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APPENDIX D REPEATED LOAD TEST SYSTEM CALIBRATION D-1
APPENDIX D REPEATED LOAD TEST SYSTEM CALIBRATION Introduction Proper test system calibration is essential to obtain accurate resilient modulus test results. A full calibration sequence should be carried out regardless of how axial deformation is measured during repeated load testing. Many calibration steps, however, are extremely critical if external EVDTs are used to measure axial deformation. The use of external EVDTs are not recommended. This appendix presents procedures for (1) preliminary testing system and test apparatus stiffness and alignment checks and (2) calibration using synthetic specimens. Both of these phases of calibration should be meticulously carried out by senior laboratory personnel. The calibration procedures, in general, are given in He form of a number of steps to be followed in sequence. Before resilient modulus testing using a closed loop testing system, the start up procedure developed for the Federal Highway Administration should be carefully followed [D-11. PRELIMINARY TESTING SYSTEM VERIFICATION Alignment 2. 4. Indexing Device. Assemble a machinist's indexing device as follows: Use a machinist's magnetic indicator block wig an adjustable extension rod extending upward. Place an accurate dial indicator (or over displacement measuring device) on the end of the extension. The dial indicator should measure directly to at least 0.001 in. (estimate 1/10,000's). Preferablv. the indicator holder should have a magnet which can be turned on and off. Flatness. Check with a mach~n~st's steel 3 ft. straight edge to be sure the base of the testing load frame is flat. If not, have the base accurately surface ground in a machine shop. Load Frame Alignment. Place the indexing device on the base of the testing load frame. Adjust the indexing device so as to be vertical in two planes. Measure the relative difference in height (i.e., the change in dial readings between various locations over about the middle I/2 of the frame base. Changes in dial readings should be about 0.0001 in. or less. Any change in reading indicates He base and upper plate are not parallel. If off by more than about 0.0001 to 0.001 in., adjust the top plate to obtain parallel surfaces. Triaxial Cell Piston Alignment. Place a dummy sample consisting of an aluminum or steel cylinder in He biaxial cell and assemble it. Insert the loading piston so that it sits on top of the sample. Clamp a steel or aluminum bar to the piston so that the bar is horizontal. Place an indexing device between the bar and the base of the testing frame. Slowly rotate the bar (and piston) and move He indicator block observing He change in reading. Keep the dial indicator the same distance from the piston as it is rotated around through 360°. This procedure checks alignment of the piston with the base of the system. Any change in dial indicator reading indicates misalignment. D-2
5. Semen Preparation Mold Alignment. Clamp the mold used for preparing granular specimens on the base of the biaxial cell. Set the moldibase assembly on a flat surface such as a machinist's layout table or the base of the load frame. Using a machinist's height gage, obtain readings at 4 to 6 approximately equally spaced points on the top of the mold. Any deviation in readings indicates tilt of the mold. Note: The indexing device will have to be set up different than in the other experiment. Stiffness of System and Apparatus I. Measure the actual stiffness of the testing frame by placing a jack between the two platens of the machine. Measure the load and displacement between the top and bottom platens of the testing frame at 4 equally spaced loads up to about the maximum to be applied in the test. For example, for 0.6 in. diameter specimen use 300 Ibs., 600 Ibs., 900 Ibs. and 1200 Ibs. Calculate the stiffness which equals load divided by displacement. System stiffness should be at least sxio6 Ibs./in. to loxI06 Ibs./in. For this and other calibrations, a machinist's indexing device can be used to good advantage. 2. Tighten the bottom pedestal on the base of the biaxial cell. Place the pedestallbase in a testing machine and measure the displacement at 4 equal load increments up to the maximum load to be applied during testing. For example, for a 6 in. diameter specimen use 300 Ibs., 600 Ibs., 900 Ibs. and 1200 Ibs. Deflections should be on the order of 0.0001 in. to 0.0002 in. or less. 3. Measure the deformation of the top plate ofthetriaxial cell. It also should be approximately 0.0001 to 0.0002 in. or less. 4. If possible, remove the porous stones and measure their deformation. The deformation of similar new porous stones could be measured as an alternative. Bronze stones 0.25 in. Wick, in general, should be used in this test. Bronze porous stones are available, for example, Tom Geotest Instrmnent Corp., IS40 Oak Avenue,, Evanston, IL 60201. Ph: 800/523-5883 (ext. 800~. Other porous stones having similar deformations can also be used. 5. If He porous stones are not new, boil Hem in water which has a small amount of detergent added to it. Porous stones become clogged wig tines and do not function properly. Therefore, clean Hem frequently by boiling. If water does not readily flow into He stone, it needs to be cleaned and/or replaced. Note: At high loads, important deformations can occur in small diameter pistons (0.5 in. for base materials) that transmits the load to He top platen. Therefore, a large diameter piston should be used to minimize this displacement. Furthermore, the biaxial cell, platens, etc. should be made as rigid as practical. CALIBRATION WITH ALUMINUM OR STEEL SPECIMEN Introduction This calibration is to be carried out only if axial deformation is to be measured outside the D-3
