by William M. Kelly
Chapter 6: Hot Mix Asphalt
Hot mix asphalt (HMA, asphalt concrete) is a mixture of coarse and fine construction aggregate mixed with asphalt binder, a petroleum derivative. Typically, the mix is 5 percent binder and 95 percent aggregate. It is widely used for pavement, and is placed and compacted at elevated temperature, typically 135°C (275°F) to 163°C (325°F). Hot mix asphalt is usually applied in layers 4 to 8 inches thick. The lower (base) layer(s) are typically composed of angular aggregates chosen to resist failure. The base layer is coarser-grained than the top layer but fine aggregate is added in the mix to fill voids between the larger particles and provide load transfer to the larger particles. The top layer, called the top course or friction layer, is made of finer aggregate that is durable and has good friction properties to prevent vehicles from skidding.
Crushed stone is commonly used for aggregate, but other materials such as reclaimed asphalt pavement, crushed concrete, foundry sand, coal fly ash, and slag can be used (Industrial Resources Council 2010). New York State specifications allow for local high-quality aggregate use from sand and gravel deposits, limestone and dolostone units, metamorphic rocks such as schist and gneiss, and igneous rock such as diabase, for aggregate sources. There are various “friction” levels required in roads and most of these aggregates meet Department of Transportation quality specifications. One exception is for the highest friction surface. In this case, noncarbonate stone is necessary in the surface layer. Crush-count requirements can limit the use of fine-grained gravels for roads with very high traffic loads (B. Barkevich, pers. comm., 2010).
Asphalt plants now operating in New York may be up to fifty or sixty years old. New asphalt plants are typically of the drum-type with capacities of 408 metric tons (450 short tons) per hour or more. Most of these can use reclaimed asphalt pavement (RAP) in the mix. Many batch plants are being refitted to handle RAP as well. The New York State Department of Transportation allows 20 percent RAP in surface and binder courses of pavement and 30 percent in base courses. New York City allows 40 percent in some mixes. New York towns and counties do not specify proportions but generally follow state specifications. Hot mix asphalt for commercial projects, such as driveways and parking lots, can contain whatever amount the producer believes is suitable (B. Barkevich, pers. comm., 2010).
Polymer modifiers are added to modern asphalt to improve the elasticity of the asphalt and increase durability. Modifiers add to the cost of the asphalt (five to ten dollars per ton) but the increased cost is mitigated by the longer life of the pavement. An additive which is being investigated and which is coming into increased use is recycled roofing shingles. The large amount of high-quality asphalt in shingles makes them an attractive addition to the hot mix asphalt mix. Currently, the preferred variety of shingles is manufacturers’ waste but shingles removed from buildings by contractors are also being investigated for use. Adding 5 percent recycled shingles to hot mix asphalt can reduce the cost by $1.00 to $2.80 per ton and improve the quality of the mix used in paving (Northeast Recycling Council 2007).
Asphaltic concrete mixtures were first used in the United States in the late 1860s for sidewalks, and to a limited degree, roads. The first true asphalt pavement, a mixture of asphalt and sand, was installed in Newark, New Jersey, in 1870. In the late nineteenth century, all asphalt was derived from an asphalt lake in Trinidad or from one in Venezuela. Since builders quickly realized the advantages of asphalt paving, they created competing proprietary brands of hot mix asphalt, which were aggressively marketed. The first of these patents was filed in 1871, by a resident of Brooklyn, New York. However, as requirements for pavement became more stringent, including warrantees, the proprietary pavements were forced from the market by the 1920s. The early-twentieth-century use of refined petroleum asphalt surpassed the use of natural asphalt as oil refineries proliferated (National Asphalt Pavement Association 2010). In New York, coal tar was routinely used as a binder prior to 1950. In 1976, New York State prohibited the use of coal tar in base course, road shoulders, and all roadway paving (Mundt etal. 2009).
