WARM MIX ASPHALT IS THE ASPHALT OF THE FUTURE

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1 WARM MIX ASPHALT IS THE ASPHALT OF THE FUTURE Trevor Distin, National Technology Manager, Boral Asphalt Abstract Warm mix asphalt is essentially the same as hot mixed asphalt except that it has been mixed at a temperature which is 20 to 40 C lower than hot mix asphalt by using new warm mix technology. This paper will explain how asphalt can be produced at a lower temperature without compromising the performance properties of the asphalt. In doing so it will show what the benefits are for; the environment and the community, the asphalt manufacturer and the paving contractor, and most importantly the asset owner. The paper will share information on international and local best practices with regards to the design, manufacture and performance of warm mix asphalt incorporating the use of RAP. This will be supported by data on performance tests carried out on locally produced warm mixes using two different technologies; an organic wax additive and foamed bitumen. Some of the paving sites have been retested to monitor the performance of the warm mix asphalt during its in-service life. Warm mix asphalt offers local Councils an opportunity to provide an asphalt surfacing with a longer life because the binder is less aged during mixing, has lower permeability and less binder absorption in the aggregates. Key Words: warm mix asphalt, green house gas, performance testing, performance benefits 1. Introduction Warm mix asphalt (WMA) represents a range of technologies which allow asphalt to be produced at a lower temperature than typical hot mix asphalt (HMA) which ranges from 150 to 170 C. WMA is produced at a temperature which is generally 20 to 40 C lower than HMA. In the manufacture of HMA the aggregates are heated to the equiviscous temperature of the binder to ensure that it adequately coats the aggregates. The use of foamed bitumen or special additives are used to lower the binder s viscosity so that the aggregates can be coated at a lower temperature. These technologies also allow the asphalt to be paved and compacted at lower temperatures. Once the asphalt cools down it behaves the same as normal hot mix asphalt in the pavement. In essence WMA uses the same component materials (binders, aggregates, RAP and fillers) in the same proportions but the aggregates are heated to a lower temperature when drying them prior to mixing. This paper sets out to describe the benefits of warm mix asphalt to the environment and the community, the asphalt manufacturer and the paving contractor, and most importantly the asset owner. In doing so it will look at the use of WMA and its development in Australia. The paper will report on test results of WMA placed in Australia using the foaming and Sasobit technologies to demonstrate its performance. 2. Use of WMA The first WMA pavements were constructed in Europe back in 1995 using Aspha-min zeolite. In response to the 1997 Kyoto treaty on climate change each country within the European Union was faced with greenhouse gas reduction targets. In 1997 Sasobit wax was used in Hamburg, Germany and Shell s WAM foamed bitumen was used by Kolo Veidekke in Norway in Following on from a European scanning tour by NAPA members the first WMA trials were conducted in the USA in NAPA reported the annual usage of WMA in 2011 was 68.7 million tonnes which equated to 19% of the total asphalt market in the USA (or seven fold the annual HMA production in Australia).

2 In March 2010 Austroads constructed the first field trials to validate the suitability of warm mix technologies for Australian conditions. AAPA member companies in conjunction with VicRoads paved various WMA and HMA mixes to compare their performance over a 2 year period. Two foaming and two additive systems with and without RAP were placed on a section of the Hume highway in Melbourne. The laboratory test results showed inter alia that: While most mixes did not conform to the target design air voids range, the values for both the HMA and WMA were comparable. The Marshall stability and flow results for all the mixes were satisfactory with no discernible difference between the HMA and WMA mixes. There was no discernible difference in the indirect tensile modulus values between the HMA and WMA mixes. There was no discernible difference in the moisture sensitivity (laboratory and field samples) results though the results for one company were lower than the specification. All the mixes satisfied the wheel tracking specification criteria except for one WMA mix. The fatigue life results were mixed; some of the results may have been affected by low air voids. The mixes incorporating RAP generally had a higher binder viscosity than the HMA or WMA mixes. In summary, although there were some uncertainties associated with the test data, within the range of tests conducted the laboratory performance of the HMA and WMA mixes was similar. This was also reflected in the validation trial, where the field performance of the various WMA surfacings over two years was satisfactory and very similar to the performance of the HMA control surfacings. Subsequent to the findings from these validation trials the following State and Local Government specifications have been amended or introduced to allow the use of WMA: Aus Spec 1144 Asphalt Concrete (Roadways) October 2012 RMS QA Specification R116 Heavy Duty Dense Graded Asphalt June 2013 RMS QA Specification R117 Light Duty Dense Graded Asphalt June 2013 VicRoads Standard Specification 409 Warm Mix Asphalt Queensland Main Roads Technical Standard 30 Dense Graded and Open Graded Asphalt Main Roads Western Australia Specification 510 Full Depth Asphalt Pavement and Specification 504 Asphalt Wearing Courses The above specifications allow WMA to be used as a substitute for HMA under certain circumstance and conditions by lowering the temperature regimes for their manufacture and placing. 3. WMA technologies WMA technologies are generally classified into 3 main categories namely: surfactants, foaming processes and non-foaming additives. All these technologies work on reducing the binder s viscosity or surface tension thereby allowing the binder to coat the mineral aggregates at a lower aggregate temperature. The actual temperature reduction will depend on a number of factors from mix type, binder viscosity, RAP content, ambient temperature and haul distance.

