In environments characterized by repetitive freeze/thaw cycles, coarse aggregate containing pores with throat diameters between 50 nm and 10 µm can cause premature pavement deterioration. As a result, the Iowa Department of Transportation (Iowa DOT) developed the Iowa Pore Index (IPI) in the 1980s to differentiate durable vs. non-durable coarse aggregates for use in portland cement concrete. In 2016, the IPI test was accepted as an American Association of State Highway and Transportation Officials standard (AASHTO TP 120-16) because of its straightforward, fast, inexpensive, nondestructive, and nonhazardous procedure for quantifying coarse aggregate pore distributions.
The newly released third-generation (3G) IPI test device and procedures provide faster and more accurate and reliable assessment of coarse aggregates’ freeze/thaw susceptibility by measuring the macropore/micropore volume ratio at time intervals ranging from 0.1 to 2.0 seconds via measuring water intrusion at variable pressures up to 70 psi (480 kPa). The 3G IPI reduces soak time as well as allows for smaller sample sizes and the automation of previously manual processes.
ABSTRACT:
Coarse aggregate, depending on intended usage, constitutes roughly 20–45% of portland cement concrete as well as being a major component in the construction of granular surface roads and shoulders for paved roads. However, coarse aggregate quality greatly varies among sources based on its petrophysical properties. Therefore, it is important to understand how these properties emerge from the depositional and diagenetic history of a deposit in order to accurately predict pavement durability, which can be negatively impacted by oscillating freeze/thaw cycles. To derive more information about a coarse aggregate’s pore system, this study used a “third generation” Iowa Pore Index (IPI) device capable of measuring the volume of intruded water at various time intervals ranging from 0.1–2.0 seconds, as well as measuring intrusion at variable pressures up to 70 psi (480 kPa). Using this new device, 21 carbonate samples (10 dolostones and 11 limestones) were compared to “traditional” IPI measurements. The new method gave 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 macroporous limestone sources. By decreasing apparatus chamber size, higher total intrusion was recorded and IPI values were more correlative with traditional measurements. However, by analyzing the effect of variable pressure intrusion (15, 35, and 60 psi), it was observed that the transition point between intrusion of macropores and micropores was sample-dependent based on lithological properties (i.e., porosity, connectivity of pores, and pore-throat sizes). Although prior methods utilized 60 seconds as this transition point, incremental intrusion data suggest most samples complete macropore intrusion within the first 12 seconds. Therefore, by assessing the incremental intrusion of each source, new primary and secondary loads were calculated, which may be more characteristic of individual lithologies. As a result, secondary load values increased as more intrusion was accounted to micropores than through utilization of the previous method.
With further study, this method could better predict the longevity and overall durability of coarse aggregate based on pore structure in a more individualized fashion than the previous IPI method.
For more information about the 3G IPI test device and procedures, please go to: https://intrans.iastate.edu/research/completed/next-generation-iowa-pore-index-phase-iii/
To download the report, please go to: https://intrans.iastate.edu/app/uploads/2019/08/next_gen_Iowa_pore_index_phase_3_w_cvr.pdf
Cover photo: Third-generation IPI device with
(a) 1,650 cm3 chamber and
(b) 1,100 cm3 chamber