# Cost-Effective Temperature Sensor for Monitoring the Setting Time of Concrete
## 2 Materials and Methods  
Four mixtures were prepared with different percentages of cement substitution by zeolite (15% and 25%) and two reference cements (CEM I and CEM II). In addition, 20% of the coarse aggregate was replaced by recycled concrete aggregate in one of the samples to evaluate whether there were significant differences in the performance of the concrete. Nine specimens were prepared for each of the mixtures following the UNE-EN 12390-2 standard of 2020 [34]. The specimens were cured in water inside a humid chamber to ensure the control of environmental factors such as temperature (20 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C) and humidity (\(\geq\)95%). Of the nine specimens, three were used for each age of breakage (28 and 180 days) and one was monitored for each age. All specimens (with and without sensors) were subsequently subjected to the hardened concrete test to determine their compressive strength as described in the UNE-EN12390-3 standard of 2020 [35]. A total of twelve sensors (one sensor for each mixture and curing age) were used to measure the temperature every five minutes during the setting process. In addition, an environmental sensor was used to measure the water temperature. The dosage used for the preparation of the 9 cylindrical specimens of 150 \(\times\) 300 mm is shown in Table 1.  
From the above dosage, substitutions of different materials were made to obtain the different mixtures as shown in Table 2.  
Table 3 shows the characterization of the composition of the materials used by X-ray diffraction. Particle size analysis of the aggregates used was carried out, with the results shown in Figure 1. \begin{table}
\begin{tabular}{c c} \hline
**Material** & **Quantity (kg)** \\ \hline Cement & 17.2 \\ Fine aggregate & 55 \\ Coarse aggregate & 43.5 \\ Water & 9.7 \\ Superplasticizer & 0.14 \\ \hline \end{tabular}
\end{table} 
Table 1: Dosage for 9 cylindrical specimens. For temperature monitoring, the DS18B20 sensor (Zhiwei Robotics Corp. from Shanghai, China), was used, due to its versatility, simple programming, low cost, and resistance to humid environments. It is a digital Celsius thermometer that provides 9 to 12 bits, with 64-bit memory (equivalent to 8 bytes) to store the unique identifier or address of every sensor and a non-volatile top alarm function programmable by the user. It is compatible with Arduino and has three pins: Vcc, GND, and DQ, which in this case corresponds to the data pin. The sensor can be encapsulated, i.e., sealed in a watertight wrapper, and there is also waterproof probe. The DS18B20 sensors support 3 to 5.5 V power and 9-bit, 10-bit, 11-bit, and 12-bit resolutions (12-bit resolution is the default). The range of temperature values it can measure are from \(-55\)\({}^{\circ}\)C to 125 \({}^{\circ}\)C, with an accuracy of \(\pm 0.5\)\({}^{\circ}\)C from \(-10\)\({}^{\circ}\)C to +85 \({}^{\circ}\)C; outside this range, the variation could be greater. The sensor was made of stainless steel, and was 6 mm in diameter and 30 mm in length. The behavior of the sensor was previously analyzed by the Testing and Calibration Laboratory (LECEM) of the ETSIME-UPM, where the ENAC calibration certificate was obtained. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Sample 1 \({}^{*}\)** & **Sample 2 \({}^{*}\)** & **Sample 3 \({}^{*}\)** & **Sample 4 \({}^{*}\)** \\ \hline CEM II & 100\% & 75\% & & \\ CEM I & & & 85\% & 85\% \\ Zeolite & & 25\% & 15\% & 15\% \\ Fine aggregate & 100\% & 100\% & 100\% & 100\% \\ Coarse aggregate & 100\% & 100\% & 100\% & 80\% \\ Concrete aggregate recycled & & & 20\% \\ Water added (mL) 1 & & 600 & 300 & 800 \\ \hline \end{tabular} \({}^{*}\) Every sample is made up of three specimens named as 1-2-3; 1 For samples with zeolite content, water was added to the original dosage until a consistency similar to that of the reference sample was achieved.  
\end{table} 
Table 2: Dosage of mixtures.