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TECHNICAL PAPER

Table 1: Condition of corroded PTC structures at various stages of corrosion detection

NAME AND LOCATION OF STAGE OF OBSERVATION UPON DETAILED CONSEQUENCE (AGE AT THE
BRIDGE (REFERENCE) DETECTION INVESTIGATION TIME OF DETECTION, YEARS)
Boundary Creek Bridge, New Zealand [1] Cracking • Localized wire breakage Strand removal
• Hidden corrosion (≈ 48)

Tiwai Point Bridge, New Zealand [1] Cracking • Up to 60% estimated section loss Decommissioning
• Hidden corrosion (≈ 40)
Hamanatua Stream Bridge, New Zealand [2] Spalling • Up to 10% estimated section loss Significant repair
• Hidden corrosion (≈ 38)
Lowe’s Motor Speedway Pedestrian Bridge, Collapse • Pre-mixed Calcium Chloride set accelerator Human injury
North Carolina [3] • Hidden corrosion (≈ 5)

bridges are young (say, couple of decades old), their long- Also, if the flow of corrosion along the length of the interstitial
term performance issues are not yet widely reported. Some spaces (in the direction of strands) is considered, the critical loss
well-documented cases of corrosion-induced premature failure in cross-section could be much more than 6%. This is evident
of PTC members in bridges are presented in Table 1. As given, from the significant cross-sectional loss shown in Figure 1(b), in
the consequences of corrosion of PTC girders in New Zealand which case no surface stains were observed. Strand breakage
were severe with significant repair and decommissioning in is also a recurrent observation from PTC structures that have
about 40 years of age. Although the corrosion-induced collapse failed much before the expected service life; and that too
of the bridge in North Carolina at five years of age was due to without any corrosion stains on the concrete surface [as shown
pre-mixed calcium chloride accelerator, which is an extreme in Figure 1(c)]. Therefore, if interventions are not made before
case, the incident provides an indication on the possible brittle/ the strand corrodes, the load carrying capacity can reduce
catastrophic failure mode and high risk/safety concerns. significantly because of the vicious series of wire breakage à
stress redistribution à strand breakage, resulting in catastrophic
In case of conventional steel rebars, the corrosion products start failures [1,3,7–9] . According to fib Bulletin 26, over 40% of failures in
exerting expansive stresses onto the surrounding concrete when PTC can be attributed to unsatisfactory corrosion protection or
the thickness of the rust layer reaches about 15 µm , leading error in construction [10] .
[4]
to cracking/spalling etc. (depending on the pore structure of
cover concrete). The corrosion products ooze out and reach 2. SERVICE-LIFE BASED DESIGN (SLD) OF PTC
the concrete surface causing brown stains, which is usually
considered as a sign of corrosion by the inspectors. However, Uncontaminated concrete cover provides a physical barrier
the case with corrosion of the seven-wire prestressed steel against ingress of corrosive species and also the alkaline
strands is different. The corrosion products fill the interstitial chemical environment (pH ≈ 13) for passivation of embedded
spaces between the wires [as shown in Figure 1(a)] prior to steel [11] . The passive film is believed to be a protective oxide
exerting pressure on the surrounding concrete and cracking it. or hydroxide film that reduces the rate of dissolution of
Therefore, visible signs of strand corrosion will not be observed steel to negligible ranges [12–14] . Once the passive film breaks
on the concrete surface at least upto about 6% cross-sectional down, active corrosion of steel bars begins and the corrosion
loss – making timely detection of corrosion challenging [5,6] . products, being expansive, can cause cracking and then further

Broken strand; but, predominantly
unstained concrete at locations away
from the stirrups

Cross-sectional loss in Filling up of the
the interstitial space interstitial space
Note: Arrows indicate severe section loss
(a) Interstitial spaces filled with corrosion (b) Severe localized hidden corrosion and section (c) The soffit of a 50 year old bridge
products prior to exerting pressure onto loss observed on prestressed wires in a 40 year old girders showing strand breakage due
[1]
[5]
the surrounding concrete (adapted from ) bridge (adapted from ) to corrosion (adapted from )
[1]
Figure 1: Insidious and hidden nature of strand corrosion in PTC systems

THE INDIAN CONCRETE JOURNAL | NOVEMBER 2020 55