Theoretically, backcalculation of each individual bituminous layer is possible, but this is generally not advised because of the complexity of evaluating more than three or four pavement layers. If the total thickness of the bituminous layer is less than 75 mm (3 inches), the modulus of the bituminous layer should be fixed (see table 9 for guidance) to allow backcalculation of the base and subgrade moduli. It can be difficult to obtain reasonably backcalculated moduli for bituminous surface layers less than 75 mm (3 inches) thick. The following subsections provide guidelines for setting the layer thickness for each pavement layer.
(25) If this is the case, other means (e.g., laboratory testing) may be required for determining layer moduli. When a pavement structure consists of a stiff layer between two weak layers, the backcalculation process may produce unrealistic moduli. (25) If unrealistic results persist, then the analysis should consider the presence of a stiff layer. In this case, the lower base moduli may indicate contamination from the underlying subgrade, resulting in weaker base moduli owing to the presence of finer material. If the backcalculation results produce unrealistic weak base moduli, it may be advantageous to eliminate the base layer and evaluate the pavement structure as a two-layer system. Ideally, no more than three (preferable) or four layers with unknown moduli should be used in the backcalculation process.
Modulus values, including seed or initial moduli, and modulus range.Ī number of different factors must be considered in establishing a model of the pavement section for backcalculation, as described in the following sections. Pavement structure (layer types (e.g., HMA, PCC, base/subbase material), layer thicknesses, Poisson’s ratio for each layer (often assumed), material density (often assumed), and subgrade). Pavement temperature at the time of FWD testing. FWD testing configuration and results (load plate diameter, sensor locations, load level, test locations, and resulting deflections). The following inputs are needed to perform a backcalculation analysis: Inputs Needed for Backcalculation Analysis Summary of available backcalculation programs. Table 8 summarizes available software programs that can be used for backcalculation of pavement deflection data that the research team was able to identify during the conduct of this research study. Over the years, researchers and practitioners have developed numerous approaches to backcalculate pavement layer and subgrade moduli, as well as numerous software programs to perform the calculations. The primary difference between backcalculation and forwardcalculation is that the former uses specific equations, while the latter uses an iterative procedure in estimating layer moduli. Forwardcalculation can be used to estimate layer moduli for the subgrade and bound surface layers, while intermediate layer (e.g., unbound base) moduli are estimated using modular ratios. In forwardcalculation, load and deflection data are entered into closed-form equations for estimating layer moduli. A solution is found when the difference between the measured and calculated deflection basin is minimized (discussed in the following sections). An iterative approach is used to vary layer moduli until the calculated deflection basin matches the FWD-measured deflection basin. In the backcalculation process, pavement deflections are determined using layer elastic theory, layer thickness, and assumed layer moduli (e.g., HMA layer, unbound base layer, and subgrade). Backcalculation Versus Forwardcalculation In addition to the guidelines on pavement backcalculation, this chapter describes the results of studies that have verified backcalculated results with instrumented pavement sections and also presents an example illustrating the interpretation of results from a backcalculation program. The guidelines are intended to assist the pavement engineer in conducting the backcalculation process, evaluating the results, and ensuring that those results are reasonable however, they should be used only as general guidance because considerable engineering judgment and expertise is still required. General Backcalculation Guidelinesīecause the most common use of deflection data is in the backcalculation of the fundamental engineering parameters of the paving layers, this chapter has been prepared to provide general guidance on performing backcalculation. Using Falling Weight Deflectometer Data With Mechanistic-Empirical Design and Analysis, Volume III: Guidelines for Deflection Testing, Analysis, and InterpretationĬHAPTER 3.
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