PREDICTIVE PERFORMANCE CLASSIFICATION OF ROCK UNITS FOR CONSTRUCTION AGGREGATE

It is necessary to understand that pre-production, root-cause, and mappable geologic factors result in a good, marginal, or poor aggregate potential, particularly for durability and strength assessed with engineering tests. Granular base (GB), asphaltic concrete (AC), or portland cement concrete (PCC) use aggregate produced from geologic deposits of rock or natural sand and gravel. What is an effective and rapid classification scheme during geologic mapping? Are the geologic classifications of igneous, metamorphic, and sedimentary adequate? Is ASTM C294 adequate when it classifies rocks? Are IUGS or Folk classification schemes too detailed? Does simple engineering data from the Los Angeles Degradation or Micro Deval tests result in rapid, effective classification for geologic mapping?

Geologic mapping for aggregate must consider the potential performance of the product. A classification method using engineering product requirements and geologic criteria can identify rock with inherent quality or marginal rock correctable during extraction and processing. For GB, AC, and PCC, the most significant mappable geologic factors influencing the quality are weathering or alteration in the rock deposit, grain size, rock fabric, inadequate cementation, shale presence, and potentially reactive minerals. These visually-identifiable criteria correlate with the results of physical engineering tests such as LA, Micro Deval, or sulfate soundness. The aggregate in PCC must not expansively degrade in the wet, high-pH, and often oxidizing environment of PCC; thus, rapid geologic identification of minerals such as laumontite, opal, pyrrhotite or strained micro-fractured quartz suggests potential performance problems. In suspect lithologies, slower ASTM C1778 criteria and testing can quantify rock components that degrade. Different engineering criteria are essential for some rock in AC paving layers; these also correlate with readily identified, inherent geologic properties. In AC, the abundance and size of quartz and mica affect the moisture susceptibility of asphalt and rock as quantified by the modified Lottman test using AASHTO T283 or T324.