The average of the expansion of the aggregate at 16 days when tested in conformance with the requirements is less than or equal to ____________ percent.

• A. 0.04 percent
• B. 0.20 percent
• C. 0.10 percent
• D. 0.15 percent

Answer: (D)

This question tests the limit used to screen aggregates for alkali-aggregate reaction by measuring how much the aggregate expands in a standard test. When the aggregate is tested under the required procedure, the average expansion after 16 days must not exceed a specific maximum. That maximum is 0.15 percent, which is set to balance safety against excessive expansion with the practical variability of materials and testing. If the measured expansion reaches 0.15 percent or more, the aggregate is considered potentially reactive and may require using a non-reactive substitute or applying mitigation. The other numbers listed aren’t the standard allowable limit, since 0.15 percent is the established cutoff.