Effect of Scalping on Shear Strength of Aggregate

The scalping of crushed rock produced in a quarry is required to produce aggregates of a range of sizes, for various applications such as use in road pavements and retaining walls. Large-scale (300 mm by 300 mm by up to 190 mm high) laboratory direct shear box tests were carried out on a crushed aggregate scalped to pass 75 mm, 37.5 mm, 19 mm, 9.5 mm, 4.75 mm and 2.36 mm, to study the effect of scalping on its shear strength. Scalped specimens were air-dried and loosely-placed in the shear box to represent loose-dumping of dry aggregate in the field, and were then subjected to single-stage testing under applied normal stresses of 250 kPa, 500 kPa or 1000 kPa. In general, the failure envelopes obtained tended to flatten with increasing degree of scalping (i.e., reducing maximum particle size). Both the inferred friction angle and the apparent cohesion tended to decrease with increasing degree of scalping. In addition, sieving analyses were carried out before and after the direct shear tests on the 75 mm, 37.5 mm, and 19 mm scalped specimens, to assess the degree of particle breakdown on loading and shearing. The results imply that small-scale laboratory testing of aggregates, necessitating scalped specimens, will generate conservative shear strength parameters.