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8 monitor horizontal and vertical deflections during the In summary, the current version of the IDT test and analy- strength test. The point of failure is defined as occurring sis procedure has been substantially improved to address when the difference between the vertical and horizontal many of the shortcomings found immediately after the con- deformations reaches a maximum. Unfortunately, keeping clusion of SHRP. The following changes have been incorpo- LVDTs in place during the strength test often results in dam- rated into the most recent version of the IDT test procedure age or destruction to these sensitive and expensive transduc- and Superpave thermal cracking software: ers. Engineers within the Superpave Centers agreed that, for practical reasons, the IDT strength test should be done without Simplified formulas have been developed for making LVDTs, and the strength based only upon the maximum load. correction factors for specimen bulging and non-uniform Although the SHRP procedure is more accurate, it appears that stress and strain distribution across the specimen; it is impractical, and damage to the LVDTs because of this The initial portion of data analysis, which involves devel- procedure could actually reduce the overall reliability of the oping a "trimmed" mean for the response of a given set IDT creep and strength tests. The relationship between cor- of specimens, has been enhanced to avoid problems that rected and uncorrected IDT strength were evaluated experi- occurred when a transducer was not responding and also mentally in this project, and a relatively accurate empirical to provide the user an overall indication of the quality of equation for estimating the true IDT strength from the un- the data being analyzed; corrected strength (based on maximum load) was developed. The procedure used to shift the individual compliance These results, along with other data and analyses constituting curves to form a master compliance curve has been sub- the laboratory testing portion of Phase III of NCHRP Project stantially improved and is more robust and produces 9-29, are presented later in this chapter. reasonable and repeatable master curves even for non- ideal data; Most or all of the minor problems ("bugs") in the original REFINEMENTS IN THE IDT TEST DURING NCHRP PROJECTS 1-37A AND 9-19 Superpave computer program have been corrected; and The entire program has been recalibrated with an ex- One of the early work elements in the Superpave Support panded data set, which includes the original mixtures and Performance Models Management Project (FHWA Con- and pavements used during SHRP and additional ma- tract DTFH61-95-C-00100, later NCHRP Project 9-19) was terials and pavements from the Canadian SHRP program. an evaluation of the Superpave low-temperature cracking model. A report on this work element was compiled, which Potential problems that have not been addressed include a documented numerous problems in the original SHRP ther- potentially inaccurate estimate of the coefficient of ther- mal cracking model (7 ). A large number of minor problems mal contraction and use of LVDTs during the IDT strength in the program and its interface with the main SHRP mixture test, which often damages the LVDTs and can result in the program were documented, along with a number of poten- collection of faulty data for subsequent creep and strength tests. tially more serious conceptual problems. One such issue was the use of an equation to estimate the coefficient of thermal contraction, , of asphalt concrete mixtures, rather than an PRECISION AND BIAS OF THE IDT TESTER actual measurement. Research has suggested that the recom- mended equation for estimating is not accurate (8), but One of the main objectives of this study was to make a pre- available experimental procedures for measuring have not liminary estimate of the precision of the IDT creep and strength been widely used and have not been thoroughly evaluated (9). test procedures. Although it had been planned to perform A simple, improved equation for estimating the coefficient of ruggedness testing using the IDT test systems at the Superpave thermal contraction for mixtures has been developed as part Centers, the many problems with these systems prevented the of this project and is presented later in this chapter in a sec- completion of a thorough ruggedness test program. However, tion devoted to theoretical considerations of IDT creep and ruggedness testing was performed on the IDT strength test strength testing. under Contract DTFH61-95-C-00055, as reported by Anderson Two other potentially serious problems noted by Janoo and and McGennis (3). The results of this testing are summarized his coauthors (7) were the use of a very short, 100-second below. As part of Phase III of NCHRP Project 9-29, creep data creep loading time and the characterization of mixture ten- were collected from six laboratories around the country and sile strength using a single measurement at -10C rather than summarized and analyzed statistically, as described below, in with a number of measurements over a range of tempera- order to provide estimates of the precision of this procedure. tures. However, improvements in the algorithm for generat- ing compliance master curves have made the use of short creep tests more reliable (5). Use of only one tensile strength Precision of the IDT Strength Test value in the computer program should also be acceptable, because tensile strength is simply one of several inputs used Three laboratories participated in the IDT strength rugged- to estimate fracture properties and predict thermal cracking ness study: FHWA's TFHRC, the Northeast Superpave Cen- using the calibrated Superpave thermal cracking model. ter (NESC), and the Asphalt Institute (TAI) (3). The objec-

