Investigating paleoenvironmental and diagenetic controls on the skid resistance of carbonate construction aggregates mined from the Lower Cambrian Kinzers Formation in the West York 7.5-minute quadrangle, Pennsylvania
Pelepko, M. Seth
University of Delaware
Wet pavement friction, or skid resistance, is one of the most important engineering considerations in highway design from a safety standpoint. For bituminous pavements, skid-resistance performance is primarily a function of the durability, texture, and composition of coarse aggregate used to construct the wearing course, or the uppermost layer of the road surface. Cambrian age carbonate construction aggregates mined in Pennsylvania, although relatively simple mineralogically, exhibit a wide variety of textures that developed in response to complex paleoenvironmental and diagenetic processes. To reveal what factors influence the petrographic properties of such aggregates and relate these attributes to skid resistance, carbonate construction aggregates currently being mined from the Lower Cambrian Kinzers Formation in the West York 7.5-minute quadrangle, Pennsylvania have been sampled and subjected to laboratory testing. Thin section analysis has revealed six individual microfacies types at the mining operation in West York: Microfacies Types I and III include massive debris flow deposits consisting either of coarser crystalline dolomite or limestone; Microfacies Type II is believed to represent a proximal turbidite facies consisting of finer crystalline dolomite; Microfacies Types IV and V are laminated to shaly dolomites and limestones corresponding to middle and distal fan turbidite environments; and Microfacies Type VI consists of hemipelagic, meta-pelitic deposits. Four complete depositional sequences and portions of two others have been identified at the West York quarry using clast composition and the presence of sharp lithofacies contacts believed to represent sequence boundaries. High energy debris flows (LST) are interlayered with more distal turbidites (LST and TST) and altered, hemipelagic shales (HST). Aggregate skid resistance has been assessed through cyclic material polishing in the micro-Deval testing apparatus and incremental microtexture evolution monitoring via two methodologies: the application of wavelet analysis to grayscale images of aggregate particles and the use of laser confocal microscopy to measure aggregate sample surface roughness. Laser confocal microscopy, which results in a direct measurement of surface texture, provides a more precise method for monitoring changes in aggregate roughness as a function of polishing time than wavelet analysis. The latter technique is adequate for assessing bulk samples, but apparently is susceptible to misinterpreting aggregate variegation as surface roughness. Stockpile samples consisting of Microfacies Type I resist polishing much more effectively than those comprised mostly of Microfacies Types IV and V, reflecting the importance of coarser crystal sizes in carbonate aggregate skid-resistance performance. Microfacies Types I and III, both initially characterized by pronounced surface roughness associated with coarser, interlocking carbonate crystals, exhibit marked differences in polish resistance. Microfacies Type I – a mostly pure dolomite – maintains its initial rough microtexture more efficiently than Microfacies Type III – a high-calcium limestone. This observation confirms that it is essential to consider mineralogy when estimating the potential for relatively pure carbonate construction aggregates to resist polishing. Finally, Microfacies Types II and IV, which are both comprised of higher percentages of dolomite than calcite and mostly consist of fine to medium crystals, appear to sustain microtexture more effectively than the coarser textured, high-calcium limestones of Microfacies Type III, indicating that the dolomite component enhances skid resistance even more noticeably than the presence of coarser crystalline material, particularly for aggregates that are not exclusively fine textured in character. Findings are used to develop a process map that may serve as a valuable tool for mining companies targeting relatively pure or argillaceous carbonate deposits for use in bituminous wearing courses and state agencies charged with evaluating construction materials prior to approval for use and to confirm ongoing, consistent performance.