Warren Carter | Cassandra Simpson Andrew Bethune Warm Mix Asphalt Warren Carter | Cassandra Simpson Andrew Bethune

Introduction WMA represents a group of technologies which allow a reduction in the temperatures at which asphalt mixes are produced and placed WMA is produced at temperatures that are 20°C - 50°C below that of hot mix

Emissions and Energy Nominal 20% fuel savings Nominal 20% reduction in CO2 emissions Note – general indication only as some WMA technologies do not reduce emissions (e.g. some surfactants)

Benefits of WMA Improved working conditions Less fumes and emissions Less energy consumption Decreased binder aging during production Early site opening Cool weather paving Compaction aid for stiff mixes Longer haul distances Improved and more consistent compaction Initially, the benefits were viewed to only favour the contractor. This perception has changed as the client (DoT’s) have realised greater durability and consistency of end product (especially compaction) i.e. improved pavement constructability. Some of the WMA technologies may not result in reduced emissions e.g. some of the chemical additives.

HMA Temp = 160oC WMA Temp = 132oC

Warm Mix Hot Mix

WMA Technologies Free water systems (mechanical foaming by water injection) – 130°C - 140°C Water carrying chemical additives (additives introduce moisture e.g. synthetic zeolites) – 115°C - 135°C Water carrying non-chemical additives (LEA process) – 95°C - 110°C Chemical additives (reduce internal friction e.g. Cecabase RT, Evotherm 3G, HyperTherm, etc) – 110°C - 135°C Rheological modifiers (reduce binder viscosity during mixing e.g. Sasobit, Rediset WMX, etc ) – 115°C - 135°C Growth in free water systems due to lower variable cost. Free water systems do not allow as great a temperature reduction as some of the chemical additives and rheological modifiers. Also, with free water systems there is a maximum production temperature (nominal 140°C) to ensure that the water doesn’t boil out, and, a minimum paving/compaction temperature (nominal 100°C) to ensure that the steam bubbles don’t collapse and loose the warm mix attributes. It was recommended that free water systems not be used in cold/freeze affected climatic zones – although it is the dominant technology in Virginia. Niche applications of some systems in non-attainment areas and over crack sealing products. Indications were that WMA have a cost premium of @ $3-$5/t over hotmix.

WMA Production Temperatures

WMA Technologies Astec Green, zeolite, Sasobit, ?, Evotherm 3G and Rediset WMX

US Experience - Evaluation European scan/study tours (2002 and 2007) Growth has been rapid Technologies: 3 (2005) – 23 (2010) Trials: 15 states (2007) - 45 states (2010) Market: 2% (2007) – 10% (2010) – 50% (2015?) Key properties being evaluated Deformation resistance/rutting Cracking/fatigue Moisture sensitivity 3 technologies – Asphamin, Evotherm and Sasobit. Rutting via HVS and Hamburg Wheel Tracker – early indications are that twice the degree of rutting is predicted in the laboratory but it is not being experienced in the field. Fatigue via 4 point bending beam – WMA has a greater initial fatigue life. Moisture sensitivity is a grey area with some WMA reporting low values i.e. 50-70%. The specifications vary considerably, ranging from minimum values of 70% to 80% and some requiring a conditioning step i.e. 150°C for 4 hours. Some states have a mandatory inclusion of hydrated lime and others require the sue of an adhesion agent to comply with their local TSR requirements. It was indicated that WMA achieves a comparable stiffness to hot mix after 2 years in performance.

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US Experience - Implementation Initial client desires were that WMA provides equal or better performance compared to hotmix The various states have approached implementation differently collaborative trials permissive specifications lengthy approval processes with laboratory and field evaluation list of approved technologies/products maximum production temperatures (non-attainment areas, over crack sealing, etc) Collaborative trials California – CALTRANS still wants the performance of WMA to be equivalent or better than hotmix. They assess the WMA technology under the HVS (Heavy Vehicle Simulator) and subject to the results, include the technology/product on an “approved list” e.g. currently only Advera is approved. Contractor approval process includes undertaking a lab evaluation (rutting, moisture sensitivity and cracking) using relevant state test methods and three placement trials with 12 months monitoring (generally only by photos). California has the most stringent requirements for WMA and are slow on the up take i.e. being cautious. Texas requires laboratory rutting and stripping (Hamburg Wheel Tracker) and field trials (three with at least one in Texas). The WMA technology must be on the “approved list” – currently includes all foam processes, Advera, Asphamin, Evotherm, Rediset WMX and Sasobit. A permissive specification exists, but a mandatory maximum temperature in non-attainment areas. Some local areas/counties have preferences, but this is mainly due to the push by the technology provider in such areas. Virginia approves the contractor to use the technology and uses a permissive specification. North Carolina has a list of approved technologies based on the use of the technology within the USA i.e. it doesn’t have to have been used in the state. A collaborative approach existed and was described as being an appropriate mechanism to assess, introduce and monitor trials – which were undertaken on secondary i.e. low traffic roads (e.g. <500vehicles/day). They have a TSR minimum requirement of 85% (no freeze cycle) – which usually requires the use of either hydrated lime or an adhesion agent. South Carolina used the NCAT approval process and currently only have foam and Evotherm on the approved list. The use of hydrated lime added to damp aggregates is mandatory (prior to drying).

US Experience - WMA Dense graded asphalt Open graded asphalt Stone mastic asphalt Crumbed rubber asphalt Conventions bitumens and polymer modified binders

US Experience - Concerns Rutting Strength issues Stripping Water injection technologies in freeze climates (ravelling) Production issues – ensuring the aggregate is dry Plant addition of WMA additives vs terminal blending Rutting concerns not validated by field performance. WMA is initially tender, with low tensile strengths being reported, but it “just needs a little more time to cure”. Stripping has not been identified as an issue during field trials, although NCAT indicated that some WMA with recycled shingles were showing some moisture related distress. Keep the voids low and stripping isn’t an issue. Few have faith in the modified Lottmann test when used to assess WMA and the Hamburg Wheel Tracking test is believed to be too harsh for WMA. Some DoT’s require the additive are added to the binder by a third party rather than the contractor (terminal blending) In some cold/freeze climates, it was reported that some ravelling had been observed when a water injection warm mix technology was used. It was recommended that consideration be given to not use this technology in such states. In saying this, they get some cold winters in Virginia and most of their WMA is via this technology.

US Experience – WMA Mix Designs Mix designs are generally the hotmix design with the warm mix technology applied Some minor allowance is made to WMA containing Advera and Asphamin as these become an inert filler (once the water is remove) Lower voids of a WMA mix design compared to the hotmix design (but binder content isn’t adjusted)

Warm Mix Hot Mix

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