Mihai O. Marasteanu

Evaluation of fatigue criteria for asphalt binders

The original SuperPave asphalt binder specification criterion for fatigue, G* sin δ, has received considerable criticism. Recently, a time sweep using the dynamic shear rheometer (DSR) has been proposed as an alternative test method for developing load-associated fatigue information for asphalt binders. This proposed test method is examined with respect to a phenomenon called edge fracture. Edge fracture is reported in the literature for steady state and oscillatory flow in DSR, but it has not been reported for asphalt binders. The modulus, when plotted versus number of cycles generated in a time sweep test, has the appearance typical of fatigue behavior; however, the actual response of the material depends markedly on the initial modulus of the material. The development of the modulus with repeated shearing is described with respect to flow of the asphalt binder at its circumference. The data are examined with respect to their validity as a measure of fatigue, and recommendations with respect to the use of time sweep data in a binder specification are presented.

Low-Temperature Thermal Cracking of Asphalt Binders as Ranked by Strength and Fracture Properties

The original Superpave low-temperature specification for asphalt binders placed limits on the low-temperature stiffness and m-value. A recently approved alternative to the original Superpave asphalt binder low-temperature specification makes use of the measured stiffness and tensile strength of the binder to determine a critical cracking temperature. Thermal cracking temperatures are presented for 42 plain and modified asphalt binders. Thermal cracking temperatures determined by the original and recently approved alternative Superpave specification are compared. The fracture toughness, KIC, can also be used to evaluate low-temperature cracking properties of asphalt binders. Fracture properties obtained for 14 asphalt binders are compared with the thermal cracking temperatures determined by the original and recently adopted alternative Superpave specifications. The set of 14 binders was produced from a common base material but modified by different means. For the set of 14 binders, there is little difference in their ranking according to both the original and recently proposed alternative Superpave low-temperature criteria; however, their ranking is quite different on the basis of the fracture properties as measured by KIC. KIC appears to provide a much more discriminating ranking of the binders than either of the Superpave specification criteria.

DETERMINING THE LOW-TEMPERATURE FRACTURE TOUGHNESS OF ASPHALT MIXTURES

There has been a sustained effort in applying fracture mechanics concepts to crack formation and propagation in bituminous pavement materials. Adequate fracture resistance is an essential requirement for asphalt pavements built in the northern part of the United States and Canada, for which the prevailing failure mode is cracking due to low-temperature shrinkage stresses. The current Superpave® specifications address this issue mainly through the use of strength tests on unnotched (smooth boundary) specimens. However, recent studies have shown the limitations of this approach and have suggested that fracture mechanics concepts, based on tests performed on notched samples, should be used instead. Research in progress at the University of Minnesota investigates the use of fracture mechanics principles to determine the low-temperature fracture properties of asphalt mixtures. A testing protocol is presented that makes it possible to obtain multiple measurements of fracture toughness as a function of crack propagation based on the compliance method to measure crack length. An increase in fracture toughness with crack length is observed, which is consistent with the behavior displayed by other brittle materials. The plateau of the curves may be representative of the asphalt concrete resistance to fracture, because the initial values can be significantly influenced by the presence of the inelastic zone at the crack tip.

PHYSICAL HARDENING OF ASPHALT BINDERS RELATIVE TO THEIR GLASS TRANSITION TEMPERATURES

Physical hardening (physical aging) is a process that occurs below room temperature in asphalt binders. Physical hardening causes time-dependent isothermal changes in the rheological behavior and specific volume of asphalt binders. The process is reversible: when the asphalt binder is heated to room temperature or above, the effect of physical hardening is completely removed. Physical hardening for amorphous materials is generally reported as occurring below the glass transition temperature (Tg), but this is not the case for asphalt binders, in which physical hardening is observed both above and below Tg. The glass transition temperature of asphalt binders is measured by using three different techniques: dilatometry, differential scanning calorimetry, and rheological considerations (peak in the loss modulus versus temperature). These three techniques give roughly equivalent estimates of the glass transition temperature. The behavior of physical hardening in asphalt binders is somewhat different than that reported for polymers and other organic materials. This difference is explained in terms of the presence of crystalline fractions in the asphalt binder. Techniques for modeling physical hardening are described, and possible explanations for the anomalous behavior of asphalt binders are given.

Establishing Linear Viscoelastic Conditions for Asphalt Binders

The linear viscoelastic regime is defined in terms of the constitutive relationship between the stress and the strain. The set of equations that define the fundamental linear viscoelastic material properties in the time and frequency domains and their relationship to one another is based on the validity of the linearity principle. A material must obey two simultaneous conditions to be linear viscoelastic: the homogeneity (also called proportionality) condition and the superposition principle. On the basis of these considerations a testing procedure was developed to check linear viscoelastic conditions for tests performed on asphalt binders with the dynamic shear rheometer (DSR), the bending beam rheometer (BBR), and direct tension (DT). The testing procedure for the DSR requires performing strain sweeps and multiwave single-point tests. For the BBR, tests performed using different constant loads are required. In addition, the recovery part of the specification test is recorded. For the DT, tests performed at different strain rates and relaxation tests performed at different strain levels are required. When applied to asphalt binder data, the testing procedure found no departure from viscoelastic conditions for the DSR and BBR test data. However, the DT procedure indicated a departure from linear viscoelastic conditions.

