Mechanistic-empirical Approach Research Articles

Nondestructive Deflection Testing based Mechanistic-empirical Overlay Thickness Design Approach for Low Volume Roads: Case Studies

Illinois Department of Transportation (IDOT) uses an empirical design approach to conduct thickness designs of structural overlays for low volume roads. Modified layer coefficients for a limited number of material types are utilized to assign the structural capacity of in-service pavements. Despite the ease in use and simplicity, such an empirical approach is outdated and lacking in many aspects to characterize recycled and/or nontraditional construction materials nowadays more commonly used in pavements. As far as the rehabilitation of low volume roads is concerned, the lack of testing for evaluating the structural condition of existing, in-service pavements often results in uneconomical and unreliable practices. This paper presents a mechanistic-empirical approach for overlay thickness designs of low volume pavements through a combination of nondestructive deflection testing and pre-established pavement damage models. Twenty different pavement sections were selected from six counties in Illinois with varying structural and traffic characteristics. Falling Weight Deflectometer (FWD) tests were conducted on these road segments and FWD data were analyzed with appropriate temperature correction procedures to determine and monitor the structural conditions of existing, in-service pavement sections. Then, the corresponding required overlay thicknesses were determined for these twenty case sections using three different methods commonly used by local agencies such as, AASHTO 1993 NDT method, IDOT modified layer coefficient method, and Asphalt Institute deflection approach. The M-E Overlay Design method successfully identified structural deficiencies in the original pavement configurations through FWD tests and subsequently led to more economical or safer and more reliable overlay solutions for low volume roads.

Sensitivity Analysis of Flexible Pavement Sections Using Mechanistic-Empirical Pavement Design Guide

In damage analysis of flexible pavement sections, the AASHTO 2002 design guide, also called as MEPDG (Mechanistic-Empirical Pavement Design Guide), adopted a mechanistic-empirical approach. In the MEPDG, the pavement performance is computed using different input parameters that characterize pavement materials, design features and condition. However, these input parameter values are expected to differ to varying degrees and, therefore, the predicted performance may also vary to some degree depending on the input parameter values. The current study evaluated the influence of four input parameters, namely, reliability level, climate, traffic characteristics and modulus values on the performance of selected pavement sections using MEPDG software. Knowledge gained from the sensitivity analysis of different pavement sections using MEPDG is expected to be useful to pavement designers and others using MEPDG for future pavement design. Specifically, this paper focuses on the sensitivity study of flexible pavement sections at four different locations namely, Chicago in Illinois, Grand Forks in North Dakota, Oklahoma City in Oklahoma, and Houston in Texas, for addressing sensitivity towards climatic conditions. For addressing effect of reliability and traffic three levels of reliability (80%, 90%, 95%) and traffic (low, medium, high) were used, respectively, for designing flexible pavement sections. Additionally, sensitivity towards modulus of subgrade soil was evaluated by designing pavement sections containing 6% lime, 15% class C fly ash (CFA), and 15% cement kiln dust (CKD). The performance of each pavement section was monitored for 240 months (20 years) using MEPDG software by generating plots for rutting, alligator cracking, and International Roughness Index (IRI). It was found that rut predicted by MEPDG is sensitive towards climate, modulus values of chemically stabilized layer and reliability level. IRI values showed sensitiveness toward only reliability and traffic level. Alligator cracking showed sensitiveness toward climate with unexpected trend, modulus of chemically stabilized layer, reliability and traffic level.

