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Post-tensioning (PT) has become a common aspect to many reinforcing steel applications. As a method with many construction advantages, PT is no longer an unfamiliar method of reinforcement, especially as contracting professionals now understand the process and its benefits.

 

 

  • Corona Steel, Inc. obtains some of the most experienced and talented post-tensioning installation superintendents and foremen in the industry.
     
  • We are proud of our commitment to excellence and approach each project with skill and confidence.
     
  • Our PT installation crews have a wide range of experience and can respond to any post-tensioning need.

Concrete Construction,  June, 2003  by Joe Nasvik
 
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.

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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