Transferring all the fresh property qualities of self-consolidating concrete (SCC) from the lab to the field is now much easier. The introduction of two new portable rheometers, devices that measure a concrete's viscosity, will allow technicians to correlate lab results with field performance.
Measuring viscosity offers producers important information about fresh concrete's workability. Using technical terms, these units measure a mix's strength growth, Bingham parameters of yield stress, and plastic viscosity. In layman's terms, they measure thickness, robustness, or creaminess.
Researchers have used rheometers for decades. They look very similar to a mixer. A special meter records the resistance of the blade as it rotates through a semi-liquid material. In pure research, the devices can record the mixes' stiffening as the cementitious material begins to hydrate and harden the matrix. While the spread and flow tests provide an indication of an SCC's flowability, researchers have relied on rheometers to provide a quantifiable and repeatable measurement of a concrete's viscosity.
The combination of results often leads to accurately determining a fresh concrete's ability to flow without segregation. The less viscous the material that flows well, the better the mix.
Two respected concrete instrument manufacturers have recently introduced field-ready devices that measure viscosity. Both provide strong co-relation with field and lab tests. And both units use procedures that can easily be accomplished in the field with minimal technician training.
How they work
The ICAR rheometer, distributed by Germann Instruments, of Copenhagen, Denmark, is a fully portable test device. The unit is about the size of a drill and can be operated by hand or positioned above a standard container. It is accurate in m measuring a flow curve or performing a stress growth test for almost the full range of concrete workability, ranging from a 2-inch slump to SCC. from
The device is accurate enough to detect changes in workability and rheology and is well-suited for measuring high-microfines concrete and other highly thixotropic concrete mixtures. It's portable and durable so technicians can make routine rheology measurements in the field.
The BT2 rheometer, designed by Schleibinger Geräte, of Buchach, Germany, is geared to production and field measurements. It's accuracy is sufficient to detect any change in a recipe's viscosity from batch to batch. Precast producers in Germany have used the device to help reduce variability on even small production runs of SCC.
The technician places a small volume of fresh concrete into a shallow sample container. He then positions the rheometer, centering three probes in the sample volume.
As the technician turns a hand crank one complete round at a constant speed, an internal processor monitors the resistance of each probe's movement in the mix. The processor detects the momentum and angular velocity of each probe. The battery-powered processor yields a final output average from the probes.
The output data is transferred to a companion palmtop portable computer that uses a special software program to display relative yield stress and relative viscosity.
Both rheometer manufacturers have posted technical information about their instruments at their Web sites. Both market directly to the user.
RHM-3000 ICAR Rheometer: Germann Instruments, www.germann.org, 847-329-9999.
SCConcrete Rheometer BT2: Schleibinger Geräte, www.schleibinger.com. 49-8086-94014