3 Whilst NAPA recognises 22 different technologies in use in the USA at the time of publishing their 3 rd Edition of Warm Mix Asphalt Best Practices in 2012, this paper focuses only on the use of Sasobit synthetic wax additive and the Astec Green mechanical foamed bitumen system. Sasobit reduces the viscosity of the bitumen above its congealing point of 100 C as can be seen in Figure 1. At temperatures below the congealing point the hydrocarbon wax solidifies in the asphalt without impacting on the asphalts performance at low additions of wax. Figure 1: Effect of Sasobit on bitumen viscosity Source: Early Experience with the Use of Warm Mix Asphalt Technology in NSW by Figueroa J et al (2007) AAPA International Conference, Sydney. The Astec Green system uses a multi nozzle device which injects a small amount i.e. 2% of atomised water into the heated bitumen. The injected moisture immediately causes the bitumen volume to expand by 5 to 10 fold thereby reducing the viscosity of the bitumen. The microscopic foam bubbles remain active in the asphalt until the mix temperature falls below the boiling point of water. Initially WMA additives were used as a compaction aid to improve workability of stiff and difficult mixes. They were also used when laying thin asphalt layers in cold ambient temperatures or when hauling asphalt long distances. In the latter cases the mixes were all mixed at normal hot temperatures. Figure 2 shows the impact of foaming on the field density of asphalt when mixed at different temperatures. Foaming bitumen at normal HMA mixing temperatures of 165 C versus non foaming helps improve field density of HMA by approximately 1%. By using foamed bitumen and lowering the mixing temperature to 132 C the same level of compaction can be achieved compared to the HMA manufactured with non foamed bitumen.

4 Figure 2: Compaction of foamed asphalt produced at different temperatures. Source: NAPA (2012) WMA Best Practices QIP 125, 3 rd Edition 4. Benefits of WMA The move towards lowering mix temperatures was promulgated by the desire to reduce emissions to meet targets set by the signatories to the Kyoto treaty. Jenny (2009) reported that, in Australia, about 390,000 tonnes of CO 2 are generated annually from the 8 million tonnes of asphalt produced and that a reduction in production temperature through the use of WMA technologies would roughly translate to a reduction of more than 120,000 tonnes of CO 2 per annum. The use of WMA has rendered benefits to all stakeholders which has far surpassed that which was ever originally envisaged with its implementation. 4.1 Better for the environment Lowering the temperature required to heat aggregates during the mixing with bitumen reduces both the demand for heating fuel and reduces the emissions generated during the manufacturing process. The reduction in fuel savings is dependant on various factors such as moisture content of aggregates, WMA technology used, plant design and operation. Reducing the emissions will depend on the amount of temperature reduction achieved, type of fuel used, aggregate moisture content, RAP usage, plant design and operation. The lower emissions are mainly as a result of reduced fuel combustion during the drying process. Lower temperatures also reduce the odours emitted thus improving conditions for asphalt workers and neighbours by reducing the fumes. Table 1 presents typical emissions reductions achieved with WMA.

5 Table 1: Reductions in emissions of WMA vs HMA Emission Reduction in emissions CO % SO % VOC s Up to 50% CO 10 30% NO x Up to 60 70% Source: EAPA position paper (January 2010) The use of Warm Mix Asphalt 4.2 Better production and construction The reduction in the binder s viscosity results in easier compaction of the asphalt layer at lower paving temperatures. This phenomena is also aided by the fact that WMA has a more uniform temperature across the surface of the mat than HMA (see Figure 3) and that at a lower temperature the rate of cooling is slower (see Figure 4). In 2010 Boral overlaid Queen Street in Granville, Sydney with both WMA using Sasobit and HMA. Both mixes where identical except the mixing and paving temperatures of the WMA was 30 C lower than HMA. The thermographic images taken of the heat distribution using an infrared camera of both mixes are shown in Figure 3 with the WMA showing a more uniform temperature distribution across the mat behind the paver. This will tend to lead to more uniform compaction thereby reducing the permeability of the asphalt to the ingress of water and air. Other benefits of WMA over HMA during paving operations are: Improved compaction of construction joints Improved productivity because one can pave multiple layers in one shift without having to wait for the bottom layer to cool down. One can also pave more asphalt in a given shift because the time required for the asphalt to cool before one can open it to traffic is reduced. Larger compaction window which is critical for paving thin layers because WMA cools down at a slower rate as shown in Figure 4.