Mechanistic-empirical approach to characterizing permanent deformation of reinforced soft soil subgrade

This study focuses on characterizing the permanent deformation of geogrid-reinforced soft soil subgrade by using the mechanistic-empirical (ME) approach based on both experimental measurements and results of numerical modeling. Two sets of small-scale pavement sections were built over two types of soft soil subgrade and subjected to cyclic moving loads by means of a reduced-scale accelerated pavement testing (APT) device. Each set of pavement sections included one control section and three sections reinforced by different geogrids placed at the base–subgrade interface. The pavement sections were instrumented to measure vertical deformation and vertical stresses at the top of the subgrade. Strains developed in the geogrids were also measured by strain gauges throughout the construction and accelerated testing. Simplified finite element (FE) models were created to simulate both the control and geogrid-reinforced pavement sections and to compute the pavement mechanistic responses. With the experimental measurements from the first set of tests and results of FE analysis, the permanent deformation model in NCHRP 1-37A for unbound layers was adapted and calibrated to model the permanent deformation of the geogrid-modified subgrade. The effects of geogrids on the subgrade permanent deformation were integrated into the ME performance model through both the mechanistic response modeling and the empirical calibration. Predictions from the permanent deformation model were then compared with the second set of measurements and found to underestimate the permanent deformation of the geogrid-modified subgrade. However, the model was able to distinguish the difference in performance among the sections, i.e. the predicted rank of the performance was consistent with the measurements.

A Mechanistic Empirical Approach Utilizing Controlled Tolerable Deflection for the Design of Thin Surfaced Asphalt Pavements

Thin surfaced asphalts are designed in a similar manner to spray sealed asphalts considering only rutting as the major mode of failure while there is no consideration for fatigue in the current mechanistic empirical pavement design method adopted by Austroads. However, sprayed seal surfacing are less sensitive to vertical deflection compared to thin surfaced asphalts. Thin surfaced asphalts are commonly prematurely failed by fatigue before any major rutting failure. In this research, the Austroads tolerable deflection criteria was investigated utilizing deflection and cumulative traffic data from 30 thin asphalt pavement sections from Queensland. The data showed that the current Austroads tolerable deflection criterion is largely overestimating the design life of these pavement sections. A new calibrated tolerable deflection criterion was developed. The calibrated tolerable deflection criterion was used with synthetic deflection data and pavement responses generated by multilayer elastic analysis using Circly software for 200 pavement sections to develop a new subgrade criterion. The new subgrade criterion is designed to limit the maximum pavement deflection to be within the calibrated tolerable deflection. By limiting the maximum deflection of the thin asphalt pavements to the new calibrated tolerable deflection, it is expected that surface curvature will be reduced and therefore the fatigue life of these pavements will be significantly improved and it will limit the premature failures of these surfaces. The new criterion produces a stiffer pavement structure compared to the current method. The high stiffness can be achieved by using thicker base course or stabilized bases and subbases structures.

Optimal pavement design and rehabilitation planning using a mechanistic-empirical approach

This paper presents the development of a pavement design and rehabilitation optimization decision-making framework based on Mechanistic-Empirical (ME) roughness transfer models. The AASHTOWare Pavement ME Design (the software of Pavement ME Design) is used to estimate pavement deterioration based on the combined effects of permanent deformation, fatigue, and thermal cracking. The optimization problem is first formulated into a mixed-integer nonlinear programming model to address the predominant trade-off between agency and user costs. To deal with the complexity associated with the pavement roughness transfer functions in the software and to use the roughness values as input to the optimization framework, a dynamic programming subroutine is developed for determining the optimal rehabilitation timing and asphalt concrete design thickness. An application of the proposed model is demonstrated in a case study. Managerial insights from a series of sensitivity analyses on different unit user cost values and model comparisons are presented.

