Bjorn Birgisson

Evaluation of a Predicted Dynamic Modulus for Florida Mixtures

The new 2002 AASHTO guide for the design of pavement structures is based on mechanistic principles and requires the dynamic modulus as input to compute stress, strain, and rutting and cracking dama ...

Near-surface stress states in flexible pavements using measured radial tire contact stresses and ADINA

The finite element code ADINA was used to identify the three-dimensional stress states in a typical flexible pavement configuration, resulting from measured radial tire contact stresses. The predictions show that measured radial tire contact stresses result in stress states being both larger in magnitude and more focused near the surface than those obtained from traditional uniform vertical loading conditions. In terms of effects of possible pavement damage mechanisms, predicted high near-surface shear stresses may be a part of an explanation for near-surface rutting failure modes, as supported by near-surface slip planes seen in the field.

Development of Tentative Guidelines for the Selection of Aggregate Gra¬dations in Hot-Mix Asphalt

Development of Tentative Guidelines for the Selection of Aggregate Gra¬dations in Hot-Mix Asphalt

Predicting Viscoelastic Response and Crack Growth in Asphalt Mixtures with the Boundary Element Method

It has long been accepted that cracking of hot-mix asphalt pavements is a major mode of premature failure. Many state agencies have verified that pavement cracking occurred not only in fatigue cracking, in which a crack initiates from the bottom of the asphalt layer, but also in other modes such as low-temperature cracking and the more recently identified top-down cracking. To improve current pavement designs and the cracking resistance of mixtures, it is necessary to understand the mechanisms associated with crack initiation and crack growth in hot-mix asphalt mixtures. However, the complexity of the problem and the lack of simple-to-use analysis tools have been obstacles to a better understanding of hot-mix asphalt fracture mechanics and the development of better hot-mix asphalt fracture models. Until today, the well-known finite element method has been the primary tool used for modeling cracks and their effects in mixtures and pavements. Unfortunately, it is both complex and numerically intensive for fracture mechanics applications. The displacement discontinuity boundary element method is presented, which is a numerical method that has been very successful in many other engineering fields, as a potential method for modeling cracking in hot-mix asphalt mixtures and pavements. A series of examples are provided to illustrate the effectiveness of the method in dealing with cracks, crack propagation, and visco-elasticity in hot-mix asphalt. It was concluded that the method was easy to use, resulted in accurate solutions, required minimal computation time, and significantly simplified the modeling of crack-related problems.

Micromechanical Analyses for Measurement and Prediction of Hot-Mix Asphalt Fracture Energy

A verification of fracture energy density is presented as a fundamental fracture threshold in hot-mix asphalt. Fracture energy density was evaluated with the semicircular bending (SCB) test. Experimental analyses were enhanced by a digital image correlation system capable of providing a dense and accurate displacement-strain field of composite materials at the microstructural level and suitable for describing the cracking behavior of materials at crack initiation. The resulting fracture behavior in the SCB was predicted with a displacement discontinuity method to explicitly model the microstructure of asphalt mixtures and to predict their fracture energy density. The input parameters for the displacement discontinuity micromechanical model of the SCB were obtained from the Superpave® indirect tensile test. The predicted crack initiation and crack propagation patterns are consistent with observed cracking behavior. The results also imply that fracture in mixtures can be modeled effectively with a micromech...

Windows-Based Top-Down Cracking Design Tool for Florida: Using Energy Ratio Concept

Top-down cracking has been found to be a predominant mode of distresses of asphalt pavements in Florida. Therefore, it is important to accommodate top-down cracking in the design of asphalt mixtures and pavement structures. After a multiyear study on top-down cracking supported by the Florida Department of Transportation, the University of Florida developed a top-down cracking model based on hot-mix asphalt fracture mechanics. This paper presents the implementation of the Florida cracking model into a mechanistic–empirical (M-E) flexible pavement design framework. Based on the energy ratio concept, a new M-E pavement design tool for top-down cracking has been developed. In the Level 3 M-E design, a series of semiempirical models were developed for estimation of time-dependent material properties. With incorporation of the material properties models, the design tool is capable of performing pavement thickness design as well as pavement life prediction for top-down cracking in Florida. The thickness design is optimized for different traffic levels, mixture types, and binder selections, and the optimization is an automated process. This design tool has been packed into Windows-based software, making it convenient to use for pavement design engineers.

