Last November, hundreds of researchers, producers, and material suppliers met in Chicago to discuss the current status of self-consolidating concrete (SCC) research. While not quite yet a mainstream product in North America, SCC is a material that has been, and continues to be, studied extensively by the concrete industry throughout the world.
Organized by Northwestern University's Center for Advanced Cement-Based Materials, SCC 2008 Challenges and Barriers to Application, The Third North American Conference on the Design and Use of Self-Consolidating Concrete was an international event. Presenters from all over the world shared important data.
Traditionally, this type of conference focused on the science of rheology. But this event brought in a new element. Many presenters offered information from real life operations. Several presentations offered insights on how producers might decrease the variability of fresh SCC's workability. Two papers seemed to generate great interest as they provided guidance on two of SCC's greatest production variables: air content and temperature.
Aulis Kappi and Erik Nordenswan, of Consolis Technology Oy, described efforts at their operations in Rusko, Finland. While maybe not a familiar name to many North American producers, Consolis is a leading European manufacturer of prefabricated concrete products. The company operates more than 130 plants in 25 countries.
The paper, “Online Measurement of air Content in Fresh SCC,” described instrumentation that provided online measurement of air content in fresh SCC. The authors noted that trying to maintain consistent air content in SCC during production can be difficult. Fresh SCC is sensitive to variations in raw material properties and interactions with admixtures. This problem causes unpredictable variations in air content.
To monitor air content, QC technicians frequently measure air content from each batch using the procedures outlined in C 231–97, Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method. Typically, this procedure takes five to 10 minutes to complete. In high-volume production plants, where producers typically mix up to 200 batches in a day, this need is tedious and time-consuming.
The presenters reported on a new test method by which each batch's air content was measured automatically online. Their results showed an accurate process. Also, all the measurements are automatically recorded together with the rheological parameters. The method is based on the findings that the air content is mainly related to the plastic viscosity and, to some extent, yield value. Both can be measured online in the concrete mixer using the mixer as a rheometer.
In their study, the presenters focussed on how to gain more control of the impeller's shaft speed to accurately measure the fresh concrete's shear stress. To accomplish this, they equipped the concrete mixer's electrical system with a frequency inverter.
The inverter is equipped with a programmable control with which operators can adjust the frequency of the alternating current. The goal is to target the shaft's speed so it is constant at the desired target value of shear stress. An intelligent inverter continuously measures the frequency, current, and phase lag.
From these values, it calculates the real speed of the motor. If the speed is deviating from the target, this is corrected by increasing the frequency and voltage. The inverter will also measure the torque of the motor based on the signals. The torque of the shaft is proportional to the flow resistance in the mixer and thus, also proportional to the shear stress.