Advantages and Disadvantages of Moisture Probes for the Construction Industry Moisture probes must meet diverse requirements for the deployment in sand, gravel, and fresh concrete within the construction industry. The accordance with norm-conformity at the production of ready-mixed concrete, SCC and other types of concrete is increasingly gaining in significance. Hereby, the concrete expert is frequently confronted with the question, if and how these requirements can be met. The following elaborations represent an article addressing the advantages of moisture probes based on the TDR radar technology.

The extreme ambient conditions in the construction industry are demanding consistently higher to the measuring technology. With the achievement of an accuracy of ±1 picoseconds at the measurement of the radar run-time, it was subsequently possible to deploy radar probes within the construction industry. The TDR measurement principle (Time-Domain-Reflectometry, also referred to as „cable radar“) has managed

to increasingly assert itself for demanding applications as a new and precise measurement method within the industrial sector. At the TDR method, the dielectricity constant of an electro-magnetic impulse, and consequently the moisture, is determined via the run-time (time domain reflectometry).

Why is it so important to measure the moisture in sand, gravel, fresh concrete, and other materials very precisely, i.e. with accuracies of at least ±0.3%, respectively ±0.1%?

The following simple exemplary calculation intends to illustrate the connection between %-moisture values delivered by a moisture probe which features errors and the respectively connected effects upon the w/c value: The example applies to 1 m3 of ready mixed concrete consisting of:Aggregates : 1850kg in totalSand : 700kgGravel (2..16) : 1150kg

Cement : 320kgWater content : 176 Litres (nominal value) Nominal w/c-value : 0.55

At a medium moisture content of 5% in the aggregates, this results in a water content in the aggregates of 97 litres. At a nominal/target value of 176 litres per m3, this would result in 79 litres which would have to be added to obtain a correct w/c nominal value of 0.55 at a cement content of 320 kg.

If a moisture probe measures at an error rate of ±1.5% absolute, this would result in significant deviations at the w/c content. In the event of a deviation of the moisture measurement value in downward direction, one would be issued a w/c value of 0.64, and 0.45 in case of a deviation in upward direction (remark: this would naturally be noticed immediately and according measures would be taken). At an error rate of the probe of ±0.3% (±6 litres/m3), depending on the demands made to the concrete, this could result in still acceptable deviations of the w/c value of ±0.02, i.e. the final w/c value would either be 0.57 or 0.53.

An error rate of ±1.5% of a moisture probe is equivalent to a difference of ±30 litres at the water content in sand and gravel, which would considerably influence the w/c value and the Exposition Class. A moisture probe must at least feature an accuracy of ±0.3%, preferably even ±0.1%.The cause and how easily measurement errors of ±1.5% in a moisture probe can be generated is depicted in this article.

Author: Kurt Koehler, Managing Director of the IMKO GmbH

The water quantity of a measurement error of ±1.5% of a moisture probe accords to ±30 litres at 1m3 fresh concrete.

A further issue is, if it is sufficient to merely measure the moisture in the sand but not in the gravel?

The 30 litres mentioned in the example above would also be contained, e.g. in the additional charge of 1150 kg gravel, if the same would feature a moisture content of merely 2.5%.

This means that the moisture content of larger quantities of aggregates should also be measured precisely and on a long term stable basis in order to ensure for sufficient product quality.

What are the ambient conditions a moisture probe has to deal within the construction industry?

If operated permanently, the probe is exposed to considerable •abrasion.

The measured media features differing grain sizes, from finest •particles, to sand, right up to gravel size 2-16/32.

The form of the grain may vary from round to crushed stone, grain •or split.

Both the probe and the measured media are exposed to significant •temperature variations.

Sand and gravel may contain pure water but also water with a high ion •content (e.g. recycled water or sand from areas close to the coast)

Special compositions my contain ratios of steel (steel fibre concrete)•Fresh concrete may feature a pore-water-conductivity of up to •50dS/m.

Materials such as lime sand feature significant adhesive properties. •

Some special concretes, e.g. SCC contain special additives. They •should be mixed with accuracies of ±1.5 litres per m3 in order to obtain the demanded qualities.

In order to be able to measure precisely with an accuracy of ±0.1% at these conditions and the required expedition classes, a moisture probe should meet four decisive requirements.Remark: The aforementioned extreme ambient conditions have frequently generated doubt in regard to the reliability of the measurement values delivered by conventional moisture probes already shortly after installation. The result was that the operators switched the plant from the automatic mode to the manual operation mode.

Four decisive requirements to moisture probes for the construction industry:

1. Requirement: The probe must be able to precisely measure at varying temperatures and varying ion ratios in the water. This means that in the event of aggregates containing recycled water or sand from coastal areas, the measurement value may not be falsified. High and low temperatures during winter and summer operation, e.g. in combination with exposure to steam, may not lead to faulty measurements at a probe. At conventional aggregate probes so far, even the slightest ion-contamination or higher temperature devia-tions can lead to significant measurement errors. In moist mixtures with cement, conductivities up to 50dS/m can be generated and demand a high level of precision of the probe technology. Due to an ideal frequency band, SONO radar probes are not influenced by increased conductivities in the water or temperature deviations. This results in a high level of accuracy even under the harshest ambient conditions.

The water quantity of a measurement error of +-1.5% of a moisture probe accords to +-30 litres at 1m3 fresh concrete.

