Evaluation of particle shape and texture of mineral aggregates and their blends

625362PDFProceedingsNCAT Report No. 92-4Hot mix paving mixturesAsphalt pavementsMineral aggregatesParticlesShapeTextureCoarse aggregatesNational Center for Asphalt TechnologyKandhal, Prithvi SKhatri, Maqbool A.Motter, John B.National Center for Asphalt Technology (U.S.)NTL-HIGHWAY/ROAD TRANSPORTATION-Construction and MaintenanceNTL-HIGHWAY/ROAD TRANSPORTATION-DesignNTL-HIGHWAY/ROAD TRANSPORTATION-MaterialsNTL-HIGHWAY/ROAD TRANSPORTATION-Pavement Management and PerformanceUS Transportation CollectionTwo important factors for minimizing rutting of hot mix asphalt (HMA) mixtures are: a) the fractured face count of aggregate when gravel is used as coarse aggregate, and b) the use of manufactured sand. Both these factors relate to the shape and texture of the aggregate particles. One of the objectives of this paper was to study the effect of crushed particle count on the particle shape and texture index (ASTM D3398) of gravel coarse aggregate and to determine if there is an upper limit on the crushed face count above which the particle shape and texture index of the coarse aggregate fraction would not increase significantly. A definite relationship was found to exist between the crushed face count and index of particle shape and texture of the coarse aggregate. Higher percentages of crushed particles increase the index of particle shape and texture significantly. Highway agencies also limit the amount of natural sand in HMA mixes. This is usually done by generically specifying the maximum allowable percentage of natural sand. A total of 18 fine aggregates (8 natural and 10 manufactured sands) of different mineralogical compositions were used from various sources in Pennsylvania. Particle shape and texture data were obtained using ASTM D3398 and two proposed methods of the National Aggregate Association (NAA). A particle index value of 14 based on ASTM D3398 appears to divide the natural and manufactured fine aggregates, and can be considered for specification purposes. Corresponding values for NAA's methods A and B are also reported. Test data also suggest that the major fraction particle index can be used in place of the weighted average particle index of the entire gradation when using ASTM D3398. Good correlations are found to exist between NAA's methods A and B and ASTM D3398, suggesting that the NAA methods, which are less time consuming, can be adopted as standard test methods in lieu of ASTM D3398. Equations needed to compute ASTM D3398 weighted average particle index values from NAA methods' results are given. The difference between two orifice diameters for the NAA methods was not found to be statistically significant in determining the uncompacted void content of the fine aggregates. However, the 3/8-in. (9.5-mm) diameter orifice resulted in lesser variation compared to the 1/2-in. (12.5-mm) orifice. Natural and manufactured fine sands were blended in various proportions to study the effect of using ASTM D3398. The particle shape and texture index decreases as higher percentages of natural sand are blended with the manufactured sand. There is no linear relationship, however