A NEXT-GENERATION “IOWA PORE INDEX” TEST DEVICE FOR CHARACTERIZING COURSE AGGREGATE PORE SYSTEMS USING WATER INTRUSION

Coarse aggregate, depending on intended usage, constitutes roughly 20-45% of the total compositions within both asphalt cement concrete and portland cement concrete. Therefore, it is apparent that coarse aggregate quality impacts pavement durability and cost efficiency. The Iowa Pore Index (IPI), which involves a device and a procedure for measuring the volume ratio of macro- to micropores of coarse aggregate via water intrusion, was initially developed by the Iowa DOT in the 1980’s. Although the device has been automated since, the concept has remained unchanged and has since been adopted by other state transportation departments and accepted as a standard by the American Association of State Highway and Transportation Officials. Water intrusion at a constant pressure of 35 psi is measured on a 1000 g sample of oven-dried coarse aggregate over the span of 15 minutes. With prior devices, two volumes of intrusion were measured: primary load and secondary load. These measurements were assumed to relate to macro-pores and micro-pores present within the coarse aggregate, respectively. A high secondary load (i.e., high micro-porosity) has been correlated with low service life.

In an attempt to derive more information about a course aggregates pore system using water intrusion, this study uses a device capable of measuring the volume of intruded water during intrusion at various time intervals ranging from 0.1-2.0 seconds as well as measuring intrusion at variable pressures up to 70 psi. Using this new device, twenty-one carbonate coarse aggregate samples (ten dolostone samples and eleven limestone samples) were compared to historical IPI data. The results showed slightly higher primary loads. Additionally, with cumulative volume plotted for the first five minutes of intrusion, dolostones and limestones with elevated primary loads stood apart from the remaining, less porous limestones sources. More importantly, the rate of intrusion between 60-300 seconds is directly proportional to the source’s secondary load index as measured using the traditional IPI apparatus. With further examination, this method could better predict the longevity and overall durability of coarse aggregate based on pore structure in a quicker fashion than the previous IPI device.