By Hongbin Xie, Graniterock RTS

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 is still commonly used worldwide, new binder tests and specifications are 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 is 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 on Aged Residue (AR) System. Caltrans started to use the PG system since January 1, 2006, and will start to use the PG system for polymer modified binder on January 1, 2007.

What is PG system?
Performance grading is based on the idea that asphalt binder properties should be related to the conditions under which it 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 typical set of binder testing for PG system is shown in Figure 1. 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 level 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 lay down etc. The Pressure Aging Vessel (PAV) aging is used to simulate the long-term aging of the asphalt binder after the AC pavement in service about 10 years.

How to read a PG grade?
The PG grading system is based on climate, so the grade notation consists 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 7 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 increment. The average 7 day maximum pavement temperature typically ranged from 46 to 82°C, and minimum pavement temperature typically ranged from −46°C to −10°C.

A binder identified as PG 64-10 must meet performance criteria at an average 7 day maximum pavement temperature of 64°C and also at a minimum pavement temperature of −10°C. Please note that maximum pavement temperature is typically higher than the air temperature by about 20°C since the dark color pavement absorbs the heat and retains it. The maximum pavement temperature is typically measured at about 1 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%, so that means when the PG 64-10 binder is selected, the asphalt binder in the AC pavement should perform satisfactorily under normal traffic condition at the location where the extreme pavement temperature are within the range of −10°C and 64°C throughout its service life with a minimum 98% confidence level. Where the traffic condition is not typical, such as the really heavy traffic like interstate highway, or slow traffic such as bus stop or intersection area, one or two grades stiffer asphalt binder 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, it is likely to be a polymer modified binder. 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
Note¹

The Caltrans climate regions used for selecting asphalt binder grade is shown in Figure 2 and listed in Table 1.

Asphalt Binder Grade

Binder	Conventional Asphalt					Rubberized Asphalt
	Dense Graded			Open Graded		Gap Graded	Open Graded
Climatic Region	PG	PG	PG Polymer Modified	PG	PG Polymer Modified2	PG	PG
South Coast Central Coast Inland Valleys	64-10	70-10	64-28PM	64-10	58-34PM	64-16	64-16
North Coast	64-16	—	64-28PM	64-16	58-34PM	64-16	-64-16
Low Mountain South Mountain	64-16	—	64-28PM	64-16	58-34PM	64-16	64-16
High Mountain High Desert	64-28	—	58-34PM	64-28	58-34PM	58-22	58-22
Desert	70-10	—	64-28PM	70-10	58-34PM	64-16	64-16

¹ For asphalt concrete dikes use PG 70-10. For tack coats use either 64-10 or 64-16.

² For low temperature placement

At DME discretion, PG 76-22PM may be specified for dense graded asphalt concrete for special location identified by DME.

For conventional Dense Graded Asphalt concrete, values are given 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, ramps to and from Truck Scales etc.)

Literatures

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