In 2005, contractors resurfaced this section of US 61/67 in St. Louis County, Mo., with hot mix containing 20% reclaimed asphalt. Photo: Missouri DOT
Reclaimed asphalt enters the outer drum near the burner end. Aggregate flows through the flame and mixes with the binder and used material as it moves in the outer drum from left to right. Photo: Astec Inc.
Another is the combination of warm-mix technologies with higher used-material contents. This enhances mix workability while providing the triple environmental benefits of reduced plant emissions, reduced energy demand, and conservation of natural resources.
The more reclaimed material is allowed, the more contractors can reduce bid prices.
Virgin aggregate ranges from $10 to $30/ton, depending on the region of the country. Thus, a hot mix of $15/ton aggregate and 5% $600/ton new binder costs about $44.25/ton. By contrast, a mixture of 20% reclaimed material and 5% new binder costs $37/ton — a 15% savings.
Dale Rand, PE, director of the flexible pavements branch at the Texas DOT (TxDOT), says departments may save $7 to $10/ton on hot mixes with 20% recycled material. Last year the department placed 5 million tons, about half the usual amount. “At $10/ton, that's $50 million in savings,” Rand says.
Effective Jan. 1, all TxDOT projects use reclaimed asphalt. Previously, the mixture was allowed only if project plans specifically called for it, and only about 3% of the department's hot-mix tonnage included it. Today, if reclaimed material is not fractionated, the allowable amount is 10% on surface courses, 20% on intermediate courses, and 30% on any base course that's more than 8 inches below the surface. If the RAP is fractionated, all of those percentages jump by 10%.REGULATORS CRACK DOWN
Asphalt abounds in New Jersey, and when it deteriorates the roads are milled up. Millions of tons of material are stockpiled.
“The state Department of Environmental Protection has begun to consider these huge stockpiles as more of a landfill waste than a resource,” says Eileen Sheehy, manager of the DOT's Bureau of Materials. “It's supposed to be a recyclable product, but it's not being used.”
As a result, Sheehy expects the state to increase its surface-course limit from 15% to 25% recycled material.
The industry has been working with the Arizona DOT to allow up to 15% recycled content in base and intermediate courses, but the agency has yet to decide on a maximum limit.
“We'd be comfortable with 20%,” says Scott Weinland, acting pavement materials testing engineer. “We've been reluctant to use reclaimed asphalt because it's stiff, and with the heat in Arizona we already use a stiff binder to prevent rutting. If we add RAP to an already stiff binder, the mix is more prone to cracking. In the 1980s we were producing mixes with 30% or more reclaimed material and saw some problems with premature cracking. That's why we've proceeded with more caution than other states have.”
With a 20% limit and strict end-product specifications on mixture characteristics such as density and aggregate gradation, he expects a smoother process.
“Today we have better controls and management of stockpiles and hot-mix plants in general,” says Dave Newcomb, vice president of research and technology at the National Asphalt Pavement Association. “We have better plant emissions controls, better heat distribution in the mixing drums, and better ways to size the material and feed it into the drums.”
— Brown is the owner of TechniComm, a communications business based in Des Plaines, Ill.Recycling made easier
Advances in drum technology make for more efficient mixing.
Asphalt plant mixing drums come in two types: parallel-flow, which is an older style, and counterflow.
With parallel-flow mixers, a large rotating drum sits nearly horizontal to the ground. At one end a large burner blasts a flame into the drum to dry aggregates. The aggregates are introduced close to the flame end of the drum. As it rotates, paddles inside the drum move the aggregate to the opposite end, where liquid asphalt is introduced and mixing is completed.
Counterflow plants move the aggregate toward the burner and against the hot gas stream instead of with it. Mixing of the aggregate with liquid asphalt occurs behind the flame or in an outer drum not exposed to the flame, and reclaimed material is added to the mix at varying locations depending on the manufacturer.
Because they heat aggregate and reclaimed asphalt more efficiently, counterflow drums incorporate 30% to 50% more recycled material into mixtures. Parallel-flow drums can only add about 25% to the mixture.
Astec Inc. is marketing a “green” asphalt plant that allows used material to be used in warm mixes. By adding a small amount of water to the liquid asphalt, the plant makes foam that disperses the binder throughout the mix. As a result, the plant can operate at lower temperatures — between 240° and 270° F — than the 300° to 325° F that conventional plants require.Web Extra
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