Lu, Guo, Zhang, and Yu 0 0 0 0 0 COMPREHENSIVE STUDY OF PENDULUM IMPACT TEST FOR RAISED PAVEMENT MARKERS Qing Lu, Ph.D. Assistant Professor Department of Civil and Environmental Engineering University of South Florida 0 E. Fowler Avenue Tampa, FL 0-0 Phone: -- Fax: -- Email: qlu@usf.edu Lukai Guo Research Assistant Department of Civil and Environmental Engineering University of South Florida 0 E. Fowler Avenue Tampa, FL 0-0 Phone: -0-0 Fax: -- Email: lukai@mail.usf.edu Yunlong Zhang, Ph.D. Associate Professor Zachry Department of Civil Engineering Texas A&M University TAMU College Station, TX Phone: --0 Fax: -- Email: yzhang@civil.tamu.edu Bin Yu Research Assistant Department of Civil and Environmental Engineering University of South Florida 0 E. Fowler Avenue Tampa, FL 0-0 Phone: -- Fax: -- Email: yubin@mail.usf.edu Total words = (abstract) + 0 (text) + 0* ( Figures + Tables) =
Lu, Guo, Zhang, and Yu 0 ABSTRACT Field observation of retro-reflective raised pavement marker (RRPM) performance on Florida roadways has revealed service lives generally shorter than those expected based on current laboratory test specifications. There is a need to improve current test procedures or develop new test methods to evaluate and predict RRPM performance in the laboratory. Based on the idea of a pendulum impact test proposed in a prior study, this paper focuses on the development of an improved pendulum impact test that can apply repeated loading to the specimen. Finite element models of the pendulum impact test were built to explore the effects of test parameters, including impact location, impact load, and impact speed, on test results and to help determine the appropriate test procedure. Six types of RRPMs with known field performance rank were then tested in the laboratory to verify the validity of the improved pendulum impact test. It was found that the laboratory test results are consistent with the FEM analysis results and the observed field performance rank.
Lu, Guo, Zhang, and Yu 0 0 0 0 INTRODUCTION Current specifications of laboratory tests for retro-reflective raised pavement markers (RRPMs) mainly include three categories: American Society of Testing and Materials (ASTM) standards D 0 (), National Transportation Product Evaluation Program (NTPEP) procedures (), and State DOT specifications. Among them, ASTM D 0 is most widely used. For evaluating the structural integrity of an RRPM, ASTM D 0 specifies a compressive test and a flexural test, both of which apply a static load to a marker specimen at a constant loading rate. Recently, from field observations of marker performance on Florida highways, it was discovered that the service lives of RRPMs that had met the Florida Department of Transportation (FDOT) laboratory test specifications, which is based upon ASTM D 0, were often shorter than expected. This indicates that the current ASTM D 0 mechanical tests may not adequately evaluate the structural capacity of the RRPMs. From field observation, the failures of RRPMs are frequently found to initiate with the facture of the outer shell, which may be caused by the impact of a hard small object on the surface of the RRPM, such as a stone wedged in the tread of a tire. The small-area hard to hard interacting cannot be represented by the currently available test procedures. Thus, for emulating dynamic impact from small objects on the RRPM surface in real condition, one pendulum impact test (PIT) was originally designed by Texas Transportation Institute (TTI), which was inspired and modified from the British pendulum friction test (). The principle of PIT is similar to that of the lens impact test in ASTM D 0 (). Compared to the lens impact test in ASTM D 0, there are two main advantages of PIT: ) impact location on markers can be adjusted through one adjustable marker support; ) magnitude of impact on markers can be controlled through adding different weights on the steel rods, which are installed at the end of the pendulum arm. The original PIT showed a potential of successfully evaluating the performance of RRPMs and was recommended for the purpose of marker qualification and quality control (). The test parameters (such as impact location, impact velocity, and load level) and procedure, however, have not been well studied and established. Moreover, the test has one major limitation in that only one impact load is applied, which is different from the repeated loading scenario in the field. It is expected that a marker rarely cracks/breaks under one load. Instead, cracking or breakage of a marker often occurs after many repetitions of tire loads, which is a sign of fatigue damage. Therefore, there is a need to improve the original design of the pendulum test to include the fatigue nature of marker failure. This study first developed an improved version of the pendulum test to allow for repeated loading. The effects of key test parameters on test results, including critical impact locations, impact weight, and impact speed, were then analyzed and determined through finite element modeling. Finally the improved test was verified in the laboratory with six types of RRPMs with known field performance rank. Original Pendulum Impact Test The original RRPM pendulum impact test device, as shown in Figure, was developed by TTI as part of a study of RRPM for the Texas Department of Transportation (TxDOT) (). In the test, the hitting force is delivered to the RRPM by a -inch rounded steel rod fixed at the end of a swinging arm. The marker is adjusted to the desired position by a small metal sleeve and a simple metal clip that holds it against an elastomeric pad (0.-inch, 0 Shore A). The developed device allowed both horizontal and vertical positioning of the marker relative to the steel rod and its impact point through the adjustable support, as shown in the right picture of
