Simon A. M. Hesp

Low-Temperature Fracture Testing of Asphalt Binders: Regular and Modified Systems

The results of low-temperature fracture testing of a large number of both regular and modified asphalt binders are discussed. Two Strategic Highway Research Program (SHRP) binders (Materials Reference Library Codes AAG-2 and AAN) were evaluated with 5 percent by weight of a variety of commonly used polymer modifiers. Specimens of three different sizes were tested in a three-point bend configuration, both with and without a notch. The original SHRP effort was aware of the need for a rigorous fracture mechanics-type binder test, but because of a lack of time and resources, only the bending beam rheometer and the direct tension test were ultimately developed. Some of the differences between failure strain and fracture toughness measurements are discussed, as well as how these differences may relate to pavement performance. The results of this study demonstrate that there is a large range of notch sensitivities and fracture energies for different polymer-modified binders, suggesting that the ductile-to-brittle transition (as measured with the direct tension test) may not be a totally reliable performance indicator. Fracture energy may be a better choice, since it combines the notched strength with a stiffness to yield a true material property that is independent of sample size and configuration.

X-ray fluorescence detection of waste engine oil residue in asphalt and its effect on cracking in service

This paper documents the discovery of waste engine oil residues in pavements across Ontario, Canada. We have found that recovered asphalts from a large majority of poorly performing contracts test positive for zinc through X-ray fluorescence (XRF) analysis. In contrast, neither the aggregates nor any of the well-performing asphalts showed any signs of the metal. Since zinc dialkyldithiophosphates are universal additives in engine oils, we inferred that the use of waste oil residues in asphalt must be widespread. Further analysis of 2008 quality assurance samples taken for the Ontario Ministry of Transportation substantiated this, with most samples testing positive for zinc. XRF analysis of straight waste oil residues suggests that typical modification levels are in the 5–20% range. The damaging effect of this additive through increased physical and chemical hardening is briefly discussed with reference to previous studies on unexplained, premature and excessive thermal cracking.

Asphalt Pavement Cracking: Analysis of Extraordinary Life Cycle Variability in Eastern and Northeastern Ontario

This paper documents an investigation of the performance of 20 pavement contracts in eastern and northeastern Ontario, Canada. Eleven of these pavements showed little or no distress after 7–15 years in service. The remaining nine, aged between 7 and 13 years, all cracked prematurely and excessively. Creep testing of the recovered asphalt cements, according to an extended bending beam rheometer protocol, revealed that the long-life pavements were made with materials that suffered little from reversible ageing mechanisms during cold conditioning. Ductile failure testing in a double-edge notched tension test at 15°C revealed equally significant differentiation, with the superior performing materials possessing considerably higher strain tolerances. The newly developed methods were able to explain vast performance differences with 95% accuracy. This study further validates Ontarios interest in an improved asphalt cement grading approach and provides evidence that thin pavements can have long lives, provided the asphalt cement is selected properly.

ESSENTIAL AND PLASTIC WORKS OF DUCTILE FRACTURE IN ASPHALT BINDERS

Currently, no satisfactory asphalt binder test method exists that can relate properties measured in the laboratory to fatigue performance in service. The loss modulus of the binder, G*sinδ, as proposed by the Strategic Highway Research Program, is a rheological parameter that measures the energy dissipated within the homogeneous binder at low strains under dynamic conditions. A number of publications have reported that this binder parameter provides little correlation with the susceptibility of the asphalt concrete to fracture at high strains in the nonlinear regime under simulated in-service conditions. The essential work of the fracture method, an energy-based testing approach used for the fracture characterization of ductile materials, was explored. Given that asphalt is a ductile material at ambient temperatures, it is only reasonable to assume that the essential work of fracture method yields valuable information with likely use in fatigue performance ranking. The binders investigated showed a wide range of essential and plastic works of fracture at ambient temperature and a single rate of loading. Although only six binders were evaluated, their works of fracture were contrasted with other properties.

Comparison of Polyphosphoric Acid-Modified Asphalt Binders with Straight and Polymer-Modified Materials

This paper discusses the effects of polyphosphoric acid (PPA) modification on asphalt binders. Findings are compared with those for straight and polymer-modified materials of similar grades. The effect of PPA modification was investigated by means of conventional protocols as well as three new binder tests developed in Ontario, Canada. The compact tension test and the extended bending beam rheometer protocol were used to investigate low-temperature performance properties. The double edge-notched tension test was used to study the resistance to ductile failure. The brittle state fracture properties and the reversible aging processes were largely unaffected by PPA modification. However, changes in the low-temperature grade were considerable in some instances. While the essential work of fracture increases with the addition of PPA, the yield stress increases more rapidly in most binders and thus results in an overall drop in strain tolerance in the ductile state. Recent field experience in Ontario suggests that the use of PPA may either prevent or contribute to wheelpath cracking, depending on the formulation and concurrent use of polymer.

