Performance of Three Aggregates on All-weather Roads

About the report:

Because the Virginia Department of Highways and Transportation had received complaints from the public and Department maintenance personnel concerning the durability of slate used on all-weather surface roadways, a field study was conducted in which the performance of the slate was compared to that of two other aggregates. For the study, two test sites were constructed using granite, slate, and greenstone. Prior to being placed on the road, the aggregates were tested for California Bearing Ratio (CBR), gradation, and Atterberg limits. While in service, the aggregates were sampled and tested for gradation, Atterberg limits, thickness of the layer, and skid resistance. Sampling of the aggregates in service was done on a 2-week schedule from week 3 through week 9, on a 4-week schedule from week 14 to week 22, and then at weeks 50 and 60. The composition of the Arvonia slate is very high in muscovite, a flaky mineral that is particularly susceptible to winnowing and somewhat self-lubricating. The slate had a much lower CBR than either the granite or greenstone, and this value did not improve when the slate was not soaked before testing. This absence of improvement was attributed to the lubricating effect of muscovite. The gradations of the aggregate samples obtained over time exhibited some variability that was attributed to variations in the sampling of the coarse fractions of the granite and greenstone. Also, it was observed that the slate disintegrated at a much faster rate during the first 5 weeks of service than did the granite or greenstone. Such early wear is extremely important because, normally, very light applications of crusher run are made to all-weather surface roadways, and it is necessary that the larger sizes of aggregate maintain their integrity so they can be reused when the roadway surface is reshaped. Two changes in the specifications for crusher run and a change in policy were recommended.

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