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Development of a 3/4-inch Minus Base Course Type A Specification for Montana

Project #: 4W5279
Start Date: 12/29/2014
End Date: 12/31/2016
Status: Completed

The final report summarizes the work to develop a new specification, which included conducting a review of current ¾-inch minus specifications from around the U.S., using that
information to generate a preliminary specification to create ¾-inch minus mixes, testing the material properties of these mixes, and modifying these mixes to determine the effect changes in the gradation primarily had on its strength, stiffness and permeability. Based on the results of multiple statistical evaluations as well as qualitative
comparisons, it was concluded that a ¾-inch minus gradation specification will perform at least as well as Montana’s existing CBC-6A materials and better than CBC-5A materials. Gradation limits for a new ¾-inch minus, Grade 7A, crushed base course were suggested; however, the practicality of producing mixes that fit within the suggested gradation limits still needs to be determined.


Gravel bases are a critical component of roads, providing drainage, structural support, and load distribution to reduce pressures on subgrade soils. Montana specifications currently exist for a 2-inch-minus and 1½-inch-minus crushed base course. As gravel sources dwindle, particularly in the eastern regions of the state, a ¾ inch gravel base option is becoming more desirable. This project will develop a standard specification for a new gravel base course with a nominal maximum aggregate size of ¾ in.

The results of this research will include a draft specification for a ¾-in. crushed base course that will be consistent with MDT’s current specifications. The results will also include all the information required for a Major Revision, as shown in MDT’s standard specifications revision process.


The objective of the proposed project is to develop a standard specification for a new gravel base course with nominal maximum aggregate size of ¾”.