Steel detection in reinforced concrete wall by microwave infrared thermography

• Implementation of an infrared thermography method with a new microwave excitation system.
• A method allowing to detect vertical steel reinforcements in the reinforced concrete structures.
• Determination of the bar number and spacing in a reinforced concrete wall.
• An excitation system accessible to in situ inspections.

This paper presents a NDT by infrared thermography with a microwave excitation system. The advantages of such stimulation lie in the volumetric absorption of incoming waves which lead to a greater sounded depth. The device generating the microwaves was made of a commercial magnetron 800 W at 2.45 GHz associated with a pyramidal horn antenna. This method was applied to detect the steel reinforcements (deformed bar 12 mm of diameter placing at every 10 cm) in a concrete specimen (1 m×1 m×6.5 cm) with a concrete cover of 3.8 cm. The specimen was heated with an average power of 600 W for 5 min. An infrared camera was placed on the same side as the stimulated surface and thermograms were recorded at regular intervals. The whole assembly was placed in a protective enclosure against high frequency electromagnetic fields. The method based on a contrast algorithm was used to analyze the thermogram series. The result shows a higher temperature rise in front of the steel reinforcement areas. The effect of microwave caused an increase in moderate temperature does not lead to alteration of inspected concrete.