Overlay Thickness Design for Low-Volume Roads

Identifying the appropriate overlay thicknesses is critical to a local transportation agency's ability to maintain its pavement network. Local agencies often use empirical approaches for designing the overlay thickness for low-volume pavements. For example, overlay design for low-volume roads in Illinois is currently carried out using assumed layer coefficients for a limited number of material types. Although such empirical approaches are fairly simple to use, they are often not suitable for considering the effects of recycled and nontraditional construction materials that are more commonly considered in current-day sustainable pavement applications. The lack of mechanical testing for evaluating the pavement structural condition often leads to uneconomical practices in the rehabilitation of low-volume roads. This paper presents a mechanistic–empirical approach for overlay thickness designs of low-volume pavements through a combination of nondestructive deflection testing and preestablished pavement damage models. Five pavement sections, with varying structural and traffic characteristics, were selected from two counties in Illinois. Three sets of falling weight deflectometer tests were conducted over a period of one year to monitor changes in pavement deflection responses. Structural conditions of the pavement sections in their original configuration were evaluated first. Then, the corresponding required overlay thicknesses were determined by using two methods currently used by local agencies. The inability of the currently available methods to properly account for current pavement structural conditions was highlighted. A new mechanistic–empirical overlay thickness design method introduced in this study successfully identified structural deficiencies in the original pavement configurations.

Mechanistic properties of the unaged and aged DGAMs

Short and long term aging were conducted on the dense graded asphalt mixtures (DGAMs) containing 40-50 penetration grade asphalt cement binders. The short term oven aging (STOA) was conducted on loose mixtures at temperature of 135 C for four hours and at temperature of 154 C for two hours, whereas, the long term oven aging (LTOA) was conducted at temperature of 85 C and at two periods of time between four and eight days. The performance tests includes: Marshall properties, indirect tensile strength at 25 and 60C, compressive strength at 25 and 60C, flexural strength at 0 and -10C, cohesion at 60C, tensile strength ratio, and index of retained strength were carried out on unaged and aged DGAM. A mechanistic-empirical design approach using BISAR program was adopted for estimating the improvement in service life of the pavement or reduction in thickness of DGAM and base layer for the same service life due to the aging of DGAM. The results showed that the STOA and LTOA increases: (1) resistance of DGAM against permanent deformation at high temperatures; (2) resistance to stripping phenomenon; (3) flexural strength at low temperatures; (4) traffic benefit ratio between 10 and 20%; and (5) reduces the thickness of the surface layer between 8 and 14%; and (6) the base thickness reduces between 35% and 55%.

Mechanistic–empirical approach for design of low volume pavements

This study proposes a more rational method to design low-volume roads (LVRs), which accounts for permanent deformation development in granular layers. Rutting is regarded as the main distress mode in unsealed and thinly sealed pavements. Hence, it is desirable that it be analytically approached rather than empirically. Twenty low-volume pavement sections from West Uttar Pradesh and Uttarakhand regions were considered for full scale in situ tests conducted annually for the period of 4 years to include the effect of weather and seasons on the rutting of granular pavements. The properties of the pavement material were then used in mechanistic analysis using finite element method for calculation of failure stresses and strains, and this leads to the mechanistic–empirical basis for design. Finally, the framework of the proposed method contributes to LVR pavement design procedures mainly due to its simplicity.

Modelling metal speciation in the Scheldt Estuary: Combining a flexible-resolution transport model with empirical functions

Predicting metal concentrations in surface waters is an important step in the understanding and ultimately the assessment of the ecological risk associated with metal contamination. In terms of risk an essential piece of information is the accurate knowledge of the partitioning of the metals between the dissolved and particulate phases, as the former species are generally regarded as the most bioavailable and thus harmful form. As a first step towards the understanding and prediction of metal speciation in the Scheldt Estuary (Belgium, the Netherlands), we carried out a detailed analysis of a historical dataset covering the period 1982–2011. This study reports on the results for two selected metals: Cu and Cd. Data analysis revealed that both the total metal concentration and the metal partitioning coefficient (Kd) could be predicted using relatively simple empirical functions of environmental variables such as salinity and suspended particulate matter concentration (SPM). The validity of these functions has been assessed by their application to salinity and SPM fields simulated by the hydro-environmental model SLIM. The high-resolution total and dissolved metal concentrations reconstructed using this approach, compared surprisingly well with an independent set of validation measurements. These first results from the combined mechanistic-empirical model approach suggest that it may be an interesting tool for risk assessment studies, e.g. to help identify conditions associated with elevated (dissolved) metal concentrations.

