A bilinear cohesive zone model tailored for fracture of asphalt concrete considering viscoelastic bulk material

Improvements to existing methods in the literature have been made for predicting cumulative plastic deformation for fine-grained subgrade soils. The soil deviator stress, number of stress applications, soil physical state, and soil type are considered. The improved method incorporates multilevels of deviator stresses and multisoil physical states that result from load-level variations, as well as seasonal and weather changes throughout traffic. Measurements of plastic deformation for a railroad-track subgrade are presented and show a significant influence of soil physical state, soil type, traffic tonnage, and wheel loads on the accumulation of plastic deformation. Comparisons between predicted and experimental results show good applicability of the improved method.

Cumulative plastic deformation for fine-grained subgrade soils

The research results in this report concern the characterization of physical properties of asphalt binders and the development of test procedures for aging and adhesion. Asphalt binders were shown to be linear viscoelastic materials at expected pavement strains. Test methods and models describing rheological behavior were developed with the assumption of linear behavior. A mathematical model that contains parameters that can be used to relate physical and chemical parameters was developed. The test methods and models allow the stiffness of asphalt binders to be characterized as a function of loading time and temperature with the dynamic shear rheometer at intermediate to upper pavement surface temperatures and with a newly developed bending beam rheometer at low temperatures, where thermal cracking is important. To characterize fracture properties, the project developed a direct tension test, in which failure strain, stress, and energy are determined as functions of loading time and temperature. Limited work on fatigue properties at low temperatures successfully related them to tensile properties. A new phenomenon, low-temperature physical hardening, was identified during the project. This phenomenon results in increased stiffness of the binder when it is held at a constant temperature at room temperature or below. The blister test for quantitative measurement of adhesion between asphalt binder and aggregate surfaces was found to be impractical. The investigation of various factors, including film thickness, aging temperature, the effect of oxygen versus air, and the effect of mineral surfaces, led to a new aging procedure for simulating in-service oxidative hardening of asphalt binders, which was adopted as part of the new Strategic Highway Research program (SHRP) binder specification.

Binder characterization and evaluation. volume 3: physical characterization

This is a practical paper which consists of investigating fracture behavior in asphalt concrete using an intrinsic cohesive zone model (CZM). The separation and traction response along the cohesive zone ahead of a crack tip is governed by an exponential cohesive law specifically tailored to describe cracking in asphalt pavement materials by means of softening associated with the cohesive law. Finite-element implementation of the CZM is accomplished by means of a user subroutine using the user element capability of the ABAQUS software, which is verified by simulation of the double cantilever beam test and by comparison to closed-form solutions. The cohesive parameters of finite material strength and cohesive fracture energy are calibrated in conjunction with the single-edge notched beam [SE(B)] test. The CZM is then extended to simulate mixed-mode crack propagation in the SE(B) test. Cohesive elements are inserted over an area to allow cracks to propagate in any direction. It is shown that the simulated crack trajectory compares favorably with that of experimental results.

Simulation of Crack Propagation in Asphalt Concrete Using an Intrinsic Cohesive Zone Model

A thermo-viscoelastic-viscoplastic-viscodamage constitutive model for asphaltic materials

The results of several previous investigations have led to the conclusion that rutting criteria should be included in any pavement design methodology that attempts to achieve the goal of improved pavement serviceability. Moreover, in some practical cases, it is necessary to estimate the rutting expected to occur in a pavement system during a certain period. Therefore, it is necessary to develop a scheme for the estimation of rutting. Because it is the foundation of the pavement, the subgrade makes a considerable contribution to its rutting. The objective of this research was to study the rutting in subgrade soils experimentally. The rate-process theory approach was used for the analysis of the results. Five types of silty soils were obtained from four different construction sites in Ohio, and laboratory-prepared samples were used for uniaxial dynamic testing. A direct general relationship that relates rutting to the dynamic modulus and the applied stress level was developed. On the basis of that relationship, a scheme for the estimation of subgrade permanent deformation is proposed and design nomographs for the soils studied were developed. The findings of this study are limited to the types of soils (silty and clayey) tested; further studies should indicate their applicability to other materials also. /Author/

Rutting evaluation of subgrade soils in ohio

A nationwide survey of pavement performance indicated that fatigue is the most important distress problem in U.S. primary pavements. The classical approaches to investigating fatigue problems are classified as phenomenological and mechanistic. The phenomenological approach is based on developing a distress function using laboratory simulation of third-point beam loading, beam on elastic foundation, discs, and so forth; its main drawback is that most of the fatigue life is exhausted through cyclic plasticity, and crack initiation, propagation, and ultimate failure, with the associated stress and strain redistributions, are not accounted for. The mechanistic approach is based on the laws of linear elastic fracture mechanics (LEFM) to model laboratory-simulated crack initiation, propagation, and unstable failure; its main shortcoming is that failure is assumed to be by brittle fracture and the initiation phase is not modeled because it is assumed that a crack size of "c sub o" initially exists. Moreover, the stress intensity factor requires rather specialized computational techniques like finite elements. In this paper, a cyclic plasticity fatigue model is developed through laboratory-controlled stroke fatigue testing of unnotched disc samples of asphaltic mixes. This model is statistically matched with controlled stroke fatigue testing of notched discs under the same stroke range loading condition. Another model is developed for fatigue crack propagation using the elasto-plastic fracture mechanics (EPFM) approach. The EPFM fracture criterion is the path-independent contour J-integral. Within the development of the propagation model, a method for J-integral testing of asphaltic mixtures has been established. An example is presented and worked out to illustrate how this new approach can be used in modeling fatigue life.

