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Many rock formations in the State of Washington, particularly Eocene basalts, have undergone so much weathering that the interstitial basaltic glass has been altered to a magnesium-saturated montmorillonite clay. When the altered rock is used in highway construction as base or surfacing material, the secondary clay minerals, which have a great affinity for water, break down into plastic fines which gradually permeate the entire rock mass. This degradation is brought about by the action of water and weathering plus the abrasion of rock-against-rock as the entire mass is flexed under moving loads.
When the plastic fines permeate the base course, the latter loses stability rapidly and pavement failure soon follows. Failure is characterized first by longitudinal cracks in the wheel path followed by “alligatoring” and finally by breaking of the overlying flexible pavement into separate and distinct pieces.
Many of the altered rocks pass all conventional laboratory quality tests. The paper proposes a laboratory test to detect detrimental degradation. The test is simple and rapid. Results on more than 100 aggregate sources indicate a high degree of correlation between the degradation test values and observed service records.
The degradation test consists of abrading a saturated rock sample for 1 hr in a 1-gal jar. After abrasion, the quantity of manufactured fines is measured and the “cleanness value” of the fines determined. The quantity and quality of the manufactured fines form the basis for computation of the “degradation factor.” The degradation factor ranges from 0 to 100 with good materials having high values.
Materials and Research Engineer, Washington State Highway Commission, Olympia, Wash.