ASSGNMENT HELP ON POST TENSIONING IN BUILDING STRUCTURES

Introduction
When Eugene Freyssinet developed and patented the technique of prestressing concrete in 1928 he little realized the applications to which his invention would be put in future years. Spectacular growth in the use of prestressed concrete took place after the Second World War with the material used to repair and reconstruct bridges in Europe. It is now an accepted Civil Engineering construction material.

The A.C.I. Committee on Prestressed Concrete gives one of the most apt descriptions of post tensioned concrete.
`Prestressed Concrete is concrete in which there have been introduced internal forces of such magnitude and distribution that the forces resulting from given external loadings are counteracted to a desirable degree'.

In post-tensioning we obtain several distinct advantages: -
a) Designers have the opportunity to impart forces internally to the concrete structure to counteract and balance loads sustained by the structure thereby enabling design optimization.
b) Designers can utilize the advantage of the compressive strength of concrete while circumventing its inherent weakness in tension.
c) Post-tensioned concrete combines and optimizes today's very high strength concretes and steel to result in a practical and efficient structural system.

The first post-tensioned buildings were erected in the USA in the 1950’s using unbonded post tensioning. Some post-tensioned structures were built in Europe quite early on but the real development took place in Australia and the USA. Joint efforts by prestressing companies, researchers and design engineers in these early stages resulted in standards and recommendations which assisted in promoting the widespread use of this form of construction in Australia, the USA and throughout the Asian region.

Extensive research in these countries, as well as in Europe more recently, has greatly expanded the knowledge available on such structures and now forms the basis for standards and codes of practice in these countries.

Since the introduction of post-tensioning to buildings, a great deal of experience has been gained as to which type of building has floors most suited to this method of construction. Many Engineers and Builders can identify at a glance whether the advantages of post-tensioning can be utilized in any particular situation.

Current architecture in India continues to place emphasis on the necessity of providing large uninterrupted floor space, flexibility of internal layout, versatility of use and freedom of movement. All of these are facilitated by the use of post-tensioning in the construction of concrete floor slabs, giving large clear spans, fewer columns and supports, and reduced floor thickness.

Post-tensioning in buildings can be loosely divided into two categories. The first application is for specialized structural elements such as raft foundations, transfer plates, transfer beams, tie beams and the like. For large multi-strand tendons used in these elements, 15.2 mm diameter seven wire strands are preferred.

1. ADVANTAGES OF POST-TENSIONED FLOORS
The primary advantages of post-tensioned floors over conventional reinforced concrete in-situ floors may be summarized as follows:
a. Longer Spans
Longer spans can be used reducing the number of columns. This results in larger, column free floor areas which greatly increase the flexibility of use for the structure and can result in higher rental returns.
b. Overall Structural Cost
The total cost of materials, labour and formwork required to construct a floor is reduced for spans greater than 7 meters, thereby providing superior economy.
c. Reduced Floor to Floor Height
For the same imposed load, thinner slabs can be used. The reduced section depths allow minimum building height with resultant savings in facade costs. Alternatively, for taller buildings it can allow more floors to be constructed within the original building envelope.
d. Deflection Free Slabs
Undesirable deflections under service loads can be virtually eliminated.
e. Waterproof Slabs
Post-tensioned slabs can be designed to be crack free and therefore waterproof slabs are possible. Achievement of this objective depends upon careful design, detailing and construction. The choice of concrete mix and curing methods along with quality workmanship also plays a key role.
f. Early Formwork Stripping
The earlier stripping of formwork and reduced back propping requirements enable faster construction cycles and quick re-use of formwork. This increase in speed of construction is explained further in the next section on economics.
g. Materials Handling
The reduced material quantities in concrete and reinforcement greatly benefit on-site carnage requirements. The strength of post-tensioning strand is approximately 4 times that of conventional reinforcement. Therefore the total weight of reinforcing material is greatly reduced.
h. Column and Footing Design
The reduced floor dead loads may be utilized in more economical design of the reinforced concrete columns and footings. In multi-storey buildings, reduced column sizes may increase the floor net let table area.

These advantages can result in significant savings in overall costs. There are also some situations where the height of the building is limited, in which the reduced storey height has allowed additional storey’s to be constructed within the building envelope.

