Asphalt Mixture Performance Tests

CHAPTER 2. LITERATURE REVIEW

2.3 Asphalt Mixture Performance Tests

2.2.3 Binder Oxidation Process

The oxidation process in the asphalt binder affects its physical and chemical properties by making the binder harder and stiffer [16]. The oxidation process allows carbonyl (- C=O) groups to be formed, which in turn, increases the polarity of the compounds and causes these to

associate with other polar compounds [17]. As a result, the association of the carbonyl groups with other polar compounds produces less soluble asphaltene materials, which result in asphalt hardening [17].

The reduction in the pavement performance results in the impact of the oxidation process on the binder, as the binder becomes stiffer and more brittle [18]. The viscosity and elastic properties of the binders are affected, due to the changes in the binder composition that occur during the oxidation process [19]. The high elastic stiffness that results from the oxidation process subsequently yields to a material that cannot relieve the stress, which results in a material displaying a high susceptibility of fatigue and thermal cracking.

[20]. The absolute value of the complex modulus is defined as the dynamic modulus, |E*|. The dynamic modulus is mathematically defined as the ratio between the peak dynamic stress (σo) and the peak recoverable strain (ε^o).

|𝐸 ∗ |=^σo

εo (2.2)

A dynamic modulus tests is usually conducted at -10, 4, 20, 38.8 and 54.4°C at loading frequencies of 0.1, 0.5, 1.0, 5, 10, 25 Hz at each temperature [21].

Figure 0.3. Stress-Strain Relationship under Sinusoidal Load 2.3.2 Loaded Wheel Tracking (LWT) Test

The Loaded Wheel Tracking (LWT) test is performed to evaluate HMA mixtures at high-

temperature to assess the rutting susceptibility due to traffic loading. The LWT test is performed, based on the standard AASHTO T324-04, “Standard Method of Test for Hamburg Wheel-

Tracking Testing of Compacted Hot-Mix asphalt (HMA)” [22]. The test consists of submerging two HMA cylindrical specimens under hot water with a temperature of 50°C. The specimens are subject to a steel wheel weighing 703 N (158 pounds), which repeatedly rolls across its surface.

The test is complete when the specimens have been subjected either to a maximum of 20,000

cycles or attained a 20 mm deformation, whichever is first reached. Upon completion of the test, the average rut depth for the tested samples is recorded. Figure 2.4 shows a typical LWT test.

Figure 0.4. Loaded Wheel-Tracking (LWT) Test 2.3.3 Semi-Circular Bending (SCB) Test

The Semi-Circular Bending (SCB) test is performed to evaluate HMA mixtures at an

intermediate-temperature to assess the cracking resistance of an asphalt mixture. The fracture resistance of an HMA is evaluated based on the fracture mechanics principles, described by the critical strain energy release rate, also called the critical value of J-integral (Jc) [23]. The mathematically formula to describe the critical strain energy release rate is the following:

𝐽_𝑐 = − (¹

𝑏)^dU

da (2.3) Where:

Jc= critical strain energy release rate (kJ/m2);

b = sample thickness (mm);

a = notch depth (mm);

U = strain energy to failure (N·mm); and

dU/da = change of strain energy with notch depth

Semi-circular specimens with notch depths of 25.4 mm, 31.8 mm, and 38 mm are used to determine the critical value of J-integral, using Equation 2.3 by calculating the change of strain energy with each notch depth (dU/da). A three-point bending load configuration is utilized to apply a monotonical load under a constant cross-head deformation rate of 0.5 mm/min, as shown in Figure 2.5. The load and deformation are continuously recorded and the test is performed at a temperature of 25°C. The strain energy to failure (U) is represented by the area under the loading portion of the load deflection curves, up to the maximum load measured for each notch depth. The average values of U are then plotted versus the different notch depths to compute a regression line slope, which gives the value of (dU/da). Finally, the Jc is computed by dividing dU/da value by the specimen thickness.

The semi-circular specimens are compacted in a Superpave Gyratory Compactor (SGC) to an air void level of 7 ± 0.5%. The compacted samples are aged in accordance with AASHTO PP2 by placing compacted specimens in a forced draft oven for five days at 85°C [24]. Three specimens per notch depth are utilized to control the variation in the critical strain energy release rate values (Jc). High Jc values are desirable for fracture-resistant mixtures.

Figure 0.5. Semi-Circular Bending (SCB) Test

2.3.4 Thermal Stress Restrained Specimen Tensile Strength (TSRST) Test

The low-temperature cracking susceptibility of HMA mixtures is evaluated by performing a Thermal Stress Restrained Specimen Tensile Strength (TSRST) Test based on the standard AASHTO TP 10-93 [25]. The test consists in cooling an HMA specimen at a constant rate while being constrained from contraction. The TSRST determines the tensile strength and temperature at fracture, which can be used to evaluate the susceptibility of an HMA mixture against low- temperature cracking.

The TSRST test uses rectangular specimens with dimensions of 50 ± 5 mm (2.0 ± 0.15 in.) square and 250 ± 5 mm (10.0 ± 0.25 in.) in length, which are manufactured by sawing each test specimen from a rectangular HMA slab of dimensions 260.8 mm (10.25 inches) wide, by 320.3 mm (12.5 inches) long, by 50mm (2 inches). The prepared beams are affixed at each end to platens of the test machine, and enclosed in an environmental chamber for conditioning.

Afterwards, a tensile load of 50 ± 5 N (10 ± 1 lbs.) is applied to the HMA beam specimen, and the specimen is cooled at the rate of 10.0 ± 1°C per hour until tensile fracture occurs. The thermal contraction along the long axis of the specimen is monitored electronically. A typical setup for a TSRST test is shown in Figure 2.6.

The fracture stress of the HMA beam specimen is calculated with the following expression:

𝐹𝑟𝑎𝑐𝑡𝑢𝑟𝑒 𝑆𝑡𝑟𝑒𝑠𝑠=^{𝑃𝑢𝑙𝑡}

A (2.4) Where:

Pult= ultimate tensile load at fracture (pounds); and

A= average cross-sectional area of beam specimen (mm2)

Figure 0.6. Thermal Stress Restrained Specimen Tensile (TSRST) Test