ML102871045

6.2 Inadequate Tendon

ML102871045
Person / Time
Site:	Crystal River
Issue date:	01/04/2010
From:	- No Known Affiliation
To:	Office of Information Services
References
FOIA/PA-2010-0116
Download: ML102871045 (2)

Text

6.2 Inadequate Tendon Wires

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Description:==

The tendons in a concrete post-tensioned system are made of steel wires. The wires must have a very high tensile strength and must sustain high stress levels for long time periods with minimal stress relaxation. Cold-drawn steel wires are typically used. The wire quality, strength, uniformity, and corrosion are tested during regular surveillances, as described in the ASME Code Section XI, Subsection IWL (FM 6.5 Exhibit 1). Relaxation of the tendon wires leads to reduced pre-stress levels in the concrete. Local strain-hardening (work-hardening) leads to non-uniform force along the tendon and non-uniform pre-stress levels. Tendon wires are ASTM A421-65 high-strength steel with a guaranteed ultimate tensile strength (GUTS) of 240ksi (FM 6.2 Exhibit 2).

Data to be collected and Analyzed:

1. Draw wire elongation variation along the tendon due to friction (FM 6.2 Exhibit 3);

2. Review surveillance data on tendon wires (FM 6.2 Exhibit 4);

3. Analyze wire surveillance data (FM 6.2 Exhibit 5);

4. Test wires recovered from removed tendons (FM 6.2 Exhibit 6);

Verified Supporting Evidence:

a. The strain in the wire is not uniform because the tendon force is not uniform (due to friction) (FM 6.2 Exhibit 3).

Verified Refuting Evidence:

a. Wire material ASTM A421 high-strength steel is the standard material specified in the industry. It is low relaxation and it is not subject to strain hardening / work hardening (FM 6.2 Exhibit 2);

b. Surveillance data on tendon wires show expected strengths and ductilities (FM 6.2 Exhibit 4 and FM 6.2 Exhibit 5);

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• 1 Discussion:

a. The strain of 5,000 pin/in on a 120 ft wire leads to a displacement of 7.2in. This is consistent with observations made by PSC personnel;

b. There are no cyclic stresses imparted to the tendon wires so that the effects of strain hardening / work hardening are further limited;

c. ASTM A421 has chemical requirements on Sulfur (0.050% max) and Phosphorus (0.040 % max) only for carbon steel alloys. The wire material is left to the discretion of the manufacturer to provide the wire physical requirements such as tensile, yield, and elongation. Therefore, a wire chemistry of 1080 carbon steel falls within the ASTM A421 specifications. Based on Atlas Laboratories analysis, the wire meets chemical and mechanical properties of the ASTM A421 requirements;

d. The tendon wires are stretched to 70% GUTS in service. Therefore the maximum possible over-stressing in a local area of a wire is only 30% (above that we would reach the GUTS and the wire would break). There are very few instances of broken wires and all are explained by other factors than over-stressing;

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Conclusion:==

There are no indications the tendon wires used in CR3 tendons generated the delamination.

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