F POST TENSION SERVICES INDIA PVT. LTD.
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Unbonded Post-Tensioning System

PTSI provides Unbonded Post-Tensioning System which is a means of complementing the strength of the concrete in compression and overcoming its weakness in resisting tetnsion. Post-Tensioning Systems apply compressive force to the structure through the stressing of high strength steel strand with specialized anchorage assemblies. The flexibility of the strand is a major advantage, allowing it to be profiled through the concrete element, counteracting a portion of the applied loads to provide an exceptionally efficient structure.

Choice of Post-Tensioning System

The choice of Potst-Tensioning System usually involves trade-offs between structural and construction considerations. Typically the decision is governed by economic considerations. Virtually all American experience, both in the field and the laboratory, has been with unbonded tendons. Unbonded tendons offer unique structural advantages not observed in bonded tendons, and these advantages should be recognized and carefully considered in the decision between unbonded and bonded tendons. The important technical considerations are serviceability, strength, corrosion protection, redundancy and sustainability.

History

The history of pre-stressing technology is the history of the invention of tools and techniques, and is similar in many ways to the history of concrete construction. Experience and knowledge has enabled people to generate new ideas since its inception.

Take a tour through the timeline of Pre-stressing Technology.

Pre-stressed Concrete

Pre-stressing simply means that the structural member is stressed internally before it is subjected to service loads. The Pre-stressing is done by applying an external force using a steel cable assembly, placed inside the structural member.

The design concept is to keep a management of various stresses at every cross-section of the structure. This can be done by a careful placement of cables/tendons, through which an external force can be applied to the structure resulting in the final stresses in the structure at or below the desired level.



Application of Post-Tensioning System

Post-Tensioning is a highly efficient structural system that offers many benefits in a wide range of construction. Post-Tensioning offers a perfect balance of two materials which complement each other. By combining the concrete and pre-stressing steel, a structural member can resist both compressive and tensile forces caused by various loads. This results in greater efficiency in resisting tensile as well as compressive stresses resulting from the applied load.

Post-Tensioning can be used in all facets of construction. Post-tensioned concrete is used in

  • Commercial buildings
  • Residential apartments (Flat Plate Construction)
  • Office buildings
  • Parking structures
  • Mixed-use facilities such as hotels, schools, hospitals, etc.
  • View Case Studies for more applications.

    Various Floor Systems

    Flat Plate

    Flat Plate with Column Capitols

    Flat Plate with Drop Panels

    Wide Shallow Beams

    One-way Beam & Slab

    Beam-Slab System

    Waffle Slab

    Ribbed Beam System

    Benefits of Post-Tensioning System
  • A significant reduction in the amount of concrete and reinforcing steel required
  • Thinner structural members as compared to non-prestressed concrete; resulting in lower overall building heights to comply the bye laws
  • Aesthetically pleasing structures that harness the benefits of cast-in-place structures with expected geometries, and longer, slender members with large spaces between supports
  • Superior structural integrity as compared to precast concrete construction because of continuous framing and tendon continuity
  • Monolithic connections between slabs, beams, and columns that can eliminate troublesome joints between elements
  • Profiled tendons that results in balanced gravity loads (typically a portion of dead load only), significantly reducing overall deflection
  • Better crack control, which results from permanent compressive forces applied to the structure during pre-stressing
  • Post-tensioning reduces overall building height and weight, which is important in zones of high seismicity
  • Post-tensioning also offers the following construction advantages as compared to steel, non-prestressed concrete and precast construction:
    • Faster floor construction cycle
    • Lower floor weight
    • Lower floor-to-floor height
    • Larger spans between columns
  • High early-strength concrete allows for faster floor construction cycles
  • The use of standard design details of the post-tensioned elements, minimum congestion of pre-stressed and non-prestressed reinforcement, and earlier stripping of formwork after tendon stressing can also significantly reduce the floor construction cycle
  • Greater span-to-depth ratios are allowed for post-tensioned members as compared to non-prestressed members
  • Resulting in a lighter structure and a reduction in the floor-to-floor height while maintaining the required headroom
  • (Source: Post-Tensioning Manual, 6th Edition, Post-Tensioning Institute, USA)

    Evolution of Unbonded Post-Tensioning System

    Durability has become an important issue in all types of construction as developer, engineers, and even the general public have become aware of the high cost of repairs and the potential property damage and life safety issues associated with deterioration of concrete and corrosion of the concrete reinforcing material.

