The Superpave Binder Specification uses the Bending Beam Rheometer to measure the stiffness at specified temperatures. The temperature at which the binder's stiffness exceeds 300 MPa is the limiting stiffness temperature. To address varying cooling rates, the slope of the creep curve (m) is included in the binder specification. The temperature at which the m value drops below 0.30 is a factor in determining the limiting stiffness. For most asphalt binders the m value is the controlling value for determining the limiting stiffness temperature. Determining the limiting stiffness of the binder has been made easy with the Bending Beam Rheometer. The problem arises in determining the expected critical cracking temperature of the pavement.
Several algorithms have been developed to convert lowest air temperature to lowest pavement temperature or critical cracking temperature. However, the vast majority of low temperature research has been conducted in harsh climates. Because the vast majority of work has been done in harsh climates the reliability of these algorithms is greatly diminished for warmer climates.
For northern climates the effect of using the air temperature for determining the pavement critical cracking temperature can produce differences of up to 15°C. The difference of 15°C between the air and pavement temperature can result in specifying binders up to 3 grades more restrictive than required. For extremely cold climates these differences can have tremendous economic effects. Superpave currently recommends binders with limiting stiffness temperatures of -34, -40, and -46°C for extreme northern climates. For unmodified binders typically used in dense graded applications (i.e. AC-10 or AC-20) -28°C is typically the lowest limiting stiffness temperature attainable. To attain temperatures lower than -28°C will generally require modification. Typically a very soft base asphalt, an AC-2.5 or AC-5, is used to achieve low temperature properties and a modifier is added to meet the required high temperature properties. These modifiers can add 50% to 100% to the cost of the binder. Because of these high costs it is essential that the determination of the critical cracking temperature be correct.
Until the findings from the various studies can be finalized into an accurate algorithm for converting air temperature to pavement temperature an alternative system should be used. From anecdotal data a simple system can be used for selecting low temperature binder grade. For northern cold climates, when the mean low air temperature is below -28°C, a reliability of less than 98% should be used. For example, if the mean low temperature were -29°C the 50% reliability recommendation for a low temperature binder grade would be PG XX-34. The actual reliability for the -34 grade with a mean low temperature of -29°C could very well be 75 or 80%. However, to reach 98% reliability may require a PG XX-40. This system will prevent recommendations for extremely low temperature binder grades. Low temperature requirements that exceed -40°C, i.e. -46°C, should not be considered without careful evaluation. Binders with good low temperature properties below -40°C are extremely difficult to produce and can be very expensive to use.
The purchaser will have to use discretion in selecting binder grades to perform in the wide variety of climates in North America. However, with some evaluation of existing binders and pavement performance a binder with a reasonable probability of success can be selected.
When the performance data collected from the various test sections is compiled and analyzed a new algorithm will be developed to accurately determine the critical pavement cracking temperature. The information needed to accurately predict critical temperatures should be available within 2 to 3 years. Until that time a rational alternative system can be used to avoid unnecessary expenses to acquire binders that will perform.
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