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52 show appreciable humps above the maximum density line Some fines make HMA mixtures susceptible to at about the 0.6-mm sieve produced higher VMA and lower moisture-induced damage. Water sensitivity of one Marshall stabilities than do gradations that plot as a more source of slag baghouse fines has been reported in the dense gradation. United States, and the water sensitivity of other stone The recently published Transportation Research Circular dusts has been reported in Germany. Stripping of HMA E-C043: Significance of Restricted Zone in Superpave Aggre- mixtures as related to the properties of fillerasphalt gate Gradation Specification (130) reviewed results from combinations has been reported in Japan. research relevant to the performance of mixtures with grada- tions passing above the restricted zone, below the restricted In NCHRP Project 4-19, "Aggregate Tests Related to Per- zone, cross the restricted zone or S-curve, and through the formance of Asphalt Concrete in Pavements," Kandhal and restricted zone. Independent results from the literature indicate Parker (2) conducted dynamic sheer rheometer (DSR) tests that no relationship exists between the Superpave restricted on filler-asphalt mortars to determine the rutting and fatigue zone and HMA rutting or fatigue performance. Mixes meet- properties. Fines passing a No. 200 sieve (P200 material) ing Superpave and FAA requirements with gradations that obtained from six different mineral aggregates were included violated the restricted zone performed similarly to or better in the study. The P200 materials were characterized by Rig- than the mixes having gradations passing outside the restricted den voids, particle size analysis, methylene blue test, and a zone. Results from numerous studies (3, 127, 131140) show German filler test. HMA specimens containing different that the restricted zone is redundant in all conditions when all P200 materials were tested in the Superpave shear test device other relevant Superpave volumetric mix and FAA require- for rutting and fatigue cracking. AASHTO T283 (modified ments are satisfied. Based on this research, the restricted zone Lottman test) was used to evaluate moisture susceptibility. is no longer included in AASHTO M323 (Superpave Method). It was found that the D60 size (the particle size that 60% would be passing or smaller than) and methylene blue values were related to rutting, whereas the D10 size (the particle size 2.9 EFFECT OF AGGREGATE FINES AND that 10% would be passing or smaller than) and methylene FILLERS ON HMA PERFORMANCE blue values were related to stripping. No performance-related test was identified for fatigue cracking. Mineral fillers were originally added to dense-graded HMA Anderson and Goetz (142) evaluated the stiffening effect paving mixtures to fill the voids in the aggregate skeleton and of a series of one-sized fillers in one of the first studies that to reduce the voids in the mixture. When asphalt binder is focused on determining the mechanical properties of asphalt mixed with aggregate, the fines mix with the asphalt binder filler mixtures. They concluded that both the size of the filler to form a fines-asphalt mortar. The additions of fines to the and asphalt binder composition had a significant influence on asphalt binder can have three main effects: extend the asphalt the stiffening effect. Rigden (143) conducted experiments to binder, or stiffen the asphalt binder, or both. This modifica- study the relationship between filler properties and the vis- tion to the binder that may take place because of the addition cosity of mineral fillerasphalt cement mixtures. As much as of fines could, in turn, affect HMA properties. a 1,000-fold increase in viscosity of neat asphalt cement was measured when certain fillers were added to the cement in 2.9.1 Research on Fines and Fillers ratios similar to ratios used in a typical HMA. Rigden showed a strong correlation between the voids content of dry com- Extensive research efforts on mineral filler and baghouse pacted filler and the amount of stiffening produced by the fines have been made by many researchers throughout the filler. world. Kandhal and Parker (2) and Kandhal (141) summa- In 1992, Anderson et al. (144) conducted a study to deter- rized the influences mineral filler can have on the perfor- mine whether the addition of baghouse fines affects the fail- mance of HMA mixtures as follows: ure or fracture properties of HMA mixtures. No conclusions with respect to fatigue were drawn because of the problems Depending on the particle size, fines can act as a filler encountered with the test procedures. With respect to frac- or an extender of asphalt cement binder. In the later ture properties, it was concluded that the mineral filler frac- case, an over-rich HMA mix can lead to flushing and tion could have a significant effect and that rutting. In many cases, the amount of asphalt cement used must be reduced to prevent a loss of stability or Gradation does not necessarily relate to stiffening--the pavement bleeding. finest dust acted in much the same manner as the coarser Some fines have a considerable effect on the asphalt dust--and cement, making it act as a much stiffer grade of asphalt Fracture toughness, J1c, appeared to be sensitive to the cement compared with the neat asphalt cement grade source of the aggregate as well as to the amount of and, thus, affecting the HMA pavement performance added baghouse dust. In general, the addition of the dust including its fracture behavior. increased the fracture toughness of the HMA mixture.