    The Bonded (Grouted) P-T System is used primarily in bridge decks and girders as a multi-strand system. It is also used in transfer beams, containment structures including slabs and other civil applications. The system connects bare strands which run through a steel or plastic round or flat oval duct. The strands are stressed using Hydraulic Jacks after concrete pouring and then the ducts are filled with cement grout which is known as grouting process.

    Ducts are used in Bonded P-T System, which provide protection to the post-tensioning strands after construction. The ducts are corrugated to facilitate the transfer of force between the tendons and the concrete.

    4-STRAND PLASTIC FLAT CORRUGATED DUCT

    In bonded construction the ducts contain multi-strands which are anchored with a common anchorage. These ducts are filled with cement grout as soon as possible after stressing of the tendons. Grouting is primarily necessary for protection of cables from corrosion and restricting the movement of cable within duct to surrounding concrete. Grout is injected in the ducts to bond the pre-stressing strand to the surrounding concrete after it has been stressed.

    To be effective, the grout must essentially fill the duct completely throughout the length of the tendon. To do so, it must be flow-able enough to be easily pumped over long distance in confined spaces without excessively high pumping pressure that could burst the duct and damage the system.

    Problem related to Grouting

    To be effective, the grout must essentially fill the duct completely throughout the length of the tendon. To do so, it must be flow-able enough to be easily pumped over long distance in confined spaces without excessively high pumping pressure that could burst the duct and damage the system.

    • Good expertise required
    • Considerations at high & low points
    • Provision of vent points
    • Checking of the quality of grouting activity
    • Ensuring effective grouting throughout the length of duct
    • The permissible intervals between tendon installation and grouting based on
      the exposure condition:
    • Very damp atmosphere or over salt water (humidity > 70%): 7 days.
    • Moderate atmosphere (Humidity <70% >40%): 20 days.
    • Very dry atmosphere (Humidity < 40%): 40 days

    Threats due to Improper Grouting
    Several Documented Cases of Corrosion‐Related Failures of Post‐Tensioned (PT) Strand. Corrosion identified in early 2000's attributed to grout void formation due to bleed
    water formation and chloride presence.

    Corrosion in Galvanized Steel Duct in Beams with Different Levels of Post-Tensioning

    Ref: Research Project 0-1405 Conducted for the Texas Department of Transportation in cooperation with the U.S. Department of Transportation, Federal Highway Administration

    As the level of Post-Tensioning (concrete pre-compression) increases, the corrosion protection increases. The levels of Post-Tensioning is classified in American code (ACI-318) as Class U, T & C; whereas the similar classification in Indian code (IS-1343) is Type 1, 2 and 3.

    Case of PT Bridge in Florida built in 2002, Florida, USA

    • PT segmental bridge with internal and external tendons.
    • Among first Florida bridges to use low bleed grouts.
    • External Tendons placed to reduce tensile stresses in web. Anchor caps at low elevation.
    • Severe corrosion in multiple external tendons.
    • Failure occurred after ~ 8 years' service.
    • Severe corrosion accompanied by wet plastic grout.
    (Source: CORROSION DEVELOPMENT OF POST‐TENSIONED TENDONS WITH DEFICIENT GROUT – Slide 3)
    Grout segregation characterized as:

    A. Wet plastic
    B. Sedimented Silica
    C. White chalky

    Corrosion attributed to wet plastic grout but not necessarily to void presence. Grout segregation created environment with dissimilar pore water chemistry and physical properties.

