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16 indicated that the prototype workability device could effec- vt = r tively differentiate the effects of mixture gradations and where binder grades. This led to development of the current work- vt = the tangential velocity; ability device by Instrotek, Inc. r = the radius; and Further research with the Instrotek workability device = the angular velocity. studied additional materials factors including three aggregate types (granite, crushed gravel, limestone); two NMAS (19 mm, The rotation speed for a popular model of continuous 12.5 mm); five gradations; and three binder types (PG 64-22, mix plant (Astec 400 ton/hr double barrel plant) is 7.68 rpm 70-22, 76-22). The research found that each mix has a range (0.8 rad/s). This plant has a drum radius of 1.69 m (5.56 ft). of workability as the mix temperature decreases. When the The tangential velocity at the point of mixing for this plant is same binder grade was used with three different aggregate 1,352 mm/s. In a popular size batch plant, the pugmill is driven types, each mix had different workability levels. This indicates at a rate of 33.6 rpm (3.52 rad/s). The length of the mixing that binder properties alone may not be the best measure of the compactability of an asphalt mixture. When compared arms from the center of the mixer shaft is 0.47 m (1.55 ft). with the equiviscous method for determining mixing and Therefore, the maximum tangential velocity during mixing in compacting temperatures, the workability tests resulted in this pugmill is 1,654 mm/s. If the nominal thickness of the compaction temperatures 9C to 28C (16F to 50F) lower asphalt coating on an aggregate particle during the initial mix- for modified asphalts and about the same temperature for ing is approximated as 10 microns (0.01 mm), then the instan- unmodified asphalt. The data were examined to determine taneous shear rate of the binder film on a particle in contact the temperatures at which mixtures with different binders with the pugmill tip can be estimated with Equation 1. had the same workability. The study also evaluated an ap- Using this approach, the pugmill will yield a maximum in- proach to define compaction temperatures for each mixture stantaneous shear rate of 165,400 1/s, and for the drum plant, based on the workability versus temperature data. The re- the maximum instantaneous shear rate is estimated to be search concluded that every mix has a unique relationship be- 135,200 1/s. These estimated shear rates for mixing represent tween temperature and workability based on aggregate type, the high end of a range of shearing that occurs during mixing. binder type, gradation, and NMAS. In reality, the mixing process in a plant or in laboratory mixers Table 3 provides a summary of methods in use or proposed produce a turbulent mass mixing action with an extremely by researchers for determining mixing and compaction tem- wide range in shear rates. peratures of hot-mix asphalt. For comparison, the shear rates for several laboratory mixers and binder test methods are shown in Table 4. These estimates also reveal how different the conditions may be in routine Shear Rates During Mixing and Compaction laboratory binder tests versus an HMA plant. If viscosity is the binder parameter used to establish mix- For estimating shear rates during compaction, it is necessary ing and compaction temperatures, the shear rate(s) used to at this point to only be concerned with laboratory compaction determine the viscosity should approximate the shear rates since that is the issue at hand. Although the conditions of lab- that occur during mixing and compaction. However, very lit- oratory compaction are much more controlled than in the field, tle information was found in the literature regarding the shear there are still numerous complications in estimating shear rates rates that exist during mixing and compaction in the labora- during compaction. tory or during plant production and construction. The max- In NCHRP Report 459, Bahia et al. (39) reasoned that the imum shear rate that exists in a rotating shaft mixer can be shear rate during compaction in the SGC was very low. This approximated using Equation 1 and substituting tangential logic was based on the observation that the change in specimen velocity of the mixing tip for the relative velocity, V: heights is very low during most of the compaction, especially after the first 10 or so gyrations. However, the one-dimensional V vertical strain rate of the mixture and the shear rate of the = (1) binder films coating the aggregate particles during compaction d are not the same thing. Since the aggregate particles are essen- tially nondeformable rigid bodies, the strain or deformation where only occurs due to manipulation of aggregate particles around = the shear rate; one another, thus shearing the asphalt films during those V = the relative velocity of the solid elements shearing the movements. One valid point is that the strain rate changes fluid; and throughout the compaction process. The early part of com- d = distance between solids. paction is where the binder consistency plays a greater role.

