Iowa Pore Index

MDOT Iowa Pore Index Device Instructions

Introduction

MDOT commissioned design and construction of a custom device for performing Iowa Pore Index testing of carbonate aggregate. The device has automated much of the test, requiring minimal user interaction.

These instructions cover use of the device.

No standard method for this test has been established.

Importance of Test

The Iowa Pore Index is a measure of the capacity of a carbonate aggregate to absorb water. A known mass of carbonate aggregate is immersed in water. Air pressure is applied and the amount of water pushed into aggregate pores is measured after 60 seconds and again after 15 minutes. The volumes are corrected for expansion and trapped air in the measuring system, producing two values; the Primary Load and the Secondary Load. Experience has demonstrated that these values are related to the durability of the aggregate and the development of "D" cracking in concrete pavements.

Methodology

The device requires measurement of expansion and trapped air prior to performing the first test of the day. This is performed by running a test with a dense and impervious aggregate proxy (e.g. glass marbles) and recording Primary and Secondary volume changes.

A known mass of carbonate aggregate is placed in the pressure chamber of the device and the chamber's lid is secured in place. The device is programmed to purge the system with water, during which time the operator manipulates the pressure chamber so as to remove as much air as possible. Following purging, the operator adjusts the water level, records the initial volumes from the sight glasses, and continues the test. The device will automatically apply air pressure and isolate the primary and secondary sight glasses at the appropriate times, allowing the operator to perform other functions while the test is performed. At the conclusion of the test, the operator records the final volumes from the sight glasses and drains the system.

Instructions

Expansion and Entrapped Air Correction

Prior to performing the first test with aggregate for the day, determine system expansion correction by running the test with 5 kg of an impervious and dense aggregate proxy (e.g. glass marbles). Calculate the volume changes:

P_corr = P_f - P_i

S_corr = S_f - S_i

Performing a Test

1. Weigh approximately 5 kg of the material to test, recording weight to the nearest 0.1 gram as W.
2. Place the material into the pressure chamber and tip the chamber so that space is created for the down-pipe in the chamber's cover.
3. Place the cover on the chamber and seal in place, tightening all four wingnuts.
4. Insure that the air and water supplies are attached to the device and turned on. Ensure the drain and vent lines are secured in an appropriate drain.
5. Start the test; the device will start water flowing into the system.
6. While filling with water, rock the chamber or beat it with a rubber mallet to dislodge and remove air from the chamber and aggregate.
7. Three short beeps will sound when the device has been purged with water. Adjust the water level in the sight glasses as required by pulsing the water or drain valves.
8. Click "Continue". The device will prompt to record levels.
9. Record the the level in the primary sight glass as P_i and the level in the secondary secondary sight glass as S_i.
10. Select "Continue". The device will automatically adjust valves and apply air pressure. The remainder of the test can be performed unattended and will conclude in 15 minutes.
11. Five long beeps will sound at the conclusion of the test. A prompt will appear to record levels.
12. Record the level in the primary sight glass as P_f and the level in the secondary sight glass as S_f.
13. Click "Continue".
14. Click "Drain System" to initiate draining.
15. Remove the cover from the pressure chamber.
16. Remove the wet aggregate from the chamber.

*To minimize corrosion, the chamber should be left open and dry when not in use.

1. The corrected volume changes are calculated for both primary and secondary loads:

V_P = P_f - P_i - P_corr

V_S = S_f - S_i - S_corr

Calculating Pore Index

Primary Pore Index:

P = 9000 * V_P / W

Secondary Pore Index:

S = 9000 * V_S / W