Overview
The scope of this research is to clarify whether breaks of reinforcing steel bars can be detected and localised with the magnetic flux leakage method.
Tests are executed to detect and collect typical patterns of the magnetic flux density for common and planned inhomogeneities on one hand and breaks and reductions in cross section on the other hand. With the knowledge of this pattern, the number of misinterpretations can be reduced.
The trials shall give information on which magnets are suitable to magnetise the mild steel reinforcement and which sensors work best to measure the magnetic flux density.
To guide the sensors along the concrete surfaces an appropriate positioning system has to be developed.
Whether existing/available software for data acquisition and interpretation can be used or whether new software has to be developed is also subject to further considerations.
In the long term the research aims at a non-destructive detection of breaks and deficiencies of reinforcing steel bars in bond with concrete.
Intermediate aims are:
- Identification of suitable magnets and sensors and adjustment to the specific conditions of the materials to be examined.
- Recognition and collection of typical patterns of the measured values for bar crossings, splices, bending-ups, etc.
- Development of graphical representations of the measured values, leading to an imaging technique.
- Like this a practical tool for the engineer shall be provided, which enables him on site to make a statement on the existence of breaks or deficiencies.
Following metodology was used in the project:
- Literature review,
- Lab tests,
- Graphical representation of measured value
Funding
Results
For the first tests, the magnetometer was still guided by hand, which impeded an accurate relation between measured value and location. This problem could be solved by design and construction of a measurement vehicle. By combining the magnetometer with a rotation encoder, it became possible to assign an exact position to any measured value.
A specific program was written for control and data acquisition that enables to get a first impression of the condition of the reinforcement already during the measurement.
A further step in the development of the measuring system was the development of software routines to quickly record, process and display the measured values as curves on a screen or a printer. The visualization by means of curves makes sense because like this the condition of the reinforcing bar can be roughly evaluated on the spot.
All further tests have been executed with the described measuring system. A test rig was developed for the primary trials on both, intact and broken reinforcing bars in order to be able to mount them repeatedly in the same position. In this way it could be demonstrated that under equal conditions the measurements are reproducible.
Trials to capture the surrounding magnetic field have shown that the superposition of the fields originating from the magnetized reinforcing bars with those of the environment lead to an offset in the measured values that does not need to be constant over the measured length. This kind of superposition complicates the analysis of the measurements and can sometimes lead to a situation where breaks are not detected in every case.
The execution of the large scale test with the concrete frame that has been built to research fatigue behaviour unveiled both, the potential and shortcomings of the MFLmethod. The large distance between sensor and surface of the specimen that had been determined before the test had started led to the result that not all reinforcement breaks could be detected. The visually detected breaks, however, covering ? of all existing breaks, showed that detection works and provides a good accuracy of localization with deviations of some few centimetres. The main test also showed that the magnetometer used at present allows only slow movements during measurements.
Technical Implications
The non-destructive examination plays an ever greater role in the condition assessment of existing reinforced concrete structures. With a change of use or change of effects on the structure, it is important for the engineer as well as the client to know what adjustments are necessary to the building in order to continue to meet the requirements.
The development and deployment of a nondestructive measurement system for the detection of rebar fractures, which makes use of the properties of magnetic stray fields to use, added the engineer practically working in a position to make precise statements about the state of reinforcement of a component. The client may be proposals for repair work specifically with this information. The otherwise for costly investigations, such as Inspection Windows etc., applied financial resources can be used more effectively for repairs.
As the example of the effective traffic growth and the increase in the permissible axle loads shows many bridges are subjected to loads, which could not be predicted at the time of project planning. A most accurate state detection of the reinforcement is therefore desirable. As already mentioned, are probably older buildings be repaired more frequently in the future, which means that reinforcement plans must be matched with the reinforcement layout actually exported. The stray magnetic field method is then an adequate means for this task because it can still detect the position of the reinforcement and the state for the here discussed issue.
The aim is the development and the establishment of a measurement and evaluation system be, which can be used by trained engineers in practice.