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53 In another study, Anderson et al. (145) stated that the Their results indicated that filler can promote the oxidation importance of mineral filler fraction was often overlooked and hardening of asphalt binders. Since it is generally even though it is one of the most important components of accepted that fatigue is related to hardening of the binder, HMA. Two mineral fillers, quartz and calcite, were added to fatigue must also be related to this phenomenon. four asphalt cements, and the rheological properties and fail- With the increased use of SMA mixtures in the United ure properties of the resulting mastics were determined States, the importance of the mortar in SMA mixtures had using the test methods developed by SHRP. DSR, flexural been recognized by many researchers. In SMA, the mortar is creep, and direct tension were found to be applicable to void- composed of fine aggregate, filler, asphalt cement, and a sta- less fillerasphalt cement mastics. Based on the study, it was bilizing additive. The mortar is an important component of found that SMA. It needs to be stiff to help prevent draindown and flushing during production and placement and to resist rut- The addition of the mineral filler does not affect the tem- ting during in-service life; it must also be flexible enough to perature shift factors of the rheological response but resist fatigue and thermal cracking. does change the frequency dependency by lengthening Brown et al. (147) conducted a comprehensive study to the relaxation times, thereby stiffening the asphalt. determine whether SMA mortar can be evaluated by the The presence of the mineral filler did not significantly Superpave system binder tests and to determine the manner affect the rate or level of oxidative or physical hardening. in which each of the mortar components affects the overall At low temperature the mineral filler imparts a leathery- mortar performance. In the study, for testing purposes, the like behavior to the mastic, enhancing the strain and fine mortar fraction was considered to be a binder and was energy-to-failure characteristics of asphalt cement. tested in the Superpave binder equipment before and after "Leathery-like behavior" is used to describe a tempera- aging. The total mortar fraction was considered to be more ture region for polymers between the glassy and rubbery like a mixture and was tested at low, intermediate, and high state and is also referred to as the glass-transition region. temperatures using the BBR, resilient modulus, indirect ten- sile test, and Brookfield viscometer. Test results indicated In this state, deformation is time dependent and not how each of the mortar components affects the mortar prop- totally recoverable. Normally, binder might be expected erties. Most of the stiffening effect comes from the mineral to have a glassy behavior at low temperature, resulting filler. It is believed that the finer the filler, the more stiffen- in a brittle failure. ing it will provide. However, in this study, the coarser bag- house fines stiffened the mortar more than a limestone dust The authors concluded that asphalt mastics can play a major did. This suggests that filler size is not the only important role in defining the performance of HMA. The data also led parameter in specifying fillers. The parent material from the authors to conclude that voidless mastics, similar in vol- which the filler comes as well as filler particle angularity may ume concentration to the mineral fillerasphalt fraction in also be important to SMA. The authors found that with minor typical HMA, can be characterized with the same test meth- modifications, the DSR and BBR appear to offer viable test ods as those developed for neat asphalt cement. methods for determining SMA mortar properties. The direct Gubler et al. (146) conducted DSR and bending beam tension test also may be applicable, but will require more rheometer (BBR) tests on a series of asphalt and binder com- modification to testing procedures. The Brookfield viscome- binations. Two asphalt binders and three fillers with varied ter does not seem to be applicable in its present form. free volumes and ageing conditions were included in their A study was conducted by Mogawer and Stuart (148) to study. The authors believed that stiffening is only one way in determine whether mastic and mixture properties can distin- which the addition of mineral filler changes the properties of guish good mineral fillers from poor ones. Eight mineral fillers asphalt binder. In fact, mastics behave quite differently than with known performance were obtained from three European does a binder that is simply stiffer binder. These differences countries. Mastics were tested for stiffness using the BBR, include changes in the material properties with aging and the DSR, and ring-and-ball softening point. The authors found time and magnitude of loading that are of practical and sci- that none of the tests distinguished among mastics with good entific interest. and poor mineral fillers. Unlike asphalt binder, mastics are susceptible to shear; Mixtures were tested for draindown of mastic using the their mechanical properties are changed by the application of NCAT draindown test, for rutting using the LCPC pavement stress during the test itself. Their mechanical properties are rut tester, for low temperature cracking using the indirect ten- also dependent on the amplitude of the applied stress and the sile test, for workability using the USACE gyratory testing time the stress is applied and are thus a function of the test- machine, and for moisture susceptibility using the ASTM ing history. An important decrease in complex modulus (up D4867 method. None of the tests distinguished among SMA to a 50% decrease) during testing at intermediate strains, fol- mixtures with good and poor mineral fillers. lowed by a partial recovery of the modulus during a subse- There was a good correlation between the free binder con- quent period with low strain, was demonstrated in the study. tent and the stiffness of the mastics measured by the BBR and

