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A-1 APPENDIX A REVIEW OF AASHTO T322 AND RECENT PROPOSED CHANGES INTRODUCTION 16, and 17. The sections below address the most significant parts of the specification in sequence. The primary purpose of this appendix is to summarize the procedures for performing the indirect tension (IDT) creep and strength test and the methods for analyzing the subsequent AASHTO T322 Sections data, as described in AASHTO T322, Standard Method of Test Section 4. Summary of Method for Determining the Creep Compliance and Strength of Hot Mix Asphalt (HMA) Using the Indirect Tensile Test Device. The Summary of Method in Section 4 presents a good intro- This appendix also includes recent suggested modifications to ductory description of the test procedure: this standard, which have occurred during the course of NCHRP Projects 1-37A and 9-19. This information is crit- 4.1 This standard describes two procedures. For one proce- ical to understanding the current form of the IDT test system dure, the tensile creep and tensile strength are determined on and changes likely to occur over the next few years. the same specimen for thermal cracking analyses, and for the This appendix includes a summary of AASHTO T322, a other procedure the tensile strength is determined separately for fatigue cracking analyses. section on modifications to AASHTO T322 recommended during NCHRP Projects 1-37A and 9-19, a section on related 4.2 The tensile creep is determined by applying a static load research, a section discussing the results of this review and of fixed magnitude along the diametral axis of a specimen. The horizontal and vertical deformations measured near the presenting various findings, a section presenting conclusions center of the specimen are used to calculate a tensile creep and recommendations, and a list of references. This appen- compliance as a function of time. Loads are selected to keep dix is intended to provide detailed background information horizontal strains in the linear viscoelastic range (typically supporting the findings, conclusions, and recommendations below a horizontal strain of 500 × 10-6 mm/mm) during the presented in the body of the NCHRP 9-29 Phase III final creep test. By measuring both horizontal and vertical defor- mations in regions where the stresses are relatively constant report. However, an attempt has also been made to make this and away from the localized non-linear effects induced by suitable as a stand-alone document. the steel loading strips, Poisson's ratio can be more accu- rately determined. Creep compliance is sensitive to Poisson's ratio measurements. AASHTO T322 4.3 The tensile strength is determined immediately after deter- mining the tensile creep or separately by applying a constant AASHTO T322 consists of 17 sections: rate of vertical deformation (or ram movement) to failure. 1. Scope The most important features of this test system are the indirect 2. Referenced Documents tensile test geometry, the use of both compliance and strength 3. Terminology tests, the assumption of linear viscoelastic behavior, and 4. Summary of Method the determination of not only creep compliance but also of 5. Significance and Use Poisson's ratio during the IDT creep test. 6. Apparatus One of the most important issues concerning the IDT creep 7. Hazards and strength test is that of test geometry. The IDT geometry 8. Standardization was originally selected for use in low-temperature character- 9. Sampling ization of asphalt concrete mixtures during the Strategic 10. Specimen Preparation and Preliminary Determinations Highway Research Program (SHRP) primarily because the 11. Tensile Creep/Strength Testing (Thermal Cracking specimen preparation methods available at that time did not Analysis) include ways of making specimens suited for uniaxial mea- 12. Tensile Strength Testing (Fatigue Cracking Analysis) surement of creep compliance, relaxation modulus, or strength. 13. Calculations The simple performance tests developed as part of NCHRP 14. Report Project 9-19 and the characterization methods developed for 15. Precision and Bias use in conjunction with the pavement design guide devel- 16. Keywords oped in NCHRP Project 1-37A require 100-mm diameter 17. References by 150-mm high specimens to be used in uniaxial testing. Therefore, this obstacle to uniaxial testing at low temperature Many of these sections are only of nominal significance and no longer exists. This would also potentially allow the use will not be discussed here, including sections 1, 2, 3, 5, 7, 15, of relaxation modulus tests, rather than creep tests, which
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A-2 would eliminate the need to calculate the relaxation modulus dimensional stress analyses, such as those used in the Super- from the creep compliance. However, relaxation tests have pave thermal cracking analysis, Poisson's ratio is not needed. not been widely performed on asphalt concrete mixtures; and, Furthermore, for most materials, Poisson's ratio falls for practical purposes, the creep test should probably be between about 0.2 and 0.5. For asphalt concrete, Huang (A2) retained regardless of test geometry. Phase III of NCHRP states that values typically fall in a narrower range, from 0.3 to Project 9-29 included an evaluation of the possible use of 0.4. Huang goes on to state "Because Poisson's ratio has a uniaxial creep testing as the standard low-temperature test relatively small effect on pavement responses, it is custom- for asphalt concrete. Because the uniaxial test can produce ary to assume a reasonable value for use in design, rather compliance data in the same exact format as the IDT test, than to determine it from actual tests." (A2) It appears as there would be no need for major changes in the Superpave though determination of Poisson's ratio is not critical to the thermal cracking program. prediction of low-temperature cracking, again suggesting Examination of the equipment requirements for the dynamic that perhaps uniaxial creep tests could provide the needed modulus master curve equipment as developed earlier in data more simply and more directly than the IDT creep test. NCHRP Project 9-29 indicates that this equipment should have both the load capacity and transducer resolution for properly performing the creep test on asphalt concrete at low Section 6. Apparatus temperatures; this evaluation is described in detail in Appen- dix B of this report and summarized in Chapter 2 of the There are six main components to the IDT test system: body of the report. The maximum load capacity of 22.5 kN (5 kips) is, however, too low for performing uniaxial tensile · Axial loading device, strength tests, which would require a maximum load of 70 kN · Load measuring device, (16 kips) to ensure that all or almost all mixtures could be · Deformation measuring device(s), tested to failure. Therefore, it is suggested that mixture ten- · Environmental chamber, sile strength normally be determined using either the IDT · Control and data acquisition system, and or uniaxial tensile strength tests on a high-capacity static test · Specimen loading frame (test fixture). machine separate from the dynamic modulus master curve device. However, it should also be possible to perform both This section of AASHTO T322 provides specifications for creep and strength tests on a single, high-performance servo- each of these subsystems, which are summarized in Table A-1 hydraulic system, as long as all equipment requirements are below. The term "test fixture" is used here rather than "load- ing frame" to describe the device that holds the IDT specimen met. Such a system would, however, likely be somewhat in place and transfers the load from the testing device to the more expensive than the standard dynamic modulus master specimen, as loading frame is an ambiguous term that could curve system. A new, draft test procedure should be written be confused with the loading system. In order to evaluate these for performing uniaxial creep tests, based upon AASHTO specifications, it is necessary to examine the possible range of T322 and the specifications developed for the simple per- responses for HMA at low temperature and also to understand formance tests and related procedures as part of NCHRP what ranges and sensitivities are possible and practical for the Project 9-29. systems in question. The following paragraphs address these The issue of linearity is of great practical importance. Intu- issues. itively, it should be expected that asphalt concrete at low tem- In SHRP Report A-357, the developers of the IDT creep and peratures should behave in a linear manner through loading strength testing procedure present data for a range of mixtures approaching the point of failure because of the high stiffness (A3). These show a typical range in compliance values of of asphalt concrete under these conditions and the very low about 3 × 10-11 Pa-1 to 4 × 10-9 Pa-1. Because the linear range strains. It is, however, important to verify that the loads used for HMA occurs at strains less than or equal to 0.05 percent, in the IDT test are appropriate--as high as possible, to ensure the maximum applied tensile stresses corresponding to these large deflections and good repeatability, while still remaining compliance values range from 125 kPa to 17 MPa. Based upon in the linear viscoelastic region. AASHTO T322 calls for a the relationship t = 2P/tD, the axial loads corresponding to maximum strain of 500 × 10-6 mm/mm, or 0.05 percent. This these tensile stresses are 1.5 and 200 kN, respectively, for a value is consistent with work performed by Mehta and Chris- specimen 50 mm thick and 150 mm in diameter. However, tensen (A1), who reported that deviations from linearity began another consideration is the maximum load that can be applied to occur at the same strain level of 0.05 percent. This aspect without failing a specimen. The lowest tensile strength, t, of AASHTO T322 probably does not need revision. reported in SHRP A-357 (A3) was 1.3 MPa; the highest was The final general issue in the IDT test procedure is whether 4.3 MPa. The corresponding load, P, for these tensile strengths it is truly necessary to determine Poisson's ratio when charac- can be calculated as P = ttD/2, where t and D are the speci- terizing the mechanical behavior of HMA at low temperature. men thickness and diameter, respectively. The calculated Poisson's ratio represents the ratio of lateral to axial deforma- loads based on tensile failure are between 15 and 51 kN for a tion under uniaxial loading. It is theoretically necessary to 50-mm-thick specimen. Limiting the load to one-half that know Poisson's ratio when performing stress analyses in two required to cause failure and allowing for specimens up to or three dimensions. However, in performing simple, one- 100 mm in thickness, the anticipated maximum load is then
