What is Post-Tension Concrete?

Introduction

The principle is easily observed when loading together several books by pressing them laterally.

Under such pressure the whole row gains enough stiffness and strength to ensure its integrity.

In concrete structures, this is achieved by placing high tensile steel tendons (cables) in the element before casting, when the concrete reaches the desired strength; the tendons are pulled by special hydraulic jacks and held in tensioning using specially designed anchorages fixed at each end of the tendon.

This provides compressions at the edges of the structural member that increase the capacity of the concrete resisting tension stresses.

If tendons are appropriately curved to certain profile they will exert in addition to the compression at the perimeter a beneficial upwards set of forces that will counteracts applied loads, relieving the structure from a portion of gravity effects.

Post tensioning is a process of pre-stressing with reinforced concrete or equivalent material that possesses high strength steel strands or bars commonly referred to as tendons. Concrete is strong in compression but weak in tension so to counter this, compression force is introduced to the concrete after casting, lending it strength to withstand slab weight and heavy loads.

This also helps in minimal deflection and cracking under heavy load. Post-tensioning process could be applied on any building, residential, commercial of office buildings. It could also be applied to parking structures slabs-on-ground, bridges, sports stadia, rock and soil anchors or water tanks.

Post tensioning even allows construction at sites where it is not feasible to build owing to site and architectural deficiencies.

Post-tensioning as a process requires specialized knowledge and well trained staff. Its usage makes construction easy with knowhow of fabrication, assembling and installation.

The post-tensioning systems commonly used in building and bridge construction are grouped into two principal categories. These are the unbonded and the bonded systems.
The distinguishing characteristic of an unbonded tendon is that, by design, it does not form a bond along its length with the concrete. Unbonded tendons are generally made of single strand high strength steel, covered with a corrosion inhibiting coating and encased in a plastic sheathing. The force in the stressed tendon is trans­ferred to the concrete primarily by the anchors provided at its ends. Variations in force along the tendon is effected by the friction between the strand and the tendon profile in the concrete member. Since the force in an unbonded tendon is transferred primarily by the anchors at its ends, the long-term integrity of anchors throughout the service life of an unbonded tendon become crucial.

The function of the plastic sheathing is,

  1. to act as a bond breaker
  2. to provide protection against damage by mechanical handling
  3. to form a barrier against intrusion of moisture and chemicals

The strand coating, commonly referred to as grease, functions as

  1. Reduces fric­tion between the strand and the plastic sheathing,
  2. provides added protection against corrosion. 

Unbonded tendons are typically employed as monostrands, with each tendon having its dedicated end anchors. Also, tendons are stressed individually. Recently however, unbonded tendons consisting of groups of two, or more strands, each wrapped individually, but encased in a tough group sheathing have been introduced into the market in Europe , gulf countries and overseas. 

Monostrand unbonded tendons have been in use in the United States since the late 1950s. Their application has been primarily in building construction. the unbonded tendons are sometimes referred to as debonded tendons. 
The characteristic feature of a bonded tendon is that, by design, the tendon forms a continuous bond along its length with the concrete sur­rounding it. The bond is achieved through a cementitious matrix which surrounds the strands, commonly referred to as grout. It acts with the duct which is encased in the concrete member to complete the bond path between the prestressing strands and the concrete member. After stressing of a tendon, the grout is injected into the void of the tendon duct which houses the prestressing strands.
When the grout hardens, through its bond to the strand, it locks the movement of the strand within the duct to that of the concrete sur­rounding it. Hence, the force in a bonded strand becomes a function of the deformation of the concrete surrounding it,The flat duct tendon shown is for use in thin members, such as slabs. It houses up to either 4 or 5 strands placed side by side. The strands generally share a common anchor piece at each end, but are stressed and locked off individually. Corrugated, or smooth metal ducts, as well as corru­gated plastic ducts are the materials of choice. Use of the flat plastic corrugated duct is more common in the US, whereas elsewhere, flat metal ducts are more widely used. The larger round ducts are for application in beams and deep members. The strands in these are stressed and locked off simultaneously using a specially designed multistrand stressing jack. 

In this system, the function of the grout is:

  1. to provide a continu­ous bond between the strand and the duct
  2. To increase protection against corrosion by acting as a physical barrier to moisture penetra­tion,
  3. Through its alkalinity, provide an environment non-con­ducive for corrosion.

The function of the duct is:

  1. to maintain a voided path for the strands in the concrete member during construc­tion,
  2. to transfer the bond between the grout within the duct and the concrete surrounding,
  3. to act as additional protection against penetration of moisture and chemicals into the interior of the duct.

The function of the anchor assemblies

The principal function of the anchor assemblies at the ends is to hold the forces generated in the tendon at stressing, until the grout is introduced, hardened and cured. Bonded tendons are generally multi­strands. Tendons of up to 50 strands in one duct are not uncommon. Traditionally, the principal application of bonded tendons has been in bridge construction.

