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The Iowa Pore Index (IPI) test has been useful in prediction of freezing-and-thawing damage susceptibility of aggregate used in concrete pavements. However, the index is based on larger top size gradations that have been used in the past. This study was performed to identify size corrections that would allow aggregate gradations of smaller nominal maximum sizes, other than the standard 1/2 to 3/4 in. (12.5 to 19 mm) fraction, to be used in the index test method. In addition to the size corrections, several modifications to the method were developed to reduce variability in the test, and material re-testability was shown to be feasible. A new method of additional data collection from the IPI test was developed that allows for the measurement of water that is retained in and expelled from the aggregate after depressurization. These data have shown to have a better correlation with freezing-andthawing testing data than IPI.
Keywords: aggregate; durability; freezing-and-thawing resistance.
INTRODUCTION
Freezing-and-thawing resistance of concrete pavement is of primary importance in areas where significant freezingand- thawing cycles occur. The freezing-and-thawing susceptibility of coarse aggregate in concrete is largely a function of the aggregate's pore system, which includes water escape path (pore) length,1-6 and pore size, distribution, and shape.7-12 These characteristics relate to the volumes of water absorption, adsorption, and retainage, and rates of absorption and expulsion. If the aggregate particle size is greater than its critical size, freezing-and-thawing damage can occur. The pore length is related to the critical size of the aggregate particles, thus, if the particle can be crushed to a size smaller than the critical size, the susceptibility to freezing-and-thawing damage is reduced.2,3,13,14
The Iowa Pore Index (IPI) test was developed to provide a simple test method that would identify aggregates that are prone to freezing-and-thawing deterioration (D-cracking).15 In the test procedure, the aggregate is subjected to water pressure. Large pores are filled during the initial pressurization with a volume of water called the primary load (PL); these large pores do not relate particularly well to freezingand- thawing susceptibility. The volume of water taken in by the aggregate particles subsequent to the PL is called the secondary load (SL), which is considered a measure of the absorptiveness of the ink-bottle-shaped or smaller size pores. It is...