These systems are of particular interest to precast concrete manufacturers who wish to maintain ultimate moisture control of the final mix, w/c ratio and workability to the highest level of accuracy possible.

This is achieved by measuring the moisture content of all the materials in the mixer during the mixing cycle and thereby determining the precise amount of water to be added to achieve the desired moisture content mix after mix, irrespective of the variation of water content of the aggregates.

Benefits

The benefits of achieving consistent moisture content, or in other words, consistent w/c ratio, are clear, resulting in consistent strengths, consistent colouring and consistent workability (or slump), providing the grade of the sand and aggregates remain essentially the same.

Changes in the ratio of sand to aggregate will also have an effect on the workability of the mix as well as the texture and colouring. If for any given recipe the amount of sand is increased, the mix becomes progressively more ‘stiff’ and as a consequence, more water has to be added to maintain the workability. This in turn increases the w/c ratio with consequent reduction in strength. Therefore, maintaining consistent aggregate/cement ratio is also important for the production of consistent quality concrete. For optimum control it is therefore desirable to measure and control the water addition in the mixer as well as providing on-line correction when weighing the aggregates to ensure consistent ‘dry’ batch weights of the respective materials. For further information please refer to ‘Aggregate Systems’.

Solutions

Hydronix offers a variety of solutions to meet all requirements. The Hydro-Mix VI digital microwave sensor is designed to fit in nearly all types of mixer. Using either the analogue or RS485 output, it will operate directly with an external control system or for possible optimum accuracy with the dedicated Hydronix water control systems such as the Hydro-Control V or Hydro-View.

See Hydro-Mix VI connections

Improving the performance of your mixer

Reduce your mix cycle time by optimizing the performance of your mixer:

The speed with which any water control system can work is very much dependent on the time take on obtain a meaningful reading. The moisture reading from the sensor indicates what is happening in the mixer. The speed of reading, or the time taken to reach a steady reading when the materials are homogenous reflects the effectiveness of the mixer. By taking some simple precautions, the overall performance can be considerably improved and the cycle time reduced with consequent financial savings.

Adjustment of the blades

Ensure the mixer blades are regularly adjusted to comply with the manufacturers recommendations (normally 2mm clear of the floor), with the consequent benefits:

* All the residual mix is discharged when emptying the mix.

* Mixing action close to the floor of the mixer is improved, thereby improving the reading of the sensor.

* Will reduce wear on mixer floor blades.

Plastic blades over the sensor face

It is recommended that the blade or blades sweeping across the sensor face should be of the plastic synthetic type as this helps to keep the sensor clean and can sweep closer to the sensor with less risk of damage.

Cement addition

Mixing the fine cement particles in with the relatively coarse size of the sand and aggregates is a tough job. When possible, the cement addition should commence within a few seconds after the commencement of the addition of the sand and aggregates. Folding in the materials together this way will greatly assist the mixing process.

Water addition

To facilitate the mixing action, the water should be sprayed in over as wide an area as possible rather than discharged at one point. Remember, excessively rapid water addition will increase the wet mix time required to reach homogeneity. Hence there is an optimum rate of water addition for achieving minimum mix cycle time.

Commencement of water addition should only start after the cement has been substantially mixed in with the aggregates.

NOTE: Cement powder lying on the surface of the aggregates will absorb the water turning into a wet paste, which will be more difficult to disperse uniformly throughout the mix.