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A-3 TABLE A-1 AASHTO T322 specifications for IDT apparatus Component General Requirements Range Sensitivity Axial loading Shall provide a constant 98 kN maximum load; 5 N minimum device load Displacement rate between 12 and 75 mm/min Load measuring Electronic load cell 98 kN minimum capacity 5 N minimum device Deformation Four linear variable 0.25 mm minimum 0.125 µm measuring differential transducers minimum device(s) (LVDTs) Environmental Temperature control only; -30 to +30 °C Control to ±0.2 chamber large enough to perform °C test and condition 3 specimens Control and Shall digitally record load 1 to 20 Hz sampling rate 16-bit A/D board data acquisition and deformation during test required system Test fixture As described in ASTM N/A 2 kg maximum D4123 (diametral resilient frictional modulus testing) resistance 50 kN. However, to ensure good loading system performance, where the capacity of the loading system should be about double the E = modulus, MPa (inverse of the creep compliance D); anticipated maximum load, giving a maximum capacity of P = applied load, N; 100 kN, agreeing nearly exactly with the 98 kN given in = Poisson's ratio; AASHTO T322. t = specimen thickness, mm; In evaluating the required sensitivity of the loading system, H = total horizontal deformation, mm; and the worst-case situation is for the lowest anticipated load. V = total vertical deformation, mm. Because it would be undesirable to approach nonlinearity, in some cases the applied loads might be somewhat less than the Because these equations are based upon conditions of plane estimated minimum load of 1.5 kN, say 1 kN. To calibrate to stress, which is a simplification of the actual three-dimensional this load level, ASTM E4 requires a resolution that is 1/100th state of stress during an IDT test, they should be considered of the minimum load level or a resolution of 10 N, which is approximate. However, they should be accurate enough for the significantly larger (poorer) than the 5 N resolution require- purposes of estimating the required loading rates and trans- ment given in AASHTO T322. Consideration should be given ducer sensitivities. Rearranging Equation A-1, replacing H to changing the required resolution for the IDT loading sys- with V/5.38 (from Equation A-2 for = 0.40): tem to 10 kN; this would likely reduce the cost of the equip- ment required to perform the test. V = 5.38 P ( + 0.27) Et ( A-3) Addressing the requirements for the required displace- ment rate is more complicated. Because linearity requires a Assuming a Poisson's ratio of 0.4, for the given conditions maximum strain of 0.05 percent, this represents the maxi- of P = 50 kN and D = 3 × 10-11 Pa-1, the maximum expected mum horizontal strain during the IDT creep test. In a creep vertical deformation for the IDT creep test is 0.10 mm. If this test, the load is applied very quickly during the initial part is to be applied during a maximum ramp time of one second, of the test, typically within a period of not more than one the maximum expected displacement rate is then 0.10 mm/s, second. The condition requiring the highest loading rate is or 6 mm/min. However, to ensure that the system has adequate for very stiff materials at low temperature, because in this reserve capacity for good control of the loading rate, a higher case the behavior is nearly elastic and most of the specimen maximum displacement rate is desirable, say 12 mm/min. This deformation will occur during the initial application of the corresponds exactly with the lowest displacement rate given load. Therefore, in order to calculate the vertical deforma- in AASHTO T322. It is not clear why a range is specified for tion, an applied load of 98 kN and a specimen compliance the displacement rate; there is no reason to arbitrarily limit of 3 × 10-11 Pa-1 can be assumed. Two useful equations relat- the maximum displacement rate for the IDT system. It is rec- ing load, Poisson's ratio, and horizontal and vertical defor- ommended that the required displacement rate for IDT test mations are given in ASTM D 4123, Standard Test Method systems be given as at least 12 mm/min. for Indirect Tension Test for Resilient Modulus of Bituminous Evaluation of the requirements for the IDT load cell follow Mixtures: directly from the previous discussion. The maximum applied load is 50 kN, and the load cell should have a maximum capac- E = P ( + 0.27) tH ( A-1) ity substantially higher than the maximum expected load to avoid overloading and potentially damaging the transducer. = 3.59 H V - 0.27 ( A-2) Therefore, the load cell should have a maximum capacity of at
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A-4 least 100 kN. The sensitivity of the load cell should at least control systems and the relatively large thermal mass of the match the sensitivity of the loading system, determined to be IDT specimens. Furthermore, as with the requirements for 10 N rather than the 5 N listed in AASHTO T322. the temperature chamber to be used with the simple perfor- The first issue concerning specimen deformation measure- mance tests, ambient conditions should be given under which ment that should be addressed is the type of transducer to be the specification should be met--15 to 27°C. There is no used. Currently, AASHTO T322 requires the use of LVDTs. need to require the IDT chamber to have a range extending Although LVDTs are widely used in this type of test, there are to 30°C; this means that the system must have a substantial other types of transducers that have been used with this sys- heating system in order to control temperatures at ambient tem with success, including strain-gagebased clip-on gages. temperatures and above, increasing the complexity and cost The specification should not specify the type of transducer to of the chamber. The required temperature range for the cham- be used, only the required level of performance in terms of ber should be narrowed to -30 to 10°C under the given gage length, range, and sensitivity. ambient conditions. The maximum deflection to be measured during an IDT The requirements for system control and data acquisition creep test will occur in the vertical direction. Based upon are largely acceptable but could be slightly improved. The equations given in SHRP Report A-357 (A3), the vertical use of a personal computer in the control and data acquisition strain measured during an IDT test can be nearly twice the system should be explicitly required. On the other hand, the horizontal strain, which is limited to 0.05 percent. Therefore, required sensitivity of the data acquisition system could be the maximum expected deflection during a typical test would more effectively stated to be consistent with the required sen- be 0.001 × 38 mm, or 0.04 mm. This range would however sitivity of the various transducers. The manner in which this be extremely difficult to work with in setting up and execut- is achieved should be left to the equipment supplier. ing a test. Current requirements in AASHTO T322 are for a The test fixture is specified to meet the requirements of minimum LVDT range of 0.25 mm; commercially available ASTM D4123, which is a standard test method for diame- IDT equipment used at Advanced Asphalt Technologies, tral resilient modulus testing. It is suggested that a separate, LLC, uses displacement transducers with an overall range of smaller frame be used to help meet the requirements of this 2.5 mm, which include a software window of 0.25 mm that specification. A maximum frictional resistance of 2 kg is also is enabled after initial specimen set up. This is an effective specified in AASHTO T322. If the minimum applied load is system that should be considered in the next generation of 1 kN, as discussed previously, the maximum frictional resis- HMA low-temperature testing equipment. tance should be no more than about 2 percent of this, or 20 N. Evaluation of the required sensitivity of the deformation This is in very close agreement to the 2 kg frictional resistance transducers for the IDT is straightforward. Only the case of in the current specification. However, as frictional resis- horizontal deflections needs to be addressed, because these tance is a force, AASHTO T322 should be revised to specify will always be significantly smaller than vertical deflections the maximum frictional resistance in Newtons rather than and so represent the critical situation. Linearity constraints, kilograms. A simple procedure should be given for evaluat- as discussed previously, limit horizontal strains during the ing the frictional resistance of the test fixture. ASTM D4123 IDT creep to 0.05 percent. However, it is impossible to deter- requires stainless steel loading strips one-half inch wide, with mine test conditions a priori so that strains are always close a curvature matching that of the IDT specimen. Generally, to this limit; therefore, a realistic strain at the end of the test load