ESTIMATING SURFACE AREA FOR AGGREGATE IN THE SIZE RANGE 1 MM OR LARGER

Measures of surface area are available for small particles up to 0.5 mm in diameter, using a calorimeter to measure heat of immersion, and for very small particles using nitrogen adsorption. These methods have limited or no applicability for determining surface area of particles 1 mm in diameter or larger, in part due to the sensitivity of the analytical equipment. An empirical method based on the fractal dimension is described for estimating surface area of particles with diameters greater than or equal to 1 mm. This method is compared to an existing technique to judge the quality of the surface area estimates. For aggregate pieces having isotropic shape, a single perspective is photographed in silhouette to determine the fractal dimension of the particle's surface, from which surface area is estimated. For an aggregate piece of anisotropic shape, two or three mutually orthogonal perspectives are photographed in silhouette to determine the fractal dimensions of surfaces comprising the particle. Areas are then determined for these surfaces and summed to derive the total surface area for the sample. A practical procedure for estimating surface area is discussed.