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CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH BACKGROUND Pavement design and evaluation for purposes of construction and rehabilitation require the careful evaluation of a number of factors including material properties, traffic type and volume, the environment, construction and maintenance variables, and engineering economics. Undoubtedly, material properties are one of the most significant factors in the structural design and performance of pavements. Design me~ods for flexible pavements in He past have mainly involved empirical correlation of field performance wig material properties determined either in He laboratory or He field. Extrapolation of these empirical me~ods beyond the specific conditions for which they were developed can lead to erroneous results. Also, empirical design me~ods cannot take into account all possible failure mechanisms. These deficiencies can result In either pavements that are designed with a large factor of safety or pavements that fad! prematurely thus resulting in a heavier burden to He taxpayers. Traffic conditions are changing rapidly with trends toward faster and heavier vehicles, higher traffic volumes and new types of loadings. These changing trends will encourage, in the future, the replacement of existing empirical design methods by either updated empirical methods or mechanistic based approaches. The continued use of empirical methods would require costly statistical research at frequent intervals as new vehicle configurations emerge and new materials are developed. The development of mechanistic based methods requires a fundamental scientific study to establish the correlation between pavement performance and response including He effects of material properties. The SHRP Longterm Pavement Performance (LTPP) monitoring program will provide a valuable data base for use in developing new pavement design procedures. The 1986 AASHTO Guide for the Design of Pavement Structures has incorporated the resilient modulus of component materials into the design process. Also, considerable attention in the pavement industry has recently been focused on the development of mechanistic based approaches for design and evaluation of pavements. Both the 1986 AASHTO Guide method and all mechanistic based design me~ods use He resilient modulus of each layer in the design process. Mechanistic based design methods use layered theories to predict pavement response. Good relationships have been found to exist between flexible pavement performance and the stresses, strains, and displacements calculated by lavered pavement theories using appropriate resilient moduli for the layers. _~ -~---~ r More research effort has to be directed toward the development of practical, yet reliable, laboratory and field test procedures for the characterization of pavement materials. The additional cost of developing suitable design mesons and laboratory test procedures and their implementation is justified by the ability to more efficiently use paving materials and develop cost-effective, reliable pavement designs. ' 1
BASIC CONCEPTS FOR REPEATED LOAD TESTING Vehicle Loading As traffic moves over a pavement surface, large numbers of rapidly applied stress pulses of varying magnitude are applied to each element of material below and for some distance out to the sides of the wheel path (Figure I). The presence of pavement surface irregularities causes the vehicle to bounce which leads to impact loads as great as twice the static value. For moderate vehicle speeds, the stress pulse lasts between about 0.02 to 0.4 sec., with the pulse time increasing with increasing depth below the pavement surface and decreasing vehicle speed (Figure 2~. The type and geometry of the pavement has only a secondary influence on the pulse shape and duration. Near the surface the stress pulse has a pronounced haversine shape. With depth the pulse duration becomes greater and, although it remains approximately haversine in shape, a triangular loading gives a reasonably good approximation. Repeated Load and Cyclic Testing Since pavement materials are subjected to a series of distinct load pulses, a laboratory test duplicating this condition is desirable. The repeated load type test has for many years been used to simulate vehicle loading. In the repeated load test, instead of applying a single slow loading, a series of load pulses are applied that are separated by a distinct rest period as shown in Figure 3a. The repeated load test concept can be incorporated into many conventional static types of tests such as the diametral, biaxial, beam bending and simple shear tests. The resilient modulus of the material tested is then determined from the results of these dynamic tests. A continuous, sinuso'4ally varying load (Figure 3b) is also sometimes used to characterize the dynamic response of pavement materials. This wave form is referred to as cyclic loading. Elastic Constants. For isotropic, linear-elastic behavior, a material is completely characterized by 2 elastic constants determined from suitable material tests. Usually in pavement design, the modulus of elasticity E and Poisson's ratio v are He 2 elastic constants evaluated in ache laboratory and used in layered theory. Once any 2 elastic constants such as E and ~ have been evaluated, all other elastic constants can be calculated using simple equations derived from the theory of elasticity. The bulk modulus (K) and shear modulus (G) are also sometimes used in layered pavement analyses. The constants K and G are more fundamental ones than E and ~ since they are related to volume change and shear distortion, respectively. As a result, in the future they may gain more use in pavement analysis and design. Resilient Modulus. The resilient modulus (MR) is analogous to Be modulus of elasticity (E) with bow terms having the same basic theory of elasticity definition. The resilient modulus is determined from a repeated load test. Peak values of stress and recoverable (i.e., resilient) deformation occurring in the test are use to calculate the resilient elastic constants even though peak stress and recoverable deformation do not occur at the same time in a dynamic test of this type. 2