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54 the stiffening power measured using the ring-and-ball appa- Particle-interaction reinforcement: The stiffening beyond ratus. The ramification of this is unknown since the tests did volume filling and physiochemical reinforcement. This not distinguish between good and poor fillers. The following effect increases with increasing filler content, as rigid mat- three conclusions were drawn: ter comes into contact and forms a skeletal framework. Ishai and Craus (151) summarized a long-term research 1. A poor correlation existed between the stiffening power effort conducted in Israel concerning aggregate and filler measured by the DSR and the free binder content. properties that have significant influence on the behavior and 2. A poor correlation existed between the percent rut depth durability of bituminous paving mixtures. Six types of filler measured by the LCPC pavement rut tester and the free were used in the evaluation of the physicochemical properties binder content. of the fillers, the rheological characterization of filler-asphalt 3. A poor correlation existed between the tensile strengths mastics, and the strength and durability tests on sand-asphalt measured by the indirect tensile strength test and the mixtures and HMA mixtures. Parameters such as specific sur- free binder content. face, shape factor, specific rugosity, and surface texture were evaluated for each filler type. The surface activity of the Cooley et al. (149) conducted a study to evaluate the stiff- fillers, as related to interaction with bitumen, was character- ening potential of baghouse fines using conventional and ized by either adsorption intensity or selective adsorption. Superpave binder tests and to establish a reasonable upper Criteria serving locally (in Israel) as a tool for accepting and limit on the percent bulk volume of dry compacted baghouse rejecting fillers with respect to durability were suggested. They fines, as determined by the Pennsylvania State University are based on the properties of the filler, the initial properties of modified (Penn Statemodified) Rigden void test, which would the mixture, and the durability behavior of the mixture. limit the stiffening potential. This limit could then be used in Shashidhar and Romero (152) introduced two intermedi- ate measurable parameters--the maximum packing fraction, lieu of dust proportion to more accurately reflect the influ- m, and the generalized Einstein coefficient, KE--to charac- ence of baghouse fines or filler. Variables included 10 bag- terize the asphalt-filler system. This introduction enables a house fines or fillers, 2 grades of asphalt cements, and 4 dust better understanding of the influence of various factors such proportions. Tests conducted on unaged filler-asphalt mor- as average particle size, gradation, particle shape, presence tars included ring-and-ball softening point, Brookfield vis- of agglomerates, degree of dispersion, and the asphalt-filler cometer, and DSR at 64C. Mortars aged in the pressure interface on the stiffening potential of asphalt. The following aging vessel (PAV) were also tested by the DSR at 22C and conclusions were drawn from the study: the BBR at -18C. It was concluded from this study that Penn Statemodified The stiffening effect of the fillers increased with decreas- Rigden voids test can be used to characterize the stiffening ing particle sizes below 10 m. Above 10 m, such potential of baghouse fines as measured by the softening point, dependencies were not significant. the Brookfield viscometer, and the DSR. Because of the set- The asphalt-filler interface was shown to have a signifi- tling of fines during testing with the DSR, only the softening cant effect on stiffening. The interface properties changed point test and the Brookfield viscometer gave test data that from asphalt to asphalt, and the interface can be engi- had excellent correlation with the Rigden voids. neered to yield desired properties. Buttlar et al. (150) used particulate composite micro- Fillers in asphalt had low m, indicating that they were mechanics models to investigate three reinforcement regimes poorly dispersed in asphalt. in asphalt mastics: volume filling, physiochemical effects, Agglomerates were shown to increase KE, decrease m, and particle interaction. Consistent definitions were given for and therefore to increase stiffening power. Asphalts with the three reinforcement mechanisms: agglomerated fillers were shown to have stiffness many times the stiffness of asphalt with unagglomerated fillers. An increase in the aspect ratio of the filler particles low- Volumetric-filling reinforcement: The stiffening caused ers m and increases KE. Both of these effects increase by the presence of rigid inclusions in a less rigid matrix. the stiffening power of fillers. This stiffening level was assumed to be adequately Rigden's fractional voids concept does not take into described by the generalized self-consistent scheme account the agglomeration, degree of dispersion, and (GSCS) model or by the simplified GSCS-based pre- asphalt-filler interface contributions. diction equations. The stiffness of asphalt mastic in a specific system can Physiochemical reinforcement: The stiffening caused only be predicted accurately with the measurement of by interfacial effects between asphalt and filler particles, parameters m and KE. including absorption, adsorption, and selective sorption. The altered asphalt effectively forms a rigid layer, which Shashidhar et al. (153) later developed methods evaluating leads to a higher net volume concentration of rigid mat- the parameters m and KE. The volume-filling contribution to ter, which in turn leads to increased mastic stiffness. stiffening was captured by the parameter m and the physico-