    (Source: CORROSION DEVELOPMENT OF POST‐TENSIONED
    TENDONS WITH DEFICIENT GROUT – Slide 4)
    Corrosion at Low Elevation Anchor Caps
    Corrosion and similar deficient grout characteristics are observed at low elevations, too.

    (Source: CORROSION DEVELOPMENT OF POST‐TENSIONED TENDONS WITH DEFICIENT GROUT – Slide 10)

    Unbonded Post-Tensioning System Evolved from Grouted Post-Tensioning System before 1950 for Building Construction Application. Corrosion protection in unbonded tendons is provided by a factory-applied corrosion-inhibiting coating material and an extruded plastic sheath. In highly aggressive corrosive environments the tendon can be completely encapsulated, providing additional protection against corrosion.

    The maximum achievable eccentricity is highest in case of using Unbonded System. HIGHER the ECCENTRICITY, HIGHER is the LOAD BALANCING FORCE generated by a cable. It needs lesser cables to generate certain amount of load balancing force. The gain of extra eccentricity can be utilized either by reducing the depth or reducing P-T cables or the deflections & stresses related performance is improved which enhances the durability.

    Predominant reasons for the evolution of Unbonded Post-Tensioning System:

  • Great potential of P-T in Building Construction Applications
  • To get-rid-of the uncertainties in grouting activity that could amplify the probability of corrosion during the service life of the structure
  • Difficult to assure that grouting is properly achieved
  • Difficulties in installation due to the stiffness of duct as well as in managing multi-strands in the duct
  • Lesser eccentricity achieved due to the size of duct required for grouting
  • Multi-strands make more stress concentration in the green (early age) concrete at the anchorage zones
  • Unbonded Post-Tensioning System

    Unbonded Monostrand Post-Tensioning System is quick to install; tendons can easily circumvent the cutouts/openings as well as can cope with irregular slab shapes. The system has less friction losses, achieves more eccentricity and eliminates the vulnerable grouting process. In unbonded systems, the strand is kept unbonded to the surrounding concrete throughout its service life.

    View the Material Specification

    Advantages of Unbonded P-T over Bonded P-T

    General Advantages:
  • Usually the consumption of conventional steel remains more or less same because of the practical consideration of delays in grouting (more often in case of bonded system) after the stressing activity
  • P-T cable consumption would be lesser for same level of stresses and deflections for serviceability aspects
  • Faster installation
  • Long-term Durability (Corrosion Protection by sheathing and anti-corrosive grease coating on cable)
  • Encapsulated Unbonded System for effective protection against corrosion
  • Slabs reinforced with Unbonded P-T system have better control over the tensile stresses and deflection
  • Grout testing is expensive and time consuming
  • Water cement ratio for grouting to maintain on site is difficult
  • Post construction changes, making cutouts and re-adjustments in cable profiles is not possible once the grouting is done
  • Cable experiences more frictional effects in case of Bonded system due to corrugated duct and inter-cable movements. There is a grease inside the sheathing which reduces the friction effects in case of Unbonded system
  • Post-earthquake residual deformation is minimized
  • Once grouted it is hard to fix any problem in Bonded System
  • Technical Advantages majorly observed in Two-Way P-T Slabs, an abstract from " Two-way Post-Tensioned Slabs with Bonded Tendons " by Kenneth B. Bondy (Technical Paper, PTI Journal, December 2012)

    ...The vast majority of post-tensioning tendons used in two-way post-tensioned slabs have been unbonded…

    ...Unbonded tendons offer unique structural advantages not found in bonded tendons…

    ...The most significant of these advantages is post-flexural catenary capacity. The only way to significantly increase the stress in an unbonded tendon is to increase its length between anchorages. The local strains caused by large local deformations, such as those encountered at columns in two-way slabs, are distributed throughout the entire length of the tendon and do not result in high local tendon stresses. It is virtually impossible to fail an unbonded tendon in tension due to applied load. This factor offers obvious advantages under severe overload or in preventing progressive collapse should the member suffer punching shear failure or the loss of one or more column supports due to some catastrophic event. Tests have shown that slabs with unbonded tendons possess catenary capacities three to four times the demand at factored load...

