civil engineering-->>Asphalt Binder Modifiers

Asphalt Binder Modifiers

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 tothermal 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 3.39 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.

Figure 3.39: Anti-stripping Modifier Example.  

3.7  Other Forms of Asphalt Used in Paving

Although asphalt cement is probably the most well known type of asphalt, three other forms of asphalt that are used prominently in the paving industry are emulsified asphalt, cutback asphalt, andfoamed asphalt.  These types of asphalt are not used in HMA pavements but are used extensively in pavement repairs, supporting layer or subgrade stabilization, bituminous properties, slurry seals, tack coats, fog seals, hot in-place recycling (HIPR), cold in-place recycling (CIR) and full depth recycling (FDR).

3.7.1 Emulsified Asphalts

Emulsified asphalt is simply a suspension of small asphalt cement globules in water, which is assisted by an emulsifying agent (such as soap).  The emulsifying agent assists by imparting an electrical charge to the surface of the asphalt cement globules so that they do not coalesce (Roberts et al., 1996).  Emulsions are used because they effectively reduce asphalt viscosity for lower temperature uses (tack coats, fog seals, slurry seals, bituminous surface treatments (BST), stabilization material).  Emulsions are typically either anionic (asphalt droplets are negatively charged) or cationic (asphalt particles are positively charged).

Generally, emulsions appear as a thick brown liquid when initially applied (see Figure 3.40).  When the asphalt cement starts to adhere to the surrounding material (aggregate, existing surface, subgrade, etc.) the color changes from brown to black (see Figure 3.41) and the emulsion is said to have "broken" (see Figure 3.42).  As water begins to evaporate, the emulsion begins to behave more and more like pure asphalt cement.  Once all the water has evaporated, the emulsion is said to have "set".  The time required to break and set depends upon the type of emulsion, the application rate, the temperature of the surface onto which it is applied and environmental conditions (TRB, 2000).  Under most circumstances, an emulsion will set in about 1 to 2 hours (TRB, 2000).  ASTM D 3628 contains guidance on selection and use of emulsified asphalt.

	Figure 3.40 (upper left): Freshly Placed Emulsion Tack Coat.  The brown color indicates that it has not yet broken. Figure 3.41 (second left): The Same Tack Coat After 23 Minutes.  The brown color now appears in splotches indicating it is beginning to break. Figure 3.42 (left): Tack Coat Using an Asphalt Emulsion.  The black color indicates it has broken.

3.7.2 Cutback Asphalts

A cutback asphalt is simply a combination of asphalt cement and petroleum solvent.  Like emulsions, cutbacks are used because they reduce asphalt viscosity for lower temperature uses (tack coats,fog seals, slurry seals, stabilization material).  Similar to emulsified asphalts, after a cutback asphalt is applied the petroleum solvent evaporates leaving behind asphalt cement residue on the surface to which it was applied.  A cutback asphalt is said to "cure" as the petroleum solvent evaporates away.   The use of cutback asphalts is decreasing because of (Roberts et al., 1996):

Environmental regulations.  Cutback asphalts contain volatile chemicals that evaporate into the atmosphere.  Emulsified asphalts evaporate water into the atmosphere.

Loss of high energy products.  The petroleum solvents used require higher amounts of energy to manufacture and are expensive compared to the water and emulsifying agents used in emulsified asphalts.

In many places, cutback asphalt use is restricted to patching materials for use in cold weather.

3.7.3  Foamed (Expanded) Asphalt

Foamed asphalt is formed by combining hot asphalt binder with small amounts of cold water.  When the cold water comes in contact with the hot asphalt binder it turns to steam, which becomes trapped in tiny asphalt binder bubbles (World Highways, 2001).  The result is a thin-film, high volume asphalt foam with approximately 10 times more coating potential than the asphalt binder in its normal liquid state (World Highways, 2001).  This high volume foam state only lasts for a few minutes, after which the asphalt binder resumes its original properties.  Foamed asphalt can be used as a binder in soil or base course stabilization, and is often used as the stabilizing agent in full-depth asphalt reclamation.