TRADITIONAL SOIL TESTING METHODS FOR EVALUATING CORROSIVE POTENTIAL ARE NOT USEFUL FOR SOME COARSE AGGREGATES

The corrosion of metal reinforcements in engineered structures like roads, bridges, and mechanically-stabilized walls is partly dependent on the properties of the soils/aggregates used for construction. Hence, geochemical testing of construction materials to evaluate their corrosive potential is essential. Most state Departments of Transportation (DOTs) determine the corrosive potential of aggregates through leaching experiments where pH, resistivity, and chloride and sulfate contents are evaluated. However, these traditional leach-testing methods were designed specifically for evaluating soils and fine materials, and may not be useful for coarser materials. These geochemical tests rely on the smallest size fractions of the aggregates for testing, which can exclude >99% of the rock mass for coarse materials. In these cases, the assumption is made that the chemistry of the fines is representative of the chemistry of the bulk rock. Here, we test this assumption by performing leaching tests on 5 different size fractions of coarse carbonate aggregates sieved from material collected in the field. We also compared these results to additional leach tests performed on the same materials that were crushed in the laboratory to produce the smaller fractions. The pH and resistivity of the smaller field fractions were substantially lower than the coarser field fractions. Moreover, ICP-OES and IC results suggest that the smaller size field fractions contain more calcium, magnesium, sulfate, and nitrate than their larger counterparts. The field fractions also had a lower pH and resistivity than the corresponding lab-crushed fractions, which suggests that chemical weathering reactions in the field were largely responsible for the observed differences. We suspect that SO_x and NO_x compounds in the atmosphere react with the carbonate rocks to produce reactive surface layers (typically of soluble sulfate minerals) that are easily mechanically abraded and easily chemically leached. Therefore, the finer fractions likely include some weathering products. Because many state DOT testing methods use the smallest field fractions that contain these weathering products, the results can be biased and may not reflect the chemistry of the bulk rock.