Concrete Construction, June,
2003 by
Joe Nasvik
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Post-tensioning (P-T)
dates back to the beginning of
the twentieth century but was
not seen in the United States
until the construction of a
bridge in 1949. In the 1960s,
with the development of higher
strength steel, better
attachment hardware, better
construction techniques, and
simplified design methods, the
use of P-T to reinforce
structures became more popular.
By the early 1990s the mystery
of P-T subsided with further
refinements to the
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tensioning process, the
development of more
corrosion-resistant anchorages,
and the widespread dissemination
of design software. Because of
these factors, P-T has become a
preferred method for reinforcing
concrete today. |
Post-tensioning (P-T)
dates back to the beginning of the twentieth century but was not
seen in the United States until the construction of a bridge in
1949. In the 1960s, with the development of higher strength
steel, better attachment hardware, better construction
techniques, and simplified design methods, the use of P-T to
reinforce structures became more popular. By the early 1990s the
mystery of P-T subsided with further refinements to the
tensioning process, the development of more corrosion-resistant
anchorages, and the widespread dissemination of design software.
Because of these factors, P-T has become a preferred method for
reinforcing concrete today.
Reasons to consider P-T include the following:
* Concrete slabs are thinner, and consequently lighter and less
costly.
* Slabs reinforced with P-T have less deflection.
* Joints in a P-T slab are minimized or eliminated.
* Curling in slabs is greatly reduced or eliminated.
* P-T often costs less and is much faster to install than
standard rebar methods.
* For foundations, P-T is an effective way to deal with unstable
soil.
* P-T improves the long-term durability of structures,
especially bridges.
* Span lengths can be greatly increased.
* Concrete tanks can be made as watertight as steel tanks, with
far greater durability.
Defining terms
Concrete is strong in compression and weak in tension. "Plain" (unreinforced)
concrete is placed when the structure must resist mostly
compression forces and when tensile stresses are low.
"Reinforced" concrete is used for structures that must resist
significant tensile forces. Reinforcing materials (such as steel
rebar or welded-wire fabric) that perform well in tension are
embedded in the concrete. When loads are applied to the
concrete, the bending action causes some of the concrete to be
compressed (where it performs well) and some to be in tension
(where it tends to crack). The steel reinforcement begins to
carry significant load only after the concrete cracks. For that
reason it's often referred to as a "passive" reinforcement
system.
"Prestressed" concrete is concrete that is pre-compressed by
stressing the reinforcement before loads are applied. This
greatly increases its ability to resist tensile forces without
excessive cracking. Concrete can be prestressed in a factory by
tensioning the steel reinforcement first and then placing
concrete around it--"pre-tensioned" reinforcement. Or concrete
can be cast in place and the steel reinforcement tensioned after
the concrete has reached a required strength--"post-tensioned"
reinforcement. Structural engineers calculate the limits for
tensioning. If the concrete is over-tensioned, serious problems
can result. When it's under-tensioned, performance benefits are
diminished.
The word tendon encompasses all the components of the P-T
system. This includes the "strand," which is usually 1/2 inch in
diameter, made from very high-tensile-strength steel wire,
usually seven wires twisted together. An anchorage consists of a
cast-iron bearing plate and special wedges to secure the strand
inside the anchor housing. When the concrete reaches a required
strength, one end of the strand is secured and the other end is
pulled with a hydraulic jack to its required tension and then
secured. "Strands are pulled, initially to about 33,000 pounds;
then they relax to about 27,000 pounds," says Cary Kopcyznski,
president of Cary Kopcyznski Co., Bellevue, Wash. "One strand is
capable of lifting six Chevy Suburbans." Strands are four times
stronger than rebar of the same area.
There are two types of P-T systems: bonded and unbonded.