applications to materials such as asphalt concrete must would be 0.025 percent. Also, it must be kept in mind that include some provisions for distributing the load evenly this is the strain at the end of a typical IDT creep test; the over the test specimen and avoiding stress concentrations and strain at the start of collection of data can be as much as five eccentricities. These issues are not addressed by the current times less than this, or 0.005 percent (50 parts per million). requirements of ASTM D4123. The curvature of the loading Given the standard gage length of 38 mm, this represents a min- strips, though nominally addressing the geometry of the spec- imum expected deflection of 1.9 µm. To maintain a reasonable imen, may in fact cause more problems than it solves, because resolution under this worst-case situation of about 5 percent, this could increase stress concentrations and eccentric loading would require a transducer sensitivity of 0.1 µm (4 µin.). unless the curvature and alignment of the specimen exactly The current specifications for the temperature chamber in match that of the loading strips. A more conventional approach AASHTO T322 require a range of -30 to +30°C, with a con- would be to use flat, neoprene loading strips, one-half inch trol sensitivity of ±0.2°C. Examining typical IDT creep data, thick by one-half inch wide. These strips would be compliant a temperature control sensitivity of ±0.2°C translates to a max- enough to assume the shape of the IDT specimen regardless imum potential error in creep compliance of about 3 percent. of irregularities and would greatly reduce the potential for This appears reasonable; however, a temperature chamber stress concentrations and eccentricities. with this level of control sensitivity would be prohibitively A summary of the suggested revised specifications for the expensive. A more realistic requirement for sensitivity would IDT apparatus to be used in conjunction with AASHTO be the one already established for the simple performance T322 is given in Table A-2. Many of the changes are slight, tests, ±0.5°C. This could lead to maximum potential compli- for example, giving the maximum range of the loading ance errors of about 8 percent, though the error in most cases device and load cell as 100 kN rather than the "soft" metric would be smaller because of the cyclic nature of temperature value of 98 kN. The sensitivity of the loading device and load
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A-5 TABLE A-2 Proposed revised AASHTO T322 specifications for the IDT apparatus Component General Requirements Range Sensitivity Axial loading Shall provide a constant 100 kN maximum load; 10 N or better device load Maximum displacement rate of at least 12 mm/min Load measuring Electronic load cell 100 kN minimum capacity 10 N or better device Deformation Four displacement 0.1 mm minimum 0.1 µm or better measuring transducers (LVDTs) device(s) Environmental Temperature control only; 30 to +10 °C under Control to ±0.5 chamber large enough to perform ambient conditions of 15 to °C test and condition 3 27 °C specimens Control and System shall be operated 1 to 20 Hz sampling rate Consistent with data acquisition with the use of a personal required system computer and shall sensitivity of all digitally record load and system deformation during test transducers Test fixture As described in ASTM N/A 20 N maximum D4123 (diametral resilient frictional modulus testing), but with resistance flat neoprene loading strips 12-mm thick by 12-mm wide. cell is decreased, while the sensitivity of the deformation that of a typical asphalt concrete specimen at -30 to -20°C measuring devices has been increased. The suggested use of and would exhibit stable properties with E = 69 GPa and = flat neoprene loading strips--rather than curved, stainless 0.33. Furthermore, the thinness of the specimen should pro- steel strips--should be evaluated in the laboratory testing duce conditions approaching that of plane stress, simplifying portion of Phase III of NCHRP 9-29. the analysis and providing additional certainty in the results of the verification. A full system calibration frequency of once 8. Standardization every year is probably adequate. A confidence check using the aluminum standard should be performed every time the sys- The requirements for calibration and verification of the tem is used. Verification of the load cell and LVDTs should IDT test system in the current version of AASHTO T322 are be required when the confidence check fails, at least once somewhat vague. AASHTO T322 includes the following per month when the system is being used, and prior to begin- requirements: ning tests if the system has not been used for more than 30 days. · The testing system shall be calibrated prior to initial use and at least once a year thereafter. · The temperature control in the environmental cham- 9. Sampling ber and all transducers used in the IDT system shall be This section probably needs little or no revision. Currently, verified (no frequency given). specimen preparation according to either AASHTO T312 · If the results of any verification are not satisfactory, (Superpave gyratory compactor) or AASHTO PP3 (rolling appropriate actions shall be taken to correct the response wheel compactor) is permitted. Consideration should be given of the transducer(s) in question. to requiring gyratory compaction only, in order to reduce vari- ability and promote reproducibility in IDT creep and strength Accurate execution of the IDT creep test requires that all tests. The current specification states that if cores from road- transducers in the test system be calibrated and operating prop- ways are to be tested, they should be taken following proce- erly. The calibration requirements should be more detailed, dures given in ASTM D5361. referring to appropriate ASTM standards (ASTM E4 for load and ASTM D 6027 for deflection and specimen deformation). The verification procedure and required hardware for verifica- 10. Specimen Preparation tion should also be more detailed. IDT systems should include and Preliminary Determinations a proving ring for load verification and a verification system for checking the transducers, such as a calibration block with Requirements for specimen diameter and thickness are not a very sensitive micrometer. A standard specimen, 10 mm critical to the results of the IDT creep and strength test; how- thick by 150 mm in diameter, made of 6061 T6 aluminum ever, some revisions in this section of AASHTO T322 are alloy, should also be supplied with the IDT system. Such a needed. One critical point is the smoothness and parallelism specimen would provide an effective stiffness similar to of the specimen faces; currently, AASHTO T322 only states
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A-6 that the specimen sides should be "smooth" and "parallel." 11. Tensile Creep/Strength Testing The specimen requirements given in Table A-3 are partly (Thermal Cracking Analysis) based upon those developed for the First Article Equipment Specifications for the Simple Performance Test System devel- Several changes are needed within this section of AASHTO oped earlier during NCHRP Project 9-29 and should help T322. First, the suggested test temperatures for the creep pro- ensure good test results with the IDT creep and strength pro- cedure are 0, -10, and -20°C. Because of the variability in cedure (A4). The required specimen diameter in Table A-3 binder grades and the resulting low-temperature properties of has been given as 150 to 154 mm, rather than the 150 ± 9 mm asphalt concrete, some specimens are extremely stiff at -20°C, given in AASHTO T322, to maintain consistency with the while others may be too compliant at 0°C. The test tempera- requirements of the simple performance test. The specimen tures used in the IDT creep and strength test should, therefore, thickness requirement has also been changed slightly, given change according to the binder grade used. The relationship as 40 to 60 mm, rather than as 38 to 50 mm as in AASHTO between binder stiffness and mixture stiffness is not 1:1; a T322. This change is suggested to provide a "hard" metric given change in binder stiffness will produce a somewhat specification and also to allow some margin for error in pro- lower change in mixture stiffness. Therefore, it is not neces- ducing 50-mm-thick specimens, which are considered stan- sary or advisable to link IDT test temperatures directly to low- dard for the IDT test. Specimen parallelism is specified temperature binder grade. It is suggested that the current test through the use of the standard deviation of the thickness, temperatures of 0, -10, and -20°C be maintained for mixtures which is limited to less than 1.0 mm, which corresponds to a made using PG XX-28 and PG XX-22 binders. For PG XX-16 2s limit of about 1.2 degrees, similar to the 1 degree require- and XX-10 binders, or mixtures that have been severely age- ment for the simple performance test. hardened, the recommended test temperatures should be -10, Another requirement of this section is to determine the bulk 0, and +10°C. For PG XX-34 binders (or softer), the recom- specific gravity of the specimen following AASHTO T166, mended test temperatures should be -30, -20, and -10°C. with the caveat that high-absorption specimens should be A practical problem with the current version of AASHTO tested using an impermeable plastic film rather than a paraf- T322 is that the test conditions are to be determined using a fin coating as specified in AASHTO T166. This requirement trial-and-error procedure. A load is applied to the specimen is necessary to ensure that the surfaces are clean so that the and, if the resulting strains fall outside the allowable range, LVDT gage points can be properly glued to the specimen. the test is aborted, the specimen is allowed to recover for There is also a statement here that if direct