Centrally operated hot mix production facilities existed in the late nineteenth century. Early mixing and drying equipment was modified from portland cement concrete mixers. Originally, hot mix asphalt was spread and smoothed by hand and rolled by a horse drawn or steam powered roller, a very labor-intensive process. By the early twentieth century, modified mechanical spreaders, first used for portland cement, were in use. Hot mix asphalt facilities in the 1950s were dirty, dusty industrial operations with little in the way of equipment to reduce emissions of chemicals or particulates. This is no longer the case. Centrifugal dust collectors, wet scrubbers, and large bag houses (fabric filters) now remove these materials from the exhaust gasses generated by the plant.
Fillers and fibers, including asbestos, were routinely added to asphalt products nationally. New York never used asbestos for mainline road paving but began investigating its use in 1959, and some low-volume use of this material was reported (Mundt et al. 2009). Asbestos pavement, containing 1 to 3 percent asbestos, was shown to have greater flexibility and crack resistance and reduced brittleness, and allowed higher binder content. It did, however, increase the cost. The U.S. Environmental Protection Agency proposed a ban on the use of asbestos in road construction in 1971, and by 1979 its use was widely prohibited. Tests of the use of furnace slag began in New York in 1920 and by the 1950s this material was used in 70 percent of pavements, either as a surface treatment or in the foundation material (Mundt et al. 2009). Typical fillers currently used in New York include baghouse dust and stone dust. The cost of crumb rubber is high, and its use as filler is limited in New York.
During the 1970s, the need for conservation of natural resources led to the increased use of recycled asphalt pavement in freshly produced hot mix asphalt. Currently, asphalt pavement is the most recycled material in the United States, with over 95 million metric tons being used annually (National Asphalt Pavement Association 2010). This includes 1.36 million metric tons (1.5 million tons) of RAP used in New York annually. In New York, recycled pavement is, on average, 10 percent of road paving mixes (Mundt et al. 2009). However, old pavements with coal tar are prohibited from recycling in New York.
Superpave (SUperior PERforming Asphalt PAVEments) guidelines for binder selection and mixture performance were developed to design mixes that would meet specific weather (e.g., high and low temperatures) and traffic (load) conditions. New York began implementing these in the middle to late 1990s. Using the Superpave guidelines, all mixes produced in New York use a performance-graded binder that is suitable for the specific climate and traffic volume for the pavement. The blend of aggregate, asphalt, and voids is designed to produce a road surface that will be durable and resist rutting. Typical asphalt for upstate New York use is graded 64–22, meaning that the pavement will meet its performance specifications between the temperatures of 64°C (147°F) and -22°C (-8°F). In Westchester County and southeastern New York, a 70–20 Superpave mix is used due to the warmer ambient temperatures in that region.
The primary use of hot mix asphalt is in paving. Approximately 94 percent of the roads in the United States are paved with this material. Parking lots are commonly so paved as well. It is used for small and large projects that range from residential driveways and golf cart paths to military and commercial airport runways. Asphalt pavement is used to line reservoirs and industrial retention ponds and in sea walls and groins to control shoreline erosion. It can be used in such agricultural and industrial applications as cattle feed lots, poultry and green house flooring, freight yards, and as landfill cap.
At a hot mix asphalt plant, aggregates are blended, heated, and mixed with a binder to produce a product that meets specific requirements. The plants can be stationary or portable and are generally of two types. Batch plants (Figure 22) dry, sort, if not previously sorted, and heat the aggregate. Asphalt binder is heated separately. These components are then mixed in a pugmill to make a single batch that commonly weighs on the order of 2,000 kilograms to 5,440 kilograms (4,400 to 12,000 pounds). Cold aggregate is fed into the plant by one of three methods: open-top bins fed by a front-end loader, tunnels under stockpiles fed by conveyor or loader, or bunkers or large bins with aggregate fed by trucks or dump-bottom freight cars. Feeders on the bottom of the continuous belt type or vibratory bins deposit aggregate onto a conveyor or bucketline, which carries the aggregate to the dryer. The aggregates then enter a dryer to remove moisture and heat the material (Figure 23). The dryer is a revolving cylinder 1.75 to 3.3 meters (5 to 10 feet) in diameter and 7 to 13 meters (20 to 40 feet) in length. An oil- or gas-fired burner provides the heat with fans for primary air supply and exhaust. Dryers have “flights” (longitudinal fins or channels) that lift and drop the aggregate through the flame and hot gasses. Once heated, the material is passed over screens for separation by size and then stored hot (Figure 24). Hot aggregates, fillers if any, and asphaltic binder are drawn from storage in measured amounts and thoroughly mixed in a pugmill into a batch. The hot mix asphalt can then be discharged into trucks or placed in a surge bin to await shipment to the paving site (Indiana Department of Transportation 2010).
