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40 ration conditions seldom occur. Several regions in Western Several other studies researched information in the and Midwestern U.S. states belong to this category owing to LTPP database related to resilient moduli characteristics of arid and desert-like conditions. bases and subgrades (Alavi et al. 1997; Von Quintus and Killingsworth 1998; Yau and Von Quintus 2002; Richter • Regarding the sequence of testing and magnitudes of 2006). Yau and Von Quintus (2002) studied various test confining and deviatoric stresses applied, the majority variables, including the test and sampling procedures on of the studies followed those mentioned by the stan- the measured resilient moduli. MR data from LTPP were dard protocols. A few deviated from them by using the acquired, screened, and then analyzed. Only MR results of stresses expected in the field as per the traffic loading base and subbase aggregate layers and subgrade soils were conditions. studied. • Several soil-related variables including soil type, Atterberg limits, group index properties, and compac- The Yau and Von Quintus study (2002) aimed at devel- tion conditions influence the resilient property mea- oping relationships between resilient modulus and physical surements. These observations are in agreement with properties of the unbound materials and soils. Nonlinear the past MR studies performed before 1996. Studies regression equations were developed for each base and soil conducted by Maher et al. (2000), Hopkins et al. (2001), type to determine the resilient modulus at a specific stress Masada et al. (2004), Malla and Joshi (2006), and Ping state using the physical properties of the base materials and and Ling (2007) documented resilient properties of a soils. The models developed predict the resilient proper- variety of subgrade soils. Though no overlap was noted ties reasonably; however, the authors concluded that a bias in the test results reported by these studies, the MR val- is present in the calculated values. A final important out- ues of cohesive soils ranged from 2 to 42 ksi and the come of this research is the need for additional test results MR values of granular bases and subbases as well as to improve or confirm these models. More discussion on unbound aggregates ranged from 10 to 45 ksi. Another the models from the LTPP database is covered in the later study conducted by Kim and Labuz (2007) reported sections. resilient moduli of different aggregates containing dif- ferent amounts of reclaimed asphalt pavement (RAP) Richter (2006) presented an overview of several factors aggregates. An increase in the RAP amount resulted in that influence the moduli of unbound materials from nonfro- an increase in moduli values. Kim and Labuz (2007) zen grounds (see Table 9). This report specifically focused reported that the in situ aggregate blend from full-depth on seasonal moisture content and temperature fluctuations reclamation yielded moduli similar to blend containing as well as other related factors and how they influenced the 50% of natural and 50% RAP aggregates. backcalculated MR properties. • Selection of design moduli from the laboratory tests is arbitrary and no recommended procedures are avail- able. In a few studies, the average modulus over a International Perspectives set of confining pressures is calculated and used as a design modulus for a given test conducted on a certain In this section, the MR research work performed outside moisture condition. George and Uddin (2000) have the United States is briefly summarized. This task focused used a modulus value measured at a confining pres- mainly on the resilient moduli studies used for flexible pave- sure of 14 kPa under a deviatoric stress of 35 kPa as ment design. This investigation highlights other countries’ a design modulus. Hopkins et al. (2004) performed a research that could be useful for the present synthesis. comprehensive stress analysis of a subgrade subjected to traffic loading and then determined both confining Australia, New Zealand, and the United Kingdom have and deviatoric stress conditions that are representative been actively involved in the MR research for several years. of subgrades. A modulus measured from the labora- Research reports from the United Kingdom highlight the tory data corresponding to the stresses determined moduli studies conducted on both aggregate and recycled from layered elastic analysis is used as the design pavement bases as well as subgrades (Brown 1974, 1996; modulus. Dawson et al. 1996; Fleming et al. 1998; Lekarp and Daw- son 1998; Frost et al. 2004). Similar research reports and Overall, the information compiled over the last 10-plus pavement design guides from Australia and New Zealand years during the third phase of research showed consider- describe efforts in the characterization of unbound aggre- able advances on the resilient modulus testing of various gates in pavement design (Nataatamadja 1992, 1993; Nataat- subgrades and unbound aggregates, which lead to the devel- madja and Tan 2001; Vuong 2001; Vuong and Hazell 2003; opment of a large MR database for a better interpretation of Byers et al. 2004). resilient properties for mechanistic pavement design. The LTPP database, for example, includes MR information of dif- In South Africa, the pavement design practice also relies ferent soils and aggregates tested across the United States. on the resilient moduli properties, and both laboratory- and