Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 38

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 37
37 capture measurements of shape (form), angularity, and tex- ture for both fine and coarse aggregates. The system (Figure 8) consists of a video microscope, video camera, data acquisition system, lighting system, auto- mated carriage, and associated software. The aggregate par- ticles are randomly spread on a tray. An Optem Zoom 160 video microscope is coupled with an LC-150 black-and- white CCD video camera to acquire the images. The camera is mounted on a carriage system that allows 250 mm of move- ment in the X and Y axes and 50 mm of movement in the Z axis. The Z-axis assembly can be manually moved an addi- tional 250 mm to switch from fine to coarse aggregate mea- surements. Fine aggregate measurements and coarse aggre- gate measurements of the longest and intermediate axis and information for coarse aggregate angularity are accomplished using backlighting of the aggregate tray. All other measure- Figure 8. Aggregate imaging system. ments are accomplished with top-lighting. The images are captured using a National Instruments PCI 1409 analog frame Laser-Based Aggregate Scanning System grabber. Image processing is conducted using LabView soft- ware (83). The Laser-Based Aggregate Scanning System (LASS) Fine aggregate analysis is based on 2-D images. The fine uses a laser line scanner mounted on a 2-D linear slide sys- aggregate images are acquired to produce a resolution such tem and a data acquisition system to measure aggregate par- that each pixel is less than 1% of the average aggregate diam- ticles between 1.0 mm and 100 mm in three dimensions (Fig- eter (84). At this resolution, the field of view includes 6 to 10 ure 9) (85). In the prototype laboratory version, aggregate particles. Coarse aggregate analysis is based on a combination particles are placed on a scanning platform. The laser scan- of measurements. First, aggregates are backlit, and 2-D images ner moves along the 1.5 m Y-axis on the overhead slide per- are captured to determine the largest and intermediate dimen- forming 25 scans/s. The X-axis scan width is 120 mm. The sions as well as angularity. One aggregate particle is captured laser scanner projects a stripe on the scanning platform. The in each image. The resolution of the image is set such that the reflection of the laser stripe is captured by a CCD camera. pixel size is less than 1.0% of the average aggregate diameter. Knowing the location of the laser source, the 3-D coordinates The third dimension of the aggregate is acquired during a sec- of the surface of the object can be calculated. LASS has been ond measurement pass. During this pass, the aggregates are used to measure grading, shape, angularity, and texture (86). top lit. The camera first focuses on a point on the tray. Then, the Z-axis is moved up until the top of the aggregate is in 2.5.3 Image Analysis focus. The travel of the Z-axis is the third dimension of the aggregate. Gray scale images for texture analysis are cap- The systems described above represent a sampling of the tured during this pass (84). systems currently available. Two of those systems, UI-AIA Figure 9. Laser-Based Aggregate Scanning System (85).

OCR for page 37
38 and AIMS, are under investigation as part of NCHRP Project Aggregate Shape 4-30. WipShape, UI-AIA, AIMS, and LASS can all measure the principal aggregate dimensions, length, width, and thick- Analyses of aggregate shape or form are generally aimed ness of coarse aggregate. The VDG-40 Videograder can only at replacing ASTM D4791, the F&E test. WipShape, UI-AIA, measure two dimensions--length and width--and produce a AIMS, and LASS all measure particles in three dimensions. global average of the third dimension. Image analysis tech- From these measurements, the principal dimensions of a par- niques are used to extract information about grading, shape, ticle are determined. angularity, and, in some cases, texture. The WipShape and UI-AIA systems both fit a virtual box around the aggregate particles to determine the longest, inter- mediate, and shortest dimensions (73, 81). Using these mea- surements, elongation is the ratio of the longest dimension to Aggregate Grading the intermediate dimension, and flatness is the ratio of the intermediate dimension to the smallest dimension. The Super- The VDG-40 Videograder, WipShape, UI-AIA, AIMS, pave method's specifications are based on the ratio of the and LASS will all determine the size of the aggregate parti- longest to the smallest dimension or F&E as specified by cles being evaluated. The aggregate size is used to categorize ASTM D4791. Both WipShape and UI-AIA can produce fre- particle angularity and texture measurements by size. In its quency histograms of the percent of particles exceeding var- current form, only the VDG-40 is designed to test large aggre- ious ratios of elongation, flatness, or flatness and elongation. gate samples, representative of a gradation sample. Weingart Sphericity and form factor have been proposed as indexes and Prowell (79) compared sieve results with the output of of aggregate shape (76). Sphericity is described by Equation 2: the VDG-40 for production samples of a No. 8 material. The material passing the 1.18-mm sieve was not evaluated. The ds d I results agreed well except for the 9.5-mm sieve. The devel- = 3 (2) dl2 opers of LASS also believe that their technology is adaptable to online measurements. where Problems can occur when comparing the results from dig- ital imaging to wire mesh sieves. The size of an aggregate = sphericity, particle is generally taken to be the intermediate dimension ds = smallest dimension (thickness), (dI) or the particle width. Some flat or elongated particles can dI = intermediate dimension (width), and fit through square sieve openings that are smaller than their dl = largest dimension (length). width on the diagonal (73). Rauch et al. (87, 88) completed a study to evaluate tech- Shape factor is described by Equation 3: niques to rapidly determine the gradation of unbound aggre- gates. Based on their review of potential technologies, digi- ds SF = (3) tal image analysis, and laser scanning were recommended for dl d I further research. A continuation of the study evaluated five automated gradation devices: LCPC VDG-40 Videograder, where W.S. Tyler Computer Particle Analyzer, Micrometrics Opti- sizer PSDA 5400, John B. Long Co. Video Imaging System, SF = shape factor, and and Buffalo Wire Works Particle Size Distribution Analyzer. ds, dI and dl are defined as for Equation 1. Two of the systems--the VDG-40 and the Computer Parti- cle Analyzer--use line-scan cameras. These systems evalu- Measures of aggregate shape using wavlets have been pro- ate all of the particles (greater than a minimum size) that pass posed for LASS and AIMS (83, 86, 89). Masad (83) states: in front of the camera. The remaining three systems use matrix-scan technology. Matrix-scan devices typically sam- The fundamental idea behind wavlets is to decompose a sig- ple 10% to 20% of the aggregate stream. nal or image at different resolutions. Wavlets are special func- tions, which satisfy certain mathematical conditions and are Five aggregate materials were used to prepare 15 test sam- used in representing data, which could be one-dimensional ples that were tested in each device. The samples were assem- signal (speech), or a two-dimensional signal (image). bled to provide diverse shape, color, and texture. Based on the analysis of the results, the two-line scan devices are more repeatable; however, the results from the devices using line- Angularity and Texture scan technology do not compare as well with the benchmark sieve results (88). The Micrometrics Optisizer PSDA and Several researchers have proposed methods of analyzing John B. Long Co. Video Imaging System appear to provide aggregate angularity and texture using fractals (55, 57, 90, the best overall results (88). and 91). Maerz (73) uses the minimum average curve radius