Ecological forecasting in Chesapeake Bay: Using a mechanistic–empirical modeling approach

The Chesapeake Bay Ecological Prediction System (CBEPS) automatically generates daily nowcasts and three-day forecasts of several environmental variables, such as sea-surface temperature and salinity, the concentrations of chlorophyll, nitrate, and dissolved oxygen, and the likelihood of encountering several noxious species, including harmful algal blooms and water-borne pathogens, for the purpose of monitoring the Bay's ecosystem. While the physical and biogeochemical variables are forecast mechanistically using the Regional Ocean Modeling System configured for the Chesapeake Bay, the species predictions are generated using a novel mechanistic–empirical approach, whereby real-time output from the coupled physical–biogeochemical model drives multivariate empirical habitat models of the target species. The predictions, in the form of digital images, are available via the World Wide Web to interested groups to guide recreational, management, and research activities. Though full validation of the integrated forecasts for all species is still a work in progress, we argue that the mechanistic–empirical approach can be used to generate a wide variety of short-term ecological forecasts, and that it can be applied in any marine system where sufficient data exist to develop empirical habitat models. This paper provides an overview of this system, its predictions, and the approach taken.

Designing Long Life Pavements Including Eco-Friendly ACs by Means of the Mechanistic-Empirical Approach

Two eco-compatible bituminous materials were analyzed as surface layers in two different structures under varying conditions. The pavement performances were simulated using the CalME design software with parameters obtained from Repeated Simple Shear Test and 4 Point Bending tests. The primary purposes of this study are to demonstrate the value of the eco-compatible mixes with regard to their mechanical performances and to examine the relations between their fatigue and rutting performances and the pavement structures. It was verified that the suggested asphalt thickness should correspond to a reliable pavement foundation when surface layers may suffer of stiffness and/or rutting deterioration.

Numerical integration approach to the problem of simulating damage in an asphalt pavement

A road develops permanent deformation or fatigue damage because of the stress and strain induced in its structure by surface loading and environmental change. Dynamic tyre forces generated by the vibration of moving heavy vehicles excited by the road surface profile are strongly influenced by vehicle speed and dynamic properties. A mechanistic-empirical approach is implemented here to simulate the deterioration of a pavement, taking account of dynamic excitation of the axles. This paper highlights the importance of statistical spatial repeatability in damage evolution during the pavement life. Numerical integration of the distribution of forces at each point is shown to be sufficient to predict the changing road surface and elastic modulus. This results in an approximate 100-fold increase in computational efficiency. Finally, the pattern of the forces generated by the axles of a half car is found to be a little less damaging than that of independent quarter cars. In the examples considered, the quarter car reduces calculated pavement life by an average of 6%.

Development of Mechanistic–Empirical Design Procedure for Fully Permeable Pavement under Heavy Traffic

Fully permeable pavements are those in which all layers are intended to be permeable and the pavement structure serves as a reservoir to store water during storm periods to minimize the adverse effects of stormwater runoff. Fully permeable pavements are currently used primarily for light traffic (e.g., parking lots that only allow cars, not trucks). The development of fully permeable pavement designs as a potential best management practice for stormwater management in areas that carry heavy truck traffic is of significant interest. Results are presented from one phase of a project to develop mechanistic–empirical design procedures for fully permeable pavement designs that will carry heavy vehicles operating primarily at slow speeds (e.g., in maintenance yards, parking lots, streets, and highway shoulders). Mechanistic–empirical computer modeling of the structural designs was performed for three types of surface material (open-graded asphalt, open-graded concrete, and concrete slabs with cast drainage), considering different traffic loading based on typical California highway truck traffic, different traffic speeds, and several typical California climate regions, as well as materials properties developed from laboratory testing. A preliminary design method and preliminary design charts that considered both hydraulic and structural performance were developed; some examples of their use are provided. The mechanistic–empirical approach used in this project for the development of fully permeable pavement designs could increase the speed of development and implementation of this technology.

