Sustainability through durability with Total Styrelf®
“We need a PMB for this job!” said the specifier. The assumption is that all polymer modified binders are same? This is a common misconception amongst contractors and unfortunately even specifiers! Bitumen is a complex material with very useful engineering properties particularly its adhesion and waterproofing qualities. Bitumen has thermoplastic and viscoelastic properties bringing both benefits and inherent limitations in its unmodified form.
Unlike water which is a solid at freezing temperatures and liquid at room temperature bitumen displays both solid and liquid behaviour at all temperatures. This non Newtonian behaviour is tricky to understand. There is a competition to find which characteristic will take precedence in a given situation dependent on factors such as temperature, load and frequency of loading.
Increasing temperature means the bitumen takes on more liquid behaviour. This is useful for transporting the product in pipelines and for the coating of aggregates.
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.
Superior fatigue resistance of Total Styrelf® eXtreme 100 over 35/50 grade binder.
Plastomers tend to impart stiffness which can be useful if and when this is required but are less valuable in fatigue resistance. Elastomer PMB’s impart elastic recovery characteristics where plastics resist deformation with stiffness but deformation is permanent or in engineering terms “plastic deformation” once load tolerance is exceeded.
When selecting a PMB it is important to understand which characteristic is required in the end product. Selection should be based on a holistic three dimensional design considering fatigue characteristics, stiffness, material handling and compaction capabilities. Using the softest binder that can withstand the forces to be applied is generally the best way to avoid cracking.
The production of PMB involves the dispersion of polymer units in bitumen. Performance is dependent on bitumen type and grade, polymer type, polymer size and quality of dispersion. In the case of cross linked PMB such as Total Styrelf® an additional manufacturing step which “cross-links” the polymer units is employed. Using these chemical bonds the polymers become networked and act as a single unit or “polymer matrix” within the PMB.
Sustainability through durability!
As understanding of PMB performance spreads it is certain that asphalt products will benefit in terms of durability. This in turn implies the whole life performance and asset management of a structure can become more predictable.
A recent scheme benefiting from Styrelf technology is The Mersey Gateway toll bridge connecting Widnes and Runcorn in Cheshire. A strategic route for South Liverpool as such requiring minimal future maintenance interventions. Total Styrelf eXtreme 100 PMB was chosen in the asphalt layers for over ten kilometers of network here by contractors Tarmac. This highly modified cross linked PMB is a premium product with high elasticity and fatigue resistance. Used on the bridge decks for these reasons it will give years of enhanced service extending the design life of this critical pavement. This PMB grade can be used in a multitude of asphalt designs including CL943 HRA, CL942 TSC and Scottish TS2010 SMA.
The Total Styrelf® range also incorporates lower temperature technology with Total Styrelf EC02®. The addition of warm mix additives to Total Styrelf® allows a temperature reduction of up to 40°C during the mixing stage of asphalt production. This has many benefits.
Warm mix asphalt is becoming seen as beneficial not just in terms of environmental gains from energy reduction but for the benefits it brings to the material, contractor and client. Lower temperature imparts less premature ageing of the product. Improved compaction is often cited, greater tolerance to travelling and speed of opening the newly surfaced road to traffic are all tangible benefits of these systems.
A recent example of this was collaboration between Highways England, Tarmac and Total Bitumen on the refurbishment of the A66, Penrith to Kirkby Stephen. Productivity gains allowed 1000 tonnes of asphalt per shift to be installed. Total ECO2® technology was used in the binder course and surface course. Despite Storm Callum hitting during the scheme works were completed on schedule.
Pre blended warm mix PMB removes the manual addition of additives at the asphalt plant while reducing energy consumption and emissions moving the industry in a responsible sustainable direction.
It is possible now to engineer PMB’s to specific applications. The Total Styrelf® portfolio includes tailored PMB’s such as Styrelf Intakt® for resistance to fuel and chemical corrosion such as encountered in airfield applications. Motorsport specific Total Styrelf GP® is designed to withstand the stresses, strains and shear forces encountered on Formula 1 tracks being applied to areas such as the Monaco GP.
Sustainability through durability is the responsible direction of the modern supply chain which is driven by the current drive for improved highways asset management.
If you would like to learn more about bitumen technology why not try the “MOOC” the Massive Open Online Course. MOOC is free to access and can be completed in your own time.
For more information visit www.total.co.uk/our-products/bitumen