biaxial cell. This method is not recommended, but if it is used place the load cell outside of He biaxial cell above the location of axial displacement measurement. The purpose of this calibration is to determine the extraneous deflection of Be testing system for the bull range of stresses to be used In the resilient modulus test. For a given stress state (i.e., a specified value of confining pressure and deviator stress), the extraneous system deformation equals the total measured asocial deformation minus the calculated deformation of an aluminum or steel calibration specimen. A separate calibration must be performed for each size specimen used to evaluate MR A concentrated effort should be made toward minimizing the undesirable extraneous system deformation measured in the previous section. Now consider a production resilient modulus test on an aggregate specimen. The axial deformation to use in calculating resilient modulus is obtained by subtracting the extraneous system deformation determined from this calibration procedure from Me axial deformation measured in He aggregate base resilient modulus test. Calibration Procedure Verify that the ends of the aluminum or steel calibration specimen are parallel. This check can be performed, for example, by using a mach~n~st's indexing device. Place the calibration specimen and indexing device on a machinist's layout table (or any flat surface-ground surface). Take readings from He indexing device across the top of the specimen. All readings should be He same. Perform He standard resilient modulus test for all stress levels using the aluminum or steel synthetic specimen. The test procedure for the standard resilient modulus test is given in Appendix E. .3 To obtain He extraneous system deformation, subtract the theoretical axial specimen deformation from He measured deformation. Determine the theoretical axial deformation (~) of the specimen from the following formula: 6. = s~c~ic · L/E where: 6~ = theoretical calibration deformation Sgrclic = applied cyclic deviator stress = height of aluminum or steel specimen E = modulus of elasticity of calibration specimen (E= loxI06 psi for aluminum and E=29xI06 psi for steel). (Dot) 4. By using a regression analysis, determine the constants Kit, K,3 and K,4 in the following relationship between extraneous system deformation (obtained from Step 2) and stress state D-4
System = K12 SpyclicK13 S3 K14 6. (D-2) In determining the regression constants in equation (D-2), use all of the data from the resilient modulus test performed on the calibration specimen. The r2 value obtained from the regression analysis should be greater Man about 0.95. Use equation (D-2) to calculate the extraneous system deformation for each of the stress states used in resilient modulus tests performed on pavement materials. Then subtract the system deformation from the measured axial deformation before calculating the resilient modulus. The system should be recalibrated following steps ~ through 4 every 3 months or whenever a change is made to the system (i.e., the sample pedestal is removed or tightened, new instrumentation is added, etc.~. CALIBRATION WITH SYNTHETIC POLYMER SPECIMENS Introduction The purpose of this calibration sequence is to determine if all aspects of the testing system, calibration and data reduction are meshing together to give acceptable values of resilient modulus. This test sequence should preferably be performed on two synthetic specimens. For unstabilized aggregate base specimens, synthetic specimens having a resilient modulus of about 8,000 psi to 10,000 psi and 50,000 to 60,000 psi are recommended. For subgrade soils, synthetic specimens having 2,000 psi to 4,000 psi and 20,000 psi to 30,000 psi are appropriate for many subgrade soil conditions. Specific moduli selected for subgrade soils would be dependent upon the specific soils encountered. As stiffness of the specimen increases, measured resilient deformation decreases for a given condition of stresses. Therefore, the stiff specimen in general should provide a more severe test of the instrumentation sensitivity, calibration, and operation of the testing system. This statement is true since system compliance and hence small errors in calibration percentage wise have a much greater effect on the corrected axial resilient displacement and hence on calculated specimen resilient modulus stiffness. If EVDT mounted clamps are used, He soft specimen may also be a limiting condition. Overview For synthetic specimen calibration, first measure the axial resilient displacements of the synthetic polymer specimens when subjected to a range of stress conditions. If axial deformation is measured on the specimen, the resilient modulus is equal to the cyclic stress applied to the specimen divided by He measured resilient strain. Remember that resilient strain is equal to the measured resilient deformation divided by He distance between the points at which deformation is measured. If axial deformation is measured externally to the biaxial cell, then determine, using equation (D- 2), the extraneous resilient deformation for the same stress conditions observed in the aluminum or steed specimen calibration sequence. Subtract this extraneous deformation from that observed for the synthetic polymer specimen. Use this corrected resilient deformation to calculate the "best estimate" resilient modulus. As discussed in the next section, close temperature control is important during a synthetic specimen test. D-5
