Long-Term Pavement Performance Materials Characterization Program: Verification of Dynamic Test Systems With An Emphasis On Resilient Modulus

Chapter 1. INTRODUCTION

GENERAL

The Long-Term Pavement Performance (LTPP) program resilient modulus test protocols were developed to ascertain the stiffness of pavement surface (asphalt concrete), base, subbase, and subgrade materials. The resilient modulus testing process, generally regarded as a research-type procedure, has historically been performed in a university setting and on a relatively small number of samples. Because the modulus value derived from this testing process is a key parameter for pavement design, the test is being performed for the LTPP program in a production testing environment in what may be the largest single resilient modulus testing program ever undertaken. It is of paramount importance to provide LTPP researchers with the highest quality data possible. As such, a quality control/quality assurance (QC/QA) procedure was developed to verify the ability of the laboratory equipment and personnel to perform resilient modulus testing for the LTPP program.

The original procedure was documented in FHWA-RD-96-176, LTPP Materials Characterization Program: Resilient Modulus of Unbound Materials (LTPP Protocol P46) Laboratory Startup and Quality Control Procedures. It was developed primarily for the verification of base, subbase, and subgrade resilient modulus procedures. Since issuance of that report, many lessons have been learned and procedures have been added in an ongoing process-improvement cycle. As a result of these improvements and enhancements, this document has been prepared to present a more comprehensive test system evaluation. It should be noted that this document is a revision and expansion of FHWA-RD-96-176, and some portions are not necessarily the original work of the authors of this report. It is intended to be used as a generic dynamic (primarily closed-loop, servohydraulic) test system evaluation procedure; however, the user will notice the strong emphasis on resilient modulus procedures resulting from the development history of the procedures.

The primary reasons for the development of this document were to:

• Revise the procedure to reflect lessons learned since the issuance of the original document.
• Incorporate enhanced and expanded instructions.
• Add examples to illustrate the processes.
• Expand the data-analysis procedures.
• Make the process as generic as possible to increase the use of the procedure in the dynamic testing community.

The procedures outlined here have been modified to reflect the lessons learned from numerous implementations of the procedure and to address the most frequently asked questions concerning the use and conduct of the processes. This document does not supercede FHWA-RD-96-176; most of those processes and acceptance criteria are still valid. Rather, this document expands, clarifies, and enhances FHWA-RD-96-176.

These procedures are meant to be used for verification of system performance and not calibration of the system. Calibration of the system should be performed by the manufacturer or other trained personnel.

These procedures were developed to ensure the accuracy and reliability of the raw measurements produced while testing materials using closed-loop servohydraulic systems. They are based on the premise that any engineering analysis requires reliable raw data, and the prerequisite for reliable raw data is properly configured equipment. The procedures were designed to verify the operating accuracy of all essential system components in a logical manner. Each part of the system is verified individually and then the entire system is checked to make sure all parts work together properly. As part of the electronics system verification procedure, the signal conditioning channels, data-acquisition processes, and transducers are checked for proper operation. Following the electronics system verification procedure, the calibration check and overall system performance verification procedure is performed. Load and displacement measuring devices (i.e., load cells, linear variable deformation transducers (LVDTs)) are checked for linearity and proper calibration. The ability of the software to control and acquire data is also assessed. When the process of verifying the individual system components is complete, the overall capability of the machine to conduct a specific experiment is assessed through specially designed static and dynamic experiments on materials with known properties. After the system has been evaluated, the proficiency phase of the procedure will address the competence of the laboratory personnel to prepare and test samples. Through the use of this procedure, all components necessary to obtain repeatable, accurate test results are verified.

The procedure enables laboratories to verify their testing systems and procedures before starting production testing by using a comprehensive and logical process. The procedure also can be used to perform ongoing quality control checks of the laboratory’s equipment and testing processes during the production testing process.

The equipment required to conduct the procedure was specifically chosen to be readily available in the market at reasonable costs. This equipment includes instruments such as an oscilloscope, function generator, and a computer, which are available in most testing laboratories. This procedure has been successfully implemented at Federal Highway Administration (FHWA) facilities in McLean, VA, State departments of transportation, universities, and in two commercial laboratories under contract to FHWA. Although originally intended for the resilient modulus program, this procedure can be implemented to verify most closed-loop servohydraulic testing systems.

STRUCTURE AND USE

The procedure is divided into three distinct components:

1. Electronics system performance verification procedure.
2. Calibration check and overall system performance verification procedure.
3. Proficiency procedure.

