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How to measure aggregate moisture in a concrete mixing plant?

Inaccurate measuring of aggregate moisture increases production costs

The biggest problem in connection with the concrete production is the variation in aggregate moisture and therefore it is an important task to make sure that measuring of the aggregate moisture is easy and as accurate as possible.

To be able to use aggregate moisture values for the calculation of material amounts for the actual concrete batch, reliable real time information of the moisture content is required. The moisture-% and with this the amount of water contained in the aggregates massively influence the water content and at the same time the consistency and the quality of a concrete mix.

For dosing the right amount of water according to the target W/C- respectively W/B-value

  • the target amount of total water needs to be set referring to the actual mix design,
  • the total amount of water contained in the different aggregate fractions needs to be identified,

which then allows to calculate

  • the target amount of water to be dosed from the water scale and added to the mix.

For example:

There is a 3 m3 concrete batch, containing 6,000 kg aggregates with an aggregate moisture of 4%, which results in 240 kg water brought in by the aggregates.

Given that the deviation of the aggregate moisture measuring is +-1%, the variation of the aggregate water content is plus or minus 60 kg.

In daily work in a concrete plant, inaccurate moisture measuring is compensated by manual water reduction or dosing additional water based on the ampere meter showing the power used by the mixer motor/s. This manual adjusting leads to inconstant consistency and concrete quality, prolongs the dosing and mixing time and by this reduces the production capacity and generates unnecessary wear in the mixer.

Different type of moisture measuring sensors

There are many types of moisture sensors and probes available on the market:

  • Radar sensors
  • Microwave sensors
  • Optical sensors

1. Radar sensors

Radar

  • Measuring by using radar technology
  • Accuracy up to 0.1%
  • For aggregates with a moisture of 0-90%
  • Radar sensors provide accurate measurement values even if aggregates are exposed to steam in winter and sand and the gravel are differing in grain sizes
  • Even in the event of abrasion at the sensor head, the automatic measurement compensation enables significantly longer operating periods without the necessity of recalibration
  • Sensor head can be exchanged easily, dismantling of the sensor is not necessary
  • Up to 15 calibration curves for the calibration already conducted in the sensor itself; usually, external evaluation devices are not required
  • Possibility to use just one „sliding“ sensor for all aggregates (only in connection with the Tecwill „flying“ aggregate dosing and weighing system!)
  • Easy connection to any currently existing control system

2. Microwave sensors

Microwave

  • Measuring by using microwave technology
  • Accuracy up to 0.3%
  • For aggregates with a moisture of 0-15%
  • Microwave sensors, properly recalibrated provide accurate measurement values even if aggregates are exposed to steam in winter and sand
  • With progressive abrasion at the sensor head and any noteworthy change in the aggregate grain curve recalibration is required
  • Sensor head can be exchanged easily, dismantling of the sensor is not necessary
  • Possibility to use just one „sliding“ sensor for all aggregates (only in connection with the Tecwill „flying“ aggregate dosing and weighing system!)
  • Easy connection to any currently existing control system

3. Optical sensors

Optical sensors

  • Measuring by using optical technology
  • Accuracy up to 0.3%
  • For aggregates with a moisture of 1 – 20%
  • Optical sensor needs more space than other sensors, measuring distance 0,6 – 1,6 m
  • Optical sensors, properly recalibrated provide accurate measurement values but results may not be accurate if the sensor head is dirty or the environment is dusty or steamy
  • Calibration of the sensor is difficult
  • Easy connection to any currently existing control system

All moisture measuring systems promise a lot but measuring results depend on

  • the measuring system itself, but also and even more on
  • the aggregate dosing system (dosing gates or frequency controlled belt feeders)
  • the location where the sensor/s is/are installed
  • the calibration as required by the system

The cost of moisture measuring is influenced by

  • the number of probes required and in detail
    • the purchase price
    • the installation cost and the
    • cost of calibration

The measuring results of sensors can vary a lot, which is why it is not a big surprise that in many concrete plants sensors are not connected to the automation system because of unreliable results.

Usually it is not the sensor itself being the problem, it does it work as good as possible, measuring the fraction of the aggregate for which it is calibrated at the point where it is installed.


Sensors can be installed as follows:


1. Sensor installed in an aggregate bin cone

Sensor installed in an aggregate bin cone

Sensor is installed with the sensor head protruding into a bin cone.

  • Inaccurate moisture measuring due to measuring of only the moisture content of the material close to the cone wall however, aggregates are flowing from the center of the bin
  • The sensor is not visible and its calibration is quite difficult, the sensor wears and the actual state of wearing is difficult to check
  • Expensive solution as several aggregate bins need a sensor

2. Sensor installed at the bottom of a bin cone underneath a dosing gate

Sensor installed at the bottom of a bin cone underneath a dosing gate

Sensor is installed under a dosing gate measuring the moisture of the aggregates falling onto the weighing belt conveyor.

+ The sensor is visible and its state of wearing can be checked easier

+ Reduced measuring accuracy, due to material unsteadily falling down from the bin opening

+ High wear of the sensor head

+ Expensive solution as several aggregate bins need a sensor


3. Sensor installed at the outlet above a belt feeder

Sensor installed at the outlet above a belt feeder

Sensor is installed at the bin outlet above a belt feeder measuring the moisture of the aggregates flowing from the bin with the moving of the belt conveyor.

+ The sensor is visible and its state of wearing can be checked easier

+ Increased measuring accuracy, due to material steadily flowing out of the outlet

+ High wear of the sensor head

+ Expensive solution as several aggregate bins need a sensor


4. Sensor installed above the weighing belt conveyor

Sensor installed above the weighing belt conveyor

Sensor is mounted above the weighing belt conveyor, during the dosing process flowing on top of the aggregate bed, measuring the moisture of each aggregate fraction one after the other for the next batch (only possible in connection with the Tecwill “flying” aggregate dosing and weighing system!)

+ Economical solution when all aggregates are measured using one sensor only

 +Easy to check the state of wear and to replace worn heads

+ Easy to calibrate and recalibrate (with sensors that require recalibration)


Summary

In summary, all types of sensors have their own good and bad sides and all (more or less) require calibration and eventually recalibration. The accuracy of the different systems varies.

Very important is that the sensor/s is/are installed in a way enabling them to best possibly measure the aggregate moisture.

The most economical solution is to install just one sensor above the weighing belt.

Learn more about ready mix concrete plant > 


Tecwill Moisture Measurement

Tecwill uses the “flying” aggregate dosing and weighing system.
Only this system makes it possible to use a “ski”-type sensor, sliding on the relatively thin aggregate bed continuously flowing on the weighing belt conveyor, accurately measuring the moisture of all different aggregates dosed for the next batch.

This method represents a new kind of green technology in concrete mixing, not only lowering the investment and the installation costs but also helping saving cement and with this reducing CO2-emissions.

If you would like further information please click on the link below or contact the Tecwill team.

Read more about Tecwill “Flying” weighing system and aggregate measuring


Topics: Concrete plant

Janne Tuomikko
Janne Tuomikko

VP Business development and Sales

+358 10 830 2911
janne.tuomikko@tecwill.com

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