An Automated Simple Shear Apparatus for Testing Asphalt Concrete and Large Aggregates
by
Muniram Budhu, Professor, Department of Civil Engineering & Engineering Mechanics, University of Arizona, Tucson, AZ 85721

Abstract

In this contribution, a new simple shear apparatus for testing asphalt concrete and large aggregates is presented. This apparatus subjects a cuboidal sample of plan area 15.24 cm. (6 in.) x 15.24 cm (6 in.) and heights up to 7.62 cm. (3 in.) to simple shear deformation. The test specimen is enclosed by a rigid rectangular metal box that could be easily dismantled and reassembled for positioning field samples. The top and bottom of the specimen can be independently heated to simulate thermal conditions in the field. The horizontal displacement is applied by a stepper motor controlled by a personal computer. The shear load is monitored by a load cell at the base of the sample and the vertical load is applied by a pneumatic cylinder which is monitored through a load cell. The shear and vertical load cells are wired into a closed loop loading system. Tests can be conducted under various shear loading and/or vertical loading sequence. For example, the vertical load can be cycled in phase or out of phase with a cyclic or monotonic shear load. The specimens could be tested as constant volume (constant height) or dilation could be allowed to take place (constant load). Load cells are embedded into the lateral sides to measure the lateral stresses x and the intermediate principal stress 2. The complete stress state of a specimen can be determined. Specimen compaction is achieved in the apparatus by using a rolling device mounted on the apparatus. The base of the apparatus is moved in a cyclic manner allowing the roller to roll the specimen similar to field compaction. During compaction, the amount of compaction (vertical displacement), the vertical load, shear load, number of cycles of load application and the lateral stresses are monitored. The results of a series of tests on compacted asphalt concrete mixtures are presented.
Sample size: 15.24 cm x 15.24 cm x 0.5 to 7.62 cm high.
Compaction: Hot mix is compacted in the apparatus using a rolling device. During compaction, the amount of compaction, the vertical load, shear load and the vertical displacement are continuously recorded by a computer.
Loads: Pneumatic - cyclically varied up to 1750 kPa.
Cyclic horizontal displacement applied by stepper motor.
Temperature: Temperature at the bottom and top of the specimen can be controlled independently.
Research Determination of residual simple shear modulus for use in mechanistic models. Investigation of the behavior of asphalt concrete under differential temperature. Determination of the properties of asphaltic concrete under stresses similar to those in the field Study of rolling compaction to maximize field compaction from commercial compactors Study the effects of rolling compaction on permanent deformation (shakedown)