Several hundred quarries throughout Iowa mine carbonate bedrock for aggregate, which is used in the construction of pavement and structures, as granular surface materials (i.e., “gravel roads”), or as rip rap to prevent erosion. This study focuses on coarse aggregate used in concrete pavement. The Iowa Department of Transportation employs a set of tests to predict the durability of pavement constructed with these aggregates. These tests factor in the bulk elemental chemistry of the carbonate (calcium-rich, magnesium-rich, or a mixture), the alumina percentage (a proxy for clay content), and the pore system (the volume of “micropores”). While these tests have been generally accurate in predicting the durability of a pavement since the 1980s, there have been some exceptions that require more specific exploration in a geologic and structural context.

The studied quarry removes the bulk of its material from the Spring Grove Member of the Middle Devonian Pinicon Ridge Formation, Wapsipinicon Group, with some from the Davenport and Kenwood Members of the same formation. Coarse aggregate from this source qualifies for the highest Iowa DOT quality classification, meaning it can be used on interstate highways and be expected to last at least 30 years. Despite this classification, anomalous behavior has been observed with this aggregate, including spalling during some freeze-thaw tests on durability beams (ASTM 666) and bubbling when placed in water. There is currently no evidence that it deteriorates unusually in roads—the Iowa DOT’s preferred method for predicting durability. To elucidate the reasons behind these anomalous behaviors, we compare it to another highly rated source (Late Devonian Owen Member of the Lime Creek Formation, Central Iowa) that performs equally well in aggregate testing yet does not spall in durability beams.

The Lime Creek Formation was selected for comparison to the Spring Grove because both were deposited in similar environments, tidal flats adjacent to shallow marine basins that show evidence of possible restriction during deposition. Both sources have relatively low clay content, and both are pure limestone lithologically. Helium porosity of Spring Grove material ranges from 0% to 26%, and modal pore throat size measured using mercury porosimetry ranges from 0.02 à µm to 1 à µm. The Iowa DOT’s pore system test measures the material as having a very low volume of micropores (average value: 11 mL). Owen Member coarse aggregate ranges in porosity from 2% to 5%, and a similarly small volume of micropores in the DOT’s pore system test (average value: 14 mL). It has a mercury-measured modal pore throat size of 0.03à µm.

One facies present in the Spring Grove is unique in comparison to the Lime Creek Formation. The high porosity peloidal algal grainstone from the Spring Grove has a uniquely interconnected and pervasive system of micropores that are hosted among calcite microcrystals, and may be responsible for the anomalous spalling behavior in durability beams, as well as other anomalous behaviors. This pore system likely stems from dissolution associated with episodes of karstification that have occurred in the vicinity of the Spring Grove source.