    ...In a typical case, the bonded steel will fail in tension before the concrete crushes and the catenary capacity will be lost. In such case, if the tendons were unbonded, the steel would not fail in tension at the point of concrete crushing; in fact, it would not fail at a load three to four times higher, with catenary capacity available throughout that entire range...

    ...A common criticism of unbonded tendons is that a failure at one point results in the loss of pre-stress force for the full length of the tendon. A failure at one point in a fully bonded tendon results in the loss of pre-stress force for only a development length on either side of the failure point—a total distance of 6 to 7ft for a 0.5 in. diameter, 270ksi strand...

    ...However, two-way slabs pre-stressed with unbonded tendons are highly redundant and intrinsically provide alternate load paths in the event of a catastrophic event, which results in significant loss of tendons...

    ...Two-way post-tensioned slabs with unbonded tendons offer further advantages in the prevention of local or progressive punching shear collapse...

    ...ACI 318-11, Eq. (18-1), permits a substantially higher tendon stress at nominal strength fps in bonded tendons than in unbonded tendons (ACI 318-11, Eq. (18-2) and (18-3)). This means that a member with bonded tendons will have a larger nominal strength than a member with the same number of unbonded tendons (all other things being equal in both members). This is often cited as an advantage of bonded tendons. However, the flexural capacity of unbonded tendons can be supplemented with non-prestressed reinforcement. The cost of the supplemental non-prestressed reinforcement required in the unbonded member to increase its capacity to that of the bonded member with the same number of tendons is generally less than the cost of routing the tendons. Therefore, there is no economic advantage in favor of tendons, resulting from the fact that they develop a larger nominal tendon stress. Further, the incremental flexural strength is more reliably achieved with non-prestressed reinforcement, which can be visually inspected, than with grouting, which cannot be visually inspected...

    ...The two-way slabs with bonded tendons must satisfy ACI 318-11, Section 18.8.2. This section requires that members with bonded tendons contain sufficient pre-stressed and non-prestressed reinforcement to develop a flexural capacity at every section of at least 1.2Mcr, where Mcr is the moment that produces first cracking at the section (based on a modulus of rupture).This requirement is waived for unbonded members because the type of undesirable behavior addressed (sudden transfer of tensile force from concrete to reinforcement at first cracking) does not occur in unbonded tendons...

    Practical Aspects of Using Unbonded System in Residential Projects

  • Unbonded System can be used more effectively and efficiently where the slab thickness requirement is lesser
  • Each cable can be managed independently for more complex requirements
  • Anchorage is specially designed to optimize the level of transfer of pre-stressing force to concrete at early days of pouring (around 6-7days). Even distribution of force transferred to concrete is achieved more by using Monostrand (Unbonded P-T System) than Multi-Strand System
  • Stressing can be commenced earlier than the Multi-Strand System which saves time of construction cycle
  • Bursting Steel Arrangement is not required in case of anchorages at the cantilever slab edges
  • Encapsulated System makes it "Corrosion Resistant" system
  • Minimal obstruction by column longitudinal bars which makes possible to keep atleast two cables passing through column which is a part of integrity reinforcement requirement. This kind of arrangement is not possible in bonded system with flat ducts
  • Quicker construction (floor-to-floor cycle is faster)
  • Table-form shuttering system is possible (for quick cycles)
  • All wooden supports/props will have same heights (material handling is easier)
  • Saving of time and material as shuttering for side-faces of beam is eliminated
  • Better finishing of placed concrete (chances of Honeycombing is minimized in absence of beams and variation in depth as well as levels)
  • Improved sustainability aspects using P-T Technology by controlling stress and deflection
  • Better performance under earthquake and wind forces as shear-wall provides necessary stiffness to the structure than the column-beam moment frame
  • There is always a Flexibility for the changes / modifications in the wall locations based on revised planning and interior design at later stage after construction
  • Saving of filling material as well as reduces its weight by eliminating the sunken slabs and introducing suspended plumbing system
  • Maintenance of (suspended) plumbing system is simpler
  • Rare chances of leakage
  • Easy access for repair works
  • Accessibility from bottom rather than breaking sanitary flooring at top
  • Chemical water-proofing shall be applied directly on slab top surface
  • Labor cost can be minimized due to reduction in the shuttering and bar binding
  • Better space utilization for vehicular movement at parking level


