Abstract
Reinforced concrete structural elements, as an important component of buildings and structures, require inspection for the purposes of crack detection which is an important part of structural health monitoring. Now existing crack detection methods usually use a single technology and can only detect internal or external cracks. In this paper, the authors propose a new sensing system combining BOFDA (Brillouin optical frequency-domain analysis) and FBG (fiber Bragg grating) technology, which are used to detect internal and surface cracks and their development in reinforced concrete structures, and an attempt is made to estimate the width of surface cracks. In these experiments, a special reinforced concrete beam structure was designed by the author for crack detection under load. Four continuous distributed optical fibers are fixed on the steel skeleton, which is located within the reinforced concrete beam. Three FBG sensors are fixed on the lower surface of the beam, near its centre. By analysing the sensor data, it can be found that the BOFDA-distributed fiber can be used to detect internal cracking before surface cracking, and the difference between scans can be used to judge the time of onset of internal cracking, but the relative error in position is about 5%, while the FBG sensor can detect the cracking time of microcracks on the lower surface in near-real-time and can be used to calculate the crack width. Through the experiment, it is found that if the combination of BOFDA and FBG technology is adopted, we can initially use the strain data obtained by multiple groups of BOFDA monitoring to predict the general location of the internal cracks, then to monitor the exact location of the surface cracks by FBG in the medium term, and to estimate the width of the final expansion of the cracks finally.
Highlights
Reinforced concrete structures are widely used [1, 2] due to load and other mechanical factors, and the concrete structure may crack [3, 4]. ese cracks may develop into large cracks that can pose a threat to the safety of the structure with time, so it is necessary to monitor the cracks [5,6,7]. ere are two types of cracks in reinforced concrete beams, surface cracks [8, 9] and internal cracks [10,11,12]
Distributed optical fiber sensing technology for crack monitoring includes OTDR [24, 25], BOTDR (Brillouin optical timedomain reflectometer) [26, 27], BOTDA (Brillouin optical time-domain analysis) [28,29,30], BOFDA (Brillouin optical frequency-domain analysis) [31], and BOCDA (Brillouin optical correlation-domain analysis) [32]. ey can allow continuous monitoring, but their accuracy is low. e pointbased fiber optic sensors used in crack monitoring are mainly fiber Bragg grating (FBG) technology, and a large number of FBG sensors are used for crack monitoring in concrete structures [33,34,35,36,37,38]
A reinforced concrete beam designed by the author is used in the paper, in which distributed fiber and FBG sensors are fixed inside and on the surface, respectively. e positions of these sensors are obtained from force analysis of the beam during loading. rough the step-by-step loading of reinforced concrete beams, the information such as the cracking time, position, and width of internal and surface cracks are calculated according to the data collected using the sensors
Summary
Reinforced concrete structures are widely used [1, 2] due to load and other mechanical factors, and the concrete structure may crack [3, 4]. ese cracks may develop into large cracks that can pose a threat to the safety of the structure with time, so it is necessary to monitor the cracks [5,6,7]. ere are two types of cracks in reinforced concrete beams, surface cracks [8, 9] and internal cracks [10,11,12]. E use of optical fiber sensing for crack detection is a new technology developed in recent years. It has the advantages of real-time monitoring, dynamic data logging, and high precision. E pointbased fiber optic sensors used in crack monitoring are mainly FBG technology, and a large number of FBG sensors are used for crack monitoring in concrete structures [33,34,35,36,37,38]. There is little reported research on the monitoring of cracks in reinforced concrete structures using a combination of distributed (BOFDA) and point (FBG) methods. BOFDA is a distributed technology which can detect the location of cracks in the whole optical fiber area but has low spatial resolution. FBG has a high position measurement precision, and it should be placed in a critical stress location to compensate for the lack of spatial resolution of BOFDA. e authors will try to use both BOFDA and FBG techniques to monitor the cracks at the same time in this paper
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