6 Figure 3: Thermographic image of WMA vs HMA Figure 4: Cooling rate/compaction time of WMA vs HMA 4.3 An opportunity for longer service life During the manufacture of WMA the binder is exposed in a thin film to a lower aggregate temperature than HMA which results in less hardening of the binder. The benefit of this process is that: The binder will be less aged and therefore be less prone to fatigue cracking and ravelling in its in-service life.

7 Less binder will be absorbed into the aggregate resulting in more binder being available to retard ageing and cracking. The down side of manufacturing asphalt at lower temperatures raises concerns regarding the longer term performance of WMA in that it increases the potential for rutting and moisture damage on the basis: Less aged binder will be more prone to rutting under heavy traffic loading; Aggregates may not be completely dried during mixing at lower temperatures although the temperature is well above the boiling point of water. These concerns can be allayed through the use of RAP which can help stiffen the binder properties in WMA. The use of RAP will also require the virgin aggregates to be super heated. Super heating will help dry the moisture out of the virgin aggregate even although the overall temperature of the WMA is lower. Boral have undertaken numerous WMA trials and projects in Australia since This paper will report on the performance of three of these projects with the focus being on the performance of the asphalt to investigate if there was any evidence of premature rut deformation and/or moisture damage in the early life of WMA using both Sasobit and Astec foaming technologies. 5. Case studies 5.1 Botany Bay Road, Sydney - early life performance properties using foamed bitumen WMA In December 2012, as part of the overlay on Botany Road near Sydney airport for RMS, Boral manufactured and paved 800 tonnes of WMA using the Astec Green double drum process. The WMA mix was HD14DG with AR450 bitumen containing 20% RAP and 1% hydrate lime. Samples were taken from the plant to measure deformation and moisture sensitivity of the WMA. These results are compared in Table 2 with those achieved for the HMA mix placed on the project. The properties measured on the WMA exceed those required for HMA specifications and compare favourable to those measured for the HMA equivalent mix type. Table 2: Performance test results on HD14DG placed on Botany Bay Road Property WMA HMA Retained strength ratio, % 90 (RTA T649) 81 (RTA T640) Wheel tracking 60 C, mm (AG:PT/T231) Gallipoli Grove, Adelaide performance properties after 2 years using foamed bitumen WMA Boral was the first company in Australia to install a foaming system on a batch plant at their Gepps Cross plant in Adelaide. Boral was awarded a contract to supply HMA for the construction of the DTEI s South Road Superway project in December DTEI permitted 3000 tonnes of WMA to be placed as a full depth greenfields pavement on the detour road which would carry about 18,000

8 vehicles per day until the Superway was complete. The WMA was produced from C320 bitumen with 15% RAP and the asphalt was mixed at a temperature of 135 C using the Astec Green foaming system. The decision was made to return to the site two years later in 2013 and take cores from the pavement to measure the performance of the WMA as far as rutting and moisture damage was concerned. To this end the Coopers wheel tracking test and RTA s resilient modulus ratio test was done after a freeze thaw cycle. The average wheel track rut measured was 4.7mm on 2 cores and the average resilient modulus ratio was 83.5% on the 10 cores. These values meet the specification requirements for newly produced HMA. Austroads Guide to pavement technology part 4B: asphalt states that a rut value between mm is considered as a good performance and DTEI s specification for wheel tracking is < 6mm for AC14M with C320. A resilient modulus ratio greater than 70% is considered as a good result by RMS. The full set of test results are shown in Table 3 and 4. As far as compaction is concerned DTEI s requirements for AC14 dense graded asphalt are minimum characteristic air voids value of 4% and maximum of 8.5%. Even after two years of inservice performance the in-situ air voids fall within this range (3.8% to 7.4%) with most of the cores being taken from the wheel path see photograph below.

