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Asphalt Pavement Technology
Bituminous Mixtures Laboratory (BML)
Equipment SUPERPAVE Shear Tester
Used to evaluate permanent deformation (rutting) and fatigue cracking susceptibilities in asphalt mixtures. It provides vertical and horizontal loads to a cylindrical specimen in various confining conditions and at different temperatures. The data is used in models that predict pavement performance. (AASHTO TP7)
The Superpave Shear Tester (SST) is a closed-loop feedback, servo-hydraulic system that can apply axial loads, shear loads, and confinement pressures to asphalt concrete specimens at controlled temperatures. The response of asphalt concrete to these loads can be used as inputs to performance prediction models such as Superpave.
The SST has six main components: testing chamber, test control system, environmental system, hydraulic system, air pressurization system, and measurement transducers.
The testing chamber includes a reaction frame and a shear table. It also houses various components that are driven by other systems such as temperature control, pressure control, hydraulic actuators, and input and output transducers. The reaction frame is extremely rigid so that precise specimen displacement measurements can be achieved without problems related to displacements in the frame itself, which is called machine compliance. The shear table holds a specimen during testing and is capable of applying shear loads. The specimens normally have a diameter of 150 mm and a height of 50 mm; however, specimens with diameters and heights up 200 mm can be tested with only minor modifications to the system. The temperature inside the testing chamber is precisely controlled by the environmental system. A diagram of the testing chamber is shown in figure 1.
The test control system consists of hardware and software. The hardware interfaces with the testing chamber through input and output transducers. It consists of controllers, signal conditioners, and a computer and its peripherals. The software consists of preprogrammed algorithms required to provide the load and acquire the data during testing. Closed-looped feedback control allows the SST to make adjustments during testing so that the machine precisely performs the programmed test.
The environmental system is a forced-air conditioning unit and an insulated enclosure surrounding the testing chamber. Temperatures from 0 to 70 1C can be provided. The unit can be controlled through the SST computer. The insulated enclosure keeps the temperature constant during testing.
The hydraulic system provides the loads. Interlaken Series 3410 hydraulic motors power two actuators, each with a capacity of approximately 32 kN. One actuator applies the vertical, axial force. A horizontal actuator moves the shear table, which applies the shear loads.
The air pressurization system consists of an air compressor, a storage tank for pressurized air, hoses, and filters. The air storage tank is needed because pressurized air must be supplied to the testing chamber at high rates. A rate of 70 kPa/s is used in some Superpave tests.
The measurement transducers consist of two load cells, one for the vertical axial load and one for the horizontal shear load, several linear variable differential transformers (LVDT's) with ranges from 0.05 to 50 mm, and a pressure cell with a range from 0 up to 1000 kPa. LVDT's mounted on the specimens measure the response of the specimen to the applied testing loads. Three types of LVDT's are employed: shear LVDT's that measure the relative deformation of the specimen at two heights; axial LVDT's that measure the relative motion of the upper and lower platens, and circumferential LVDT=s that measure changes in the diameter of the specimen. Any of these LVDT's can be used as feedback signals for the closed loop control. The system is designed to compensate for the effects of the moving masses of the machine on the load measurements during testing because these effects can be relatively high compared to the load measurements.
Six Superpave tests are preprogrammed in the SST:
These tests are described in detailed in AASHTO Provisional Standard TP 7. Other tests can be programmed using the QuikTestTM software provided with the SST.
The SST in the Bituminous Mixtures Laboratory, shown in figure 1, was purchased from Interlaken Technologies in 1995 at a cost of $230,000. Identical SST's were purchased for the five Superpave Centers located in the United States.