Better Roads Staff
Today, a CHFRS-2P asphalt emulsion – which, following the guidelines above, is a cationic, high-float, rapid-set emulsion in the No. 2 viscosity range with polymer added – is solving adhesion and slow break problems endemic to lower-performance emulsions that had been experienced by the district.
“It’s from a local supplier, it’s easy to get, and it’s on our low-bid list of items to buy, as is the conventional CRS-2P,” Bohuslav says. “The difference between the two is the high-float characteristic that grabs the rock better. The guys like it, it sets well, and it grabs the rock and holds it. The work I’ve seen them do is outstanding, I think our crew does some of the best seal coats in the state.”
In addition to its better adhesion and durability attributes, the cationic emulsion allows the state to place seal coats on the hottest days, of which there are many in the San Antonio region.
“On hot summer days, when it’s in the 100 degree F range and we are trying to shoot seal coat, the anionic emulsions won’t break and set quickly enough,” Bohuslav says. “They just sit there. But the cationic will break and set much faster, so we can get the work done, finish and move on.”
For these important applications TxDOT’s San Antonio District is using a CHFRS-2P asphalt emulsion from Ergon Asphalt & Emulsions. It’s made possible by a polymer, Butonal NX 1122 from BASF.
“For years we’ve had both anionic emulsions with high-float structures, and cationic polymer-modified emulsions,” says Cary Brownlee, regional sales manager, Ergon Asphalt & Emulsions, Inc. “They represent a value-added product for chip seals with specific benefits to the user. CHFRS-2P marries the properties of both, resulting in a cationic emulsion with high-float attributes.”
Emulsions for Reclamation
Asphalt emulsions – polymer-modified or not – constitute an important alternative as a stabilizing agent for full-depth reclamation (FDR) of decayed asphalt roads, aggregate bases, and sandy soils.
“In my former role as a contractor, about 25 percent of the FDR work we did involved injection of asphalt emulsions to stabilize a base,” says COLAS’ Mike Buckingham. “These emulsions are an alternative to chemical additives like lime, cement and fly ash. Asphalt emulsions constitute one of the choices that a buyer agency has for a stabilizing agent.”
Despite being a value-added manufactured product, asphalt emulsions don’t represent a more expensive quantity than the commodity-priced bulk lime or cement. “Emulsions are not necessarily more expensive,” Buckingham tells Better Roads. “It depends on the makeup of what you are trying to stabilize, and what will build strength better to the level you are trying to achieve. If you are doing dirt or sand products you might be more apt to use a dry stabilizer with water, rather than emulsion. Otherwise we might use emulsion by itself, or in combination with the other products.”
Asphalt emulsion helps to increase cohesion and load-bearing capacity of the mix, according to the Federal Highway Administration (FHWA). It also helps in rejuvenating and softening the aged binder in the existing asphalt material.
“The advantage of using emulsion is that emulsions are low in viscosity and very suitable for application through an on-board liquid additive system in the recycling equipment,” FHWA says in its Pavement Recycling Guidelines for State and Local Governments. “After the blending of the base material and emulsified asphalt, the emulsion ‘breaks’ and water separates out from the asphalt cement. This water is forced out of the base during compaction or will evaporate out during the curing period. The resulting residual asphalt cement has high viscosity and, therefore, helps in improving the cohesion of the base material.”
Alternatively, portland cement increases base compressive strengths, lime mitigates reactive, expansive clay in base materials, and fly ash increases impermeability and strength of the recycled mix.
“Emulsion-treated FDR may be cheaper than dry chemical reclamation when considering the entire structure — reclaimed base plus overlay — as emulsion-treated bases have a higher structural value,” Buckingham says.
Cutting Back on Cutbacks
A different way of creating a thin asphalt surfacing material is by use of solvents to dilute the liquid asphalt. These “cutback” asphalts are blends of asphalt cement mixed with solvents, which make the asphalt cement fluid for spraying or mixing. The solvents then evaporate, leaving the base asphalt cement in place to bind stone or chips.
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