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Carbonate Aggregate in Concrete


Principal investigators:

Co-principal investigators:

Project status


Start date: 08/09/12
End date: 05/31/15


Report: Carbonate Aggregate in Concrete (NA pdf) May 2015


Sponsor(s): Minnesota Department of Transportation

About the research


The current Minnesota Department of Transportation (MnDOT) specification for coarse aggregate to be used in concrete pavements is based on either a maximum absorption of 1.75% (for Class B aggregate) or a maximum carbonate content of 30% by weight (for Class C aggregate). There is a concern that these specified limitations may reject potentially acceptable aggregates. This study was designed to evaluate the available methods for effectively characterizing the freeze-thaw (F-T) durability of aggregates.

In this study, aggregates from 15 different sources in Minnesota were investigated. The experimental program included measuring the carbonate content, absorption, desorption, specific gravity, Iowa pore indices (i.e., primary pore index [PPI] and secondary pore index [SPI]), and pore size distribution of the carbonate and non-carbonate fractions.

The results indicate that carbonate aggregates generally have higher absorption, lower specific gravity, and higher PPI and SPI than non-carbonate aggregates. Although there is no direct correlation between absorption, carbonate content, and SPI, a combination of high specific gravity (>2.60) and low absorption (<1.75%) often indicates a low SPI value. Carbonate aggregates generally have a much finer pore structure than non-carbonate aggregates, and the characteristics of their pore size distributions are clearly different. PPI correlates well with pores in the size range of 0.1–100 mm, and SPI correlates well with pores in the size range of 0.01–1 mm.

Further research is necessary to identify the critical pore size range that has the most significant effect on aggregate F-T durability and to determine the critical SPI value below which an aggregate will be truly F-T durable.