Installation of thermocouples in precast products

Last update: 03/11/2021
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Before installation

Consider the correct positioning of the thermocouple

In order for Maturix Precast to monitor the temperature, maturity, and strength development of a precast product, a thermocouple must be inserted into the element and then connected to the transmitter. The installation of the thermocouple can happen before pouring the concrete (e.g. reinforced products) or after casting concrete (e.g. prestressed hollow-core slabs).

When installing the thermocouple it is important to consider if the element is influenced by an external heating source such as overhead heating, warm water passing through the table/bed, or other heating solutions. This can influence how the cement reacts with the water in the concrete – and cause certain areas of the element to develop strength faster than others.

Before installing the thermocouple, please consider the position to be representative of the monitoring philosophy you are trying to implement. If this is not done, you may end up with incorrect measurements – not because the result is wrong, but because the position of the thermocouple is not representing what you are trying to monitor.

If you want to measure e.g. the place with the slowest strength development, then this would be at the coldest spot in the element – opposite of the heating source. In this case, the tip of the thermocouple must be positioned inside the element relative to the heat development.

The tip should be positioned and secured at the exact spot where you want to measure the concrete temperature.

The concrete temperature is measured at the tip of the thermocouple. Consider the correct positioning of this prior to installation.
It is important that the cable is positioned correctly to obtain correct measurements.

The installation of type K thermocouples is different depending on the production method. Some concrete elements are made with fluid/self-compacting concrete poured into a mold, where others are made by extrusion of dry concrete.


Installation in reinforced products (before pouring)

Precast products with reinforcement, e.g. walls or columns, are often made with fluid or self-compacting concrete. The easiest way to install the thermocouple is before concrete pouring.

Step 1
Consider the correct positioning of the thermocouple.

Step 2
Find the best route for the thermocouple to get into the measuring point, e.g. through a small opening like a hole or a slit in the mold.

Step 3
Fix the thermocouple to the reinforcement with plastic cable ties or strong tape so it stays in place during concreting operations. During the pouring, a substantial mass will inflict the cable and try to move its position, so ensure that it is fixated to the best of your ability.

Do not use metal rebar ties or alike as they might cause damage to the cables!
The tip of the thermocouple must not touch the reinforcement or the metallic bed, otherwise, the measurements might be incorrect.

Step 4
Shelter the thermocouple from direct impact during pouring by fastening it along the bottom of the reinforcement (also using plastic cable ties or strong tape). If you are using cable ties, cut and discard the excess plastic part not used.

Step 5
Once the thermocouple has been installed correctly, please continue with your concrete pouring processes.

Step 6
When the concrete has been poured into the mold (and potential leveling vibration has been finished), immediately connect the thermocouple to the Maturix™ transmitter and place it at a secure and visible location.

Installation in prestressed hollow-core slabs (after extrusion)

Prestressed hollow-core slabs are made with dry concrete using an extruder. The easiest way to install the thermocouple is after concrete extrusion.

Step 1
Consider the correct positioning of the thermocouple.

Step 2
Use a tool to penetrate the dry concrete and make a small hole (e.g. using a screwdriver/thin metal rod slightly larger than the thermocouple). Consider the right depth of the hole for correct positioning of the thermocouple’s tip, which is where the temperature will be measured, e.g. just above the prestressed wire.

Step 3
Insert the thermocouple into the concrete.

The tip of the thermocouple must not touch a prestressed wire or the metallic bed, otherwise, the measurements might be incorrect.

Step 4
Close the hole thoroughly by pushing the concrete around it to ensure that the thermocouple stays in place.

Step 5
Once the thermocouple has been installed correctly, immediately connect it to the Maturix™ transmitter and place it at a secure and visible location.

Example of positioning in prestressed hollow-core slabs

The tip of the thermocouple must be positioned inside the element relative to the heat development. Also, the tip must not touch any metal. It is important that the thermocouple is positioned correctly to obtain correct measurements.

If you want to monitor the slowest strength development in a prestressed hollow-core slab where curing is accelerated through heating in the bed, it makes sense to position the thermocouple in the end of the element, at the coldest spot, just above the wire (relative to the heated bed).


Video showing installation example in hollow-core slabs

The beginning of the video below shows how to install the thermocouple according to the above example:

Remove monitoring equipment after use

Before removing the thermocouple and transmitter, always evaluate whether the results are valid. If in doubt, please contact your quality manager or another responsible.

ASTM C1074:
“9.5 Before performing critical operations, such as formwork removal or post-tensioning, that are based on estimated strength from the concrete maturity, perform other tests to ensure that the concrete in the structure has a potential strength that is similar to that of the concrete used to develop the strength-maturity relationship.”

Before demolding, you must make sure that the strength prediction is accurate. The maturity calibration used for the strength prediction must be validated by the laboratory.

When you are done monitoring and the target strength has been reached:

  1. Disconnect the thermocouples from the transmitters
  2. Cut the thermocouples close to the surface of the concrete and discard/recycle/reuse the leftover cable part
  3. Clean the transmitters and return them back to the storage
  4. Ensure that there are no visible damages, loose parts, water inside the compartment with the device, or other errors that could affect the operation next time

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