Performance Grade Bitumen Description
The performance grade bitumen is based on the evaluation of the material performance when in use, unlike being rational as in the viscosity grading system. The viscosity grading system is more into the experience-based method of grading. And this has proved to have excellent performance for over 20 years in US pavement construction.
The Superpave grading was developed as a part of a 5year strategic highway research planning (SHRP) from 1987 to 1992, to have a performance-based grading system for bitumen. These were developed based on the engineering features that will help in solving many of the engineering problems.
Penetration grading and viscosity grading are somewhat limited in their ability to fully characterize asphalt binders for use in HMA pavement. Therefore, as part of the Superpave research effort, new binder tests and specifications were developed to more accurately and fully characterize asphalt binders for use in HMA pavements. These tests and specifications are specifically designed to address HMA pavement performance parameters such as rutting, fatigue cracking, and thermal cracking.
Superpave Performance Grade Bitumen is based on the idea that an HMA asphalt binder’s properties should be related to the conditions under which it is used. For asphalt binders, this involves expected climatic conditions as well as aging considerations. Therefore, the PG system uses a common battery of tests (as the older penetration and viscosity grading systems do) but specifies that a particular asphalt binder must pass these tests at specific temperatures that are dependant upon the specific climatic conditions in the area of use. Therefore, a binder used in the Sonoran Desert of California/Arizona/Mexico would have different properties than one used in the Alaskan tundra. This concept is not new – the selection of penetration or viscosity graded asphalt binders follows the same logic – but the relationships between asphalt binder properties and conditions of use are more complete and more precise with the Superpave PG system. Information on how to select a PG asphalt binder for a specific condition is contained in the Superpave mix design method. Table 1 shows how the Superpave PG system addresses specific penetration, AC, and AR grading system general limitations.
Performance Grade (PG) bitumen is bitumen which is graded based on its performance at different temperatures. The Long-Term Pavement Performance(LTPP) has given certain algorithm to calculate the temperature of the pavement based on the temperature of the air above. From this, the highest and the lowest temperatures of the pavement is calculated and the bitumen that performs well in that temperature range is selected.
|Limitations of Penetration, AC and AR Grading Systems||Superpave Binder Testing and Specification Features that Address Prior Limitations|
|Penetration and ductility tests are empirical and not directly related to HMA pavement performance.||The physical properties measured are directly related to field performance by engineering principles.|
|Tests are conducted at one standard temperature without regard to the climate in which the asphalt binder will be used.||Test criteria remain constant, however, the temperature at which the criteria must be met changes in consideration of the binder grade selected for the prevalent climatic conditions.|
|The range of pavement temperatures at any one site is not adequately covered. For example, there is no test method for asphalt binder stiffness at low temperatures to control thermal cracking.||The entire range of pavement temperatures experienced at a particular site is covered.|
|Test methods only consider short-term asphalt binder aging (thin film oven test) although long-term aging is a significant factor in fatigue cracking and low temperature cracking.||Three critical binder ages are simulated and tested:Original asphalt binder prior to mixingwith aggregate.Aged asphalt binder after HMAproduction and construction.Long-term aged binder.|
|Asphalt binders can have significantly different characteristics within the same grading category.||Grading is more precise and there is less overlap between grades.|
|Modified asphalt binders are not suited for these grading systems.||Tests and specifications are intended for asphalt “binders” to include both modified and unmodified asphalt cements.|
How to read a Performance 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 7-day maximum pavement temperature is used for describing the high-temperature climate. On the low-temperature side, thermal cracking can happen during the one really cold night; therefore the minimum pavement temperature is used for describing the low-temperature climate. For both high and low-temperature grades, PG grades are graded in a 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 is 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 an 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 temperatures are concurrently required in the same application, the polymer modification is generally required.
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 Performance Grade bitumen 76-22 is likely to be a polymer modified binder since the sum is 98, while a Performance Grade Bitumen 64-10 is likely to be unmodified since the sum is 74.
PG Bitumen – Unmodified CHARACTERISTICS
|PG 46-34||PG 46-28||PG 52-28||PG 58-28||PG 58-22||PG 64-22||AASHTO Method|
|Flash Point, COC,°C||230||T 48|
|Flash Point, P-M, °C||NS||≥ 204||ASTM D93|
|Rotational Viscosity @ 135°C, Pa·s||3.00||T 316|
|Dynamic Shear @ Grade Temperature,°C||46||46||52||58||58||64||T 315|
|G*/sin @ 10 rad/sec, kPa||≥ 1.00|
|Mass Loss, %||≤ 1.00||T 240|
|Dynamic Shear @ Grade Temperature,°C||46||46||52||58||58||64||T 315|
|G*/sin @ 10 rad/sec, kPa||2.20|
|PAV Residue (Aging Temperature, °C)||90||100||R 28|
|Dynamic Shear @ Grade Temperature,°C||10||13||16||19||22||25||T 315|
|G*/sin @ 10 rad/sec, kPa||5000|
|S, Mpa||≤ 300||T 313|
|m-value||≥ 0.30||T 313|