Rongzong Wu

CalME, a Mechanistic-Empirical Program to Analyze and Design Flexible Pavement Rehabilitation

A computer program known as CalME has been developed for analysis and design of new flexible pavements and rehabilitation of existing pavements. The paper describes the overlay design procedure and the calibration of the models for reflection cracking and permanent deformation through heavy vehicle simulator (HVS) tests. To simplify the input process, the program includes databases for traffic loading, climatic conditions, and standard materials. A companion program was developed for backcalculation of layer moduli, and the results may be automatically imported into the CalME database. The program incorporates the existing, empirical California Department of Transportation design methods as well as an incremental–recursive analysis procedure based on the mechanistic–empirical method. The effects of different pavement preservation and rehabilitation strategies on pavement damage may be studied with several options for triggering timing of placement. The influence of within-project variability on the propagation of damage can be evaluated using Monte Carlo simulation. The program also permits importation of the results of HVS or track tests into the database and simulation of the experiments on the computer. This feature is useful for the calibration of the mechanistic–empirical models but may also be used for in-depth interpretation of accelerated pavement testing results. An HVS experiment that was used for calibration of the reflection cracking and the permanent deformation models is described.

Comparison of Full-Depth Reclamation with Foamed Asphalt and Full-Depth Reclamation with No Stabilizer in Accelerated Loading Test

Full-depth reclamation (FDR) with foamed asphalt has been successfully used as a rehabilitation strategy in California since 2001. Long-term field monitoring on several projects and a comprehensive laboratory study resulted in the preparation of guidelines and specification language in 2008. However, the design criteria were essentially empirical, in line with California design procedures for this level of rehabilitation project. Recently, there has been growing interest in the use of cement, engineered emulsion, and no-stabilizer full-depth reclamation strategies in addition to foamed asphalt and in the use of mechanistic design in rehabilitation projects. Consequently, the research initiative was extended to a second phase to include accelerated load testing on an instrumented test track constructed with these four FDR strategies to gather data for the development of performance models that can be included in mechanistic–empirical rehabilitation design procedures. This paper summarizes the results of the first two tests in this accelerated loading study, which compared no stabilizer and foamed asphalt–cement strategies. The foamed asphalt section outperformed the unstabilized section in all measured aspects. The most notable observation was in rutting performance: the unstabilized section reached a terminal rut depth of 13 mm after approximately 490,000 equivalent standard-axle loads had been applied, compared with the foamed asphalt and cement section, which had a rut depth of only 4.3 mm after more than 17.7 million equivalent standard-axle loads. No cracking was observed on either section. The advantages of using foamed asphalt with cement over unstabilized pulverized material are clearly evident from the results.

Composite Pavement Systems Volume 1: HMA/PCC Composite Pavements

Composite pavements have proved in Europe and the United States to have long service life with excellent surface characteristics, structural capacity, and rapid renewal when needed. This project developed the guidance needed to design and construct new composite pavement systems. Volume 1 presents the state of the practice and guidelines for designing and constructing new hot-mix asphalt (HMA) concrete over a portland cement concrete (PCC) composite pavement that takes full advantage of using differing materials. Volume 2 provides guidance on the design and construction of two-layer, wet-on-wet PCC pavements where the upper layer is a thin high-quality layer (hard nonpolishing aggregate, higher cement content, higher quality binder) and excellent surface characteristics with the lower layer containing a higher percentage of local aggregates and recycled materials. Both volumes detail performance data on existing composite pavement systems and provide step-by-step guidance on the design of composite pavements using mechanistic-empirical design methods for both types of new composite pavements.

Evaluation of Embedded Discontinuity Method for Finite Element Analysis of Cracking of Hot-Mix Asphalt Concrete

Cracking is a major source of distress in hot-mix asphalt (HMA) pavements. Various approaches have been proposed to describe crack initiation and propagation in HMA. This paper evaluates a finite element analysis technique that uses the embedded discontinuity method (EDM) for model cracking. The purpose of this study is to identify the strengths and potential weaknesses of the approach and investigate its applicability in general crack simulation for HMA pavements. An alternative formulation of EDM is adopted to make the approach easier to understand. The cohesive-crack model is used to describe development of HMA cracking. Numerical examples are presented to demonstrate the ability of EDM to simulate uniaxial-tension, three-point bending, and semicircular beam bending tests. It is shown that EDM is a promising finite element analysis technique, but additional research is needed to make it more robust.

Effects of Binder, Curing Time, Temperature, and Trafficking on Moduli of Stabilized and Unstabilized Full-Depth Reclamation Materials

This paper provides a summary of in situ layer moduli of several full-depth reclamation (FDR) materials used in an accelerated pavement test (APT) track. The reclaimed layers were constructed during the FDR of four lanes of 60 mm of rubberized hot-mix asphalt (HMA), 60 mm of HMA, and 130 mm of aggregate base. The FDR layer served as the base under a new HMA layer. Each lane had a different stabilization strategy, namely, unstabilized, stabilized with foam asphalt plus cement, stabilized with cement only, and stabilized with engineered asphalt emulsion. Falling weight deflectometer testing was conducted on the test track at various times during the course of the study, and the data were used to backcalculate the FDR layer moduli. The results were used to investigate the effects of each stabilization strategy, loading temperature, curing time, and trafficking on the in-place layer moduli of the FDR layers and to provide critical inputs for mechanistic–empirical pavement design of FDR layers.

