• the detection and identification of the defects that exist in this particularly heterogeneous material. They result from mechanical and/or chemical aggressions.

• the characterization of the concrete from its early age to its maturity along with the control over its ageing.

These objectives justify the implementation of many techniques of control. Correlation between experimental measures and values of resistance to compression have often been tried out. The ultrasonic wave propagation in particular is a technique that allows a sounding of a sample or a structure. The temporal measurements of the speed and attenuation of the waves flowing in the material are used industrially after calibrating. The Fast Fourier Transformations have recently made it possible to complete the temporal signature by spectral signature and have paved the way to new prospections.

The two works we are presenting are based on the spectral modification of the waves propagation due to the scattering behaviour of the concrete:

1. Following in real time the setting of roller compacted concretes (RCC's): this is not made possible for some quick-setting kinetics RCC's from the measurements of the transmitted waves velocities. The analysis of the power spectral density (PSD) of the transmitted signal on a limited bandwidth and the visualization of its variations as a function of the setting time under the form of a two-dimensional image enhance a new ultrasonic indicator. By correlation with mechanical resistance tests and SEM observations, we define the setting time for different types of RCC's in the case of multi-layered structures.

2. Studying the alkali-silica reaction: the frequency spectrum of ultrasonic signals may be affected by the making of particular gels into concrete. It is the case during alkali-silica reactions. The microstructure modifications being small in size, that kind of damaging mainly affects high frequencies. We are studying it by the processing of the backscattered waves and of the transmitted signals through a sample and we are trying to characterize it.

The concrete is thus considered as a spectral filter evolving with the variable natures and sizes of the constituents and of the defects. Spectral analysis has become the measurement tool.