Evaluation of The Laboratory Asphalt Stability Test

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1. INTRODUCTION

The National Cooperative Highway Research Program (NCHRP) Project 9-10, "Protocols for Modified Asphalt Binders," was initiated to determine whether the binder and mixture test methods of Superpave®, an asphalt-aggregate mixture design and analysis system developed under the Strategic Highway Research Program (SHRP), are as effective for modified binders as they are for the unmodified binders for which they were initially developed. One of the tasks (Task 4SS) focused on measuring special characteristics of modified binders and developing specific testing protocols that are not part of the current Superpave testing protocols. Because most modified binders are multiphase systems, one of the subtasks involved assessing the storage stability of the modified asphalts by evaluating the possible phase separation under static and agitated conditions. Consequently, the Laboratory Asphalt Stability Test (LAST) was developed to address one of the deficiencies of the current testing protocols. ⁽¹⁾

Based on the review of research done in the past (see "Additional Sources") and on evaluation of storage conditions in typical storage tanks in actual practice, the NCHRP 9-10 researchers concluded that the new test for storage stability should incorporate an evaluation of the following effects:⁽¹⁾

• Extended storage at high temperatures in the range of 160°C to 180 °C.
• Mechanical agitation
• Storage conditions on performance-related properties measured at multiple temperatures and frequencies in the dynamic shear rheometer.

This device features an internal heating element controlled by an electronic temperature-control feedback system to maintain isothermal conditions and a constant speed, double-propeller agitator centered in the middle of the cylindrical container. The dimensions are such that a sample of 400 milliliters (ml) is used, and the sampling is done periodically using a pipette from the top and bottom of the container without interrupting the conditioning process. The thermal stability is determined by measuring the rheological properties of binders using the Dynamic Shear Rheometer (DSR)-selected conditions before and after conditioning in the LAST device.

Table 1. Dimmensional details of the LAST device.

* rpm = rotations per minute

Figure 1. Schematic diagram of the original prototype LAST device (1)

Changes in properties of samples extracted from the top and bottom of the LAST device as a function of time are used to determine the potential for phase separation or degradation in the field.

The Storage stability of the asphalt binders was evaluated using two ratios:

• A separation ratio, Rs, that is calculated by dividing the response (G*, d, or a combination of these parameters) of a sample taken from the top portion of the LAST device by the response of another sample taken from the bottom portion of the LAST device under the same conditions.

• A degradation ratio, Rd, that is calculated by dividing the average of the top and bottom responses (G*, , or a combination of these parameters) at a given sampling time by the initial response at time t=0 just before the start of the conditioning process in the LAST device.

The parameters investigated were thus,

(1)

(2)

(3)

(4)

The binder is considered to have a potential for separation and for degradation if the ratios are not within 0.8 and 1.2. The other parameters that were determined include Ksi, which is the separation rate, and K_si, which is the degradation rate. These are defined below.

(5)

Where T_cs is the critical time for separation.

(6)

Where T_ds is the critical time for degradation.

Federal Highway Administration (FHWA) was assigned the responsibility for evaluating the LAST device. This involved three tasks:

• Procuring a commercial version of the LAST device.
• Correcting perceived deficiencies in this device.
• Conducting a thorough evaluation of the test method.

This report details the work at FHWA's Turner-Fairbank Highway Research Center (TFHRC) in evaluation of the thermal stability of modified binders using the LAST device.