General remarks
The main difficulty in assessing the condition of prestressed concrete structures is that failure of the prestressed members may occur as a sudden collapse without any visible warning signs. An example is the Ynys y Gwas bridge in Great Britain. It collapsed in 1985 after 32 years in service as a result of tendon corrosion, a short time after a regular visual inspection in which no evidence for any damage was discovered.65 There are further examples of similar collapses in other countries.6 Visual examination of prestressed concrete members, which includes the detection of moisture, cracking and spalling on the concrete surface, is therefore not sufficient and so non-destructive testing methods are necessary for the following purposes:
· detecting areas with corrosion
· locating tendons and ducts
· detecting voids and grouting defects
· detecting cracks in tendons.
There is not enough space in a brief section like this to provide a compre- hensive review of the wide range of non-destructive testing methods used internationally. The following methods are briefly reviewed:
· potential mapping
· pulse techniques
· geo-radar/ground penetrating radar
· radiography
· ultrasonic methods
· impact-echo method
· magnetic flux leakage (MFL) measurement.
Other methods, such as acoustic emission, are not discussed here, but are covered elsewhere.
