“Contemporary architectural trends favour large uninterrupted floor areas, flexible internal layout and spaces that are easy to modify and move around in, while considerably improving the durability of buildings. All of these requirements can be met through the use of prestressing by post-tensioning (both unbonded and grouted method), which enables the creation of large unobstructed floor areas with a minimum number of columns and reduced floor thicknesses”
Prestressed Concrete is a very efficient form of construction; it takes advantage of the strength of concrete in compression. Developed mainly over the second part of the 20th century, it has proven to be reliable and durable. One reason for the increasing use of posttensioning is the advancement in technology in recent years. While older post-tensioning systems focused more on obtaining the desired prestress forces and less on durability, by improving the systems used to protect the prestressed steel from corrosion, the industry can now offer systems that deliver both. The advancements in corrosion protection are especially important in areas that experience significant exposure and damage from freeze thaw cycles, de-icing salts, seawater, salt spray and other deterioration mechanisms.
Snapshot of Pre & Post Systems
Both pre-tensioning and post-tensioning systems are used to create prestressed concrete. Pre-tensioned systems, however, must be fabricated in a precast plant and are limited to straight, harped or circular tendons. This method is also limited to straight or circular members. Because pre-tensioning is used only in precast elements, it is more difficult to construct continuous structures because of the necessary connections. Additionally, though the tendons in pre-tensioned concrete are protected from corrosion because they are in direct contact with concrete, the steel itself is not able to be encapsulated in any other manner. As such, any moisture migrating to the steel through cracks in the concrete could cause the tendons to corrode. Post-tensioning, on the other hand, can be performed on the project site or in a precast yard. Post-tensioning tendons can be configured into almost any shape. This flexibility allows the posttensioning to match exact design requirements with few limitations. And, depending on project needs, the tendons in a post-tensioned system can be unbonded or bonded. For corrosion protection, whether unbonded or bonded – post-tensioning has superior features.
Forms of Post-Tensioning Systems
In post tensioning there are two different techniques; bonded and unbonded. Due to these techniques – used in tendons/strands – more flexible and fast construction is possible when compared to RCC. (Refer Figure 1)
- Unbonded Post-Tensioning: typically consist of single (mono) strands or threaded bars that remain unbonded to the surrounding concrete throughout their service life – giving them freedom to move locally relative to the structural member. The strands in unbonded monostrand systems are coated with specially formulated grease, with an outer layer of seamless plastic extruded in one continuous operation to provide protection against corrosion. Depending on the application and the level of protection that is needed, the anchorages of unbonded monostrand systems may also be encapsulated. Unbonded monostrand systems are typically used in new construction for elevated slabs, slabs-on-grade, beams and transfer girders, joists, shear walls and mat foundations. Light and flexible, unbonded monostrand can be easily and rapidly installed – providing an economical solution.
- Bonded Post-Tensioning: comprises of tendons from one to multiple strands (multistrand) or bars. For bonded systems, the prestressing steel is encased in a corrugated metal or plastic duct. After the tendon is stressed, cementitious grout is injected into the duct to bond it to the surrounding concrete. In addition, the grout creates an alkaline environment which provides corrosion protection for the prestressing steel. An advanced duct system encases the prestressing steel in a corrugated duct and plastic coupler system.
Bonded strand post-tensioning systems can range from a single strand to 55 or more strands in a single tendon, while the anchorage assembly consists of local zone confinement reinforcement, bearing plate, anchor head, wedges and grout cap. Bonded multistrand systems, while used extensively in new construction of bridges and transportation structures, can be and have been successfully applied to commercial building structures. When these multistrand systems are used for large structural elements such as beams and transfer girders, design advantages include increased span lengths and load carrying capacity and reduced deflection.
Challenges Inspiring Technological Advancement
Since invention, post-tensioning industry has seen many technological advances. Improvements in systems include seven-wire strand with wedge-type anchorages, low relaxation strand, and the use of banded tendons in flat plates. Analysis techniques and design software have advanced, as have techniques for improving durability. These include extruded sheathing for unbonded tendons and encapsulated anchorages for enhanced corrosion resistance, plastic duct systems, and the development of non-bleed grouts.