J-integral and cyclic plasticity approach to fatigue and fracture of asphaltic mixtures

Results of an evaluation of permanent deformation, or rutting, in asphalt concrete pavements subject to traffic load and environmental conditions are presented. Because previous studies have shown rutting to be a major mode of distress in various pavement systems, rutting criteria should be considered in any rational method of pavement design. For practical applications, it is often necessary to estimate the rutting expected in a pavement for a certain period or set of conditions. To accomplish this, a model of rutting estimation is necessary. An approach to rutting estimation applied successfully to subgrade soils in research at Ohio State University was used to study the variation of rutting parameters with asphalt content, temperature, and loading conditions for two mixtures that met Ohio specifications for surface-course mixtures. The data and results were obtained from uniaxial dynamic tests performed on laboratory-prepared samples and from previous research conducted at Ohio State University. The findings of the study, and comparisons with results of previous similar studies, indicate that a direct general relation exists between rutting parameters, dynamic moduli, and applied stress. The rutting parameter m was found to be almost constant within the range of study, which confirms previous results on subgrade soils that showed m to be an almost universal constant independent of soil type, stress, dynamic modulus, or saturated environmental conditions. A method for estimating permanent deformation in asphaltic concrete layers is proposed on the basis of the relation found in this study, in which the dynamic modulus was found to characterize such mix properties as density, asphalt content, gradation, air voids, and temperature with respect to rutting criteria. A general relation was also found between the rutting parameter A and the dimensionless ratio of dynamic modulus to applied stress. This relation satisfactorily describes the test results regardless of other influential factors such as type of material. (Author)

Evaluation of permanent deformation in asphalt concrete pavements

Constitutive equations used in solving the boundary value problem of flexible pavements employ linear elastic or viscoelastic theory. Accordingly, permanent deformations are calculated based on elastic or viscoelastic deformation laws. Advances in the field of constitutive modeling of materials indicated the need to develop a constitutive relationship that better replicates asphaltic mixture responses under various loading and environmental conditions. In this paper a one-dimensional combo viscoelastic-plastic constitutive model composed of Burger-type mechanical elements connected in series with a friction slider is used. The friction slider is the mechanical representation of plasticity with a DruckerPrager yield criterion. This model is solved under creep phase loading conditions, and the solution is used to develop a rutting model that incorporates a densification phase represented by a relaxing spring. Within the verification of the constitutive model a true yield line has been identified and used instead of the Mohr-Coloumb failure line. The two developed models are supplemented by appropriate experimentation phases to identify and numerically evaluate the relevant parameters. Experimentation is based on actual existing routine methods, with proper adjustments, modifications, or extensions to comply with proper evaluation of the model parameters, and kept as simple as possible to encourage wider user acceptance. An example using actual data is worked out and compared with results obtained from the VESYS III structural subsystem program.

Combo viscoelastic-plastic modeling and rutting of asphaltic mixtures

THE PRIMARY PURPOSE OF THIS INVESTIGATION WAS TO STUDY THE BEHAVIOR OF THIN FILMS OF ASPHALTS SUBJECTED TO TENSILE STRESSES. A NUMBER OF THEORIES THAT MIGHT EXPLAIN THE BEHAVIOR OF ASPHALTS OR SIMILAR MATERIALS IN THIN FILMS ARE REVIEWED AND THE ADVANTAGES AND DISADVANTAGES OF EACH THEORY ARE DISCUSSED. TO VERIFY THE HYPOTHESES AND THEORIES RELATED TO TENSILE PROPERTIES OF ASPHALTS, LABORATORY EXPERIMENTS WERE CONDUCTED ON THIN ASPHALT FILMS UNDER CONTROLLED TEST CONDITIONS. THE VARIABLES STUDIED, SELECTED ON THE BASIS OF THEORETICAL CONSIDERATION, WERE FILM THICKNESS, RATE OF EXTENSION, TEMPERATURE, AND SIZE OF SPECIMEN. THE FAILURE OF MECHANISM OF ASPHALT FILMS CAN BE PLACED IN THREE CATEGORIES - BRITTLE FRACTURE, TENSILE RUPTURE, AND SHEAR FLOW - ALL OF WHICH ARE INFLUENCED BY THE VARIABLES STUDIED. EQUATIONS ARE PRESENTED FOR PREDICTING THE LIMITING VALUE OF THE FILM THICKNESS CORRESPONDING TO EACH TYPE OF FAILURE. A HYPOTHESIS IS ALSO DEVELOPED THAT EXPLAINS THIS FAILURE MECHANISM AND THE BEHAVIOR OF ASPHALTS OVER A WIDE RANGE OF FILM THICKNESSES. IT IS ALSO SHOWN THAT TWO THEORIES - THE HYDRODYNAMIC THEORY AND THE THEORY OF POTENTIAL ENERGY AND CAVITIES - COULD BE USED TO PREDICT THE TENSILE STRENGTH OF ASPHALTS IN THICK AND THIN FILMS, RESPECTIVELY.

Kamran Majidzadeh

Papers

Rutting evaluation of subgrade soils in ohio

J-integral and cyclic plasticity approach to fatigue and fracture of asphaltic mixtures

Evaluation of permanent deformation in asphalt concrete pavements

Combo viscoelastic-plastic modeling and rutting of asphaltic mixtures

Modes of failure and strength of asphalt films subjected to tensile stresses