3. Asphalt Materials and Mix Design
3.6 ADDITIVES
Additives are sometimes used in asphalt mixtures to enhance various properties and/or mitigate environmental impacts. Numerous products are on the market, but this section will discuss the most common additives.
3.6.1 Warm-Mix Asphalt
WMA includes both additives and plant technologies that allow asphalt mixtures to be produced at lower-than-normal temperatures while maintaining mix workability, extending compaction windows and often acting as a compaction aid.
3.6.1.1 Foaming
Foaming is a common method of warm mix production that can reduce the required mixing temperature to as low as 250 °F (121 °C) in some instances. Foaming uses an injection of small amounts of water into the hot asphalt binder as it flows through special foaming injection nozzles. The rapid expansion of the steam creates microscopic bubbles in the binder that expand the volume of liquid and allow mixtures to be thoroughly coated at a reduced temperature.
Additives are sometimes used in asphalt mixtures to enhance various properties and/or mitigate environmental impacts.
3.6.1.2 Chemical Additives
A multitude of different chemical additives are available in the WMA market. These products are typically added by either premixing with the asphalt binder at the supplier’s facility or by addition at the mix plant. The use of these products can reduce mixing temperatures significantly lower than the foaming process. Those products premixed at the asphalt binder terminal are handled and used without special equipment needs. Other products must be stored separately at the plant and accurately metered into the plant as directed by the WMA product supplier. Asphalt mix producers should refer to the additive manufacturer’s recommendations when using these specialty products. Project specifications may dictate which additives can be used.
3.6.1.3 Organic Additives
Organic additives typically come in the form of paraffin wax and low molecular weight esterified wax with higher viscosity than asphalt binders below their melting point and lower viscosity at temperatures above their melting point. When mixed at temperatures above their melting point, they make the resulting mix more workable at lower temperatures. It is important that the melting point of the organic additive be higher than the expected in-service temperatures of the pavement to avoid potential issues related to rutting or deformation under load.
3.6.2 Liquid Anti-Stripping Additives
Chemicals known as liquid anti-stripping (LAS) additives have been used to treat moisture susceptibility in asphalt pavements. They function by reducing the surface tension between the asphalt binder and the aggregates, promoting better adhesion. Some of the chemical classes of amines that are used as LAS additives include fatty amines, amidoamines, and imidazolines. They all contain a hydrocarbon that has similar properties to asphalt and one or more amine groups. In addition to their chemistry, LAS additives can be characterized by properties such as heat stability and storage, concentration, viscosity, and odor. LAS additives are often added at the refinery or supplier’s terminal if requested. The method and rate of incorporation during the mix design should conform to the recommendations of the binder and LAS suppliers. Regardless of the type of LAS, the actual dosage rate should be determined by testing. It is possible for an increased dosage to lower test results.
Compatibility issues often need to be resolved; certain aggregate geologies may perform better with certain LAS formulations. Most agencies maintain a qualified product list for LAS.
3.6.3 Hydrated Lime
Hydrated lime is sometimes used in asphalt mixtures, primarily to improve resistance to moisture damage by reacting with clay in or on aggregate particles. It can also be used to stiffen the binder for improved rut resistance and potentially reduce age hardening. USACE reports that hydrated lime is also used to reduce bacterial deterioration of asphalt in warm, wet climates.
The amount of hydrated lime needed to improve the moisture sensitivity of an asphalt mixture generally ranges from 0.5 to 1.5 percent by dry weight of aggregate. It is typically added either dry or mixed with water as a lime slurry.
The addition of hydrated lime has been known to impact volumetric properties in the mixture. It is recommended that the manner of introducing lime during the mix design process be similar in nature to the method used in the field during plant mix production. It is also recommended that the mix designer optimize the amount of lime to be added to the mix. The amount of lime required will depend on the binder and aggregate combination used in the mix.
For additional information on LAS additives and hydrated lime, see National Cooperative Highway Research Program (NCHRP) Synthesis 595, Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements.
3.6.4 Fibers
Fibers for asphalt mixtures can generally be grouped into two categories:
- Fibers intended to stop or mitigate draindown in SMA mixtures.
- Fibers intended to stabilize the mix and inhibit rutting, cracking, and shoving.
Fibers to mitigate draindown in SMA are typically either cellulose fibers or mineral fibers. Cellulose fibers are plant-based fibers made from woody plants, although some are finely shredded recycled newspaper. Cellulose fibers have high absorption, which helps them maintain high binder contents without draindown. Cellulose fibers can be provided in loose form or in pellets and are typically added at a rate of about 0.3 percent by weight of the total mix mass.
Mineral fibers (also called mineral wool or rock wool) are manufactured by melting minerals then physically forming fibers by spinning or extruding. Minerals used to create mineral fibers include slag or a mixture of slag and rock, basalt, brucite, and carbon. Mineral fibers are less common and can be provided in loose form or in pellets and are typically added at a rate of about 0.4 percent by weight of the total mix mass.
Fibers intended to stabilize the mix are typically polyester, polypropylene, aramid, or combinations of these. The fibers used for this purpose are strong, heat-resistant, and from 5 to 40 mm in length. Polyacrylonitrile fibers can have a softening point as low as 430 °F (220 °C), while aramid fibers have softening points more than 800 °F (425 °C). Although these fibers may not significantly increase the tensile strength of the asphalt mixture, research has shown them to significantly increase the fracture energy, thereby improving crack resistance and crack propagation. These fibers are most commonly added at a rate from 0.065 to 0.1 percent by weight of the total mix mass.