Lu, Guo, Zhang, and Yu Figure (). The hitting force can be adjusted by both the weight of the steel ring that is attached to the rod or the speed of the rod hitting the marker which is determined by the height where the rod is released. The swing arm itself weights. lb. 0 Figure Overall view of the original pendulum impact device (left) and the marker adjustable support (right) () Revised Pendulum Impact Test For the original PIT, its device could only apply one single impact load to RRPM specimens. However, it is rare that RRPMs installed in the field would fail under just one single load impact. Generally RRPM damages develop from micro cracks or flaws to stages manifested as observable failures, under repeated tire impact loading. Therefore, a modification was made to the original pendulum test device to enable it to apply load impacts repeatedly, using a power motor that lifts the impact arm in cycles, as shown in Figure. An RRPM can be tested for a given number of impacts, or be tested until it fails with the number of impacts recorded. 0 Figure Revised pendulum impact device Similar to the original PIT, the use of the adjustable marker mount under revised PIT allows for a variation of impact locations on the marker. With this setup, impact tests can be done to any impact point on the surface of the marker. In addition, different weights can be
Lu, Guo, Zhang, and Yu 0 0 added to the end of the pendulum arm to increase the force exerted on the marker at impact. The device also has the option of selecting different loading frequencies and it has a counter to record the number of impacts. METHODOLOGY Finite Element Model (FEM) of Pendulum Impact Test In a related study, the critical stress/strain distribution under tire loading in an RRPM installed on a pavement was examined through finite element model (FEM) analysis of the tire/rrpm/pavement system and through field measurement using strain gauges installed on RRPMs in the field (). To investigate the stress distribution and magnitude produced in the PIT, the PIT with its devices was modeled by finite element, using the commercial FEM software ANSYS. To simplify the test in ANSYS, the height of the steel rod is set very close to RRPM, with an initial velocity of around m/s, which is equivalent to that from free falling from one fixed height. The point mass function in ANSYS is used to get variation of weights at the end of the steel rod, instead of real weight component (). Taking the impact location at the RRPM top corner for example, the FEM of the pendulum test is illustrated in Figure. More details of the modeling process can be found in the reference (). Verification of Pendulum Impact Test To verify the findings from the FEM analysis of the PIT, six RRPM models (M 0, M 0 PSA, Ennis 0, Ennis C0, Rayolite RS, and Apex AR) that are commonly used on Florida roadways () were tested by the revised pendulum impact device. The effects of testing locations, the selected speeds, and the loads from the end of steel rod were all examined in the laboratory. Totally, there are ***= tests for each marker type, and for all six markers. The details of this verification are described in the analysis result section. Figure Pendulum impact test model in ANSYS
Lu, Guo, Zhang, and Yu 0 ANALYSIS RESULTS PIT Simulation Results in Comparison with FEM Results for Field Conditions FEM analysis of the tire/rrpm/pavement system has revealed that the critical stress frequently occurs at the RRPM top corner under field conditions (). For this reason, the impact location at the RRPM top corner is selected to compare the stress distributions/magnitudes in the RRPM under the PIT and those under field conditions. The FEM deliberately locates the steel rod around the corner of the marker, which produced similar stress concentration area as the tire/marker impact in the field. The stress distributions are shown in Figure and the critical values are given in Table. 0 Figure Stress distributions in a marker in the pendulum impact test As results, the differences between the stress magnitudes from the pendulum impact test setup and from the field condition setup are -%, %, %, and -0% for the four stress indices (i.e., von Mises, maximum principal, minimum principal, and maximum shear stresses) in sequence. Both the difference ranges and the critical stress impact locations of the PIT resemble the field conditions, and therefore the PIT is deemed as a good candidate test for evaluating RRPM performance in the laboratory.
