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OHMPA seminar goes green

With a theme of Sustainable, Economical and Environmentally Friendly Hot Mix Asphalt, the 2007 Fall Seminar of the Ontario Hot Mix Producers Association (OHMPA) was firmly centred on the environment.

Held on December 12, 2007, at Le Parc II Conference Centre in Thornhill Ont., the seminar was, once again, the best attended asphalt seminar in Canada with 475 attendees from across the industry. Presentation topics included the constructability of asphalt pavements, recycling and perpetual pavements, all in addition to the latest developments in warm mix asphalt, porous mixes and quiet mixes.

Always a popular event, the silent auction at the  OHMPA Fall Seminar supports asphalt research and development.

Keynote speaker Larry Michael provided a comprehensive overview of current industry issues, first noting that asphalt was green when "green" was just a colour, being 100 per cent recyclable, sustainable and environmentally friendly. But what is environmental sustainability? In Michael’s view, the goal of environmental sustainability is to maintain the quality of life we need to "sustain" or take care of our environment and decrease the negative environmental impacts of human existence. Achieving these goals is a difficult balance between environmental impacts, trade-offs, and cost. The industry goal is to create a product with consistent high quality in a sustainable manner and involving the use of recycled materials.

Asphalt has a long history, with the first recorded use of asphalt in 2500 BC as a reservoir sealant. It was first used for roadbuilding in 625 BC in Babylon and was first used for roadbuilding in the United States. in 1870. In recent years, the challenge of the 1973 crude oil embargo and quadrupled oil prices was met by sources changes and new asphalt cement test methods from penetration grading to viscosity grading. In terms of the environment, the industry has, since 1970, decreased total emissions from plants by 97 per cent while increasing production by 250 per cent.

Over the same time frame, one of biggest challenges is increasing traffic congestion. Since 1970 the U.S. population has increased by 32 per cent but drivers are up 63 per cent, vehicles up 90 per cent, miles traveled up 132 per cent and tonne miles up 400 per cent, although highways have increased just 6 per cent.

The industry is meeting the challenge in a number of ways. New technologies include Superpave, a new asphalt mixture design system based on: environment, traffic level and speed, pavement structure and a new binder specification based on environment and traffic level. Stone Mastic Asphalt (SMA) is delivering rutting and deformation resistance, increased durability, good frictional properties, reduced water spray and lower traffic noise. Pavement performance is increasing through the use of perpetual pavement designs having a 50-year design life, requiring excellent design, quality materials and quality construction. The New Jersey Turnpike illustrates the perpetual pavement concept well, although it was not built as such. Constructed in 1950-52 the 190-km long Turnpike carried 17.9 million vehicles in 1952 or an average of 49,000 per day. By 2006, the annual figure had increased to 205 million vehicles, or 560,000 per day. Despite the more than tenfold traffic increase, there have been no failures in the pavement structure. Further developments include storm water management from the drainable asphalt bases of the 1970’s to the porous asphalt of the 2000’s, while recent developments in quiet pavements include open graded friction course (OGFC) and 9.5 mm SMA.

Returning to the environment, asphalt pavements are America’s most recycled product with less energy consumed in building pavements as well as less energy consumed by the traveling public. Michael noted that 100 000 tonnes of Reclaimed Asphalt Pavement (RAP) contains 5000 tonnes of binder worth $1,500,000 at $300/tonne. It also contains 95 000 tonnes of aggregate worth $1,492,500 at $15/tonne. Turning to warm mix asphalt, its advantages include reduced greenhouse gases, reduced (or eliminated) fumes, reduced fuel consumption and improved mix compaction density. The new technology of intelligent compaction can provide both short term and long term benefits. Short-term benefits include improved density, a more efficient compaction process and the measurement of dynamic stiffness of compacted material "on-the-fly" which may correlate to percent compaction. Long-term benefits include continuous compaction control specifications; possible estimates of pavement moduli; a possible tie to design guide to verify design and possible performance specifications.

So what are the weaknesses of our current systems? According to Michael, these include the availability (or rather potential unavailability) of asphalt, the price of asphalt, lack of funding, artificial limits in RAP, loss of expertise, quality of construction and the low bid system.

On the positive side, strengths include innovative technologies, the availability of millions of tonnes of RAP, modified binders, skilled people, pavement management systems and global research. Michael’s summary of topical issues leading to greener asphalt included storm water management and porous pavement design; quieter and safer pavements; cooler pavements to reduce urban heat islands, reducing greenhouse gas emissions; increasing recycled material use in asphalt pavements with RAP delivering environmental and energy benefits; the Leadership in Energy and Environmental Design (LEED) credit for asphalt pavements including RAP, albedo (the fraction of incident sunlight that is reflected) and porous asphalt.