12t 10 8 Creep Speed_ ~ /~ CONVENTIONAL / /' 1 ~ \~x \ 1\ ( 18 KS ) /~; '~/ - \ ~ \\ \ l ~ /A _ At ·'\~X if 40 '` 40 30 20 10 0 10 2 030 LEfT TRANSVERSE DISTANCE, INCHESRIGHT Figure 1. Measured vertical stress pulse applied to the subgrade due to a slowly moving vehicle - 23 in. thick pavement (after reference 1 ) 1~0 . 1 1 ANGLE ~ MEL MEL Led - _ _ _ VEllICLE VELOCITY. V _ ,DE~H lENEAIN ,AVE~E" S0Rf4CE I1N`N~) - VERTICAL ~N~ - tU5£ ran -- VERTICAL no HEW Figure 2. Variation of equivalent vertical stress pulse time with vehicle velocity and depth (after reference 2) 3
Although this approach neglects energy loss effects, use of the resilient response appears to give satisfactory results for calculating pavement response. Rotation of Principal Stress Axes When a wheel load moves toward and then past an element of material, this element is subjected to stress states similar to those shown in Figure 4. Each element of material is subjected to a simultaneous buildup in both the major principal stress (o,) and the minor principal stress (03~. As those stresses build up, a rotation of the principal stress axes also takes place Figure 4~. A complete reversal of shear stress also occurs. Unfortunately, as discussed later, these stress rotation effects are not duplicated in the commonly used repeated load diametral and biaxial tests. Initial Stress State Most pavement design approaches, including the AASHTO methods, presently use in the thickness selection process a single value of the resilient modulus of each layer. Therefore, to select design resilient moduli the representative stress state acting upon each layer must be either known or assumed. The complete stress state consists of the combined effect of the initial residual stresses existing abler construction and the dynamic stresses caused by traffic loading. Temperature and moisture induced stresses in stabilized layers are also important, but have received almost no attention. During construction, heavy compaction equipment is used to density in thin lifts the subgrade, base, and surfacing. The heavy construction equipment then use each completed lift as a temporary working surface. Usually the greatest stresses to which a particular layer is ever subjected are applied during either compaction or else by construction equipment before the pavement is completed. The application of large vertical stresses during this stage of construction cause lateral stresses to develop which can become locked Into the layer [4, 5, 61. These locked in stresses are called residual stresses or residual lateral stresses. Residual Stress. Uzan [4] has pointed out that residual lateral stresses of 2 psi and 6 psi have been observed for cohesionless and cohesive soils. Methods of analysis proposed by Uzan [4], Selig [7] and Duncan and Seed [5] are quite encouraging for predicting residual lateral stresses due to compaction of both granular and cohesive soils. Selig [7] concluded that the residual lateral stress is the most important factor limiting permanent deformation in the bottom of a granular base. Residual lateral stress is also an important factor determining the appropriate confining pressure at which to evaluate He resilient modulus. The residual lateral stress oh, is relatively large and can be expressed as follows [71: ~hr=Ko(7o (~) where: o0 = vertical overburden stress 4
LOAD c~a~ LOAD (~) Repeated L~ding ~n ~: _ J ·\ ~ '.W . .' .,. .~.. J~ · LOAD ~E B · RECOVERY ll~E C · CYCL£ ~ 1., \ R£P£ATED / \ LO" p ~ · .\ ~ "s, - - ~nNG~ r~ INPU7: ~ ~tsoSJN( o3T ) (b) Cyclic Loading - Sinusoltal Wa~ve Fora Figure 3. Repeated load and cyclic loading wave forms (after reference 3 ~ VEH1CI-E aonoe ~nos ~ ~T10N ~llm tIlll1 /~/_ //~% I/~_ /~y '% 1' %`O1 q' I q. `~ 4 V-~ ~ /~// ~ STATIOIIARY ELElilEllT ~< Of I"TERIAL @3 q. ~` I ~ r llAJOR FRINCIPAL STRESS AXIS, FOS~10# ~ \~ BLUOR PRINCIPAL STR£SS AXIS, FOSITION S Figure 4 . Rotation of principal stress ax~s of an element as a vehicle moves over the surface 5
Ko = coefficient of lateral earth pressure considering residual stress The coefficient of lateral earn pressure associated with residual stress is greater than unity but less than the passive coefficient of earth pressure (Kp). For a granular material Me passive coefficient of lateral earth pressure Kp = tan2 (45-~/2) where ~ is the angle of internal friction of the material. Residual Stress Experimental Study. A fi~-scale field study was conducted as a part of this study to evaluate residual compaction stresses In a I.5 in. maximum size crushed granite gneiss base 12 in. thick. The base was placed in 6 in. lifts and compacted with a 10 ton smooth drum, vibratory roller. Lateral compaction and residual stresses were measured using a free field pressure gage consisting of an aluminum cube approximately 2.5 in. on each side. The cubic-shaped pressure gage was designed to be as rigid as practical to simulate an aggregate. The pressure gage was partially hollow on the inside, and the hollow space was filled with a mixture of water and anti-freeze. An active diaphragm was placed in one face of the gage wig fluid between it and a low displacement, miniature pressure cell located on the other side. Measurements made in the field on two bases, wig the active diaphragm of the gage oriented horizontally, indicated that average residual lateral pressures of about 0.5 psi exist in the middle of a 6 in. aggregate base or the bottom 6 in. of a 12 in. base. In a 12 in. base compacted in two 6 in. lifts, a 3 psi residual stress was observed in the top 6 in. An additional confining pressure of 3 psi can cause an increase in resilient modulus