Better Roads Staff
Mineral Modifiers and Extenders
There are many kinds of asphalt modifiers or additives other than polymer modifiers. These can include mineral modifiers such as hydrated lime, fly ash or portland cement; a liquid mineral additive, polyphosphoric acid (PPA); and extenders like sulfur, itself often a product of petroleum refining.
The more familiar polymer modifiers mix or dissolve with the liquid binder to help it perform better. Mineral modifiers perform physically to enhance adhesion of binder to aggregate, or keep liquid asphalt binder from draining from an aggregate structure, as with stone matrix asphalt.
Hydrated lime is pre-eminent among the modifiers used principally to fight moisture damage to asphalt pavements. Moisture damage – the loss of strength and stiffness of the mix due to water being present in different forms or phases – is a syndrome that can threaten long-term durability of asphalt pavements.
Moisture can penetrate in the liquid phase from the surface, rise via capillary action from the base, and also diffuse in the vapor phase throughout the pavement structure. This water can cause a loss of adhesion of the asphalt binder to the aggregates, resulting in the condition known as stripping.
One view is that in the presence of moisture, stripping is driven by the aggregate surface’s affinity for water. Mineral- or polymer-based additives can change the surface of the aggregate from hydrophilic (water-loving) to hydrophobic (water-hating). Simply put, some aggregates prefer water over asphalt, tending to be acidic and suffer from stripping after exposure to water. Other aggregates prefer asphalt to water, tend to be basic ( versus acidic) and tend not to suffer from stripping.
In the presence of hydrophilic aggregate, liquid antistrips and polymers are added to asphalt binder, while dry antistrips like hydrated lime, portland cement or fly ash are added to the aggregates. They operate by boosting adhesion between aggregate surface and binder, by reducing binder surface tension and thus helping the binder more completely enrobe the aggregate particles, or improving the chemical properties of both.
Polyphosphoric acid, or PPA, despite its name beginning with poly, is not a polymer. “The longest PPA chain with the same units repeating are in the neighborhood of five or six units,” says Blacklidge Emulsions’ Daranga. “To be a polymer you need to be at least in the neighborhood of 50.”
Furthermore, PPA is an inorganic compound, so it is not hydrocarbon-based. It’s an inorganic acid, just like hydrochloric acid. It modifies properties of asphalt, bringing subpar binder up to a PG level. “Its method of activity is elusive and it’s difficult for us to understand exactly how it works,” Daranga says. “For this reason some agencies are afraid of it. But contractors would like to use it because it’s not expensive.”
Polyphosphoric acid has been proven as a successful modifier for asphalt either by itself or in combination with polymers, but the word on its performance needs to penetrate the public agency level, says Darrell Fee, Rene Maldonado, and Henry Romagosa, ICL Performance Products, and Gerald Reinke, Mathy Construction, in their 2010 Transportation Research Board paper, Polyphosphoric Acid Modification of Asphalt.
“In fact, over the past five years, an estimated 3.5 to 14 percent of the asphalt pavement placed in the United States contained PPA,” they say. “This represents up to 400 million tons of asphalt mix. However, for any particular pavement project, the information on the use of PPA remains largely in the private sector. The information in the public sector is mainly limited to laboratory studies of asphalt modified with PPA. There are only a few reports on the field performance of asphalt containing PPA or polymer and PPA, primarily from the test tracks at National Center for Asphalt Technology (NCAT) and The Minnesota Road Research Project (MnROAD). Due to the lack of data in the public domain, there are concerns about the moisture sensitivity of PPA modified asphalts.”
The mechanism by which polyphosphoric acid interacts with asphalt to improve rheology and other properties is still under investigation, the authors say. “One theory suggests that polyphosphoric acid reacts with various functional groups in the asphalt, breaking up the asphaltene agglomerates and allowing the individual asphaltene units to form a superior dispersion in the [surrounding] maltene phase. Once dispersed, the individual asphaltene units are relatively more effective in forming long-range networks and affecting the rheology and physical characteristics of the asphalt.”
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