Stand der Entwicklungen zur zerstörungsfreien Bestimmung der Längsspannung in Bewehrungsstäben von Betonbauwerken

State of Development of the Nondestructive Evaluation of the Longitudinal Stress State of Steel Bars Reinforcing Concrete Structures

Constructional changes of buildings in use have become an important task, in particular in case of reinforced concrete structures. Changes in use of existing buildings require an analysis of the load state of each component, especially of the ceilings. The longitudinal stress in the reinforcing bars is helpful information for the estimation of the remaining bearing capacity. Nondestructive micromagnetic and ultrasonic methods are in use to evaluate the stress state of machinery components already. Their potential to evaluate the longitudinal stress of reinforcing bars is the objective of this paper which also describes adaptations needed for a reliable stress analysis on the site.
Micromagnetic techniques allow for the analysis of surface near layers to a depth of about 1 mm. The influence of stress gradients caused by the manufacturing process has to be studied and limited.
Ultrasonic techniques enable the evaluation of the longitudinal stress in the volume of the bar resulting in a value averaged over the part of the volume penetrated by the ultrasound. Local changes of the elastic material properties along the length of the bar, as they are most likely caused during the manufacturing process remain to be studied.
The on site evaluation of the stress of a reinforcing bar by applying micromagnetic or ultrasonic techniques yield results with unacceptably large errors. Adaptations of the sensors and improvements of the preparation of the measuring positions have more accurate results in prospect.