in a base on the order of 10 to 159S or more compared to neglecting this effect. OBJECTIVES OF RESEARCH The resilient modulus of pavement materials is usually evaluated using the repeated load biaxial test. A large variation in results, however, has been observed to occur between different test methods and testing laboratories. The ~rimarv objective of this study is to develop and recommend laboratory test en ~ e' · ~ · ~ ~ , · e es e , ~ ~ e ~ . . e ~ e ~ en ~ procedures for determining resilient moduli of component materials In a flexible pavement structure. Another objective is to access the applicability and constraints of using He resilient modulus to establish structural coefficients for He flexible pavement procedure in the 1986 AASHTO Guide. Note Hat this second objective was accomplished and the results were submitted separately as changes to the 1986 AASHTO Guide. The specific tasks of the research as given in the REP are as follows: I. Review the state-of-the-art procedures and equipment for laboratory resilient modulus testing. 2. Develop detailed Laboratory test procedures for evaluating the resilient modulus of asphalt concrete, aggregate base/subbase materials, and subgrade soils. 3. Perform limited multi-lab validation testing to refine the proposed test procedures. 4. Compare and analyze field determined moduli using common nondestructive devices with moduli determined using validated test procedures. 6
Review the 1986 AASHTO GUIDE FOR DESIGN OF PAVEMENT STRUCTURES assessing the applicability and constraints of using resilient moduli values to establish structural coefficients of base and subbase materials. RESEARCH APPROACH The overall organization used in carrying out the resilient modulus research is shown in Figure 5. To approach this problem In a systematic maimer, consideration was given to a large number of factors potentially affecting the laboratory measured value of the resilient modulus of each layer in the pavement. The more important factors considered included equipment type, equipment and instrumentation calibration, specimen preparation, test conditions, test procedure details and presentation of test results. Careful equipment and instrument calibration are extremely important in obtaining reliable values of resilient moduli and their importance cannot be overemphasized. Improvements to the proposed test procedures were made by conducting limited inter-laboratory studies and also from He valuable comments from the NCHRP review panel. The evaluation of resilient modulus in the laboratory is a complex problem. Furthermore, the influence of the environmental conditions, which are hard to predict over He life of He pavement, have a significant influence on pavement thickness requirements and must not be neglected in design. For example, the resilient modulus of an aggregate base or subbase can increase by a factor of 5 or more as the material goes from a wet to a dry state. Environmental effects of base and subgrade materials were therefore investigated as a part of this study. In following the intent of He REP, emphasis in this study was placed on resilient modulus determination. Permanent deformation characteristics of pavement materials, however, are often more important than for resilient moduli. Therefore, permanent deformation testing of base and subgrade materials was also investigated since permanent deformation can be determined as an extension of the resilient modulus test. The accuracy required for the resilient modulus test is a very important practical question. To study this important aspect, a Monte CarIo reliability simulation using 10,000 trials for each simulation was performed to determine He effect of testing variability on required pavement thickness. The effect of resilient modulus measurement variability on required pavement thickness was compared with both the thickness required if no variability was present and also considering variability from all sources. The results of the Monte CarIo reliability simulation were also compared wig those from the simplified AASHTO type reliability analysis. The reliability study gives valuable insight into the level of sophistication to which the resilient modulus test should be performed. REPORT ORGANIZATION Chapter 2 describes diametral tests performed on asphalt concrete specimens and presents and discusses the results. Chapter 3 summarizes the findings of repeated load biaxial tests performed on granular bases, lime stabilized granular materials and subgrade soils. Primary emphasis is placed on resilient modulus test results although permanent deformation is also included. 7
Laboratory Resilient Modulus Testing Resilient Modulus Testing . AC | Base | Subgrade · ~ \ 1 /- Limited Round Robin Testing . ~ ~ ~ it' ~ Reliabilit y Analysis | | Synthesis | 1 ~- Overall Recommendation | Recommended Testing Procedures | Comparison With Field Measured Moduli Figure 5. Organization of research approach for NCHRP Project I-28 8
The broad overall findings of the study are discussed, interpreted and developed into practical recommendations in Chapter 4, and a general summary and conclusions for all the findings are given in Chapter 5. The appendices include the recommended detailed resilient modulus test procedures for asphalt concrete (Appendix C) and for aggregate base and subgrade soils (Appendix E). For asphalt concrete, specimen preparation methods are given in Appendix B and resilient modulus analysis me~ods in Appendix A. Appendix G gives procedures for preparing subgrade soils. Equipment calibration procedures are presented for both the diametral test (Appendix C) and the repeated load test (Appendix E). The detailed approach used in He reliability analysis is given in Appendix F. Appendix H summarizes~e multi-Laboratory validation study, and Appendix ~ compares laboratory measured resilient moduli with moduli back-calculated from FWD tests performed in the field. 9
