Virtual reality for rocks?

Feb. 17, 2004

New developments in imaging technology promise to move the production of aggregates toward real-time analysis of the aggregates used in asphalt pavement mixes. While this may not sound earth-shattering to anyone other than an aggregate producer—or a hot-mix asphalt (HMA) producer who uses the aggregates—consider the way the shapes of aggregates are currently evaluated and how long it takes for test results to be obtained.

New developments in imaging technology promise to move the production of aggregates toward real-time analysis of the aggregates used in asphalt pavement mixes. While this may not sound earth-shattering to anyone other than an aggregate producer—or a hot-mix asphalt (HMA) producer who uses the aggregates—consider the way the shapes of aggregates are currently evaluated and how long it takes for test results to be obtained.

Most tests involving particle shape (currently flat and elongated) require measurement of approximately 100 particles taken from an aggregate sample. Each particle is carefully measured one piece at a time. This is one of the more difficult aggregate tests because it is so time consuming and boring for the technician. This is especially true when handling and measuring small pieces retained on the 4.75- mm (No. 4) sieve. Now consider that for a medium-sized quarry producing an ASTM No. 8 product, approximately 25,000 4.75-mm pieces are produced per second. Does the sample of 100 pieces tested represent what is actually in the finished product? We believe so, but new imaging technology will give us a capability we have not previously had to measure larger samples more frequently to better characterize what we are producing.

All shapes, forms and sizes

Most producers and users of aggregates probably do not realize the effort that is ongoing to better measure and describe aggregate particles. Computer imaging technology is being developed through NCHRP Project 4-30, Improved Testing Methods for Determination of Critical Shape/Texture Factors for Aggregates, to give us better ways to evaluate particle form (flat and elongated, or in the future “sphericity”), particle angularity (fractured faces) and for the first time particle surface texture. This new technology should lead to better understanding of how aggregate particles affect HMA performance, and most likely will lead to new and better specifications.

Currently, to determine the form or shape of an aggregate particle used in HMA, we compare the particle’s length to the particle’s thickness. This gives us a ratio between the maximum dimension and the minimum dimension and is known as the “flat and elongated” ratio. A flat and elongated ratio of 10% 5:1, our traditional aggregate specification, means that an aggregate sample cannot have more than 10% of the particles with a length greater than five times the thickness.

Stone-matrix asphalt (SMA) mixes are becoming increasingly popular in some parts of the country. SMA mixes require more cubical particles and specify 20% 3:1 particle shape. The SMA particle shape requirements can be a real challenge to produce for some types of geologic materials. For example, the hard, abrasive granites have historically been produced from compression crushers as compared to impact crushers. Compression crushers literally squeeze particles causing these hard materials to shatter, generating some flat and elongated particles, but do not have the high wear costs associated with impact crushers if used with abrasive materials.

It’s interesting to note that a recently developed concept called multiple ratio analysis revealed that in a sample of aggregate, particles are usually present in a variety of particle shapes or flat and elongated ratios. Using this concept, a particle shape profile can be graphed to show the percentage of particles present in each of five different flat and elongated ratios from less than 2:1 to greater than 5:1.

With the new automated imaging technology under development, we will be able to measure the length, width and thickness to calculate sphericity of each particle in an aggregate sample. This gives us a better idea of the volume of each particle, which is important since HMA mix designs are volumetric. Presumably something like a multiple sphericity analysis will evolve to describe the different volumes of the aggregate particles found in a sample, leading to improved HMA mix designs with better performance.

By zooming in on an aggregate particle with the new imaging technology, it’s now possible to make surface texture measurements. The peaks and valleys can readily be seen, just as when looking at the surface of the moon through a powerful telescope. Surface texture will prove to be an important property, but HMA performance ultimately depends on the interaction of surface texture, form, angularity and gradation. Any future specifications developed for surface texture also should take into consideration that surface texture can’t be modified during the production process as can form and angularity, and is a function of the geology of the deposit. Is there a difference in performance with smooth texture and sharp edges (basalt) as compared to a rough surface with more rounded edges (dolomite)? Both have a history of good performance.

Taking X-rays?

These new tools and potential specifications for particle shapes will influence aggregate production in the future. Today it’s possible for an aggregate plant to be fully automated and run unattended for a full production shift. Multiple sensors monitor and adjust the process and can even shut down the process automatically if needed. Now imagine increasing that capability with scanners that measure particle form, angularity and gradation on a moving production conveyor belt. The scanners could be digital, laser or perhaps even use X-ray tomography like a CAT scan to give three-dimensional images of the particles in a sample of asphalt pavement. The sensors would control the production process to give the ultimate in production consistency.

Segregation of particle sizes is sometimes a problem at aggregate plants and during the production of HMA. Multiple cold feed bins and skillful stockpile recovery techniques are usually necessary for uniform gradations. We will probably see more fractionated aggregate plants in the future to make the best use of investments in sophisticated controls for particle shape, angularity and gradation. Having the capability to evaluate the form, angularity and texture of the various products produced at an aggregate location will allow the gradation to be optimized for best use of the particle characteristics from that individual source. A sophisticated virtual reality design process will probably be used someday to assist with designing HMA mixes. In fact, virtual reality design also is currently under development.

Now imagine the future is almost here.

About The Author: Jahn is director of product development, MidAmerica Region, Martin Marietta Aggregates.

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