3. Requirement: The reliable moisture measurement at varying grain sizes. Sand and gravel feature different grain sizes. Moisture probes should not be influenced by the particle size and form and the subsequent-ly connected high-frequency stray. SONO radar probes, in regard to the grain size distri-bution, do feature some interference which is however insignificant, even at sands with finest fractions. Microwave probes in comparison, due to a high level of high-frequency stray, feature a distinctive dependency in regard to the particle size and form.

The SONO radar technology offers high reliabilities at the measurement of sand and gravel, as differing grain sizes, in contrary to other measurement method, do not lead to a falsification of the measurement results.The calibration curve Cal1 „Universal Calibration“ is suited for the measurement of the moisture in sand (finest grain sand), gravel, and gravel/sand mixtures. The deviations of the six sand/gravel types presented below to the calibration curve Cal1 are so insignificant that the SONO Probe only require the deployment of one calibration curve.

Different measurement field, depending on the abrasion

Constant measurement field, regardless of abrasion

Microwave and capacitance method SONO Sensors

dielectricbarrier

(sensorhead)before abrasion after abrasion before abrasion after abrasion

Moisture probes are usually equipped with a ceramic face-plate which may lead to errors in the %-range in the event of abrasion. At the conventional moisture probes so far, the measuring field lines had to pass through the ceramic plate before entering the sand and gravel. Even the slightest abra-sion lead to a change of the measuring field and, depending on the moisture content, generate measuring errors in the %-range.

At the SONO radar technology, an innovative probe design featuring a centrally located conductor foil on the extremely robust and stable ceramic cover ensures that the field line strength does not change in spite of abrasion. An auto-correction function in the SONO probes ensures for consistent reliabilities and maintenance cycles. Only this innovative invention made the long-term stable deployment of SONO radar probes within the construction industry possible in the first place.

2. Requirement: Long-Term Stability at Abrasion. This requirement is of particular significance. What use is a moisture sensor, if it is only able to deliver precise/constant measurements for a short period and delivers faulty values after only a few weeks of deployment?

4. Requirement: The additional Determination of the Cement Content in a Mixture. The radar method deployed in the SONO probes, in addition to the measurement of the moisture, also offers a completely novel method for the determination of the material composition in fresh concrete. The novel mixer probe SONO-MIX delivers two parameters:

P1: Moisture Value P2: Cement Value via the measurement of the RbC

(Radar-based-Conductivity) Currently, there are investigations on how this

novel measuring method could be directly deplo-yed during the mixing process itself, in order to receive additional information regarding the w/c value and the slope of the fresh concrete.

The additional Determination of the Cement Content in a Mixture. The radar method deployed in the SONO probes, in addition to the measurement of the moisture, also offers a completely novel method for the determination of the material composition in fresh concrete. The novel mixer probe SONO-MIX delivers two parameters:

P2: Cement Value via the measurement of the RbC (Radar-based-Conductivity) Currently, there are investigations on how this novel measuring method could be directly deplo-yed during the mixing process itself, in order to receive additional information regarding the w/c value and the slope of the fresh concrete.

With the SONO probes, IMKO is presenting a new generation of moisture probes for the construction industry. Their secret: the deployment of most modern radar technology.

The novel and innovative radar measuring method, at the SONO probes, in addition to the moisture value, also delivers a radar-based guide value (RbC– Radar based Conductivity). This value allows for important conclusions to further parameters of a material recipe, e.g. the cement content or the slope of fresh concrete. Consequently, it represents a significant contribution to the prevention of cost-intensive quality errors.

Significant cost savings by using state-of-the-art SONO moisture probes!

For plant manufacturers and plant operators:

Elimination of errors, less waste, and lower costs with an optimum use of resources,

considerably less acquisition costs,

easier and quicker to put into operation,

reduced maintenance due to long-term stability without recalibrations,

also usable with recyclingwater,

high security by quality production according to standards

What enterprises from the concrete industry say:

Numerous enterprises from the concrete industry have in the mean time convinced themselves of the advantages of the SONO radar probes. One of these is the company Peterbeton in Karlsruhe which, in regard to their plant control system, does not make any compromises. They are deploying three SONO moisture probes for the precise measurement of the aggregates sand and gravel. As the plant director Herr Kütemeier confirms, „Over the years, we have tested a multitude of moisture sensors. Apart from the SONO probes, no other system has managed to really completely convince us. The feature most astounding to us is the low maintenance expenditure. Whereas in former times we had to recalibrate the sensors every 10 days, we can now fully concentrate on the important matters within our production”.

Abb.: SONO-Probes are measuring the moisture on the discharge and on a conveyor belt.

Concluding Remark :

The general demands made to moisture sensors within the construction industry are very high. Faulty sensors can lead to cost-intensive non-norm-conform concrete. In times in which quality and reliability are consistently gaining in significance, new measurement technologies such as the SONO radar measurement technology represent a decisive contribution to improve quality. Users should however also be clear in regard to the limitations of the individual measuring methods in order to be able to identify possible disappointment in the forefront.

IMKO GmbH Im Stöck 2 Germany-76275 Ettlingen Phone +49-(0)72 43-59 210 Fax +49-(0)72 43-59 21 40 e-mail INFO@IMKO.de www.IMKO.de

www.imko.de

SONO Moisture Sensors for industrial Deployment The sensors‘ decisive lead is generated by the deployment of state-of-the-art TRIME®-Radartechnology

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