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3 CHAPTER ONE INTRODUCTION Introduction and Definitions Other forms of moduli and their definitions are presented in Figure 2. The initial slope of the “stress–strain” curve Historically, flexible pavement design practices were typi- is termed as initial tangent modulus (Emax), and the secant cally based on empirical procedures, which recommend modulus (E1) is termed as the slope of the line that joins the certain base, subbase, and surface layer types and their origin and a point on the stress–strain curve that represents thicknesses based on the strength of the subgrade. Recom- mendation of layer types and their dimensions were estab- lished based on AASHO road tests performed during the 1950s. The often-used soil strength parameters in this pave- ment design practice are California Bearing Ratio (or CBR) value, Hveem R value, and Soil Support Value (SSV). All these soil parameters are based on the failures of subgrade soil specimens in the laboratory conditions. However, flex- ible pavements seldom fail owing to subgrade strength fail- ures during their service life (Huang 1993). Most of the flexible pavements fail owing to either exces- sive rutting or cracking of pavement layers as a result of fatigue, temperature changes, and/or softening caused by the surface layer cracking (Barksdale 1972; Brown 1974, 1996). Because flexible pavements do not fail as a result of soil strength failure, the 1986 AASHTO interim pavement design guide and subsequently the 1993 AASHTO pave- ment design guide recommended the use of a soil parameter known as the Resilient Modulus (MR) to replace strength- based parameters such as CBR and SSV (Brickman 1989; Mohammad et al. 1994; Maher et al. 2000). Several other FIGURE 1 Definition of resilient modulus. investigations also refer to this modulus parameter as MR in their studies. The resilient modulus is analogous to the elastic modulus used in elastic theories and is defined as a ratio of deviatoric stress to resilient or elastic strain experienced by the material under repeated loading conditions that simulate traffic load- ing. Figure 1 presents a schematic of the resilient modulus parameter (MR). Most bases and subgrades are not elastic and they experience permanent deformation under repeated loads (Uzan 2004). However, because loads applied in the laboratory test for resilient modulus are small when com- pared with ultimate loads at failure and also the result of the application of a large number of cycles of loading that reduces the plastic deformation, the deformation measured during test cycles is considered as completely recoverable or elastic and hence the recovered deformations are used to estimate the resilient modulus or elastic modulus (or simply FIGURE 2 Definitions of other elastic moduli parameters modulus or stiffness). (Nazarian et al. 1996).
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4 50% of the ultimate deviatoric stress at which sample fails. Subsequent pavement design guides, including the 1993 The resilient modulus (MR) parameter is close to Emax for AASHTO Pavement Design Guide and the current Mech- stiff materials, and for soft soils the modulus lies in between anistic-Empirical Pavement Design Guide (MEPDG), E1 and Emax. Also, the laboratory methods using repeated describe design methodologies that use resilient modulus load triaxial (RLT) tests measure resilient moduli of tested as the primary input parameter for characterizing subgrade, materials whereas the field nondestructive methods using subbase, and base materials. Resilient modulus values of backcalculation subroutines and other approaches provide these materials are typically determined by performing lab- the elastic moduli of subgrades and bases. oratory tests using RLT tests simulating traffic loading con- ditions. Different resilient modulus test protocols, including The main reason for using the resilient modulus or modu- AASHTO test methods T-292, T-294, and T-307 as well as lus or stiffness as the parameter for subgrades and bases is TP-46, have been used by the departments of transporta- that it represents a basic material property and can be used in tion (DOTs) in the laboratory conditions. All these test the mechanistic analyses for predicting different distresses procedures have certain differences with respect to test such as rutting and roughness. This parameter has been used sequences of applying confining and deviatoric stresses, to directly determine the structural capacity of the subgrade measurement of deformations either inside the triaxial cell or determine the structural coefficient of the untreated base or outside the triaxial cell, application of seating stresses, and subbase layer. Based on the structural number value, specimen preparation steps, and conditioning methodol- pavement layers and their dimensions are designed in the ogy applied before the actual testing. These differences do AASHTO pavement design guide. contribute to several uncertainties, which are explained in chapter three. Table 1 presents a chronology of how these The pavement design approach is termed as mechanistic test methods were developed in the estimation of resilient because the design is based on the mechanics of materials moduli of subgrades. that relate traffic characteristics and information to pavement response output parameters, such as stress or strain of mate- Also, a CD-ROM guide developed by the FHWA as a part rials. Pavement response parameters are used to estimate or of Long-Term Pavement Products (LTPP) presents several predict distress using laboratory- and field-measured moduli interactive tutorials on resilient modulus of unbound mate- values. For example, the estimation of vertical compressive rials. Three videos that depict the MR test procedures were strains of subgrade and the moduli properties can be used to developed to aid practitioners, laboratory managers, and understand plastic deformation of subsoil, which contributes technicians (FHWA 2006). to the overall rutting of the pavement system. Such design approach is considered as mechanistic empirical because Irwin (1995) presented various limitations in both cur- statistical empirical relationships are used to correlate pave- rent test methods with respect to applied confining and devi- ment response parameters and distress magnitudes. atoric pressures and current nonlinear regression models Table 1 Chronology of AASHTO Test Procedures for MR Measurements Test Procedure Details Earliest AASHTO test procedure; No details on the sensitivities of displacement measurement devices AASHTO T-274-1982 were given; Criticisms on test procedure, test duration (5 hours long test) and probable failures of soil sample during conditioning phase; testing stresses are too severe. AASHTO procedure introduced in 1991; Internal measurement systems are recommended; Testing AASHTO T-292-1991 sequence is criticized owing to the possibility of stiffening effects of cohesive soils. AASHTO modified the T-292 procedure with different sets of confining and deviatoric stresses and their sequence; Internal measurement system is followed; 2-parameter regression models (bulk stress AASHTO T-294-1992 for granular and deviatoric stress model for cohesive soils) to analyze test results; Criticism on the analyses models. Procedural steps of P-46 are similar to T-294 procedure of 1992; External measurement system was Strategic Highway Research allowed for displacement measurement; Soil specimen preparation methods are different from those Program P-46-1996 used in T-292. T-307-1999 was evolved from P-46 procedure; recommends the use of external displacement AASHTO T-307-1999 measurement system. Different procedures are followed for both cohesive and granular soil specimen preparation. This recent method recommends a different set of stresses for testing. Also, a new 3-parameter model NCHRP 1-28 A: Harmonized is recommended for analyzing the resilient properties. The use of internal measurement system is Method-2004 (RRD 285) recommended in this method.