Chief, Petrographic Section, U.S. Army Corps of Engineers, Missouri River Division Laboratory, Omaha, NE
Evaluation of ASTM Tests for Potential Reactivity of Chemical Aggregates (Chemical Method) (C 289), Potential Alkali Reactivity Cement-Aggregate Combination (Mortar-Bar Method) (C 227), and Petrographic Examination of Aggregates for Concrete (C 295) for predicting alkali-silica reactivity potential of aggregates was undertaken using a natural siliceous sand and a manufactured limestone sand to which 2, 5, 10, and 20% by weight of reactive materials (chert and opaline sandstone) were added. Three aggregate gradings were tried in the mortar-bar test. None of the test methods by themselves are adequate to predict alkalisilica reactivity potential; furthermore, the chemical test did not correlate well with the limit of mortar-bar expansion at an age of six months as indicative of reactive aggregate. There is a linear trend between the amount of reactive material and the amount of soluble silica by weight added to the sands and reduction of alkalinity determined by the chemical test. A similar linear relationship is shown between the amount of soluble silica and the amount of reactive material in the aggregate as determined by petrographic examination. The pessimum proportion of the reactive material was found to be around 10% by weight. Coarser graded aggregate tended to produce greater expansion, but this was not definitely established. Mortar-bar expansion tends to be highly variable with increase of amount of reactive material over 2%. The test data indicates that the limit of mortar-bar expansion of 0.1% at six months does not adequately define potentially reactive material. It is suggested that the limit should be lowered to 0.075% at six months. Because of the effect of grading and the variability of expansion, consideration should be given to replacing the mortar-bar test with a concrete-prism test. Limestone aggregate appears to have an inhibiting effect on alkali-silica reactivity, therefore, adding limestone to reactive aggregates may be a practical way to make them usable in concrete. Further testing should be conducted to establish the minimum amount of limestone required to effectively inhibit reactivity.
Paper ID: CCA10251J