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Post-Tensioned Concrete
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> Post-Tensioned Concrete
 Designers
use post-tensioning as a way to reinforce concrete by prestressing
it. In prestressed members, compressive stresses are introduced
into the concrete to reduce tensile stresses resulting from applied
loads including the self weight of the member (dead load). Prestressing
steel, such as strands, bars or wires, is used to impart compressive
stresses to the concrete. Pre-tensioning is a method of prestressing
in which the tendons are tensioned before concrete is placed and
the prestressing force is primarily transferred to the concrete
through bond. Post-tensioning is a method of prestressing in which
the tendons are tensioned after the concrete has hardened and the
prestressing force is primarily transferred to the concrete through
the end anchorages.
Post-Tensioning Explained
 Unlike
pre-tensioning, which can only be done at a precast manufacturing
facility, post-tensioning is performed on the jobsite in cast-in-place
applications. The concrete component is cast with steel reinforcing
strands installed in a way that protects them from bonding with
the concrete. This practice gives designers the flexibility to further
optimize material use by creating thinner concrete members.
The materials used to post-tension concrete members are ultra-high-strength
steel strands and bars. Horizontal applications (like beams, slabs,
bridges, and foundations) typically employ strands. Walls, columns,
and other vertical applications usually utilize bars. Steel strands
used for post-tensioning typically have a tensile strength of 270,000
psi, are about a half-inch in diameter, and are stressed to a force
of 33,000 pounds.
Benefits
While concrete is strong in compression, it is weak in tension.
Steel is strong under forces of tension, so combining the two elements
results in the creation of very strong concrete components. Post-tensioning
can help create innovative concrete components that are thinner,
longer, and stronger than ever before.
Many of today’s “high-performance” concrete structures,
including many landmark bridges and buildings, employ some type
of prestressing. Parking garages, high-rise residential towers,
and many other kinds of structures also employ post-tensioning techniques.
Industry Resources
The Post-Tensioning
Institute provides a variety of resources online for architects
and engineers.
The Concrete Reinforcing
Steel Institute offers an extensive collection of publications,
including its Ready
Reference Reinforcing Steel Resource Guide.
| Publications |
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PCA
Notes on ACI 318-05 Building Code Requirements for Structural
Concrete with Design Applications (EB705)
Designed to help you apply the provisions of ACI 318-05 to the
design and construction of concrete structures, these Notes were
prepared with engineers and architects in mind. PCA Notes is also
a valuable aid to educators, undergraduate and graduate students,
contractors, materials and product manufacturers, building code
authorities, and inspectors. The 34 chapters of PCA Notes provide
detailed coverage of ACI 318-05’s 22 chapters and four appendixes
dealing with strut-and-tie models, alternative provisions for
reinforced and prestressed concrete flexural and compression members,
alternative load and strength-reduction factors, and anchorage
to concrete. Most chapters are introduced with a summary of code
changes in 2005 and contain numerical examples that help design
professionals to better understand and apply the code provisions.
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Design,
Construction, and Maintenance of Cast-in-Place Post-Tensioned
Concrete Parking Structures (LT261)
This manual provides a comprehensive reference for the design,
construction, and maintenance of free-standing, cast-in-place
concrete parking structures. The emphasis is on design, construction,
and maintenance practices that will ensure long-term durability
and minimize life cycle costs. Standard details and case histories
are also provided. Published by the Post-Tensioning Institute.
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Concrete
Floor Systems—Guide to Estimating and Economizing (SP041)
Recently updated, this 45-page publication highlights four
popular reinforced, cast-in-place concrete floor systems:
flat plate, flat slab, one-way joist, and two-way joist. Preliminary
estimates of material quantities (concrete, reinforcing steel,
and formwork) for various span and loading conditions help
with planning. The effects of different concrete strengths,
column dimensions, and floor aspect ratios on material quantities
are discussed to facilitate decision making. Charts containing
relative cost indices help in making comparisons and selecting
the most economical floor system.
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Long-Span
Concrete Floor Systems (SP339)
Two increasingly popular long-span concrete floor systems
are covered in this 100-page publication: banded-beam and
wide-module joist. Preliminary estimates of material quantities
(concrete, reinforcing steel, and formwork) for various span
and loading conditions help with planning. The effects of
different concrete strengths, column dimensions, and floor
aspect ratios on material quantities are discussed to facilitate
decision making. Charts containing relative cost indices help
in making comparisons and selecting the most economical floor
system.
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Post-Tensioning Manual 5th Edition
The 5th edition of the Post-Tensioning Manual was
published in November, 1990. The chapter on post-tensioning
systems (154 pages) has been extensively revised and updated
to reflect details of systems available from members of the
Post-Tensioning Institute, and the design chapter has been
edited to conform to the 1989 ACI Building Code. The 406-page
soft-cover book is available from the Post-Tensioning
Institute.
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