Olga Selezneva

Development of a Mechanistic-Empirical Structural Design Procedure for Continuously Reinforced Concrete Pavements

The concepts and approach used to develop a mechanistic-empirical structural design procedure for continuously reinforced concrete pavements (CRCP) are presented. Key aspects of the CRCP design procedure— including design overview, design inputs, structural response model, incremental damage analysis, and prediction of CRCP deterioration— are addressed. The mechanism of punch-out development is the foundation of the structural design procedure. Punch-out development is modeled by using mechanistic principles and damage accumulation over the design life. The incremental damage is used to account for changes in many factors throughout the entire design period, including material properties (portland cement concrete strength and modulus, erosion of base), seasonal climatic conditions, traffic loadings, crack load transfer, subgrade support, and others. Each analysis increment represents a specific combination of these factors during a selected period of time. Finally, accumulated damage is correlated with CRCP punchouts by using extensive field data, and a sensitivity analysis is provided to show that the procedure is reasonable.

DEVELOPMENT OF RAPID SOLUTIONS FOR PREDICTION OF CRITICAL CONTINUOUSLY REINFORCED CONCRETE PAVEMENT STRESSES

A rapid solution is presented for predicting critical tensile stresses on the top surface of continuously reinforced concrete pavements (CRCP). These tensile stresses are responsible for the development of CRCP punchouts and have to be considered in a mechanistic-based design procedure. The solution is based on a combination of the neural network (rapid solution) and finite element (numerical analysis) techniques. This approach combines the convenience and computational efficiency of neural network solutions with the flexibility and power of the finite element analysis. Such a combination is quite efficient for analyzing damage accumulation in CRCP, which requires predicting portland cement concrete tensile stresses for a large number of loading and site condition combinations. The procedure for stress prediction is based on the finite element model developed with ISLAB2000. The neural network has been trained with the results from ISLAB2000. A concept developed specifically for this study—the equivalent CRCP structure—was used extensively to reduce the number of independent parameters of the neural net work and speed up its training. The proposed rapid solution provides a good match of the ISLAB2000 stress values for a small fraction of the computation cost. This makes the rapid solution CRCP a natural choice for analyzing CRCP stresses and for inclusion in a mechanistic-empirical CRCP design procedure.

Characterization of Transverse Cracking Spatial Variability: Use of Long-Term Pavement Performance Data for Continuously Reinforced Concrete Pavement Design

Mechanistic-empirical design procedures for continuously reinforced concrete pavement (CRCP) require characterization of variations in major design parameters so that a new or rehabilitated pavement can be designed for a desired level of reliability. Transverse cracking is an important CRCP design parameter affecting the prediction of crack width, crack load transfer efficiency, and critical stresses leading to longitudinal cracking and punchout development. The primary focus of this study was to investigate spatial characteristics of transverse cracking occurring in CRCP and to develop a theoretical model that would provide a means for systematic characterization of transverse crack spacing variability along the pavement length. Long-term pavement performance distress data were utilized to analyze transverse crack spacing characteristics for CRCP sections. From the results of the field data analysis, a theoretical model utilizing a Weibull distribution was developed to characterize the along-the-section transverse crack spacing frequency distribution. This theoretical model could be incorporated into the mechanistic-empirical procedures for CRCP structural design. The relationship between transverse crack spacing characteristics and punchout development and the location of the longitudinal cracks, which are a punchout precursor, were also analyzed using field survey data. Conclusions derived from field data analysis and the theoretical model presented here will be of interest to practicing engineers and researchers involved in CRCP design and performance modeling.

ESTIMATION OF PAVEMENT LAYER THICKNESS VARIABILITY FOR RELIABILITY-BASED DESIGN

Estimating the variability of key pavement design inputs is essential to reliability-based pavement design. The thickness of most pavement layers has a great impact on the outcome of practically all analyses of pavement performance. The within-section layer thickness variability is investigated here, as is the extent of the mean layer thickness deviation from its design thickness. Pavement layer thickness data (elevation and core measurements) from a large number of newly constructed flexible and rigid pavement sections in the Long-Term Pavement Performance (LTPP) program were examined. To determine the distribution type of the thickness data, a combined statistical test for skewness and kurtosis showed that (a) thickness variations within a layer indicate a normal distribution for 86% of 1,034 layers and (b) the mean thickness deviations from the design values may be assumed to be normally distributed for a layer having a given type and design thickness. The estimated thickness-within-layer variability values and the estimated typical thickness deviations derived from LTPP data may serve as benchmarks for use in pavement design reliability, construction quality assurance specifications, and other research studies. In addition, statistical comparisons of layer thickness variability indicators were made between the elevation and core layer thickness data to determine whether there are systematic differences between these two measuring methods. These results will be very useful to both researchers and practitioners who develop or use reliability-based pavement design procedures.

Performance of Recycled Hot-Mix Asphalt Overlays in Rehabilitation of Flexible Pavements

The most frequent application of recycling materials in pavements is the reuse of reclaimed asphalt pavement (RAP) to produce recycled hot-mix asphalt (HMA). When designed properly, RAP mixes have demonstrated quality comparable to virgin HMAs in laboratory tests. Despite all the information available about the quality of RAP mixes, obstacles still promote their more frequent use in pavement engineering. Short- and long-term field performance of RAP mixes was investigated compared with virgin HMA overlays used in flexible pavements. Data from the 18 Specific Pavement Studies-5 (SPS-5) sites from the Long-Term Pavement Performance program located across the United States and Canada were used. Performance data were collected during periods ranging from 8 to 17 years. Repeated measures analysis of variance was the statistical analysis tool chosen, pairing distress measurements with survey dates to compare performance and response. The results suggest that in the majority of scenarios RAP mixes have performance statistically equivalent to virgin HMA mixes. The statistical equivalency of deflections suggests that RAP overlays can provide structural improvement equivalent to virgin HMA overlays.

