Abstract.A new remote nondestructive inspection technique, based on thermoelastic temperature measurement by infrared thermography, is developed for detection and evaluation of fatigue cracks propagating from welded joints in steel bridges. Fatigue cracks are detected from localized high thermoelastic temperature change at crack tips due to stress singularity under variable loading from traffics on the bridge. Selfreference lock-in data processing technique is developed for the improvement of signal/noise ratio in the crack detection process. The technique makes it possible to perform correlation processing without an external reference signal. It is very difficult to detect through-deck type fatigue cracks in steel decks by the conventional NDT technique, since they are not open to the inspection. In this paper, self-reference lock-in thermography is applied for detection of through-deck type fatigue cracks. Experiments are carried out to steel deck sample, which simulates an actual steel bridge, during crack propagation test. It is found that significant stress concentration zone can be observed near the crack front, which enabled us to detect through-deck type fatigue cracks and to estimate its size.

Synopsis: By analysis of the data of periodic inspections of national highway bridges in Japan, the actual condition of concrete members and the characteristic of degradation were clarified. Many concrete girders have a cracked skin, which is conjectured to have occurred soon after the completion of construction. Moreover, the data showed that the kind of cracked skin that tends to occur differs for pre-tensioning girders and post-tensioning girders. Furthermore, it became clear that degradation speed differs for differing forms of the component section. Multiple linear regression analysis was conducted using many form-related items, such as the environment conditions, the bridge types and the applied design criteria. Notwithstanding, the results did not allow identification of the dominant factor that influences the degradation characteristics.

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In the last decade, the Ministry of Land, Infrastructure, Transport and Tourism of Japan has inspected 24 000 highway bridges with a new ‘segmental’ data-recording protocol for bridge maintenance. The data-recording units (‘segments’) are subdivisions of structural components and elements, each containing information on the extent of 26 designated types of damage. This paper analyses the deterioration characteristics (such as average deterioration trend equations and variabilities) of steel I-girder highway bridges. Inspection data show that the new protocol can express the difference in deterioration states evolving within the same element, such as girder ends and span centres for girders, and the influence of the simultaneous existence of different damage types on the rate of deterioration, for example that efflorescence seems to accelerate the development of reinforced concrete deck fatigue. The data also show that deterioration trend curves sometimes mislead data-driven management due to uncertainty in damage extent probability distributions.

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