Brillouin OTDR The Concrete Crack Detection

Concrete is one of the building materials and is most widely used in world's building construction. The concrete has the characteristic that the compressive strength is great and the tensile strength is very small. Once the stress exceeds the tensile strength which the concrete can bear, the cracks will emerge and reduce the impermeability and anti-fatigue properties of the concrete structures, lead water and harmful substances to infiltrate the concrete, accelerate the steel corrosion and induce the natural aging of concrete, and ultimately damage the using functions and durability of the concrete structures. So the safe of concrete structures will be hazarded because of the cracks. The concrete crack is an important indicator in the safety test of the large concrete structures. Full-line monitoring the concrete cracks has a vital significance for indicating the dangerous in concrete projects, then analyzing the causes and repairing the cracks according to the monitoring results are effective measures for protecting the stability and safe operation of concrete structures.In this paper, the glass fiber reinforced plastic (GFRP) single strain sensing cable is used for the optical signal transmission path and the sensor of the Brillouin OTDR distributed sensing technology. The theoretical analysis and the experimental research related to the measurement of concrete cracks are implemented. The glass fiber reinforced plastic (GFRP) single strain sensing cable is buried in the middle of the reinforced concrete specimen by using attachment method, and the GFRP single strain sensing cable and the longitudinal reinforcements in the concrete specimen are into the 30°,60°,90°. By the temperature and applied stress, the Brillouin frequency of the backscattered light in optical fiber is shifted correspondingly. The main researched work as follows:(1) Unloading tensile test of GFRP single strain sensing cable is carried out, and the strain sensing parameter is 1.04. Bath temperature experiment is done three times, and the temperature sensing parameter of GFRP single strain sensing cable is 0.93.(2) The size of the concrete specimen and the laid line of the GFRP single strain sensing cable in the concrete specimen are designed. The GFRP single strain sensing cable is buried by using attachment method. The relationship between the distributed monitoring line and the concrete specimen is established.(3) The temperature cracks during the process of concrete setting and curing and the stress cracks under the external force after the concrete sets are monitored. The monitoring results indicate that the strain tends to increase and the strain variation is between 520μεand 2200μεin the 24 hours after the concrete specimen has been poured. With the gradual setting of concrete, the strain tends to be stable. After the concrete sets, the strain shift tends to increase and the maximum strain variation (2318με) emerges at the position of 433.8 m as the crack emerged under the external force.(4) The system based on Brillouin OTDR is established for monitoring the concrete cracks. The software for monitoring the cracks in the concrete cushion is designed and developed by using the.NET technology. Through this software, the data about the cracks in the concrete cushion is analyzed and processed. In this paper, the functions of Indicating and monitoring the cracks in the concrete cushion are achieved.