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CHAPTER 5 SUMMARY, CONCLUSIONS AND GENERAL RECOMMENDATIONS OPTIMUM RESILIENT MODULUS TESTING SYSTEM For production resilient modulus testing, a completely automated, modern electro-hydraulic loading and data acquisition system is a necessity to maximize the number of tests performed and to minimize the potential for testing and data reduction errors. The loading system should be programmed to automatically perform the complete stress sequence required for a resilient modulus test. Data should also be automatically collected and saved using an analog to digital data acquisition system. Excellent success, including high production output, has been reported using fully automated testing systems of this type. Ideally, after a test is set up and started, resilient moduli should be printed out at the completion of the test without the need for operator intervention. By using an automated testing/data acquisition system, approximately 6 to ~ resilient modulus tests, under ideal conditions, can be performed and the data reduced in one day. One person performs the tests while a two person team prepares specimens. Recommendation. Use a fully automated testing system for routine resilient modulus testing or extensive research applications. A complete, automated testing/data acquisition/data reduction system is necessary to achieve reliable resilient modulus test results, especially when conducted as a routine test by a technician. TESTING SYSTEM SET-I]P AND CALIBRATION Accurate, reproducible resilient moduli cannot be measure by sending an inexperienced technician or engineer into the laboratory and having hither start running resilient modulus tests even using a new system. An automated testing system is complicated to set up, and the electronic measurement and data acquisition systems must be thoroughly understood. The entire operation of the testing system must be verified by quantitative measurements. The effects on resilient modulus of poor or lack of system calibration and choice of instrumentation far outweigh the influence of most testing details. System Set Up Before beginning production resilient modulus testing, a careful shakedown is required of both (~) the system electronics (including the closed-Ioop testing system, load cell, axial deformation measurement system and data acquisition system) and also (2) the mechanical aspects of the testing system such as load alignment and extraneous system deformations. A complete check out of the testing system is required even if the system is new. Verifying the proper operation of the electronics and mechanical aspects of the testing system requires supervision by an engineer knowledgeable in these types of systems as well as the availability of a senior level laboratory technician. 278
Recommendation Laboratories have significant problems in properly setting up and making operational the resilient modulus test including testing equipment, data acquisition apparatus, specimen preparation methodology, specimen set up and data reduction. Synthetic Specimens System calibration by testing synthetic specimens having known resilient moduli is absolutely essential to insure reliable resilient modulus test results. If external deformation measurements are made In the repeated load biaxial test, which is not recommended, system compliance must be accounted for in reducing the data. Each laboratory should own their own synthetic specimens. These specimens are easy and inexpensive to make or can be purchase ready to test. Synthetic specimens sent from one laboratory to another take a tremendous beating and often become unreliable as a laboratory standard. Each laboratory should have the capability of measuring a reference resilient modulus by making accurate deformation measurements directly on the synthetic specimen. The importance of thorough testing system calibration cannot be overemphasized. Proof testing using synthetic specimens also serves to train laboratory technicians. Human Related Problems The laboratory validation study clearly showed several considerations to be important in resilient modulus testing. Before resilient modulus testing is begun, laboratories need to develop a well-planned and carefully supervised program which includes using synthetic specimens for calibration. Some test equipment does not work as advertised, and technicians often have too little experience to identify and correct the source of problems. Also, not all laboratories carefully follow calibration and/or testing procedures. A rushed laboratory testing schedule frequently leads to problems. The test of reasonableness of measured resilient moduli was found to be all to frequently absent from laboratory test procedures. _, _ - or . - Both ~e electronics and mechanical performance of even a new testing system must be carefully validated by knowledgeable engineers and senior laboratory technicians. The laboratory should own synthetic specimens for verifying the overall reliability of the entire resilient modulus measurement process. Serious consideration should be given to obtaining outside help in setting up, calibrating and establishing He resilient modulus test as a routine laboratory procedure. Calibration procedures are given in Appendix C for asphalt concrete and Appendix D for base and subgrade materials. ASPHALT CONCRETE RESILIENT MODULUS TESTING METHODS AND PROCEDURES Based on findings from this study presented and discussed in Chapter 2, a new protocol for resilient modulus testing of hot mix asphalt concrete was developed and presented in Appendix C. The Protocol has been written by incorporating the findings of this study into the final version of SHRP P07 Protocol (November I, 1992~. It was decided to rewrite SHRP P07 instead of the existing ASTM D4123 procedure, as the SHRP protocol had already made significant improvements to the ASTM standard. 279