DEVELOPMENT OF EFFICIENT CRACK GROWTH SIMULATOR BASED ON HOT-MIX ASPHALT FRACTURE MECHANICS

It has long been accepted that cracking of hot-mix asphalt (HMA) pavements is a major mode of premature failure. Many state departments of transportation have verified that pavement cracking occurred not only in fatigue cracking in which a crack initiates from the bottom of the asphalt layer but also in other modes such as low-temperature cracking and the more recently identified top-down cracking. Recent work at the University of Florida has led to the development of a crack growth law based on viscoelastic fracture mechanics that is capable of fully describing both initiation and propagation of cracks in asphalt mixtures. The model requires the determination of only four fundamental mixture parameters, which can be obtained from less than 1 h of testing using the Superpave® indirect tensile test (IDT). These parameters can account for microdamage, crack propagation, and healing for stated loading conditions, temperatures, and rest periods. The generalization of the HMA crack growth law needed for its successful implementation into a displacement discontinuity boundary element method is described. The resulting HMA boundary element approach is shown to predict the crack propagation of two coarse-graded mixtures under cyclic IDT loading conditions.

Development of New Moisture-Conditioning Procedure for Hot-Mix Asphalt

The presence of pore water in mixtures can cause premature failure of hot-mix asphalt pavements. The processes typically associated with moisture damage are complex and occur over a long period of time in the field. Short of being able to simulate each of the possible mechanisms of moisture damage directly, the ideal laboratory-based conditioning system should accelerate the penetration of moisture through the asphalt film and at the same time minimize complicating effects. This paper presents the results of an experiment conducted to determine whether it was possible to use cyclic pore pressures to induce enough damage to distinguish between mixtures known to be highly resistant from mixtures known to be susceptible to moisture damage. Experimental constraints included requirements that conditioning be accomplished within a reasonable length of time and that typical laboratory equipment be used. Evaluation of the resulting effects of moisture damage included the use of the Superpave® indirect tension test and the energy ratio parameter. Findings show that cyclic pore pressures can be used to accelerate moisture damage enough to distinguish between mixtures known to be strippers and those known to be highly resistant to moisture damage. The use of cyclic pore pressures to accelerate moisture damage in mixtures may minimize the introduction of other confounding damage effects on the mixtures.

Numerical Implementation of a Strain Energy-based Fracture Model for HMA Materials

ABSTRACT This work combines a new strain energy-based fracture criterion with a viscoelastic displacement discontinuity boundary element method to investigate crack growth in hot mix asphalt (HMA) materials. The study employs a fundamental crack growth threshold and simulates crack growth by accumulation of the dissipated creep strain energy (DCSE)—below this threshold, only healable micro-damage develops, and non-healable crack initiation or growth occurs, otherwise. A critical zone is introduced ahead of the crack tip to represent the portion of the material being damaged. Once the cumulative micro-damage inside the critical zone reaches the cracking threshold, the crack extends by the length of the critical zone. An HMA fracture simulator is developed by incorporating the DCSE threshold concept into a numerical framework based on a viscoelastic displacement discontinuity method, which has proven its convenience and efficiency in crack modeling. Numerical analyses are performed to predict piecewise crack propagation in asphalt mixtures using the HMA fracture simulator, and laboratory experiments are also conducted to verify and validate the numerical model.

Identification of a Physical Model to Evaluate Rutting Performance of Asphalt Mixtures

The objective of this study is to identify a physical model that can provide reliable predictions about a mixtures ability to resist permanent deformation under realistic stress states. Key differences were identified between stress states under the existing Asphalt Pavement Analyzer (APA) loading device (hose) and stress states under radial truck tires, which may indicate potentially different rutting mechanisms. It was shown that the APA hose was not capturing the critical lateral stresses found to be detrimental to rutting and cracking of HMA pavements. A new loading device (rib) was designed and constructed for use in the APA that more closely represents stress states found under radial tires. Contact-stress measurements under the two loading devices - hose and rib - showed that the rib was able to reproduce the lateral stresses found under individual ribs on a radial-tire tread. Subsequent finite element modeling also showed that the rib appeared to generate similar shear stress patterns to those found under the modeled radial-tire load. A new method was developed to measure deformations on the surface of APA specimens, where a contour gauge was used to record and store the entire surface profile of the sample throughout the progress of the test. An area-change parameter, which reflects volume change, was introduced to calculate the volumetric changes in the specimen. The area-change parameter can be used to determine whether specimen rutting is primarily due to shear instability or consolidation. Two mixtures of known field performance - poor and good - were tested to evaluate the tests ability to predict performance with the new loading device and the new measurement and interpretation system. Results showed that the new system (loading strip and profile measurement method) appears to have greater potential of evaluating a mixtures potential for instability rutting than the original (hose and single rut-depth measurement) configuration.