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85 to address compaction quality control. The majority it is time to separate the current database of soil and base of the correlations with DCP are local and empirical layer properties that are obtained using the AASHTO types. Reasons for these are a lack of standardization T-307 test method from other test laboratory test meth- (this method was recently standardized by ASTM and ods and field methods. This separated database should currently there is no standardized AASHTO test pro- be used to develop better statistical and intelligent for- cedure) and different modes of devices being used in mulations to predict moduli. A few such formulations various states. As a result, measured data from various are discussed in the chapter five summary. materials are different, and hence the use of local cor- relations or the need for additional tests for verification should be reviewed and considered. Future Research Needs • FWD studies are being used with varying levels of success in the present field studies. This was evident One of the problems learned from the Geotechnical/Materi- from the state surveys, as nearly 60% of the survey als group survey is the fatigue in a small group of geotechni- respondents (equivalent to 50% of the total DOTs in cal/materials users with respect to resilient modulus testing the United States) noted that they use FWD for dif- and implementation of this method in the flexible pavement ferent applications, including new pavement construc- design. One respondent noted during telephone communica- tion and pavement rehabilitation projects. Still, there tion (in the follow-up surveys) that the test methods are being appears to be some variation with respect to the back- constantly revised, which leaves users confused. Several oth- calculation programs used for determining moduli of ers are not interested in exploring both laboratory and field layers. Three programs—MODULUS, EVERCALC, methods for moduli determination, because they mostly rely and ELMOD—are being used or mentioned as a pre- on indirect correlations using group index parameters, or R ferred program by more than one DOT in their survey or California Bearing Ratio values. All these point to the responses. need for better training modules to explain the importance of • Other nondestructive methods such as GeoGauge and resilient moduli or stiffness of the materials in a mechanistic SPA have been used successfully to determine the com- pavement design approach. posite moduli and soil moduli (in the case of DSPA). As long as the soil moduli determined by these devices To achieve these goals, it is important to develop a series are defined properly with respect to design resilient of action items, including a few research needs, to promote moduli, the use of these devices should be encouraged the use of moduli of unbound bases and subgrades in the within the DOTs. flexible pavement design. These action items include the • LWDs have become powerful tools for quick assess- following: ments of moduli in the field. However, only a few DOTs are using them or addressing them in a research study • Explain the importance of Level 1 input parameters for environment. More studies will lead to potential imple- better pavement design. mentation of these devices in both determining the • Encourage the use of Level 2 moduli input from corre- moduli of compacted subgrades and bases and assess- lations assuming the correlations are statistically supe- ing the compaction quality conditions. These devices rior and provide reasonable moduli values. may offer an alternate to nuclear-based gauges that are • Standardize test procedures, both in laboratory (cur- currently used in the practice for determining the rela- rent method of T-307 vs. Harmonized method) and tive compaction of the compacted materials. field conditions (Research Need). • From the studies and surveys, the nonlinear nature • Promote acceptance of field nondestructive stud- of resilient moduli appears to be well accepted by the ies and intrusive studies with standardized protocols DOTs as the majority of them have been using various (Research Need). three-parameter models that account for confining and • Address seasonal moisture variations and their effects deviatoric stresses of the subgrades and bases. One of on moduli of soils and unbound bases along the lines the concerns from this synthesis is the availability of suggested by the MEPDG (Research Need). several three-parameter models that have been used in • Define design moduli and their correlations with vari- the practice. These models along with the current three- ous moduli determined from other field devices and parameter model recommended by MEPDG create a methods (Research Need). considerable dilemma for the users when choosing an • Emphasize the cost-effectiveness (life-cycle cost- appropriate correlation to analyze the moduli data. benefit studies) and non-nuclear-based methods for • From both surveys and modeling results of chapters two quality control of compaction of subgrades and bases and five, it is apparent that the development of universal (Research Need). resilient moduli will be a difficult task with the current • Develop training modules that emphasize the test meth- stiffness database because of a high level of scatter in ods cited earlier and how they can be used for a realistic the test data. Local correlations are essential; however, pavement design using MEPDG (Research Need).
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86 Simplification of test procedures and design methods to address the use of nondestructive studies for moduli needs to be considered, but it should not be the sole high interpretations of subgrades and unbound bases and com- priority owing to the complex nature of a performance- paction quality assessments. based mechanistic pavement design method using moduli properties. A Transportation Pooled Fund program initi- The majority of the synthesis information show consider- ated a research study in 2007, which is aimed at improving able advances made by several DOTs in the flexible pavement the resilient modulus test procedures for unbound mate- design area using moduli of subgrades and unbound bases. rials. The goals of this 5-year pooled fund study are (1) Focusing on these positives along with a universal imple- to reduce the variability associated with resilient modulus mentation of a standardized MR measurement approach in testing of unbound materials, (2) to conduct a precision both laboratory and field conditions will lead to a better and and bias study of the test procedure, and (3) to provide reliable moduli database. Such a database should be used assistance to states to properly equip and set up a labora- to derive universal statistical correlation models for better tory for successful MR testing. Similar studies are needed interpretation of moduli properties of bases and subgrades.