An aggregate dryer in which the aggregate and the hot gas steam flow in the same direction. Counterflow dryers are inherently more efficient than parallel-flow dryers for making HMA. However, a parallel-flow dryer is well suited to soil remediation applications.

Pavement. A hard, smooth surface, especially the surface of roads traveled by the public. The most common pavement materials used in modern roads are HMA and concrete. Concrete roads that need repair are often re-paved with HMA.

An asphalt binder designed to produce HMA that meets certain performance standards. Designations for performance-graded asphalt binders are prefixed with PG. Each grade designation also includes two sets of numbers that denote a temperature range. This is a range of climate temperatures to which the road may be exposed and still be expected to give superior performance. The PG numbers do not indicate viscosity as in conventional liquid asphalt designations.

Consider, for example, the binder designated PG 58-46. The first set of numbers 58 refers to the high temperature in degrees C. The second set—46 (minus 46) refers to the low temperature in degrees C. Thus, this binder should perform well in climates where temperatures range from plus 58 to minus 46 degrees C (plus 136 to minus 51 degrees F).

The points where the temperatures are measured and the average duration of time that the temperatures are sustained are detailed in PG specifications.

Note that both modified asphalts and conventional, unmodified virgin asphalts (commonly known as neat asphalts) are assigned PG designations. The PG designations of the unmodified asphalts reveal that they perform well in a more limited range of climatic temperatures than modified asphalts.

A rule of thumb is sometimes used to indicate whether an asphalt incorporates modifiers. Just add the two sets of numbers of any PG designation. (Disregard the minus sign.) If the sum is 92 or greater, that asphalt probably includes modifiers.

(See facility.)

Programmable Logic Controller. A small electronic processor that can be programmed to automate controls. It has a wide variety of applications. PLCs can be used to automate production of hot mix at HMA plants or virtually any portion of the process. They can also be used to automate a polymer blending process. PLCs provide an economical alternate to a conventional desktop computer system, especially for processes where such a computer is not warranted.

PLCs have modular construction allowing the addition of modules to suit the application. Various modules provide a wide selection in the number of inputs and outputs and whether they are analog or digital.

Each PLC must be programmed for its application. It is usually programmed by a systems engineer and has no provisions for program changes by operators in the field. However, a systems engineer can alter old programs and send new software to users in the field where operators can implement the new program.

Astec's model designation for the Process Mate computer used to automate continuous HMA facilities. The latest version (PM-96A) incorporates an IBM-compatible desktop computer, color monitor and mouse.

The PM-96A control system automates blending of the mix. It automatically blends virgin aggregate, liquid AC, RAP, and additives, such as lime and anti-strip agents. Blending is based on material flow rates measured by the aggregate weigh scale.

The desktop computer uses Astec software which runs under Microsoft Windows, an extremely popular computer operating system. Computer controlled functions are backed up by manual backup units (MBUs).

The system produces and stores delivery ticket data in a format that can be copied onto a floppy disk and used with other programs.

The computer uses either metric units or U.S. units. It uses any of eight languages: English, German, French, Danish, Hungarian, Turkish, Spanish or Swedish. As an option the computer can use languages other than the eight named.

Any of numerous natural and synthetic compounds of usually high molecular weight consisting of up to millions of repeated linked units, each a relatively light and simple molecule.

When polymer is mixed with asphalt cement the mixture is known as PMAC or polymer modified asphalt cement. Rubber and plastic are polymers commonly used to produce modified asphalt. Rubber is an elastomer and includes natural latex, synthetic latex, block copolymer, and reclaimed rubber. Examples are natural rubber, styrene-butadiene-rubber (SBR), styrene-butadiene-styrene(SBS) and recycled tires.

Plastic polymers include polyethylene, polypropylene, ethyl-vinyl-acetate (EVA), and polyvinyl chloride (PVC).