Performance grading of the Lamont, Alberta C-SHRP pavement trial binders

This paper documents and discusses an investigation into the improved performance grading of asphalt binders that were used in the Canadian Strategic Highway Research Program (C-SHRP) pavement trial in Lamont, Alberta. Binders were assessed for their tendency to reversibly age at low-temperatures. While the correlation between cracking severity and regular AASHTO M320 and MP1a grades was weak (r 2=0.73 and 0.49, respectively), the predictive ability of the BBR method increased significantly when binders were tested after 1, 3 and 8 days of conditioning (r 2=0.94–0.97). Brittle and ductile fracture energies were measured in single-edge-notched bending (SENB) and double-edge-notched tension (DENT), respectively. In the brittle state, the SENB test revealed little difference between the binders. However, in the ductile state, the DENT test revealed significant differences between materials, with binders that showed the least amount of cracking having the highest works of fracture at 0°C.

Crack pinning in asphalt mastic and concrete: Regular fatigue studies

The effects of finely dispersed fillers on the fatigue performance of asphalt binders and asphalt concrete mixes at relatively low temperatures are examined. A series of model binder systems containing glass spheres with narrow particle size distributions were used to study the effect of filler particle size on the fatigue performance of the asphalt mastic. Two mastic systems containing ground limestone fillers, which possessed significantly different gradations, also were tested. Fatigue performance was evaluated by applying a constant torsional strain to each specimen in a dynamic rheometer at 10°C and 40 Hz. Testing at various strain levels allowed the relationship between fatigue life and strain to be determined for the different systems. The results indicate that as the particle size of the filler decreases, the fatigue life of the asphalt mastic increases. This observation is a direct result of the mode of fatigue failure in the asphalt mastics and is in agreement with Evans’s theory on crack pinning for failure in filled brittle solids. Constant stress asphalt concrete fatigue tests on both dense- and gap-graded systems prepared with the two different ground limestone fillers show that the particle size does not significantly affect the fatigue life of the mixes. These results also confirm that crack pinning is the major mechanism responsible for improved fatigue performance.

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.

Asphalt Cement Loss Tangent as Surrogate Performance Indicator for Control of Thermal Cracking

This paper documents and discusses the field validation of a simple performance indicator for specification grading of asphalt cement for thermal cracking. The loss tangent, tan(δ), as defined by the ratio of the viscous over elastic modulus, G”/G’, is able to provide a quantitative measure of the sol and gel nature of asphalt cement. As such, tan(δ) relates closely to how well a material is able to relax stress and is therefore investigated as a surrogate performance indicator for thermal cracking. Asphalt cements from 20 contract sites in Ontario, Canada, were tested in torsion bar geometry to determine their viscoelastic properties. The findings show that tan(δ) was able to distinguish good from poor performers for this set of materials with 95% accuracy, which is a considerable improvement over the current bending beam rheometer protocol. In addition to the use of tan(δ), it is suggested that a measure of the critical strain tolerance in the ductile state could be included to provide a significantly improved performance grading method. Most of the worst performing contracts were found to contain zinc. The presence of this element suggests that the use of waste engine oils in asphalt production is likely widespread, since zinc is a universal additive in engine oil and is never found in straight asphalt cement

Time–temperature superposition in rheology and ductile failure of asphalt binders

This paper documents and discusses an investigation into the time–temperature superposition principle as it relates to ductile failure in asphalt. Seven binders of approximately the same low and intermediate temperature and varying high temperature Superpave® grades were tested in a dynamic shear rheometer (DSR) and double-edge-notched tension test to determine their rheological and failure energy master curves. Master curves typically permit the prediction of rheological properties at very long or short timescales from measurements at higher or lower test temperatures over more accessible testing timescales. It has been suggested in the Strategic Highway Research Program final report A-369 that rheological shift factors obtained from a DSR experiment can be used to predict failure master curves from experimentally accessible data at various temperatures. The findings of this study suggest that this substitution is not generally accurate. For straight asphalt binders the difference can be relatively small but for more highly modified materials serious errors would be introduced.