In construction and mining industry, concrete reinforcement has rebar and prestressing methods. However, prestressing has dominated market share because of its uncompetitive advantages. Furthermore, the prestressing method has pre-tensioning and post-tensioning two ways. The pre-tensioning is a reinforcing method in which the steel bar or strand is tensioned before concrete is cast in place. On the other hand, post-tensioning is that the steel bar or strand is tensioned after concrete hardened.
According to the definition from Post tension Institue, a Post-tensioning system is a tendon of a particular size, including prestressing steel, anchorages, local anchorage zone reinforcement, duct, trumpets, couplers, grout caps, inlets, outlets, etc.
While allowing concrete members in any location, the post-tension anchorage system has all of the advantages of prestressed concrete. As such, it has wide applications in civil engineering, including department buildings, bridges, slabs on the ground, etc.
Post Tensioning System Terminology
To understand the theory and practice of the Post-tensioning system, it would be beneficial to know the basic concept of the industry terminology. The list below is some frequently used and may be found in the Post Tension Manual published by PTI.
Anchorage (assembly): a mechanical device that consists of all components required to transfer the post-tensioning force from the prestressing strand to the structure, including all accessories for encapsulation or grouting.
Bar: bars in tendons conformed to ASTM A722, and has a minimum tensile strength of 150,000psi
Barrel Anchor: a cylindrical metal device housing the wedges and normally used with a bearing plate to transfer prestressing force to concrete
Bearing Plate: a plate that bears directly against concrete and is part of an overall anchorage system.
Bonded tendon: tendon in which prestressed steel is bonded to concrete either directly or through grouting
Duct: A conduit (plain or corrugated) to accommodate prestressing steel for post-tensioning installation.
Monostrand: one single strand
Post-tensioning: method of prestressing steel strand after the concrete has hardened
Strand: high strength steel wires that turn around a center wire helically. Strand is typically low relaxation 7 wire steel with an ultimate tensile strength of 270ksi. Unbonded tendons use 7 wire strands typically.
Tendon: a complete tension system that consists of anchorages, prestressing steel, post-tension wedge, and sheathing with coating for unbonded application or ducts with grout for bonded application
Unbonded tendon: tendon in which prestressing steel is prevented from bonding to concrete, and is free to move relative to concrete. The prestressing force transfers to concrete permanently only at the tendon ends by the anchorages.
Wedge：Pieces of tapered metal with serrations that bits into prestressed strand during transferring prestressing force
Post-Tension Anchorage Component
Post-tensioning systems, normally referred as "tendon", have three catalogs in general: monostrand (single strand), multistrand, and steel bar. It depends on the type of prestressing steel used, and consists of a fixed end and stressing end anchorage assembly.
For single strand tendon, it includes prestressing strand, post tension wedge and monostrand anchor.In addtion, a pocket former is put onto anchor to creat a pocket in concrete for fitting stresssing jack.
In multistrand system, strands are typically installed in a duct without P/T coating. Anchor head and anchorage block are specially designed steel components to anchor strand to concrete member. Once strands stressed, grout will be filled into duct to bond the strands to the surrounding concrete and protect the strand from corrosion.
Bar Tendons have either single or multiple bars. The tendons consist of high-strength steel rods, bearing plate, and anchor nut.
The critical components of anchorage assembly are specially designed castings. As such, quality control of these castings is critical to the reliability and durability of construction projects.
Post-Tensioning Systems Type
In most post-tensioned construction, the prestressing tendons are embedded in the concrete before the concrete is cast. These internal post-tensioning systems can be either bonded or unbonded.
In addition, in some bridge or retrofit field applications, the post-tensioning tendons are mounted outside the structure member. These are referred to as external post-tensioned systems.
Bonded and unbonded
In unbonded systems, the strand keeps unbonded to the surrounding concrete. As result, it allows relative movement between the strands and surrounding concrete. Most monostrand tendon system and all external post-tensioning system fall under this unbonded catalog. Strand is coated with corrosion-resistant coating, and protected by extrusion plastic sheathing to prevent the ingress of water.
In the bonded system, the prestressed strands or steel bars in the duct bonds with grout and the surrounding concrete after it has been stressed. Once the grout cured, the system behaves as an integral system without any relative movement between the steel and concrete. Ducts material is corrugated galvanized steel, high-density polyethylene (HDPE), or polypropylene (PP) in general.
Internal and external
In most post-tensioning construction, post-tensioning tendons are embed in concrete. These internal post-tensioning systems can be either bonded type or unbounded type
In some bridge and retrofit applications(as the right picture showed), post-tensioning systems are installed outside of concrete members, these are referred to as external post-tensioning systems.
Prestressed steel in external post-tensioning systems is either grease coating and plastic sheathing or enclosing in a duct.
Benefit and Advantages
Rebars are typical embedding in concrete and reinforce concrete passively to limit crack. However, it does not carry any force until the concrete has already deflected enough to crack.
On the other hand, the post-tensioning system reinforces concrete actively and makes constructions less deflective in counteracting with the environment loading force. The pictures below give a clear demonstration of how both systems work against the loading force.
Because post-tensioning enhances concrete members actively, so it allows longer spans, thinner slabs, fewer beams, and more slender elements in construction. Also, thinner slabs mean fewer construction materials, this reduces building weight and foundation load. So it can be a major advantage in seismic areas.
Post-tensioning allows bridges construction to very demanding geometry requirements, such as long-span between beam, complex curve and significant grade changes.
How does Post Tensioning Tendon Works
A complete tendon system includes anchorages, prestressed strand or bar, cable wedge, and sheathing coating for unbonded application or ducts with grout for bonded application.
In the construction field, tendons layout in forms according to installation drawings. Drawing defines how tendon system space, what their profile should be against side forms, and where to stress.
After concrete inside the side forms has reached over 70% nominated design strength, the strands are tensioned at the “stressing end“ through a hydraulic pump and stress jack. Once stressing strands reach designed strength, wedges will be put into anchor block tapered hole cavities. As soon as pump pressure relief, wedges will hold up the strand to prevent it back to its original point.
As a result, the pre-tensioning strand will apply a compressive force on concrete through post tension anchorage. Consequently, the compressive force will counteract the tensile forces from application field loadings, and this increases the loading capacity of concrete significantly.
The choice of Post Tensioning System
Unbonded tendons are common in building construction, and bonded systems are typical in bridge construction, usually in larger members such as beams and girders.
External post-tensioning tendon is used extensively for repair and rehabilitation applications in bridge and building structures.
Bar systems, bonded or unbonded, are typically feasible in relatively short members where harped or straight tendons are required and are common in vertical applications such as walls, piers, and reinforced masonry.
In conclusion, the choice of the post-tensioning system involves trade-offs between structural and construction considerations. Usually, more than one type of system may be feasible for a particular application. The technical considerations are strength, corrosion protection, and design redundancy. Over the years, the projects that Sipaitech Mfg have developed covers all range comonents of post tension system. Visit our site to get more about the specific projects and relevant technical articles.