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Aggregate Testing with High Capacity, High Precision Scales
An aggregate material, when referring to construction materials, is a mixture of a variety of different solid materials. These are mixed together and used for different construction purposes, including for septic systems and other types of drainage, as a component of concrete, and as a material used in manufacturing asphalt for road and other paving projects.
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The aggregate material itself is specified for the specific application. It may include crushed and recycled concrete, sand and gravel, and crushed rocks and stone. These crushed stones may include granite, marble or limestone. The types of material that are locally available will often dictate the type of aggregate produced. Recycled materials have become attractive ingredients, as they fulfill the dual role of disposing of unwanted waste materials while producing new construction materials. Popular materials in this group include slag from steel production, along with concrete that is recovered from demolition. A common property of any of these materials is that they generally will not retain significant amounts of water, as opposed to most soils.
The characteristics of the finished concrete or asphalt material is dependent on the type, amount and size of each of the aggregates that are used to manufacture it. There are often complex specifications carefully detailing the composition. Contractors must often analyze their materials to prove that they conform to the requirements.
One tool that may be used for this type of analysis is an ultra precision industrial scale. Standard digital scales may not be suitable, because the scale must have the ability to weigh a large quantity of material, and must also be able to have a very high resolution. A typical high quality electronic scale with a capacity of 100 lb may have the ability to determine weight to within one or two hundredths of a pound, or about 0.01 lb. While this may be suitable for most requirements, the analysis of the aggregate material requires even finer reading in order to measure some of the smaller components. An ideal precision scale would be able to weigh ten or even twenty times more accurately.
A regular industrial scale uses a weighing technology that is based on a strain gage load cell. The scale platform is supported by this load cell, which consists of a machined metal bending beam with resistive strain gages bonded to it. While the cell only bends a few thousandths of an inch, it is enough to cause the resistance of the strain gages to change. High quality electronic circuits are used to amplify these changes of resistance, and produce a signal that is accurately digitized. The resultant values are proportional to the weight of the material on the scale platform. This type of scale is limited in accuracy for a number of reasons. While the load cell does not bend very much, even this small amount causes errors due to the elastic properties of the metal used. Special aluminum, steel or stainless steel alloys are chosen to reduce these errors. The resistive strain gages are also subject to error because of issues such as temperature changes. Thermal compensation is included on the load cell to minimize this type of error. But even the best quality strain gage load cells are limited in their accuracy specifications.
There is another type of weighing technology that can produce much higher levels of accuracy. These types of industrial scales are known as “force motor”, or “force restoration” scales. While they offer the types of accuracy required for aggregate analysis, they are extremely expensive for the required weight capacity. Typical prices range from $3000 to $5500.
Arlyn Scales offers their Serial Acoustic Wave (SAW) type of industrial scales. These ultra precision scales use semiconductor transducers to measure the deflection of a load cell at a precision level that cannot be obtained by other means. The load cells are designed to experience only five percent of the stress of a standard strain gage load cell. This eliminates the material errors that are seen with strain gages. Because these transducers are completely digital, they do not experience the type of electronic errors that occur with the digitizing process of resistive strain gages. They are also much less affected by temperature variations. The SAW electronic scales have a similar cost to the strain gage scales, with a price of about $1100 for a scale with 100 lb capacity. Larger capacities and platform sizes are also available. As an added benefit, the SAW load cells have an extremely high safe overload factor. This is particularly desirable when large loads of aggregates are dumped onto the scale.
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