In this section, we will examine the different types of bituminous binders used in SMAs and methods for selecting one.
3.3.1 Types of Applied Binders
Various SMA binders may be seen in the highway engineering practices of many countries. These binders can be divided into paving grade bitumens (unmodified), polymer-modified bitumens (PMB), and special binders (multigrade and others).[16]
Paving grade bitumens are frequently used. In Europe the most commonly used binder is the penetration graded 50/70 type, and to a lesser extent the 70/100 type. Performance graded binders are routinely used in the United States and may or may not be polymer modified, depending on the base asphalt, the desired range of temperatures at which the binder is expected to perform, and the anticipated traffic level.
Polymer-modified bitumens are increasingly being used. They are found mostly in mixtures laid on roads with high traffic loadings, in special conditions (e. g., road crossings, slow traffic lanes) or on special pavements, although one should remember that PMBs require a suitable technological regime. U. S. experts recommend that highly modified PMBs with polymer contents over 5% (m/m) should not be used (Asphalt Review, December 2004). High polymer contents create problems with fast stiffening and increased difficulties during compaction. Use of highly- modified PMBs combined with other stiffness enhancers should be especially avoided.
Properly chosen and tested modified binders could increase rutting resistance and decrease the risk of low-temperature cracking of an SMA pavement.
Among the special binders, multigrade ones are sometimes used in SMA mixtures; for example, in Australia (NAS AAPA, 2004) they are used for heavily trafficked pavements.
