LASER, X-RAY SCANNING PROVIDE RELIABLE AGGREGATE CHARACTERIZATION

Both laser and x-ray scanning provide reliable methods of quickly determining aggregate characteristics, say Linbing Wang, Ph.D., P.E., You Zhou and Christopher Harris, Virginia Polytechnic Institute and State University; and Cristian Druta, Ph.D., Virginia Tech Transportation Institute, Center for Sustainable Transportation Infrastructure, in their paper, “Three-Dimensional Aggregate Evaluation Using Laser and X-ray Scanning.”

Accurate and quick characterization of aggregates is an important part of aggregate production quality control for pavements, the researchers say. “Aggregates constitute the major part of a pavement structure; thus, properties such as shape, angularity and surface texture substantially affect the overall performance of pavements, the bond with cementing materials, and the resistance to sliding of one particle over another,” they write. “Flat or thin aggregates, as well as long, needle-shaped aggregates tend to break more easily than cubical aggregates. Those with rough and fractured faces allow a better bond with asphalt binder than rounded aggregates and also allow higher friction strength to develop when aggregates slide against each other.”

Automated, accurate and easy to use scanning systems may offer the ultimate capabilities for evaluating the shape, angularity and texture of aggregates, they say. To better identify an optimal system, the writers studied two commercially available scanning systems, one x-ray-based and the other laser-based, and evaluated them in terms of their capabilities in accurately acquiring three-dimensional surface data.

Imaging systems that employ laser and x-ray are frequently used in mapping three-dimensional surfaces and volumes. Their primary functions reside in acquiring images of the objects of interest and subsequently processing them with the help of analysis tools, which are computer programs used to analyze the acquired images in order to obtain the desired information, such as dimensions, shape, texture, or angularity.

The Surveyor 3D laser scanner selected for this study utilizes a line-range laser probe (RPS 120) for profile measurement of objects ranging from 1mm to over 10 in. in size. The probe is capable of acquiring inputs for surface point reconstruction, as the position of a profile can be calculated by triangulation method, since the position relative to the laser is known.

The x-ray technique used the Skyscan 1174 x-ray micro-CT scanner, employing a sensitive 1.3 megapixel camera that allows scanning of a 4cm × 4cm × 4cm sample in a few minutes. Also, a variable magnification (6 to 30 um pixel size) is combined with object positioning for easy selection of the part of an object to be scanned. The full range of the Skyscan software includes fast volumetric reconstruction, 2D/3D quantitative analysis and 3D visualization.

“Their potential in characterizing fine and coarse aggregate's shape properties was assessed as part of the [National Cooperative Highway Research Program] NCHRP 4-34 research [on developing and evaluating 3D aggregate characterization systems and analysis methods], with very good correlation,” they write.

The aggregates used were crushed limestone and granite and uncrushed gravel. Two sets of aggregates, each containing nine particles randomly selected, were chosen for results comparisons, ranging from 1 in. (25 mm) to No. 16 (1.18 mm), and were measured using three means: electronic calipers with a precision of four decimals, Skyscan 1174 Analyzer software and RPS-120 Geomagic software.

“The dimension measurements, surface area and volume quantification capabilities of the two systems were evaluated by comparing these measurements with caliper measurements, theoretical calculations and other proved methods,” the researchers say. “The evaluations indicate that both the x-ray scanning method and laser scanning method can yield very high resolution, from 6 to 60 um, in determining the dimensions of aggregates, allowing for accurate characterization of the derived characteristics of aggregate shape, angularity and texture in true 3D.”

The two scanning systems can successfully be used in evaluating mineral aggregates, they conclude. Both dimensioning methods, Skyscan 1174 CT Analyzer and RPS-120 Geomagic, agreed with the caliper results. The CT Analyzer software is more accurate in dimensioning samples than Geomagic software. Also, scanning with Skyscan takes less time than scanning with the Surveyor laser scanner.

But there is a very good correlation between both methods, with dimension values featuring R2 values being around 0.95 or higher. There are very good results regarding the ability of the two systems in calculating the surface areas and volumes of aggregates. The evaluations also indicate that both scanning methods can be used to assess the derived aggregate characteristics such as shape, angularity and texture.