Typical components of a batch-type hot mix asphalt plant. (Indiana Department of Transportation 2010.)
The dryer in a batch hot mix asphalt plant heats and dries the aggregate. Also shown: conveyor for aggregate feed (left) and baghouse (rear). (Courtesy Northern Bituminous Mix, Inc.)
Baghouse dust collection system (center). Also aggregate hoppers (foreground) and hot screen and pugmill (left
background). (Courtesy Northern Bituminous Mix, Inc.)
Hot mix asphalt plant operators have recognized the potential for air pollution and have developed equipment to mitigate the problems. Close attention is paid to the burners so that they do not become dirty or clogged, and air-fuel mixtures are kept properly adjusted to avoid the emission of excessive smoke or other deleterious products of incomplete combustion. Dust control systems are integrated into the design of the plant and its operations. Dust collectors can be one of two types in New York: wet scrubbers or baghouses (fabic filters). Sometimes more than one type is used, particularly if the aggregate is very dusty. The primary type of collector used in New York is the baghouse (Figure 25). While a small number of plants use wet scrubbers, even newly constructed plants install baghouses (B. Barkevich, pers. comm., 2010). A baghouse allows the accumulated dust cake to be reclaimed and used in the hot mix asphalt as filler or it can be discarded. In a wet scrubber, the dust is trapped in water and is not recoverable. Furthermore, the waste water containing the dust from a wet scrubber must be properly handled to prevent pollution. The amount of water that requires treatment can be considerable since a wet scrubber can consume about 1,136 liters (300 gallons) per minute (Indiana Department of Transportation 2010).
Hot screen deck and mill of batch-type hot mix asphalt plant. Baghouse also shown (left center). (Courtesy Northern Bituminous Mix, Inc.)
Drum-type plants (Figure 26) heat and dry previously sorted aggregate with binder in a drum. Drum plants differ from batch plants in that the aggregate is not only dried and heated in the dryer drum but the binder is mixed there as well. The processes are the same at both types of plants including cold aggregate storage and feeding, dust collection, and storage of the mix. Drum-type plants have no hot gradation screens or pugmills. Aggregates in various size gradations are withdrawn from stockpiles and placed in a multiple-bin feed system. Precision feeders control the amount of aggregates that are delivered and fed cold into the drum. Typically, the burner that heats the material is located in the feed end of the drum, but other arrangements (e.g., counter-flow units) are possible. The interior of the drum is equipped with flights that direct the motion of the aggregate and lift and drop the material through the burner flame. As the aggregate is heated and dried, weighed amount of binder is introduced into the drum where it is thoroughly mixed. A dust collection system captures the dust produced by the drum. The hot mix asphalt product is discharged continuously into a surge bin and subsequently loaded into trucks for delivery.
Typical components of a drum-type hot mix asphalt plant.
Indiana Department of Transportation 2010.
No matter which type of plant is used to produce the hot mix asphalt, fillers and modifiers are routinely added to the mix to improve performance. Recently, the use of polymer modified asphalt has increased dramatically with approximately 1.36 million metric tons (1.5 million tons) of material being used annually. It has been shown that polymers aid in the elastic recovery of the asphalt. The materials used for fillers and modifiers, and the purpose of the addition of these, are given in Table 9. Crude oil from which New York’s asphalt is produced has multiple sources, including Venezuela, Mexico, Canada, and the U.S. mid-continent region. Most of this material is brought by barge, rail, or trucks to refineries in the Northeast. The asphalt is either shipped directly to an HMA plant or to an intermediate company which will modify the asphalt to meet New York State specifications. Final delivery is typically by tanker truck.
