Measuring Rheological Properties of Asphalts

“Belt tightening” seems to be the buzzword of the new millennium for road builders all across America these days. More often than not, each and every design Engineer is forced to make the most out of every paving dollar in both new construction and rehabilitation. Justifications for funding seem to be less tied to need and more to the overall cost than ever before. These limits on funding can create difficulties throughout the business cycle effecting margins and profitability as well as the balance for meeting on target quality control goals. Typically, there is little room for error in any stage of the process. In some cases, careers can be made or lost on the outcome of a single, visible, paving project.

Asphalt is the solution. With more than 90% of roadways in the Unites States being built with asphalt materials today, improved performance can make the most of the investment. While the use of cost effective asphalts are typical, field performance for similar grades may not be. Well engineered asphalts may have a greater upfront cost, with savings not always initially obvious. These savings are realized by both the Contractor and Owner Agency. Pavements can be built with improved performances which can cost less to produce, as well as, maintain over time. Decreased variability in quality control makes for more efficient construction and, in many cases, even pay bonus to the Contractor. Longer times between maintenance and replacement are direct savings for the owner agency as well.

Measuring rheological properties was introduced in large scale to the paving industry by the U.S. Government funded Strategic Highway Research Program (SHRP) in 1993. SHRP brought new rheology focused tools and procedures to the market to improve performance and commerce with Superpave™ Performance Graded Binders. Since that time, more was gained than just the improved performance grade selection process and the clear cut buy/sell criteria that the SHRP program implemented. Over time, these new tools have broadened the industry’s understanding and use of rheological properties leading to better characterization and identification of improved asphalt binder performance qualities.

So how do we know which asphalt to use? Determining the grade is a relatively straight forward process when following the Superpave™ guidelines. However, many similar grades can vary widely both in purchase price and performance, which, can make it even more difficult to choose. Using price as the deciding factor alone does not always take in to consideration the value added benefits of a well engineered modified binder.

In order to improve the performance life of asphalt road systems, pavement failures such as rutting, pushing, shoving; thermal and fatigue cracking must be eliminated or minimized by design. Knowing the rheological properties, or viscoelastic behavior, of the binder are the keys to understanding and predicting pavement performance over a wide range of climatic, environmental, traffic and loading conditions. Range and degree of stiffness, elasticity, and relaxation times tell a great deal of the binder performance when measured over the working temperatures. The measurements of these rheological properties are also the most accurate method of comparing competitive products, formulation variation, and even effects of additive interactions.

The Bohlin line of asphalt testing systems are widely used by most of the major refiners, producers, additive suppliers, contractors along with state and federal departments of transportation for quickly and easily measuring a variety of rheological properties including:

• PG Grade determination to Superpave™ specifications for QA/QC acceptance.
• True grade analysis for greater process quality control.
• Master Curve generation for complete product ‘finger printing’. Provides insight to formulation, product interaction, bending and optimization.
• Creep & recoverable compliance and zero shear viscosity for mix & compaction temperature determination.
• Accumulated strain analysis for rutting behavior on either binder or mix samples
• Temperature dependence for crude selection and processing conditions.
• Binder and mix sample fatigue behavior.
• Viscosity profiles for performance prediction of spray application of tack coats & emulsions.
• Yield stress determination for emulsion coating behavior to reduce sag, optimize film thickness and improve leveling.
• Binder strength, relaxation time, retardation time, thermal stress accumulation, coefficient of binder thermal expansion/contraction.
• The use of rheological engineering methods to replace the old empirical tests such as, elastic recovery, forced ductility, and toughness & tenacity.

Bohlin rheometers are the most relied upon instruments for binder assay in the world and are widely used to characterize the rheology of asphalt binders.

Available products:

	Bohlin DSRII Asphalt Rheometer Industry standard dynamic shear rheometer for SHRP and AASHTO asphalt tests.
	Bohlin ADS Automated Dynamic Shear Asphalt Rheometer Industry standard for automated characterization of asphalt rheology.
	Bohlin Visco 2000 Asphalt Binder Viscometer Newest SHRP asphalt binder testing equipment specifically designed for high temperature rheology testing of asphalt binders.