Center for Transportation Infrastructure Systems - ctis.utep.edu Understanding Sources of Variability of Overlay Test Procedure SPTC Student Paper and Poster Award Ceremony By Victor Garcia and Alejandro Miramontes April 29, 2016

Center for Transportation Infrastructure Systems - ctis.utep.edu 2 Introduction The Overlay Tester (OT) currently measures the cracking resistance of asphalt specimens as a function of the number of cycles to failure

Center for Transportation Infrastructure Systems - ctis.utep.edu 3 Specimen Preparation Critical aspects of the specimens preparation that may contribute to the high variability Quantity and proper mixing of the glue - Spreading of the glue (area covered) - Type of the glue in term of setting/curing time - Gap or spacing of the plates - Placement and alignment of the specimen. - Proper weight for specimen adhesion to the plate - Adequate amount of torque on the plates

Center for Transportation Infrastructure Systems - ctis.utep.edu Preliminary Testing Results Multi-laboratory Study Reproducibility: The average number of cycles were similar for CST and UTEP Repeatability: The COV values for the number of cycles to failure from both institutions were high Gap or Spacing of the plates 2mm gap shows a higher maximum load Less number of cycles to failure 4 GapParameters Cycle to Failure Max Load (lbs) Last Load (lbs) UTEP 4 mm gap Average COV, %8378 UTEP 2 mm gap Average COV, % 7810 CST 4 mm gap Average COV, % 7378

Center for Transportation Infrastructure Systems - ctis.utep.edu Elapsed Time Effect on Number of Cycles 5 The number of cycles seems to be independent of the elapsed time after molding and the density of specimens. Preliminary Testing Results cont. Density of Specimen Effect on Number of Cycles

Center for Transportation Infrastructure Systems - ctis.utep.edu 6 MaterialPolyurethane NameVery SoftSoftMediumHardVery Hard ColorBlackRedNaturalOrangeRed Durometer90A95A55D65D75D Ultimate Tensile Strength, psi Modulus of Elasticity, psi Elongation, % Summary of Synthetic Specimens Properties and Illustration Evaluation of OT Test Procedure

Center for Transportation Infrastructure Systems - ctis.utep.edu 7 12 grams: Insufficient to cover critical areas uniformly 14 grams: Maximum load was reached 16 grams: Low variability but excessive spillage of glue Effects of Glue Quantity on OT Max Load Amount of Glue: Specified 14 grams of glue is adequate

Center for Transportation Infrastructure Systems - ctis.utep.edu 8 (Effect of Glue Curing Time on OT Max Load) Four synthetic specimens vs three different curing times 10-hrs cure time: Lower Maximum Load 12-hrs and 24-hrs: Maximum load was reached Glue Curing Time: Specified 12hrs minimum cure time seems adequate

Center for Transportation Infrastructure Systems - ctis.utep.edu 9 Four different amount of torque (5, 10, 15, and 20 lb-in.) Torque Required to Attach OT Specimen Specified 15lbs of torque is adequate

Center for Transportation Infrastructure Systems - ctis.utep.edu 10 An LVDT was placed on top of the specimens to investigate the behavior (especially under first cycle - crack initiation). Evaluation of OT Testing Mechanism

Center for Transportation Infrastructure Systems - ctis.utep.edu 11 Compression Tension Effect of Specimen Stiffness

Center for Transportation Infrastructure Systems - ctis.utep.edu 12 A dense-graded mix was analyzed under the OT test with the LVDT installed on top of the specimens Specimen 1 was found on compression and lasted 45 cycles Specimen 2 was in pure tension and lasted 25 cycles Effect of Specimen Stiffness cont. Compression Tension

Center for Transportation Infrastructure Systems - ctis.utep.edu Key Findings and Recommendations 13 Operational Parameter Variables Investigated Key Findings/Conclusions Gap or Plate Spacing  2 mm  4 mm The load obtained from the specimens with 2-mm gap were consistently higher than the load from specimens with 4-mm gap, while the number of cycles were less for specimens with 2-mm gap than 4-mm gap. Specimen Sitting Time and Specimen Density  2, 3, 4, 5 and 6 days  92% to 94% density The OT specification provides for the sample to be tested within 5 days from preparation. The OT specification recommends to prepare the OT specimens at a target density of 93% ±1% Mechanism of OT Equipment Extra LVDT on Top of Specimen Our preliminary findings suggest that placing an LVDT on the top of the specimen seem prudent. The fact that a bending moment is introduced during testing of certain HMA mixes can be a major deficiency in the protocol to handle all mix designs. Amount of Glue  12 grams  14 grams  16 grams An optimum amount of glue is required to obtain the OT maximum performance. It is critical to apply a proper amount of glue to reliably cover the contact area to attach specimens to the OT plates. As a result of this study, the use of 14±1 g of Devcon 2 ton epoxy for gluing is recommended to obtain the most repeatable initial load. Glue Curing Time  10 hrs  12 hrs  24 hrs For practical workable hours the curing time of glue can range between 12 hrs to 24 hrs without influencing the results. Amount of Torque  5 lbs torque  10 lbs torque  15 lbs torque  20 lbs torque As recommended in the current OT specifications, 15 lbs.-in. of torque showed to be an acceptable amount of torque applied to each bolt for fastening the base plates to the machine.

Center for Transportation Infrastructure Systems - ctis.utep.edu Thank you! 14 Acknowledgements Sonya Brindle SPTC Program Coordinator Advisers: Dr. Soheil Nazarian and Dr. Imad Abdallah