Lu, Guo, Zhang, and Yu 0 0 Index Compressive Test Tire/Marker Impact Table Result of the Pendulum Impact Test Maximum Maximum Minimum Principal Maximum Principal Stress Principal Stress Stress Principal Stress (MPa) (MPa) (MPa) (MPa) Critical Critical Critical Critical Location Location Location Value Value Value Value Location Contact boundary Two tips and one rim Contact boundary Two tips and one rim - - Contact boundary Surrounding two tips Contact boundary Two tips Differences -.%.0%.% -0.% Determination of Critical Impact Locations on RRPMs To identify the critical impact locations on RRPMs under the PIT, the same five impact levels, which were initially selected by TTI, with six impact locations, were chosen to search for the most potential fragile one (). The impact levels are shown in Table and the impact locations are shown in Figure. Thus, there are a total of 0 tests. These test IDs are listed as ij, where i represents location ID and j represents weight ID. Specifically, in this study, the potential fragile part on the RRPM can be initially defined as follows: under the same impact condition, different impact locations contribute to various magnitudes of stresses. Higher stress magnitude indicates more damage risk, which seems more fragile. The stresses, produced by impacts at different RRPM locations with different magnitudes, were obtained with ANSYS and listed in Table. For clearly observing different stresses magnitudes in different scenarios, one line is plotted for each impact location, as shown in Figure. Based on Figure, the following findings are obtained: The order of location ID s by von Mises stress magnitudes is:. It means, under the same impact condition, impact on the corner of top shell (location ) will produce the most critical von Mises stresses and the highest damage risk. The impact location also is the critical stress location under the field loading condition. Table Pendulum Impact Device Weight Weight ID Weight (kg) 0. 0. 0. 0. 0. Moreover, compared to other points, except corner or side, the center point has relatively higher risk for failure. This result can explain the observation from field survey: most cracks started from RRPM corners and/or middle of edges.
Lu, Guo, Zhang, and Yu Figure RRPM impact location ID s for the pendulum impact test Table Results from FEMs of Pendulum Impact Tests Test ID Von Mises Stress (MPa) Maximum Principal Stress (MPa) Minimum Principal Stress (MPa) Test ID Von Mises Stress (MPa) Maximum Principal Stress (MPa) Minimum Principal Stress (MPa) 0 0.. -. 0.. -... -...0 -.0.00. -.0.. -... -0... -...0 -... -0... -... -. 0.. -0..0. -.0..0 -... -0...0 -.0.. -... -00.0.. -0.0.. -...0 -. 0.0. -.0.. -... -... -... -... -... -... -.0 Generally, the impact on the lens edge can generate higher von Mises stresses. Furthermore, when the weight is very low (0. kg), the von Mises stresses at locations on the same longitude are very close, such as locations,, and. However, when the weight increases, the von Mises stresses are proportionally increased if the locations are on the same latitude, such as locations and, locations and, and locations and. In other words, statistically, the impact location and the weight have no interaction on stress magnitude.
Lu, Guo, Zhang, and Yu Figure Critical stresses vs. weights and locations
Lu, Guo, Zhang, and Yu 0 0 0 0 0 In this case, Figure shows that the von Mises stress from impact location increases to be very close to that at location. Figure also shows that, based on the trends, impact on location also keep producing the highest von Mises stresses, no matter how much the weight increases. Based on the maximum principle stress distribution, the critical stress occurs on the edge of contact area between steel rod and markers, which is very close to impact location. When the weight is 0. kg, the order of location ID by maximum principle stress magnitudes is:. However, when the weight increases to 0. kg, this order is altered to be:. Based on this variation, location reveals its lowest sensitivity on weight impact. Moreover, these orders show that impact on the middle of non-lens edge, where the finger grips exist, always can generate much higher tensile stresses than that on all other locations. In other words, this result indicates that the cracks from finger grips are more probably caused by tensile stress. Because the tensile stress generated by impact on point is also high, the crack will have high risk to extend from point to point. Moreover, based on the stress magnitudes generated by impact on point and point, it is safe to say that the edge and corner are not key factors to produce high tensile stresses, although their von Mises stresses are relatively high. The order of location IDs by minimum principle stress magnitudes is:, which have similar order with von Mises stress. This order illustrates that, without considering the disturbance from the tensile stresses, the compressive stresses generated by impact on locations and are very close. In the field loading condition, because of the tire deformation and its contact points on RRPM, there is almost no chance that locations and have the same impact situation, unless some stones wedged in the middle of vehicle s tire tread. Thus, location still has the highest risk of compressive failure compared to other locations. Critical Impact Location Analysis Based on the above findings, location is the critical impact location for compressive failure, and location is the critical one for tensile failure. Both locations are in accordance with the field loading condition (; ). For setting the proper weight and initial velocity of the steel rod on these locations, through comparing with the field conditions especially under heavy truck scenarios, this section aims to obtain the relationship between critical stress magnitudes with steel rod s weight and initial velocity at these two critical impact locations. In this section, the critical stresses produced under field condition refer to the results from one relevant final report (). Weight Effect on Pendulum Test In this section, the impact weights were increased to observe the trends of stress increase, and it was found that the critical stresses generated by impact on location and location increase proportionally, as shown in Figure. Based on Table, at locations and, the slopes of stress between all weight intervals and their variances can be calculated and shown in Table. Thus, based on Figure, if the velocity of steel rod is fixed as m/s, the minimum principal stress and maximum principal stress generated by impact on locations and can be roughly estimated by the following two equations: Minimum principal stress at location and : ( m m) (.) Maximum principal stress at location and : ( m m).