Conclusions. The following general conclusions are made concerning resilient modulus testing of asphalt concrete specimens: I. Resilient modulus decreases when testing is repeated on an axis mutually perpendicular to the axis initially tested. 2. 3. 6. 7. 8. 9. The resilient modulus decreases significantly with increase in temperature. Thus, it is important to run the resilient modulus test at the desired test temperatures. Poisson's ratio is one of the most important parameters influencing the resilient modulus. The variation in MR values due to the testing axis dependency and different lengths of rest periods are almost negligible compared to He magnitude of difference in the MR values from assumed and calculated Poisson's ratios. Poisson's ratio should be evaluated using the EXSUM deformation measurement system. A mountable extensometer device, compared to the stand-alone EVDT measurement device, provides less variance and hence better repeatability within the five consecutive cycles used for resilient modulus determination. However, using the SHRP EG device EVDTs gave comparable performance to the mountable extensometer. Mountable deformation measurement devices are recommended for resilient modulus testing because of the smaller variability. The SHRP EG device minimizes rocking of the specimen. The main features of the SHRP EG device are the use of two guide columns, a counterbalance system, an innovative semi-rigid connection between the upper plate and the load actuator, and its sturdiness. The disadvantages are its bulkiness, complication of use, possible inertia from the counter-balance system, friction in the guide columns, and limitation of the size of the sample that can be tested. The concept is sound behind the use of EVDTs mounted along a small gage length (l in.) on the surface of the specimen, as in the Gage-Point-Mounted setup. The main drawbacks for its use in repetitive testing are its heavy dependence on the alignment and homogeneity of specimens. The gage length of ~ in. seems to be too small for reasonable results with the asphalt concrete specimens used in this study. The proposed measurement system, the EXSUM setup, provides a promising measurement method for determination of consistent and reasonable Poisson's ratios. At 41°F, however, increase in variability occurs due to misalignment and rocking. Use of the SHRP EG device, or its modification, together with the EXSUM setup ensures reasonable values even at low temperatures. The use of the EXSUM setup requires an increase in testing time compared to conventional systems because of the significant time required for mounting the EVDT on the specimen. For research applications, improved reliability can be obtained by mounting an L~VDT on both the front and back surfaces of the specimens. A square load pulse produces significant specimen damage and smaller resilient moduli compared to a haversine pulse. The haversine pulse also better simulates the field loading condition than a square pulse. As a result, the haversine load pulse is recommended for resilient modulus testing. The loading time significantly affects the MR values. A loading time of 0.2 sec. considerably reduces MR' and produces more damage as compared to a shorter loading time of 0.05 sec. A shorter loading time of 0.05 sec. is representative of high vehicle speeds, but is hard to accurately 280
apply and monitor. Also, accurate load control at higher temperatures is difficult using very short loading times. The usually used loading time of 0. ~ sec., represents slow traffic conditions that cause significant damage to the pavement and should be continued to be used for resilient modulus testing. 10. Rest period to loading period ratios of 4, 9, 19, 24, and 29 used in the study did not make a significant difference in the resilient moduli. Also, a rest period to loading period ratio greater than ~ has bun shown to generate no significant beneficial effect by past research. A rest period to loading time ratio of 9 gives a rest period of 0.9 second and a loading frequency of ~ Hz. This is He loading condition specified by SHRP P07 and a change in it is not justified. . Three levels of preconditioning were studio. There was no significant difference in the variation of resilient moduli and Poisson's ratio between five cycles for the selected preconditioning levels 2 and 3. However, MR values did decrease with increasing number of preconditioning cycles. One-hundred preconditioning cycles are recommended at 41 and 77°F and 50 cycles at 104°F. 12. A significant difference exists between resilient moduli and Poisson's ratio values computed using the SHRP P07 analysis and the elastic analysis which is similar to the ASTM analysis. The SHRP approach gives higher values when an assumed Poisson's ratio is used as compared to the elastic analysis with an assumed Poisson's ratio. 13. A 4 in. diameter specimen Is acceptable for testing medium gradation mixes, but a 6 in. diameter specimen should be used to test coarse gradation mixes (mixes with more proportion of coarse aggregate or mixes with large aggregate such as base courses or large-stone mixes). Medium and coarse gradations are given in Appendix B. Table Bet. 14. SHRP protocol P07 recommended load amplitudes are suitable for testing at 41°F and 77°F, but at 104°F a smaller load should be used. Load levels corresponding to 30, 15, and 4 percent of the indirect tensile strength at 77°F are recommended for testing at 41°F, 77°F, and 104°F, respectively. 