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WISCONSIN STRUCTURES & MATERIALS TESTING LABORATORY (WSMTL)

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Critical Evaluation of Using the SUPERPAVE Volumetric Mixture Design Procedure for Modified Binders
Research by
Dong-Woo Cho/ Prof. Hussain Bahia
Dept of Civil & Environmental Engineering

Asphalt concrete is the composite material composed of aggregate and asphalt binder to pave driving ways. It also can be categorized as flexible pavement due to asphalt binder. Most roads have been paved with asphalt concrete because asphalt concrete provides smooth driving, easy construction and maintenance, and 100% recycling.

There are three different distresses to give detrimental effect to asphalt concrete:

1) Rutting is its permanent deformation due to heavy traffic.
2) Fatigue cracking is based on accumulated repeated traffic.
3) Thermal cracking includes low-temperature cracking and thermal fatigue cracking.

Crystallization by low temperature causes fracture failure and it is called thermal cracking. Thermal fatigue cracking based on asphalt aging or hardening is very similar with fatigue cracking. These distresses are the indicators to evaluate the performance of asphalt binder or asphalt concrete and are all based on temperature change. Because conventional asphalt binder is very susceptible to temperature change, the research for modified binder has been studied and the approach to evaluate the modified binder performance has been developed.

This study was conducted to examine the possible interference of modified binders with the standard procedure for volumetric mixture design used in the Superpave system. Sensitivity of volumetric properties and moisture damage performance was evaluated using four binders modified with two different technologies (Polymer and no-additive modifications).

The study included three tasks to evaluate effects of compaction temperatures (in the range of 72C to 148C), effects of vertical pressure (in the range of 200 kPa to 600kPa), and moisture damage according to the AASHTO T283 procedure (is a testing procedure for moisture damage test).

The Zero Shear Viscosity concepts were used to estimate mixing and compaction temperatures, compaction energy index was used to study changes in compaction effort, and cohesion and adhesion of binders were measured. The results indicate that temperature effects, within a reasonable range, are somewhat marginal and that unless temperatures are reduced to below 80 C, the effects on volumetric properties are rather small. It was also found that using a target viscosity of 1.5 Pa•s for mixing and 3.0 Pa•s for compaction can provide very reasonable compaction temperatures, particularly if Zero Shear Viscosity is used. These criteria for temperatures worked well for the grades and type of modification used in the study.

Photo:  equipment to fabricate the asphaltThe vertical stress used in the gyratory compactor showed very significant effects on volumetric properties. Changing pressure from 600 kPa to 300 kPa resulted in 3.0 to 4.0 % increase in air voids, which is more significant than changing viscosity by more than 10 times. It is therefore postulated that the focus on keeping temperature high during compaction in the lab and in the field could be un-founded. With increasing pressure during compaction in the lab or weight of roller in the field, significant changes in density could be achieved. Moisture damage does not appear to be affected significantly by the variation in binder viscosity profiles for the binders and aggregates used in this study. Because of the limited sample size, more work is needed in this area to study effect of mixing and compaction
temperatures on moisture damage.

Key:
Pa = Pascal = N/m^2 = Newton per square meter (a unit of pressure or stress)
Pa * s = Pascal times second (a unit of viscosity)
kPa = kilo Pascal = a thousand times of Pascal (a unit of pressure or stress)

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Wisconsin Structures & Materials Testing Laboratory
University of Wisconsin-Madison
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Madison, WI 53706-1691

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E-mail: wsmtl@engr.wisc.edu

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