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https://nap.nationalacademies.org/catalog/27432/critical-issues-in-transportation-for-2024-and-beyond

Phase 2 - A Comparison of Optical Gradation Analysis Devices to Current Test Methods

Optical devices are being developed to deliver accurate size and shape of aggregate particles with, less labor, less consistency error, and greater reliability. This study was initiated to review the existing technology, and generate basic data to determine when and where such devices are appropriate from the standpoint of both economies and performance. The study was taken up in two phases. Phase-1 incorporated a review of the existing devices for gradation analysis. Following a review of available optical devices, an optical device called Computerized Particle Analyzer (CPA) was selected for laboratory evaluation to determine its suitability for gradation analysis. This device examines particles as they freely fall in front of a light source, while a camera capable of making 10,000 scans per second captures 2-D images. Aggregate samples were collected from 46 different sources in Ohio. These sources included various sizes of lime stone and gravel with varying amounts of crushed faces. Gradation tests were first performed according to standard American Society for Testing and Materials/American Association of State Highway and Transportation Officials (ASTM/AASHTO) procedures. The samples were then tested in CPA. The results of the tests showed that the maximum difference between the test results was less than 1%. Phase-2 was initiated: to validate the gradation results derived from CPA ; and to investigate its suitability for the measurement of shape characteristics of aggregates such as % Fractured Face (FF) (ASTM D5821) and Flakiness and Elongation Index (F/E) (ASTM D4791). The Fractured Face test is subjective and requires visual inspection of aggregate surface to estimate percent of fractured surface. The CPA device is not configured to directly output this property. However, the device measures another property termed ‘Spherecity’. The researchers attempted to establish a correlation between %FF and the Spherecity values. To pursue this goal, a total of 202 aggregate samples (78 gravel and 204 limestone) were tested. Flat and elongated test requires the measurement of largest and least dimension of each particle. A free falling particle always falls with its largest dimension exposed to the camera. However, the width exposed to the camera may not always correspond to the least dimension. This is particularly true for flaky particles which lie on their flat surface. In order to overcome this limitation and to capture the least dimension, the feeder bin was redesigned so as to allow only one particle to fall at a time. The effort culminated in the redesign of feeder so as to allow only one particle to fall at a time. The study led to the following conclusions: 1. CPA device is capable of producing gradation results with great repeatability, reproducibility, reliability, and precision. The results clearly demonstrate the capability of CPA in matching traditional sieve analysis results. 2. The device is rugged, durable, and user friendly. 3. A primary advantage of CPA is its capability to be installed as in-line systems for continuous monitoring at the crusher and/or asphalt plant. With such systems, the results are continuously transferred to the control plant for making necessary adjustments for process control. However, the present study focused on the use of CPA device in the laboratory environment. 4. In its current format, the CPA is not capable of determining %FF and F/E Index.

Language

  • English

Media Info

  • Media Type: Web
  • Edition: Final Report
  • Features: Appendices; Figures; Photos; References; Tables;
  • Pagination: 109p

Subject/Index Terms

Filing Info

  • Accession Number: 01374405
  • Record Type: Publication
  • Report/Paper Numbers: FHWA/OH-2012/5
  • Contract Numbers: 134330
  • Files: NTL, TRIS, USDOT, STATEDOT
  • Created Date: Jun 29 2012 2:45PM