The Iowa Pore Index test is a method employed by several Midwestern State Departments of Transportation to determine the volume ratio of macropores-to-micropores in a rock aggregate by means of water intrusion. This method, when combined with x-ray diffraction (to measure the dolomite mineral structure using the peak shift) and x-ray fluorescence (to measure the calcite-to-dolomite ratio and the clay content using alumina), has been shown to be effective in predicting the performance of aggregates in Portland cement concrete.

In this test, 4.5 kilograms of oven-dried crushed carbonate is intruded with water progressively at 240 kilopascals. Readings of intruded volume are taken at 1 and 14 minutes corresponding to macropore and micropore volumes, respectively. The Iowa Pore Index test is interesting more broadly because it is fast, non-destructive, inexpensive, and environmentally friendly, hence it has the potential to replace mercury porosimetry and be integrated in any petrophysical lab.

This research aims to understand the geological factors (depositional environment; facies; grain and pore types; texture; and paragenesis) responsible for the results of the Iowa Pore Index test. End-member samples of various geologic ages are collected around Iowa to represent different combinations of accepted and rejected porosity and clay contents. The pore index of each sample is calibrated quantitatively via helium and mercury porosimetry and qualitatively via thin section petrography.

Findings of this research show that even the most homogeneous sources have at least three different rock types, or groups of pebbles characterized by distinctive physical and textural attributes observable at hand-specimen scale. Petrographic analysis shows that limestones with a sparite matrix, peloidal grains, and low matrix-to-allochem ratio (i.e., grainy) are better for road constructions than limestones with a micrite matrix, skeletal grains, and high matrix-to-allochem ratio (i.e., muddy). Dolostones with fine-to-coarse grains, crystal-supported euhedral-to-subhedral rhombs and porous intercrystalline areas are more desirable than dolostones with very fine grains, a tightly interlocking crystal mosaic in anhedral form.

Each rock type occurs in different abundance and has different porosity. Several linear models have been developed to relate IPI to helium porosity. Limestones with a helium porosity less than ~7% and dolostones with a helium porosity greater than ~3% were found to be desirable for use in road construction. The critical range of pore-throat size was found to be between 0.02 to 0.1 à µm.