immersion is 5 minutes, and the test is then repeated at an adjusted load used to determine the bulk specific gravity, the specimen level. No suggestions are given concerning what the appropri- must then be dried to a constant weight prior to fastening of ate applied loads should be for different combinations of mix- the LVDT gage points. In the interest of ensuring consistent ture types and test conditions. Given the suggested revised bulk specific gravity measurements and also to ensure rapid protocol recommended above, Table A-4 presents guidelines and consistent specimen preparation, it is suggested that for the applied load. this part of AASHTO T322 be revised to require that all These guidelines are based upon typical ranges for asphalt bulk specific gravity measurements be made using imper- concrete modulus under the conditions likely under the pro- meable plastic rather than the saturated surface-dry method posed protocol. The maximum allowed deformation cor- or the paraffin coating technique. responds to the maximum allowable horizontal strain for TABLE A-3 IDT creep and strength specimen requirements Item Specification Remarks Average diameter 150 to 154 mm See Note 1 Standard deviation of diameter 1.0 mm See Note 1 Average thickness 40 to 60 mm See Note 2 Standard deviation of 1.0 mm See Note 2 thickness Smoothness 0.3 mm See Note 3 Table A-3 Notes: 1. Measure the diameter at the center and third points of the test specimen along axes that are 90 degrees apart. Record each of the six measurements to the nearest 1 mm. Calculate the average and the standard deviation of the six measurements. The standard deviation shall be less than 1.0 mm. The average diameter, reported to the nearest 1 mm, shall be used in all material property calculations. 2. Measure the thickness of the specimen to the nearest 1 mm at 8 equally spaced points along the circumference of the specimen, using a pair of calipers or other similar device. Calculate and report the average thickness to the nearest 1 mm. The standard deviation of the specimen thickness shall be less than 1.0 mm. The average thickness shall be used in all material property calculations. 3. Check this requirement using a straight edge and feeler gauges.
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A-7 TABLE A-4 Guidelines for applied load in the IDT creep test Test Temperature Initial Applied Other Possible Applied Loads Load (kN) (kN) Lowest 40 Deformation < 0.01 mm: 80 Deformation > 0.02 mm: 20, 10 Intermediate 10 Deformation < 0.01 mm: 20, 40 Deformation > 0.02 mm: 5, 2 Highest 5 Deformation < 0.01 mm: 10, 20 Deformation > 0.02 mm: 2, 1 linearity, 0.05 percent, rounded up from 0.019 to 0.02 mm. 2-hour failure time during a thermal cracking event would be The lower limit represents one-half this value, which is nec- 7200 s / [10(-0.2)(-18)] = 1.8 s. A typical failure strength for essary to ensure adequate resolution of the deformation data asphalt concrete at low temperature would be 3 MPa (A3, A5, during the test. In the final version of the IDT software, it A7 ). Because for diametral loading, x = 2 P/tD, the corre- might be possible to provide a utility that estimates the spec- sponding vertical load for a typical IDT strength would be imen compliance from the binder grade (or bending beam 35 kN. Using a typical low-temperature asphalt concrete rheometer test data) and mixture composition and uses this modulus value of 14 GPa, a Poisson's ratio of 0.4, and a spec- information to calculate the initial load for the test. Additional imen thickness of 50 mm, Equation A-3 can be used to esti- software controls could be designed to monitor the progress of mate the vertical displacement at failure for a typical IDT the test and make adjustments to the applied load as needed. strength test as 0.18 mm. Because the estimated equivalent Another important issue in executing the IDT creep and failure time was found to be 1.8 sec, the loading rate for the strength test is the temperature and rate for IDT strength test- IDT strength test should be 0.1 mm/sec. The IDT strength ing. In the original conception of the IDT procedure and in test should, therefore, be performed at a vertical displace- the current version of AASHTO T322, the strength test was ment rate of approximately 0.1 mm/sec or 6 mm/min, which to be performed at the same three temperatures as the creep is somewhat slower than the 12.5 mm/min currently specified test--typically, -20, -10, and 0°C. However, partly because in AASHTO T322. Considering the approximate nature of of the irregular relationship between temperature and tensile this analysis and the fact that the Superpave thermal cracking strength and probably to make the entire test procedure more model has been calibrated using strength data collected at efficient, most laboratories perform the strength test at -10°C 12.5 mm/min, no change to the strength test loading rate in only. The specified loading rate in AASHTO T322 for the AASHTO T322 is recommended. strength test is 12.5 mm/min. The assumption in this approach The specimen conditioning time given in AASHTO T322 is to testing is that the IDT strength test should be performed 3 hours ±1 hour. Three hours is probably an acceptable time quickly, to eliminate time dependency from the result. How- for temperature equilibration, but the range of ±1 hour is prob- ever, because the strength of HMA, like modulus or compli- ably too large given the potential for possible physical hard- ance, is time and temperature dependent, an effort should be ening of the specimen at low temperatures. It is suggested that made to make the time and temperature conditions for the IDT this range be reduced to ±0.5 hours. AASHTO T322 should strength test at least approximately representative of what also include an alternate approach using a dummy specimen occurs in the field during low-temperature cracking events. with an embedded temperature sensor, which could be used to The analysis of a suitable loading rate for the IDT strength provide additional assurance of proper specimen equilibration. test can only be done in an approximate manner, but should If the dummy specimen is used, the test should be completed help obtain reasonable test conditions. Examination of thermal within 1 hour of reaching equilibration. Some mention should stress development curves shows that at cooling rates of be made here of the possibility for steric hardening under con- 5°C/hr, most of the tensile stress in the mixture is generated tinued storage at low temperatures, so engineers and techni- during the last two hours of cooling. This representative load- cians have some understanding of the reason for this limitation ing time agrees with the 2-hour effective loading time used in and the possible consequences if it is ignored. This section of most limiting stiffness approaches to controlling thermal crack- AASHTO T322 also states that the test should not begin until ing (A5 ). However, it is suggested that the IDT strength test be the chamber is within ±0.2°C of the target temperature. As performed at the middle creep test temperature, which is 12 to discussed previously, this requirement is too stringent; the 18°C higher than the minimum binder grading tempera- allowable temperature range should be increased to ±0.5°C. ture. Considering that the actual cracking temperature should generally be several degrees below the grading temperature, the IDT strength test would normally be performed at about 12. Tensile Strength Testing 15 to 21°C above the anticipated cracking temperature. Typ- (Fatigue Cracking Analysis) ically, shift factors for asphalt binders at low temperature vary -0.2 log shift factors per °C (A6 ). Therefore, the fail- Tensile strength is not required information for the fatigue ure time for an IDT strength test roughly equivalent to the analysis to be used in the pavement design guide developed
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A-8 in NCHRP Project 1-37A. Therefore, there is no longer a The details of the calculations presented in AASHTO T322 need for this section in AASHTO T322. have been modified somewhat over the past 6 years and so will not be discussed in this appendix. This section of the specifi- cation needs to be edited to ensure that it represents the latest 13. Calculations version of the calculation procedure as developed during NCHRP Project 9-19 (A8). This section of AASHTO T322 describes in detail the pro- cedure for organizing data and calculating creep compliance and Poisson's ratio. The procedure for data collection is not 14. Report explained; the specification should provide information con- cerning the standard structure for data files, including times at This section of AASHTO T322 is straightforward but does which data should be collected, and what properties should be need some revision. Because the reporting of creep compli- reported and in what format. A key issue in this section of ance is relatively complicated, the standard format for such a AASHTO T322 is the data trimming process, in which arrays report should be given here, including the times at which test of data are collected representing six cases: two sides for each results are to be reported and the properties to be included in of three specimens. The highest and lowest values are some- the report. This section should also include information con- what arbitrarily discarded, and the remaining four arrays are cerning the standard format for input into the pavement design used to estimate average values. This procedure was appar- guide developed in NCHRP Project 1-37A for analyzing ther- ently needed because of the high variability in IDT data dur- mal cracking. There are references to the Superpave software ing early versions of the test. There are several problems in this section of the specification that should be deleted. with this approach. As the hardware and procedures used in this procedure have