Some asphalt cements require modification in order to meet specifications. Asphalt cement modification has been practiced for over 50 years but has received added attention in the past decade or so. The added attention can be attributed to the following factors (Roberts et al., 1996):
- Increased demand on HMA pavements. Traffic volume, loads and tire pressures have increased substantially in recent years, which can cause increased rutting and cracking. Many modifiers can improve the asphalt binder’s stiffness at normal service temperatures to increase rut resistance, while decreasing its stiffness at low temperatures to improve its resistance to thermal cracking.
- Superpave asphalt binder specifications. Superpave asphalt binder specifications developed in the 1990s require asphalt binders to meet stiffness requirements at both high and low temperatures. In regions with extreme climatic conditions this is not possible without asphalt binder modification.
- Environmental and economic issues. It is both environmentally and economically sound to recycle waste and industrial byproducts (such as tires, roofing shingles, glass and ash) in order to gain added benefit. Thus, when they can benefit the final product without creating an environmental liability they are often used as additives in HMA.
- Public agency willingness to fund higher-cost asphalt additives. Modified asphalt cement is usually higher in initial cost than unmodified asphalt cement, but it should provide a longer service life with less maintenance.
There are numerous binder additives available on the market today. The benefits of modified asphalt cement can only be realized by a judicious selection of the modifier(s); not all modifiers are appropriate for all applications. In general, asphalt cement should be modified to achieve the following types of improvements (Roberts et al., 1996):
- Lower stiffness (or viscosity) at the high temperatures associated with construction. This facilitates pumping of the liquid asphalt binder as well as mixing and compaction of HMA.
- Higher stiffness at high service temperatures. This will reduce rutting and shoving.
- Lower stiffness and faster relaxation properties at low service temperatures. This will reduce thermal cracking.
- Increased adhesion between the asphalt binder and the aggregate in the presence of moisture. This will reduce the likelihood of stripping. Figure 1 shows two aggregate samples from the same source after they have been coated with asphalt binder. The asphalt binder used with the sample on the left contain no anti-stripping modifier, which resulted in almost no aggregate-asphalt binder adhesion. The asphalt binder used with the sample on the right contains 0.5% (by weight of asphalt binder) of an anti-stripping modifier, which results in good aggregate-asphalt binder adhesion.
Common Types of Asphalt Modifiers
The following table lists some common asphalt cement and HMA modifiers and their general purpose/use.
|Type||General Purpose or Use||Generic Examples|
|Filler||Fill voids and therefore reduce optimum asphalt content
Meet aggregate gradation specifications
Improve the asphalt cement-aggregate bond
|Extender||Substituted for a portion of asphalt cement (typically between 20 – 35 % by weight of total asphalt binder) to decrease the amount of asphalt cement required||Sulfur
|Rubber||Increase HMA stiffness at high service temperatures
Increase HMA elasticity at medium service temperatures to resist fatigue cracking
Decrease HMA stiffness at low temperatures to resist thermal cracking
(see Figure 2)
(e.g., Polychloroprene latex)
(e.g., Styrene-butadiene-styrene (SBS))
(e.g., crumb rubber from old tires)
Ethylene acrylate copolymer
Polyvinyl chloride (PVC)
Ethylene propylene or EPDM
|Rubber-Plastic Combinations||Blends of rubber and plastic|
|Fiber||Improving tensile strength of HMA mixtures
Improving cohesion of HMA mixtures
Permit higher asphalt content without significant increase in draindown
|Oxidant||Increase HMA stiffness after the HMA is placed||Manganese salts|
|Antioxidant||Increase the durability of HMA mixtures by retarding their oxidation||Lead compounds
|Hydrocarbon||Restore aged asphalt cements to current specifications
Increase HMA stiffness in general
|Recycling and rejuvenating oils
Hard and natural asphalts
|Antistripping Agents||Minimize stripping of asphalt cement from aggregates||Amines
|Waste Materials||Replace aggregate or asphalt volume with a cheaper waste product||Roofing shingles