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| HISTORY OF POST TENSIONING |
Reinforced concrete is a very popular choice of material in construction of variety of structures. Concrete is reinforced with conventional rebar or with Prestressing wire or cables. While conventionally reinforced concrete construction hasn't changed much in the past four decades, prestressed concrete construction has become more and more popular. Prestressed concrete dates back to the beginning of the twentieth century but became popular only after World War-II in the bridge industry. In the 1960s, with the development of higher strength steel and better attachment hardware, the use of prestressed concrete to reinforce commercial structures gained popularity. Better construction techniques and simplified design methods have fueled the growth of prestressed concrete in construction industry ever since. By the early 1990’s the popularity of prestressed concrete increased with further refinements to the tensioning process, the development of corrosion-resistant anchorages and availability of design software. As a result prestressing has become a preferred method for reinforcing concrete today in most developed countries.
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| PRESTRESSED CONCRETE |
Concrete is pre-stressed using high strength steel wire. Other names for this reinforcing are tendon, cable and strand. Concrete can be prestressed in two distinctly different ways, by stressing the reinforcement prior (pre) to pouring the concrete and tensioning the reinforcing after (post) concrete has hardened. Prestressed concrete utilizes bare strand stressed between two abutments and inside concrete formwork. This technique is popular in the precast concrete industry. In post-tensioned reinforcing application, there are two types of systems: bonded and un-bonded. In an un-bonded system, the reinforcing strand is surrounded with special corrosion-inhibiting grease and encased in waterproof plastic sheaths. Concrete is placed around the cable assembly, similar to standard reinforced concrete. In a bonded system, steel or plastic ducts are placed in the formed area. After concrete is placed and gains strength, bare reinforcing strands are threaded through the ducts and tensioned. The ducts are subsequently grouted to develop bond and prevent corrosion. Un-bonded systems are nearly always used for poured in place concrete construction, while bonded systems are utilized in poured in place as well as Precast industries. Both un-bonded and bonded systems provide similar results. The structural engineer chooses the system based on its cost, his expertise and availability of the material. The advantages of prestressed concrete over reinforced concrete are far too many to be overlooked in the design and construction of concrete structures.
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| Where prestressed concrete is currently being used ? |
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- Commercial construction
- Office Buildings & I.T. Parks
- Residential construction
- Industrial construction
- Transportation industry
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Monostrand Unbonded Post Tensioned (P-T) construction is fairly easy to install and hence is popular amongst concrete contractors.
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| REASONS TO CONSIDER MONOSTRAND UN BONDED PT V/S BONDED PT INCLUDE THE FOLLOWING |
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- Concrete slabs can be achieved thinner up to 5"(125mm) minimum thickness
- Column size can be reduced as the self weight of slab gets reduced
- Members with lesser deflection ( lesser nominal thickness of sheathing than ducts used in bonded PT hence greater drape and weight balancing)
- Fewer construction Joints
- System often costs less conventional steel
- Faster installation than Bonded PT as well as conventional steel systems
- Shallow foundation systems
- Long-term durability ( Corrosion protection by sheathing and anticorrosive grease coating on cable)
- Greater Span lengths ( Ideal for the spans with grid between 6m to 8m- a typical grid for office building and I.T. parks)
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