been improved, the quality of the data has also improved, to the point where the data trimming might in 15. Precision and Bias most cases represent an unnecessary discarding of otherwise useful and perfectly accurate data. On the other hand, it is con- This section currently contains no information. Although ceivable that in some cases perhaps only one or as many as some limited information is now available, it probably is not three data arrays might be faulty. An alternate approach is sug- extensive enough to include in a precision and bias statement. gested to ensure that the quality of IDT creep data is acceptable: Perhaps a note could be included here giving preliminary estimates of the precision of the IDT creep and strength tests. · Data for each test should be analyzed as the test is run, to ensure that they are of good quality. The IDT software AASHTO T322 Summary should automatically verify that load and deformation data are reasonable and produce sensible results. If not, There are a number of important issues concerning the operator should be informed that the test data gener- AASHTO T322. The most fundamental issue is whether the ated were of poor quality, and the test should be repeated. low-temperature creep compliance of asphalt concrete If an additional test fails, the specimen should be dis- should be determined using the IDT geometry or whether a carded and only two specimens used in the analysis. uniaxial creep test should be used. This is especially perti- · Upon completion of the test and analysis of the data, the nent as the simple performance tests being developed as part creep compliance, m-values, and Poisson's ratio values of NCHRP Project 9-19 are uniaxial tests, and, as a result, in for each specimen should be compiled, and the average a few years, equipment for preparing specimens and per- and standard deviation reported for the complete set of forming uniaxial creep tests should be commercially available tests. The software should notify the operator if the val- at a reasonable cost. Using the same test geometry for both the ues appear unusual or otherwise of poor quality. simple performance tests and the low-temperature creep com- pliance test would simplify implementation activities and Analyzing the data in this way would ensure that if an indi- potentially reduce the cost of equipment and training for lab- vidual test is suspect, it is repeated immediately rather than oratories wishing to have the capability of performing both waiting until all tests are completed to evaluate the data and procedures. realizing that there were one or more suspect test results. Fur- Various other relatively minor issues have been identified in thermore, analyzing the replicate specimens separately and the review of AASHTO T322. Some of the existing require- reporting statistics on these data allows the technician and/or ments for the loading system, environmental chamber, and engineer to evaluate the overall quality of the data and the load and deformation transducers should be revised; suggested repeatability of the results. This is particularly important in changes were presented previously in Table A-2. Existing situations where the IDT procedure is being used to compare requirements for IDT specimen dimensions are largely sub- two different mixtures. For example, without appropriate test jective. Specific requirements for specimen dimensions and statistics, it would be very difficult to evaluate if a difference uniformity were given in Table A-3 and were based upon of 20 percent in the creep compliance of two such mixtures requirements developed for use in conjunction with the sim- represents a statistically significant difference. ple performance test. The current test protocol involves test-
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A-9 ing at three temperatures (-20, -10, and 0°C) regardless of ratio (A3). Within 2 years of the completion of SHRP, a sim- the binder grade used. This sometimes results in marginal plified procedure was developed for accounting for nonideal data for one of the temperatures, where the compliance of the conditions during the IDT test (A7). An empirical set of equa- specimen was either too high or too low to be of value in the tions was developed based upon the results of the finite element analysis. A more efficient system would be to link the creep analysis, which avoided the iterative procedure in calculating compliance test temperatures to the low-temperature binder compliance and Poisson's ratio. grade used in making the asphalt concrete. This would ensure A second area of modification occurred in the manner in that the creep data would almost always be in the desired which calculated creep compliance data are used to generate a range. Statistical analyses should be provided in calculating master curve, providing creep compliance data at a selected compliance data, so that the technician or engineer running reference temperature (-20°C in this case) over a wide range the test can immediately evaluate the quality of the data and of loading times. In producing such master curves, use is made repeat the test if needed. of time-temperature superposition, which essentially involves In general, most of the required modifications in AASHTO shifting log compliance-log time functions determined at T322 are minor, other than the fundamental issue of whether several temperatures along the log time axis until a single func- the IDT test is the most efficient method for determining the tion is created. Often this procedure is done visually, which creep compliance of asphalt concrete mixtures at low temper- leads to substantial differences in results generated by differ- atures. That issue can be best addressed through experimental ent engineers; and it also requires substantial overlap among testing to compare creep compliance data at low temperatures the compliance curves for best results. During SHRP, creep determined using both procedures. Provided that the results of tests were performed for 1,000 seconds, which generally pro- such testing suggest that the IDT test be retained, the sug- duced good overlap of data. Details of the procedure used to gested modifications in AASHTO T322 could be easily made develop compliance master curves were not provided in the and should not be controversial. If the laboratory testing final SHRP reports, but later publications provided such infor- supports the use of uniaxial compression in low-temperature mation. Also, at the conclusion of SHRP, it was decided that creep tests, then a new standard would have to be developed, the length of the IDT creep test should be reduced from 1,000 although much of it could be borrowed from AASHTO T322 to 100 seconds to shorten the test time required to complete and from existing proposed standards for the simple perfor- the test. This unfortunately meant that the compliance curves mance tests. determined at the three test temperatures often provided lit- tle or no overlap for developing the master compliance curve. This required development of new algorithms for extrapolat- RECENT RELATED CHANGES TO THE IDT TEST PROCEDURE, EQUIPMENT, AND ANALYSIS ing the master curve and shifting the resulting data. The IDT creep and strength procedure was developed dur- IDT Research at the Superpave Regional Centers ing SHRP, which took place 10 to 15 years ago. Since the conclusion of SHRP, there have been numerous substantial A third area of research, unfortunately of limited scope, changes in the test procedure, equipment, and analysis meth- occurred under the auspices of the Regional Superpave Cen- ods used in performing the IDT creep and strength test and ters established by the FHWA in 199596. There were five interpreting the resulting data. The following subsections of such regional centers throughout the country: the Northeast the report discuss the various changes that have occurred, Superpave Center, located at the Pennsylvania Transportation organized more or less chronologically: post-SHRP devel- Institute of the Pennsylvania State University; the Southeast opments, IDT research at the Superpave Regional Centers, Superpave Center, located at the National Center for Asphalt and modifications during NCHRP Projects 1-37A and 9-19. Technology at Auburn University; The Northcentral Super- pave Center, associated with the Indiana Department of Trans- Post-SHRP Developments in the IDT Procedure portation and Purdue University; the Southcentral Superpave Center at the University of Texas; and the West Coast Super- The modifications in the IDT test and analysis procedure in pave Center, which was divided between the University of the first several years following completion of SHRP primar- California at Berkeley and the University of Nevada at Reno. ily involved improvements in the methods used to calculate All of the Superpave Centers, except for the West Coast creep compliance and Poisson's ratio from load and deflection Center, were given IDT creep and strength test systems de- data. During SHRP, finite element analyses performed on the signed and manufactured by Instron Corporation. These IDT test geometry indicated that the simple, plane stress analy- systems were unique in that they were closed-loop electro- sis typically used in the past to analyze the results of the test mechanical ("screw") test machines; most closed-loop test can produce substantial errors. These errors result from two systems are servo-hydraulic. It was believed that these systems sources: horizontal and vertical bulging of the specimen and would potentially be less expensive to purchase and operate, nonuniform strains across the vertical and horizontal diame- and also easier and safer to operate, especially in a state high- ter. Correction factors were developed for use in a cumber- way or contractor's laboratory that might lack experienced test some, iterative calculation of creep compliance and Poisson's engineers.
