**5. The Description of the Extradosed Bridge**

The research works on the longest span in the European extradosed bridge located in Poland were carried out (Figure 5). The aim of the studies was to determine the compressive strength of young concrete embedded in the bridge's span in real time. Laboratory tests included compression tests of mortar and concrete samples and allowed it to develop a maturity–strength relationship for the considered concrete. Field works were focused on temperature measurements of concrete bridge deck and also on compression tests of validation specimens.

**Figure 5.** View of the object and monitored zone.

The bridge object (Figure 5) was monitored three times. For every considered section, the formwork of the system was made of 3-SO plywood and H20N timber formwork beams spaced at a distance of about 30 cm. The first stage involved monitoring of the bottom slab of the starting section (No. 4.1–2, Figure 6a). The temperature sensors were mounted in two cross-sections (A–A and B–B) of the bottom plate (points p1–p6). The ambient temperature was also monitored (o1, o2). In two cubes and cylinders, the temperature measurements of the concrete (p7–p10) and the measurements of the water temperature (o3) were carried out. Figure 6b illustrates the location of the measurement points.

**Figure 6.** The monitored section No. 4.1–2: (**a**) View and cross section and (**b**) localization of temperature measurement points.

The second stage concerned the overhanging section (No. 4.4) with variable height (Figure 7a). In this case, the concrete temperature in the top slab and the web within the prestressing cables zone was monitored. This segment was concreted in two stages: The bottom plate and webs at first, and a day later, the top plate. The temperature sensors were installed in the inner web (points p1–p7) and in the top slab (p8–p14; Figure 7b). The measurements, with dedicated prefabricated modules consisting of nine sensors (seven for concrete temperature and two for air temperature) were realized. The p7 and p8 sensors were destroyed during the constructions works.

**Figure 7.** The monitored section No. 4.4: (**a**) View and cross section and (**b**) localization of temperature measurement points.

The last, third part of the in-situ research, was dedicated to the overhanging section (No. 3.18) with a fixed height (Figure 8). The entire segment was concreted in one cycle. The temperature sensors were installed in the top plate (p1–p5), the inner web (p6–p8) and the bottom slab (p9–p13). Especially for this monitoring a prefabricated measurement module consisted of 13 sensors was designed (Figure 9b). The temperature in the cylindrical and cubic samples (p14, p15) and air temperature (o16–o19) was also registered.

**Figure 8.** The monitored section No. 3.18.

**Figure 9.** Localization of measurement points—section No. 3.18: (**a**) Global view and (**b**) detailed view.

Especially for field tests, the own system was designed. The recorder makes it possible to measure concrete temperature at 20 points simultaneously and works on water-resistant, digital, 1-wire sensors type ds18b20, which do not need to be calibrated. 1-wire interface requires a single digital pin, and it is possible to connect multiple ones to the same pin, each one has a unique 64-bit ID burned in at the factory to differentiate them. The ds18b20 sensor provides a temperature measurement in the range from −55 to +125 ◦C with an accuracy of ± 0.5 ◦C. The sensor is fairly precise and can give up to 12 bits of precision from the onboard digital-to-analog converter. It is powered from 3.0 V to 5.5 V. Since the sensor is digital, it does not get any signal degradation even over long distances. Measurement points can be assembled using individual sensors or by using a prefabricated measurement strip with a series of sensors. The device can be powered with a battery for about 30 days. Temperature data is transmitted via a GSM modem.
