American Association Of State Highway And Transportation Officials Design Research Articles

Scaled Sensitivity of Pavement–Mechanistic-Empirical Transfer Function Coefficients for Flexible and Rigid Pavements

The American Association of State Highway and Transportation Officials (AASHTO)Ware Pavement–Mechanistic-Empirical (ME) transfer functions need local calibration for reliable performance predictions. It is often not viable to calibrate all coefficients at the same time. Therefore, it is crucial to identify the most sensitive transfer function coefficients. Moreover, the sensitivity also indicates the impact of each coefficient on the performance prediction. Several studies have shown the sensitivity of the Pavement-ME design inputs, but limited research is available on the sensitivity of transfer function coefficients. Typically, the sensitivity is obtained using a normalized sensitivity index (NSI). This paper estimated the sensitivity of the Pavement-ME transfer function coefficients using scaled sensitivity coefficients (SSCs). NSI values are compared with the SSCs. Ten sections, each from flexible and rigid pavements, were used to calculate the NSI values. These are existing pavement sections from the Michigan Department of Transportation (MDOT) pavement management system (PMS) database, designed using the 1993 AASHTO design guide. Results show that SSCs provide a more reliable sensitivity on a range of independent variables rather than a point estimate, unlike NSI. NSI values showed variability among different sections and magnitudes of predicted performance. Calculation of SSCs is a forward problem and does not require any input data. A mathematical model (transfer function) is needed; therefore, SSCs can be calculated on any range of independent variables. It also shows the potential correlation between different coefficients and accuracy in estimation.

Evaluation of Design Procedure and Performance of Continuously Reinforced Concrete Pavement According to AASHTO Design Methods.

The Guide for Design of Pavement Structures (AASHTO 86/93) and Mechanistic Empirical Pavement Design Guide (MEPDG) are two common methods to design continuously reinforced concrete pavement (CRCP) published by the American Association of State Highway and Transportation Officials (AASHTO) in the USA. The AASHTO 86/93 is based on empirical equations to assess the performance of highway pavements under moving loads with known magnitude and frequency derived from experiments on AASHTO road tests. The MEPDG is a pavement design method based on engineering mechanics and numerical models for analysis. It functions by incorporating additional attributes such as environment, material properties, and vehicle axle load to predict pavement performance and degradation at the selected reliability level over the intended performance period. In order to evaluate the CRCP design procedure and performance, crack width (CW) and crack spacing (CS) from five examined test tracks in Europe with different climate condition, base layer, geometry, and materials were collected in this paper and compared with predicted distresses as well as CW and CS from AASHTO 86/93 and MEPDG design methods. The results show that the interactions between geometrics, material properties, traffic, and environmental conditions in the MEPDG method are more pronounced than in the AASHTO 86/93 and the prediction of CS and CW based on MEPDG matched closely with the recorded data from sections.

INFLUENCE OF RAILING STIFFNESS ON WHEEL LOAD DISTRIBUTION IN TWO-SPAN CONCRETE SLAB BRIDGES

The American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications or LRFD do not account for the presence of railings in the analysis and design of concrete slab bridges. This paper presents a parametric investigation of the influence of railing stiffness on the wheel load distribution in simply-supported, two-equal-span, and one-and two-lane reinforced concrete slab bridges using the finite-element analysis (FEA). A total of 160 bridge cases were modeled and bridge parameters such as span lengths and slab widths were varied within practical ranges. Various railing stiffness were investigated by assuming railings built integrally with the bridge deck and placed on both edges of the bridge. The FEA wheel load distribution and longitudinal bending moments were compared with reference bridge slabs without railings as well as to the AASHTO design procedures. Accordingly, the presence of railings reduced the FEA negative moments by a range of 54% to 72% and the FEA positive moments by a range of 40% to 61% depending on the railing stiffness. This reduction in slab moments due to the presence of railings could be considered an increase in the bridges load carrying capacity. The results of this investigation will assist bridge engineers in better designing and/or evaluating concrete slab bridges in the presence of railings. This could also be considered an alternative for strengthening existing concrete slab bridges.

