Evaluation and Quality Control of
Asphalt Mixture Additives

The development of the Superpave asphalt binder specification has made the evaluation of asphalt binder modifiers much less mystifying. The Superpave mixture analysis system, however, is not as well defined. The Superpave mixture analysis system, though not as well defined as the binder system, has identified many new tests to evaluate the performance of asphalt mixtures. These new tests have not been well validated and their ability to actually predict performance has been questioned. These tests can, however, indicate the difference in performance of asphalt mixtures and provide the information needed to determine if a modifier has improved the performance of a mix. The Superpave mix analysis tests are new and the output from these tests are not familiar to most highway engineers. Before these tests will be used on a standard basis a detailed plan establishing a program on how to use them is needed. Once a plan has been established, the evaluation of mixture modifiers to determine if they will improve performance can become standard practice.

This paper will establish a program highway agencies may elect to use for the evaluation of mixtures to demonstrate their properties against a standard for various failure mechanisms.

Each agency should have several standard mixes with known documented performance. These mixes will be used as a basis for the evaluation of modified mixes.

Mixes should be evaluated for the specific performance criteria the modifier is intended to improve: such as permanent deformation, fatigue, and low temperature cracking. Evaluation of all criteria should be included to assure fixing one problem does not create another.

Quality control is an integral part of the system. Quality control will be established based on the family of modifiers used and production processes for introduction into the mix.

A listing of performance criteria and tests include:

• Performance Tests High Temperature:
  • Superpave Shear Tester-
  • Tri-axial testing
• Performance Tests Low Temperature:
  • Superpave Indirect Tensile Tester
• Performance Tests Intermediate Temperature:
  • Beam Fatigue
  • Superpave Shear Tester- Frequency Sweep

Performance Evaluation:

Low temperature evaluation should be performed using the Indirect Tensile Test. The indirect tensile creep and the indirect tensile strength tests at three temperatures are used to characterize the low temperature properties of the modified mix. This testing should be performed on mixes produced using the agency=s standard gradation.

High temperature evaluation should be performed using the Superpave Shear Tester or a tri-axial tester. The repeated shear at constant height or dynamic creep performed at Tcritical for permanent deformation is used to evaluate the additives ability to improve the mixture resistance to permanent deformation. The simple shear, repeated shear at constant height, and the frequency sweep are the actual tests used to evaluate performance. This testing is performed on mixes produced using the agency=s standard gradation.

Intermediate temperature evaluation should be performed using the Superpave Shear Tester or tri-axial tester. The frequency sweep performed at Tcritical for fatigue is used to evaluate the additives ability to improve the mixtures resistance to fatigue cracking. This testing is performed on mixes produced using the agency=s standard gradation. The beam fatigue test can also be used to evaluate fatigue response of mixtures with modifiers. This testing is also done at Tcritical for fatigue. The SHRP procedure should be used for beam fatigue.

Quality Control Testing:

The best of designs will not perform if the production process is out of control and construction practices inadequate. To assure that a well designed mix will perform a detailed quality control program is required.

The contractor is ultimately responsible for the quality of the products produced. The contractor shall develop and submit to the agency a detail quality control plan. As a minimum the contractors quality control plan should address the following:

1. Shipping, storage and handling of materials to be used in the production process.
2. All material feed systems should be attached to continuous read totalizers monitored and controlled by computer, to assure proper addition of any mixture modifiers.
3. Quality control testing of the plant produced mixture should be part of the plan such as volumetric properties of gyratory compacted plant mix.
4. Quality control testing of the manufactured product for performance criteria should be performed, were applicable, to assure the desired properties are being achieved during production.

Note: Highway agencies are currently evaluating many varieties of loaded wheel testers. These devices have been used in the highway industry for many years with varying amounts of success. The devices use empirical evaluations of some measured response to loaded wheels as an indicator of performance. Minor variations in test parameters can cause widely variable test results. Close evaluation of actual performance on the roadway to performance in the wheel testers in required to gain confidence in the results of the tester. Where mixes have been placed on the roadway and detailed comparisons made to actual performance, loaded wheel testers can be useful for mixture evaluation and quality control testing of plant produced mix. For these reasons loaded wheel testers may note be useful in evaluating new materials or mix types.