William G. Buttlar

Disk-shaped compact tension test for asphalt concrete fracture

A disk-shaped compact tension (DC(T)) test has been developed as a practical method for obtaining the fracture energy of asphalt concrete. The main purpose of the development of this specimen geometry is the ability to test cylindrical cores obtained from in-place asphalt concrete pavements or gyratory-compacted specimens fabricated during the mixture design process. A suitable specimen geometry was developed using the ASTM E399 standard for compact tension testing of metals as a starting point. After finalizing the specimen geometry, a typical asphalt concrete surface mixture was tested at various temperatures and loading rates to evaluate the proposed DC(T) configuration. The variability of the fracture energy obtained from the DC(T) geometry was found to be comparable with the variability associated with other fracture tests for asphalt concrete. The ability of the test to detect changes in the fracture energy with the various testing conditions (temperature and loading rate) was the benchmark for determining the potential of using the DC(T) geometry. The test has the capability to capture the transition of asphalt concrete from a brittle material at low temperatures to a more ductile material at higher temperatures. Because testing was conducted on ungrooved specimens, special care was taken to quantify deviations of the crack path from the pure mode I crack path. An analysis of variance of test data revealed that the prototype DC(T) can detect statistical differences in fracture energy resulting for tests conducted across a useful range of test temperatures and loading rates. This specific analysis also indicated that fracture energy is not correlated to crack deviation angle. This paper also provides an overview of ongoing work integrating experimental results and observations with numerical analysis by means of a cohesive zone model tailored for asphalt concrete fracture behavior.

Understanding Asphalt Mastic Behavior Through Micromechanics

The mechanical properties of asphalt-mineral filler mastics have long been known to significantly influence the overall performance of paving mixtures. However, reinforcement mechanisms associated with the presence of fillers in asphalt mastics are not well understood. Particulate composite micromechanical models are shown to be a powerful tool for separating various reinforcing mechanisms in asphalt mastics, including volume filling, physiochemical, and particle-interaction reinforcement. The generalized self-consistent scheme model is shown to predict very reasonable baseline reinforcement levels for asphalt mastics, and simplified prediction tools are presented as an alternative to the cumbersome micromechanical solution. An experimental program was conducted to evaluate micromechanical predictions of mastic properties over a broad range of temperatures and filler concentrations. A new equivalent rigid layer modeling technique was developed, which suggests that stiffening effects observed in mastics beyond those due to volume filling may be largely explained by an effective increase in volume concentration of rigid inclusions due to a rigidly adsorbed asphalt layer just 0.02 to 0.10 μm thick. Particle-interaction reinforcement appears to play a smaller role, possibly as a result of the interaction of partially altered asphalt layers, and was observed to be significant only at very high filler contents. More work is needed to better understand the nature of physiochemical reinforcing and to study other possible stiffening mechanisms in mastics such as agglomeration, state of dispersion, and particle-size distribution.

DISCRETE ELEMENT MODELING OF ASPHALT CONCRETE: MICROFABRIC APPROACH

Micromechanical modeling has tremendous potential benefits in the field of asphalt technology for reducing or eliminating costly tests to characterize asphalt-aggregate mixtures for the design and control of flexible pavement structures and materials. In time, these models could provide a crucial missing link for the development of true performance-related specifications for hot-mix asphalt. A microfabric discrete element modeling (MDEM) approach is presented for modeling asphalt concrete microstructure. The technique is a straightforward extension of a traditional discrete element modeling (DEM) analysis, in which various material phases (e.g., aggregates, mastic) are modeled with clusters of very small, discrete elements. The MDEM approach has all the benefits of traditional DEM (e.g., the ability to handle complex, changing contact geometries and the suitability for modeling large displacements and crack propagation). These models also allow for the simulation of specimen assembly (e.g., laboratory compaction of the asphalt mixture). By modeling inclusions such as aggregates with a “mesh” of small, discrete elements, it is also possible for one to model complex aggregate shapes and the propagation of cracks around or through aggregates during a strength test. A commercially available DEM package was used to demonstrate the usefulness of the MDEM approach. A method was also presented to obtain the properties of the matrix material in an asphalt mixture, which is typically difficult to determine experimentally. This study was limited to two-dimensional analysis techniques and involved the simulation of small test specimens. Follow-up studies involving larger specimen models and three-dimensional modeling capabilities are under way.

