Pavement Design Methodology Research Articles

Effect of Thermal Stresses on Pavement Performance under Mild Climate Conditions

Thermal stress on the surfaces of pavements located in mild climates has been commonly considered to have a negligible effect on pavement performance. Current pavement design methodology focuses on maximum tensile stress rather than tensile stresses occurring over the design life of pavements. To identify the significance of thermal stress on pavement performance through structural stress analysis, this study investigated the predictive capability of the enhanced integrated climate model by comparing it with measured temperature data and introducing an analytical methodology of estimating thermal stress development using the dynamic modulus test. The identified thermal stresses were compared with the tensile stresses occurring at the bottom of an asphalt layer through the analysis method employed in the Guide for Mechanistic–Empirical Design of New and Rehabilitated Pavement Structures. Damage caused by the thermal and bottom stresses was evaluated on the basis of the damage ratio derived from the hot-mix asphalt fracture model. The conclusion drawn from the investigations was that the effect of thermal stress on pavement cracking performance, evaluated through the damage analysis considering the time-and temperature-dependent nature of asphalt mixtures, is much less significant than that of bottom stress under mild climate conditions. Under the nonaging condition, 1-year average damage occurring due to thermal stress at typical pavement structures would be slightly less than 0.5% of that which would be expected to occur at the bottom.

Resilient Modulus of Clay Subgrades Stabilized with Lime, Class C Fly Ash, and Cement Kiln Dust for Pavement Design

The effects of different percentages of one traditional additive—hydrated lime—and two by-products—Class C fly ash (CFA) and cement kiln dust (CKD)—on the resilient modulus (Mr) of four different clay subgrades were investigated. It was found that at lower application rates (3% to 6%), the lime-stabilized soil specimens showed highest enhancement in the Mr values. At higher application rates (10% to 15%), however, CKD treatment provided maximum improvements. Three log-log and one semi-log stress-based models were evaluated. Validation of selected models was conducted by using additional soil data. Overall, a semi-log deviatoric and confining stress model was found to show the best acceptable performance. Additional analyses were performed to develop correlations of Mr with compacted specimen characteristics and soil and additive properties. To illustrate the application of the semi-log model, the AASHTO flexible pavement design methodology was used to design asphalt concrete pavement sections.

Repeatability of Asphalt Strain Measurements under Falling Weight Deflectometer Loading

Embedded instrumentation has become an important tool at accelerated load facilities as states begin implementing mechanistic–empirical pavement design and analysis methodologies. Instrumentation can provide valuable information to help validate mechanistic models and develop a deeper understanding of pavement response under a wide range of conditions. Before these types of investigations are conducted, however, it is critical to determine if the response measurements are accurate and precise. Although both accuracy and precision are important, this investigation centers on establishing the practical level of within-gauge precision for asphalt strain measurements made at the National Center for Asphalt Technology Pavement test track. Ten test sections, each containing 12 strain gauges, were included in this study. Falling weight deflectometer testing was conducted on strain gauges, and the absolute differences were calculated to determine individual gauge variability. It was found that within-gauge variability was not significantly affected by gauge orientation, load level, or pavement condition. Overall, strain readings with differences less than 12 μ∈ were set as a practical benchmark for the test track for within-gauge variability. It is recommended that, as the sections continue to deteriorate, further testing be conducted to better quantify the relationship between variability and pavement distress.

Repeatability of Asphalt Strain Measurements under Full-Scale Dynamic Loading

Embedded instrumentation has become an important tool at accelerated load facilities as states begin implementing mechanistic-empirical pavement design and analysis methodologies. Instrumentation can provide valuable information to help validate mechanistic models and develop a deeper understanding of pavement response under a wide range of conditions. Before such investigations are conducted, however, it is critical to determine whether the response measurements are accurate and precise. Although both accuracy and precision are important, this investigation centers on establishing the practical level of between-gauge precision for asphalt strain measurements made at the National Center for Asphalt Technology Pavement Test Track. Eleven test sections, each containing six paired gauges, were included in this study. Absolute differences between paired strain readings were computed on a per-truck and per-axle basis during 8 months of testing that included approximately 80,000 strain readings. It was found that longitudinal strain was slightly less variable between gauges than transverse strain. Steer axles had the least differences in strain between gauges, followed by the tandem and trailing single axles. The condition of the pavement (i.e., extent of cracking) was found to be very important to the measured differences between gauges. Overall, 80% of strain readings with differences less than 30 μ∈ were set as a practical benchmark for the test track

Strain Distribution in the Asphalt Layer under Measured 3-D Tire-pavement Contact Stresses

ABSTRACT Most pavement design methodologies assume the tire-pavement contact stress is equal to the tire inflation pressure and uniformly distributed over a circular contact area. However, tire-pavement contact area is not in a circular shape and the contact stress is neither uniform nor equal to the tire inflation pressure. To precisely account for the effect of actual contact stress on pavement response, this study evaluates the horizontal strains at the bottom of the asphalt layer produced by measured 3-D non-uniform stresses applied on the pavement surface. A multi-layer linear-elastic computer program, CIRCLY, was used to estimate horizontal strains in longitudinal direction and transverse direction under a number of combinations of load, tire pressure and asphalt thickness. Results show that the uniform stress model underestimates the longitudinal strains under a combination of thin surface layer and low tire pressure, and underestimates the transverse strains under a combination of thin surfacing, ...