15. The relatively large seating loads recommended by the SHRP P07 protocol may not be necessary as high seating loads seem to damage the specimen at higher temperatures. Instead, 5, 4, and 4 percent of the total load are recommends at 41°F, 7T and 104°F, respectively. However at 104°F a minimum load of 5 Ibs. must be maintained to avoid the possibility of separation of the loading strip from the sample surface. The maximum seating load should not exceed 20 lbs. to ensure minimum damage to the specimen. 16. The following configuration of test apparatus is recommended for use in resilient modulus testing: Load Device: A device comparable to He SHRP EG device, possibly with He following modifications: I. Reduction of the upper plate weight using high strength, light-weight materials and thus elimination of the counterbalance weights, 2. Reduction of the size of the device so that it can be easily used in commonly available environmental chambers, and 281
3. Capability for He testing of 6 inch diameter specimens. The MTS diametral testing device was used for the final phase of the testing program mainly due to time and budget constraints. Although the control of rocking is a little inferior to the recommended diametral device, the MTS testing device gives comparable results, especially as extensometers are to be used for measurement of horizontal deformation. Also, the testing device can be used in a typical environmental chamber. Measurement System: The EXSUM setup described in Chapter 2 is recommended for use. However, a faster curing glue with non-sagging properties is required to reduce the time required for testing. Also, in~epth finite-element analyses might be required to make corrections for bulging and non-uniform stress distributions. The capability to use two mounted EVDTs, one each at the front and back face of the specimen, might make results more trustworthy. Although accurate and convenient, extensometers are expensive, and a cheaper mountable measurement system wig comparable accuracy should be developed. BASE AND SUBGRADE RESILIENT MODULUS TESTING METHODS AND PROCEDURES The repeated load biaxial test is recommended to evaluate the resilient modulus of base, subbase and granular subgrade materials. A repeated load test performed on an unconfined specimen is recommended for cohesive subgrade soils. The round-robin tests (Appendix H) show for base and subgrade materials that very large variations in MR values were observed between labs when axial deformation is measured bow outside and inside the biaxial cell using current procedures. Testing details are considered in Chapter 3, and testing details compared with the AASHTO and SHRP procedures in Chapter 4. Recommendation The recommendation is made to make axial deformation measurements inside the cell. An optical extensometer, non-contact proximal gages and light, sensitive EVDTs mounted on lightweight clamps can all be used. Use of He optical extensometer, however, is recommended. The use of an inside deformation measurement system neither eliminates nor reduces the need by careful calibration for minimizing system compliance (i.e., extraneous deformation in the loading and testing system) or measuring test apparatus alignment (Appendix D). Major Issues The following major resilient modulus test issues completely overshadow other test details which usually have relatively minor influence on the measured resilient modulus: (~) fully automated loading and data acquisition system to minimize errors, (2) accurate measurement of axial deformation including end bedding effects of cohesive soils, (3) cohesive specimen aging, (4) environmentally induced changes in MR and (5) soil structure of compacted cohesive specimens. Failure to properly account for any of the above major factors can easily lead to errors of 30 to 100% or more in the measured resilient modulus. These important issues are discussed in Chapters 3 and 4. 282
Optimum Granular Base and Granular Subgrade Test Method The optical extensometer can be used to measure deformation of a granular specimen subjected to confining pressure by applying a vacuum pressure to the inside of the specimen. This test eliminates the need to use a biaxial cell and hence is both simple to set up and requires no special changes in equipment to use the optical extensometer. The vacuum type repeated load test has been found to be a practical method for testing granular materials. Application of a vacuum inside the specimen causes virtually the same effects on the specimen as applying an external confining pressure. Neither the AASHTO nor SHRP procedures consider or allow this type system. A quasi-static test can, if necessary, be used to satisfactorily measure the resilient modulus of granular materials. The quasi-static test does not require either a sophisticated testing system or data acquisition system. The coefficient of variation of resilient moduli, with careful equipment calibration, is as small as about 11% if one specimen is used and 8% for two specimens. The popular K-8 resilient modulus model cannot always distinguish one material from another at the 95% confidence level. The K-8 model does not work at all with stabilized materials. The more accurate Uzan, U.T-Austin or FHWA models are recommended to characterize resilient modulus behavior. Cohesive Soil Optimum Test Method An unconfined repeated load test is proposed for both undisturbed and compacted cohesive subgrade specimens. The considerably more complicated biaxial test is specified by both the AASHTO and SHRP test