Time-Temperature Superposition and Physical Hardening Effects in Low-Temperature Asphalt Binder Grading

During the development of the Strategic Highway Research Program low-temperature binder specifications, in an effort to propose practical laboratory tests that require less time to perform, the time–temperature superposition principle was used to show that the stiffness after 2 h of loading at the performance-graded (PG) low temperature can be approximated by the stiffness after 60 s of loading at 10°C above the PG low temperature. This equivalence principle was developed on the basis of test results from the eight core asphalts and is widely accepted today. However, actual 2-h tests were not performed to experimentally validate this equivalence. Furthermore, the effect of physical hardening on time–temperature superposition was not considered. The validity of the time–temperature equivalence factor used in the low-temperature specification criterion and the ways in which the deviations could affect the current specification are evaluated.

Field Performance of Modified Asphalt Binders Evaluated with Superpave Test Methods: I-80 Test Project

In the early 1980s heavy-duty pavements in Pennsylvania showed evidence of excessive rutting. As a consequence, the Pennsylvania Department of Transportation adopted several changes in its materials specifications and mixture design procedures. In addition, a number of modified binders were evaluated in an experimental test road that was constructed in 1989 in Clearfield County on Interstate 80. The construction was a 175-mm thick asphalt concrete overlay over an existing portland cement concrete pavement. Although the construction predated Superpave, original samples of the asphalt binder and loose asphalt mix were retained and were characterized using Superpave test methods. Field performance evaluations were performed immediately after construction and in subsequent years, giving a record of rutting, cracking, raveling, and overall visual performance. Overall, the mixtures have performed well during their 9 years of service. However, differences in the performance of the mixtures with the different binders are evident. These differences are related to the properties of the binder and the properties of the mixture as measured with the Superpave mixture and binder tests.

Time–Temperature Superposition and AASHTO MP1a Critical Temperature for Low-temperature Cracking

Thermal stress calculations performed as part of the AASHTO MP1a low-temperature criterion are based on stiffness master curves generated from test data obtained at two temperatures. The time–temperature superposition (TTS) principle is considered valid although no verification of the validity has been reported so far. This paper addresses this issue by comparing the master curves obtained from 4-min creep tests performed at two test temperatures with data obtained in a 2.5-h creep test. Due to the presence of physical hardening a limited analysis is conducted to quantify the effects of physical hardening on the calculation of the critical temperature, T CR. Two methods for calculating T CR are investigated: the Dual Instrument Method and the Single Asymptote Procedure. The analysis indicates that T CR is not sensitive to small deviations from TTS validity. However, T CR as well as the limiting temperatures from Bending beam rheometer stiffness and m-value and Direct tension failure strain are considerably affected by physical hardening.

Investigation of the effectiveness and mechanisms of enzyme products for subgrade stabilization

The subgrade stabilization effectiveness and mechanisms of two enzyme products (enzyme A and B) were investigated using chemical analysis and resilient modulus testing. Two types of soil were tested in this study. A soil with a high percentage of fines (96.4% passing 200 sieve) and high clay content (75.2%) (Soil I) and a soil with a relatively low fines content (59.7% passing 200 sieve) and low clay content (14.5%) (Soil II). The addition of enzyme A did not improve significantly the resilient modulus of Soil I, but increased the resilient modulus of Soil II by an average of 54%. On the other hand, the addition of enzyme B significantly increased the resilient modulus of both soils. The soil clay content and percent of fines appear to play an important role in the effectiveness of enzyme-based stabilizer treatment. The limited effectiveness of enzyme A (for low clay content soil) appears to be due to its surfactant-like characteristics while enzyme B, which was effective for both soils, exhibited no surfactant-like characteristics.

Techniques for determining errors in asphalt binder rheological data

Numerous new methods for specifying asphalt binders, based on more complex theoretical approaches, have emerged in the past years, and researchers are increasingly using these methods for studying asphalt binders. These methods require an increased degree of accuracy and precision in the laboratory data. A number of simple techniques are detailed that can be successfully used to identify errors in rheological data obtained for asphalt binders. Asphalt binders do not exhibit sudden changes in their behavior with respect to time or temperature. Therefore, any discontinuities revealed by the visual inspection of the test data in graphical format can be attributed to testing errors. A powerful yet simple tool in identifying potential problems with the dynamic shear rheometer test data is the Black diagram. The Black diagram is a plot of phase angle versus log |G*|, which, unlike master curves, does not require shifting of the data generated at different temperatures. However, pseudo Black diagrams should not be used with the bending beam rheometer (BBR) data because of the errors in calculating m-values associated with the use of the polynomial approximation. The c-coefficient can be used for a quick check of BBR data on the basis of the fact that the slope of the m-values decreases as the test temperature decreases. For the direct tension test, the secant modulus calculated from the stress-strain data provides a better tool to identify errors in the test data.