Partial-depth paved shoulder effect on part-time shoulder use pavement bearing capacity

In areas of high population density, widening all classifications of roadways for growing traffic volumes becomes increasingly complex and expensive to complete. For major highways in Europe and the United States (US), use of the shoulder as a part-time lane is becoming progressively common and may be a solution to growing traffic on lower roadway classifications. This approach is known as hard shoulder running internationally and part-time shoulder use (PTSU) in the US. For interstates or other major highways, the shoulder design is a full-depth paved shoulder, the same pavement cross-section as the mainline lanes, and is structurally sound. For arterials and collectors, the shoulder design commonly has a different pavement cross-section from the mainline lanes, a partial-depth paved shoulder design. This study focused on evaluating the design strength of typical partial-depth paved shoulder designs in the US for PTSU using the American Association of State Highway and Transportation Officials (AASHTO) Guidelines for Design of Pavement Structures 1993 design. The study sites were also redesigned according to the AASHTO design methodology to handle the PTSU design traffic volumes. This study found that two of the nine states evaluated could withstand PTSU with minor to no reconstruction.

Service life of bonded concrete overlay

More than 60% of highways in Korea are constructed with cement concrete and when their condition has deteriorated rehabilitation and reconstruction are required. Recently, bonded concrete overlay has been considered as a possible alternative material for use in pavement rehabilitation as its material properties are similar to those of the existing concrete pavements. The present study investigated the service life of bonded concrete overlay and evaluated the appropriateness of the American Association of State Highway and Transportation Officials (AASHTO) prediction for bonded concrete overlay life based on a comparison with the measured life of bonded concrete overlay obtained from long-term pavement performance data analysis. The findings of this study lead to the conclusion that there is significant correlation between the predicted life of bonded concrete overlay based on the AASHTO design method and the measured life based on long-term pavement performance data after eliminating early distressed pavement sections. It was also found that the thickness and existing pavement type have significant influence on the service life of bonded concrete overlay.

Re-Evaluation of the AASHTO-Flexible Pavement Design Equation with Neural Network Modeling

Here we establish that equivalent single-axle loads values can be estimated using artificial neural networks without the complex design equality of American Association of State Highway and Transportation Officials (AASHTO). More importantly, we find that the neural network model gives the coefficients to be able to obtain the actual load values using the AASHTO design values. Thus, those design traffic values that might result in deterioration can be better calculated using the neural networks model than with the AASHTO design equation. The artificial neural network method is used for this purpose. The existing AASHTO flexible pavement design equation does not currently predict the pavement performance of the strategic highway research program (Long Term Pavement Performance studies) test sections very accurately, and typically over-estimates the number of equivalent single axle loads needed to cause a measured loss of the present serviceability index. Here we aimed to demonstrate that the proposed neural network model can more accurately represent the loads values data, compared against the performance of the AASHTO formula. It is concluded that the neural network may be an appropriate tool for the development of databased-nonparametric models of pavement performance.

Mechanistic-empirical pavement design guide (MEPDG): a bird’s-eye view

Past editions of the American Association of State Highway and Transportation Officials (AASHTO) Guide for Design of Pavement Structures have served well for several decades; nevertheless, many serious limitations exist for their continued use as the nation’s primary pavement design procedures. Researchers are now incorporating the latest advances in pavement design into the new Mechanistic-Empirical Pavement Design Guide (MEPDG), developed under the National Cooperative Highway Research Program (NCHRP) 1-37A project and adopted and published by AASHTO. The MEPDG procedure offers several dramatic improvements over the current pavement design guide and presents a new paradigm in the way pavement design is performed. However, MEPDG is substantially more complex than the AASHTO Design Guide by considering the input parameters that influence pavement performance, including traffic, climate, pavement structure and material properties, and applying the principles of engineering mechanics to predict critical pavement responses. It requires significantly more input from the designer. Some of the required data are either not tracked previously or are stored in locations not familiar to designers, and many data sets need to be preprocessed for use in the MEPDG. As a result, tremendous research work has been conducted and still more challenges need to be tackled both in federal and state levels for the full implementation of MEPDG. This paper, for the first time, provides a comprehensive bird’s eye view for the MEPDG procedure, including the evolvement of the design methodology, an overview of the design philosophy and its components, the research conducted during the development, improvement, and implementation phases, and the challenges remained and future developments directions. It is anticipated that the efforts in this paper aid in enhancing the mechanistic-empirical based pavement design for future continuous improvement to keep up with changes in trucking, materials, construction, design concepts, computers, and so on.

