen
Reducing its temperature the bitumen takes on a more solid form and as temperatures drop further a stiff even glass like brittle nature. This is why when applied to structural design any stiffness quoted for bitumen or ultimately asphalt is only relevant for the temperature the testing is performed at.
Increasing frequency of loading (faster loads) mean the bitumen takes on more of a solid or stiff nature whilst decreasing frequency of loading means the bitumen takes on more liquid behaviour. This is why rutting is most common at traffic lights, HGV lanes and bus stops where slow or static loads are encountered.
Conventional or penetration grade binder is limited in its ability to meet a wider range of temperature and load variations. This is where polymer modified binders (PMB’s) can start to stretch the performance characteristics of the binder and ultimately the asphalt mixtures they are added to. PMB’s can increase the range of serviceable temperatures the bitumen can remain both solid enough to resist softening at high temperature but yet remain flexible enough to resist cracking at low temperature.
These shifts in characteristic performance can be identified by parameters such as plasticity range.
According to the standard BS EN14023 which is The Specification Framework for Polymer Modified Bitumens. This property is the numerical difference between the softening point (high temperature) and the Fraass point (low temperature cracking point) of the binder. Therefore a PMB with a softening point of 75°C and a Fraass of -18°C would have a plasticity range of 93°C. In comparison 40/60 penetration grade with 50°C softing point and -2°C Fraass would have a plasticity range of 52°C. Comparisons can be made between PMB products based on the various test parameters within this standard.
There are almost as many polymers as there are asphalt mixtures available and understanding this is one aspect of getting the most from an asphalt design. SBS, EVA, SBR, crumb rubber even plastic can all be added to bitumen to alter its characteristics. Understanding the contribution the particular polymer will impart to the bitumen is the key to improving the fundamental behavior of an asphalt mixture in service.
It is still possible to find specifications asking for a “PMB” in the bitumen column of the specification. This is a symptom of a lack of knowledge around the presumed “black art” of PMB selection. There are many PMB’s available and their selection should consider the application and the previous failure modes encountered on site. A blanket “PMB” specification could mean anything from “a sprinkle of unspecified fairy dust” to a highly modified elastomeric product. Get this wrong and apply an inappropriate treatment to the highway it will increase the possibility failure or introduce a previously unseen failure mode.
It is possible to prove this with Swiss’ Lavoc study, independently conducted by the University of Lausanne. In 1988, 16 sections of the highway of Canton to Valais were constructed using the same asphalt designs and foundations but with 16 differing binders. The trial used a selection of modified binders and penetration grade binders. Each section was monitored over a 14 year period with a “crack measuring index” used in each section. The test included penetration binders, additivated, plastomeric and elastomeric PMB binders. The results show some of the binders that used additives and PMB’s actually cracked sooner than the unmodified penetration grade binders! This indicates those binders displayed early ageing, brittle or over stiff characteristics poor in fatigue resistance.
After 14 years the only section still intact was the section using Total Styrelf® elastomeric cross linked PMB. All 15 of the other sections had been replaced. This section was left in place. The section was still serviceable after 20 years proving the long term durability of the Total Styrelf® product.
To further explain plastic and elastic behaviour - a polymer can be elastomeric (e.g. Total Styrelf®) or plastomeric – using plastic modifier.
Elastomeric Polymers bring additional cohesion improving strength, fretting resistance, deformation resistance and are particularly valuable in reducing the effects of fatigue. Fatigue resistance is crucial where cracking through either thermal movement or reflective cracking may have been a previous failure mode or a predicted model in situations such as concrete overlays, bridge decks or evolved roads with uncertain structural integrity.
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www.hub-4.com Nov 2018 - Issue 53