2.1 Asphalt binder characterization and tests at low temperatures

It has been shown that the characteristic of the asphalt binder can significantly impact the low temperature behavior of asphalt pavement. During the service life of the pavement, asphalt is exposed to low temperatures, which tend to alter the rheological behavior. Different studies have concluded that asphalt binder behavior is the dominant component for low-temperature performance of the asphalt pavement mixtures [30]. Therefore, it is very important to study the asphalt binder characterize at low temperatures to have a clear picture of factors that affect the low temperature behavior of pavement.

Based on the concept that asphalt binder properties play the major role in cracking, several studies have focused on investigating the effect of rheological and other parameters of asphalt binder on low temperature performance. A quantitative method is necessary in order to study the complex role of asphalt binder in the pavement and to relate its properties to low temperature cracking phenomenon.

It is believed that thermal stresses causing cracking are due to constrained thermal strains. When the temperature drops, the pavement tends to contract its volume, following its thermal expansion/contraction coefficient. However, the layer underneath opposes some resistance due to friction, therefore thermal strains within the asphalt layer are not free to take place leading to co-active stresses proportional to the stiffness of the material. Since asphalt is a viscoelastic material, part of said stresses are dissipated through relaxation, but eventually they build up until they reach the strength of the material, leading to the formation of cracks to relieve these stresses [31].

A number of methods have been introduced throughout the years to model the viscoelastic behavior of asphalt binders and mixtures to estimate the accumulation of thermal stress during cooling cycles and predict the temperature at which cracking occurs. Shoor et al. [32] studied the penetration measurement to investigate the low temperatures behavior of asphalt binder. Majidzadeh and Schweyer [33] investigated asphalt low temperature properties using more fundamental approach using viscoelastic models. They studied few asphalt binders' behavior at the temperature range of 9–5°C using cylindrical specimens. At the lowest temperatures, the asphalt binders exhibited some instantaneous elastic deformation. Pink et al. [34] used a Rheometric Mechanical Spectrometer (RMS) to make accurate lowtemperature viscoelastic measurements on asphalt down to 94°C. They developed a methodology to construct a dynamic master curve to separate the effect of time and temperature. Button et al., also used the RMS to measure viscosity of asphalts from 0 to 46°C. Thus, most of the predicting low-temperature cracking methods involve measured asphalt stiffness, predicted asphalt stiffness, or consistency and temperature susceptibility parameters that indirectly establish asphalt stiffness [35].

During the SHRP project, several studies have focused on developing a new device for measuring low temperature stiffness of binders. Finally, these studies led to the development a device to determine the properties and response of asphalt binders at low temperatures in the 1980's. This device was later modified and updated as part of the SHRP binder research [36]. The resulting machine was named the Bending Beam Rheometer (BBR).

## 2.2 Bending beam rheometer (BBR) test

The data acquisition system of the BBR records the load and deflection test results and calculates two parameters: (1) Creep Stiffness, S(t), which is a measure of how the asphalt binder resists the creep loading, and (2) m-value, which is

a measure of the rate at which the creep stiffness changes with loading time (Figure 2).

Thermo-mechanical properties of the asphalt samples can be measured using the Bending Beam Rheometer to evaluate the low temperature properties based on the ASTM D6648 and AASHTO T 313 [37, 38]. The stiffness, S(t), is a measure of the thermal stresses developed in the asphalt pavements as result of thermal contraction. Classic beam analysis theory is used to calculate the creep stiffness of the asphalt binder beam at 60 seconds loading time [5]. The BBR loads the beams for 240 seconds and report the stiffness values at loading times of 8, 15, 30, 60, 120 and 240 seconds. These values were chosen because they are fairly equally spaced on a logarithmic time scale. These data points, along with the following equation, are used to determine the shape of the stiffness (creep compliance) master curve for the asphalt binder (Eq. (10)).

$$\mathbf{S(t) = A + B \log(t) + C \left[ \log(t) \right]^2} \tag{10}$$

as the slope of the asphalt binder stiffness curve flattens, the ability of the asphalt pavement to relive thermal stresses by flow decreases. This again would increase the

Federal Highway Administration (FHWA)/Engineering and Software Consultants,

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

propensity of thermal cracking in the pavement.

BBR data analysis: (a) stiffness; (b) m-value.

