Experimental assessment of aggregates

Abstract

An extensive suite of geotechnical laboratory tests were conducted to quantify differences in engineering properties of three crushed aggregates commonly used on Montana highway projects. The material types are identified in the Montana Supplemental Specifications as crushed base course (CBC, 1.5 to 2-inch maximum particle sizes) and crushed top surfacing (CTS, 0.75-inch maximum particle size). All aggregates were open-graded and contained relatively few fines. Results from R-value tests and direct shear (DS) tests performed on large samples (12-in by 12-in) indicate the CBC aggregates generally exhibited higher strength and stiffness than the CTS aggregates. Drainage capacity was quantified by conducting multiple saturated constant head permeability (k) tests on 10-inch-diameter samples of each material type.

Hydraulic properties of the materials examined in this study did not correlate well with aggregate type, but were found to correlate with pore properties. The fine fraction void ratio was correlated to k. It is derived from gradation and density, both parameters that are commonly tested in roadway construction projects. This could allow roadway designers to incorporate an estimate of k into their design, and could also allow quick comparisons of aggregate samples and the development of aggregate specifications. X-ray CT scanning was performed to acquire pore size distributions of the materials. No differences between aggregate types could be discerned from the pore size distributions, but a strong correlation between the pore size of 80% passing and k was discovered. Additionally, an equation was presented for thresholding 2D X-ray CT soil images. This equation could be applied in future studies to help reduce the subjectivity of the thresholding process.