Note on Unbonded post tension system:

unhanded post-tensioning systems, which today are viewed as systems with inherent flaws in their durability performance, were used. These problems are now well recognized and have been effec­tively addressed. Today, through a full understanding of the struc­tural behavior, availability of strong analytical tools, matured speci­fications and codes [PTI 1993, ACI 1992, 1989], refined construc­tion techniques [PTI 1994], improved materials and hardware, own­ers and engineers can fully realise the advantages of prestressing in their building projects. This Technical Note examines the features and performance of the unbonded and the bonded systems - as they are available today. It provides a comparative review of the merits of the two systems.

however both bonded and unbonded systems if designed, detailed and constructed according to current specifications and good practice, will provide durable structures meeting code intended serviceability and strength requirements. Or, if need be, both systems are capable to reach beyond the minimums stipulated in codes and produce a user-defined level of performance, in particular with respect to dura­bility. The merits of each, and the selection of a system depends on the technology, the skilled labor and hardware readily available to the supplier, as well as the economics of construction in the local market area. None is blessed to be categorically superior to the other. The following review concludes with a numerical design example, giving the material quantities which are needed for a frame of a typi­cal parking structure. 

Analysis of Post tension concrete slabs

Analysis:

Analysis is the computation of actions (moments, shears, and forces) and deformations in the prestressed structure under applied loading. Apart from losses in prestressing, which are somewhat different in nature and magnitude, the analysis procedure for the two systems is essentially the same. 

The loss in prestressing force due to friction is higher in short and heavily profiled bonded tendons due to higher friction between the strand and its housing. For seating loss (wedge draw-in), and elastic shortening, the losses are the same for both systems. The long-term stress losses due to creep and shrinkage of concrete, and the relax­ation in strand are different. For bonded tendons, these are subject to the local strain of concrete adjacent to the tendon, whereas for unhanded tendons these are taken to depend on the average precom­pression in the prestressed member. 

Read More on our Blog on The 3D analysis of PT slabs

Design

At the design stage, using the governing building codes and construc­tion practice, the magnitude of prestressing, the amount and disposi­tion of nonprestressed steel, and the detailing of the pres tressed mem­ber are finalized.

Concrete Cover to Tendon for Fire Resistivity

Most building structures are designed for a 2-hour fire resistivity, some for a 3-hour rating. UBC Table 7-A gives the same value of cover for both bonded and unbonded tendons. 

Concrete Cover to Tendon for Protection Against Corrosion

ACI section 7 .7 and UBC do not differentiate between unbonded and bonded tendons in this respect, Permissible Service and Initial Stresses in Concrete Same for both systems

Stresses in Tendon at Strength Limit State

These are different for the two systems. For the same initial stress and tendon profile, code formulas (ACI-318 section 18.7) yield a higher stress for bonded tendons. Table 1 gives a comparison of the two values applied to the example used herein. 

Minimum and Maximum Level of Prestressing
Same for both systems

Minimum Nonprestressed Reinforcement

Bonded tendons systems 

For crack control, there is no code prescribed minimum nonpre­stressed reinforcement for members prestressed with bonded tendons.

Un-Bonded tendons systems 

Unbonded tendon construction, however, requires a code specified minimum amount of nonprestressed reinforcement installed for crack control (ACI - section 18.9). The amount and disposition of this minimum steel depends on whether the struc­ture is desinged as a one-way or a two-way. system [Aalami 1994a]. For a one-way system, the minimum is a function of the geometry of the section. It does not depend on the level of pre­stressing. For the two-way system, however, the minimum at midspan is linked to the level of in-service stresses at that loca­tion. The midspan minimum can be eliminated if the tensile ser­vice stresses are confined to a certain low level as stated in the code. 

Construction
Ease of construction depends to a great extent on the experience, the skill and the practice of the construction crew in the local of the pro­ject.

Unbonded systems are easier and faster to handle and place. Once installed and secured, the unbonded tendon seems to retain its profile more faithfully compared to plastic flat ducts. Some plastic ducts are found to be too flexible about their weak axis. This requires them to be secured at closer intervals for profile control in the vertical plane (at 3 to 4 ft.; 1.0 to 1.3 m).

Due to the greater flexibility of unbonded tendons, compared to the strong axis stiffness of flat ducts, unbonded tendons can be more readily maneuvered in the horizontal plane to avoid interference with openings and inserts.

The practice of some construction crews is to place the empty flat duct first, and thread the strands into the duct after concreting is complete. This is particularly the case for round ducts holding more than four strands. The site threading is regarded by some contractors as an additional labor operation. Other contractors consider it as a time­saving feature, in that threading of tendons can take place on-site while the concrete is hardening. In addition, it eliminated shop fabri­cation of tendons.

Some plastic ducts are found to be sensitive to daily changes in temperature and need to be secured at more frequent intervals to maintain the placement tolerance in the vertical plane. A drawback of infilled duct scheme is the occasional problems with collapse of ducts, or blockage of ducts through intrusion of concrete in poor concrete construction.

There is also the obvious added labor of grouting the bonded tendons, subsequent to stressing. In addition to the cost of the grouting equipment and its maintenance, or its rent, the quality control in the grouting operation is a skill demanding task. Good grouting in many environments is essential to the long-term performance of a tendon.

For slab construction, the strands in the flat duct of a bonded system are generally all anchored into a single assembly, but are stressed and locked off individually. A single strand stressing jack is used. The single strand jacks are light and are normally handled by one person. For the beams, commonly a round duct containing usually 5 to 12 strands is used. The round duct strands are stressed simultaneously using a multistrand jack. The multi-strand jacks normally require more than a one-man crew, and require a hoist or other equipment for handling.

One preceived advantage of the bonded systems in construction time saving is that the strands are cut from strand packs at the site. Hence, factory fabrication is eliminated, thus reducing lead time.

For more information, contact us 

Cited references

  • PTI technical notes- Issues05-1994

 

 

 

 

 

 

 

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