Calibration of Mechanistic-Empirical Performance Model for Continuously Reinforced Concrete Pavement Punch-Outs

The field calibration of a mechanistic-empirical (M-E) continuously reinforced concrete pavement (CRCP) punch-out prediction model developed under NCHRP Project 1-37A is presented. The model elaborates on the procedure adopted to obtain a national calibration curve for punch-outs, the quantity and extent of data used, and the development of a reliability design procedure based on the calibration data. The model was based on a national database of pavement performance data and advanced mechanistic CRCP structural evaluation and damage accumulation concepts presented in earlier works by other authors. Sensitivity plots are also presented to illustrate how the model responds to changes in key design features considered in the design of CRCP pavements. The model presented can be used to perform M-E design of CRCP pavements.

NATIONAL PARK SERVICE ROAD INVENTORY PROGRAM: QUALITY ASSURANCE SAMPLING CONSIDERATIONS FOR AUTOMATED COLLECTION AND PROCESSING OF DISTRESS DATA

The National Park Service Road Inventory Program (RIP) uses automated collection and processing of pavement-condition data. The study examines various approaches to quality assurance (QA) sampling used to evaluate those data. The RIP includes automated collection and processing of pavement-condition and roadside-inventory-feature data at 258 national parks, covering over 5,000 mi of paved roads. Evaluating the quality of some data elements requires manual data assessment conducted by trained data analysts. Because of the high volume of the collected and processed data, it is economically impractical to check quality of all data manually. To overcome this difficulty, a search for an appropriate QA data sampling methodology was conducted. The goal of the investigation was to determine appropriate statistical procedures and required sample sizes so that conclusions based on QA sample testing could be extrapolated to the whole data set with a certain level of confidence. Evaluation of QA sampling approaches, including the selection of statistical procedures for QA testing, determination of QA sample sizes, and development of the procedures for evaluating the QA testing results applicable to the RIP, is presented. The results will be of interest to practitioners involved in automated collection of pavement data and to researchers involved in the design of statistical testing procedures for engineering QA applications.

Dynamic performance of composite pavements under impact

The importance of developing a deep understanding of the behavior of pavement layers under the action of dynamic loads, and the availability of cutting-edge computational and visualization technologies, led to the study presented in this paper. Explicit finite-element analysis was used to investigate the propagation of dynamic displacements induced in pavement layers under the action of an impact load similar to the one applied in a falling weight deflectometer test. The time-dependent dynamic response of a rigid pavement with straight asphalt concrete overlay was studied for two cases of unbonded and fully bonded interfaces between different layers. Significant differences in behavior were observed. Three-dimensional computer graphics and animation of the deformed model were used to display the propagation of vertical dynamic displacements through pavement layers. It was found that in the absence of a perfect bond between all pavement layers, the displacements measured on the top surface correlated little with the deformation measured in subsequent layers. In this case, a complicated pattern of behavior took place between the asphalt overlay and the concrete. The time histories of vertical displacements at selected surface locations and on the top and bottom of every layer were plotted. The plots revealed the existence of time shifts between the maximum displacements experienced by each layer, irrespective of the type of bond assumed between the interfaces.

COMPARISON OF AS-CONSTRUCTED AND AS-DESIGNED FLEXIBLE PAVEMENT LAYER THICKNESSES

Sound pavement design is important for improving pavement performance, but construction is equally critical. Because of variations in pavement construction, the as-constructed pavement layer thickness varies spatially within a pavement section, and the mean constructed pavement layer thickness often deviates from the designed values. The as-constructed pavement layer thicknesses are compared to their design values by using data from the newly constructed flexible pavement sections in the Long-Term Pavement Performance program. First, the distribution type of the mean layer thickness deviation was investigated, and a combined statistical test for skewness and kurtosis showed that the mean thickness deviations may be assumed to be normally distributed for a given layer type and target thickness. Typical thickness deviation summary statistics values were derived to estimate the extent of deviations from the design thicknesses. Second, the analysis showed that elevation thickness data differ from the core examination data. Furthermore, as-constructed layer thickness values were compared to their design values by looking into the percentage of the individual measurements falling into specified ranges from the design values and by using two-sided and one-sided t-tests to compare the mean constructed thicknesses and the design values. The analyses showed that the mean constructed layer thicknesses tend to be above the design value for the thinner layers and below the design value for the thicker layers for the same layer and material type. Results from the study will be useful as inputs for reliability-based design procedures.

IMPROVING RELIABILITY OF PAVEMENT LOADING ESTIMATES WITH PAVEMENT LOADING GUIDE

The development of the Long-Term Pavement Performance (LTPP) Pavement Loading Guide (PLG) was initiated to improve the reliability of traffic load estimates for the LTPP sections that do not have measured axle load data. The PLG contains extensive traffic data obtained from the LTPP database that may constitute the best available source of traffic data at the national level, a user-friendly graphical interface, and guidelines intended to help the user with the development of axle load spectra. Because of these features, the PLG will also facilitate traffic projections for general pavement design and management purposes. The uncertainty associated with estimating annual axle load spectra was quantified by assuming that the measured data do not exist and must be estimated. The estimated data were obtained by using axle load spectra obtained at similar sites in the same jurisdiction and utilized prototype PLG software. The difference between the estimated and the measured traffic loads was quantified by expressing axle load spectra in terms of equivalent single-axle loads. The results show that reasonable traffic load estimates can be obtained by judiciously selecting replacement traffic data. Although the PLG can reduce uncertainty of traffic forecasts and facilitate traffic forecasting, surrogate data can never replace site-specific data.