procedures. The unconfined compression test is simple to perform and also allows the easy use of an optical extensometer since a biaxial cell is not required. Axial deformation can be measured directly on the specimen using either an optical extensometer, cIamp-mounted EVDTs or noncontact proximity gages. Axial deformation can also be measured between solid end platens if the specimen ends are either grouted or an empirical end correction is applied to the measured resilient modulus. Neither He AASHTO nor SHRP test methods consider the important end effects of cohesive specimens. Measured Subgrade Modulus and Observe Perfor~nance. The measured subgrade resilient modulus has sometimes been found not to give a correct indication of how the subgrade actually performs. Reasons for this apparent discrepancy include: (1) Testing problems in measuring the resilient modulus can occur as discussed throughout this report. (2) The specimen tested may not be representative of the insitu material due, for example, to sample disturbance, changes in moisture content between the field and the laboratory, also the specimen may not be representative of the poorer performing material along the route under consideration. (3) The resilient modulus does not properly depict permanent deformation characteristics of the subgrade which should be evaluated by the repeated load biaxial test using an appropriate stress state. ENVIRONMENTAL MOISTURE CYCLE The most realistic method of pavement design, as suggested in the 1986 AASHTO Guide, is to analyze the pavement for a number of different time periods throughout the year. Because of varying moisture contents throughout the year, the resilient modulus (and also the resistance to permanent deformation) is continually changing. The difference in resilient modulus for dry and wet conditions can be as large as 100% or more. 283
Recommendation Develop empirical relations for correcting the resilient modulus measured at a standard reference moisture/density state to give appropriate resilient moduli for other moisture conditions. Such empirical corrections make considering realistic variations in resilient moduli with seasons of the year practical in pavement design. EMPIRICAL RESILIENT MODULUS RELATIONSHIPS FOR USE IN DESIGN Resilient modulus testing at the level of sophistication needed to obtain satisfactory results, for most laboratories, is more suitable for a research project than for routine production type testing. The resilient modulus test is: . Not an easy task unless the test is fully automated Requires a significant amount of time and is subject to testing errors Requires equipment that is relatively expensive May not be production oriented depending upon the available equipment A very attractive approach for obtaining resilient moduli for use in design, for at least most agencies, is to determine values of resilient modulus using generalized empirical relationships. Such relationships give resilient modulus as a function of statistically relevant, easy to measure physical properties of He material such as percent compaction, water content, etc. Justification The resilient modulus has been found to vary as much as 20% to 40% along segments of highways of limited length. Also, He average variation in resilient modulus likely to occur, under favorable laboratory testing conditions, is about 10 to 15% for within laboratory tests. Considering the large variation in resilient moduli due to the environmental moisture cycle as well as the above variations, He use in design of empirical resilient modulus relationships is considered to be justified. A number of states have already developed generalized resilient modulus relationships for use in design, particularly for cohesive subgrade soils. Recommendation Give serious consideration to developing generalized resilient modulus relationships to obtain in a practical manner design values for route use. These relationships should be developed from a well planned and executed laboratory resilient modulus testing program. PERMANENT DEFORMATION The resilient modulus of pavement materials has received considerable publicity in recent years since its introduction in ache 1986 AASHTO Design Guide. The evaluation of permanent deformation characteristics of He asphalt concrete, base and subgrade materials is just as important, and usually more important, as He resilient modulus and hence should neither be forgotten nor neglected. The permanent 284
deformation characteristics of the base and subgrade can be readily determined as an extension of the proposed resilient modulus test using a repeated load biaxial test apparatus. The permanent deformation behavior of asphalt concrete can be evaluated using a loaded wheel tester or else as an extension of the repeated load diametral test. SUGGESTED ADDITIONAL RESEARCH The following research topics are considered to be important in determ resilient moduli for design and deserve additional research: . fining reliable, practical I. Develop generalized relationships between resilient modulus and easy to measure material parameters for asphalt concrete, granular base and subgrade soils and cohesive subgrade materials. 4. Study the effect of specimen aging on the resilient modulus of cohesive subgrade soils. Develop generalized relationships for correcting resilient moduli measured a short time after compaction to the long-term modulus. Develop generalized design relationships for relating fiche change in resilient modulus with change in moisture content for both subgrade and base materials. Incorporate permanent deformation using realistic material tests into He AASHTO design method. 285
