Tests to Identify Poor Quality Coarse Limestone Aggregates and Acceptable Limits for Such Aggregates in Bituminous Mixes

Over the last few years the Texas Department of Transportation has expressed concern about mineralogical segregation (variation) of coarse aggregates used in bituminous mixes; problems are associated with variation in the quality of aggregates taken from a quarry/gravel pit. The primary objective of this project was to examine the effects of poor quality coarse limestone aggregate on hotmix asphalt performance and to determine how much of the poor quality limestone can be used before adversely affecting performance. A Type C aggregate composed of a high quality limestone from one quarry was blended with soft and absorptive limestone aggregates from two other quarries in different proportions using a PG 64-22 asphalt binder. The individual aggregates were run through Los Angles abrasion, Micro-Deval, magnesium sulfate soundness, specific gravity, and absorption tests. Molded bituminous samples were tested with the Hamburg wheel tracker, dynamic modulus, and the overlay tester. In order to obtain less than 10 percent marginal Texas coarse limestone aggregate, the Micro-Deval loss should not exceed 20 percent, and the magnesium sulfate soundness percent loss should not exceed 15. The introduction of marginal coarse limestone aggregate will lower the reflection cracking life of the bituminous mix, so a maximum of 10 percent marginal (soft and absorptive) coarse limestone aggregate is recommended.