Abstract
Abstract A new nonlinear ultrasonic technique for nondestructive evaluation of concrete components is developed and implemented to characterize the effects of carbonation on concrete. The physical principle of this method is the second harmonic generation (SHG) in propagating Rayleigh surface waves which are detected by a non-contact air-coupled transducer. The nonlinearity parameter, as an indicator of material properties, is experimentally obtained from measured Rayleigh wave signals and is used to quantitatively evaluate the progress of carbonation under accelerated conditions. The experimental results show that there is a significant decrease in the measured nonlinearity parameter, most likely originated from the deposit of the carbonation product, CaCO3, in pre-existing voids and microcracks. The sensitivity of the nonlinearity parameter is also verified by comparing with the measured Rayleigh wave velocity. The results in this paper demonstrate that the SHG technique using Rayleigh surface waves can be used to monitor carbonation in concrete.
Highlights
Exposure of in-service concrete infrastructure to the natural environment causes continuous changes in microstructure and composition, often starting from their surfaces
In parallel with the nonlinear ultrasonic measurements which showed a decrease in nonlinearity with carbonation, the destructive characterization using a phenolphthalein indicator solution is performed to track the carbonation depth
nondestructive evaluation (NDE) of the microstructural behavior in the carbonated concrete is achieved by the proposed second harmonic generation (SHG) technique
Summary
Exposure of in-service concrete infrastructure to the natural environment causes continuous changes in microstructure and composition, often starting from their surfaces. Calcium hydroxide (Ca(OH)2) contained in hydrated cement paste reacts with carbon dioxide, present in air or water, forming calcium carbonate (CaCO3) and water (H2O). The gradual process of carbonation alters the surface properties and decreases the pH value of the pore solution from typical values (12–13.0) to a 9 or less [1,2,3] due to the virtually insolubility of the carbonation product (CaCO3) relative to the slightly soluble reactant [4]. Over time in reinforced concrete, as the relatively low-pH ‘‘carbonation front’’ reaches greater depths from the surface, depassivation of the reinforcement steel can occur and corrosion can initiate. Corrosion of reinforcement is considered to be a serious durability concern which limits the service life of reinforced concrete structures [7,8,9]
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