What types of evaluation techniques are available?
Once concerns are raised, the implementation of a proactive evaluation program should be initiated that treats the anchor bolts as critical components of an important asset. Several types of evaluation programs can be assembled which are generally a mix of non-destructive (NDT) and semi-destructive (SDT) techniques that work concurrently to quickly assess the type, quality, and quantity of the tested anchor bolts. Essentially the testing techniques available include:
Each anchor bolt is struck with a 3 lb. maul hammer and the audible resultant tonal variations are noted. For example, a sharp ‘metallic ring’ typically identifies a sound condition relative to the impacted anchor bolt, while a dull ‘drummy’ sound is indicative of an unsound anchor bolt, probably separated/broken along its length.
|Ultrasonic testing of anchor bolts for a tall industrial pressure vessel.|
Ultrasonic metal flaw detection (Ultrasonic)
The top of each anchor bolt is ground smooth and flush using an abrasive grinder, perpendicular to thread orientation (image). Using a piezoelectric transducer, applied to the ground portion of the anchor bolts’ top surface, a mechanical stress wave is propagated (500 KHz – 10 MHz) whereby these waves will travel through a given metallic medium at a specific speed or velocity, in a predictable direction.
Ground penetrating radar (GPR)
GPR emits an electromagnetic wave radiating from a transmitting antenna (900 – 1500 MHz), traveling through the testing material at a velocity that is determined primarily by ‘permittivity’ of the material. The wave spreads out and travels inward until it hits an object that has different electrical properties from the surrounding medium, is scattered from the object, and is detected by a receiving antenna. The output is a graphical display that can provide a 3D image of the anchor bolt embedment within the concrete.
Due to hazards associated with radiographic processes, this technique is rarely used in routine inspections, and large areas need to be cordoned off as a precautionary safety measure to eliminate radiological exposures.
Exposing sub surface regions of embedded anchor bolts, the bolts are cleaned of laminar deposits of corrosion (i.e. rust pack) and a vernier caliper is used to determine the existing diameter of the exposed anchor bolt.