Shiraz Tayabji

Evaluation of Mechanistic-Empirical Performance Prediction Models for Flexible Pavements

In recognition of the potential of mechanistic-empirical (M-E) methods in analyzing pavements and predicting their performance, pavement engineers around the country have been advocating the movement toward M-E design methods. In fact, the next AASHTO Guide for Design of Pavement Structures is planned to be mechanistically based. Since many of the performance models used in the M-E methods are laboratory-derived, it is important to validate these models using data from in-service pavements. The Long-Term Pavement Performance (LTPP) program data provide the means to evaluate and improve these models. The fatigue and rutting performances of LTPP flexible pavements were predicted using some well-known M-E models, given the loading and environmental conditions of these pavements. The predicted performances were then compared with actual fatigue cracking and rutting observed in these pavements. Although more data are required to arrive at a more conclusive evaluation, fatigue cracking models appeared to be consistent with observations, whereas rutting models showed poor agreement with the observed rutting. Continuous functions that relate fatigue cracking to fatigue damage were developed.

Concrete Slab Track State of the Practice

Railway track technology has evolved over a period of 150 years since the first railroad track on timber ties was introduced. For much of this period, the conventional track system, commonly referred to as the ballasted track system, has consisted of certain components including rails, ties, ballast, and the subgrade (roadbed). Over the last 20 years, there has been an increase in the use of concrete slab technology for transit, commuter, and high-speed train applications. Essentially, a slab track consists of a concrete slab placed on a subbase over a prepared subgrade. The rails may be directly fastened to the concrete slab, or the rails may be placed on concrete blocks or another slab system that is placed on (or embedded in) the underlying concrete slab. A version of the slab track, developed in the Netherlands, incorporates rails embedded in a trough in the slab and surrounded by elastomeric material. The slab track systems for passenger service applications incorporate several requirements to mitigate noise and vibration. The slab track system for transit applications in the United States and for high-speed rail in Europe and Japan has performed well over the last 20 years. Also, the limited application of the slab track system for mixed passenger service-freight operations has also exhibited good performance. Because of the continued increase in gross tonnage expected to be carried by railroads and the expected growth in high-speed passenger rail corridors, with the smaller deviation in the rail geometry allowed for high-speed rail, the need for a stronger track structure is apparent. At-grade concrete slab track technology is expected to fill the need for stronger track in the United States. The state of the practice related to concrete slab track technology is summarized.

Calibration of Mechanistic-Empirical Rutting Model for In-Service Pavements

Rutting is a major failure mode for flexible pavements. Pavement engineers have been trying to control and arrest the development of rutting for years. Many models are available to relate pavement rutting to design features, traffic loading, and climatic conditions. These models range from purely empirical to mechanistic models. Mechanistic-empirical models (the Asphalt Institute and Shell) were used to predict the development of rutting for 61 Long-Term Pavement Performance (LTPP) test sections. The rutting damage, calculated using these models, did not appear to be a good predictor of the observed rutting depth. A new rutting model was developed and calibrated using the data from the 61 LTPP sections. The model accounts for the plastic deformation in all pavement layers and allows the use of actual axle load and type, rather than the equivalent single axle load, in characterizing traffic.

Using Transverse Profile Data to Compute Plastic Deformation Parameters for Asphalt Concrete Pavements

Previous studies have shown that the performance of in-service pavements may deviate significantly from that predicted by use of laboratory-calibrated performance models. Therefore calibration of performance prediction models with data from in-service pavements is important. Calibration of mechanistic rutting models by use of transverse profile data is explored. A well-known family of mechanistic rutting prediction models uses plastic deformation parameters [slope of elastic or plastic strain (or both) and load hardening factor] for quantification of the amount of permanent deformation resulting from each load application. For the purpose of obtaining these parameters, two traditional methods have been used: repeated load testing in the laboratory and calibration by use of time-series data from in-service pavements. Although the first suffers from the lack of compatibility between laboratory-predicted and actual performance, the second requires collection of field data for an extended period of time (years of monitoring) and may be interrupted by rehabilitation activities. The transverse profile contains valuable information that can be used for determining the contribution of each pavement layer to the observed rutting and the plastic deformation parameters. Transverse profile data were used for calibration of rutting prediction models. The stability and sensitivity of the computed parameters were also investigated.

EVALUATION OF IN SITU MOISTURE CONTENT AT LONG-TERM PAVEMENT PERFORMANCE SEASONAL MONITORING PROGRAM SITES

Time domain reflectometry (TDR) has become one of the most reliable nondestructive methods for measuring in situ soil moisture content. TDR sensors developed by the Federal Highway Administration are being used in the Long-Term Pavement Performance (LTPP) Seasonal Monitoring Program (SMP) to monitor the in situ moisture content at 64 LTPP sites. The main goal of this study is to develop procedures to produce good estimates of in situ gravimetric moisture content. All the TDR traces in the LTPP information management system database that were recorded at LTPP SMP test sections were processed using the approach described in this paper. To estimate the in situ gravimetric moisture content, methods were selected to interpret TDR traces. An algorithm and a computer program, Moister, were developed to implement these TDR interpretation methods. Then the apparent length of the TDR trace and the dielectric constant of the unbound material were computed. Models were developed to relate dielectric constant with in situ volumetric moisture content. Finally, gravimetric moisture content was computed using the volumetric moisture content value and dry density of the soil. A diagnostic study of the computed gravimetric moisture content was also conducted to evaluate the reasonableness of the computed moisture content.