Test Procedure 2. 5. 6. Test the two synthetic specimens following the test procedures given in Appendix E, Recommended Standard Method for Routine Resilient Modulus Testing of Unbound Granular Base/Subbase Materials and Subgrade Soils. The resilient modulus of the synthetic test specimens is sensitive to temperature and may be sensitive to deviator stress level. Therefore, perform the resilient modulus tests on the two synthetic specimens at a constant room temperature (t 2°F). Before testing, the specimen should be held at the constant room temperature for at least 12 furs. Place a membrane around the synthetic polymer specimen to be tested. Carefully position the synthetic specimen inside the biaxial cell. Adjust the horizontal position of the biaxial cell until He load Is applied axially. The following three checks can be used as an aid in obtaining a load which is applied to the center of the specimen. a. Very gradually lower the loading ram. Carefully watch the ball bearing placed between the ram (or its extension) and the top plate of the specimen. Be sure symmetry is maintained (i.e., the spacing around the ball bearing relative to the ram loading piston, or its extension, is the same). Also, continually rotate the ball bearing to be sure it is free. When the ball bearing no longer freely rotates, the load piston should be symmetrically located around the ball bearing. b. Watch the specimen when a small amount of load goes onto it. If the specimens visually displaces laterally, it is being loaded eccentrically. c. After setting up the axial displacement measuring system, apply a small amount of {gad. Observe individually the displacement of each axial displacement gage. The ratio of the maximum to He minimum displacement of the two gages should be less than I. 10 before beginning the data gathering portion of the experiment. If a single optical extensometer Is used, measure the eccentricity by using two external EVDTs or dial indicators. Warm up the closed loop testing system and data acquisition electronics for at least 45 min. to ~ hour before beginning the resilient modulus test. A relatively long warm up period is required for He system to reach a stable condition. Measure the load pulse using an oscilloscope to verify a haversine-shaped load pulse is applied. Adjust the gain and other controls if required. Perform the standard resilient modulus test using the stress conditions given in Table D-. If extemally mounted EVDTs are used, the simultaneous use of EVDTs SR4 strain gages or other type gages mounted inside the biaxial cell is also encouraged but certainly not required. If external EVDTs are used, correct He measured axial displacements to account for He extraneous displacement in He system. To accomplish this, use equation ~-2) to calculate the deformation in the system. The resilient deformation fir of He specimen is equal to br bmeasured - bs3rstem D-6
where Toured = total measured displacement of the synthetic specimen and the overall system Stem = theoretical displacement of the system, equation (D-2) b~ = approximate resilient specimen deformation The system displacement (i.e., undesired extraneous displacement) is calculated using equation (D- 2) for each stress condition used in the test. Then subtract from the measured displacement for each stress state the calculated system displacement for the corresponding stress state. 7. 8. 9. References Reduce the data and calculate the resilient moduli. Fill our the data sheets given in Appendix E with the raw data and the calculated values of resilient moduli. Be sure to carefully check the data for consistency and reasonableness. Repeat the complete test including assembly of the tria~cial cell at least 5 times for each synthetic specimen tested. Figure He coefficient of variation (C V) of the resilient modulus for each stress level and obtain Be average value for all stress levels. The coefficient of variation should be less Man 6%. The maximum variation in measured MR should not exceed 7%. If this criteria is not satisfied, review the test procedures, equipment setup, etc. and repeat the test 5 more times and compare results. Repeat until a suitably low CV and maximum spread of MR IS obtained. Compare He average resilient modulus for each stress level wig the reference valuers) of resilient modulus. If a reference value is not available, determine one by using an independent measurement system. Preferred independent methods of axial displacement measurement include: (~) EVDTs located between plugs glued into the synthetic specimen, (2) large 2 in. gage length SR4 strain gages glued to He specimen, and (3) optical extensometer if it is not used as the test deformation measurement approach. Resilient moduli measured by each memos should be within about 5 to 7% of each other. D-~. (1977), ALTO Materials Characterization Program: Resilient Modulus of Unbound Materials (LTPP Protocol P46) Laboratory Start-Up and Quality Control Procedure", FHWA Pub. No. FHWA-RD-96-176, Report prepared for FHWA by PCS/I,aw. D-7
Table D-1. Stress Conditions for Synthetic Specimen Calibration Test Jest Temperature 75°F (il.5°F)). . Applied Cyclic Contact Confining Axial Stress, Stress Stress No. Specimen Pressure Sm" S~c~ic Soon Load Type (psi) (ps i) (psi) (psi) Cycles Base 1 0 3.6 1 3 0.6 1 50 6.6 6 0.6 50 9.6 9 0.6 50 28.8* 27 0.6 50 Subgrade . 0 2.6 2 0.6 50 4.6 4 0.6 50 6.6 6 0.6 50 10.6 10; 0.6 50 *Optional Stress 1 D-8