Unbonded systems use strands surrounded with special
corrosion-inhibiting grease and encased in waterproof plastic
sheaths. This assembly is positioned, and then the concrete is
placed, similar to standard reinforced concrete. With a bonded
system, before the concrete is cast, empty steel or plastic
ducts are positioned in the formed area and attached to the
anchorages at either end. After the concrete is placed and gains
strength, strands are threaded through the ducts, tensioned, and
the ducts are filled with a special grout designed to prevent
corrosion. Unbonded systems are nearly always used for building
and slab construction, while bonded systems are mostly for
bridge construction.
Stage stressing is a technique of applying stress to the tendons
in stages, as the concrete progressively gets stronger. This
technique helps to avoid early cracking in slabs. However, Jerry
Holland, Structural Services, Jonesboro, Ga., cautions that you
must plan for this carefully. He takes extra cylinders from the
last load of concrete that are then tested each day to determine
the strength of the slab and the proper tension that can be
applied to the tendons.
Where P-T reinforcement is currently being used
As a method of reinforcing, post-tensioning is growing in
popularity because it saves money, has many construction
advantages, and contractors and designers no longer regard it as
a mysterious method of reinforcement. Also, owners of structures
are beginning to understand the process and its benefits. Tendon
corrosion problems, an earlier issue, have been overcome by the
development of corrosion-resistant tendons and new materials
that electrically isolate tendons from the concrete--eliminating
the corrosion reaction. Here are the most common types of
construction where P-T is now being used.
Commercial buildings
John Purinton, a principal with Watry Design, Redwood City,
Calif., designers of office buildings and parking structures,
thinks that 50% to 60% of the structural concrete buildings in
the United States today use P-T reinforcing. The reason, says
Purinton, is that it can reduce floor thickness. "If a high-rise
office building has a 200-foot height limit, for example, owners
can often have a couple more floors of offices just by using P-T
reinforcing."
P-T reinforcement--building construction is typically used for
horizontal applications (floors and beams) but not vertically
(columns and walls). This is partly because deflection of floors
is an important issue. Kopcyznski notes that "tendons are
arranged like the cables in a suspension bridge--draping near
the bottom of the slab in the center and rising to the top part
of the slab at the supports. This is so the tendons pick up the
deflection caused by the weight of the concrete when they are
tensioned."
Ray Messer, president of Walter P. Moore & Associates, Houston,
also designs office buildings and stadiums. "P-T reinforcing is
ideal for hotels, apartments, and condominiums," he says,
"because flat plate construction is possible--meaning that the
bottom of a slab is flat. No drops are needed around columns and
beams, so the bottom of a slab becomes the ceiling of the room
below." He has also used P-T to connect vertical precast column
segments.
P-T reinforcement also makes it possible to achieve much longer
spans between columns--up to 32 feet. "For hotel construction,
this means that two rooms can fit into one bay," says Kirk
Harmon, president of Cagley, Harman & Associates, King of
Prussia, Penn. The resulting building can weigh 20% to 30% less
than with other systems.
Residential slabs and foundations
P-T reinforcement is commonly used in regions with poor soils:
expansive soils, seasonal water accumulations, collapsing soils,
and areas subject to long-term drying. P-T reinforcement is used
more in the residential market than in any other sector of
construction. "It's now being used in areas with stable soils,
too, such as central Florida, which has sandy soil," says Harley
Nethken, president of Engineering Services, Slidell, La. "It
represents a cost saving, and you get real reinforcement, not
steel mesh at the bottom of the slab." P-T stiffens slabs so
that they can serve as foundations and resist any soil
movements.
P-T reinforced concrete can have cracks, though they are
controlled. Cracks usually develop when tensioning is performed
after significant shrinkage and curling has already occurred.
When a residential floor is intended to be the finished surface,
such as with chemical staining or diamond polished surfaces,
Nethken suggests some additional precautions:
* Designs should be based on higher residual compression.
* Stage stressing should be used.
* Concrete slabs should be properly cured.
No-crack, no-curl slabs
There are tens of millions of square feet of industrial floors
with P-T reinforcement in service, and the market is growing.
"The No. 1 problem for owners of warehouse floors is joints,"
says Holland. "By using P-T, it's possible to increase the
distance between joints to as much as 500 feet."
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