When some of the earliest unbonded posttensioned buildings were about 10-20 years old, corrosion problems started to surface. It was apparent that some of the tendon corrosion protection systems used could not adequately protect the tendons in the most aggressive environments, such as where de-icing salts are used or in coastal areas that have a high salt content in the air. On the other hand, with internal bonded and external tendons, grout is a key element of the overall corrosion protection strategy. Experience gained over many decades with grouted post-tensioning tendons has proven that cementitious grout provides excellent protection for the prestressing steel. The principle objectives of grouting are to protect the prestressing steel from corrosion by encasing it in a passive environment and filling the duct to minimize voids in the completed structure. Specialized equipment has also been developed to complement the process and ensure the highest quality product.
Benefits Analysis: Unbonded Vs Grouted (Bonded) Post Tensioning System
- Unbonded Post Tensioning Benefits: Various researchers have found that
- There is reduction in deflection (~upto 10%) of unbonded beam in comparison with bonded beam.
- There is reduction in Shear (~upto 2%) of unbonded beam in comparison with bonded beam.
- There is reduction in top stress (~upto 5%) of unbonded beam in comparison with bonded beam.
- There is reduction in bottom stress (~upto 5%) of unbonded beam in comparison with bonded beam.
- There is reduction in bending moment (~upto 3%) of unbonded beam in comparison with bonded beam. But upto 20m long span beam bending moment good result in bonded beam.
- From comparison of bonded and unbonded beam it is observed that 5 to 20% reduction in depth of unbonded beam.
- For 5 to 20m span unbonded beam is better and for over 20m span bonded beam shows good results.
Bonded Post Tensioning Benefits:
- In corrosive environments, encapsulated bonded systems offer significant design advantages that lead to life-cycle savings. Because the amount of mild steel is reduced, particularly at the top zone of slabs, there is less steel to corrode should the concrete crack or spall. This is particularly important in parking garages where significant maintenance costs are due to repairs associated with spalled concrete from corroded rebar.
- Second major advantage of bonded posttensioning is the inherent capacity to provide resistance to progressive collapse. This may be especially important in the event of localized blast loading. Like mild steel reinforcement, a bonded post-tensioning tendon is capable of developing its force in a relatively short distance along its length. In the event that an anchorage fails or a tendon is severed, the loss of tendon force would be localized. The remainder of the tendon would retain its force at the development length away from the failure point and would remain functional. This functionality can be considered in the design of a structure.
- In addition, bonded post-tensioning systems also allow for flexibility when future modifications to the building are needed. Tenant buildouts, remodeling and changes in a building’s use may require modifications to the floor slabs. The use of bonded post-tensioning systems has allowed owners the flexibility to make these changes quickly, easily and cost-efficiently.
Opting for Hybrid Model with Unbonded and Bonded Post-Tensioning System:
Bonded and unbonded systems can be mixed within a structure. An example of how bonded and unbonded systems were combined for economics, efficiency and design requirements is the W Victory Hotel & Residences in Dallas, Texas. The W Victory’s concrete frame structure includes a combination of monostrand, unbonded post-tensioning systems and bonded, multistrand post-tensioning systems. The unbonded post-tensioned systems were used in typical levels, while the bonded posttensioning systems were specified for the transfer girders on three levels to provide optimum crack and deflection control features essential for transfer girders required to carry the loads from the multistory structure. Additionally, bonded posttensioning systems were used in exterior applications where corrosion could be an issue.
Post-tensioning has seen much development and many improvements over the past 50 years, resulting in the method now serving as a significant feature in mainstream construction. With Rapid urbanization and shortage of space in big cities, this technology is going to be the major contributor for the building sector. Though various researcher have done research on pros and cons of Unbonded and Bonded post tensioning system, yet carrying the better side of both the technologies and opting for a hybrid model would be the best choice.
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Sonjoy Deb, B.Tech, Civil