Special Considerations for

Post-Tensioned Structures

Early commercially available unbonded systems were susceptible to corrosion including paper wrapped, plastic push-through (stuffed) and heat-sealed (cigarette wrapped) sheathing systems due to their susceptibility to the ingress of water. Grouted (bonded) post-tensioning systems have also experienced problems due to voids, water bleeding, and grouting problems.

Many investigation and repair techniques utilized for conventionally reinforced concrete are inappropriate for post-tensioned concrete. For example, special techniques are necessary to evaluate corrosion in encapsulated systems where corrosion can occur inside sheathed cables without exposure to external chlorides (as is typical for conventional reinforced concrete). Post- tensioned steel is also more susceptible to damage from chloride-induced pitting corrosion, hydrogen embrittlement, and stress corrosion cracking.

Unbonded

Post-Tensioning

Corrosion in unbonded post-tensioning systems can occur anywhere along

the length of the tendon but typically occurs at low points where water can

accumulate, at uncoated anchors, and at locations where less concrete cover is present. Because the tendon is not in direct contact with concrete or grout, it is not protected by the alkalinity of these materials. As such, if post-tensioned steel is exposed to oxygen and moisture, corrosion can occur even in a chloride-free environment.



In some cases, components of the post-tensioning system such as uncoated

anchor plates or exposed strands in front of or behind the anchors are not

protected before being embedded directly in concrete. Environmental exposure to chloride or deteriorated grout pockets can cause localized corrosion.

Grouted Post-Tensioning

Bleed water, segregation of portland cement grout and improper grout installation can leave voids and residual moisture in many grouted structures. Formulated, prepackaged grout has been an important advancement in materials technology; however, segregation, pockets of soft grout and chloride contamination have resulted in tendon corrosion.

Visual Inspection

A visual inspection of the surface of a post-tensioned structure can be

performed to detect signs of distress. The following defects may be a

symptom of a larger problem and merit more substantial investigation.

• Concrete spalling
• Delaminated concrete
• Cracking patterns
• Efflorescence
• Rust staining
• Excessive deflection
• Tendon eruption from the structure
• Evidence of previous repairs or structure modifications such as cored holes for utilities