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A-10 Unfortunately, these systems were plagued with a wide ment design guide developed in NCHRP 1-37A to include as range of hardware and software problems and a lack of cus- an option the necessary capacity, hardware, and software tomer support. There were frequent problems with malfunc- for performing the IDT creep test, perhaps in combination with tioning LVDTs used to measure IDT deformation and with the uniaxial tensile strength. It is even possible that uniaxial conditioners used in conjunction with these transducers. Part creep tests would provide data equivalent to that provided by of this problem was related to the practice of keeping LVDTs the IDT procedure, which would mean that the same specimens mounted on the specimens during strength tests, which fre- could be used throughout the testing needed for flexible pave- quently damaged the LVDTs. Sometimes the LVDT was dam- ment design work. This is an issue that should be addressed in aged enough to be completely nonfunctional, but often times the laboratory testing to be done as part of Phase III of NCHRP it was only slightly damaged, so that it was not clear that the Project 9-29. LVDT was not functioning properly. Another source of prob- One aspect of the experience among the Superpave Cen- lems was the placement of some of the LVDT conditioning ters that should be given consideration is their abandonment circuits inside the environmental chamber, which subjected of using LVDTs during the IDT strength test to determine these electronics to frost and moisture. The manufacturer the exact moment of failure. In a standard IDT strength test, explained that the nature of the bid documents required them to the precise moment of failure, and hence the "true" tensile design the system in a less than ideal manner and indicated that, strength, is difficult to determine, because the specimen fails given more flexibility in their choice of transducer type, they very gradually and continues to carry substantial load even could have produced a significantly more reliable system. after large cracks appear. During SHRP, the suggested solu- The software supplied with these systems was inflexible tion to this problem was to use the horizontal and vertical and difficult to operate and frequently crashed. The latter LVDTs to monitor horizontal and vertical deflections during problem was possibly caused by insufficient memory in the the strength test. The point of failure is defined as occurring computer systems supplied with these test systems. Some when the difference between the vertical and horizontal defor- engineers at the Superpave Centers complained that the ramp mations reaches a maximum. This is the procedure included times required to reach specified loads for the creep tests in AASHTO T322. Unfortunately, as explained previously, were too long, though experience at the Northeast Center was keeping LVDTs in place during the strength test often results that this was a software limitation and not a limitation of the in damage or destruction to these sensitive and expensive capability of the electromechanical system. transducers. Engineers within the Superpave Centers agreed Because of the numerous problems encountered by the var- that for practical reasons, the IDT strength test should be ious Superpave Centers in operating these systems, only one-- done without LVDTs and the strength based only upon the the Northeast Center--performed IDT tests on a regular basis maximum load. Although the AASHTO T322 procedure is using this equipment; recently, the Northcentral Center also probably more accurate, it appears that it is impractical, and began using their system. The quality of the data produced at damage to the LVDTs as a result of this procedure could actu- the Northeast Center was, however, marginal, and testing was ally reduce the overall reliability of the IDT creep and strength continued only in an effort to gain experience with this system. tests. In any case, the IDT strength test is only an approxi- The Northeast Center did publish one research paper on analy- mation of the "true" tensile strength, and there is no reason sis of the IDT creep test, which was essentially a detailed to suspect that the refinement included in AASHTO T322 explanation of a simplified version of Roque and Hiltunen's provides a more accurate result. For example, it is quite pos- analysis (A3), suitable for use in estimating thermal cracking sible that IDT tensile strengths are in general lower than uni- temperatures using IDT creep and strength data (A9). axial tensile strengths. Because the AASHTO T322 "cor- In general, it appears that most of the problems encoun- rection" actually results in lower IDT strengths, this would tered in the IDT systems used within the Superpave Centers actually increase the error inherent in the test. The relationship could have been corrected, given an adequate investment of between IDT strength and uniaxial tensile strength should be time and money by the manufacturer and/or the Superpave Centers. Many of the problems were relatively minor ones evaluated experimentally by testing a range of mixtures using dealing with the LVDTs and conditioners or were related to both procedures. If necessary, empirical relationships can be the software and were not fundamental problems with the test developed among apparent IDT strength, the "corrected" system. Unfortunately, this experience has probably created strength as used in the Superpave thermal cracking program, a situation in which it would be politically inadvisable to and uniaxial tensile strength. Because the Superpave ther- continue to promote electromechanical systems for use in mal cracking program was designed to use "corrected" IDT IDT creep and strength testing. The likely market for this test strength as input, care must be taken to provide test data is probably too small to motivate any equipment manufacturer equivalent to that produced using this procedure. to provide significant custom engineering design and support for the IDT test system. The most practical approach for pave- Modification of the IDT Procedure ment engineers is, therefore, to use off-the-shelf test systems During NCHRP Projects 1-37A and 9-19 to perform the test, with a minimum of specially machined accessories. One potentially effective approach, for example, One of the early work elements in the Superpave Support would be to encourage suppliers of the frequency-sweep and Performance Models Management Project (FHWA Con- equipment to be used in characterizing mixtures for the pave- tract DTFH61-95-C-00100, later NCHRP Project 9-19) was
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A-11 an evaluation of the Superpave low-temperature cracking used in developing the master curve, and also to improve the model. A report on this work element was compiled that doc- reliability of the results. Recent improvements in the algo- umented numerous problems in the original SHRP thermal rithms used to develop master curves from IDT creep data cracking model (A10). Most of these problems were in the have probably addressed this problem. Although Janoo and computer program used to analyze the data and predict ther- associates indicated that the procedure used to estimate relax- mal cracking and have been addressed in recent modifica- ation modulus from creep compliance seemed to work well, tions of the program. However, some suggestions made in they suggested that perhaps a better approach would be to this report were not incorporated into later versions of the measure relaxation modulus directly, using a constant rate of Superpave thermal cracking model. strain test. However, recent research in which constant rate of One important issue raised in the report by Janoo and col- strain tests were performed on asphalt concrete has clearly leagues was the determination of the coefficient of thermal shown that the strain rate in these tests is difficult to control, contraction, (A10). The value of has an extremely strong and the results are, therefore, difficult to analyze and interpret. effect on the cracking temperature of asphalt concrete, and At this time, as the general approach and analysis method an accurate value for this parameter is essential to developing appear to work well, there is no reason to consider this sug- accurate predictions for low-temperature cracking. It is prob- gestion further. ably of equal importance to obtaining accurate measurements A final serious, pertinent issue raised by Janoo and his of compliance and strength. In the original SHRP procedure, associates (A10) was the inadequate incorporation of tensile was to be estimated based upon mixture composition (A3). strength in the model. Although the original intent in SHRP The accuracy of this procedure, however, was never verified. was to use tensile strength data at -20, -10, and 0°C, this Kwanda and Stoffels actually measured the coefficient of apparently proved impractical. Later versions of the Superpave thermal contraction of the mixtures used in developing the low-temperature cracking model used only tensile strength at SHRP low-temperature cracking test procedures and models -10°C. As pointed out in the report by Janoo and colleagues, and found very poor correlation between the predicted and the tensile strength of asphalt concrete increases with measured values of (A11). Mehta et al. later presented a decreasing temperature, up to a certain point, after which the procedure based upon Kwanda and Stoffel's, in which was tensile strength begins to decrease slowly (A10). Although measured using the instrumentation used in the IDT creep test this would appear to create a significant problem in the (A12). However, the accuracy of this procedure