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5 CHAPTER 1 Background Introduction tify shortcomings of this method, and identify potential new methods that may be evaluated or explored. A survey of state The use of modified asphalt binders in hot-mix asphalt highway agencies and materials suppliers also was conducted (HMA) has steadily increased over the past several decades. to evaluate the current practices for determining mixing and The Association of Modified Asphalt Producers (AMAP) esti- compaction in the United States and abroad. mates that modified asphalt binders currently make up ap- The literature review is organized into the following topics: proximately 20% of paving grade asphalt sales (1), and the quantity of modified binders used in HMA is increasing at an A note on the units of viscosity. annual rate of about 4%. However, a suitable method for de- Background on the development of mixing and compaction termining mixing and compaction temperatures for these temperature criteria. binders has not been established. The traditional equiviscous Effect of temperature on degradation of asphalt binders. principle used to determine mixing and compaction tempera- Mixing and compaction temperatures for modified asphalts. tures was developed using unmodified binders having New- Survey on current practices for selecting mixing and com- tonian behavior. However, using this method with many paction temperatures. modified asphalt binders often results in excessively high mix- Recent research on proposed new methods for determining ing temperatures that have caused concerns with emission mixing and compaction temperatures. problems and degradation of the binder's properties. Shear rates during mixing and compaction. The objective of this research was to identify or develop a Summary of key findings. simple, reliable, and accurate procedure for determining the mixing and compaction temperatures applicable for modified A Note on Units of Viscosity and unmodified asphalt binders in HMA. The research project consisted of six tasks: Viscosity may be expressed in several different ways, and this often leads to some confusion. One of the early methods for Task 1-Literature Search, measuring asphalt viscosity was the Saybolt Furol viscosity test, Task 2-Design of Experiments, which measured the time for a given volume of asphalt to flow Task 3-Interim Report, through an orifice of specific dimensions. Thus the results of Task 4-Conduct Experiments and Analyze Results, this test were reported as seconds Saybolt Furol (SSF). Task 5-Prepare Procedure in AASHTO Format and Field Another expression of viscosity is kinematic viscosity, which Validation Plan, and is actually the viscosity divided by the material's density. The Task 6-Final Report. most commonly used unit of kinematic viscosity is the centis- tokes, which is written as mm2/s. The original equiviscous cri- teria in AASHTO T 312 used ranges of kinematic viscosity to Literature Search define mixing and compaction temperatures. These criteria A review of available literature was conducted on the subject were 170 20 mm2/s and 280 30 mm2/s for mixing and com- of mixing and compaction temperatures for asphalt mixtures. paction, respectively. The purpose of the literature review was to gain an understand- The measurement of viscosity using a rotational viscometer ing of the historical development of the current method, iden- is often recorded in units of centipoise (cP). However, the SI