Lu, Guo, Zhang, and Yu Where: m is the mass of designed weight; m is the known mass of weight; is the stress generated by the weight with known weight. Figure Maximum principal stress distributions at Location (a) and minimum principal stress distributions at Location (b) Table Slopes of Stress Increase with Weight Interval Location ID Interval ID Slope (MPa/kg) Maximum Principal Minimum Principal (Weight ID to Weight ID) Stress Stress to. -. to. -0. to 0.0 -.0 to. -. to. -. to. -0.00 to. -.0 to. -. Standard Error.00. Average Slope. -.0
Lu, Guo, Zhang, and Yu 0 0 Figure Critical stresses vs. weights at locations and Based on these two equations, if impact is applied on location, with a m/s initial velocity of steel rod, the weight should be. kg to reach its critical compressive stress (-. MPa). For getting critical tensile stress (. MPa) at location,. kg weight with m/s initial velocity of steel rod should be used to impact location. It is worth pointing out that, although the critical stresses calculated by FEM on field condition are varied by specific traffic condition, the above equations are still available for estimating the proper weight to reach other critical stress magnitude. Steel Rod Velocity Effect on Pendulum Test In this section, the impact weight is fixed at 0. kg, and the initial speed of steel rod is changed to test the stress variation. The results are shown in Figure. Figure verifies these two critical stress locations: compressive stresses generated by impact on location remain higher than those at location, and on the contrary, tensile stresses generated by impact on location keep higher than those at location. However, differing from the influence of weight variation, the difference between the compressive stresses generated by impact on these two locations is enlarged with increased initial velocity of steel rod. For the
Lu, Guo, Zhang, and Yu difference of tensile stresses generated by impact on these two locations, the velocity of steel rod has relatively slight influence. Thus, compared to setting proper weight of the steel rod, the proper velocity of the steel rod can be more difficultly set to approximate to real condition,. 0 0 Figure Initial velocity of steel rod vs. critical stresses at locations and Figure also shows that, compared to field loading condition, the critical tensile stress (. MPa) at location can be reached through pendulum test on location with. m/s initial velocity of steel rod and 0. kg weight. Similarly, Figure illustrates that the critical compressive stress (-. MPa) at location under field loading condition are equivalent to that from pendulum test with. m/s initial velocity of steel rod and 0. kg weight on location. Revised Pendulum Impact Test Verification In this study, six RRPM models (M 0, M 0 PSA, Ennis 0, Ennis C0, Rayolite RS, and Apex AR) that are commonly used on Florida roadways were tested by the revised pendulum impact device. Six locations, as shown in Figure, were tested for each type of marker. The effects of speed and load were examined in the tests. The speed at which the hammer hit the marker was adjusted with a knob on the machine itself, and the load was adjusted by adding a. lb weight on the hammer. In the tests conducted the load has two levels: low (L) and high (H). The speed also has two levels: slow (S) and fast (F). Each combination of location, load, and
Lu, Guo, Zhang, and Yu 0 speed was repeated with markers. Hence there are ***= tests for each marker type, and for all six markers. As above mentioned, the marker was placed vertically, with its top facing the hammer. It can be adjusted horizontally or vertically, to meet the requirement on the hitting location. A clipper was used to prevent the marker from moving. The machine also has a counter to record the number of hits on the marker. After starting the machine, the hammer hit the marker at the desired location at a desired speed level, and the machine stopped after breakage occurred. The number on the counter was then recorded. To save time, the tests were conducted from the heaviest and fastest combination. If no breakage occurs at this combination, it is reasonable to believe that the markers tend not to break at lighter weights or lower speeds. For each marker, the test was stopped after 0 hits if no breakage occurred. If breakage occurred in 0 hits, the lighter weight and slower speed combination was tested. Three tests were conducted for each combination, and their mean and standard deviation are listed in Table. Table Results of Revised Pendulum Impact Test (Number of Hits) Marker Type Load Location /Speed M 0 LS 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) LF (.) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) HS. (.) 0 (0) 0 (0) 0 (0). (.) 0. (.) HF. (.) 