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9 temperature. Whilst the mixing temperature must be sufficiently because of "the relatively high shear forces seen at the manu- high to allow rapid distribution of the bitumen on the aggregate, facturing level compared to the lower shear forces used in lab- the use of the minimum mixing time at the lowest temperature oratory viscosity measurements." Shuler et al. (35) showed possible should be advocated. The higher the mixing tempera- ture the greater will be the oxidation of the bitumen exposed in that viscosities increased with higher polymer contents for thin films on the aggregate surface. . . . There are, therefore, upper Kraton and Styrelf modified binders. Based on the viscos- and lower limits to mixing temperature. . . . These different con- ity tests, significantly higher mixing and compaction temper- siderations combine to give an optimal bitumen viscosity of atures would be required. However, experience with these 0.2 Pa s (2 poise) at mixing temperatures. . . .When materials are binders on previous construction projects indicated that the being laid at low ambient temperatures, or if haulage over long distances is necessary, mixing temperatures are often increased extremely high mixing and compaction temperatures were to offset these two factors. However, increasing the mixing temper- not necessary. For environmental reasons, they considered ature will considerably accelerate the rate of bitumen oxidation the upper limit for field mixing to be 160C (320F). New- which will increase the viscosity of the bitumen. Thus a significant comb et al. (11) conducted laboratory and field studies of proportion of the reduction in viscosity achieved by increasing mixtures containing polyolefin and styrene-butadiene latex. the mixing temperature will be lost because of additional oxida- In the field, normal construction procedures were used, and tion of the bitumen. . . . Once the mat has been spread it must still be sufficiently workable to enable the material to be satisfacto- they noted that the only modified mixtures that created any rily compacted with the available 30 Pa s (300 poise). At viscosi- handling difficulty were mixtures with 3% latex that tended ties lower than 5 Pa s the material will probably be too mobile to cause sticking problems in the trucks and screeds. Other- to compact and at viscosities greater than 30 Pa s the material wise, modified mixtures behaved similar to conventional will be too stiff to allow any further compaction. unmodified mixtures. Shenoy (36) studied the temperature-viscosity relationships of two polymer modified asphalts, Styrelf and Novophalt. Mixing and Compaction Temperatures Both of these binders have been studied extensively at the for Modified Asphalt Binders FHWA Turner Fairbank Research Center's Accelerated Load- ing Facility and laboratories. The Novophalt binder used in The use of polymer-modified asphalt binders has become this study was an AC-10 modified with about 6.5% low-density much more common over the past two decades. Many types of polyethylene. The Styrelf binder was an AC-20, which was polymers have been used in paving asphalts to enhance the per- air blown to AC-40 and then modified with styrene-butadiene. formance of asphalt pavements in a wide range of climates and Sulfur was also added as a cross-linking agent. Testing utilized loading conditions. The AMAP now estimates that modified a Brookfield viscometer with three spindles to generate a wide asphalt binders make up about 20% of paving grade asphalt range of shear rates. Arrhenius plots for the binders showed sales in the United States (1). that the viscosity-temperature relationships are influenced by Most modified binders require higher temperatures for mix- the melting points of the polymers. For the Novophalt, the ing and compaction in the field and the laboratory to achieve polyethylene has a melting point around 125C (257F). For the same workability as mixes with unmodified binders. In the Styrelf, the melting point of the polystyrene occurs in the "Using Additives and Modifiers in Hot Mix Asphalt," Terrel range of 115C to 150C (239 to 302F) and the polybutadi- and Epps (34) include construction guidelines for a number of ene melts between 150C and 163C (302F and 325F). Ev- specific modifiers. The guidelines regarding temperatures for idence of polymer degradation also was presented. Aging of mixing and placing vary widely for the several specific HMA the binders and breakdown of the polymers occurred in the polymers. For example, mixing information for Butonal NS same temperature range. Aging tended to cause the viscosity 175, a styrene/butadiene latex, states that "the temperature of to increase whereas polymer degradation caused viscosity to the aggregate, when introduced into the mixture should not ex- decrease. ceed 182C (360F), and the temperature of the mixture when In a field trial of various modified binders and one unmodi- discharged from the hauling unit shall be 149C (300F) mini- fied control binder, Albritton et al. (37) used a rotational mum." Another polymer required much higher temperatures. viscometer to determine viscosity of the different modified Rosphalt 50, a virgin polymeric additive, primarily used as an binders. At the lower temperature of 135C (275F), the vis- additive to HMA for bridge deck sealing, recommended the cosities of the binders range from a high viscosity value of discharge temperature be at least 199C (390F). 2.60 Pa s for the multigrade binder to a lower value of For Kraton, a block copolymer, mix preparation instruc- 0.50 Pa s for the unmodified binder control section. At the tions simply state that "it may be necessary to adjust the mixing higher temperature of 190C (374F), the viscosities of the and compaction temperatures when conducting laboratory modified binders grouped closer together, ranging from a high work." For plant operations, asphalt mixtures containing value of 0.40 Pa s for the Novophalt modified binder to a Kraton should be as workable as a typical asphalt mixture lower value less than 0.10 Pa s for the control and multigrade