Choosing the right concrete sensor

Choosing which concrete sensor to buy can be a challenge. Multiple solutions are offered on the market and their advantages and disadvantages remain unclear. In this article, we are going to summarize some of the aspects that you should consider when looking for a concrete monitoring solution. Things such as:

How easy is it to install?

Every concrete sensor has a slightly different installation process. One major difference between the hardware is if they use wires (Wired sensor solution) or not (Wire-free) to monitor the temperature inside the structure.

Wired sensor solutions are typically either an industry-standard thermocouple or a proprietary sensor with internal memory. The difference between these is that thermocouples do not have an integrated memory. Therefore, thermocouples need to be connected to a data logger, to store the data, or a data transmitter, to transmit the data to the cloud, to provide the measurement data.  The sensor with internal memory, as the name indicates, has the capacity to measure the temperature history and store it until the data is collected manually on-site.  The installation for wired sensors generally requires attaching the wires to the structure (e.g. rebar) and connecting the wires to the data loggers or data transmitting device.

Sensor with internal memory

Wire-free sensor solutions combine the sensor, internal storage, and the transmitting electronics in a single device. These are usually attached to the rebar and are fully cast into the concrete.

How many times can you (re-)use them?

Reusable components

When using a thermocouple-solution, this is connected to either a data logger or data transmitter, only the economical thermocouple is embedded in the concrete. The remaining electronics, either the data logger (to store the data) or the data transmitter (to send the data), stay outside of the concrete and can be reused over and over again.

Reusable components

The sensors with internal memory need to be connected to a logger reader. In every monitoring, you lose the sensor cable, as this cannot be reused, but the logger reader can be used multiple times.

Non-reusable components

Wire-free sensors are not reusable as all the electronics are combined in one device and everything is cast into the concrete. Therefore, you lose the complete hardware for every monitoring you do.

How can you collect and access the data?

Depending on the technology used in the solution, the data can be transferred manually (with a handheld device, such as a data logger or a smartphone USB cable or Bluetooth) or automatically (with GSM or LPWAN).

When using rather traditional data loggers or logger readers, you manually collect the data by plugging a USB cable or connecting to the device via Bluetooth. This method will require you or someone on the team to personally get to every sensor and obtain the data.

Wire-free sensors

When using wire-free sensors, you have to get close to the sensor location and connect to each monitoring sensor via Bluetooth to collect the data. Therefore, you will gain the advantages of a wireless connection, but being present on-site is still a requirement.

Automatic data transmitters

When using automatic data transmitters, they send the data continuously to the cloud or to another storage device using Wifi, GSM, or an IoT network. The transmitters do not store the data, therefore they require an active network connection in order to work properly. With this option, the data will be available from anywhere – with no need to be on-site or close to the sensor to collect measurements.

Do you need to make the calculations manually?

Traditional data loggers are able to record the temperature history. However, the measurements are transferred to a computer, and the maturity and the strength calculations need to be done manually in a calculating software like Excel. Below, you can see one of the maturity functions, the Freiesleben Hansen & Pedersen (Arrhenius) function, used to calculate maturity:

${t_{e}=\sum_0^t{e} ^{\frac{-E}{R} \left( \frac{1}{T}\frac{1}{T_{r}} \right)}\Delta t\ }$
\begin{align*} { t_{e}=} \qquad &\text{the equivalent age at the reference temperature}\\ \text{E =} \qquad &\text{activation energy}\\ {R =} \qquad &\text{universal gas constant (8.314 J/mol-K)} \\ {T=} \qquad &\text{average temperature during Δt in Kelvin} \\ {T_{r}=} \qquad &\text{reference temperature in Kelvin} \\ {\Delta t =} \qquad &\text{the time interval}\\ \end{align*}

More advanced solutions, such as Maturix, automatically calculate the maturity and strength based on the recorded temperature history and display the results in a software platform. This speeds up the process as the strength estimations are displayed in real time and accessible from anywhere, with no need to be on-site.

How much does it cost?

The overall cost for concrete monitoring with sensors differs a lot depending on the choice of the specific solutions and the number of monitorings. Some sensors are sold as stand-alone units while others are a part of an end-to-end system combining hard- and software. It makes sense to evaluate your specific needs, such as the number of monitorings that you plan to do or the importance of real-time and location independent data access. Another important aspect when choosing a concrete monitoring sensor solution is the sensor setup: Do I lose one complete device per monitoring or just a sensor cable? Gauging these aspects helps you to choose the right concrete monitoring solution with a cost-benefit-calculation, according to your needs.

If your project is small and you just need to monitor in a few positions, manual data loggers or stand-alone wireless sensors are probably your best choice. With manual data loggers, you can only get the raw data from the logger by being on-site and you need to do the calculations yourself. If you do not want to do the (sometimes quite complex) calculations yourself, stand-alone wireless sensors are a good choice. These are fast and easy to install and do not require additional infrastructure.
To collect the data, you need to be on-site to get the data from each sensor (if it uses Bluetooth max. 8 m to the sensor) and the maturity and strength are automatically calculated in the software.

However, if you have a large project, where many monitorings are required, a dedicated solution (hard- and software) with reusable components will be the most cost efficient choice. Thermocouples connected with transmitters are a solution that offers a low cost per monitoring as the transmitter can be reused, and the only part remaining in the concrete is the industry-standard thermocouple. Therefore, this solution will allow you to use the same transmitters over and over again instead of losing one full device per monitoring and thereby achieve an extremely low cost per monitoring.

Summary

Installation​

The concrete sensors can be broadly classified in:

Wired sensor solutions

Wired sensor solutions, either using thermocouple sensors or custom-made sensors with internal memory, are attached to the rebar and connected to a data logger or transmitter. The sensors are then embedded into the concrete while the data logger or transmitter stays outside the concrete.

Wire-free sensor solutions

Wire-free, these sensors combine the sensor, internal storage, and transmitting electronics in one single device. These are usually attached to the rebar and are fully cast into the concrete.

Reusable components

When using wired sensors, the data logger or transmitter stays outside the concrete which means that these can then be reused again and again.

Non-reusable components

When using wire-free sensors, all the components, which are contained in a single device, are embedded in the concrete. Therefore, you lose one device in every monitoring.

Data collection

When using data loggers, you or someone on the team needs to personally get to every sensor and obtain the data manually.

When using wire-free sensors, to get the data you need to be on-site to connect to each sensor via Bluetooth (distance max. 8 meters).

When using wireless transmitters, the data is available from anywhere, as the measurements are sent to the cloud continuously – with no need to be on-site to collect measurements.

Cost

When choosing the right solution for concrete monitoring, in-depth research pays off – as the multiple solutions offer different benefits and have advantages and disadvantages and there is not a “one fits all” solution. Evaluate how many monitorings you’ll need to do in total and simultaneously, how important real-time insights and wireless data access from anywhere is for you – and of course, what your budget is.

Maturix - most cost-efficient solution for multiple monitorings

Maturix is an end-to-end solution providing real-time insights with access from anywhere using an IoT network. As the solution uses thermocouples and connects them to wireless data transmitters, it combines modern IoT technology with cost-efficient thermocouples. This leads to a low cost per monitoring while having all the comfort of automatic analysis with rich features such as alarms, easy data sharing and creation of customizable reports.

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