Hamburg Wheel-Track Test Equipment Requirements and Improvements to AASHTO T 324 (2016)

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5 2. INTRODUCTION AND RESEARCH APPROACH This report presents the results of NCHRP Project 20-07/Task 361, Hamburg Wheel- Track Test Equipment Requirements and Improvements to AASHTO T324. This chapter describes the problem statement, objective, and research approach. 2.1. Problem Statement The Loaded Wheel Test (LWT) is a laboratory-controlled rut depth test that uses loaded wheel(s) to apply a moving load on hot-mix and warm-mix asphalt (HMA, WMA) specimens to simulate traffic load applied on asphalt pavements. In the 1970s, Helmut-Wind Incorporated of Hamburg proposed a test method and developed specification requirements to measure the combined effects of rutting and stripping susceptibility. The equipment developed was named the Hamburg Wheel Tracking Device (HWTD) and has been used for over four decades worldwide. The HWTD measures the combined effects of rutting and moisture damage (stripping) by rolling a steel wheel across the surface of an asphalt concrete slab that is immersed in a temperature-controlled water bath. The interest and use of LWT in performance specifications, alternatively referred to as rut testers or torture testers, has seen an increase in recent years. This interest can be attributed to several factors, including the use of such devices by FHWA and many state Departments of Transportation (DOTs). Other important factors in this increased popularity are the ease of use and good correlation to field performance, which led many DOTs to incorporate LWT tests in their specifications as a pass or fail acceptance criteria. As the popularity of this test equipment increased, several manufacturers started producing their own variation of the LWT, while others adapted their existing designs from a load over a rubber hose to deadweight loading from a steel wheel. Those machines were built using various solutions for controlling the wheel speed, measuring the rut depth, water bath temperature control, and reciprocating mechanisms, to name a few. These different machines are all currently being used by highway agencies and research centers. Despite the aforementioned discrepancies among the different LWT machines, no comprehensive study has been conducted to compare the results from different manufacturers. In 2010, Shiwakoti et al. carried out a research study focused on wheel tracking devices to develop a rapid test method to evaluate moisture sensitivity (1). The Asphalt Pavement Analyzer (APA) and the Hamburg Wheel Tracking Device (HWTD) were used for this research. Compacted cylindrical samples were fabricated using the Superpave Gyratory Compactor. However, the APA tests were carried out using the rubber hose instead of the metal wheel. Results showed major differences on the stripping behavior. APA results did not indicate any stripping inflection points, contrary to the HWTD results that showed significant stripping susceptibility. A recent study carried out by the Iowa DOT (2) statistically evaluated the results from 150 test runs on gyratory specimens using a two‐wheel HWTD manufactured by Precision Machine and Welding (PMW). Linear variable differential transducers (LVDTs) were used to measure rut depths at eleven locations across the wheel track per pass. Measurements were recorded to the nearest 0.01 mm every 20th pass for the first 1,000 passes. The frequency was reduced to every 50th pass thereafter. Results indicated that the impression measurement location was found to be a source of significant variation in the HWTD. The study suggests that the differences are likely due to

6 the non-uniform wheel speed across the specimen, geometry of the specimen, and air void profile. 2.2. Research Objective The objectives of this research as stated in the project description are to [1] document the capabilities of available commercial Hamburg test equipment, [2] determine Hamburg test equipment capabilities, components, or design features that ensure proper testing and accurate, reproducible results, and [3] provide proposed revisions with commentary to AASHTO T 324 to enable the use of a performance type specification for Hamburg test equipment. In this study, reference to AASHTO T 324 implies reference to the latest standard published in 2014, AASHTO T 324-14. 2.3. Research Approach The approach to be followed in this research project was consistent with the guidelines outlined in the project description. The proposed research activities were divided into five tasks. Task 1 consisted of collecting and critically reviewing all available Hamburg test equipment capabilities, and specifications. In Task 2, laboratory experiments were conducted to determine the capabilities of available Hamburg equipment and the adequacy of AASHTO T 324. In Task 3, and based on the results of Task 2, revisions were proposed to AASHTO T 324 to ensure repeatability and accuracy of measurements. In Task 4, a statistically based experimental plan was developed to validate proposed requirements for Hamburg equipment and for specimen preparations and their impacts on test results and acceptance test criteria. Finally, Task 5 consisted of preparing a final report that summarizes the project findings and conclusions, document the study results, and presents recommended revisions to AASHTO T 324. Detailed descriptions of the proposed research effort are presented in the following sections. 2.3.1 Task 1 – Available Hamburg Test Equipment Specifications The objective of this task is to conduct a critical review of the test capabilities, specifications, and similarities and differences of available Hamburg test equipment in the US. AASHTO T 324 “Hamburg Wheel-Track Testing of Compacted Hot Mix Asphalt (HMA)” establishes the testing protocol as well as the data-reporting format. However, the standard in its most current version vaguely describes some critical aspects of the testing procedure and data analysis, leaving room for ambiguous interpretation. Further, some commercially available machines do not fully comply with its equipment requirements resulting in discrepancies in the reported results. A comparative review of some of the critical technical aspects of the representative equipment in the US was carried out and presented to what is required by AASTHO T 324. It is worth noting that results of this task, which are presented subsequently in this report, identified four major manufacturers of HWT in the US. These vendors are referred to as vendors A, B, C, and D to protect the anonymity of the vendors. 2.3.2 Task 2 – Engineering Desk Analysis of Existing Hamburg Test Systems In this task, engineering desk analysis was conducted to identify potential issues on different aspects of the AASHTO T 324 procedure, mainly on its specifications of what

7 needs to be measured, and the needed accuracy and resolution of the measurements. As these critical points are identified, the work progressed to evaluate the capability of the existing equipment to accurately measure, control, and maintain the desired test conditions. Finally, the minimum equipment capabilities, components, and design features to ensure the consistency and accuracy of the test were presented. This task concentrated on the following items of the current AASHTO T 324: • Loading mechanisms; • Temperature measurement and control system; • Impression measurement system; • Specimen dimensions; and • Data collection and reporting. Other factors within the current standard were also analyzed to accommodate any changes and new recommendations as needed. The current issues on each of these sections, as well as potential improvements were evaluated and presented. 2.3.3 Task 3 – Propose Revisions to AASHTO T 324 Based on the results of Task 2, revisions to AASHTO T 324 were proposed to incorporate the equipment capabilities, components, or design features that ensure proper testing and accurate, reproducible results. Modifications were based on the aforementioned components and whether existing HWT equipment possess the needed technologies to meet the required specifications. 2.3.4 Task 4 – A Framework for Future Laboratory Evaluation Upon completion of Tasks 3, Hamburg test equipment capabilities, components, and specifications would have been reviewed and modified to ensure proper testing, accurate, and reproducible results such that it may be used in performance-based specifications. In Task 4, a detailed experimental plan was developed to validate the proposed equipment configurations and specifications developed in Tasks 2 and 3 and to meet the recommended modifications to AASHTO T 324. 2.3.5 Task 5 – Prepare and Submit Final Report The objective of Task 5 was to complete a final report documenting the entire research effort. Task 5 was divided into two subtasks that include preparation of a draft and a final report. The report summarized the findings and conclusions and presented the recommended modifications to AASHTO T 324 along with the developed research framework. Task 5 also included a detailed review of the capabilities, specifications, similarities, and differences among the available Hamburg test equipment in the US.