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17 Table 3. Summary of researched methods for determining mixing and compaction temperatures. Method Description Advantages Disadvantages Equiviscous The rotational viscometer is Simple to obtain Assumes linear Temperatures used to determine the viscosity and analyze results relationship between at 2 temperatures and 1 shear Can be completed viscosity and rate. The viscosities are in less than 1 hour temperature plotted vs. temperature and a Assumes that all temperature range asphalt binders are corresponding to 0.170.02 Newtonian liquids; Pas is chosen for mixing and a does not account for temperature range shear rate dependency corresponding to 0.280.03 Can result in Pas is chosen for compaction. unnecessarily high mixing and compacting temperature for some modified asphalt binders High Shear Uses the rotational viscometer Takes into account Requires extrapolation Rate to determine the shear rate the shear rate of results to a high Viscosity dependency of an asphalt dependency of shear rate binder at 2 temperatures modified asphalt (135C, 165C). For each binders temperature, the data is fit to Testing is simple to an inverse power curve and perform extrapolated to estimate the Does not require viscosity at a shear rate of 490 complicated 1/s. The high shear viscosities modeling are plotted versus temperature, and mixing and compaction temperature ranges are determined at target values of 0.170.02 Pas and 0.280.03 Pas, respectively. Steady Shear Uses a DSR Steady State Flow Simple to perform, Can be time Flow test at 76C, 82C, 88C, and uses standard DSR consuming for 94C. Measurements of steady equipment and modified asphalts state viscosity are made over a testing procedures Not all modified range of 0.16 to 500 Pa stress. asphalts reach a state The viscosity values at 500 Pa of steady shear by 500 are plotted versus temperature Pa and mixing and compaction Requires extrapolation temperature ranges are of viscosity to much determined at target values of higher temperatures 0.170.02 Pas and 0.350.03 Pas, respectively. Zero (Low) Uses the rotational viscometer Takes into account May not accurately Shear to determine the shear rate the shear rate represent the shear Viscosity dependency of an asphalt dependency of thinning behavior of binder at 3 temperatures (120, modified asphalt modified asphalt 135, 165C). The Cross- binders binders Williams model is used to fit a Testing is simple to Requires extrapolation curve to the data at each conduct of results to a low temperature from which the Results in lower shear rate viscosity at a shear rate of mixing and Cross-Williams 0.001 1/s is estimated. The compaction regression model is low shear viscosities are temperatures for complicated plotted versus temperature and mixing and compaction modified asphalt No clear agreement on binders definition of zero temperature ranges are determined at target values of shear viscosity 3.0 Pas and 6.0 Pas, Results for some respectively. binders yield unrealistically low mixing and compaction temperatures (continued on next page)

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18 Table 3. (Continued). Method Description Advantages Disadvantages Mixture Uses a large stirring device to Considers the New equipment Workability measure the torque required to effects of aggregate Time-consuming stir a mix as it cools. Torque is particle shapes and procedure inversely proportional to size on Not practical for workability. The relationship compactability of routine use between workability and mixtures temperature can be used to Aggregate help establish temperature characteristics and range where a mix is easiest to gradation may work. overwhelm binder effects Compaction A standard mix is compacted Easy to analyze Time-consuming Test with an unmodified "control" based on density procedure binder to establish a baseline and volumetric SGC is insensitive to density. The modified binder properties binder consistency is then added to the standard Only provides results mix and samples are for compaction compacted at temperature temperature intervals. The temperature that provides the same density as Results are dependent on the "standard" the control binder is the compaction temperature for the mixture. Other mixes may provide different modified binder. results. As the aggregates compact more closely together, the com- is compacted, the shear moves in the opposite direction paction resistance and mixture strain is dominated more by for a total displacement of 4.86 mm within one-half of a the aggregate texture, shapes, and gradation. In a typical gy- gyration, which occurs in 1 second. Thus, at the top of the ratory compaction record, the height change during the first specimen, there is an approximate horizontal shear rate of gyration is 2.8 mm, and by the tenth gyration the height 4.86 mm/s. change is down to about 0.4 mm. The speed of gyration for Resolving the vertical and horizontal shear movements SGC compactors is specified to be 30 gyrations per minute yields or 0.5 gyrations/sec. Multiplying the height changes by the gyration rate gives an approximate instantaneous vertical (1.4 )2 + ( 4.86)2 = 5.06mm s. velocity within the compacting mixture. For the first gyra- tion, it is 1.4 mm/s and for the tenth gyration, 0.2 mm/s. Dividing this velocity by a nominal film thickness of 10 mi- However, due to the fixed tilting angle, there is also a rota- crons yields estimated shear rate of 506 1/s for the first gyra- tional shear. Using a 2-D approximation of this 3-D prob- tion, with a slight reduction in shear rates as the compaction lem, the horizontal displacement for a 120-mm tall speci- process continues. Note that this is very similar to the shear rate men gyrated at 1.16 is 2.43 mm. Figure 7 illustrates the estimated for gyratory compaction by Yildirim et al. (43, 44) shear movement during SGC compaction. As the specimen based on experimental analyses. Table 4. Summary of shear rates for some field and lab equipment. Tangential Radius velocity shear rate Laboratory Device Model # RPM (in.) (mm/s) (1/s) Bucket mixer KOL M-60 65 5.6 961 96,100 Pugmill mixer 7590-H 128 3.9 1341 134,100 Workability device Instrotek 20 6.0 319 31,919 Bowl mixer Hobart A200 48 4.0 425 42,558 Rotational Brookfield 20 8.4 mm 17.5 6.8 viscometer DV-II+ Dynamic Shear 25 mm 125 10rad/s Rheometer 8 mm 20