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Publication Number: FHWA-04-111
Date: February 2005

Evaluation of The Laboratory Asphalt Stability Test

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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)

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.

  Original Prototype Commericial Unit
Internal height of the container
166 mm 203 mm
Internal diameter of the container
62 mm 63.5 mm
Gross volume of the container
501 cm3 643 cm3
Diameter of the propellers
22 mm 35 mm
Propeller (1) location on the shaft
85 mm from bottom 19 mm from bottom
Propeller (2) location on the shaft
20 mm from bottom Not applicable
Baffle width
4 mm Not applicable
Amount of binder to be tested
450 g 450 g
Agitation speed
2000 rpm* 365 rpm*
Heating temperature for the test
165°C 165°C

* rpm = rotations per minute

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

The parameters investigated were thus,

  R subscript S G asterick equals G asterisk separation ration equals G asterisk subscript top over G asterick subscript bottom (1)


  R subscript S delta equals delta separation ratio equals delta subscript top over delta subscript bottom (2)


  R subscript d G asterick equals G asterick degradation ratio equals 0.5 (G asterick subscript top plus G asterick subscript bottom) over G asterick subscript initial (3)


  R subscript d delta equals delta degradation ratio equals 0.5 (delta subscript top  plus delta subscript bottom) over delta subscript initial (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 Ksi, which is the degradation rate. These are defined below.

  K subscript si equals G asterick / Sin delta (at T subscript cs)/G asterick/Sin delta subscript initial over T subscript cs (5)

Where Tcs is the critical time for separation.

  K subscript di equals G asterick / Sin delta (at T subscript cd)/G asterick/Sin delta subscript initial over T subscript cd (6)

Where Tds is the critical time for degradation.

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

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.


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