First of all, it’s not “tar.” Asphalt is a natural substance that has some amazing physical properties. It’s sticky (adhesive) and it’s elastic, able to stretch, bend and flex without breaking (cohesive). This material does an excellent job of waterproofing. At air temperatures, asphalt cement is a very, very thick liquid (highly viscous). When heated, it becomes thinner and easier to use. Asphalt has been used since before Roman times as a glue and for water proofing. In a few places in the world, it’s naturally occurring, such as in a lake on the island of Trinidad and in the LaBrea “tar pits” in downtown Los Angeles. Almost all of the asphalt used today for paving comes from petroleum crude oil. Liquid asphalt is the heaviest part of the crude—what’s left after all the volatile, light fractions are distilled off for products such as gasoline. In Europe and Canada it is commonly called bitumen.
Asphalt is supplied in several different grades. Generally softer asphalts are used in colder temperatures and harder asphalts in hotter climates. The US government sponsored a multi-million dollar research project (Strategic Highway Research Program, SHRP) in the 1990’s which developed new standards for asphalt binders called performance grade (PG) binders. For example, a PG 64-22 is meant for use where average surface (high) pavement temperatures in the sun reach 64°C (147°F) and lows reach -22°C (-8°F). Premium grades usually have polymers or other modifiers for use in heavy duty applications such as intersections on city streets or airports or in extreme climates.
At normal temperatures, asphalt is too stiff to mix with the aggregates. There are three ways to thin it enough to make it mixable—heat it, dilute it with a solvent (cutback) or emulsify it in water.
By the way, “tar” (or coal tar) is a product made from coal.