## 3 Research Methodology
### Sensor Module Design and Application Development  
This study used three different sensors to collect humidity and temperature data from the tested concrete. The first sensor was an SHT21, which can collect both temperature and humidity information and produce digital output according to an I2C communication protocol. This sensor has a 14-bit resolution for temperature and 12-bit resolution for humidity. Its operating range is \(-40\)\({}^{\circ}\)C to 125\({}^{\circ}\)C for temperature and 0% to 100% for humidity. The Ds18b20 sensor can measure temperatures between \(-55\)\({}^{\circ}\)C and \(125\)\({}^{\circ}\)C. It operates based on a one-wire communication protocol connected to a board via  
Figure 1: Overview of research methodology. three wires (i.e., VCC, GND, and Date). This sensor has a nominal error of \(\pm\)0.5 \({}^{\circ}\)C in the temperature range of \(-\)10 \({}^{\circ}\)C to 85 \({}^{\circ}\)C and an error rate of up to \(\pm\)2 \({}^{\circ}\)C in the rest of its operating range [51]. The third sensor was an LM35, which can produce the analog output. The voltage output increases linearly by 10 mV with each 10 \({}^{\circ}\)C increase in temperature. It has an error of \(\pm\)0.5 \({}^{\circ}\)C at 25 \({}^{\circ}\)C. After connecting the LM35 to a micro-analog/digital converter, the voltage outputs can be turned into temperature data with 12-bit resolution [29]. The combination of these three sensors made the proposed system capable of monitoring temperatures ranging between 0 \({}^{\circ}\)C and 80 \({}^{\circ}\)C while the concrete cured. High-performance sensors capable of indicating high or low temperatures between \(-\)100 \({}^{\circ}\)C and 300 \({}^{\circ}\)C were not required for this research. The sensors used in this study have a relatively high rate of precision with regards to temperature (\(\pm\)0.5 \({}^{\circ}\)C); the level of accuracy required is at least \(\pm\)1 \({}^{\circ}\)C to determine the concrete's maturity. Also, these sensors resist changes to the properties of the concrete that occur during hardening; previous studies have also used these sensors for similar purposes. The hardware module consisted of data acquisition, data storage, and wireless transmission circuits. The module stored the retrieved information on an SD card mounted on the board. The data were processed by software designed with a.NET framework, using Android for visualization of the server or mobile device. This small module was equipped with a self-designed ad-hoc board appropriate for concrete formwork on construction sites. The module program was developed using Cube software (see Figure 2a). The microcontroller employed on the board was a STM32F130 RET6, which is a 32-bit microcontroller from the Cortex M3 family. The microcontroller had an operating frequency of 72 MHz and enjoyed very high processing speed and precision. The IEEE 802.11 (Wi-Fi) topology and, specifically, an HLK-M35 module were used to establish a connection with the computer or smartphone. The module used the TCP/IP operating protocol and universal asynchronous receiver/transmitter communication protocol to link with the microcontroller. Figure 2 shows the HLK-M35 communication module setup (see Figure 2b) and the configuration of the microcontroller connection (see Figure 2c). The Wi-Fi module was connected to the microcontroller through the serial port, which was specified on the PA0 to PA3 bases. Two analog inputs were inserted into PA5 and PA6 to connect the sensors. A one-wire protocol was used on the PA9 base to connect the DS18b20 sensor. The SDIO protocol developed by STMicroelectronics was connected through the PC to PC12 bases for connection with the SD card. Pull-up 47K resistors were used to reduce noise. Figure 2: Sensor module program and design. The data logger system was designed in Visual Studio with C#, and the DevExpress software suite was used alongside to improve the user interface (UI) design.