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- Wireless sensor networks for temperature and humidity monitoring within concrete structuresPublication . Barroca, Norberto; Borges, Luís M.; Velez, Fernando J.; Monteiro, Filipe; Górski, Marcin; Castro-Gomes, JoãoThis paper presents the development of an automatic wireless sensor monitoring system for civil engineering structures. The objective is to provide a solution to measure both temperature and humidity inside a concrete structure. The research has been focused in the early age and curing phase period. Four solutions have been addressed. The first one involves the use of a negative temperature coefficient (NTC) thermistor and an IRIS mote allowing for the creation of an IEEE 802.15.4 network. However, the results have shown that the sensor measurements present a 5 !C standard deviation between the actual and the experimental values. The second one considers the use of the SHT15 (humidity/temperature) sensor, together with the PIC18F4680 microcontroller or the Arduino platform. The third solution involves the use of the SHT21S humidity/temperature) sensor and the eZ430-RF2500 wireless development tool platform for the MSP430 microcontroller. In this case, the temperature readings were successfully performed for the first 16 h, while the humidity values were successfully obtained for the first 24 h of the experiment. Although the set of measured values is very promising for the SHT15 and SHT21S sensors, both sensors have stopped working after some time, showing that direct contact between the sensor and the concrete alkaline environment causes its breakdown. Finally, the fourth solution considers both the SHT15 and SHT21S sensors completely shielded allowing for the creation of a long-term solution. As, the SHT15 and SHT12S sensors have not been affected by the alkaline environment for more than two months of operation, enabling real-time and continuous monitoring with almost non-intrusive tiny devices, the potential of applying the proposed inexpensive wireless sensor network approach is verified.
- Carbon Fiber Epoxy Composites for Both Strengthening and Health Monitoring of StructuresPublication . Salvado, Rita; Lopes, Catarina; Szojda, Leszek; Araújo, Pedro; Górski, Marcin; Velez, Fernando J.; Castro-Gomes, João; Krzywon, RafalThis paper presents a study of the electrical and mechanical behavior of several continuous carbon fibers epoxy composites for both strengthening and monitoring of structures. In these composites, the arrangement of fibers was deliberately diversified to test and understand the ability of the composites for self-sensing low strains. Composites with different arrangements of fibers and textile weaves, mainly unidirectional continuous carbon reinforced composites, were tested at the dynamometer. A two-probe method was considered to measure the relative electrical resistance of these composites during loading. The measured relative electrical resistance includes volume and contact electrical resistances. For all tested specimens, it increases with an increase in tensile strain, at low strain values. This is explained by the improved alignment of fibers and resulting reduction of the number of possible contacts between fibers during loading, increasing as a consequence the contact electrical resistance of the composite. Laboratory tests on strengthening of structural elements were also performed, making hand-made composites by the “wet process”, which is commonly used in civil engineering for the strengthening of all types of structures in-situ. Results show that the woven epoxy composite, used for strengthening of concrete elements is also able to sense low deformations, below 1%. Moreover, results clearly show that this textile sensor also improves the mechanical work of the strengthened structural elements, increasing their bearing capacity. Finally, the set of obtained results supports the concept of a textile fabric capable of both structural upgrade and self-monitoring of structures, especially large structures of difficult access and needing constant, sometimes very expensive, health monitoring.