David E. Newcomb

CONSIDERATION OF HOT MIX ASPHALT THERMAL PROPERTIES DURING COMPACTION

A computer program was developed at the University of Minnesota to predict asphalt concrete cooling times for road construction during adverse weather conditions. Cooling models require extensive experimental data on the thermal properties of hot-mix paving materials. A sensitivity analysis was performed to determine which thermal properties affect pavement cooling times significantly. The results indicated that more information on asphalt thermal conductivity and thermal diffusivity is required. Two suitable test methods for determining these properties at typical paving temperatures and densities were developed, and preliminary results for dense-graded and stone-matrix asphalt (SMA) mixes agreed well with values reported in the literature.

Interpretation of Indirect Tension Test Based on Viscoelasticity

The diametral indirect tension test is a convenient configuration for determining the modulus of asphalt concrete samples. The resilient modulus test has been a traditional approach to characterizing the stiffness of asphalt concrete, but it leaves much to be desired when considering the viscous behavior this material exhibits, even at low temperatures. A method for determining the complex compliance, complex modulus, and phase angle of asphalt mixtures using the indirect tensile test and a haversine load history is presented here. This test may be performed over a range of frequencies and temperatures as demonstrated on materials used in the Minnesota Road Research Project. The use of the haversine loading simplifies the test when compared with the pulse loading and rest time used in the resilient modulus test, and it allows for the characterization of the elastic and viscous components of the materials overall behavior, which is very difficult, at best, with the current test methods.

Babbitt, Minnesota: Case Study of Pretreated Crumb Rubber Modified Asphalt Concrete

A 2.38-mm (No. 10) mesh crumb rubber from waste passenger tires was pretreated with a low-viscosity petroleum-based product and used as an aggregate replacement in asphalt concrete mixtures. The preconstruction testing consisted of the development of an appropriate aggregate gradation, mix design information, and fundamental mixture properties such as temperature susceptibility, moisture sensitivity, low temperature behavior, and permanent deformation characteristics. Five crumb rubber modified (CRM) asphalt concrete plus two control test sections were placed in Babbitt, Minnesota, in the fall of 1993. Variables in the CRM sections include pretreated and untreated crumb rubber and CRM mixtures in just the wear course or throughout the 150-mm (6-in.) pavement section. Laboratory results indicated that there was little difference in the temperature susceptibility between the mix design and behind-the-paver mixtures. Although the mix design samples indicated some increase in the moisture sensitivity of the C...

THE USE OF FALLING WEIGHT DEFLECTOMETER DATA IN MONITORING FLEXIBLE PAVEMENT SYSTEMS

Pavement performance studies are being conducted in the states of Nevada and Washington for the purpose of developing mechanistically based pavement design procedures for highways. A critical element of each of these studies is the statewide characterization of the pavement materials. The materials characterization process has included laboratory and field testing for 16 sites in Washington and 27 sites in Nevada. This paper presents the general approach used by these states in the development of mechanistic design methods and how resilient moduli inferred from falling weight deflectometer (FWD) data have been used in the process. The FWD has been used to monitor the variability of roadway materials and to ascertain the seasonal changes in material properties. Comparisons of laboratory-determined and field-estimated resilient moduli are discussed. It was found that there is generally excellent agreement between the laboratory and field results on subgrade modulus. The accuracy of granular base course modulus estimates primarily depends upon the asphalt concrete modulus values can differ significantly from laboratory values. However, if care is taken to account for the effects of temperature and rate of loading, the agreement for uncracked asphalt concrete can be good. One of the important assets of the FWD is that it allows the evaluation of in-place materials variations. Thus, it is possible to realistically sample and test materials for design purposes. This, in turn, means that material properties can be treated more realistically than they have been in the past. In the development of a mechanistically based pavement design procedure, it is necessary to account for seasonal variations in material properties. The FWD can be used for the purpose of monitoring how materials change with the environment. This information can be used to develop algorithms which describe these changes. Washington State has accumulated over two years of data regarding seasonal changes, and Nevada has just begun to gather these data. This paper presents the approaches used by these two states in the development of pavement design procedures. It is hoped that the information contained herein will be of use to others considering similar efforts.