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Post Tensioned Foundations

POST-TENSIONING INSTITUTE CERTIFICATION

Established in 1976, the Post-Tensioning Institute is recognized as the world-wide authority on post-tensioning. PTI is dedicated to expanding post-tensioning applications through marketing, education, research, teamwork, and code development while advancing the quality, safety, efficiency, profitability, and use of post-tensioning systems. Members of the Institute include major post-tensioning materials fabricators in the U.S., Canada, and Mexico, and manufacturers of prestressing materials in the U.S., Canada, Mexico, Japan and Europe, and companies supplying materials, services and equipment used in post-tensioned construction. In addition, PTI has more than 500 professional engineers, architects, and contractors.

Most residential foundations in the United States are constructed with rebar or wire mesh as their reinforcement to minimize shrinkage cracks and to resist the stresses created by the weight of the building. An alternative is the use of post-tensioned strands in lieu of conventional bonded reinforcing. Post-tensioned foundations have been around since the 1960s and historically have been used for expansive and compressive soil conditions that are prevalent in Texas, California, and other parts of the Southwest. Contractors and engineers have been taught that concrete is bad in tension and good in compression, so it is in the best interest of the concrete to be in compression.

The general philosophy of post-tensioning is to apply a large enough force to the concrete such that any tensile stresses are reduced to acceptable levels. Placing the concrete in a compressed state will minimize the potential for cracking and improve the integrity of the slab. With their excellent performance, relative ease of construction, and economy, post-tensioned residential foundations are becoming a more standard foundation system for residential construction regardless of soils classification.

Post-tensioned foundations are designed using the Post-Tensioning Institute (PTI) method first published in 1980 and recently updated with its 3rd edition. Aside from the reinforcement, the construction of the foundation is the same as a conventional system. For most single-family residential construction, the slabs are 4 to 5 inches thick and have tendons at approximately 4 feet on center in each direction. The tendons are placed in the center of the slab and continue at this elevation across the foundation. The specific location of the tendons is relatively arbitrary. Provided the correct number of tendons are installed and effectively equally spaced, variations in tendon spacing will not affect the performance of the foundation. Gaps between adjacent strands of up to 6 feet are acceptable. The relatively small number of strands allows for easier inspection, structural observation, and identification of problems. In addition, the workers have plenty of room to move during placing of the concrete to avoid stepping on the strands and pushing them into the dirt. Since their exact location is flexible, contractors will often place the tendons to avoid penetrations and other embedded items. If a conflict does occur, the strands can curve around the obstruction, provided the bend is in a smooth and gradual manner. The majority of post-tensioned foundation plans will show every strand across the slab, a callout for its length, a color code for identification, and the expected elongation from a successful stress. In contrast to elevated construction, the engineered drawings effectively serve as shop drawings and the post-tensioning materials should be able to be fabricated and placed directly from the permitted plans.

The tendons can be 15 to 200 feet long and will be delivered to the site as one continuous piece. They are shipped in rope-like coils to the jobsite and take up a relatively small amount of room in comparison with a series of 60-foot-long pieces of rebar. The contractors must take care to place the tendons essentially straight with no localized "kinks" vertically or horizontally along their length. During the stressing operation, the kinks in the tendons will try to straighten which can cause localized cracking if the discontinuity is severe.

Except in cases of extremely large expansive soils, the tendons do not extend into the footings. The numbers of tendons are calculated to provide a minimum of 50 pounds per square inch of compression over the entire foundation and must account for frictional losses of the concrete grinding against the subgrade. When a post-tensioned foundation is used on stable soils, maintaining the 50-psi minimum is the only additional calculation from a conventional reinforced foundation.

The PTI design method incorporates slabs with exterior and interior footings (ribs) that extend from one end of the foundation to the other. The interior footings occur in both directions and add strength and stiffness to resist any applied loading. Depending on the weight of the building and soil parameters, the footings are typically 18 to 24 inches deep and 12 inches wide. The PTI method requires minimal rebar, if any, however most engineers include one or two #4 bars in the bottom of the footings. Most geotechnical reports require a minimum embedment of 12 to 18 inches below the lowest adjacent grade. This depth activates the appropriate bearing pressure and provides a partial water stop to minimize moisture intrusion under the foundation. The footings are typically located under bearing or shear walls and are spaced approximately 12 feet on center in each direction. If longer spanning joints or trusses are used, the interior footings may not align with any structural or architectural element.