New Mechanistic–Empirical Approach for Estimating the Effect of Roughness on Vehicle Durability

Vehicle manufacturers place a major focus on improving the design of vehicle components to respond better to changes in road surface profiles. Nevertheless, changes in the surface profile still directly affect user costs including repair and maintenance (R&M) costs and damage to goods. For example, AASHTO reported that poor road conditions added an estimated $76.8 billion to transport costs annually. The objective here is to estimate the effect of roughness, as expressed by the international roughness index, on vehicle durability and R&M costs. First, the R&M costs of Zaniewski et al. (the latest comprehensive research conducted in the United States) are updated by multiplying their reported costs by the inflation rate of R&M costs between 1982 and 2007. Then a mechanistic–empirical (M-E) methodology is proposed to estimate the effect of roughness on R&M costs. The proposed approach is based on fatigue damage analysis by using numerical modeling of the vehicle response. Finally, the results from the M-E approach are compared with the empirical results (updated Zaniewski tables) and found to be very close up to 5 m/km. The standard error is about 2%. Also, a case study of the I-69 section near Lansing, Michigan, is presented. This detailed analysis is useful for identifying those segments of the road within a project that cause higher operating costs to the traveling vehicles. These localized rough sections could then be subject to maintenance activities in order to remedy the problem.

Comparative Performance of the SMAC Made with the SBS- and ST-Modified Binders

Styrene-butadiene-styrene (SBS) and starch (ST) have been used by many to modify asphalt cement and to improve the properties of asphalt concrete. In this investigation, SBS and ST were used to modify asphalt cement. The basic properties of modified asphalt binders and stone mastic asphalt concrete (SMAC) containing such asphalt binders were studied and compared with those of asphalt cement. Marshall stability, Marshall Quotient, tensile strength, tensile strength ratio, rutting resistance, flexural strength, and resilient modulus were carried out on the SMAC. A mechanistic-empirical design approach was used for estimating the improvement in service life of the pavement or reduction in thickness of SMAC and base layer for the same service life due to modification the SMAC. In addition, an optimization table was prepared to select the type of SMAC for any field applications. The analyzes of test results show that the performance of SBS-modified SMAC is slightly better when compared to ST-modified SMAC. The temperature susceptibility can be reduced by the inclusion of SBS and ST in the SMAC. The results of multilayer elastic analysis presented herein indicate that the pavement consisting of SBS and ST-modified SMAC as a surface layer is beneficial in reducing the construction materials. Actual savings would depend upon the option exercised by the designer for reducing the thickness of an individual layer.

Growth of truck traffic volume for mechanistic-empirical pavement design

Pavement design is moving towards the mechanistic-empirical approach, which requires very detailed truck traffic information, including truck traffic volume. This paper focuses on analysis of truck traffic volume growth, specifically in three areas: characteristics of the growth pattern, sensitivity of pavement responses to the growth rate, and prediction of the growth rate based on road way characteristics and socio-economic data. The truck traffic data collected by weigh-in-motion (WIM) stations on California highways were used in the analysis. Results showed that both linear and compound growth models well fit the truck traffic growth trends. Growth rates estimated from less than six years data may have large variation, which can lead to significant errors in pavement response prediction. Various truck classes have distinct growth patterns and are affected differently by roadway and socio-economic characteristics.

