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3501-21 - Volumetric Properties of Asphalt Mixtures A...

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1 Volumetric Properties of Asphalt Mixtures A compacted asphalt mixture consists of three main volumetric components: (1) Air (2) Asphalt (3) Aggregate However, the complications in volumetric properties arise due to the fact that part of the asphalt may be absorbed into the aggregate. The following figure illustrate the various volumes within a compacted asphalt mixture. Volumetric Relationships in a Compacted Asphalt Mixture VOLUMETRIC PROPERTIES OF COMPACTED ASPHALT MIXTURES (1) Air Voids, P a (expressed as a %) = (Vol. of air) / (Bulk vol. of compacted mix) = V a / V mb X 100% - Too high an air voids in an asphalt mixture can lead to problems of high rate of asphalt aging, water damage and possibly higher consolidation of the asphalt mixture in service. - Too low an air voids can lead to problem of bleeding due to increase in temperature, or additional compaction of the mixture in service.
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2 (2) VMA (Voids in Mineral Aggregate ) = (Vol. of air + effective asphalt) (Bulk vol. of compacted mix) = (V a + V b - V ba ) / V mb X 100% = (V a + V be ) / V mb X 100% - Adequate VMA is needed to ensure that adequate amount of asphalt could be added to the mixture without overfilling the voids and resulting in asphalt bleeding. - When VMA is not adequate, two possible problems are: (A) When enough asphalt to coat the aggregate is added , low air voids and bleeding will result. (B) When not enough asphalt is added, low durability will result. (3) VFA (Voids Filled with Asphalt) = ( Vol. of eff. asphalt) / (Vol. of eff. asphalt + air) = V be / (V a + V be ) X 100% = (VMA - P a ) / VMA X 100% where V be = volume of effective asphalt = V b - V ba - VFA is related to VMA and Air Voids (P a ) , as can be seen from the above equation. - A low VFA may result in a high air voids, and a high VFA may result in a low air voids COMPUTATION OF VOLUMETRIC PROPERTIES OF MIXTURES Maximum Specific Gravity , G mm Specific gravity of an asphalt mixture when there is no air voids. This can be measured by ASTM 2041 test procedure, which is also known as the Rice Test. Effective Specific Gravity of Aggregate, G se (for 100 g of mix) = P s / (100/G mm - P b /G b ) __________________________________________ 100/G mm = Volume of mixture with no air voids P b /G b = Volume of asphalt
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3 Maximum specific gravities of mixtures, G mm , at different asphalt contents can be computed by assuming that the asphalt absorption is constant and thus the effective specific gravity of the aggregate is constant at all asphalt contents. G mm = 100 / (P s /G se + P b /G b ) _________________________________ P s /G se = Effective volume of aggregate P b /G b = Volume of asphalt Asphalt Absorption , P ba is expressed as a percent by weight of the aggregate P ba = (100/G sb - 100/G se ) G b where 100/G sb = bulk volume of aggregate 100/G se = effective volume of aggregate ______________________________________ Effective Asphalt Content , P be = P b - P ba P s /100 where P ba P s
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