Porous mixes

In "Introduction to Porous Mixes" Susan Tighe of the University of Waterloo reviewed the context of sustainable HMA, porous pavement design, selection of materials, asphalt mix design, performance testing and next steps in porous pavement development. Tighe noted that in traditional dense graded asphalt pavements fluid flows along the asphalt surface to drainage facilities whereas a porous pavement permits fluids to flow through the structure. In the context of sustainability, current goals are to investigate an emerging SWBP for the Canadian climate, increased environmental awareness and to provide guidance for Canadian engineers, contractors, and government agencies in dealing with porous technologies. The characteristics of porous pavements include reduction in noise levels and reduction of water-retention areas, thereby increasing parking areas. Porous pavements also improve conditions for drivers and pedestrians through reduced spray during rain, as well as the reduced potential for black ice and a potential reduction in associated de-icing activities. Suitable locations for porous pavements include municipalities, residential areas, universities, libraries, religious centres, prisons, industrial parks, commercial plazas, low volume roads, parking lots, pathways and trails. Their functional considerations include freeze-thaw performance, draindown of the asphalt cement, ravelling and coarse aggregate loss, lack of technical expertise and the potential risk of groundwater contamination. Porous pavements also require greater site evaluation and design effort (both structural and storm water design), demand high level of construction skill, inspection, and attention to detail. Once in service, porous pavements require routine maintenance to minimize clogging to ensure long-term infiltration.

A typical porous pavement cross section has three courses; surface course, filter course and reservoir course, laid on subgrade, while design options for water treatment
require consideration of both water infiltration and exfiltration. The subset of four infiltration designs include the capture/infiltrate all water from an entire design storm, infiltrate increased runoff from surrounding impervious surfaces, infiltrate runoff fixed volume from every storm or infiltrate sufficient water to control the peak rate of discharge. The three options for exfiltration design include full exfiltration, partial exfiltration or no exfiltration.

Areas where porous pavements should not be used include high water tables or high depth to bedrock, expansive soils, storm water hotspots, industrial sites, fuel stations or fleet storage areas, areas which drain pesticides or fertilizers, areas near drinking water wells (30 m minimum distance) or areas having an increase in impervious cover that exceeds filtering and storage capacity.

Tighe reviewed the scope and objectives of current porous pavement research and also described the design, testing and required characteristics of the surface and reservoir courses. Going forward, the next steps in porous pavement development will examine freeze-thaw performance, quantify clogging potential, quantify performance, quantify contribution to SWBP and examine life cycle cost.

Increased percentages of recycling

In Introduction to Increased Percentages of Recycling in OPSS, Paul Lum of Lafarge Canada noted that HMA recycling is using any waste material and/or a supplemental product that can be recycled into HMA, with the important requirement that the recycle mix provides equal or better performance. The use of RAP in HMA provides the highest value by allowing a reduction in both mix asphalt and aggregates, with field performance and laboratory testing showing that recycled HMA performs very well when properly designed. Besides RAP, other supplemental materials have included recycled shingle tabs (RST) from new shingle manufacturer’s waste, crumb rubber, glass and toner. Lum concluded by reviewing the maximum permitted percentages of RAP allowed by mix type under Ontario Provincial Standard Specifications (OPSS) 1150 and 1151 and encouraged producers to take full advantage of the opportunities provided by these specifications to use recycled HMA.

Task Force on Constructability

Murray Ritchie of the Murray Group Ltd. updated attendees on the work of the MTO/OHMPA Task Force that is looking at ways of improving the constructability of HMA pavements. The task force was formed as both the MTO and the hot mix industry recognize that there are opportunities for constructing better longitudinal joints in asphalt pavements. This forum allowed for the discussion of ideas that will raise the quality of these joints and recommended changes could result in cost savings, shorter paving dates, increased safety and better performance. The task force has focussed on ways of obtaining better longitudinal joints by examining potential changes during every stage of a project, from development to construction, to enhance pavement constructability. The advantages and disadvantages of any potential changes are reviewed as well as their impacts on resource utilization and possible stakeholder involvement. The goal is to identify best construction practices that would assist contractor to do a better job and to date the extensive list of task force discussion topics has included full closure-detours, partial closure, use of shoulders, lane shifts, sharing and availability, extended lane access, staging, asset management, noise exemptions, equipment capability, shoulders and ramp mix type, surface mix temperature restrictions, profile grade constraints, paving in echelon for widening, tapers, turning lanes, policy on matching lanes at the end of the day, grade exposure time, hours of operation, traffic speed and the uniformity of contract language.