Micromechanical Modeling Approach to Predict Compressive Dynamic Moduli of Asphalt Mixtures Using the Distinct Element Method

A clustered distinct element method (DEM) approach is presented as a research tool for modeling asphalt concrete microstructure. The approach involves the processing of high-resolution optical images to create a synthetic, reconstructed mechanical model that appears to capture many important features of the complex morphology of asphalt concrete. Uniaxial compression tests in the laboratory were employed to measure the dynamic modulus of sand mastic (a very fine sand-asphalt mixture) and asphalt mixtures at three temperatures and four loading frequencies. For a coarse mixture considered in this study, it was found that a two-dimensional (2-D) clustered DEM provided good estimates of mixture dynamic modulus across a range of loading temperatures and frequencies without calibration. However, for a fine-grained mixture, the uncalibrated predictions of the 2-D model were found to reside near the lower theoretical bounds and well below experimentally determined moduli, most likely because of current limitation...

Application of Discrete Element Modeling Techniques to Predict the Complex Modulus of Asphalt-Aggregate Hollow Cylinders Subjected to Internal Pressure

An extension of the discrete element modeling (DEM) approach, or clustered DEM, was used to simulate the hollow cylinder tensile (HCT) test, in which various material phases (e.g., aggregates, mastic) are modeled with bonded clusters of discrete elements. The basic principle of the HCT test is the application of internal pressure to the inner cavity of a hollow cylinder specimen, which produces circumferential strain. In the present study an experimental program was conducted to measure the complex modulus of asphalt concrete mixtures at various loading rates and temperatures. The HCT test was then modeled with a two-dimensional, linear elastic DEM simulation. The current approach uses the correspondence principle to bridge between the elastic simulation and viscoelastic response. The two-dimensional morphology of the asphalt concrete mixture was captured with a high-resolution scanner, enhanced with image-processing techniques, and reconstructed into an assembly of discrete elements. The mixture complex moduli predicted in the HCT simulations were found to be in good agreement with experimental measurements across a range of test temperatures and loading frequencies for the coarse-grained mixtures investigated. Ongoing work in the area of viscoelastic constitutive modeling, fracture modeling, and three-dimensional tomography and modeling will extend the capabilities of this promising technique for fundamental studies of asphalt concrete and other particulate composites.

Assessment of Existing Micro-mechanical Models for Asphalt Mastics Considering Viscoelastic Effects

ABSTRACT Micromechanical models have been directly used to predict the effective complex modulus of asphalt mastics from the mechanical properties of their constituents. Because the micromechanics models traditionally employed have been based on elastic theory, the viscoelastic effects of binders have not been considered. Moreover, due to the unique features of asphalt mastics such as high concentration and irregular shape of filler particles, some micromechanical models may not be suitable. A comprehensive investigation of four existing micromechanical methods is conducted considering viscoelastic effects. It is observed that the self-consistent model well predicts the experimental results without introducing any calibration; whereas the Mori-Tanaka model and the generalized self-consistent model, which have been widely used for asphalt materials, significantly underestimate the complex Youngs modulus. Assuming binders to be incompressible and fillers to be rigid, the dilute model and the self-consistent model provide the same prediction, but they considerably overestimate the complex Youngs modulus. The analyses suggest that these conventional assumptions are invalid for asphalt mastics at low temperatures and high frequencies. In addition, contradictory to the assumption of the previous elastic model, it is found that the phase angle of binders produces considerable effects on the absolute value of the complex modulus of mastics.

Effective thermal conductivity of two-phase functionally graded particulate composites

A multiscale modeling method is proposed to derive effective thermal conductivity in two-phase graded particulate composites. In the particle-matrix zone, a graded representative volume element is constructed to represent the random microstructure at the neighborhood of a material point. At the steady state, the particle’s averaged heat flux is solved by integrating the pairwise thermal interactions from all other particles. The homogenized heat flux and temperature gradient are further derived, through which the effective thermal conductivity of the graded medium is calculated. In the transition zone, a transition function is introduced to make the homogenized thermal fields continuous and differentiable. By means of temperature boundary conditions, the temperature profile in the gradation direction is solved. When the material gradient is zero, the proposed model can also predict the effective thermal conductivity of uniform composites with the particle interactions. Parametric analyses and comparisons ...

