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Post-Tension Systems & Materials

PTE Strand’s Post-Tension Materials provide advantages over convention designs (rebar & concrete) and pre-cast structures for several reasons. Concrete is very weak in tension but extremely strong in compression. Post-tensioning acts on the concrete structure or elements by compressing the cast-in-place concrete to high pressures. Some advantages of post-tensioned concrete include crack control and lower construction costs.

Post-tension materials (PT) can provide thinner slabs and is especially important in high rise buildings in which floor thickness savings can translate into additional floors for the same (or lower) cost. PT can optimize constructability by having fewer joints since the distance that can be spanned by post-tensioned slabs exceeds that of reinforced construction with the same thickness. Architects like having a PT option since increased span lengths also increases the usable floor space in buildings and provides more parking spaces in garages. Post-tensioning has advantages over pre-cast since it has lower maintenance costs over the life-cycle of the structure. Architectural shapes other than rectangles give post-tensioning an advantage over pre-cast, as well, since strands in cast-in-place construction can be curved or swept into place.

Post-tension systems use high strength steel strands that are comprised of seven individual wires spiraled together tightly. The most common diameter of the strands are 0.5” and 0.6”. These strands are stressed under high forces and an anchoring device, along with wedges, clamp down on the strand to retain the force applied while stressing. These anchorages help to distribute these forces into the concrete resulting in the desirable concrete compression.

There are basically two types of post-tension system categories – “Bonded Multi-strand” and “Un-bonded Mono-strand”. Both types have a wide range of uses and applications. Another post-tensioning material is the PT Threaded Bar. It is a piece of steel rod that has threads that are cold-rolled into the rod. These bars can be stressed and anchored similar to the strands.

Utilizing post-tension materials in designs and construction is a very economical choice. PT designs lend themselves to speedier construction and easier installations.

Un-bonded Mono-strand  · Bonded Multi-Strand  · PT Threaded Bars

Un-bonded Mono-strand
Most common diameter utilized is 0.5” although 0.6” has been used to decrease congestion in designs that have high design forces resulting in increased materials. The seven-wire strand goes through an extrusion process that covers them with corrosion-inhibiting grease along with a seamless plastic sheathing. Common applications for this type of PT are in parking garages, condominiums, hotels, offices, mixed-use buildings and slab-on-grade construction. These systems are further categorized into “Standard” or “Encapsulated”
  • Standard System has anchoring devices that are most commonly cast pieces of steel. The anchor has a “Wedge Cavity” where the wedge set seats down and bites into the strand. Although the strand is coated by grease and plastic, the anchorage, along with its wedges, are bare steel. Used in the correct applications and the proper environments, this system has proven to be very versatile.
  • Encapsulated System is very similar to the Standard System but has anchoring devices that are encased in a thick plastic coating. This system is most widely used in corrosive or aggressive environments such as near salt water, ground moisture, de-icing materials, industrial compounds, etc. This type system is commonly considered “water-tight” since the strand also utilizes grease filled tubes and a grease filled cap to connect to the plastic coated anchors. These all work together to help prevent corrosion of the PT system. Although the Encapsulated System is sometimes considered as a premium system, the added cost for this system is roughly only 7% to 10% more than the Standard System. The Encapsulated System helps to increase the life-cycle of the post-tensioned structure as well.

Bonded Multi-Strand
Bonded post-tensioned concrete utilizes higher capacity 0.6” strands and multi-strand anchors. The terminology is slightly different than mono-strand systems, as well.
  • A “Tendon” in a Mono-Strand System is defined as a 7-wire bare strand that has had grease and plastic extruded over the steel strand so the stressing operations can occur after the “Tendon” is cast in concrete.
  • A “Tendon” in a Multi-Strand System is defined as multiple 7-wire strands in plastic, steel or aluminum curved duct. The multiple strands within the duct are now deemed a “Tendon” in multi-strand systems.
  • - This duct material creates a void within the cast concrete so bare strand can be fished through after the concrete reaches an acceptable strength. Depending on the design requirements, and accompanying loads for a given structure (or concrete member), anywhere from 2 to 39 individual strands are strung through this planned void in the concrete.
    - Much larger anchors are used in multi-strand systems compared to mono-strand anchorage. Once the strands are in place, they are usually stressed at the same with high capacity multi-strand hydraulic jacks capable of pulling the strands in unison. Some lighter multi-strands systems can have their strands stressed individually with a special mono-strand jack.
    - After the strands are stressed, the duct is then pumped with grout so as to achieve a homogenous bond between the cast concrete outside the duct along with the grout inside the duct. Since the strands are under tension, and grout has been pumped into the remaining space within the duct, the bare steel strands achieve a highly compressive force along the whole concrete member. The grouting of the duct also helps to protect the strands from corrosion.
    - “Tendons” that are cut, corrode or damaged within a Mono-Strand System usually lose their force over the whole length of the tendon. A multi-strand “Tendon” generally will only lose compression in the concrete local to the cut, corrosion or damaged structure.
  • Bonded multi-strand systems are utilized in many structures and applications:
  • - Bridges such as: Stay-cable, Incremental Launch, Segmental, Cantilevered, Box Girder, and Bridge Decks.
    - Containment Tanks such as: Large capacity cast-in-place circular and rectangular shapes, LNG Tanks, Water and Sewage Tanks, Silos, and liquid or dry materials. Also used in pre-cast post-tensioned tanks to help lock the pre-cast panels together.
    - Nuclear Containment Vessels, Domes and Buildings.
    - Transfer Plates and Transfer Girders: Mono-strand structures that have an irregular load path utilize multi-strand designs to help distribute the loads to the vertical members of the structure. These transfer members are such important elements of the design and should utilize multi-strand tendons when considering high design forces and long life-cycle corrosion. Too often transfer girders in high-rise structures are designed as mono-strand tendons when a multi-strand system may be a better design option.
    - Rock anchors, Tunnels, Retaining Walls, Sports Stadiums, and Mat Foundations.
  • PTE strand offers Multi-Strand Systems with Flat Ducts (slabs, walls, exterior applications) and Round Ducts (high capacity tendons for higher loads) along with the appropriate anchorage. We offer design and supply services along with leasing of stressing and grouting equipment.

PT Threaded Bars
High-strength, high capacity threaded bars are cold-rolled steel and each are supplied with a bearing plate, hex nut, couplers and grout caps. PTE Strand’s Threaded Bar Systems have many applications including:
  • Geotechnical uses such as Soil & Rock Anchors, Tie-backs, and Tower Crane Foundations.
  • Retaining Walls and Tunnels.
  • Attaching Steel elements to concrete structures.
  • Our bars come in 75ksi and 150 ksi strengths with diameters from 1” to 3 ½”.

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PTE Strand Company | Post Tension Systems