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35 2.5 IMAGING METHODS FOR THE tablet (75). The technique was viable for particles larger than ASSESSMENT OF AGGREGATE SHAPE, the No. 8 sieve. Aggregate particles were first placed in clear ANGULARITY, AND TEXTURE trays in a "flat" orientation such that their minimum dimen- 2.5.1 Introduction sion was orthogonal to the surface of the copier. A photocopy was then made of a group of 50 aggregates producing two The proceeding sections discussed some of the shortcom- dimensional images of each aggregate particle. The minimum ings of the indirect methods of measuring aggregate shape, (i.e., the third) dimension of the aggregates was then mea- angularity, and texture. For example, the uncompacted void sured with a vernier caliper. Both the photocopied image and tests for fine and coarse aggregate do not separate the effects the thickness determined with the vernier caliper were digi- of shape, angularity, and texture. Further, the indirect tests tized by means of a digitizing tablet. Measurements on aggre- can be time consuming and are subject to testing variation gates smaller than the No. 8 sieve were made with micro- based upon the experience of the technician. The sample size photographs. The minimum dimension of these particles was evaluated can be small in proportion to the quantity of mate- measured by evaporating a thin film of metal onto the slide rial produced: for example, the percent F&E is only based on and measuring the shadow of the particle. Once the images the shape of 100 particles of a given size fraction. The rela- were digitized, the data could be manipulated to determine tively poor precision statements for the uncompacted voids shape factors such as elongation ratio, flatness ratio, shape in fine aggregate (AASHTO T304) and F&E (ASTM D4791) factor, or surface roughness (75). demonstrate the magnitude of the test variability. By compar- A black-and-white charged couple device (CCD) camera ison, Maerz (73) outlines the advantages of digital systems: coupled with an image analysis system replaced the use of dig- itizing tablets and photocopied images. Frost and Lai (76) cap- Reduced unit testing cost, tured static images using a Sony black-and-white camera and Reduced technician subjectivity, a Cambridge Instruments Quantiment Q570 Image Analysis Faster results, and System. Coarse aggregate particles were adhered to two pieces Ability to test larger sample size to improve statistical of Plexiglas joined at a 90 angle. The Plexiglas fixture was validity. placed on a light box, which backlighted the sample to pro- duce a high contrast between the particles and the back- These advantages are somewhat offset by additional capital ground (77). Two dimensions were acquired: the longest costs for the equipment. Digital equipment may also be more dimension, dL, and the intermediate dimension, dI. The Plexi- complicated, requiring a greater degree of technician train- glas bracket was rotated 90, and the shortest dimension, dS, ing. Finally, digital systems do not always provide measure- was captured. From this data, the ratio of the principal ments that are directly comparable to those of currently dimensions--elongation and flatness--could be calculated accepted techniques. For instance, there can be differences in along with several other measures of shape (76). gradations based on digital data as compared with wire-mesh Broyles et al. (78) used two black-and-white video cameras sieves with square openings because F&E may fit through simultaneously to capture static images in three dimensions. the sieve opening on the diagonal (e.g., a 1/2-in.-wide particle Rows of aggregates were arranged on a stepped platform so may fit through a 3/8-in. sieve. that they could be viewed by two cameras at 90 to one Several researchers have evaluated digital imaging methods another. Using this technique, the authors could complete 100 to measure aggregate shape, angularity, and texture. Some of measurements of the principal dimensions of a particle in these methods have been introduced previously where they fewer than 10 min. This system could be used to calculate fre- have been used in performance studies in conjunction with quency distributions of flat or elongated particles for a range the currently accepted methods. NCHRP is currently sponsor- of ratios. In addition to shape parameters, analysis methods ing Project 4-30A, "Test Methods for Characterizing Aggre- were developed for roughness and angularity (77). gate Shape, Texture, and Angularity." The objective of this research is to identify or develop test methods for both cen- tral and field laboratories to measure shape, angularity, and 2.5.2.2 