Mechanistic analysis of ST and SBS-modified flexible pavements

This paper deals with the viability of using starch (ST) and styrene–butadiene–styrene (SBS) as an additive in stone matrix asphalt (SMA) mixtures. A 5% of ST and SBS by weight of bitumen were blended with (70/100)-paving grade asphalt. Unmodified and modified bitumen were subjected to physiochemical tests. The performance tests including, Marshall stability, tensile strength, tensile strength ratio and resilient modulus were carried out on unmodified and modified SMA mixtures. A mechanistic-empirical design approach was used for estimating the improvement in service life of the pavement or reduction in thickness of SMA and base layer for the same service life due to modification the SMA mixtures. The analyses of test results show that the moisture damage and temperature susceptibility can be reduced by the inclusion of ST and SBS in the asphalt mixture. The results also revealed that ST-modifier can be used as anti-stripping agent, and reduced plant emissions and energy consumption by 30%, as well as it shows resistant to fuels, most common chemicals and solvents. The results of multi-layer elastic analysis presented herein indicate that the pavement consisting of ST and SBS-modified SMA as a surface layer is beneficial in reducing the construction materials. In addition, the safe pavement design criteria of unmodified and modified SMA mixtures based on the anisotropic elasticity analysis through finite-element simulation were obtained and reported herein.

Development of a Permanent Deformation Model of Asphalt Mixtures for South Korean Pavement Design Guide

The Korean Pavement Design Guide (KPDG) was developed based on a mechanistic–empirical approach. This paper presents the permanent deformation model of asphalt mixtures in the KPDG. The permanent deformation model for various types of asphalt mixtures was developed by using the results of triaxial repeated loading tests. This model was calibrated with the accelerated pavement test (APT) and the Long-Term Pavement Performance (LTPP) database. Through the triaxial repeated loading tests, the permanent deformation model of asphalt mixtures was developed to address the effects of temperature and initial air voids. The model coefficients for the temperature and air voids obtained from the triaxial tests do not need to be calibrated. However, the model coefficients for the shift factor and number of load repetitions should be calibrated with APT data. This study found that the model requires a correction factor to consider the effects of total asphalt concrete (AC) layer thicknesses. The correction factor is useful for simulating the distribution of a real vertical plastic strain for different AC layer thicknesses. A shift factor was found by comparing the measured rut depth from the LTPP sections and predicted values from the KPDG program. A verification study showed that the final permanent deformation model developed in this study can predict the field rut depth of various AC pavements quite accurately.

Mechanistic approach for polypropylene-modified flexible pavements

The present study investigates the benefits of modifying the asphalt and stone matrix asphalt (SMA) mixture in flexible pavement. Fifty/sixty penetration grade asphalt cement and four proportion of pyrolisis polypropylene (PP) were selected. Unmodified and modified asphalt binders were subjected to rheological and homogeneity tests. The performance tests including, Marshall stability, tensile strength and compressive strength were conducted on unmodified and modified SMA mixtures. The regression relationships between the performance tests were obtained. A mechanistic-empirical design approach was used for estimating the improvement in service life of the pavement or reduction in thickness of SMA and base layer for the same service life due to modification the SMA mixtures. The analyses of test results show that the performance of PP-modified asphalt mixtures are better when compared to conventional mixtures. The temperature susceptibility can be reduced by the inclusion of PP in the asphalt mixture. A PP content of 5% by weight of asphalt is recommended for the improvement of the performance of asphalt concrete mixtures similar to that investigated in this study. The results of multi-layer elastic analysis presented herein indicate that the pavement consisting of PP-modified SMA as a surface layer is beneficial in reducing the construction materials. Actual savings would depend upon the option exercised by the designer for reducing the thickness of an individual layer.

Optimal Asphalt Pavement Design Considering Cost and Reliability

A typical asphalt pavement design based on the mechanistic-empirical approach offers multiple design alternatives for a given set of input values. The present paper emphasizes the fact that the cost as well as the reliability levels of these design alternatives are different. A simple methodology has been suggested in this paper to assist a pavement designer in selecting an optimal pavement design thickness which is cost effective yet does not compromise the reliability of the pavement design. Pavement design charts have been developed as an illustrative example to explain how the proposed methodology can be considered as an improvement over the deterministic design.