Investigation of the Effect of Fines on Base Course Performance

Crushed aggregate which met the gradation requirements of Arkansas Class 7 base course was sampled from five quarries in the state of Arkansas. The material taken from these quarries had a wide variety of mineral composition (limestone, dolomite, sandstone, syenite, novaculite) which represents a large portion of the mineral types used as Class 7 base material in the State of Arkansas. Three to five-ton samples from each quarry were fractionated and subsequently reblended in smaller batches so that the resulting gradations replicated the average of the historical gradation from each quarry, with the exception that the fines content (minus-#200 material) was varied from 6 to 16 percent in 2 percent increments. Replicate specimens for hydraulic conductivity testing and triaxial strength testing and suction testing were created for each of the six new gradations plus gradations at the upper and lower boundaries of the AHTD specification. Specimens were compacted in 6 inch diameter molds at optimum moisture content to 98 percent of maximum dry density, as determined by AASHTO T-180 method D. Hydraulic conductivity was measured on 6 inch diameter by 4 or 4.625 inch high specimens using both constant and falling head methods at relatively low gradients. Shearing strength was measured on 6 inch diameter by 12 inch high specimens using consolidated-drained triaxial testing at confining pressures of 5, 10 and 20 psi. The results of testing showed that the hydraulic conductivity of all specimens was so low that none of the base materials could be considered drainable at any of the tested fines contents. In addition, the decrease in hydraulic conductivity for fines contents of 10 to 16 percent was essentially negligible. The results of strength testing showed that the strength and modulus of these materials actually increased as the percent of fines increased from 8 percent up to 12 percent. Beyond 14 percent fines there was a slight decrease in both strength and modulus for three of the five quarries tested. As a result of this testing, recommendations were made to the Arkansas Highway and Transportation Department to increase the percentage of allowable non-plastic fines from 10 to 12 percent for Class 7 base course materials..