Strain Distribution in the Asphalt Layer under Measured 3-D Tire-pavement Contact Stresses

ABSTRACT Most pavement design methodologies assume the tire-pavement contact stress is equal to the tire inflation pressure and uniformly distributed over a circular contact area. However, tire-pavement contact area is not in a circular shape and the contact stress is neither uniform nor equal to the tire inflation pressure. To precisely account for the effect of actual contact stress on pavement response, this study evaluates the horizontal strains at the bottom of the asphalt layer produced by measured 3-D non-uniform stresses applied on the pavement surface. A multi-layer linear-elastic computer program, CIRCLY, was used to estimate horizontal strains in longitudinal direction and transverse direction under a number of combinations of load, tire pressure and asphalt thickness. Results show that the uniform stress model underestimates the longitudinal strains under a combination of thin surface layer and low tire pressure, and underestimates the transverse strains under a combination of thin surfacing, low tire pressure and high wheel loading.

Comparisons of Flexible Pavement Designs

Flexible pavement designs and performance predictions derived from the empirical 1993 AASHTO pavement design methodology and the mechanistic–empirical NCHRP Project 1–37A approach are compared for a range of locations within the United States, each with its own climate, subgrade, and other material properties and local design preferences. Particular emphasis is devoted to the influence of traffic and reliability levels. The results suggest that relative to the NCHRP Project 1–37A predicted performance, the 1993 AASHTO guide overestimates performance (i.e., underestimates distress) for pavements in warm locations and at high traffic levels. Trends of pavement performance with reliability level were similar for both methodologies. The results suggest that the default design criteria incorporated in the NCHRP Project 1–37A software are broadly consistent with what would be observed historically from pavements designed with the 1993 AASHTO guide.

Comparisons of Flexible Pavement Designs: AASHTO Empirical Versus NCHRP Project 1-37A Mechanistic-Empirical

Flexible pavement designs and performance predictions derived from the empirical 1993 AASHTO pavement design methodology and the mechanistic-empirical NCHRP Project 1-37A approach are compared for a range of locations within the United States, each with its own climate, subgrade, and other material properties and local design preferences. Particular emphasis is devoted to the influence of traffic and reliability levels. The results suggest that relative to the NCHRP Project 1-37A predicted performance, the 1993 AASHTO guide overestimates performance (i.e., underestimates distress) for pavements in warm locations and at high traffic levels. Trends of pavement performance with reliability level were similar for both methodologies. The results suggest that the default design criteria incorporated in the NCHRP Project 1-37A software are broadly consistent with what would be observed historically from pavements designed with the 1993 AASHTO guide.

Axle Load Spectra Characterization by Mixed Distribution Modeling

Traffic comprises a primary input to any pavement design methodology. As pavement design evolves from traditional empirically based methods toward mechanistic-empirical (M-E), there will be a greater emphasis placed on accurate characterization of the distributions of axle loads, because they are entered directly into pavement response models. Known as load spectra, these distributions of axle weight are often complex and not well represented by individual theoretical statistical distributions. This research proposes a mixed distribution model of two or more theoretical distributions to accurately characterize axle load spectra. This paper details the model development and demonstrates its predictive capability using weigh-in-motion data from 13 sites in Alabama. The data show the predictive capability of the mixed distribution model through high R2 values, exceeding 0.96 in all cases. The typical axle load distribution in Alabama is comprised of a combination of lognormal and normal distributions.

Connectionist Approach to Improving Highway Vehicle Classification Schemes: The Florida Case

The mechanistic-empirical pavement design methodology being developed under NCHRP Project 1-37A will require accurate classification of vehicles to develop axle load spectra information needed as the design input. Scheme F, used by most states to classify vehicles, can be used to develop the required load spectra. Unfortunately, the scheme is difficult to automate and is prone to errors resulting from imprecise demarcation of class thresholds. In this paper, the classification problem is viewed as a pattern recognition problem in which connectionist techniques such as probabilistic neural networks (PNN) can be used to assign vehicles to their correct classes and hence to establish optimum axle spacing thresholds. The PNN was developed, trained, and applied to field data composed of individual vehicles' axle spacing, number of axles per vehicle, and overall vehicle weight. The PNN reduced the error rate from 9.5% to 6.2% compared with an existing classification algorithm used by the Florida Department of T...