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9 tive of ruggedness testing is to evaluate the effect of slight so detailed information concerning the various laboratories variations in important aspects of test conditions on the test cannot be provided. The nature of the six organizations is results. An estimate of the precision of the test method is summarized briefly in Table 5. also generally possible. Factors evaluated in the IDT strength Data were requested for 5 or 6 different mixtures; each ruggedness testing included air voids, preload, temperature, laboratory submitted data for 2 to 12 mixtures. No more than temperature preconditioning, temperature stabilization time, 6 mixtures were analyzed from each lab. Most of the labora- loading rate, and specimen orientation. These tests were con- tories performed tests at -20, -10, and 0C. Laboratory L3, ducted at a nominal temperature of -10C, which is the stan- however, performed tests at temperatures 10 to 20 degrees dard temperature for performing the IDT strength test (3). lower than this, perhaps because the binder grades repre- Anderson and McGennis found that none of the main fac- sented by their mixtures were softer than those normally used, tors evaluated had a statistically significant effect on the IDT although the resulting compliance data were significantly strength test (3). However, it should be kept in mind that lower than typical. Most of the laboratories performed three improvements in the precision of this procedure could result replicate tests at each temperature, except Laboratory L4, in different conclusions in the future. It was recommended which performed four replicates. Extensive information con- that current tolerances on test temperature (0.2C) and cerning the nature of the mixtures was not provided by the requirements for preconditioning time (3 1 hour) be main- laboratories, though typical data were requested and for the tained. It was also suggested that specimens be stabilized most part were submitted. Table 6 is a summary of the data for 45 minutes prior to testing, unless a given laboratory submitted by the six laboratories. The compliance data sub- can document that shorter conditioning times are effective, mitted by the various laboratories were in a similar range, though specimens should in any case be conditioned for at with the exception of Lab L3, which as mentioned previ- least 15 minutes prior to testing. Current requirements for ously, performed their tests at significantly lower tempera- loading rate and initial preload appeared to be adequate, as did tures than normal. the tolerance for air void content (3). Anderson and McGennis The replicates referred to in Table 6 were individual tests suggested that further studies be conducted to evaluate an air on the same mixture, which are normally averaged when void tolerance of 7.0 1.0 percent, in order to simplify spec- reporting the final results of the IDT creep compliance test. imen preparation (3). In other words, this data set does not include full "true" repli- The overall average value of tensile strength for the rugged- cation, in which the same mixture was tested repeatedly, in ness study was 2870 kPa (415 lb/in2). The pooled standard each case using three replicate measurements. However, the deviation was 346 kPa (50.1 lb/in2), which is for a single repli- three individual measurements comprising a normal IDT cate determination (3). Normally, three independent deter- creep test contain perfectly useful statistical information and minations are averaged in an IDT strength test, so statistics can be treated as replicates for the purposes of evaluating the should be calculated for n = 3. In this case, the standard error precision of the IDT creep test. In this study, the three (or in would be 200 kPa (29.0 lb/in2), and the coefficient of variation 7.0 percent. A common and convenient statistic for character- TABLE 5 Description of laboratories participating izing the precision of a test method is the d2s precision. The in the precision study term "d2s" stands for "difference, 2 standard deviations," Laboratory Type of Organization Type of Test System and represents the maximum expected difference between Code two independent measurements, in this case for a single oper- L1 University Servo-hydraulic ator within one laboratory. The d2s precision is calculated as L2 Commercial Servo-hydraulic Engineering, Research, 2 2 SE, where SE