Resilient Moduli of Granular Base Materials Using a Modified Type 5 Gradation

David Newton Richardson, Missouri University of Science and Technology
S. M. Lusher

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The Missouri Department of Transportation (MoDOT) in collaboration with the Missouri Limestone Producers Association (MLPA) was interested in determining what effect a change in the Type 5 aggregate base gradation specification would have on the resilient modulus (Mr) of said aggregate. The proposed change would lower the minimum allowable total percentage of material passing the #4 (4.75 mm mesh) sieve from 35% to 25%, and the #30 (0.600 mm mesh) sieve from 10% to 5%. The remainder of the gradation specification would remain unchanged. The rationale for this proposed change is that some aggregate producers believe the change could help lower their costs of producing a Type 5 aggregate base material. To investigate the proposed gradation specification change, an experimental gradation was devised which followed the lower bounds of the proposed gradation specification on the #4, #30, and #200 sieves, and approximated the as-delivered gradations of two aggregate formations previously tested for MoDOT on the 3/8, ½, ¾, and 1 inch sieves, making it a relatively open-graded material. Thus, two different aggregate sources were tested. In the previous study for MoDOT, two gradations were analyzed: as-delivered Type 5 materials, and gradations with elevated fines contents. Both gradations could be considered to be high-fines content materials, with minus #200 contents between 11 and 18%. As a result of changing the gradation to fit the lower proposed specification limits, the experimental gradation in the present study contained no minus #200 material, and had significantly more #4 retained material, but less #8 retained material. The resulting Mr values in this study were greater than the results from the previous study for the same aggregates. Besides a change in gradation, the degrees of saturation for the proposed, more open-graded gradation were significantly lower than seen in the previous study for the same aggregate types.