Investigating the accelerated deterioration of flexible pavement using two-stage design analysis approach

The accelerated deterioration of flexible pavement and its relation to design strength requirements is a major problem facing highway engineers. The goal of this research is to investigate a possible relationship between the accelerated pavement deterioration rates and pavement design strength using a two-stage design analysis approach. The approach in this study applies a two-stage solution to the three popular design methods of flexible pavement (i.e. California Department of Transportation (Caltrans) method, the American Association of State Highway and Transportation Officials (AASHTO) method, and the Asphalt Institute (AI) method) to yield pavement designs using stage load applications values specified for each investigated design load applications level. The resulting two-stage pavement designs are then used to obtain an indicator called the stage design strength ratio (percentage) defined as the ratio of the relative strength change to the design relative strength. The sample results indicate that the stage design strength ratio is relatively low compared to the corresponding increase in load applications especially at advanced service times. This could be a major contributing factor to the accelerated deterioration of flexible pavement. Therefore, it is recommended to provide initially stronger pavement structures. This can be done by either designing flexible pavement for a longer design period than the typical 20 years suggested by most design methods or using a higher terminal present serviceability index as in the case of the AASHTO design method.

An Optimum Design Approach for Flexible Pavements

An optimum approach for the design of flexible pavements has been developed which utilizes the anticipated performance of pavement and its life-cycle cost. The optimum approach developed has been applied to the design method recommended by the American Association of State Highway and Transportation Officials (AASHTO) for the design of flexible pavements. Pavement performance, defined using the initial and terminal serviceability indices, is a major design parameter that directly affects future pavement condition, initial construction cost and maintenance and added user costs. The optimum design is the one associated with the optimum terminal serviceability index and corresponds to the most cost-effective design. Cost-effectiveness is defined using a parameter called pavement life-cycle disutility which is the ratio of the pavement life-cycle cost to the pavement life-cycle performance identified by the area under the corresponding performance curve. The optimum pavement design is the one associated with the minimum pavement life-cycle disutility value and yields the optimum terminal serviceability index. The optimum terminal serviceability index value replaces the general AASHTO design index recommendations of 2.0 and 2.5 for minor and major roads, respectively. A performance curve is generated for a particular pavement structure using an incremental solution of the AASHTO basic design equation. It is shown that pavements should be designed for higher terminal serviceability index values than currently recommended.

Influence of Bridge Approach, Surface Condition, and Velocity on Impact Factors for Fatigue-Prone Details

An impact study on a long-span continuous highway steel girder bridge is described. Crawl and full-speed test data were analyzed to examine the influence of bridge approach, surface condition, and truck velocity on impact factors for fatigue-prone details in both main and secondary bridge members. The measured impact factors for fatigue stress categories A, B, E, and E’ details were compared with values obtained using the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications formula. For fatigue-prone details, the test results showed that surface roughness, truck velocity, and bridge approach affect the impact factor. Measured impact factors varied with the location of the detail. Also, the study showed that the AASHTO design impact factors were unconservative for details located in the bridge approach or end span.

Use of Nonlinear Subgrade Modulus in AASHTO Design Procedure

Resilient modulus ( M r ) of subgrade is a very important factor in the pavement design and evaluation process. Typically, this factor is evaluated using simple empirical (non–stress-dependent) relationships with CBR (California-bearing-ratio) values as defined by several agencies. However, with the current state of knowledge, it is evident that the response of typical unbound subgrade materials may be nonlinear and hence, a function of the state of stress. Because of this material's stress dependency, different pavement cross sections (due to change in layer thickness) and/or layer moduli will cause a change in the M r response of the subgrade material under given moisture-density conditions. Three approaches are presented in this paper to estimate the nonlinear resilient modulus response of fine grained soils to be used in the flexible design procedure from the American Association of State Highway and Transportation Officials (AASHTO). These approaches are based on stress-dependent Lotfi and Moossazadeh predictive equations for estimating nonlinear modulus of subgrade. A close comparison between the three approaches appears to be very encouraging in the characterization of nonlinear subgrade modulus.

Climatic Considerations in New AASHTO Flexible Pavement Design

This research is undertaken to study and assess carefully the effect of climatic factors on the new American Association of State Highway and Transportation Officials (AASHTO) design for flexible pavements. Climatic factors considered include seasonal changes in the subgrade moisture content and the annual variations in the ambient temperature. Through a sensitivity analysis of the new AASHTO equation, the change in the structural number required to offset a reduction in the subgrade resilient modulus due to an increase in moisture content is evaluated; an adjusted structural number is thus calculated. The impact of the ambient temperature on the asphalt‐concrete layer is evaluated by considering the interactive result of temperature and fatigue damage; the concept of weighted effective dynamic modulus is thus used. A rational method is proposed to select the appropriate asphalt grade to minimize both low‐temperature cracking and rut depth. By knowing the asphalt grade and the mix properties, the weighted, effective asphalt‐layer coefficient is calculated. An example to illustrate how climatic considerations can be incorporated into the AASHTO design is presented.