is the standard error based upon an aver- and Testing age of several measurements--in this case three measure- L3 Commercial Servo-hydraulic ments. The d2s can be expressed in absolute terms--in units Engineering, Research, and Testing of kPa in this case--or as a percentage of the mean response. L4 Material Supplier Servo-hydraulic For the strength data reported by Anderson and McGennis, L5 Material Supplier Servo-hydraulic the d2s precision is 19.7 percent, expressed as a percentage L6 Superpave Center Electro-mechanical of the mean (3). Considering the generally high variability observed in strength test data, this level of precision is prob- TABLE 6 Summary of data submitted ably acceptable. for compliance precision study Total Lab. No. of No. of No. of No. of Minimum Maximum Precision Evaluation of the IDT Creep Test Code Mixes Temp. Reps. Tests Compliance Compliance (1/GPa) (1/GPa) L1 4 3 3 33 0.031 0.583 As part of Phase III of NCHRP Project 9-29, numerous L2 4 3 3 36 0.030 0.511 laboratories that have IDT creep and strength test systems L3 6 3 3 54 0.027 0.188 L4 6 3 4 96 0.032 0.543 were contacted and asked to provide data for the purposes of L5 2 3 3 18 0.053 0.875 evaluating the precision of IDT creep data. These laborato- L6 4 3 3 36 0.046 0.737 ries were told the results of the study would be anonymous, All labs 26 3 3-4 273 0.027 0.875

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10 one case four) individual measurements on each mix were 0.10 analyzed separately to provide independent replicate deter- Low Temp. minations. These were then used to estimate an average 0.08 Mid. Temp. d2s Precision for m(t) value and a standard deviation for each mixture for a given High Temp. laboratory. These values were then averaged over all mix- 0.06 tures for a given laboratory. For standard deviation, the 0.04 average was calculated as the square root of the average variance, which is the correct way of calculating an average or 0.02 pooled standard deviation. Statistics were calculated for compliance, m-value (log-log slope of creep compliance), 0.00 and Poisson's ratio (). L1 L2 L3 L4 L5 L6 Avg. Because a normal IDT creep test consists of an average of Laboratory three measurements, the standard deviation and coefficient of variation calculated as described above (for a single mea- Figure 2. D2S precision for m(t) for six laboratories. surement) overestimate the variability in the standard pro- cedure. In order to estimate the standard deviation for the electro-mechanical test system used there, which, as dis- complete procedure (including full replication), the standard cussed previously, was one of the prototypes procured for the deviation for an average containing three replicates must be Superpave Centers that exhibited many software and hard- calculated--this is simply the standard deviation divided by ware problems. The average d2s precision for all laboratories the square root of 3. This is referred to in this report as the was 22, 28, and 32 percent at the lowest temperature, the standard error (SE). Finally, the d2s precision was calcu- middle temperature, and the highest temperature, respec- lated for each laboratory and temperature. tively. Excluding data from Lab L6 would probably reduce Figures 1 through 4 graphically represent d2s precision esti- these values to about 20 to 30 percent, which is somewhat mates for compliance, m-value, Poisson's ratio, and critical high for single-operator precision values. If it is assumed that temperature (respectively) for the six laboratories involved in this study. For compliance (Figure 1) d2s precision is given as a percentage, while for m-value (Figure 2), Poisson's ratio 0.25 (Figure 3), and critical temperature (Figure 4), it is in absolute Low Temp. 0.20 Mid. Temp. terms. Critical temperature is the temperature at which the cal- d2s Precision for mu High Temp. culated thermal stress equals the tensile strength; it represents 0.15 the expected cracking temperature during a single extreme low-temperature event (10). This value was estimated using 0.10 typical values for tensile strength (3.0 MPa) and coefficient of thermal expansion (1.1 10-5 m/m/ C), so that the variability 0.05 was from compliance measurements only. The variability in compliance, in general, appears to in- 0.00 crease with temperature and is generally in the range of about L1 L2 L3 L4 L5 L6 Avg. 10 to 30 percent, though it is somewhat higher for Lab L5 and Laboratory Lab L6. Because data for only two mixtures were submitted for Lab L5, the variability estimates are not completely reli- Figure 3. D2S precision for Poisson's ratio for six able. The higher variability for Lab L6 is probably due to the laboratories. 12 d2s Precision for Tc, C 70 10 d2s Precision for D(t), % 60 Low Temp. 8 Mid. Temp. 6 50 High Temp. 4 40 2 30 0 20 L1 L2 L3 L4 L5 L6 Avg. Avg. w/o Labs 3 10 and 6a Laboratory 0 a The reasons for excluding the results from Labs 3 and 6 from the average are L1 L2 L3 L4 L5 L6 Avg. given in the text. Laboratory Figure 4. D2S precision for critical temperature for six Figure 1. D2S precision for compliance for six laboratories. laboratories.