Early Experience with the Use of Warm Mix Asphalt Technology in ...

For both the Aspha-min® zeolite and Sasobit wax methods, the situation isdifferent. For example, the National Center for Asphalt Technology (NCAT) in theUSA conducted laboratory studies on both the Aspha-min® zeolite and Sasobitwax methods and reported them in 2005 [6,7]. Another report on the performanceevaluation on mixes containing Sasobit has been done for the Maryland StateHighway Administration.3. EvolutionThe concept of lowering the heat used to produce asphalt mixes is not something new.The idea of saving energy and lowering emissions in the asphalt industry has beendiscussed for decades.It is known that as asphalt temperature decreases, its binder becomes more viscousand resistant to deformation, which results in a smaller reduction in air voids for agiven compactive effort. Eventually, the binder becomes stiff enough to prevent anyfurther reduction in air voids regardless of the applied compactive effort. Thetemperature at which this occurs is known as the “cessation temperature” which isestimated to be about 79°C for dense graded asphalt. High temperatures for hot mixasphalt are required to achieve the right balance between:• Low viscosity of the bitumen to obtain full aggregate coating;• good workability during laying and compaction;• rapid increase in mechanical strength, and;• durability during traffic exposure.Warm mix technology allows the temperatures of the HMA to be reduced duringmanufacturing and still achieve all of the above. Effectively this is carried out by thereduction of the viscosity of the asphalt via the foaming process of the binder or bythe physical viscosity reduction of the asphalt by the incorporation of wax additives.Amongst all of the technologies capable of achieving this, wax additives and theZeolite technologies were found to be the most practical for evaluation and trialpurposes in Australia. In addition to the lowering of asphalt manufacturingtemperatures the wax additives also, allow asphalt to be carted over long distancesand then laid whilst still achieving all of the relevant specification parameters. On thisbasis, Sasobit was chosen as the additive for these trials.Sasobit is a product of Sasol Wax International and has been marketed in SouthAfrica, Europe, the USA and Asia since 1997. Sasobit is a mixture of long chainhydrocarbons produced from coal gasification or the conversion of natural gas usingthe Fischer-Tropsch synthesis process. A product is produced that is highly crystallinein structure, giving a very hard material (due to its solid condition) in relation tobitumen, but it is also very easily melted giving it a very low viscosity. It is this lowviscosity that allows asphalt to be manufactured at lower temperatures and still becompacted.3

5. Case Studiesa) Deep Heavy Patching (Woodville Road, Sydney NSW) – 1.5% Sasobitadded.This trial served to demonstrate the benefits of warm mix asphalt technology whenconstruction timeframe is critically tight.The job comprised of full pavement reconstruction involving milling 410mm ofexisting pavement, placing 200mm of stabilised dense graded roadbase and 210mm ofasphalt in four layers (two layers of 60mm of AC20 and two layers of AC14 withAR450 binder).The asphalt was mixed at 130 °C (the lowest temperature that would allow thebitumen to be pumped efficiently). Lower temperatures may not be recommended incases in which the aggregates show relatively high moisture contents.The main risk in this job was rutting as a consequence of low stability due to retainedheat in the underlying layers. Conventional hot mix asphalt is usually overlaid whenthe surface temperature is below the softening point of the binder. In this case, thetarget was to overlay each layer when the surface temperature was around 50 °C orlower. Road Occupancy approval to carry out asphalt work was given from 8pm to5am. The effective timeframe is reduced by approximately 1 hour due to allow trafficcontrol set up and removal.140130120110Temperature (°C)10090807060Probe-1aProbe-2aProbe-2bProbe-3aProbe-3bProbe-4a50400 50 100 150 200 250 300Time (minutes)Figure 3 – Temperature profiles6