Fillers and Modifiers Added to Asphalt Cement (after Roberts et al. 1996). Note that while extenders, oxidants, antioxidants,
and hydrocarbons are used in asphalt products for surface treatments, micro-surfacing, and stone penetration, these compounds find only limited use in hot mix asphalt.
Virtually all of the hot mix asphalt produced in New York is dense-graded mix, a relatively impermeable product suitable for all pavement layers and all traffic conditions. They mixes are used for structural, high friction, patching, and leveling needs. These contain well-graded aggregates, asphalt binder with or without modifiers, and reclaimed asphalt pavement (Washington Asphalt Paving Association 2010).
Open-graded mixes use only crushed stone or gravel and a small amount of manufactured sand. Consequently, they are porous and allow water to penetrate. This product, depending on the specific mix, can be used for surface course paving or as a drainage layer below dense-graded asphalt. Open-graded mix is reported to reduce road spray from precipitation and decrease road noise by up to ten decibels (National Asphalt Producers Association 1995). These mixes are becoming popular in New York as their use garners LEED (Leadership in Energy & Environmental Design) points in “green” building certification, but to date their applications are currently limited (B. Barkevich, pers. comm., 2010). Open-graded mixes have voids, typically 15 percent, that are critical to the proper function of this type of asphalt. Anything that leads to clogging of the voids, such as de-icing sand, will degrade performance.
Stone matrix (gap-graded) asphalt was earlier used in New York. This material was developed to allow for stone-to-stone contact in the aggregate. Since the crushed stone does not deform as much as asphalt, this should reduce rutting and increase durability. Stone matrix asphalt is more expensive than dense-graded mixes due to the need for more durable aggregate, higher asphalt content, and the addition of modifiers and fillers (National Asphalt Paving Association 2001). In New York they have fallen from favor due to higher costs (B. Barkevich, pers. comm., 2010).
Warm mix asphalt is rapidly increasing in use in New York. Warm mix is spread at temperatures of 93°C (200°F) to 135°C (275°F), which is on the order of 25 percent lower than typical hot mix asphalt. Reduced temperature results in less fuel usage and decreased fumes and greenhouse gas emissions from the plant. Warm mix is reported to provide better compaction on the road. Its use can increase the haul distance for paving mixes. Reducing the temperature at which the hot mix asphalt is produced will reduce the level of oxidation of the asphalt and lead to better long-term pavement performance. Warm mix permits the use of increased amounts of recycled asphalt pavement.
New York State has recently developed special specifications for warm mix that allows the New York State Department of Transportation to call for bids on projects that use this material. In 2010, about fifteen projects were slated to be completed with warm mix. In the past five years, approximately 227,000 metric tons (250,000 tons) of warm mix have been spread in New York. Currently, four companies are approved for use of warm mix on NYS DOT projects. This number is expected to double by the 2011 paving season and it is projected that in ten years more than half of the blacktop produced in the United States will be warm mix.
There are approximately 200 hot mix asphalt plants currently operating in New York (Figure 27). In a pattern similar to ready mix concrete plants, HMA producers are distributed across the state with increased numbers concentrated near population centers. As hot mix asphalt has to be spread while it is within a certain temperature range, the distance that it can be transported is limited. Maximum transport distance is roughly 120 kilometers (75 miles). Longer hauling is possible but it is expensive and the quality of the material at delivery may be jeopardized. Average transportation distance is 80 kilometers (50 miles) or less with 48 kilometers (30 miles) being preferred (B. Barkevich, pers. comm., 2010).
Location of hot mix asphalt plants that serve New York. Note that some are located outside state borders but still serve the New York market. (Data: New York State Department of Transportation.)
The price for hot mix asphalt varies by region within New York. Material used in the New York City Metropolitan Region and in southeastern New York can be significantly higher in cost than the same product in upstate regions. In part, this is due to the cost of shipping for the aggregate component of the HMA. In addition, labor and hauling costs are higher in downstate areas. On average, prices range statewide between $50 and $100 per ton.