Connectionist Approach to Improving Highway Vehicle Classification Schemes

The mechanistic–empirical pavement design methodology being developed under NCHRP Project 1–37A will require accurate classification of vehicles to develop axle load spectra information needed as the design input. Scheme F, used by most states to classify vehicles, can be used to develop the required load spectra. Unfortunately, the scheme is difficult to automate and is prone to errors resulting from imprecise demarcation of class thresholds. In this paper, the classification problem is viewed as a pattern recognition problem in which connectionist techniques such as probabilistic neural networks (PNN) can be used to assign vehicles to their correct classes and hence to establish optimum axle spacing thresholds. The PNN was developed, trained, and applied to field data composed of individual vehicles’ axle spacing, number of axles per vehicle, and overall vehicle weight. The PNN reduced the error rate from 9.5% to 6.2% compared with an existing classification algorithm used by the Florida Department of Transportation. The inclusion of overall vehicle weight as a classification variable further reduced the error rate from 6.2% to 3.0%. The promising results from neural networks were used to set up new thresholds that reduce classification error rate.

Investigation of Relationship Between Roughness and Pavement Surface Distress Based on WesTrack Project

Modern pavement rehabilitation and design methodologies require an adequate evaluation of the functional capacity of pavements. A key component of this functional capacity is the roughness of the pavement. The current standard for characterization of a pavement’s roughness is the international roughness index (IRI). Pavement roughness measurements were conducted at regular intervals during the application of approximately 5 million equivalent single-axle loads at the WesTrack Project, a full-scale flexible pavement accelerated loading facility located near Reno, Nevada. The results are presented of an investigation into the relationship between pavement roughness and pavement surface distress using WesTrack data. With a sample population of 317 observations, a relationship was found among the roughness (IRI) and the initial IRI, percentage of fatigue cracking, and average rut depth. A test of the relationship with data collected as a part of the Long-Term Pavement Performance Program indicates favorable results.

Analysis of Current State Rigid Pavement Design Practices in the United States

Current portland cement concrete pavement design practices and the key concrete pavement design features used by state highway agencies in the United States are summarized. This information was obtained from a comprehensive survey conducted in 1994 and 1995 under an NCHRP project. Pavement types, design methodologies, and reliability levels are included, along with many design inputs. Parameters that the states use to characterize pavement site conditions, including climate, subgrade, and traffic conditions, are given. The designed concrete slab thicknesses for different site condition combinations are compared. An analysis of variance was conducted to compare the mean slab thicknesses designed in different climatic regions. This examination and summary of the details of current pavement design practices and design features for concrete pavements in the United States will be of interest to both pavement researchers and practitioners.

Analysis of Current State Rigid Pavement Design Practices in the United States

Current portland cement concrete pavement design practices and the key concrete pavement design features used by state highway agencies in the United States are summarized. This information was obtained from a comprehensive survey conducted in 1994 and 1995 under an NCHRP project. Pavement types, design methodologies, and reliability levels are included, along with many design inputs. Parameters that the states use to characterize pavement site conditions, including climate, subgrade, and traffic conditions, are given. The designed concrete slab thicknesses for different site condition combinations are compared. An analysis of variance was conducted to compare the mean slab thicknesses designed in different climatic regions. This examination and summary of the details of current pavement design practices and design features for concrete pavements in the United States will be of interest to both pavement researchers and practitioners.

Structural Design of Concrete Block Pavements

Several methods for designing concrete block pavements (CBP) for roads and streets are presently available. All recognize the need to consider the subgrade soil, paving materials, environment, and anticipated traffic. There are a number of limitations associated with each, including inadequate characterization of the subgrade soil and paving materials, lack of pavement performance prediction capabilities, and inability to specify desired pavement failure and reliability level. This paper presents a method to overcome these limitations. The AASHTO flexible pavement design methodology is used as the fundamental framework for developing this procedure. Several modifications allow for the characterization of concrete block pavers, as well as for various levels of engineering analysis, depending on the availability of information. The proposed design methodology is demonstrated through a numerical example.

Airfield Pavement Evaluation—FAA Viewpoint

The FAA is currently revising the theory and design procedures utilized for the structural design and evaluation of airport pavements. The new pavement design methodology employs several recommended changes to the theory under which previous design curves were based, i.e.: (1) Use of measured rather than assumed soil strength in flexible pavement design; (2)assumption of jointed edge loading of slabs instead of interior slab loading in rigid pavement design; and (3)employment of nondestructive testing techniques (in conjunction with destructive tests) to evaluate load carrying capacity of pavements and in the design of pavement overlays. The net result in the new methodology will be a slight increase in runway pavement thickness requirements.