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Asphalt Pavement Technology
Bituminous Mixtures Laboratory (BML)
Equipment SUPERPAVE Indirect Tensile Test
Evaluates the thermal cracking susceptibility of asphalt mixtures. It loads a cylindrical asphalt concrete specimen through its diametrical axis, and the resulting deformations are used to determine the viscoelastic material properties used in models that predict pavement performance. (AASHTO TP9)
The Superpave Indirect Tensile Test (IDT) is used to determine the creep compliances and indirect tensile strengths of asphalt mixtures at low and intermediate pavement temperatures. These measurements can be used in performance prediction models, such as Superpave, to predict the low-temperature thermal cracking potential and intermediate-temperature fatigue cracking potential of asphalt pavements. The Superpave IDT, shown in figure 1, was developed under the Strategic Highway Research Program. The Superpave IDT in the FHWA Bituminous Mixtures Laboratory consists of a vertical loading device, specimen deformation measuring devices, an environmental chamber, and a data acquisition and control system. The equipment cost $125,000.
The loading device is a 100-kN capacity electromechanical test frame, Instron Corporation Model 5583. This type of load frame applies a load through a set of rotating screws that lower or raise a crosshead with an attached load cell. Unlike servohydraulic-based loading systems, this load frame has no high pressure hydraulic lines. This reduces noise, provides a safer system, and eliminates the need to cool the hydraulic fluid using an external medium such as water. However, due to the inertia of the crosshead, the time of loading in the Superpave creep compliance test is slower than for servo-hydraulic-based systems. This means that the two types of equipment could potentially provide slightly different test data. A load cell with a sensitivity of 5 N controls the load applied to the specimen.
The specimen deformation measuring device consists of four linear variable differential transformers (LVDT's) that are capable of measuring up to 0.25 mm. They have a resolution of ð"0.000125 mm. Two LVDT's are placed at right angles on each side of a specimen to measure both horizontal and vertical deformations. See figure 2.
The environmental chamber includes a digitally controlled refrigeration and heating system capable of achieving a temperature in the range of -30 to +30°C, and maintaining the required temperature within ð"0.2°C. The chamber is also capable of storing specimens prior to testing. It includes a viewing window, system for keeping the air dry, interior lights, and an interface for remote control.
The data acquisition and control system records and stores the loads and deformations during the test and provides closed-looped feedback control to the loading frame. Closed-looped feedback control allows the Superpave IDT to make adjustments so that it precisely performs the programmed test. Position, load, or strain can be used as feedback. The system operates under a Microsoft Windows environment with displays of data and real time plots.
The specimens tested by the Superpave IDT can be cored from pavements or laboratory slabs, or can be prepared in the laboratory using the Superpave Gyratory Compactor. The specimens must have smooth, flat, parallel faces, to which brass "buttons" are glued. These buttons have a diameter of 8 mm and a thickness of 3 mm. The LVDT's are attached to these buttons. The standard specimen has a diameter of 150 mm and a thickness of 50 mm.
Creep compliances and indirect tensile strengths at low temperatures, such as -20, -10 and 0°C, are used for thermal cracking analyses. Indirect tensile strengths at temperatures in the range of -10 to 30°C are used for fatigue cracking analyses. The testing sequence for the creep compliance test consists of cooling the specimen to the desired temperature, applying a compressive creep load for 100 s, and measuring the time dependent deformation using the LVDT's. Knowing the load and deformation as a function of time, the creep compliance of the material can be calculated. Creep compliance is the time dependent strain divided by the applied stress. The indirect tensile strength of the specimen for thermal cracking analyses is determined by applying a load at a rate of 12.5 mm per minute of crosshead movement until the load starts to decrease because of specimen failure. For fatigue cracking analyses, a faster rate of 50.0 mm per minute of crosshead movement is used. The test procedures are described in American Association of State Highway and Transportation Officials (AASHTO) provisional standard TP 9-95.