Though there are several methods of blending materials, few are well suited to the mixing of low slump, low moisture materials. Achieving the desired mix results requires two primary components; a mixer and a material feed system. The mixer can be either a drum or a pugmill and the feed system either batch or continuous. Because a drum mixer folds the same material repeatedly, it is of the batch type. This leaves three possible combinations, batching drum, batching pugmill and continuous pugmill. Within these three types material can be measured either volumetrically or gravimetrically.
Drum mixers used for conventional concrete simply fold the material to be blended. This requires the high water content of conventional concrete to allow the folding action to be effective and efficient. The time that is required for a drum mixer to attempt to completely and uniformly mix low slump or no slump low moisture materials (if even achievable) is prohibitive, limiting production or requiring much larger equipment and the associated costs. The preferred method of mixing low slump materials is therefore a well-designed pugmill that will provide the essential violent mixing action required.
There is one characteristic of the batch system that on the surface seems to give the batch system an advantage. This is the fact that each ingredient in every batch is statically weighed. While it is easy to verify the calibration of these static weigh hoppers, there are many problems that arise when these systems are in production. The fact that a weigh bin can be shown to be accurate in a static condition does not mean it will dose the proper amount of ingredient to each batch. There are several factors that influence batch accuracy, including the reality that overdosed ingredients cannot be returned to their hoppers, material may lodge in the bins after being weighed and vibrations and movements accompanying mixing action may introduce inaccuracies not seen during static calibration. In order to achieve a completely homogeneous mix all ingredients must enter the mixing chamber at the same time and place and at the precise designed proportional rate. This requires measuring and controlling the flow rate of each input feed on a continuous basis. It is virtually impossible to achieve a constant input flow rate in a batch type system because they contain no devices to measure the flow rate of the input feeds. Since these systems also require a batch time, inconsistencies in the mix will likely result if all of the weighed materials do not arrive in the mixer before this timing begins. Batching pugmills or “Compulsory” mixers utilize a pugmill, but because the pugmill mixing action involves moving the material in one direction, the non-consistent flow rates of the input feeds prohibits the pugmill from achieving true uniformity in the mix.
This being the case the ideal system for this application is a continuous mixing pugmill. We chose the Aran ASR 280B as a base for the AccuMix™ 750XB because it is the best continuous mix pugmill design in the industry. This plant is designed to completely and uniformly mix low or no slump low moisture materials.
Volumetric vs. Gravimetric
With the increasing reliability and accuracy of in line weighing systems, combined with pressure from customers specifying their use, many continuous mix manufacturers are building their systems with gravimetric feeders. While weighing a material adds the benefit of accuracy regardless of flow consistency, any weighing system applied to a continuous metering system or a batching system, is by nature retrospective. Weighing only takes place after the metered stream has left the feed hopper or vessel. A retrospective weighing system can add little benefit to a poorly designed and inherently unstable continuous metering system. If the feed rate is constantly varying because of changes in density, flow characteristics or partial bridging in the metering zone, than a loop feedback based on material weight can correct for improper flow rates only after the fact. The only way to ensure stable and uniform proportioning of the ingredients is to make sure that the material in the metering zone is uniform and free of blockages and that the metering device itself is inherently stable.
In order to reap the rewards of a gravimetric system you must still maintain the consistency required of a volumetric system. A successful continuous metering system begins with the design of hoppers and silos so that material of uniform characteristics is provided in the metering zone. This is another primary reason we chose the Aran ASR 280B as a base for the AccuMix™ 750XB. It’s volumetric measuring system has given priority to insure the materials presented to the metering zones are uniform and that the metering feeders extracting it have dimensional and speed stability. The AccuMix™ 750XB utilizes the consistency and uniformity of this industry proven volumetric design. To this we have added the benefits of gravimetric in line weighing systems with premium state of the art Ramsey in line weighing systems to the aggregate, fine particulate and discharge feeds. In addition we have added Hydronix microwave moisture sensors on aggregate feeds and Endress+Hauser Electromagnetic flow meters to the water and admixture feeds.