Slide 6 of 17
Field measures of gravel aggregate quality are illustrated by examples from the South Platte valley north of Denver:
1-Particle size, not shown here, is the most important measure. Particle size will be discussed later, in conjunction with sampling plans and results.
2-Lithology is the basis for predicting expected quality (durability, chemical reactivity, etc). South Platte gravels consists mostly of igneous and metamorphic rocks such as granite, pegmatite and gneiss. Abundance of volcanic rocks, which might be chemically deleterious, is small. South Platte lithologies yield a combined Los Angeles degradation value of 40-44 pct, suitable for most aggregate applications.
3-Shape is defined by axial ratios, where A = long axis, B = intermediate axis, and C = short axis. The main concern is whether flat, bladed particles that might be physically weak are abundant. Weak shapes are not abundant in South Platte gravels. Sphericity, the tendency toward equidimensional shape, can be calculated from axial ratios.
4-Roundness is the absence of angular corners. Roundness is determined by visual comparison with standards. Angular particles interlock; well-rounded particles tend to shift under load, as in road base. South Platte gravel particles tend to be well-rounded, but we know of no problems affecting applications such as road base. Roundness categories are: A = angular, B = subangular, C = subrounded, D = rounded, and E = well rounded.
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Last updated 10 Jan 2013