This technique enables to take a photograph showing details of the inside of a concrete member. It is used to determine the location and size of reinforcement, to check for the existence of voids and areas of poor compaction in concrete where other NDE tests methods are not suitable, such as massive old concrete structural members when being assessed for structural safety. It is also used for checking for voids in the grouting in prestressing ducts.
BS 1881: Part-205 gives detailed information / recommendation for radiographic inspection of concrete.
This method uses sources of gamma rays for concrete upto about 500 mm thick and above this thickness, the standard recommends the use of high energy X-rays.
Due to the inherent danger of using gamma rays and high energy X-rays, the use of this method is confined to investigation which justify the cost of the special precautions which have to be taken.
Impact – echo Test:
Most often non – destructive evaluation technique based on use of transient stress waves, has been used for detecting flaws in many different types of structures including plate like structures such a bridge decks, slabs and walls beams and columns, layered structures and hollow cylindrical structures. To detect hidden damage and determine the extent of damage inside a concrete cross section (delamination, honey-comb, cracks etc).
Dynamic Testing of Structures:
The dynamic characteristics of structures depend on its stiffness and mass. Frequency of a structure gives a good indication of deteriorating or improvement of its stiffness. The dynamic testing of a structure before and after repairs will give an idea of the deterioration of structures and effectiveness of the repairs. The different techniques available for dynamic testing of structures are following:
- Frequency Shift: The method is base on the comparison of the natural frequency of the damaged structure with the undamaged structure. In the absence of the knowledge of the natural frequency of the undamaged structure an estimate has to be based on theoretical computations. Care should be taken for the non-structural member on the frequency. The frequency being a global property of a structure, this method can not be used for locating the damage.
- Mode – Shape Change: The change in mode shape is more sensitive to damage than the frequency. The damage location using mode shape can be done without prior knowlefge of the undamaged structure or the theoretical calculations. An alternative to mode shape is to use derivatives of the mode shapes such as curvature.
- Change in Dynamically Measured Flexibility: The flexibility matrix of a structure can be synthesized using the frequency and mode shape in various modes. The flexibility matrix of the damaged structures compared with that of undamaged structure can indicate the damage. The same comparison before and after the repairs will indicate the effectiveness of repair.
In addition to the above mentioned techniques a few techniques are available for continuous /periodic health monitoring of a structure. The problem with dynamic testing is the absence of the knowledge of the undamaged structure. However, the testing can be very useful in assessing the effectiveness of repairs, as the testing can be done before and after the repairs.