9 Table 3: Wheel tracking results on AC14 WMA cores taken from the intermediate layer on Gallipoli Grove, Adelaide (test method AG:PT/T231) Property Core # 11 Core # 12 Core diameter 200mm 200mm Core thickness 75mm 75mm Air voids 5.2% 6.3% Rut depth after 10, C 3.6mm 5.7mm Source: Boral Material Testing Laboratory report # Table 4: Propensity for moisture damage on AC14 cores taken from WMA in Gallipoli Grove, Adelaide (Test method RTA T649) Core # Air voids, % Dry modulus, MPa Thaw modulus, MPa Resilient modulus ratio, % Source: Boral Material Testing Laboratory report # Sphinx Avenue, Bankstown, Sydney early life performance properties using Sasobit WMA In July 2011 Boral was tasked by Bankstown Council to rehabilitate Sphinx Avenue by removing 200mm of asphalt from the existing pavement and replacing it with 2,300 tonnes of WMA. Three layers of different asphalt mixes produced with AR450 bitumen and Sasobit were paved at night in the middle of winter without any compaction problems. Samples of all three asphalt mixes were taken and performance tests were conducted on them. The lowering of the mixing temperature did not result in lower recovered binder viscosities, reduced asphalt stiffness or higher rutting deformation. The viscosities of the binder recovered from the WMA mixes are in the range of Pa.s which is in line with RTFOT limits specified by RMS in QA Specification 3253 for AR450 bitumen. The wheel tracking results are all below 3.5mm which is considered as superior performance by Austroads Guide to pavement technology part 4B: asphalt Table A30. The resilient modulus values obtained compare favourably with what is considered as typical values for HMA in Table A23 of Part 4B. It is clear that the addition of RAP helps stiffen the bitumen in the WMA which has a positive impact of the performance properties of WMA mixes. A summary of the test results are shown in Table 5.

10 Table 5: Performance test results on WMA placed on Sphinx Avenue Bankstown Property AC14 AC20 AC28 % RAP Recovered bitumen 60 C, Pa.s Wheel tracking 60 C, mm Resilient 25 C, MPa Source: Boral Warmpave case study Sphinx Avenue Conclusion WMA offers local government the benefit of providing a more sustainable paving material over HMA without compromising the long term performance of asphalt on their road network. WMA validation trials undertaken by Austroads support international evidence that performance properties measured on WMA projects incorporating RAP have at least the equivalent performance of HMA. Concerns over possible moisture sensitivity and early rutting of WMA have not been found in local works executed by Boral using Sasobit and foamed bitumen technologies. On the environmental side WMA helps satisfy the community need to reduce green house gas emissions and preserve of our non-renewable resources. This is achieved mainly through the lowering of mixing temperatures and reduced fuel consumption which also improves working conditions for workers at the plant and paving sites. WMA has the potential to increase the use of RAP in a symbiotic manner in that it helps increase the stiffness of the lesser aged virgin binder and the superheated RAP helps with the drying of the moisture of the virgin aggregates at lower mixing temperatures. Given that WMA provides an asphalt material which is easier to compact and has the potential of a longer in-service life, why would Councils not want to use WMA on their road network. WMA is the new asphalt of the future as it addresses a wide range of challenges faced on the environmental, construction and performance side when using HMA. 7. References 1. National Asphalt Pavement Association, USA (April 2013), 2nd Annual Asphalt Pavement Industry Survey on Reclaimed Asphalt Pavement and Warm Mix Asphalt, Information Series National Asphalt Pavement Association, USA (2012) Warm Mix Asphalt: Best Practices Quality Improvement Publication 125, 3 rd Edition. 3. Austroads Technical Report AP-T (March 2013) Laboratory Evaluation of Warm Mix Asphalt Mixes. 4. European Asphalt Pavement Association (January 2010) Position paper on the use of Warm Mix Asphalt

11 5. Boral Asphalt Warmpave Case Study, Queen Street, Granville, City of Parramatta Boral Asphalt Warmpave Asphalt Case Study Gallipoli Grove, Kilburn, Adelaide Jenny, R (2009), CO2 reduction on asphalt mixing plants potential and practical solutions, 13th AAPA International Flexible Pavements Conference, Surfers Paradise, Queensland. 8. Roads and Maritime Services (RMS) (2009), QA Specification 3253 Bitumen for Pavements 9. Figueroa J, Hennessy G and Hiley R (2007), Early Experience with the Use of Warm Mix Asphalt Technology in NSW, 12th AAPA International Flexible Pavements Conference, Sydney. 8. Author Biography I joined Boral in 2009 as their National Technology Manger for their asphalt and spray sealing business. Prior to that held positions as Executive Director of Southern African Bitumen Association, Marketing Director for Colas Southern Africa, Bitumen Technical Services Manager for Mobil Oil South Africa. Started my working career in the Roads Department of the Cape Town City Council. Have over 30 years experience in bituminous materials testing, manufacture and construction. Currently serve on Austroads Asphalt and Bitumen Surfacings working groups and on the AAPA board. Hold a Masters Diploma in Civil Engineering and Bachelors Degree in Commerce.