VDG-40 Videograder texture (74). These methods are to be applicable to HMA, hydraulic cement concrete, and unbound base materials. The LCPC developed a videograding device designed to rapidly following sections provide a brief overview of the major provide a gradation analysis of a large sample (Figure 6) (68). types of digital image or digital vision systems. The device is commercially available. Prowell and Weingart (41) and Weingart and Prowell (79) investigated the use of the VDG-40 videograder for determining aggregate shape. As dis- 2.5.2 Video Imaging Systems cussed previously, the device was primarily developed to mea- 2.5.2.1 Early Imaging Systems sure aggregate grading of particles larger than 1 mm (No. 16 sieve), but it can also measure shape properties. A sample of The first attempts to use digital imaging to quantify aggre- the aggregate (up to approximately 50 lbs) is loaded into a hop- gate shape involved a photocopy machine and a digitizing per. A vibrating feed tray orients the aggregate particles such

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36 larity is determined by analysis of the average radius of cur- vature of the particle (73). 2.5.2.4 University of Illinois Aggregate Image Analyzer The University of Illinois Aggregate Image Analyzer (UI- AIA) is similar in concept to the first WipShape prototype. However, the UI-AIA, which is shown in Figure 7, uses three orthogonally mounted cameras: a top camera, side camera, and front camera. This allows an accurate determination of the volume of the particles, which in turn increases the accu- racy of mass-based calculations such as gradation and per- cent F&E by mass because the volume and mass of the par- Figure 6. VDG-40 Videograder. ticle are related by the specific gravity (81). The aggregates particles are fed onto a conveyor belt moving at approxi- mately 8 cm/s with 25 cm spacing between particles. One of that they lie flat (i.e., the longest and intermediate dimensions two sensors triggers the cameras to capture the image in are visible). The aggregates fall off a rotating wheel; this pre- sequence using LabView software. An imaginary box is fit- vents the aggregates from tumbling as they fall in front of the ted to the captured images to determine the principal dimen- camera. Aggregates are backlit as they fall in front of a lin- sions of a particle. Then, the volume of the box not occupied ear CCD camera, which produces a line-scan image of the by the aggregate is subtracted from the volume of the virtual aggregate. An ellipse having the same length and area as the box to obtain the volume and, from that volume, the mass of image is fit to each particle. The device produces a sample the particle. An angularity index was also developed for the gradation and two estimates of aggregate shape. The ratio of device to supplement coarse aggregate angularity measure- the length to the width of each particle is reported as the SR. ments (ASTM D5821) (82). The SR may be determined as a distribution or average. The flatness factor is a property for the group of aggregates tested related to the ratio of the average width to average thickness of the particles. 2.5.2.5 Aggregate Imaging System The preceding systems are primarily designed to evaluate coarse aggregate particles. The Aggregate Imaging System 2.5.2.3 WipShape (AIMS) contains both a fine aggregate and a coarse aggre- gate module (83). These two modules allow the system to The WipShape device, developed by Maerz (73, 80), uses two orthogonally mounted video cameras to capture aggre- gate images. The prototype used a vibrating feeder to pro- duce approximately a 2-in. separation between aggregate par- ticles on a black conveyor belt (80). The aggregate particles were lit from the side and above using two lamps. Problems were observed with the contrast between dark or mottled aggregates and the black feed belt (73). This led to the devel- opment of a final prototype with a circular rotating table (73). The table is translucent and allows backlighting of the aggre- gate particles. Images are captured at 60 frames/s using a pair of Sentech STC 1000 cameras. An Imaging Source DFG-BW1 digitization board captures the image from both cameras simultaneously. Custom software manages the data acquisi- tion. "Thresholding" or the identification of the grayscale pixel value that separates the aggregate particle from the back- ground, is accomplished automatically. The software fits a virtual "box" around the aggregate to determine the principal dimensions. The software determines aggregate size (grad- Figure 7. University of Illinois Aggregate Image ing), aspect ratio (elongation or flatness), and angularity. Angu- Analyzer.