Marc Goueygou

Detection of chemical damage in concrete using ultrasound

This research deals with a non-destructive method for characterizing the degraded cover of concrete structures using high-frequency ultrasound (0.5-1 MHz). Although such a frequency range is unusual in civil engineering, it is well suited to the kind of defect to be detected, as it corresponds to a thin near-to-surface layer with increased porosity and density of microcracks. In order to assess the feasibility of detecting concrete cover degradation, velocity and attenuation measurements were made on both halves of a concrete slab. One half was immersed into an acid solution for 15-45 days, while the other half remained sound. These measurements were made for longitudinal, transverse and surface waves. The results obtained show a 23% decrease of ultrasonic pulse velocity and a 1000% increase of attenuation in the degraded material relative to the sound material. It is thus possible to detect and characterize concrete cover degradation using high-frequency ultrasound. Although attenuation measurements in heterogeneous media are difficult, their sensitivity to degradation is very high.

Non destructive evaluation of degraded concrete cover using high-frequency ultrasound

In this paper, the feasibility of detecting and characterizing concrete cover degradation by ultrasonic testing is investigated. The on-site nondestructive evaluation of concrete cover is vital to monitor the integrity of concrete structures and prevent irreversible damage. Given the thickness of the degraded layer - which can be as small as a few millimeters - the sensitivity of ultrasound propagation to cover degradation is expected to increase with frequency. Therefore, high-frequency ultrasound (0.5 to 1 MHz) is used. In order to assess the feasibility of detecting concrete cover degradation, velocity and attenuation measurements were made on both halves of a mortar slab. One half was chemically degraded, while the other half remained sound. The pulse velocity and the attenuation coefficient were measured on both halves for compression, shear and surface waves. After 45 days of degradation, up to 24% velocity decrease was observed, while attenuation increased up to 10 times. High-frequency ultrasound is thus able to detect changes in the microstructure of the concrete cover even at an early stage of degradation.

Measurement of ultrasonic attenuation and Rayleigh wave dispersion for testing concrete with subsurface damage

This paper describes a non-destructive method for characterizing the cover of concrete structures using high-frequency ultrasound (0.5 to 1 MHz). Although this frequency range is unusual for such a material, it is well suited to the kind of defect to be detected, i.e. a thin damaged subsurface layer. This research has been carried out in three directions: (i) characterization of concrete samples through measurement of porosity and ultrasonic pulse velocity, (ii) adequate signal processing to access the extremely high attenuation to be measured and (iii) testing of concrete samples using Rayleigh waves. The tested samples have been submitted to chemical degradation. Measured velocities of compression and shear waves are used to derive estimates of elastic moduli. Porosity measurements have also been performed, showing that the observed velocity and stiffness decrease are related to an increase of the damaged layer thickness, not to an increase of porosity in this layer. Furthermore, a frequency domain system identification approach is used to derive an estimate of ultrasonic attenuation from multiple transmitted signals. Finally, high-frequency Rayleigh waves are generated into mortar samples by the wedge method. Several transducer/wedge combinations are tested and the optimal configuration is used to yield dispersion curves. Rayleigh wave dispersion evolves as expected with increasing damage and gives access to depth-dependent characteristics of the degraded layer.

ASSESSMENT OF DETERIORATED CONCRETE COVER

ABSTRACT Many reinforced concrete structures are suffering from deterioration occurring earlier than their expected service life. The first barrier against attacks of external environmental agents is the concrete cover. The damage mechanisms depend on the microstructure of the cover concrete. This microstructure can be assessed with various measurements. In the present research, concrete and mortar were artificially deteriorated by acid attack. Samples were attacked during different time inducing different depth of deterioration. Different parameters were measured: deterioration depth, velocity and attenuation of compressive, shear and surface high frequency waves, porosity, elastic modulus, compressive strength, water absorption. These measures are conducted on deteriorated layer, or sound layer, or on a combination of the both layers. The depth of deterioration is well correlated with the acoustic velocities decrease for the three types of waves. The high-frequency ultrasonic wave is thus able to detect changes in the micro-structure of the concrete cover. The surface wave seems to be the most appealing for concrete cover evaluation. The correlations between ultrasonic parameters, and civil engineering parameters like porosity or elastic modulus or compressive strength are evaluated in order to perform the correct interpretation of ultrasonic measurement. The distinctive characteristics of each layer must be deduced from homogenized values measured on multi-layer samples. For that series or parallel models have been successfully used.