Effect of Factors Affecting Fracture Energy of Asphalt Concrete at Low Temperature

ABSTRACT Low temperature cracking is considered one of the primary distress modes of asphalt pavements built in northern climates. The detrimental effects of low-temperature cracking of asphalt pavements have motivated new work in fracture testing of hot-mix asphalt (HMA). A comprehensive study was conducted to investigate the effect of various factors on the fracture resistance of asphalt mixtures at low temperatures. The Semi-Circular Bending (SCB) and Disc-Shaped Compact Tension Test (DCT) fracture tests were performed at three low temperatures to measure the fracture energy for 28 asphalt mixtures, which represent a combination of factors including binder type, binder modifier, aggregate type, asphalt content and air voids. In this study, the analysis from the experimental data indicates that fracture energy is strongly dependent upon temperature and significantly affected by type of aggregate and binder modifier. The results of the analysis show the significance of air voids in the SCB test, but no statistical difference was found for the DCT test data. The analysis also illustrates that richer mixtures, ones with more asphalt than the design optimum, are not necessarily more crack resistant.

Effect of Binder Type, Aggregate, and Mixture Composition on Fracture Energy of Hot-Mix Asphalt in Cold Climates

Detrimental effects of low-temperature cracking of asphalt pavements and overlays have motivated new work in fracture testing of hot-mix asphalt (HMA). Recent work has indicated that the fracture behavior of asphalt concrete at low temperatures can be accurately predicted with a testing and modeling system that, along with viscoelastic bulk material properties, relies simply on fracture energy and material strength. In this study, the disk-shaped compact tension test is used to measure fracture energy of 28 HMA mixtures designed for cold climates. Four parameters are investigated: aggregate type (limestone and granite), temperature (three temperatures, encompassing the Superpave® performance graded binder low temperature grade for each binder tested), asphalt content (Superpave design asphalt content and Superpave design content plus 0.5%), and air voids (4% and 7%). A statistical analysis of results demonstrates the significance for fracture energy of binder content at higher temperatures, aggregate type, and temperature. The air void levels selected and binder content at lower temperatures, however, did not lead to a significant difference in fracture energy. An extrapolation technique is presented that was found to present a rational means for interpreting data from tests that were not finished because of equipment constraints.

Thermal reflective cracking of asphalt concrete overlays

Reflective cracking of asphalt concrete (AC) overlays is one of the most extensive pavement distress and damage mechanisms in composite pavement structures. Numerous studies have been performed to evaluate the reflective cracking potential of AC overlays under different loading scenarios. Most of these studies have focused on reflective cracking due to tyre loading. A very limited amount of work has been performed to evaluate non-load-associated thermal reflective cracking of overlays. Thermal reflective cracking mechanisms have been studied and are described in this paper using recently developed hot-mix asphalt mixture tests and fracture models. A series of finite-element-based pavement simulations were performed in an effort to better understand thermal reflective cracking mechanisms as a function of several key material and pavement structure variables. The enhanced integrated climatic model was used to estimate pavement temperature gradients as a function of position and time. A fracture mechanics-based cohesive fracture model was used for the simulation of damage and cracking, which was tailored for use with quasi-brittle materials such as AC. The pavement simulation model utilises creep and fracture properties from American Association of State Highway and Transportation Officials and American Society for Testing and Materials-specified tests and analysis procedures. Three asphalt mixtures manufactured with Superpave low-temperature performance grades of -22, -28 and -34 were studied in pavement structures with three distinct overlay thicknesses. Simulations were conducted with three Portland cement concrete (PCC) slab conditions to study the effects of joint spacing and rubblisation on thermal reflective cracking. The simulation results provide a new insight towards the mechanisms underlying the development of thermal reflective cracking. The curling of PCC slabs due to temperature differential and joint opening caused by pavement cooling was found to be critical in the initiation of thermal reflective cracking. This effect is greatly minimised or eliminated in the case of pavement rubblisation.