After four years of development and mixer fleet installations in 17 countries (and counting), the drum-mounted IBB Rheological Probe is shaping up as an alternative to conventional slump and spread testing. The device uses principles similar to a rheometer: Movement of an “impeller” within the concrete load at different speeds creates a flow curve that can be used to gauge viscosity and yield stress—the true behavior of fresh concrete. Most importantly, workability can be accurately estimated on a wide range of slump values by using a calibration with the load measured at low drum speed.
The IBB system has three main components: a stainless steel probe installed inside the mixer truck drum, plus exterior-mounted receiver and solar panel. The probe works using a variety of sensors (accelerometer, thermistor and load cell) and electronic components powered by rechargeable batteries. Collected data from many sensors are processed, stored and sent to the receiver using radio signal. Measurements can be displayed, stored and further made available for an alternate communication system, such as GPS, for real time monitoring.
The receiver has memory to store data for up to approximately two days of normal truck utilization. It can be connected to a computer, on a temporary basis, to collect the stored information, or to a GPS communication system (not provided by IBB) to transfer that data on a real time basis or for further processing. The receiver is powered by truck battery, but also has a cable that can be used to recharge if that power source is not available.
The rheological probe does not require any sampling and can continuously monitor fresh concrete properties without any waste of the material or human interference. In addition to slump and rheological properties, it also determines concrete temperature, movement characteristics of the drum (angle, speed and direction) and concrete volume inside the drum. The range for slump determination is 1.2-11 in. (30–280 mm) and for spread 12-22 in. (300–550mm; per German DIN Flow Table).
Depending on the drum speed and the workability of the concrete, the force acting on the side of the probe by the moving concrete varies. It can easily be transformed to an equivalent normal pressure in kPa. Using the pressure instead of direct load will allow comparison of the results with other similar devices of different sizes and shapes.
The receiver has sufficient storage capacity to keep data for several days of constant usage and includes a serial port that can be used to directly download the data or to connect to a GPS-based transmission system to send data in real time. Increasing numbers of concrete producers have GPS systems that will greatly benefit from the addition of a system that can be used as a standalone device with the receiver.
ASTM C143 FULFILLED
Globally, the two most popular tests to measure workability are slump (ASTM C143) or spread (either for self- consolidating mixes or per the German DIN EN 12350 Standard). The IBB probe can be calibrated for either method.
Tables 1 & 2 show the IBB probe slump measurements compared to cone-measured slump. The data in the tables have been collected by producers in the Middle East and other regions. Improvement in the probe performance is believed to be tied to the receiver mounting at the back of the truck (2012), rather than in the cab (2011), where the truck driver cannot adjust the speed precisely.
Table 3 shows the IBB probe-measured spread compared to a DIN Flow Table measurement. All spread figures are the result of only one manual measurement. The point in diamond yellow from “Austria 1” is to be neglected since that was too much segregation. The real value was definitely less. The series labeled “Switzerland 2” have been gathered by the producer alone (no IBB dealer representative present). A careful analysis of these data has shown that the point in square yellow has a wrong manual reading and should be moved on toward the equality line. As more data are coming in, the calibration of the probe with the German spread is excellent.
From the preliminary tests conducted, IBB finds the workability probe a) can estimate the slump with a good accuracy; b) has a lower workability limit around 30 mm slump; and, c) can estimate the German spread as measured by the DIN Flow Table with very good accuracy. The potential of this technology suggests that concrete workability in the near future could be specified by using the probe-measured pressure in lieu of slump or spread test readings.