- Author
- Shenton, H. W., III | Cassidy, M. M.
- Title
- Field Evaluation of the System for Calibration of the Marshall Compaction Hammer.
- Coporate
- National Institute of Standards and Technology, Gaithersburg, MD
- Sponsor
- Federal Highway Administration, Washington, DC
- Report
- NISTIR 5553; FHWA-RD-95-063, February 1995, 76 p.
- Distribution
- Available from National Technical Information Service
- Contract
- DTFH61-92-Y-30052
- Keywords
- compaction hammer | Marshall method | asphalt | bituminous | building technology | calibration | pavement design | standards | tests
- Abstract
- A system for calibrating the Marshall compaction hammer has recently been developed at the National Institute of Standards and Technology, in collaboration with the American Association of State Highway Transportation Officials (AASHTO), Materials Reference Laboratory (AMRL). The calibration system consists of a spring-mass device with an integral force transducer and a high-speed data acquisition system. The force delivered by the hammer to the calibration device is recorded as a function of time and analyzed to determine the peak force and impulse. Time histories from a series of hammer blows are analyzed to determine the average peak force, average impulse, and cumulative impuse. The proposed calibration procedure is based on adjusting the number of hammer blows delivered to a specimen, such that a standard compactive effort is supplied during the compaction process, regardless of slight variations in the Marshall hammer. In an earlier laboratory evaluation program, the calibration system and procedure proved to effective in reducing the variability of Marshall test results. Presented in the report is a summary of a field evaluation program of the calibration system and procedure. In this study, Marshall specimens were prepared in bituminous laboratories using "production" Marshall hammers: twelve laboratories, or field "sites", participated in the study. Sixteen Marshall specimens were prepared at each site. Four specimens were prepared using a standard 50-blow Marshall procedure and four using a standard 75-blow Marshall procedure; these are referred to as the uncalibrated specimens. Four specimens were prepared using a calibrated blow count corresponding to a standard 50-blow cumulative impulse, and four were prepared using a calibrated blow count corresponding to a standard 75-blow cumulative impulse; these are referred to as the calibrated specimens. Height, air voids, flow and stability were determined for each of the specimens. Results were compiled and analyzed to determine the between-laboratory variability of the data for the uncalibrated and calibrated specimens. The system was, in general, ineffective in reducing the between-laboratory variability of the test results in the full data set. The variability of the calibrated test results increased or decreased relative to the uncalibrated results for different specimen properties. The system was effective, however, in reducing the variability of the test results when evaluated in the reduced data set, that included results from nine sites; data from three sites were eliminated in the statistical analysis because these results were believed to be flawed, in a way that may have compromised the Marshall test results. In the reduced data set, the variability of the calibrated test results decreased by as much as thirty percent, relative to the uncalibrated results. One possible explanation for the marginal reduction in variability with calibration is that the study sample of Marshall hammers was typical of the total population: nine of the twelve machines were from the same manufacturer, and six of those were less than three years old. The machines provided reasonably consistent results, thus, there was little room for improvement. This is supported by comparison of the uncalibrated test results to data from the AASHTO Materials Reference Laboratory, Proficiency Sample Program.