Full-Scale Rutting Tests of Large-Stone Asphalt Mixtures

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Repetitive loaded-wheel tests were conducted on three large-stone mixtures: (a) a dense mixture designed using a new method that guarantees stone-on-stone contact, (b) a dense mixture designed to exhibit poor stone-on-stone contact, and (c) an open-graded mixture commonly used by the Indiana Department of Transportation (INDOT). All three mixtures had 38-mm (1.5-in.) nominal maximum size aggregate. Test pads 76-mm (3-in.) thick, 1.5-m (5-ft) wide, and 6.1-m (20-ft) long were constructed by a local contractor using a batch plant, a conventional paver, and a static steel wheel roller. No serious difficulties were experienced during placing or compacting the mixtures. The compacted surfaces were quite rough due to the large particle sizes. This rough texture apparently introduced significant variability in the density of the compacted test pads. The finer mixture designed to have poor stone-to-stone contact gave the highest density. Five thousand repetitions of a 4082-kg (9,000-lb) force were applied to each pavement through dual truck tires inflated to 620 kPa (90 psi). Cross-section profiles were measured periodically throughout testing. Rut depths measured in the mixture designed using the new design (Texas coarse) were significantly less than those in the other two mixtures. The mixture made purposely with poor stone-on-stone contact (Texas fine) exhibited the highest rut depths, which were similar to those measured in conventional binder coarses in the INDOT data base. This indicates that large stones floating in a matrix of conventional mix will have little effect on resisting rutting. The INDOT No. 2 open-graded mixture exhibited more rutting than the Texas coarse mixture but less than the Texas fine mixture. The use of this open-graded mixture as a surface coarse may have allowed unnatural displacement of large aggregate near the surface at the sides of the applied loads, which contributed to greater-than-expected rut depths.