2. BONDED OR UNBONDED TENDON SYSTEMS
Post-tensioned floors can be constructed using either bonded or unbonded tendons. The relative merits of the two techniques are subject to debate. The following points may be made in favour of each.

a) Bonded system
For a bonded system the post-tensioned strands are installed in galvanized steel or plastic ducts that are cast into the concrete section at the required profile and form a voided path through which the strands can be installed.

The ducts can be either circular- or oval-shaped and can vary in size to accommodate a varying number of steel strands within each duct. At the ends a combined anchorage casting is provided which anchors all of the strands within the duct. The anchorage transfers the force from the stressing jack into the concrete. Once the strands have been stressed the void around the strands is filled with a cementitious grout, which fully bonds the strands to the concrete. The duct and the strands contained within are collectively called a tendon.

The main features of a bonded system are summarized below.
1. There is less reliance on the anchorages once the duct has been grouted.
2. The full strength of the strand can be utilized at the ultimate limit state (due to strain compatibility with the concrete) and hence there is generally a lower requirement for the use of unstressed reinforcement.
3. The prestressing tendons can contribute to the concrete shear capacity.
4. Due to the concentrated arrangement of the strands within the ducts a high force can be applied to a small concrete section.
5. Accidental damage to a tendon results in a local loss of the prestress force only and does not affect the full length of the tendon.

b) Unbonded system
In an unbonded system the individual steel strands are encapsulated in a polyurethane sheath and the voids between the sheath and the strand are filled with rust-inhibiting grease.
The sheath and grease are applied under factory conditions and the completed tendon is electronically tested to ensure that the process has been carried out successfully. The individual tendons are anchored at each end with anchorage castings.
The tendons are cast into the concrete section and are jacked to apply the required prestress force once the concrete has achieved the required strength.

The main features of an unbounded system are summarized below.
1. The tendon can be prefabricated off site.
2. The installation process on site can be quicker due to prefabrication and the reduced site operations.
3. The smaller tendon diameter and reduced cover requirements allow the eccentricity from the neutral axis to be increased thus resulting in a lower force requirement.
4. The tendons are flexible and can be curved easily in the horizontal direction to accommodate curved buildings or divert around openings in the slab.
5. The force loss due to friction is lower than for bonded tendons due to the action of the grease.
6. The force in an unbonded tendon does not increase significantly above that of the prestressing load.
7. The ultimate flexural capacity of sections with unbounded tendons is less than that with bonded tendons but much greater deflections will take place before yielding of the steel.
8. Tendons can be replaced (usually with a smaller diameter).
9. A broken tendon cause’s prestress to be lost for the full length of that tendon.
10. Careful attention is required in design to ensure against progressive collapse.

Why a bonded system?
This is another question that arises. Why do we use bonded tendons? Well there are a number of advantages; higher flexural capacity, good flexural crack distribution, good corrosion protection and flexibility for later cutting of penetrations and easier demolition.

However there are some disadvantages such as an additional operation for grouting and a more labour intensive installation.

However, the main reason why bonded tendons are preferred relates to the overall cost of the structure and not just of the post-tensioning. With unbonded tendons it is usual to have a layer of conventional reinforcement for crack control.

Using bonded tendons there is no such requirement and therefore the overall price of bonded post-tensioning and associated reinforcement is less than for bonded tendons. For unbonded tendons the post-tensioning price may be less, but the overall cost of reinforcing materials is greater.

3. MATERIALS

Post-tensioned floors use all the materials required in a reinforced concrete floor formwork, rod reinforcement and concrete- and, additionally, they use high tensile steel strand and the hardware specific to post-tensioning.

As a material, rod reinforcement in post-tensioned floors is exactly same as that in reinforced concrete in every respect. The normal high tensile steel, as used in rod reinforcement, has a yield stress of 460 N/mm2 and a modulus of elasticity of 200 KN/mm2. It has a poisons ratio of 0.3 and a coefficient of thermal expansion of 12.5 x 10-6 per degree centigrade. The strength of high tensioned steel is affected by rise in temperature, dropping from 100% at 300 degree centigrade to only 5% at 800 degree centigrade.

The technology for the production, compaction and curing of concrete is well understood and is not discussed here. Only the properties of concrete which are important for post tensioning are considered.

Normal dense concrete, 2400 kg/m3 density, is more common in post-tensioning. Light weight concrete, however, has certain advantages in the right circumstances. Both are dealt with in separate sections.

The properties of two concretes are quite different and it is not a good practice to use the two side by side; there may be problems from differential movement and the difference in their module of elasticity, shrinkage and creep.