National Center for Wood Transportation Structures

About the research

This study evaluated the effects of coarse aggregate pore system properties on the permeability and chloride ingress of high-performance concrete (HPC) mixes used in bridge decks in Iowa.

Five coarse aggregates typically available in Iowa were studied, including three limestone and two dolostone aggregates with different water absorption capacities and porosity values. These coarse aggregates were used to make five concrete mixes with a given HPC mix proportion. (That is, the five concrete mixes differed only in terms of the coarse aggregate used.) A corresponding mortar mix was sieved from one of the concrete mixtures to eliminate coarse aggregate. The aggregates were analyzed for their pore characteristics, and the mortar and HPC mixes were evaluated for chloride intrusion using electrical and non-electrical tests. Relationships were determined between the aggregate properties and the results of the concrete chloride intrusion tests.

The results showed that the aggregate critical and threshold pore sizes were strongly related to concrete surface resistivity and initial sorptivity but did not strongly affect the chloride penetration results of salt ponding tests. The different chloride intrusion tests indicated that the concrete mixes made with limestone and dolostone aggregates exhibited different behaviors. For a given coarse aggregate type (limestone or dolostone), concrete initial sorptivity increased with increasing aggregate critical pore throat size.

Although aggregate absorption should not be used as the sole means of concrete durability control, it should be considered a key parameter for concrete aggregate evaluation. For concrete, the short-term initial sorptivity test (which takes six hours) can be used instead of a 90-day salt ponding test for a quick evaluation of chloride intrusion. This test can be used together with the aggregate absorption test to evaluate aggregate quality.