Abstract. This paper deals with the use of embedded piezoelectric transducers to monitor the ultrasonic P -wave velocity evolution during the setting and hardening phases of concrete since casting time. The main advantage of the technique is the possibility to overcome the limitations of traditional methods which do not allow to apply specific mechanical boundary conditions during the measurement. The embedded transducers are based on the "Smart Aggregates" concept previously developed at the University of Houston, Texas. Two piezoelectric transducers are embedded in a prismatic mold and the evolution of the P -wave velocity is recorded for the first 24 hours in concrete after casting time. The results are very promising and show a good agreement with classical ultrasonic tests using external transducers.Confidential: not for distribution.
Abstract.On-line Damage Detection is of high interest in the field of concrete structures and more generally within the construction industry. The current economic requirements impose the reduction of the operating costs related to such inspection while the security and the reliability of structures must constantly be improved. In this paper, non destructive testing is applied using piezoelectric transducers embedded in concrete structures. These transducers are especially adapted for on-line ultrasonic monitoring, due to their low cost, small size and broad frequency band. These recent transducers are called Smart Aggregates (SMAGs). The technique of health monitoring developed in this study is based on a Ultrasonic Pulse Velocity (UPV) test with embedded ultrasonic emitter-receiver pair (pitch-catch). The damage indicator focuses on the early wave arrival. The Belgian company MS3 takes an interest in evaluating the quality of the concrete around the anchorage system of highway security barriers after important shocks. The failure mechanism can be viewed as a combination of a bending and the failure of the anchorages. Accordingly, the monitoring technique has been applied both on a three-points bending test and several pull-out tests. The results indicate a very high sensitivity of the method which is able to pick-up the crack initiation phase and follow the crack propagation over the whole duration of the test.
This paper deals with the use of embedded piezoelectric transducers for ultrasonic monitoring of cracking in concrete. Based on the previous developments of our research team on that topic, we design a new data acquisition system which is able to interrogate the emitter-receiver pair up to 150 times per second. The system is based on low-voltage actuation (up to 20 Volts) and the signal-to-noise ratio is excellent due to the use of a voltage amplifier at the receiver side and the possibility to perform averages. With such a high measurement rate, we are able to follow brittle failure events such as the failure of a concrete cylinder in compression, which is the application example presented. In this application, we show that, in addition to the ultrasonic active monitoring of cracking, the system is also able to record the passive acoustic emission events which can be used as a complementary indicator of damage in the cylinder. We also demonstrate that because of the high level of stresses in compression, the damage indicator defined in our previous studies is not suited for crack monitoring due to the elastoacoustic effect. The amplitude of the first wave arrival is shown to be a robust alternative damage indicator allowing to follow accurately the three successive phases of cracking leading to the failure of the cylinder.
The design of concrete structures is based on calculation rules, which often do not take into account the very early age behaviour of the material. However, during this period, structural concrete is subjected to strains due to the hydration process of cement. If these strains are restrained by concrete itself or surrounding boundaries, stresses start to build up that can lead to the formation of cracks. Among the parameters involved in the stress build up, the stiffness evolution is of major importance. This paper reports the use of eight different techniques aimed at 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 equivalent age. Both the loading stress rate and amplitude are observed to have an effect of limited importance on the determination of the quasi-static elastic modulus, which might be explained by very short term creep. Ultrasonic measurements provide values of E-modulus that are higher than the values provided by the quasi-static tests at the time of the concrete setting. Similar mechanisms associated to very short term creep could explain the difference between the quasi-static and high-frequency elastic modulus.
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