6. LPC Bituminous Mixtures Design Guide, Sept 2007. - Aapaq.org

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LPC Bituminous Mixtures Design Guide- Mix Design Procedure -this step, the mix is typically submitted to the Gyratory Compactor test (or anotherdesignated test). If the expected result is not the one ultimately obtained, the mixdesigner must modify the composition, which requires understanding the effect ofcomposition variables.3.4.1 Effect of mix variables (general remarks)The mechanical characteristics of the mix stem from the combined effect of internalfriction on the granular skeleton, binder content and binder characteristics (includingthe additives eventually introduced).3.4.2 Effect of dimension DIf D were to be increased, the richness modulus (and thus binder content) wouldhave to drop.With all other parameters held unchanged, by increasing D, the percentage of voidsin the mix decreases.As the D value of the mix rises, it becomes easier to obtain a stable material, yetattention must then be paid to the risk of segregation during placement.– Example –A AC10-BBSG mix with 5,6% bitumen is equivalent to a AC14-BBSG with 5,4% bitumen.A AC10-BBSG with 5,6% bitumen leads to a percentage of voids at 60 gyrations V60 = 9%, while aAC14-BBSG with 5,4% yields a characteristic percentage of voids at 80 gyrations V80 = 4%.3.4.3 Effect of granular proportionsWith greater granular fractions, mix design setting becomes facilitated.For a 0/D mix with 10 mm < D < 20 mm, the percent passing at 2 mm can displayone of the four following cases:• > 35% percent passing at 2 mm very high curve: mortar content is in excesswithin the granular skeleton and stability relies solely upon the mortar.• 30 to 35% percent passing at 2 mm high curve: the mortar fills nearly all interaggregatespace. The mortar plays a key role in overall mix characteristics.• 25 to 30% percent passing at 2 mm average curve: coarsely-graded design.Mortar influence is attenuated due to effect of the granular skeleton.• < 25% percent passing at 2 mm low curve: the mortar exerts a secondaryinfluence, with mix stability obtained by means of inter-aggregate setting.3.4.4 DiscontinuityA discontinuity (gap-grading) within the particle size distribution curve leads to anincrease in workability along with a drop in percentage of voids, although the ruttingrisk rises as a result.- 100 -
LPC Bituminous Mixtures Design Guide- Mix Design Procedure -– Example –A AC14-BBSG or a AC14-BBM with a continuous curve yields a rut depth of 3 mm at 30000 cycles.The rut extends to 4 mm with a 2/6 discontinuity and to 10 mm with a 2/10 discontinuity.Should the material be used as a thick layer, it would be preferable to apply a gapgradedcurve with a relatively low threshold, e.g. 25% at 2 mm.3.4.5 Incorporation of rounded particle aggregateThe inclusion of rounded particles, generally limited to 10% for wearing courses,serves to increase mix workability while reducing the percentage of voids. On theother hand, the risk of rutting naturally rises quite considerably as a consequence.This type of mix formulation is not be used on roads submitted to heavy traffic loads.For AC-EME or AC-BBME however, blunt-edged materials, and in particular fineaggregate with totally rounded particles, may be used to enhance compacity, withbinder stiffness serving to reduce the risks of rutting. Yet when employing such anapproach on surface materials, the durability over time may be compromised.3.4.6 Percentage of fillersFillers, in combination with bitumen, make it possible to fill the inter-granular voids.As total filler percentage increases, the percentage of voids in the mix will drop, butthe mastic will harden. The optimal value lies between 6,5% and 7,5% for averagecurves and typical bitumen contents.3.4.7 Percentage of bitumenBitumen, when combined with fillers, plays the role of lubricant for the granularskeleton and thus enables compacting the material. Beyond an optimal content, itsfunction is limited to filling voids in the granular skeleton. As the percentage ofbitumen increases at low bitumen content levels, the stiffness modulus rises as well;whereas with higher bitumen content, this modulus tends to fall.As the percentage of bitumen increases, the inter-granular mastic film becomesdenser and thicker, which makes for improved fatigue and water resistance, with thedownside being that rutting resistance decreases.These trends have been laid out under very general conditions. In practice, certainmix component characteristics can modify expected behavior. As an example, itwould be necessary to incorporate the effect of absorbent materials (porousaggregates, fibers).3.5 Gyratory Compactor compactibility study3.5.1 General remarksThe test is conducted on a composition (test interpreted in isolation) or on anexperimental design (2 or 3 different granular compositions) containing, for example,a 5% deviation in the fine aggregate concentration.- 101 -
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