The Bending Beam Rheometer

Introduction

This page is meant to provide a summary of the current Bending Beam Rheometer (BBR) procedure, Figure 1, while providing explanations for specifications within the procedure. The BBR is used within the asphalt industry as a relatively inexpensive way to find the low temperature material characteristics of asphalt binders and asphalt mixes. A brief history of the BBR’s development is also provided.

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Figure 1- The Bending Beam Rheometer Version 1.2f

Background

Rheology is the study of the deformation and flow of a material. Asphalt has been characterized as a linear visco-elastic material meaning it behaves like both a viscous and an elastic material. “Although no material is perfectly linear under all conditions, linear visco-elastic characterization has been shown to be adequate for representing rheology of asphalt binders, in a large number of studies.” [1] Nearly a century ago, engineers began to recognize the link between low-temperature cracking in asphalt pavements and the asphalt binder’s rheological properties.

History and Development

In order to test these cold temperature properties, expensive dynamic material analyzers were borrowed from the polymer industry with limited applicability. In the 1970’s, two devices were made to test asphalt rheological properties at low temperatures. The Schweyer force capillary rheometer, Figure 2was not widely accepted because it was only capable of testing at over 0.0° C. The sliding plate rheometer, developed by Fenijin and Krooshof, ran into sample preparation and loading issues therefore was never used in industry. [1]

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Figure 2- Schematic of Schweyer Rheometer
In 1990 Pennsylvania State University research group led by D. Anderson introduced the BBR as we know it today. The Strategic Highway Research Program (SHRP) A-002A project introduced a completely computerized BBR and ran extensive testing to improve the procedure and result repeatability.

Test Procedure and Results


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Figure 3- BBR Three Point Loading (Photo taken outside of bath due for clarity)
In a BBR test, a sample is placed on the supports inside an alcohol bath at the desired low in-service temperature. A constant load is applied for four minutes and deformation is measured by an internal Linear Variable Differential Transformer (LVDT). The load is then removed; however, the deformation of the specimen is still recorded for another four minutes. The deformation is recorded for an additional four minutes to measure the elastic response on the specimen. Figure 4 shows the loading verses time plot for a typical BBR test. The example plot, shown below, utilized a constant load of 2400 mN.


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Figure 4- Load v. Time
As the constant load is applied the material will begin to deform. The total deformation will increase with the loading time. To help explain the variations within results three example results have been plotted together to show the effect of various mixes. The samples utilized for this comparison are Oil Sand Only, Oil Sand Asphalt, and Oil Sand Recycled Asphalt Pavement (RAP). The Oil Sand Only sample is made up of 100% Uintah Basin oil sand that was compacted at 220° F. The oil sand has an oil content of 13% oil. This sample was selected because it easily deforms even at room temperature. The Oil Sand RAP mix was selected because it is a relatively stiff mix and is a good example of a high end stiffness value. The Oil Sand Asphalt was selected to provide an intermediate value between the two extremes. Figure 5 is a plot of the deflection of each sample after 60 seconds of loading.


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Figure 5-Deflection at 60 Seconds at -12 °C
The total deformation at 60 seconds is used to calculate the estimated stiffness of the sample. The estimated stiffness at 60 seconds is reported automatically on the BBR results page. This value is the reported result of the test because of the initial development of the BBR. When the tests originated the data from asphalt testing showed that thermal cracking occurred with loading times from one to five hours. A two hour loading period was then selected, however two hours was impractical and the time-temperature superposition was used to enable a more practical loading time. “Testing showed that for most asphalt binders, if the test temperature were increased by 18°F (10°C) the BBR stiffness at 60 seconds loading time could be equated to the asphalt binder stiffness at 2 hours in the field at the low temperature specification. Therefore, the BBR test takes 60 seconds and is conducted at a temperature 18°F (10°C) higher than the low temperature specification.” [2]
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The asphalt industry has moved away from reporting data only at 60 seconds. In Superpave specifications the BBR is used to determine the creep stiffness as a function of time. The creep stiffness is then converted into a stress relaxation modulus and a predicted thermal stress is found. The predicted thermal stress is then compared to results of a Direct Tension Test on the same material and the binder grade is selected based off this comparison. [2]
The entire test results can be seen below in Figure 6. Notice there is a major increase in deformation and slope variations than the plot shown in Figure 5.


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Figure 6- Complete Test Results at -12° C
From the deformation time results the following plot, Figure 7, was made using the derivation in Equation 1 for maximum deflection at the center of a prismatic beam.
Equation 1- Derivation for Stiffness

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Figure 7-Creep Stiffness v. Time at -12° C
The BBR should then be run at different temperature to find the deformation-loading time relationship at three different temperatures. Once the creep stiffness verses time curves are found at multiple temperature the data is then combined into a master curve using shift factors.


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Figure 8- Stiffness Curves at Varying Temperatures

Sample Preparation

The size of the sample tested in the BBR should be ¼” thick x ½” wide and 5” long. The size of the sample was determined by the following criteria:
  1. Dimension should allow simple application of Bernoulli-Euler theory of bending.
  2. Minimize the amount of material required.
  3. Dimensions should be large enough such that experimental variability will be kept to a minimum.
  4. Small dimensions should not make the specimen fragile to handle.
  5. It should be large enough to allow load resolution but small to deflect.
Based off the above criteria and a support span of 4”, the dimensions were selected to have a span to depth ratio of 16/1 and a depth to width ratio of ½. [1]
Asphalt binder samples and asphalt mixture samples are prepared differently for testing in the BBR. Binder samples are made by heating the asphalt binder until it is viscous enough to flow. Once the binder is heated, it should be poured into the mold apparatus shown below in Figure 9. The aluminum pieces are held together by the rubber O-rings and the end pieces are made of polystyrene sheets. After the hot binder if poured into the mold, the entire mold and sample are cooled to the testing temperature and then the specimen is de-molded.

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Figure 9- BBR Asphalt Binder Mold
The use of the BBR to test asphalt mixture beams was recently submitted to the Journal of Road Material sand Pavement Design in 2010. This paper was submitted by C.H. Ho and Dr. Pedro Romero at the University of Utah. Asphalt mixture samples are prepared for the BBR through a regular laboratory mixing process and compaction in a gyratory compactor. The six inch diameter gyratory puck is the cut down to the 5” long x ¼” thick x ½” wide beam in a tile saw. The journal recommends a procedure to use the BBR for QC/QA on asphalt mixture beams. [3]

References


[1]
H. U. Bahia, D. A. Anderson and D. W. Christensen, "The Bending Beam Rheometer; A Simple Device for Measuring Low-Temperature Rheology of Asphalt Binders," Journal of the Association of Asphalt Paving Technologists, pp. 117-153, 1992.
[2]
P. Interactive, "Bending Beam Rheometer," 21 April 2011. [Online]. Available: http://www.pavementinteractive.org/article/bending-beam-rheometer/. [Accessed 16 April 2012].
[3]
P. Romero and C.-H. Ho, "Using Asphalt Mixture Beams in the Bending Beam Rheometer: Experimental and Numerical Approach," Journal of Road Materials and Pavement Design, p. 26, 2010.




Work created by
Michael Vrtis