Structural Layer Coefficients Research Articles

Use of Long-Term Pavement Performance Data for Calibration of Pavement Design Models

The benefits of long-term pavement performance data for calibration of the AASHTO flexible pavement design model for Ontario conditions are documented. To ensure that the AASHTO-Ontario pavement design model reflects Ontario pavement design practice and matches the observed pavement performance, long-term pavement performance data for 65 flexible pavement sections were used. Thirty-nine sections had original construction, and 26 sections received one rehabilitation treatment (overlay). The data for each section included the type and thickness of paving materials, subgrade type, pavement performance, and traffic loads. The results of the calibration and verification process indicate that for new flexible pavement design the AASHTO-Ontario model yields results that are in good agreement with the observed pavement performance. For overlays, there was a large difference between the predicted and observed performance. This indicates that considerably more observations (pavement sections) are required for assessment of rehabilitation designs than for the original construction because the overlays are not always built for structural reasons only. Possible avenues for using long-term pavement performance data to establish the values of input parameters such as structural layer coefficients and modulus of subgrade are also described. Even relatively limited long-term pavement performance data provide valuable information for the calibration and verification of pavement design methods.

Procedure for Use of Falling Weight Deflectometer to Determine AASHTO Layer Coefficients

The AASHTO pavement design model is one of the most widely used empirical design models for flexible pavement design. A factor complicating its utility is the use of an abstract quality, the structural number (SN), to quantify the strength of the total pavement structure. A consequence of the use of SN is the need for structural layer coefficients to characterize the component materials of the pavement structure. These layer coefficients are difficult to assess directly. However, a method for determination of layer coefficients with a falling weight deflectometer (FWD) that are representative of the in situ behavior of the pavement materials is presented. This method is based on a model provided in the 1993 edition of the AASHTO Guide for Design of Pavement Structures for assessment of the effective SN. The Vermont Agency of Transportation (VTrans) has established a procedure that uses FWD data and the AASHTO effective SN model to characterize layer coefficients for Vermont pavement materials. VTrans found that the layer coefficients determined for unbound subbases were reasonable, whereas layer coefficients estimated for asphalt cement concrete materials were generally 25 to 35 percent higher than AASHTO’s implied maximum of 0.44. However, a statistical analysis indicates considerable support for the predictive qualities of FWD-derived layer coefficients to approximate the in situ layer coefficients estimated by an elastic layer simulation.

Simple Approach to Estimation of Pavement Structural Capacity

The use of the falling weight deflectometer (FWD) as a tool for investigating the structural strength of flexible and rigid pavements is now common. The backcalculation of layer moduli is the primary technique for data interpretation. This technique is cumbersome and expensive to apply to low-volume roads. A simple way of relating the FWD surface deflections to the performance of pavement structures, expressed by the structural number (SN), is presented. The technique is proposed as an alternative to existing methods in Louisiana using Dynaflect data. The relationship was deduced by statistical analysis, using the data collected at the Louisiana Transportation Research Center Pavement Research Facility. The study found that the correlation between the SN and the FWD deflections is satisfactory. Two different relationships are recommended for flexible and semirigid pavements. Both relationships can be applied in a pavement management environment because they allow evaluation of the required thickness for the asphalt overlay. It was found that the structural layer coefficients must be assigned functions of the laboratory moduli determined by the indirect tensile test, not functions of the backcalculated moduli.

Structural Layer Coefficients of Crumb Rubber–Modified Asphalt Concrete Mixtures

Structural layer coefficients for crumb rubber–modified (CRM) asphalt concrete mixtures were developed from the backcalculated moduli values using the falling weight deflectometer (FWD) test results on in situ pavements. Several test sections of recently built crumb rubber–modified pavements on three routes in Kansas (I-135, K-32 and US-56) were selected for this study. I-135 is a newly built asphalt pavement and the other two are gap-graded CRM overlays. Deflection data were collected with a Dynatest 8000 FWD at 21 locations at 7.5-m intervals on each test section on I-135, 22 locations on K-32, and 11 locations on US-56. For CRM asphalt mix overlays, the average surface layer coefficients from the equal mechanistic approach of analysis were found to vary between 0.11 and 0.46 with most values falling around 0.30. This indicates a lower structural layer coefficient value for the asphalt-rubber mix compared with the conventional asphalt concrete. For newly constructed CRM asphalt pavements, the structural layer coefficients varied from 0.25 to 0.48, with the average value around 0.35. These values are close to the design layer coefficient values used for conventional asphalt concrete layers. Large variabilities in computed structural layer coefficients for the rubblized jointed reinforced concrete pavement were observed. The structural layer coefficients computed for this layer varied from 0.10 to 0.35.

Characterization of Base/Subbase Materials under Repetitive Loading

Abstract Testing of granular materials for evaluation of Resilient Modulus (MR) suffers from the lack of a standardized test method. Although AASHTO over the past six years (1986 to 1992) has proposed three different testing procedures (T274-86, T292-91I, and T294-92I), some state transportation departments have adopted their own MR testing procedures. In this study, the factors affecting the measurement of MR such as the testing method used and stress applied and its sequence are investigated. In addition, the updated methodologies proposed by various agencies for MR of granular materials and the MR values obtained are reviewed and evaluated. It is found that the applied stress sequence has some effects on MR of granular materials. For the two aggregate types investigated in this study, with the same location of LVDT and dynamic waveform, the T294-92I testing procedure yields higher MR than that obtained by using the T292-91I testing procedure. The stress sequence in T294-92I infers a stiffening and strengthening effect on the specimen structure as the stress level increases from low to high. The variation of MR values due to testing procedure is found to be higher than those due to the aggregate source. The MR values obtained in this study are found to be lower than those reported in the literature. Furthermore, with the updated testing procedure for MR, the correlation between the structural layer coefficients and MR of granular base and subbase proposed by the AASHTO needs to be modified accordingly.

Minimum‐Cost Design of Flexible Pavements

The formulation of a mixed integer‐linear programming model to determine a minimum‐cost flexible pavement design is presented. The model identifies the number, type, and thicknesses of paving materials required to meet the structural strength requirements of the pavement system at a minimum initial cost to the highway agency. Policies regarding the designation of minimum and maximum layer thicknesses are accommodated in the model formulation, including the establishment of variable minimum and maximum thicknesses based on the underlying material. In addition, the structural layer coefficients are allowed to vary with the pavement configuration, including the number and type of constituent layers. The model can be used to calculate a marginal‐cost function which has applications in the selection of cost‐effective measures designed to strengthen the structural pavement capacity.

Structural Layer Coefficient for Flexible Pavement

In order to design flexible pavements employing the AASHO Interim Guide, one must know the layer coefficients of pavement materials. The results of a study to derive structural layer coefficients of several Mississippi pavement materials—surface mixture and base mixture of asphalt concrete, soilcement, and soil‐lime—are presented. Fatigue cracking is the criterion employed in deriving the structural layer coefficients. A model is developed to estimate fatigue life or performance or both of pavements treating traffic, material characteristics, and environmental effects as stochastic variables. Material characteristics that govern the pavement performance include resilient modulus and fatigue sensitivity. Although the inputs are probabilistic, the resulting model equation is deterministic and amenable to direct solution for the analysis/design of flexible pavements. Using this model with the stipulation that fatigue cracking in the wheel paths be less than 45%, the researcher establishes equivalent pavement...