In the last five years, the precast industry has embraced self-consolidatingconcrete (SCC) in a big way. Some experts estimate as much as 75% of all concreteproduced by precast plants in 2009 could be classified as SCC.

The rapid acceptance was facilitated by the ease of SCC's introduction to castingoperations. Other than perhaps some adjustment to the admixture additionsequence, most producers typically only changed the mix recipe.

Precast producers typically were able to batch and mix SCC using existing plantequipment. And for the most part, their existing control and monitoringdevices provided adequate information to create quality mixes. Thus, mostplants continue to use existing electric motor devices to judge when the ingredientsare fully mixed.

But thanks to a revolutionary sensor recently introduced to North America, manyproducers may opt to gain even tighter operational controls. The ViscoProbe, developed by SKAKO, a Danish manufacturer of concrete mixing equipment, providesin-time measurement of fresh concrete's rheology.

The product measures fresh concrete's viscosity and workability during mixing. Ina series of field tests, the Visco Probe registered a change of consistencyresulting from the addition of as little as 26 ounces of water in acubic yard of homogeneously mixed concrete. The unit not only increases thequality of the final mix, but can also aid in streamlining production.

Workability and flowability

The system does not use power readings from the mixer motor. Its measurementsare not affected by the wear of any parts inside the mixer or oil temperaturein the gearbox. The unit combines the latest advances in material flowmeasurement technology with concrete industry-focused software to calculatethe concrete's consistency. The probe measures relative values for two basicmixing parameters: internal forces of mixing that are important to workabilityand flowability.

The probe assembly can be installed in practically all current models of counter-currentplanetary and turbine mixers. The in-mixer assembly includes twoparts. A vertical arm support attached to the rotating drive positions acast-steel ball containing an electronic probe in the mixer's agitation zone. Thisprobe, traveling in the same path as the mixing shovels, stays incontact with the fresh concrete with no effect on the mixing action. The secondpart of the assembly senses force with a strain gauge. The sensor, protectedin the upper part of the probe, never enters the concrete mixing zone.

A transmitter contained in the sensor assembly transfers the data in real timevia wireless signals to a receiver/automatic process controller which usesspecial software to analyze data. The controller can interface with mostexisting batching and mixing controls.

With this software, the operator watches color graphics as raw ingredients aretransformed into a consistent mix. The operator can establish parametersfor each recipe by matching permissible measurement tolerances compared tothreshold value and dynamic viscosity parameters of reference batches.

When the computer display turns green, it informs the operator that the SCCbatch is in compliance and ready to be discharged. If the sensors report anonconformance, the software supplies a graph that depicts the causes of errors, focusingon water and admixture proportioning and quantity of fines.

To learn more about the Visco Probe assembly offered by Control RetroFix, anew service from SKAKO, visit www.controlretrofix.com. You can see the productdemonstrated at booth #1148 at The Precast Show, Feb. 18-20, in Phoenix.