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APPENDIX I COMPARISON OF LABORATORY AND BACKCALCULATED RESILIENT MODULI I-1
APPENDIX I COMPARISON OF LABORATORY AND BACKCALCULATED RESILIENT MODULI Introduction Resilient moduli for use in design are presently determined by both direct laboratory measurement and by backcalculation from measurements made in the field using, for example, the falling weight deflectometer (FOOD) test. The purpose of this experiment is to compare resilient moduli determined using the proposed laboratory test procedures with FWD backcalculatecl values. The proposed laboratory resilient modulus testing ~ . ~ ~ ., t ~ ~ ~ . . · · · A ~ . · procedure for asphalt concrete is given in Appendix ~ and for base, subbase and subgrade materials In Appendix E. Both laboratory tests and backcalculation procedures from field data have important advantages and also important limitations. Important problems with resilient modulus measurement in the laboratory include fabricating specimens to duplicate field conditions and then simulating during testing the field stress states and environmental factors such as temperature and water content. In backcalculating resilient moduli from FWD data, the theory use assumes very ideal behavior of each material (i.e., the material is homogeneous, linear, elastic, isotropic). Such ideal materials simply do not exist in the field. Also, the applied loading is considered to be static which can result in large errors when the dept to rock is shallow. The limitations given above, as well as others, for both the field and laboratory approaches can result in the materials in the field performing coherent than from the laboratory field testing program. As a result, a direct comparison of FWD backcalculated moduli with laboratory determined values is considered to be a worthwhile exercise since both methods are used in design. . . Test Sections The field test pavements used in this stucly are locater] on a two lane, unctivicIed portion of US Highway 421 near Siler City, North Carolina. Test Sections ~ and 7 were used for the FWD backcalculation analysis study (Table IBM. Section ~ is a flexible section consisting of 3.5 in. of asphalt concrete and 12 in. of aggregate base. Section 7 is a 9 in. thick full depth asphalt concrete section. Both sections overlie a micaceous silty clay subgrade. The laboratory resilient moduli values for the asphaltic layers are given in Table I-2. Since the surface and binder AC courses were heavy duty mixes, these two layers were combined to give a more simple theoretical model for the FWD analysis. FWD Study Surface deflections measured with a FWD are now routinely used in evaluating structural conditions of pavement systems. The FWD imparts an impact load to a pavement with a loading time of about 0.3 sec. The resulting surface deflections are measured by varying offset distances from the loading point. The measured surface deflections are used in backcalculating "effective" moduli of various layers in the pavement, which are assumed to represent the in-si~ strength or condition of the layers. Backcalculation procedures are characterized by He type of forward structural analysis model employed and the techniques used in matching measured and calculated deflection basins. I-2
Backcalculation Analysis The MODULUS computer program, developed by Texas Transportation Institute [I-~], was used as the backcalculation procedure in this study. This method is based on multi-layered elastic theory and a database approach to matching deflection basins. Resilient moduli were backcalculated using the following three options in the computer program: (~) fixed analysis, (2) alternate fixed analysis, and (3) full analysis. For the fixed analysis, the user selects the material types, layer thicknesses, and test temperature. The program then assigns the range of acceptable moduli and Poisson's ratios to be used in the analysis. The program has built-in equation stiffness versus temperature for typical mixes found in Texas (crushed lime stone or river gravel mixes). Also, equations which give a reasonable range of stiftnesses are available in the program. Option ~ is intended for the user who is unfamiliar with typical ranges of moduli. For Option I, He AC modulus is fixed by We backcalculation program based on the given temperature. On the over hand, during backcalculation using Option 2, equations which represent the reasonable range of stiffnesses are used by the backcalculation program. For the full analysis (Option 3), however, the user inputs all of the parameters including resilient moduli of Be AC needed to perform the analysis. Once the user is familiar with the modulus backcalculation procedure, Option 3 is usually the most frequently used option. Results The mean values of the laboratory-deter~nined AC layer moduli and backcalculated values are presented in Table I-3. The preconditioning level of 100 cycles and the axis of 0 degree were chosen to determine the laboratory resilient moduli regardless of the selectee! testing temperatures. For Section I, the modulus determined using Option ~ gives the best agreement with the laboratory resilient modulus. This observation is not surprising since Option ~ uses the moclulus-temperature equation developed from laboratory testing. Since the AC layer thickness is thinner than 4 in. for this section, moduli determined from the Options 2 ant] 3 are in question since these options are for AC layers greater than 4 in. thick. This probably partially accounts for the poor agreement for Options 2 and 3 between backcalculated and measured values Of MR In Section 7, the total thickness of AC layers if in., which is in the range of thicknesses for which Option 3 was intended. However, a fairly significant clifference is observed between measurer! MR values of HDS and HE layers and the backcalculatec! mocluli values. Option I, however, shows reasonably good agreement between resilient moduli from the backcalculation method and laboratory measured values. Summary -- The limited results of this study suggest it may be desirable to use a resilient modulus backcalculation procedure, such as Option ~ of the Texas Transportation Institute method. Since the Option ~ type backcalculation methoc! is tied in with laboratory resilient modulus measurements, in general better agreement between He two methods would be expected. Resilient moduli from backcalculation and laboratory measurements have in the past been relatively poor. A detailed stucly of this important problem should probably be carried out using He extensive field ant! laboratory data collected from He LTPP study. I-3
REFERENCES Uzan, J., Scullion, T., Michalek, C.H., Parades M. and Lytton, R.L. (1988), "A Microcomputer Based Procedure for Backcalculating Layer Moduli Prom FWD Data", Texas Transportation Institute, 1988. Table 1-! Structural design of Sections ~ ant] 7 user] in FWD study Thickness (in.) Section 1 Section 7 AC Surface Course 2.0 2.0 AC Binder Course I.5 I.5 AC Base Course N/A 5.5 Aggregate Base Course 12.0 N/A Total AC Layer 3.5 , . 9.0 I-4
Table I-2. Summary of laboratory-determined resilient modulus test results: Siler City test sections Section | Mix Test' | Calculated MR (ksi) | Assumed MR(ksi) | No. | Type Temp | (it) (Calc, It) | (it) (Assm, it) l (°F) I 1 1 HDSa 87 1 0.47 440 1 0.35 366 7 1 HISS 72 1 0.44 1 943 1 0.32 773 7 1 HBb 1 67 1 0.49 1 1,371 1 0.25 1 969 Note: a2 in. heavy duty surface ~ 0.5 in. heavy duty binder. b2.s in. asphalt-stabilized base. CEstimatec} from the pavement temperatures by considering the position of the mid-point of the individual specimens in the pavement Table I-3 Comparison of the laboratory-determ . ined versus backcalculated moduli values l Section | Mix | SurfaceC | Ma (ksi) | Option 1C | OPl;On2f | Option 3g || NoO Type Temp (Assm, flu) Mg (ksi) Mg (ksi) MR (ksi) (°F) 1 1 HDS ~T 90 1 366 1 350 1 619 170S . 7 1 HDS' 1 4 1 773 1 39 1 396 1 474 HB 1 74 1 969 1 543 1 773 1 729 Note: a2 in. heavy duty surface + 0.5 heavy duty binder. b2.s in. asphalt-stabilizec] base CThese are the temperatures measured from the field. Laboratory-determined resilient moduli based on assumed Poisson's ratio. eFixed analysis, range of modulus based on temperature. Fixed analysis, range of modulus based on temperature. "Full analysis. I-5