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Active thermal imaging is an effective nondestructive technique in the structural health monitoring field, especially for concrete structures not exposed directly to the sun. However, the impact of meteorological factors on the testing results is considerable and should be studied in detail. In this study, the impulse thermography technique with halogen lamps heat sources is used to detect defects in concrete structural components that are not exposed directly to sunlight and not significantly affected by the wind, such as interior bridge box-girders and buildings. To consider the effect of environment, ambient temperature and relative humidity, these factors are investigated in twelve cases of testing on a concrete slab in the laboratory, to minimize the influence of wind. The results showed that the absolute contrast between the defective and sound areas becomes more apparent with an increase of ambient temperature, and it increases at a faster rate with large and shallow delaminations than small and deep delaminations. In addition, the absolute contrast of delamination near the surface might be greater under a highly humid atmosphere. This study indicated that the results obtained from the active thermography technique will be more apparent if the inspection is conducted on a day with high ambient temperature and humidity.

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Summary A novel concept denoted as unscented Kalman filter with unknown input and weighted global iterations (UKF‐UI‐WGI) to assess health of large structural systems is proposed. It incorporates the basic features of UKF to identify systems in the presence of severe nonlinearity but then added a few desirable features to increase its implementation potential. Because the information on excitation and unknown initial state vector is necessary to implement any UKF‐based approach, a substructure concept is introduced to generate them. The traditional UKF concept attempts to identify small structural systems using large duration of response histories in one global iteration. Because it fails to assess the defective states of large structures in most cases, a weighted multiple global iterations procedure with objective functions using short duration responses is introduced. The superiority of UKF‐UI‐WGI over the traditional UKF is demonstrated with the help of several illustrative examples using single and multiple substructures in identifying both defect‐free and defective states. With the help of the same examples, the superiority of the proposed method over the extended Kalman filter‐based method developed by the team earlier is conclusively documented. With the help of parametric studies, it is documented that the proposed method is robust, accurate, and stable. The study confirms that UKF‐UI‐WGI can identify large structural system using only limited response information measured at a small part of a structure without using any excitation information. The concept significantly advances the state‐of‐the‐art in UKF‐based nonlinear system identification and considerably improves its implementation potential. Copyright © 2015 John Wiley & Sons, Ltd.

A finite-element-based procedure is proposed to identify defects in existing structures and evaluate their extent at the local element level for the purpose of health assessment. The procedure is a time-domain system identification technique where input excitation is not required to identify a structure. It estimates the dynamic properties of a structure in terms of stiffness and damping at the element level in a finite-element representation. The method can be used to precisely locate a defective spot in an element. Although input excitation information is not required, examples are used to show that the algorithm is robust enough to identify a structure excited by different types of loading. Structures can be excited simultaneously by multiple loadings, and the response information can be noise-free or noise-contaminated. Defects can be small or relatively large. In all cases, the algorithm identified the structures correctly. The error in the identification is considerably smaller than that of other available methods where input excitation information is used to identify a structure. With the help of examples, it is shown that the algorithm can potentially be used as a nondestructive evaluation technique for health assessment of existing structures with minimum disruption of operations. Since the procedure is very simple and requires only a few seconds of response information, it is expected to be very economical and efficient.