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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., Fundação para a Ciência e a Tecnologia (FCT)

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.

The feasibility of using air jet pulses to actively control the vibrations of simple flexible structures subjected to harmonic and earthquake-like base excitations is explored. A laboratory experiment has been built to control the vibrations of a structure that is represented by a flexible vertical beam with a mass attached on the top free end, while the other end is attached to a shake table. A pair of opposite jets has been attached to the free end of the structure and pressurized air flow controlled by on-off solenoid electro-valves provides the actuating forces controlled by the acceleration measurements of the structure and the shake table. A Bounded State Control (BSC) algorithm is explored for its effectiveness in suppressing the vibrations of the system. The experimental results demonstrate the effectiveness and reliability of Jet Pulse Control Systems (JPCS). A parametric study is considered to investigate the effect of air pressure, control level and characteristics of the base excitations on the reduction of vibration levels. The vibration levels of the controlled structure have been reduced by 50% and 33% for harmonic and earthquake base excitations, respectively. JPCS appear to be promising control devices for reducing the vibration levels of flexible structures.

This paper investigates the problem of identifying the location, size and orientation of cracks in structures using an array of strain measurements. It presents statistical identification methods and computational tools that are required to identify cracks from strain measurements. It also addresses the experimental design problem related to finding the optimal location, orientation, number and density of sensors for reliable crack detection, along with the computational difficulties involved. The effectiveness of the proposed methodologies is illustrated using simulated data from a single crack in a plate subjected to static loading.