Indirect Tensile Tester (IDT)

The Indirect Tensile Test (IDT) has the ability to test as asphalt sample under three different loading modes: 1) Creep Compliance, 2) Diametric Resilient Modulus, and 3) Tensile Strength. (Figure 8)

Figure 8 – Indirect Tensile Test Machine with Environmental Chamber Ready to Perform the Creep Compliance Test

Creep Compliance

A static load of fixed magnitude is applied along the diametral axis of a preconditioned specimen for a fixed duration of time, 1,000 seconds. The horizontal and vertical deformations measured near the center of the specimen are used to calculate a tensile compliance at a particular duration of time. Loads are selected to keep strains in the linear
viscoelastic range (typically below 300 microstrains). By measuring both horizontal and vertical strains in regions where the stresses are relatively constant and away from the localized non-linear effects induced by the steel loading strips, the Poisson's ratio can be more accurately determined. The Poisson's ratio is then used to better predict three dimensional stresses and strains. The failure portion of the test immediately follows the creep portion. Without releasing the creep load, a constant rate of vertical deformation (or ram movement) is applied to the specimen to failure. The failure limits determined can be input into mechanistic-based models for thermal cracking or used to compare relative qualities of mixtures. The failure portion of the test method can be performed separately when only the tensile strength properties are needed.

The values of creep compliance can be used to evaluate the relative quality of materials, as well as to generate input for pavement design and evaluation models. From creep testing, these values include the intercept and straight line slope of log creep compliance versus log loading time, or the entire master-compliance curve. The test can be used to study effects of temperature, load magnitude, binder content, and loading time. When used in conjunction with other mixture physical properties, the creep compliance may contribute to the overall mixture characterization. It is one factor for determining a mixture's suitability for use as a highway paving material under given traffic and environmental conditions.

Diametric Resilient Modulus

The asphalt sample is subjected to a haversine wave shaped load pulse over a range of load durations, load levels, and rest periods. The test series consists of testing at 5, 25, and 40oC. Each stress levels to be applied to the specimens at each of the testing temperatures are determined from the Indirect Tensile Strength Test. The repeated cyclic stress has a load duration of 0.1 seconds, and a cycle duration of 1.0 seconds. The instantaneous and total resilient (recoverable) vertical and horizontal deformation responses of the specimen are measured and used to calculate both an instantaneous and total resilient modulus. Values of resilient Poisson’s ratio are calculated using the measured recoverable vertical and horizontal deformations. The resilient modulus values are subsequently calculated using the calculated Poisson’s ratio.

The value of the resilient modulus determined from this test method is a measure of the elastic modulus of the HMAC materials recognizing certain nonlinear characteristics. Resilient modulus values can be used with structural response analysis models to calculate the pavement structural response to wheel loads, and with pavement design procedures to design pavement structures.

Indirect Tensile Strength

The Indirect Tensile Test (IDT) is conducted on HMA samples to attaint the tensile stress characteristics of the mix. The results from the IDT can be used in the Superpave predictive models to determine fatigue and low temperature cracking. The test procedure is also used to determine the moisture sensitivity of HMA after the volumetric design is completed (AASHTO T283).

The IDT test consists of applying load at a constant rate of 12.5 mm or 50mm/minute. Testing for fatigue cracking is done at a rate of 50 mm/minute at test temperatures of -10, 4, and 20oC. Testing for low temperature cracking is conducted at a rate of 12.5 mm/minute at test temperatures of -20, -10, and 0oC.