has not been Superpave thermal cracking model, the tensile strength data fully evaluated. Also, Janoo and associates (A10) pointed out are in fact used only to estimate the fracture parameter, A, that the coefficient of thermal contraction of asphalt cement from an empirical equation. Because this equation was devel- binders and asphalt concretes is not constant, but varies with oped based upon -10°C IDT strength data, altering the data temperature. Typically, is relatively constant at high temper- used as input would result in substantial errors in the proce- atures, but begins to reduce as temperature is lowered, reach- dure. Because the thermal cracking model has been calibrated ing a value at lower temperatures which is substantially based upon IDT strength data at -10°C, this approach should lower than that at high temperatures (A10). However, assum- continue to be used. ing a binder -value typical for temperatures above the glass Partly in response to the report by Janoo and colleagues, transition is a conservative approach. Furthermore, mixture Witczak et al. (A8) made a considerable effort to refine the -values measured by Kwanda and Stoffels (A11) suggest that thermal cracking program. The simple errors identified in in the temperature range of -20 to 0°C this assumption appears the program were corrected. Minor refinements were made to be reasonable, as discussed in Chapter 2 of this report. in the data reduction module. For example, the calculation of The change in the coefficient of thermal contraction of mix- compliance is based upon using "trimmed" means of deflec- tures with temperature is due entirely to the properties of the tions, which for the IDT test generally means averaging the binder, as the value of for aggregates is constant and inde- data from four transducers after discarding the lowest and pendent of temperature. Furthermore, it should be kept in mind highest transducer outputs. This procedure originally did not that the value of for asphalt binders is much greater than for properly handle LVDTs that were erroneously providing no aggregates, and as a result, the coefficient of thermal contrac- output; improvements in the data reduction procedure han- tion for mixtures is mostly a function of the binder properties. dled this situation appropriately and apparently provide the Thus, if the value of for mixtures is to be estimated, a typi- operator with some indication of overall data quality, though cal value for for the aggregate can probably be assumed, and the nature of this information is not yet clear. Equations for what is then critical is assuming the correct relationship making corrections for bulging and for nonuniform distribu- between and temperature for the selected binder. Using these tion of stress and strain across the IDT specimen were empir- assumptions, a simple and reasonably accurate equation for ically simplified into forms that allowed direct calculation of estimating the coefficient of thermal contraction for mixtures the factors, rather than iterative calculations as initially has been developed as part of Phase III of NCHRP Project required (A8). 9-29 and is presented at the end of Chapter 2 of this report. Significant improvements were made in the procedure used Another important suggestion made by Janoo and his in developing master compliance curves from IDT creep data coauthors (A10) was to increase the time of the creep test to during NCHRP Project 9-19 (A8). In developing a master 1,000 seconds, rather than 100, to simplify the procedure curve, compliance data at several temperatures is shifted with
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A-12 respect to the time (horizontal) axis to form a single curve rep- curves to generate a master compliance curve. The NCHRP resenting creep compliance as a function of time. In analyzing Project 9-19 report also includes the results of an evaluation IDT data, the reference temperature is usually -20°C. To form of MASTER. This program appears to work effectively in the IDT master curve, the creep data at -10 and 0°C are shifted reliably producing effective creep curves. The only potential to form a unified curve with the data at -20°C. This shifting is problem at this point appears to be with the shift factors. In equivalent to dividing the actual loading times for a given MASTER, shift factors are determined individually for each test by a constant called the shift factor, a(T). Figure A-1 is set of temperatures; there is no assumed function (exponen- a sketch showing graphically the construction of a master tial, Arhennius, etc.) used to fit shift factors as a function of compliance curve from IDT data. temperature. For very stiff mixtures, because of the very Although the construction of a master curve is not difficult, small slope in the creep compliance data, shift factors at the producing master curves in a standardized manner can be lowest temperatures can become unreliable. Although it is difficult, especially if the data are noisy or otherwise non- not fully explained in recent NCHRP Project 9-19 reports, it ideal. Often, experienced engineers will develop master curves appears that in order to evaluate shift factors at temperatures graphically, using a trial-and-error procedure involving sub- other than those used in IDT testing, a polynomial is fit to the stantial judgment. In order to make use of a master curve calculated shift factors and is then used to interpolate or within a computer program, this process must be implemented extrapolate shift factors at other temperatures. This proce- through a series of algorithms, which apply logic and math- dure can potentially produce substantial errors, though such ematics rather than judgment and experience to automati- errors should be infrequent and should only occur with poor- cally generate a master curve. It is essential that such a pro- quality data. cedure be robust and repeatable. That is, such an algorithm This potential shortcoming in MASTER could be avoided should, from a similar set of data, produce a comparable mas- by two changes: (1) linking the IDT test temperature to the ter curve, even with a substantial amount of variation in the low-temperature binder grade used, so that excessively low data. Another problem in generating master curves is that, compliance values are avoided, and (2) using a linear fit to ideally, the compliance curves at each temperature should the log a(T)-temperature data. Using IDT test temperatures overlap slightly in order to produce the most accurate master related to the binder grade would also tend to produce much curve. However, the current IDT creep testing protocol does better quality data in general, as this protocol would tend to not always produce compliance curves with such overlap. An result in compliance data in the ideal range for the test sys- effective automated procedure must, therefore, also address tem. It would also simplify the test procedure, as the response this shortcoming. of different mixtures would tend to be similar regardless of The initial algorithms used in generating master curves from the binder used, so that it will be easier for the technician per- IDT creep data were not always effective, resulting in sub- forming the test to establish appropriate stress levels. stantial errors in the shift factors, which in turn produced errors in the calculation of thermal stresses and the resulting cracking. Buttlar and Roque addressed this problem in the development Summary of Recent Changes of a computer program called MASTER, which was designed in the IDT Procedure to reliably generate master curves from IDT creep data even when substantial noise was present or when the data did not The current version of the IDT test and analysis procedure overlap. The details of the algorithms used in this program are have been substantially improved and have addressed many described in detail in a NCHRP Project 9-19 report (A8). In of the shortcomings found immediately after the conclusion summary, MASTER functions by considering a full range of of SHRP. The following changes have been incorporated into ideal and nonideal situations, evaluating an IDT data set to the most recent version of the IDT test procedure and Super- determine what potential problems are present, and then pave thermal cracking software (A8): implementing an effective algorithm for shifting the creep · Simplified formulas have been developed for making cor- rection factors for specimen bulging and non-uniform Log D(t) stress and strain distribution across the specimen; · The initial portion of data analysis, which involves devel- oping a "trimmed" mean for the response of a given set of specimens, has been enhanced to avoid problems that -log a(0 C) occurred when a transducer was not responding and also 0C -10 C to provide the user an overall indication of the quality of -log a(-10 C) the data being analyzed; -20 C · The procedure used to shift the individual compliance curves to form a master compliance curve has been sub- Log Time, s stantially improved and is more robust and produces Figure A-1. Schematic of master curve construction from reasonable and repeatable master curves even for non- IDT data. ideal data;