If a ribbed foundation is designed, the system can be converted into a uniform thickness mat. The conversion requires that the section properties of the mat be equal or greater than the rib design. Instead of having a 5-inch-thick slab with several 24-inch-deep footings, a 10-inch-thick solid slab can be used. The uniform thickness slabs will have more concrete and therefore more tendons but substantially less trenching. In most cases, the only trenching required will be on the exterior of the foundation. I recommend a minimum uniform thickness of 8 inches but with larger buildings with severe soil conditions, slabs in a 14- to 18-inch range have been used in residential construction. Both methods are acceptable and are typically chosen by the owner/contractor based on their preference of placing more concrete or performing more trenching. From an engineering standpoint, the primary difference is that the mat foundation will require additional detailing for anchor and hold-down bolts. Standard off-the-shelf hardware will typically require more embedment than the thickness of the uniform mat. Washers connected with a nut to the bottom of the longer bolts in conjunction with additional rebar are common in uniform mat construction.

In addition to resisting soil movement, post-tensioned slabs have excellent load capacity and effectively remove the need for isolated pad footings from the foundation. A good rule of thumb is for every inch of slab thickness, a 1000-pound post load can be supported. For example, a 5-inch slab can support a 5000-pound post load without additional concrete or rebar. An experienced post-tensioning engineer will probably be able to generate larger capacities depending on concrete strength and force applied from the tendons. Typical interior and exterior footings support a 15,000-to 20,000-pound post load without additional reinforcement. In addition to resisting post loads, the typical post-tensioned slab will support standard bearing wall loads for residential construction without a deepened section. Both the Uniform Building Code (UBC) and the International Building Code (IBC) provide formulas to compute the load capacity of the slab to support bearing walls. This inherent load resistance has proven very useful in additions where new post and bearing walls are often introduced and typically require removing some portion of the existing concrete and building new footings.

The typical post-tensioned strands are comprised of seven high tensile strength steel wires that are coiled together to create a ½-inch-diameter strand. The wires are high tensile steel having a yield strength of 270 kips per square inch (ksi), which is substantially larger than the standard 40- and 60-ksi rebar. In contrast to conventional reinforcing, the strands have no direct bond to the concrete along their length. The strands are encased in grease and covered by an extruded plastic sheathing. The sheathing prevents a bond between the strand and the concrete, while the grease allows the strand to slide inside the sheathing. The ability of the strand to move freely is critical because each tendon will be stretched by a hydraulic jack after the concrete has been placed. Each strand will be loaded to 33,000 pounds at stressing. Unlike rebar that is activated only once the concrete begins cracking, the force from the strands is always present in the slab to resist applied loading and minimize cracking.

The force in the strands is transferred to the concrete by an anchor at each end that is embedded into the slab. The anchor is a ductile iron casting with a plan dimension of 5¼ x2 inches and a tapered hole in the center. Two small wedges placed around each side of the strand physically clamp the steel wires to the anchor. The anchor and wedges may appear small, but this system has been used for decades and is the same assembly that is used in elevated concrete construction.

The Post-Tensioning Institute recommends that the tendons be stressed between 3 to 10 days after the concrete has been placed and has obtained a minimum compressive strength of 2000 psi. The time between placing and stressing is an issue because the tendons are "unbonded" to the concrete and are effectively useless for crack control as the slab cures and shrinks. Until the tendons are stressed, the slab is essentially un-reinforced, and, if left in this condition, noticeable cracking is to be expected.

There is no difference in the concrete used between post-tensioned and conventional foundations. I recommend a minimum compressive strength of 3000 psi to allow the mix to reach 2000 psi within a few days and the tendons to be stressed within a week of pouring the slab.

The jacking system is typically operated by a two-person crew. One person handles the jack and the other operates the hydraulics. An experienced crew can stress a single-family home in less than an hour. Once the jack is attached to the tendon and is bearing against the anchor, the hydraulics are activated. When the appropriate gauge pressure is achieved, the jack will hold the force while the wedges are pushed into the tapered hole, locking the strand to the anchor and their force to the foundation. To achieve the 33,000 pounds of load, each strand is stretched approximately 0.08 inch for every foot of length. For a 50-foot-long strand, the required elongation will be 4 inches. After the tendons are stressed, the deputy inspector will measure the elongation and as long as they are within 7% of the calculated value, the tails of the strand can be removed. A rust-resisting spray is applied to the exposed anchor and tail, and the stressing pocket is filled with a grout to provide cover to the end of the assembly.