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1998 ICRI Award of Excellence
Industrial Category
In 1957 the use of pre-stressed or post-tensioned
concrete systems was relatively new to Southern California, and
the turbine deck at Ocean Vista Power Generation Station was one
of the first post-tensioned decks to be built there. This deck was
originally designed and built by the Bechtel Corporation for Southern
California Edison and consists of an 11" (275 mm) thick post-tensioned
concrete deck supported by structural steel framing. The overall
dimensions of the deck are approximately 127' x 280' (39 m x 85
m).
The pre-stressing system used 7-wire, button-headed
tendons spaced at approximately 3 feet (1 m) on center parallel
to the span, and 6 feet (2 m) on center perpendicular to the span.
The prestressing cables appear to be located ½" (13 mm) below the
center of the slab, with no drape. Traditional deformed reinforcing
steel was omitted from the design.
In 1983, excessive cracking and localized delaminations
and spalling of the deck became a concern. Because much of the original
construction documentation was missing, determining the most probable
root causes for the deterioration had to be based on recent data,
i.e. post 1983. Three areas that were just beginning to affect the
design of reinforced and pre-stressed concrete at the time were:
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The
determination of losses in the pre-stressing force which could
expect to occur
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The
causes and effects of corrosion of pre-stressed systems
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The
effects of certain types of aggregate used in concrete, which
have since been proven to be reactive |
Although the effects of creep and shrinkage were theoretically assumed
during this early era of post-tensioned construction, historical
performance to verify these assumptions was limited.
Evidence of the concern over pre-stressing loss
can be found in an earlier letter written by one of the engineers
involved in the original construction, in which it was suggested
that the deck be monitored annually to check the loss of pre-stressing
as compared to the anticipated design loss. This was due, in part,
to what he considered early loading of the concrete. The original
design assumption for loss in the pre-stressing force was 15%. The
actual force, however, is unknown.
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