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A-13 · Most or all of the minor problems ("bugs") in the original than those for which the model was fitted. This problem is ana- SHRP computer program have been corrected; and lyzed in detail in Chapter 2 of this report. It is most likely to · The entire program has been recalibrated with an ex- occur for unusually stiff mixtures, and so using the adjustable panded data set, which includes the original mixtures test temperature protocol described previously would help to and pavements used during SHRP and additional ma- reduce or eliminate this problem. If necessary, the Superpave terials and pavements from the Canadian SHRP program. thermal cracking program should be modified to provide a power-law extrapolation of the compliance data to reduced Potential problems that have not been addressed include times well beyond those used to fit the master curve, to ensure potentially inadequate characterization of the coefficient of that this problem does not occur. thermal contraction and use of LVDTs during the IDT strength The use of the LVDTs to determine the precise moment of test, which often results in damage to the LVDTs, which can failure in the IDT strength test must be abandoned; it results in then result in the collection of faulty data for subsequent creep damage to the LVDTs that can then create severe problems in and strength tests. data quality in subsequent IDT creep and strength tests. Empir- ical relationships should be established between IDT strengths determined in AASHTO T322 and (a) those based upon max- DISCUSSION AND FINDINGS imum load during the IDT test and (b) those determined using The review of the original IDT strength and creep test and a direct tension test with a 100-mm diameter by 150-mm high data analysis methods and subsequent modifications and specimen, as will be used in the proposed Superpave simple related research indicate that the current procedure and analy- performance tests. This will simplify the IDT test and allow sis are much improved over the original SHRP version and engineers to use a test procedure consistent with what will should in most cases provide reliable results. A number of probably become standard test procedures and geometries in minor changes in AASHTO T322 have been suggested to the future. improve the specifications for the IDT equipment and pro- Because the barriers that existed during SHRP to developing cedure. Many of the problems pointed out in the report by procedures for uniaxial tests at low temperatures no longer Janoo and colleagues (A10) have either been effectively exist and because such uniaxial tests will become standard pro- addressed or are no longer pertinent. One issue that requires cedures in the near future, it is suggested that uniaxial creep and additional attention is the characterization of the coefficient strength become the standard test method for low-temperature of thermal contraction. Although Witczak and his associates characterization of asphalt concrete mixtures. However, the apparently believe that the equation for estimating is reason- IDT procedure as currently used should be retained for use on ably accurate (A8), research by Kwanda and Stoffels suggests field cores. Laboratory testing should be performed to evaluate otherwise (A11). A simple and reasonably accurate equation for the relationship between data produced using uniaxial and IDT estimating the coefficient of thermal contraction for asphalt procedures and to develop empirical corrections if necessary. concrete mixtures has been developed as part of Phase III of NCHRP Project 9-29 and is presented in Chapter 2 of this CONCLUSIONS AND RECOMMENDATIONS report. More reliable data and more consistent results from subse- Based upon a review of AASHTO T322, and related papers quent analysis of these data can probably be obtained by using and reports documenting changes in the IDT creep and strength an IDT testing protocol in which the test temperatures are test procedures and analysis, the following conclusions and linked to the low-temperature binder grade used in the asphalt recommendations are made: concrete. This would ensure that the compliance values for a given mixture would be either within or close to an ideal range · A number of minor changes in AASHTO T322 have for measurement and subsequent analysis. In order to simplify been suggested and should be made in the next version implementation, it is suggested that the basic test protocol of of the standard. testing at -20, -10, and 0°C be maintained for PG XX-22 and · The proposed specification for the dynamic modulus PG XX-28 binders. For PG XX-16 binders (and harder), the master curve test equipment, as developed during NCHRP test temperatures should be -10, 0, and +10°C. For PG XX-34 9-29, should be revised to include optional requirements binders (and softer), the test temperatures should be -30, -20, for equipment intended to perform not only the dynamic and -10°C. Furthermore, it is suggested that for severely aged modulus test but also uniaxial creep tests and IDT creep mixtures (either from pavement cores or from an accelerated tests at low temperature. laboratory aging procedure), the test temperatures be increased · Mixture tensile strength at low temperatures should be by 10°C. Tensile strength tests should be performed at the mid- determined using either the current IDT procedure or dle test temperature, usually -10°C. uniaxial tensile strength. Normally, these tests should For some mixtures, use of the Prony series to characterize be performed on a large, static test system separate from the creep compliance of asphalt concrete mixtures can poten- the dynamic modulus master curve/low-temperature tially cause problems in that the Prony series predicts rapidly creep system. However, all tests could be performed on increasing compliance when extended to longer reduced times a single high-performance system if desired.
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A-14 · A draft specification should be developed for uniaxial Paving Mixtures," Report SHRP-A-357, Washington D.C.: creep and strength testing at low temperatures, based upon Strategic Highway Research Program, National Research AASHTO T322 and the specifications for the dynamic Council, 1993. modulus master curve test equipment as developed as part A4. Bonaquist, R. F., D. W. Christensen, and W. Stump, "Simple Performance Tester for Superpave Mix Design: First Article of NCHRP Project 9-29. Development and Evaluation," NCHRP Report 513, Trans- · The relationship between uniaxial compliance and IDT portation Research Board, National Research Council, Wash- compliance at low temperature should be experimentally ington, D.C., 2003, 54 pp. evaluated, and empirical equations developed for estimat- A5. Anderson, D. A., D. W. Christensen, R. Dongre, M. G. Sharma, ing IDT compliance from uniaxial compliance should be J. Runt, and P. Jordhal, Asphalt Behavior at Low Service Tem- developed if needed. perature, Report FHWA-RD-88-078, Final Report to the · Empirical relationships between the SHRP "corrected" FHWA, Springfield, VA: National Technical Information Ser- IDT strength, the uncorrected IDT strength, and uniaxial vice, March 1990, 337 pp. tensile strength should be developed so that strength tests A6. Christensen, D. W., and D. A. Anderson, "Interpretation of can be performed using the IDT geometry without attach- Dynamic Mechanical Test Data for Paving Grade Asphalts," ing LVDTs or using a uniaxial test geometry. Journal of the Association of Asphalt Paving Technologists, · An improved procedure for either calculating or mea- Vol. 61, 1992, pp. 6798. A7. Buttlar, W. G., and R. Roque, "Development and Evaluation suring the coefficient of thermal contraction of asphalt of the Strategic Highway Research Program Measurement concrete mixtures has been developed and is presented and Analysis System for Indirect Tensile Testing at Low in Chapter 2 of this report. Temperature," Transportation Research Record No. 1454: · Test temperatures for low-temperature creep tests should Asphalt Concrete Mixture Design and Performance, Wash- vary according to the binder grade. PG XX-22 and PG ington, D.C.: National Academy Press, 1994, pp. 163171. XX-28 binders should be tested at -20, -10, and 0°C; A8. Witczak, M. W., R. Roque, D. R. Hiltunen, and W. G. Buttlar, PG XX-16 binders should be tested at -10, 0, and "Modification and Re-Calibration of Superpave Thermal +10°C; PG XX-34 binders should be tested at -30, -20, Cracking Model," NCHRP 9-19 Project Report, Arizona State and -10°C. Test temperatures for severely aged mixtures University Department of Civil And Environmental Engineer- should be increased 10°C above these temperatures. ing, Tempe, Arizona, December 2000. A9. Christensen, D. W., "Analysis of Creep Data for Indirect Ten- Tensile strength tests should be performed at the middle sion Test on Asphalt Concrete," Journal of the Association of creep test temperature. Asphalt Paving Technologists, Vol. 67, 1998, pp. 458492. A10. Janoo, V., T. Pellinen, D. Christensen, H. Von Quintus, "Eval- uation of the Low-Temperature Cracking Model in Super- APPENDIX A REFERENCES pave," Draft Report to the Federal Highway Administration, Contract DTFH61-95-C-00100, undated (ca. 1997). A1. Mehta, Y. A., and D. W. Christensen, "Determination of the A11. Kwanda, F. D., and S. Stoffels, "Determination of the Coeffi- Linear Viscoelastic Limits of Asphalt Concrete at Low and cient of Thermal Contraction of Asphalt Concrete Using the Intermediate Temperatures," Journal of the Association of Resistance Strain Gage Technique," Journal of the Association Asphalt Paving Technologists, Vol. 69, 2000, pp. 281-312. of Asphalt Paving Technologists, Vol. 65, 1996, pp. 7392. A2. Huang, Y. H., Pavement Analysis and Design, Englewood A12. Mehta, Y., S. Stoffels, and D. W. Christensen, "Determina- Cliffs, N.J.: Prentice-Hall, Inc., 1993, p. 366. tion of Coefficient of Thermal Contraction of Asphalt Con- A3. Lytton, R. L., J. Uzan, E. G. Fernando, R. Roque, D. Hiltunen, crete Using Indirect Tensile Test Hardware," Journal of the S. Stoffels, "Development and Validation of Performance Pre- Association of Asphalt Paving Technologists, Vol. 68, 1999, diction Models and Specifications for Asphalt Binders and pp. 349367.