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The aim of the paper is to study the possibility of implementing modal filteringtechniques for damage detection in the presence of non-linearities in the recordedsignals. Initially designed for linear damage detection the method is based on thelinear combination of the sensors responses, a transformation to the frequencydomain, and the computation of peak indicators which are used subsequently in anoutlier analysis process. The efficiency of the method to detect both linear and nonlineardamage scenarios is assessed using data recorded on the three-storey framestructure previously developed and studied at Los Alamos National Labs.Experimental data consists in four acceleration records. Besides the baselinecondition, both linear (mass and stiffness changes) and non-linear (bumper device)changes have been considered. The results obtained using the modal filteringapproach are compared to the ones obtained based on auto-regressive models,considering either the auto-regressive parameters or the time-domain residuals.

Early age cracking of structures often leads to aesthetic problems and service life reduction. Among the parameters involved in the stress build-up that causes this cracking process, the stiffness evolution is of major importance for models and numerical computations. This paper reports the use of six different techniques aimed for stiffness evolution assessment, applied on the same concrete mix, in a round robin experimental test within three laboratories. The observations are compared after having expressed the results at the same maturity. Some of the reported techniques provide original means for Young's modulus monitoring of concrete at early age both for industrial and research applications. Two sets of results emerge. Ultrasonic measurements provide values of Young's modulus much higher than the values provided by the static or quasi static tests at the time of the concrete setting. This difference decreases as the concrete hardens.