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Iowa State University--Becoming the Best

Calibrating the Iowa Pore Index with Mercury Intrusion Porosimetry and Petrography


Principal investigator:

Project status

In progress

Start date: 07/15/15
End date: 06/30/19



About the research


Iowa weather is tough on pavements. Multiple freeze-thaw cycles during our winters cause some road aggregates to crumble. To overcome this problem, the Iowa Department of Transportation (DOT) has developed a method to predict the ultimate durability of a concrete road based on the nature of the crushed rock used for the pavement's construction. This method combines mineralogical analysis, chemistry, and the character of the rock's pore system. The pore system test is called the Iowa Pore Index test and was developed by the Iowa DOT as an environmentally friendly test method.

The Iowa test method has gained acceptance by several midwestern DOTs (such as Michigan and Ohio), while other states still rely on durability beams. There is national interest in this test to the extent that an American Association of State Highway and Transportation Officials/ASTM International  (AASHTO/ASTM) standard test method has been drafted.

Past investigations by other states have relied on comparing the Iowa Pore Index test with results from durability beams (ASTM Method 666). The experience of the Iowa DOT has been that the ASTM 666 test method is not a reliable predictor of pavement and aggregate performance. What is needed to increase acceptance of the Iowa Pore Index test is a rigorous comparison to other methods that directly measure the pore structure of an aggregate.

Aggregate samples representing the spectrum of qualities commonly seen by the Iowa DOT will be analyzed using mercury intrusion porosimetry to obtain the distribution of pore sizes (the device can measure pore sizes from 1 mm to 10 nm). Mercury porosimetry is the research standard for determining the pore structure of various materials. Samples will also be made into thin-sections for geological description of rock texture, grain types, and pore types. Finally, samples will be analyzed using a scanning electron microscope (SEM) to visually describe pore types that are too small to see in thin sections and compared to the bulk chemistry of the aggregate, determined by X-ray fluorescence (XRF).

Ultimately this research may allow for the Iowa Pore Index test to provide additional information in the evaluation of aggregates (such as grain size) and may suggest improvements to the test (such as variations in water pressure) to better analyze the pore structure. The nature of the crushed rock is a primary factor in determining the ultimate service life of a road. A better understanding the nature of crushed rock therefore has the potential to reduce the cost of road building in Iowa by better characterizing the rocks that make long-lasting roads. Because other midwestern states have problems with poor performing aggregates, validation of this test method has the potential to raise the profile of the Iowa DOT among its peer institutions.