Takao Harada

Performance of a BOTDR optical fibre sensing technique for crack detection in concrete structures

Strain distribution measurements and problematic crack detection are important issues in the damage detection and performance evaluation of concrete, or reinforced concrete, structures. In recent years, the Brillouin Optical Time Domain Reflectometry (BOTDR) based optical fibre sensing technique has attracted great attention as a distributed monitoring method. Current BOTDR instruments are suitable for strain measurements over a certain distance (termed spatial resolution), but damage such as cracks in concrete structures are local. It is crucial to find an effective method to detect local damage in concrete. In this study, two basic optical fibre installation methods, overall bonding (OB) installation and point fixation (PF) installation, are proposed. Then, several unique installation methods (one-round, one-round superposition and two-round superposition) are proposed and investigated experimentally for a reinforced concrete bending beam. The efficiency of the proposed installation methods and the effect of the length of the sensing region on the measurement accuracy are also discussed. Experimental results show that the n-round superposition installation method can effectively and correctly detect the total crack width within a relatively local region. The performance of the overall bonding and point fixation installation methods with different sensing region lengths, or gauge lengths, for local crack initiation and total crack width measurement is also discussed.

A Soft Post-Earthquake Damage Identification Methodology Using Vibration Time Series

A neural-network-based post-earthquake damage identification methodology for smart structures with the direct use of vibration measurements is developed. Two neural networks are constructed to facilitate the process of post-earthquake damage identification. The rationality of the proposed methodology is explained and the theory basis for the construction of an emulator neural network (ENN) and a parametric evaluation neural network (PENN) are described according to the discrete time solution of the structural state space equation. An evaluation index called the root mean square of the prediction difference vector (RMSPDV) is presented to evaluate the condition of different associated structures. Based on the trained ENN, which is a non-parametric model of the object structure in a healthy state, and the PENN that describes the relation between structural parameters and the components of the corresponding RMSPDVs, the inter-storey stiffness of the damaged object structure is identified. The accuracy, sensibility and efficacy of the proposed strategy for different ground excitations are also examined using a multi-storey shear building structure by numerical simulations. Since the methodology does not require the extraction of structural dynamic characteristics such as frequencies and mode shapes from measurements, it has the potential of being a practical tool for health monitoring of smart engineering structures.

Electrical and Mechanical Characterization of Hybrid CFRP Sheets

This study brings forth the electrical and mechanical models of a new kind of hybrid carbon fiber-reinforced polymer (HCFRP) composites consisting of several types of carbon fibers with different strengths and Young’s moduli. Then, an experimental program with some main variables including the carbon fiber types and their volume fractions has been carried out. Six types of specimens have been fabricated to investigate the mechanical properties and self-health monitoring function of the HCFRP sheets with different types of carbon fiber and volume fractions. The experimental results indicate that the hybridization of several types of carbon fibers is an effective method for upgrading the mechanical properties of the HCFRP sheets. The electrical resistance of the HCFRP sheets changes with the strain in a stepwise manner due to the gradual macro-fracture of different types of carbon fibers. The number of steps is related to the number of carbon fiber types. It has also been shown that the ruptured carbon fibers can still contribute to the current conduction through transverse electrical conduction. The good correlations between the mechanical and electrical behaviors reveal the self-health monitoring function of the HCFRP sheets.

Self-diagnosis of structures strengthened with hybrid carbon-fiber-reinforced polymer sheets

The correlation of mechanical and electrical properties of concrete beams strengthened with hybrid carbon-fiber-reinforced polymer (HCFRP) sheets is studied in this paper. Two types of concrete beams, with and without reinforcing bars, are strengthened with externally bonded HCFRP sheets, which have a self-structural health monitoring function due to the electrical conduction and piezoresistivity of carbon fibers. Parameters investigated include the volume fractions and types of carbon fibers. According to the investigation, it is found that the hybridization of uniaxial HCFRP sheets with several different types of carbon fibers is a viable method for enhancing the mechanical properties and obtaining a built-in damage detection function for concrete structures. The changes in electrical resistance during low strain ranges before the rupture of carbon fibers are generally smaller than 1%. Nevertheless, after the gradual ruptures of carbon fibers, the electrical resistance increases remarkably with the strain in a step-wise manner. For the specimens without reinforcing bars, the electrical behaviors are not stable, especially during the low strain ranges. However, the electrical behaviors of the specimens with reinforcing bars are relatively stable, and the whole range of self-sensing function of the HCFRP-strengthened RC structures has realized the conceptual design of the HCFRP sensing models and is confirmed by the experimental investigations. The relationships between the strain/load and the change in electrical resistance show the potential self-monitoring capacity of HCFRP reinforcements used for strengthening concrete structures.

Identification of the Causes of Fracture in the Steel Pipe Column of a Pedestrian Bridge

A serious fracture, extending halfway through the circumference of a steel pipe column, was discovered in a pedestrian bridge across a prefectural road in Japan. This paper presents the examination of the causes of the fracture and its retrofitting. In order to identify the causes of the fracture, observation of the crack surface, SEM observation, material property tests and measurement of strain were carried out. According to observation of the crack surfaces, the cracks originated from the toes of the boxing welds at the end of the rib plates. The failure surface was covered with a thick layer of corrosive products, but a large section of the crack surface in the fractured area was estimated as brittle failure and there was no evidence of cracks due to fatigue. However, the crack surfaces near the boxing welds on the opposite side of the fractured area were evaluated as having being caused by fatigue failure. The results of the measurements of stresses demonstrated that a relatively high stress range was caused near the boxing welds, when several people ran across the pedestrian bridge. The base metal was found to have very low Charpy-absorbed energy. The fractured steel pipe column of the pedestrian bridge was retrofitted by applying bolted splice rings and plates.

OPTIMIZATION SYSTEM OF MAINTENANCE STRATEGIES FOR ROAD PAVEMENT BY CONSIDERING FLEXIBILITY OF ALTERNATIVES

Maintenance strategy might be changed in future due to uncertainty included in cost of maintenance and operation, degradation of infrastructure, budget, change of social system, technology innovation and so forth. The present value (PV) method commonly used for alternative evaluation cannot consider uncertainty that is expected to occur in future. On the other hand, real options (RO) method is new technique that can consider the value of flexibility of alternative. In this study, optimization system of maintenance strategies for road pavement by using real options has been developed. Through the numerical simulations on road pavement, it was confirmed that the alternatives which have flexibility more abundantly were drawn as optimum solution by considering of option values.