H11*>100.0°C100.090.080.070.066.060.050.040.030.020.080.675.085.060.277.888.464.6*

AC14DG + Sasobit ex Dubbo3.5Tracking Depth (mm)32.521.510.500 2000 4000 6000 8000 10000 12000PassesFigure 13 – Wheeltracking graphDiscussion: The asphalt was still able to be handworked and there were minimallosses due to cooled asphalt. Total asphalt losses were estimated to be 5% or less andmost of this was due to overrun on the ends of the area that was paved. Paving,including handwork, and compaction were carried out as normal with good results.Based on the success of this project, further long haul projects using Sasobitmodified asphalt were carried out in NSW and in other states such as WesternAustralia and Queensland.d) Long Haul and Cold Weather Paving: Canberra to Jindabyne.Asphalt paving was required on the upgraded dam wall at Jindabyne. This work wasto be carried out in winter (August 06) and comprised two layers of AC10 at nominalthicknesses of 40mm and 35mm.The asphalt was produced at Boral Asphalt’s Canberra plant and hauled 200km southto the foot of the Australian Alps at Jindabyne. The travel time was 2.5 to 3 hours andthe asphalt was produced at around 175 o C. To improve workability and to allow forcool weather paving, 1% Sasobit was pre-blended in Class 320 binder.Ambient temperatures were around 4 o C with a pavement temperature of minus 1 o C.Asphalt surface temperatures in the truck at Jindabyne ranged from 104 o C to 135 o Cwith truck core temperatures from 145 o C to 165 o C.16

Figure 14 - Paving of one layercompletedFigure 15 - AmbienttemperaturesDiscussion: Workability of the mix was reported as good and there was minimal lossof material due to cooling of the mix. No slumping of the mix was observed and therewas minimal sticking of the asphalt to the truck sides and front as is normallyassociated with these types of long haul projects. In spite of the long haul and pavingin extremely cold conditions, the asphalt was placed and compacted successfully.6. ConclusionsWarm asphalt mix technology, and in particular Sasobit, offers a number ofadvantages as follows:• It allows for the construction of deep lift, multi layered asphalt heavy patchingat lower temperatures. This consequently allows for earlier re-opening of theroad to traffic due to a reduction in retained heat, which minimises the risk ofearly life rutting.• Based on limited experience, the use of warm asphalt additives provides for amore homogeneous material and improved workability. Initial trials show thatpaving and placement can be successfully carried out at ambient temperaturesthat are approaching zero.• Warm asphalt additives, such as Sasobit, allow for reduced paving androlling temperatures of up to 30 degrees C. This allows for greatly increasedhaulage times and initial trials show that asphalt can be placed successfullyafter haulage times of 10 hours or more.• Lower production temperatures represent a reduction in production energycosts.• In lowering these temperatures there is a resultant reduction in fuming andexposure to asphalt contaminants. This has been successfully been applied tocrumb rubber asphalt mixes. Additional trials are being carried out in NewSouth Wales.17

7. References1. Kristjansdottir, O. Warm Mix Asphalt for Cold Weather Paving. Thesis inpartial fulfilment for the degree of Master of Science in Civil Engineering,University of Washington, 2006.2. Maccarrone, S. Cold asphalt systems as an alternative to hot mix. AAPAInternational Conference, November 1994.3. Jenkins, K., De Groot, J., van de Ven, M. & Molenaar, A. Half-warmfoamed bitumen treatment, a new process. 7 th Conference on asphaltpavements for Southern Africa (CAPSA), Victoria Falls, August-September 1999.4. Koenders, B., Stocker, D., Bowen, P., de Groot, P., Larsen, O., Hardy, D.& Wilms, K. Innovative processes in asphalt production and applicationto obtain lower operating temperatures. 2 nd Euroasphalt & EurobitumeCongress, Barcelona, Spain. September 2000.5. Barthel, W., Marchand, J., Von Devivere, M. Warm asphalt mixes byadding a synthetic zeolite. Eurovia www.asphamin.com6. Hurley, G. & Prowell, B. Evaluation of Aspha-min ® zeolite for use inwarm mix asphalt. NCAT Report 05-06. National Center for AsphaltTechnology, Auburn University USA. June 2005.7. Hurley, G. & Prowell, B. Evaluation of Sasobit TM ® zeolite for use inwarm mix asphalt. NCAT Report 05-06. National Center for AsphaltTechnology, Auburn University USA. June 2005.8. Bornmann, F., Strydom, S, Hiley, R. Fischer-Tropsch wax bitumenmodifier: Overcoming temperature and time constraints. Sabita Digest2006. South African Bitumen Association, Cape Town, March 20079. RTA QA Specification RTA 3253 Bitumen for pavements. Edition 11.Approved 01 July 2006. Roads and Traffic Authority NSW.10. RTA QA Specification R116 Asphalt (Dense Grade And open Graded).RTA www.rta.nsw.gov.au18