  • Technical Information

    The Post-Tensioning system has gone through great evolutions over a period of time. Post-Tensioning Institute, ACI and Indian Standards have laid the specific requirements for the components of the system and keeps revising it.

    Monostrand Unbonded Tendons

    Prestressing Steel:

    • Low-Relaxation 7 wire Strand of Class II (Grade 270) with 12.7 mm nominal diameter used in monostrand unbonded post tensioning tendons should conform to the requirements of IS 14268:1995 (reaffirmed 2013)
    • Sectional Steel Area of Strand: 98.7mm2
    • 0.2% Proof Load: Not less than 165.3kN
    • Ultimate Strength: Not less than 1860N/mm2
    • Minimum Breaking Force: Not less than 183.7kN
    • Modulus of Elasticity: At least 196,500N/mm2
    • Minimum Elongation: 3.5% for gage length of 600mm
    • Relaxation at 1000 hours: Less than 2.5% @ 70% Minimum Ultimate Tensile Strength
    • Nominal Mass of Bare Strand: 0.775 kg/m

    Sheathing Specifications:

    • Sheathing Material: Polyethylene or Polypropylene
    • Minimum Density: 0.941gram/cm3
    • Minimum Thickness: 1.27mm
    • Inside Diameter: At least 0.76mm greater than the maximum diameter of the strand
    • Appearance: Sheathing should provide a smooth circular outside surface and should not visibly reveal lay of the strand.
    • Coverage: Sheathing should be continuous over the entire length to be un-bonded, and should prevent intrusion of cement paste or loss of PT coating.

    Grease Coating Specifications:

    • Grease coating should provide protection against corrosion to the Pre-stressing steel
    • It should provide proper lubrication between the strand and sheathing
    • It should resist flow within anticipated temperature range of exposure
    • It should provide continuous non-brittle coating at lowest anticipated temperature of exposure
    • It should be chemically stable and non reactive with Pre-stressing steel, reinforcing steel, sheathing material and concrete
    • It should be a compound with appropriate moisture‐displacing and corrosion‐inhibiting properties
    • Minimum weight of the grease coating on the Pre-stressing strand should not be less than 1.14kg per 30.5m (37.4 grams/m) for 12.7mm diameter strand
    • The coating material should completely fill the annular space between the strand and sheathing and should extend over the entire tendon length

    (Source: PTI Specification, ACI 423.6‐01, IS 14268:1995 (reaffirmed 2013) )

    ANCHOR PLATES

    Microstructure:
    • A. Graphite Type (As per ASTM A247 Plate I & III)
      • I. Form I & II (Spheroid or Nodular type)
      • II. Distribution A (Uniform Distribution)
      • III. Size: 6 – 8
    • B. Nodularity: 90 – 95%
    • C. Carbide: Less than 3%
    • D. Pearlite: 35 – 40%

    WEDGES

    Hardness:

    1. At Surface: 56 – 65HRC
    2. At Core: 40 – 46 HRC

    Material Grade:
    IS:9175 (Part 20)-1986 Grade 20MnCr5


    Mechanical Properties:
    Hardness Number (BHN): 170 – 230

    Material Grade:
    ASTM A 536 Grade 80-55-06
    OR
    IS 1865 Grade SG 500/7

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