Asphalt Material Creep Behavior

DOI: http://dx.doi.org/10.5772/intechopen.85783

\*Address all correspondence to: amir.golalipour.ctr@dot.gov

provided the original work is properly cited.

Author details

Figure 3.

Amir Golalipour

Inc., McLean, VA

29

where S(t) = asphalt binder stiffness, T = time (s), A, B and C = constants.

The slope of the stiffness curve, m, is a measure of the rate of stress relaxation by asphalt binder flow. The m-value indicates the rate of change of the stiffness, S(t), with loading time. In other words, the m-value is the slope of the log creep stiffness versus log time curve at any time. Since the time dependency of asphalt binder varies, the shape of the stiffness master curve as well as the stiffness at 2 hours loading time are important to take into consideration. Therefore, the slope of the stiffness master curve is also used for specification purposes [39].

In the current PG specification, two parameters from BBR test are used to characterize the binder low temperature rheological behavior. Apparent stiffness, S, and the slope of the log stiffness versus log time, the m-value are determined at a loading time of 60 seconds. The temperature at which S(60) ≤ 300 MPa and m (60) ≥ 0.3 is specified as the critical temperature + 10°C (Figure 3). These limits were established based on data from previous studies as well as the data obtained by SHRP (Bahia and [6]).

The effect of these two specification parameters, S(t) and m-value, on thermal cracking is analogous to the effect of G\* and δ on rutting and fatigue cracking. As S(t) increases, the thermal stresses developed in the pavement due to thermal shrinking also increase, and thermal cracking becomes more likely. On the other hand, as the m-value decreases, so does the rate of stress relaxation. In other words,

Figure 2. BBR test principles.

Asphalt Material Creep Behavior DOI: http://dx.doi.org/10.5772/intechopen.85783

a measure of the rate at which the creep stiffness changes with loading time

Thermo-mechanical properties of the asphalt samples can be measured using the Bending Beam Rheometer to evaluate the low temperature properties based on the ASTM D6648 and AASHTO T 313 [37, 38]. The stiffness, S(t), is a measure of the thermal stresses developed in the asphalt pavements as result of thermal contraction. Classic beam analysis theory is used to calculate the creep stiffness of the asphalt binder beam at 60 seconds loading time [5]. The BBR loads the beams for 240 seconds and report the stiffness values at loading times of 8, 15, 30, 60, 120 and 240 seconds. These values were chosen because they are fairly equally spaced on a logarithmic time scale. These data points, along with the following equation, are used to determine the shape of the stiffness (creep compliance) master curve for the

where S(t) = asphalt binder stiffness, T = time (s), A, B and C = constants. The slope of the stiffness curve, m, is a measure of the rate of stress relaxation by asphalt binder flow. The m-value indicates the rate of change of the stiffness, S(t), with loading time. In other words, the m-value is the slope of the log creep stiffness versus log time curve at any time. Since the time dependency of asphalt binder varies, the shape of the stiffness master curve as well as the stiffness at 2 hours loading time are important to take into consideration. Therefore, the slope of the

In the current PG specification, two parameters from BBR test are used to characterize the binder low temperature rheological behavior. Apparent stiffness, S, and the slope of the log stiffness versus log time, the m-value are determined at a loading time of 60 seconds. The temperature at which S(60) ≤ 300 MPa and m (60) ≥ 0.3 is specified as the critical temperature + 10°C (Figure 3). These limits were established based on data from previous studies as well as the data obtained by

The effect of these two specification parameters, S(t) and m-value, on thermal cracking is analogous to the effect of G\* and δ on rutting and fatigue cracking. As S(t) increases, the thermal stresses developed in the pavement due to thermal shrinking also increase, and thermal cracking becomes more likely. On the other hand, as the m-value decreases, so does the rate of stress relaxation. In other words,

stiffness master curve is also used for specification purposes [39].

S tðÞ¼ <sup>A</sup> <sup>þ</sup> B log tð Þþ C log t ½ � ð Þ <sup>2</sup> (10)

(Figure 2).

asphalt binder (Eq. (10)).

Creep Characteristics of Engineering Materials

SHRP (Bahia and [6]).

Figure 2. BBR test principles.

28

Figure 3. BBR data analysis: (a) stiffness; (b) m-value.

as the slope of the asphalt binder stiffness curve flattens, the ability of the asphalt pavement to relive thermal stresses by flow decreases. This again would increase the propensity of thermal cracking in the pavement.
