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15 Extensional Viscosity and the distribution of the air voids within specimens. The vertical gradient of the air voids was higher than the lateral Sudduth et al. (46) evaluated eight polymer additives used gradient in the gyratory-compacted specimens. The vertical to modify an AC-20 asphalt binder using a technique referred air void gradients (top to bottom) in the SGC specimens were to as "Pseudo Extensional Viscosity, ext." ext is defined as the lowest for samples compacted at 139C. Lateral air void gra- difference in the viscosity measured using the Rotational Vis- dients (side to side) were of a much smaller magnitude but cometer for the smaller 27 spindle and the viscosity measured were lowest at the highest compaction temperature of 179C. using the larger 21 spindle. The asphalts were evaluated at Aggregate orientation, measured as vector magnitude, peaked strain rates of 1 1/sec and 100 1/sec. It was concluded that a at 159C. A stepwise regression was performed to relate the positive value for ext was most desirable for roadway paving, physical properties (air voids, air void distribution, aggregate and a negative value would be undesirable as it would indi- orientation, and binder shear stiffness) to the mix shear prop- cate a softer asphalt pavement. erties sin/G, Gsin, and permanent shear strain. Binder stiffness and the distribution of the air voids were important Equivalent Mixture Properties factors that affected the mechanical properties. All shear prop- erties except Gsin improved with increasing compaction Stuart (47) used an SGC to compact three mixtures with temperature. For this binder, they recommended an optimum an unmodified binder over a range of compaction temper- compaction temperature in the range of 139C to 159C atures, then substituted the unmodified binder with two dif- (282 to 318F). ferent modified binders (Novophalt and Styrelf) at the same asphalt contents and repeated the SGC compaction tests. The temperature range that yielded the same volumet- Workability ric properties as the unmodified binder was determined for One of the earliest attempts to evaluate workability of each modified binder. Rheological properties of the binders HMA was performed in 1979. Marvillet and Bougalt (48) and mastics were measured to determine which property developed a prototype stirring device for loose mixtures to provided the same temperature ranges given by the com- evaluate the factors that affect workability. They defined work- paction process. The results of the compaction tests indi- ability as the inverse of the torque required to rotate the stir- cated that a compaction temperature of 145C (293F) could ring blade through a sample of mixture. The prototype device be used for each of the binders and achieve the same air void was used to test various combinations of materials and it was content. The allowable compaction temperature range was found that found to be 20C to 40C (36F to 72F), which indicates that compaction was insensitive to temperature. The tempera- Workability increased as viscosity decreased. ture at which smoking was observed for each of the binders Workability was unaffected by changes in asphalt content. during mixing or short-term oven aging was used to set the Workability was reduced as the dust content (percent pass- maximum compaction temperature. Stuart concluded that ing the No. 200 sieve) was increased. a single binder viscosity range could not be used to select Mixes with angular aggregate particles were less workable the laboratory compaction range for all binders. He also than mixes with rounded particles. commented that the viscosity range of 1.4 0.10 Pa s, as recommended by the NCHRP Project 9-10 study, was too Their work showed that an increase in workability resulted low for the unmodified binder. in a corresponding increase in compactability. However, they Azari et al. (13) conducted a follow up study to the work cautioned that just because two different mixes have the same by Stuart. They tested mechanical properties and physical workability does not necessarily mean the mixes will have the characteristics of the limestone-Novophalt mixture from same compactability. It was pointed out in the discussion of Stuart's study compacted at four different compaction tem- the paper that two mixes can start out with the same den- peratures. Mixture specimens were fabricated by mixing sity and reach the same final density but have different com- at 145C (293F) and then short-term oven-aged and com- paction slopes. The steeper the compaction slope, which was pacted at the following temperatures: 119C, 139C, 159C, referred to as coefficient of compactability, the easier the and 179C (246F, 282F, 318F, and 354F). The samples material would be to compact. were analyzed using computer-aided tomography to study Gudimettla et al. (49, 50) developed another prototype the distribution of air voids and aggregate orientation. The workability device similar to the French workability machine. shear testing conducted was repeated load at constant height The first study evaluated several operational parameters such and frequency sweep at constant height at 25 and 50C. The as paddle configuration and speed of rotation and their effect compaction temperature affected the total air void content on torque measurements with a few mix factors. This work