RUTTING OF RUBBERIZED GAP GRADED AND POLYMER MODIFIED DENSE GRADED ASPHALT OVERLAYS IN COMPOSITE PAVEMENTS

The rutting performance of polymer-modified dense-graded and rubberized gap-graded asphalt mixes used in composite pavement was evaluated with full-scale accelerated pavement testing with the heavy vehicle simulator (HVS) and laboratory test results. The effect of asphalt layer thickness on measured surface deformation for both mix types was also investigated for recommendations for future design. In addition, the progression of the rutting failure mechanism for both mix types was evaluated with transverse cross sections measured with a laser profilometer at various HVS load repetitions. The polymer-modified dense-graded mix performed better than the rubberized gap-graded mix under both laboratory and HVS testing. The greater shear movement of the rubberized gap-graded mix under HVS loading caused larger humps, which consequently increased the maximum deformation and resulted in earlier failure. Larger aggregate size and denser gradation for the polymer-modified dense-graded mix resulted in more efficient dissipation of shear stresses and created greater permanent deformation resistance.

Extended Kalman Filter and Its Application in Pavement Engineering

Kalman filter is a signal processing technique that estimates the state of a dynamic system from a series of noisy measurements. It is used in a wide range of engineering applications from radar to computer vision. This chapter demonstrates the application of a model identification procedure based on extended Kalman filter (EKF) and weighted global iteration (WGI) technique in pavement engineering. In particular, EKF-WGI is used to perform layer moduli back-calculation from falling weight deflectometer (FWD) data and to identify model parameters for Generalized Maxwell Model for hot mix asphalt using frequency sweep test data. In both cases, EKF-WGI is shown to provide consistent results that are independent of the seed values for both linear and nonlinear problems. It is believed that EKF-WGI provides an efficient, consistent and robust tool for optimization that has many potential applications.

The Logit Model and the Need to Reproduce the Stiffness Degradation Curve of Asphalt Specimens During Fatigue Testing

Asphalt fatigue cracking is widely recognized as one of the most important pavement distresses, which is typically evaluated in the laboratory by conducting repeated load fatigue tests. There is no simple model that can fully reproduce the evolution of the stiffness of asphalt specimens during fatigue testing. This reality limits analysis and interpretation of asphalt fatigue data and the implementation of test results in mechanistic–empirical modeling. Two simple models, based on the logit function, are presented in this paper. These models, called logit and logit-sigmoidal, were found to reproduce almost exactly the evolution of specimen stiffness during flexural fatigue testing. This capability was used to locate critical points of the stiffness degradation curve, including failure points according to complex failure criteria. The mechanical meaning of the parameters of the logit model is analyzed in this paper. One of the parameters was related to the ability of the asphalt mix to sustain damage before crack initiation. Another parameter was related to the presence of reversible phenomena—heating and thixotropy—in the first part of the stiffness degradation curve. The applicability of the logit model to mechanistic–empirical pavement design is also evaluated in this paper on the basis of implementation in the CalME mechanistic–empirical software.

Calibration of asphalt concrete cracking models for California Mechanistic-Empirical Design (CalME)

Cracking is one of the major distress mechanisms for pavements with asphalt concrete surfaces. Given the composite nature of asphalt concrete, simulation methods such as the Discrete Element Method (DEM) that can incorporate material microstructure are required for properly describing the formation and progression of cracking in flexible pavements. These methods are however typically too time-consuming for use in routine design. As a trade off, a simplified approach based on continuum damage mechanics is taken in the California Mechanistic-Empirical method, called CalME, for practical considerations. The effect of cracking (broken contacts) is described as decreases in overall stiffness, which is indicated by damage. The rate of damage increase is in turn empirically related to peak strain energy endured by the material. The format and constants of this relationship are determined from laboratory fatigue testing of the asphalt concrete. Except for the first few loads, where temperature effects may be pronounced, all of the stiffness versus number of load applications curve are used. Once damage history is calculated, visual surface cracking history can be derived as an empirical function of damage and asphalt layer thickness. This paper presents the CalME fatigue and reflective cracking model and its calibration process using deflection data collected from various Heavy Vehicle Simulator (HVS) tests and the WesTrack accelerated pavement testing experiment.

Sine versus Haversine Displacement Waveform Comparison for Hot Mix Asphalt Four-Point Bending Fatigue Testing

An experimental study was conducted to determine the effect of haversine and sine displacement waveforms on four-point flexural beam fatigue test results for hot mix asphalt. Seven asphalt mixtures with different gradations, binder types, and binder contents were tested for this study. Four of the mixes were tested in California and three in Australia. The mixes were tested at different strain levels under both waveforms in displacement-controlled mode without rest periods. The comparison showed no differences between haversine and sine testing modes for six of the seven mixtures. Fatigue life, the shape of the stiffness reduction curves, initial dynamic moduli and phase angles, and Black diagrams were essentially the same for both testing modes. This similarity was attributed to the viscoelastic nature of the asphalt mix. Because of asphalt’s viscoelasticity, the beam at-rest position in haversine testing will rapidly move to halfway between zero and maximum displacement, and so the same stress is produced by haversine and sine displacement waveforms as soon as the peak-to-peak amplitudes are equal. For one of the seven mixes, sinusoidal testing produced considerably longer fatigue lives, although it is believed this mix is an outlier and can be ignored. Based on these results, it was concluded that there is no compelling reason to recommend sinusoidal over haversine testing mode, or vice versa. A practical reason to recommend the sine testing mode is that the stress in the haversine alternative becomes sinusoidal after a small number of load repetitions.