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11 Summary of Findings
- The inspection procedures closely followed the NCHRP Guidelines and current practice in the United States and can be considered reliable and accurate.
- The inspectors "shaded" the distinction between Stage 3 and Stage 4 corrosion in order to ensure a sufficiently conservative projection of potentially cracked wires. This was a reasonable practice.
- The removal of wire samples was limited to eleven wires deep from the outer surface of the cable. This is contrary to usual practice in the United States and may limit the reliability of assessing the extent of cracked wires.
- The wire testing program followed the NCHRP Guidelines and usual practice and the results appear to be reasonable and reliable.
- The calculation of the current cable strength in general followed the NCHRP Guidelines except it was assumed that cracking occurs in a limited proportion of the Stage 4 wires, which is a departure from NCHRP. This results in an estimated loss of strength of 6.7% whereas a strict adherence to the NCHRP method would yield a projected strength loss of 15.4%. 7 In view of the conditions observed this may be considered a reasonable approach.
- The calculation of future strength losses due to continued degradation may be conservative when projected more than a few years albeit that there is significant uncertainty in the methods of calculation available 8. These calculations also departed from the NCHRP method in the assumption that initially only a portion of the Stage 4 wires were susceptible to cracking, thus yielding somewhat better strength than a strict adherence to NCHRP would have.
- Although a lower bound current strength loss of 6.7% has been examined, a figure of around 8% is best supported by the data presented in the audit process.
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