The electronics system performance verification procedure characterizes the frequency response of the signal conditioners and data-acquisition system of the test system. This procedure is generally used prior to the initiation of a resilient modulus testing program. As long as all electronic parts of the test system remain the same, this procedure does not necessarily need to be repeated on a continuing basis (e.g., monthly). However, the procedure should be conducted at least annually to verify that the equipment meets the acceptance criteria indicated in this document, or when any part of the electronics is replaced or modified. Also, this procedure should be performed when other circumstances suggest that the electronics may be suspect. Generally, an electronics technician well-versed in data-acquisition systems is needed to perform these experiments. The time required to perform this procedure depends on the complexity of the test system and the experience of the electronics technician. On average, this procedure should take approximately 8 to 10 hours to complete (including data analysis). If problems are found with the system, troubleshooting intended to isolate the problem usually has been found to take 24 hours, but is wholly dependent on system configuration, complexity, access to manufacturer’s specifications, and other factors.

In this document, the electronics verification procedure has been described to allow as much flexibility as possible; two methods are discussed. The first is an electrical approach that is accomplished by using electrical instruments to simulate load cells and deformation devices. The second is a mechanical approach in which the load cells and deformation devices are actually stimulated by a mechanical device. Either method can be used to evaluate the system electronics.

Dynamic testing procedures require a system made up of many different pieces of equipment: load frame, load cells, hydraulic system, deformation devices, triaxial pressure chamber, temperature chambers, computer, etc. For the calibration check and overall system performance verification procedure, individual elements of the test equipment are checked first, followed by the overall test setup. This verifies that the test system is producing the expected responses. When the individual components of the test system are checked first, many problems that would be encountered during actual dynamic testing likely can be identified and eliminated prior to an overall system check. This procedure is generally used prior to initiation of a testing program and subsequently on a continuing basis (e.g., monthly) to verify the system response. On average, the procedure requires approximately 16 hours to complete.

The ability of laboratory personnel to conduct dynamic testing is evaluated in the proficiency procedure. Again, while this procedure has been developed primarily for resilient modulus testing, the concepts introduced here can be applied to many test programs. This procedure is generally used prior to initiation of a testing program and subsequently on a continuing basis (e.g., quarterly) to verify the operator’s ability to conduct resilient modulus testing. The procedure requires approximately 2 days to complete.

The two primary goals of this process are to: (1) ensure that the test system and technicians are capable of performing a test procedure, and (2) develop a benchmark performance standard against which the laboratory can be evaluated on an ongoing basis. This is a very important part of any quality control/quality assurance system.

Certain procedures outlined here may not be necessary to verify a system. For example, if the user is performing resilient modulus testing of soils, a temperature chamber is not used, so this procedure is not necessary. Users should use engineering judgment to select the processes and experiments that best meet their objectives.

WHO SHOULD USE THE PROCEDURE

This procedure can be used by any organization performing dynamic testing procedures. It is primarily designed to be used on closed-loop, servohydraulic test systems, but potentially the concepts can be used for other test equipment. Historically, the procedure has been used by departments of transportation, universities, and consultant laboratories to verify equipment for testing resilient modulus, creep compliance, and indirect tensile strength.

WHEN THE PROCEDURE SHOULD BE USED

The procedure should be used prior to starting a testing program, every year during production testing, and after periods of system inactivity (e.g., 6 weeks of downtime). It can also be used when equipment is replaced, moved, or whenever a suspected overload or malfunction occurs. Another important use is to verify the operation of new machines delivered by a manufacturer. Conversely, the procedure can be used to verify the ability of older machines to perform new applications.

BENEFITS OF USE

Using the procedure detailed here has several obvious benefits. The procedure provides guidelines for standardization of an entire test process. It also provides a benchmark performance standard for equipment. If implemented correctly, it can minimize equipment and operator variability and thus provide greater confidence in the test results and their application in research or design.

DISCLAIMER

This procedure was designed based on three criteria: effectiveness, simplicity, and low cost. It was formulated to be as general as possible so it could be implemented by a variety of testing laboratories. Its growing implementation by laboratories nationwide is an indication of the procedure’s reaching its main objective, which is to reduce test variability as much as possible. Nonetheless, with the wide range of technology used in testing laboratories, each laboratory may have to adapt different methods to perform this procedure, especially the electronics verification procedure. The purpose of this procedure is not to verify the manufacturer’s specifications nor to set new specifications for manufacturing equipment. This procedure is merely a powerful tool for the equipment operator to verify equipment accuracy before and regularly during production testing. A certain level of expertise is required at each laboratory to ensure proper implementation of the electronics system performance verification procedure.

This procedure should be implemented with caution and by expert technicians or engineers. Laboratories should use this procedure at their own discretion, and neither FHWA nor the contractor team is responsible for any personal injury or property damage due to the use of this procedure.