Performance Grade Systems for Asphalt Binders

Background

Asphalt binders are most commonly graded by their physical properties. An asphalt binder’s physical properties directly describe how it will perform as a constituent in asphalt concrete (AC) pavement. Although asphalt binder penetration and viscosity grading are still commonly used worldwide, new binder tests and specifications have been developed to more accurately characterize asphalt binders for use in AC pavements. The most advanced asphalt binder grading system is Performance Graded (PG) System, which resulted from the $50-million Strategic Highway Research Program (SHRP), and has been adopted by all of the other 49 states in the U.S. In the past, Caltrans has classified binder using viscosity grading based the Aged Residue (AR) System. Caltrans began using the PG system January 1, 2006, and began to use the PG system for polymer-modified binder on January 1, 2007. 

What is the PG system?

Performance grading is based on the idea that asphalt binder properties should be related to the conditions under which the binder is used. PG asphalt binders are selected to meet expected climatic conditions as well as aging considerations with a certain level of reliability. Therefore, the PG system uses a common set of tests to measure physical properties of the binder that can be directly related to field performance of the pavement at extreme temperatures. A binder graded by PG system should meet all specified criteria by this common set of tests.

The Rotational Viscometer (RV) test is used to evaluate the pumpability at the asphalt plant. The Dynamic Shear Rheometer (DSR), tests with different levels of aging are used to evaluate the binder properties at high or intermediate temperature, which are related to the rutting or fatigue cracking of pavement failure modes. The Bending Beam Rheometer (BBR) and Direct Tension Test (DTT), with certain levels of aging are used to evaluate the binder properties under low temperature; which are related to the low temperature cracking of pavement failure mode. The Rolling Thin Film Oven (RTFO) aging is used to simulate the short-term aging of the asphalt binder during the construction phase, including the mixing, silo, transportation and placement. The Pressure Aging Vessel (PAV) is used to simulate the long-term aging of the asphalt binder after the AC pavement has been in service about 10 years.

How to read a PG grade

The PG grading system is based on climate, so the grade notation consists of two portions: high- and low-pavement service temperature. The major concern for high-temperature performance is rutting, which typically takes time to cumulate, therefore an average of seven-day maximum pavement temperature is used for describing the high temperature climate. On the low temperature side, thermal cracking can happen during one really cold night; therefore the minimum pavement temperature is used for describing the low temperature climate. 

For both high and low temperature grade, PG grades are graded in 6°C increments. The average seven-day maximum pavement temperature typically ranges from 46 to 82°C, and minimum pavement temperature typically ranges from -46°C to -10°C. A binder identified as PG 64-10 must meet performance criteria at an average seven-day maximum pavement temperature of 64°C and also at a minimum pavement temperature of –10°C. Note that maximum pavement temperature is typically higher than the air temperature by about 20°C since the dark colored pavement absorbs the heat and retains it. The maximum pavement temperature is typically measured at about one inch below the pavement surface. However, the minimum pavement temperature occurs on the surface of the pavement and is equal to the air temperature. The common minimum reliability used is 98%. 

When the PG 64-10 binder is selected, the asphalt binder in the AC pavement should perform satisfactorily under normal traffic conditions at the location where the extreme pavement temperature is within the range of -10°C and 64°C throughout its service life, with a minimum 98% confidence level. Where the traffic conditions are not typical, such as the heavy traffic interstate highway, or slow traffic such as bus stop or intersection, an asphalt binder one or two grades stiffer may be used to help prevent the rutting problem. 

Polymer-modified binders are used wherever extra performance and durability are desired. Improvement in resistance to rutting, thermal cracking, fatigue damage, stripping, and temperature susceptibility have led polymer modified binders to be substituted for asphalt in many paving and maintenance applications. Especially when good rutting resistance for high temperature and good thermal cracking resistance for low temperature are concurrently required in the same application, the polymer modification is generally required. A rule of thumb to differentiate the polymer-modified binder from unmodified binder is to add both low and high temperature grades together. If the sum is greater than 90, the binder is likely to be polymer-modified. For example, a PG 76-22 is likely to be a polymer modified binder since the sum is 98, while a PG 64-10 is likely to be an unmodified since the sum is 74. Polymer-modified binders used in Caltrans projects will be noted with Polymer-Modified (PM), such as PG 64-28PM, or PG 76-22PM. 

Caltrans Climate Regions

The Caltrans climate regions used for selecting asphalt binder grade are shown in Figure 2 and listed in Table 1. For conventional dense graded asphalt concrete, values are given in Table 1 for typical and special conditions. Special conditions are defined as those roadways or portion of roadways which meet any of the following criteria:

• Truck/bus traffic (over 10 million ESALs for 20 years);

• Truck/bus stopping areas (parking area, rest area, loading area, etc.);

• Truck/bus stop and go areas (intersections, metered ramps

Resources

1. http://www.dot.ca.gov/hq/oppd/pavement/ Pavement_Climateregions_100505.pdf

2. Caltrans Policy Directive letter dated Nov 21, 2006. Attachment 2 “Selection of Asphalt Binder Grade”