Going forward, the MTO is to develop design guidelines for better longitudinal joints, the industry is to develop best practices and a communication package is to be developed to disseminate information.

Warm Mix Asphalt

In his second presentation of the day, keynote speaker Larry Michael reviewed the evolution of Warm Mix Asphalt in the U.S., spurred by the promise of reduced emissions, fumes and fuel consumption. Depending on the approach used, other potential benefits include improved mix workability, an extended paving window, cold weather paving and increased percentage of RAP. Last but not least, warm mix asphalt promises improved quality. Michael noted that there are four basic methods of producing warm mix asphalt.

The first method introduces moisture to create a foaming process to coat the binder such as Advera, Aspha-min, Double Barrel Green and Low Energy Asphalt. The second method is based on a two stage process with hard and soft binder, such as WAM-Foam. The third method utilises a mix additive such as Sasobit to reduce viscosity and enhance flow. The fourth method utilises an emulsion/ chemical package such as Evotherm, delivered as a high residue emulsion.

There is now a significant database of WMA projects and methods in the U.S. Michael listed nearly 30 projects that have been completed, based on a number of mix design methods and including Superpave, Marshall, Open Graded, SMA and State mixes, with depths ranging from 25 to 87.5 mm and RAP contents ranging from zero to 45 per cent. Binders have included rubber asphalt cement (RAC), performance based asphalt binder (PBA) and a range of Performance Graded binders.

These projects have delivered reduced production plant energy costs, reduced fumes, reduced emissions, and reduced mixing and compaction temperatures. Some concerns remain, both within industry and departments of transportation (DOT’s). Industry concerns include the impact of WMA production on baghouses, plant modifications, available binder storage tanks, changing behaviours, changing paving/compaction procedures, costs and DOT acceptance. DOT concerns include costs, testing, long-term performance and an AASHTO provisional standard.

Michael concluded that the WMA market will be driven by a number of factors including emissions, worker safety, the increased use of RAP, density specifications, higher fuel costs, extended paving window, cold weather paving and the need to improve quality.

LEED system

Sandy Brown of OHMPA had a busy seminar, making three presentations. The first provided an overview of the potential role of the Leadership in Energy and Environmental Design (LEED) system. This scoring system is used to rank the environmental impact of various construction technologies and points are awarded under various subheadings for the environment choices made by a building’s designers that will affect its construction and operation. HMA can generate up to nine points out of potential seventy and so make a significant contribution to a project’s award level under the system.

Quiet pavements

In his second presentation, "Asphalt – The Quiet Pavement", Brown described the principal sources of vehicle generated noise, the measurement of sound pressure levels and some different ways to reduce the vehicle noise. The nature of the road surface has an impact and The National Highway Cooperative Research Program (NCHRP) Synthesis 268, has concluded that, "In general, when dense-graded asphalt and Portland Cement Concrete (PCC) pavements are compared, the asphalt pavement is quieter by 2 to 3 dBA". Brown pointed out that this is a significant difference as the scale used to measure sound pressure level is not linear, with a 3 dBA reduction corresponding to a doubling of the distance between source and recipient or reducing traffic volume by 50 per cent or reducing traffic speed by 25 per cent.

Hot Mix Asphalt and Sustainability

Brown’s third and final presentation of the day, "Hot Mix Asphalt and Sustainability" reviewed the potential for using RAP using current technology. With current technology we can use RAP to: reduce the use of aggregate resources thus extending current resources, reduce the cost and fuel use to transport aggregates from further away, reduce the use new binder thus extending current resources. Using HMA pavement puts the binder and aggregate in the bank for the use of future generations. On that note, Brown added that we must take care not to reduce the ability to recycle HMA in the future by adding other waste streams now.

Other current sustainable HMA Initiatives include porous asphalt, warm mix asphalt and agricultural asphalt, with the latter used for silage floors and feeding bunkers to provide cleaner and healthier conditions for livestock.

With respect to greenhouse gases, a recent New York Times article noted that "cement plants account for five per cent of global emissions of carbon dioxide, the main cause of global warming. Cement has no viable recycling potential; each new road, each new building needs new cement." By contrast, asphalt cement is 100 per cent recyclable and the only fraction of the crude oil refining process that is currently recycled to any significant degree.