CONCRETE AGGREGATE ANALYSIS VIA THIN-SECTION PETROGRAPHY: A SUPPLEMENTARY METHOD AIDING EVALUATION OF CARBONATE AGGREGATE SOUNDNESS

A preliminary study to design an alternate or supplemental technique as an aid to evaluate carbonate concrete aggregates was completed for the Florida Department of Transportation (FDOT). Petrographic number (PN) systems have been used for decades in other states and regions to characterize potential performance of concrete aggregates. Unfortunately, this method is difficult to apply to the carbonates typically used as aggregates in Florida. The primary reason for the difficulty is the reliance on macroscopic characteristics to assign a PN value since the Florida carbonates are often too fine-grained to allow appropriate macroscopic differentiation among the available carbonate aggregates. A second reason for this new approach is the limited variation in lithologies used as FDOT aggregates. Prior applications of the PN technique described numerous primary lithologies rather than just one and, thus, the traditional PN method was not appropriate for use by the FDOT. Therefore, we developed a modified PN analysis procedure using thin-section (i.e., microscopic) descriptions rather than the traditional hand-sample (macroscopic) descriptions.

Petrographic numbers are values derived by multiplying lithologic characteristics of aggregates by a "factor weight" (FW) relative to its physical or chemical durability. Lithologic variables represent a sum total of 100% and, thus, with minimum factor weights of 1.0, the minimum PN is 100 for the most "ideal" aggregates while less ideal aggregates have progressively larger PNs. The group of lithological characteristics used as defining criteria for PN determination of Florida carbonate aggregates include: 1) allochem type, 2) porosity type, 3) cement or matrix variety, and 4) any additional non-carbonate minerals or rock fragments. Earlier research (completed using the traditional PN method) indicated aggregates with PN values below 140 yield good field performance, 140-160 have fair to poor field performance, and above 160 normally have poor field performance. Using FWs developed for the Florida carbonates, four of the eight FDOT samples are good, one is fair, and three are at the poor level. However, to fully validate this technique and the resulting data, field performance data for these aggregates must be gathered and compared with these results.