0 (0) 0 (0) 0 (0) () M 0 PSA LS 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) LF 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) HS (.) 0 (0) 0 (0) 0 (0). (.) 0 (0) HF. (.) 0 (0) 0 (0) 0 (0) 0 (.). (.) Ennis 0 LS 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) LF 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) HS 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) HF 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) Ennis C0 LS 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) LF 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) HS 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) HF 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) Rayolite RS LS (.) (.). (.) 0 (0) 0 (0) 0 (0) LF (). (.) (.). (.) 0 (0). (.) HS (). (0.) (). (0.). (.). (0.) HF. (0.) () (0). (0.). (0.) (0) Apex AR LS. (0.) (.). (.). (0.) 0 (0) 0 (0) LF. (0.) 0 (0). (.) 0 (0). (.) 0 (0) HS. (0.). (0.) (0). (0.) (.) (0) HF (0). (.) (.) (0) (0). (0.) Note: the numbers in each cell represent mean (standard deviation).
Lu, Guo, Zhang, and Yu 0 0 The numbers in Table are the number of hits before the marker breaks. A number 0 in the table indicates that the marker still did not break after 0 hits. In the Load/Speed column, an L represents a lower load and an H represents a higher load. An S represents a slower hitting speed and an F represents a faster hitting speed. The cells in yellow are the combinations where the tests were not actually done, since the markers did not break at the fastest and heaviest combination at this location. Based on the results of pendulum impact tests, the performance of the six RPM models are ranked as Ennis series > M 0 series > Rayolite RS > Apex AR. This rank is similar to that from the ASTM standard flexural test (), except that Rayolite RS performed not extremely better than Apex AR in the pendulum impact test. The average results for M series (M 0 and M 0 PSA), Rayolite RS, and Apex AR are also plotted in Figure 0, from which it can be seen that generally higher impact load or speed leads to earlier failure of RRPM in the pendulum impact test. This is consistent with the FEM results presented in the previous section. Figure plots the results averaged for impact location. It can be seen that for M series, Location is the weakest spot among the six impact locations, and for Rayolite RS and Apex AR, Locations,, are weaker than Locations,,. These are generally consistent with the rank based on the maximum principle stress distribution calculated from FEM. This result also indirectly illustrates that the tension causes RRPM failure more than compression. Figure 0 Average results of the revised pendulum impact test
Lu, Guo, Zhang, and Yu 0 Number of Hits 0 0 0 0 M Rayolite RS Apex AR 0 0 0 0 Test Location Figure Revised pendulum impact test results averaged by location CONCLUSIONS Generally, as results of impacting the RRPM at its top corner under PIT, the critical stress location with its magnitude resembles the field conditions. Therefore, PIT is recommended as a candidate laboratory test. After hitting different locations on RRPMs, it can be found that, under the same impact condition, no matter how much the weight increases, impact on the corner of top shell will produce the most critical von Mises stresses with the highest damage risk, which is in accordance with the field loading condition. Moreover, the center point also has high risk for failure, which can explain the observation from field survey: most cracks started from RRPM corners and/or middle of edges. Through detecting the critical impact locations on RRPMs, this study also finds that the cracks from finger grips are more probably caused by tensile stress, and these cracks have high risk to extend to the center of top shell. Moreover, it is safe to say that the edge and corner are not key factors to produce high tensile stresses. The trends of stress increase are proportional to adding the weight of steel rod. Based on their proportional relationship, the proper weight of the steel rod under PIT can be more easily set to approximate to real condition, compared to setting proper velocity. For the verification of revised pendulum impact test, the result of RRPM performance rank from pendulum impact tests is similar to that from the field performance rank. Moreover, for tensile stress, the weak spots on markers observed from laboratory test are generally consistent with the FEM results. ACKNOWLEDGMENT The authors acknowledge the Florida Department of Transportation (FDOT) for providing funding this research during two years from 0. The authors are especially indebted to Mr. Paul Vinik, State Structural Material Systems Engineer, FDOT, Gainesville, Florida, for his project management efforts and patience. REFERENCES. Standard Specification for Extended Life Type, Nonplowable, Raised Retroreflective
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