MECHANISTIC EVALUATION OF TEST DATA FROM LONG-TERM PAVEMENT PERFORMANCE JOINTED PLAIN CONCRETE PAVEMENT TEST SECTIONS

Over the years, pavement engineers have attempted to develop rational mechanistic-empirical (M-E) methods for predicting pavement performance. In fact, the next version of AASHTO’s Guide for Design of Pavements is planned to be mechanistically based. Many M-E procedures have been developed on the basis of a combination of laboratory test data, theory, and limited field verification. Therefore, it is important to validate and calibrate these procedures using additional data from in-service pavements. The Long-Term Pavement Performance (LTPP) program data provide the means to evaluate and improve these models. A study was conducted to assess the performance of some of the existing concrete pavement M-E-based distress prediction procedures when used in conjunction with the data being collected as part of the LTPP program. Fatigue cracking damage was estimated using the NCHRP 1–26 approach and compared with observed fatigue damage at 52 GPS-3 test sections. It was shown that the LTPP data can be used successfully to develop better insight into pavement behavior and to improve pavement performance.

Precast Concrete Pavement Technology

Precast concrete pavement (PCP) systems have shown great potential for rapid rehabilitation and reconstruction of deteriorated pavement sections. Strategic Highway Research Program 2 (SHRP 2) Renewal Project R05, Modular Pavement Technology, was aimed at developing the necessary information and guidelines to encourage adoption of this new technology. This report provides an assessment of the state of the practice for PCP technology and guidance on the design, fabrication, installation, and maintenance of PCP systems. Two PCP applications were investigated: 1) intermittent repairs for full-depth repairs or full slab replacement, generally used on jointed concrete pavements; and 2) continuous applications for longer-length or larger-area rehabilitation of asphalt concrete or cement concrete pavement. Jointed PCP and precast prestressed concrete pavement were used for these applications. Field testing performed at 16 PCP projects as part of SHRP 2 R05 indicates that the currently used PCP systems are capable of performing well under traffic loading. The performance of projects constructed in the United States indicates that sufficient advances have been made to reliably achieve four key attributes of PCPs: constructability; concrete durability; load transfer at joints; and panel support conditions. The behavior and performance of constructed PCP systems appear to be similar to that of cast-in-place concrete pavements.

Joint Load Transfer and Support Considerations for Jointed Precast Concrete Pavements

Many agencies recently have started investigating strategies for pavement rehabilitation and reconstruction that are faster to implement and can produce longer-lasting pavements than previous strategies. Most highway agencies no longer consider expedient rehabilitation that results in a shorter pavement lifespan acceptable. One promising alternative rehabilitation strategy is the effective use of modular pavement technologies, principally precast concrete pavement (PCP) systems, which provide for the rapid repair and rehabilitation of pavements and also result in durable, long-lasting pavements. Rapid construction techniques can significantly minimize the impact on the driving public because lane closures and traffic congestion are minimized. Road user and worker safety also are improved by reduced road users’ and workers’ exposure to construction traffic. The renewal focus area under Strategic Highway Research Program 2 (SHRP 2) emphasizes the need to complete highway pavement projects rapidly, with minimal disruption to highway users and local communities, and to produce pavements that are long lasting. One goal of this focus area includes applying new methods and materials to preserve, rehabilitate, and reconstruct roadways. The effective use of PCP technologies for rapid repair, rehabilitation, and reconstruction of pavements addresses this goal. One of the projects funded under SHRP 2 is Project R05, Modular Pavement Technology. The objective of Project R05 was to develop better guidance for use by highway agencies to design, construct, install, maintain, and evaluate modular pavement systems, principally PCP systems. Findings related to joint load transfer and support considerations for jointed PCP from the Project R05 study are presented.

Performance of Precast Concrete Pavements

Precast pavement technology is a new and innovative construction method that can be used to meet the need for rapid pavement repair and construction. Precast pavement systems are fabricated or assembled off-site, transported to the project site, and installed on a prepared foundation (existing pavement or re-graded foundation). The system components require minimal field curing time to achieve strength before opening to traffic. These systems are primarily used for rapid repair, rehabilitation, and reconstruction of asphalt and portland cement concrete (PCC) pavements in high-volume-traffic roadways. The precast technology can be used for intermittent repairs or full-scale, continuous rehabilitation. As part of the US Strategic Highway Research Program 2 (SHRP 2), a study (Project R05) is underway to develop tools for the design, construction, installation, maintenance, and evaluation of precast concrete pavements. As part of this study, testing was conducted to obtain field performance data from selected precast concrete pavement projects constructed throughout the US. This paper summarizes the field test data collected from intermittent repair projects as well as from continuous application projects and presents the findings of the data evaluation.

VARIABILITY OF CONCRETE MATERIALS DATA IN LONG-TERM PAVEMENT PERFORMANCE PROGRAM

As part of a study sponsored by NCHRP, research was conducted to evaluate the variability of concrete materials data in the Long-Term Pavement Performance (LTPP) program. The following materials-related data were studied: (a) compressive strength, (b) flexural strength, (c) split tensile strength, and (d) modulus of elasticity. The variability was determined in terms of the standard deviation and coefficient of variability. The strength and stiffness data used came from the General Pavement Study (GPS) as well as the Specific Pavement Study (SPS) test sections. The analysis of data indicates that, in general, the LTPP program GPS test sections as well as the LTPP program SPS test sections exhibit characteristics of well-controlled construction projects with respect to the strength- and stiffness-related properties of concrete. The results of the analysis indicate that on well-controlled concrete pavement construction projects it would not be unreasonable to produce concrete that has a coefficient of variation of 15% or less for compressive strength, flexural strength, split tensile strength, and the modulus of elasticity. The findings from the analysis can be used to refine statistics-based quality assurance/quality control procedures for concrete acceptance and to define the measures of variability to be used in mechanistically based concrete design procedures.