Asphalt Pavement Technology
Bituminous Mixtures Laboratory (BML)
Equipment Temperature Stress Restrained Specimen Test
Figure1: TSRST environmental cabinet with specimen |
Evaluates the low temperature cracking susceptibility of asphalt paving mixtures. The device cools down a beam specimen while restraining it from contracting. As the temperature drops, thermal stresses build up until the specimen fractures. (AASHTO TP10)
The Thermal Stress Restrained Specimen Test (TSRST) is used to determine the low-temperature cracking susceptibility of asphalt concrete. It is an automated, closed-loop system designed to measure the tensile stress in a specimen that is cooled at a constant rate while being restrained from contracting. It also provides the temperature at which fracture occurs. The TSRST, shown in figures 1 and 2, was developed as part of the Strategic Highway Research Program and costs $42,500. It consists of a specimen alignment stand, load/displacement system, temperature control system, and data acquisition system.
Specimens for the TSRST are cored or sawed from slabs. Cores have a length of 250 mm and a diameter of 60 mm. Alternatively, beams with a length of 250 mm and a width and depth of 50 mm can be tested. The specimens are glued to two aluminum platens prior to the test using the specimen alignment stand.
The load/displacement system includes a load frame, step motor, load cell, two linear variable differential transformers (LVDT's), and two swivel connectors. The load frame consists of the two aluminum platens glued to the ends of the specimen and spring-loaded rods that connect them. These rods minimize the stresses in the specimen due to the pull of gravity. The step motor is attached to the top platen through a swivel connector that maintains alignment and guarantees that there will be no eccentricity in loading. The load cell is attached to the bottom platen, also through a swivel connector. The step motor keeps the specimen at a constant length during the test. It is controlled by a computer and operates in response to changes in electrical signals from the two LVDT's located on opposites sides of the specimen. The LVDT's are attached to the top platen and react against two InvarTM rods attached to the bottom platen. When the LVDT's measure a reduction in specimen length of 0.0025 mm due to cooling, the step motor pulls the specimen back to its original length. Tensile stresses are induced in the specimen because it is restrained from contracting. The load cell is used to monitor these stresses versus temperature and time.
Figure 2: Instrumented specimen |
The temperature control system consists of a liquid nitrogen tank, environmental chamber, a temperature controller, and a resistance temperature device. The cooling process is performed by vaporizing compressed liquid nitrogen into the environmental chamber through a solenoid valve. The cool air is circulated with a fan so that the temperature distribution is uniform. The resistance temperature device connected to the temperature controller monitors the temperature in the environmental chamber and regulates the amount of liquid nitrogen required to reach or maintain a specified temperature. Four thermistors are attached to the specimen at different locations to monitor the surface temperature.
The data acquisition system consists of transducer signal conditioners, a data acquisition and control unit, and a personal computer. All electrical signals from the load cell, LVDT's, resistance temperature device, and the four thermistors are processed through this system and stored in the computer for analysis.
The TSRST procedure is given in the American Association of State Highway and Transportation Officials Method TP10, "Test Method for Thermal Stress Restrained Specimen Test." The test requires approximately 6 hours to perform.
Overview | Equipment | BML Publications
