# Comparison of Lab vs. Backcalculated Moduli of Virgin Aggregate and Recycled Aggregate Base Layers
## 1 Introduction  
The stiffness characteristics of unbound layers play vital roles in the structural integrity of flexible pavements. The resilient modulus (\(M_{R}\)), as a measure of unbound layer stiffness, was initially introduced by the American Association of State Highway and Transportation Officials (AASHTO) Pavement Design Guide in the 1986 Edition [1] and is also used as an essential input in the pavement analysis/design procedures, such as the Mechanistic-Empirical Pavement Design Guide (MEPDG) [2]. The \(M_{R}\) of a material is commonly estimated in a laboratory using sensitive equipment, making it time-consuming and expensive, and requiring well-trained personnel to carry out the testing [3]. Estimating a reasonable value for the MR of unbound layers, which closely reflects the actual behavior of unbound materials in the field, is a prerequisite for an efficient pavement analysis/design. For this purpose, applying falling weight deflectometer (FWD) testing for estimating the \(M_{R}\) of unbound layers has gained attention among highway agencies [4, 5]. This method uses the FWD test results to backcalculate the stiffness of the unbound layers (\(E_{FWD}\)) using the deflection basin on the pavement surface. Several studies have compared the laboratory-measured \(M_{R}\) of unbound layers with their corresponding \(E_{FWD}\) from FWD testing. Along with considerable variation in the conversion factors, many researchers have also proposed wide ranges of values in the literature. This can be attributed to different types of materials, variations in the field and lab environment, and the difference in time between testing for the \(M_{R}\) in the lab and FWD in the field [6]. It is pertinent to mention that the AASHTO guide [7] allows for both modulus types and suggests a factor of 0.33 to be used with the \(E_{FWD}\). In relevant research, [PERSON] et al. evaluated subgrade samples from the existing pavements. They found that the laboratory \(M_{R}\) values were almost two times greater than those backcalculated from FWD testing [8]. [PERSON] and [PERSON] studied different base and subgrade materials from the Long-Term Pavement Performance (LTPP) Program database. They proposed a conversion factor ranging from 0.1 to 3.5 between the \(M_{R}\) and backcalculated \(E_{FWD}\) values. It has been reported that the conversion factor is highly dependent on the pavement type and temperature gradient within the AC layer [9]. In another study by [PERSON] et al., the mechanical properties of about three dozen specimens with considerable diversity in stiffness and material type (from clayey subgrade to high-quality bases) were studied. The results showed that using the seismic method, the conversion factor between the laboratory-measured MR and the field modulus ranged from 0.1 to 1.0 [10]. [PERSON] et al. reported a conversion factor of 1.6 between the laboratory-measured \(M_{R}\) and the backcalculated \(E_{FWD}\) of granular materials [11]. In another study using the LTPP database, it was reported that the backcalculated \(E_{FWD}\) values were, on average, about 1.7 and 1.9 times greater than the laboratory-measured \(M_{R}\) values for fine-grained and coarse-grained soils, respectively [12]. While every study presents a related reasoning behind the suggested conversion factors, more deliberation and research are still required to correlate the two values of different magnitudes, i.e., the \(M_{R}\) and \(E_{FWD}\). Conversion between the two may only be validated if the pavement layers, with particular \(M_{R}\) and \(E_{FWD}\) values, show a similar extent of strain response to loading in both cases. A more realistic conversion can be expected because such responses are the key parameters for calculating the damage accumulation in the mechanistic-empirical (ME)-based pavement analysis/design procedures.