Post-Tensioned Construction Technology For Multi-Storeyed Buildings
Dr. K. M. Soni, Chief Engineer, WZ-I, CPWD, Mumbai
Multi-storeyed construction is not done by choice but a necessity due to scarcity and high cost of land. Large module and span are now preferred particularly in office and commercial buildings due to economy in construction. It is also desired that more units can be accommodated in the same height of the building to minimise the cost per unit area of constructed space. In offices, commercial and institutional buildings, centralised airconditioning is also required due to temperature shooting up during summer in most parts of the country. Therefore, architects and engineers must adopt a technology by which size of air-conditioning duct is minimised and beams are either eliminated or their sizes reduced. Even in case of residential buildings if beams are eliminated, higher headroom can be achieved and within the same height, more number of storeys can be planned, thereby reducing the cost of a unit.
Post-tensioned construction technology offers the advantage of elimination or reduction in sizes of beams and slabs; hence in future, such technology is going to replace traditional RCC construction in India. In Mumbai, most of the construction is already using post-tensioned (PT) construction technology as it is readily available. Post-Tensioning – Need for Better Co-ordination The key to the successful and efficient construction of a building starts from its planning, which can be achieved only with better communication and co-ordination between architect, designer and construction engineer. In case of a private building, co-operation of the owner is also essential. In Mumbai, adoption of post-tensioned construction technology in buildings is the outcome of close interaction and co-ordination of all the stakeholders. Even if the design codes
Dr. K. M. Sonifrom Bureau of Indian Standards are not available, the architects take advantage of the technology, structural designers design the structural members and engineers incorporate the items in the tender documents and execute the works. This shows the will of using new technology by the stakeholders. Post-tensioning helps to meet the design objectives, both architectural as well as functional. Some of the benefits of posttensioning can be listed as:
- Post-tensioning allows the floor framing to be slenderer.
- With post-tensioning, small structural depth is feasible for long spans.
- Post-tensioning leads to thinner sections thereby reducing the quantities of concrete and steel.
- Post-tensioning imparts high early strength in concrete.
- It helps in reduction of dead load of the building.
- It provides considerable column free space thus larger useable space.
- It provides larger floor to floor space i.e., higher headroom for the services like air-conditioning ducts else more floors can be accommodated in same height.
- Post-tensioning allows earlier stripping of formwork.
- As significant part of the load is resisted by post-tensioning, handling of non-pre-stressed reinforcement is simple.
- It helps in speedy and quality construction.
- Post-tensioning reduces the chances of corrosion.
- Post-tensioning enhances durability.
Post-tensioned Construction and Durability
Durability of concrete in RCC construction depends on many factors like aggregates, size and their proportion, type of cement, its composition, fineness, water, water-cement ratio, cement admixtures ratio and air voids. It also depends upon the reinforcement and cover to the reinforcement used in concrete. With adverse atmospheric conditions and reactivity of cement, aggregate, water or harmful minerals present in any constituent of the concrete reacts with reinforcement under favourable conditions, and corrosion starts in the reinforcement leading to spalling of concrete affecting the durability. Cracks, carbonation, permeability, and chemicals further enhance the corrosion. Post-tensioning in slabs provides higher cover to strands as compared to conventional RCC. The galvanized or lead coated sheathing protects strands from corrosion. Duct filled with cement grout has lesser chances of air entry. The area of duct being very small results into uniform quality compared to large area in case of conventional RCC. Post-tensioning being a specialized work also ensures proper quality control in field. Thus, post-tensioning helps in durability of concrete (Soni & Kataria, 2014).
Pre-stressed Concrete and RCC
In pre-stressed concrete the entire section of the concrete becomes effective where in reinforced cement concrete (RCC), only the portion above neutral axis is supposed to act. High strength concrete that cannot be economically utilized in RCC is desirable and essentially required in pre-stressed concrete. In pre-stressed concrete, high strength concrete is required to match with high strength steel and to resist high stresses at the anchorages.
For a cast-in-situ structure, pre-stressed concrete must be designed for at least two stages: the initial stage during pre-stressing and the final stage under external loadings. In the initial stage, the member or structure is under pre-stress but not subjected to any superimposed external loads. Before pre-stressing of the concrete, it is quite weak in carrying load, hence the yielding of its supports must be prevented. Provision is also to be made for the shrinkage of concrete if it occurs. During pre-stressing, tendons are subjected to the maximum stress. Pre-stressing operations impose a test on the bearing strength at the anchorages. Since concrete is not aged during pre-stressing, crushing of concrete at the anchorages is possible if its quality is poor or has insufficient strength, and thus high strength concrete is to be used. In pre-tensioning, the transfer of pre-stress is accomplished in one operation and within a short period. For posttensioned members, the transfer is often gradual, the pre-stress in the tendons being transferred to the concrete one by one. In both the cases, there is no external load on the member except its own weight. If a member is cast and pre-stressed as per the requirements, it becomes self-supporting during and after pre-stressing. Thus centring/form work can be removed after pre-stressing.
Pre-tensioning and Post-tensioning
Pre-tensioning is the method of pre-stressing in which tendons are tensioned before the concrete is placed. The tendons are temporarily anchored when tensioned and the pre-stress transferred to the concrete after it has set. Post-tensioning is a method of pre-stressing in which the tendons are tensioned after the concrete is hardened. Thus, pre-stressing is almost always performed against the hardened concrete and the tendons are anchored against it immediately after pre-stressing. When post-tensioned, the tendons are anchored at their ends by means of mechanical devices to transmit the pre-stress to the concrete. Such a member is called end-anchored. In pre-tensioning, the tendons generally have their pre-stress transmitted to the concrete simply by their bond action near the ends.
Bonded tendons mean that they are bonded throughout their length to the surrounded concrete while non-bonded tendons are to be protected by galvanizing, greasing, or some other means from corrosion. Non-end anchored tendons are necessarily bonded ones while end anchored may be bonded or unbounded. Where the cables are contained within the ducts inside a concrete section, cement grouts are pumped for protection from corrosion. But a fundamental requirement for pre-stressing tendons is that they must be protected against corrosionto maintain integrity of the structure.
Central Public Works Department (CPWD) has adopted posttensioned (PT) construction technology in most of its major multistoreyed buildings in Mumbai. Some such buildings recently completed are:
1. Central Bureau of Investigation (CBI) office building at BKC, Mumbai
2. Income Tax building at BKC, Mumbai
3. IDBI building at Belapur, Navi Mumbai
4. Academic Blocks of National Institute of Securities Markets (NISM), Mumbai
In NISM, PT slab construction was adopted to achieve higher headroom though the buildings were only G+2 storeyed. A case study of CBI building is discussed in detail.
Details of Post-tensioning in CBI Building
CBI office building is 2B+G+13 storeyed RCC framed construction in which all the floors are constructed with PT construction technology. The pre-stressing strands having seven wires were used in pre-stressing tendons conforming to ASTM 416/90 specifications. 7 wires (super type of strands) had 15.24 mm nominal diameter and 140 sq mm steel area with ultimate tensile strength of 260.7 kN and modulus of elasticity as 195 kN/sq mm. Strand diameter at anchorage relaxation was 6 mm.
used was suitable for the pre-stressing system and strong enough to withstand the placement and compaction of the concrete without suffering damages or deformation. The sheathing and all splices were mortar-tight having friction factor as 0.21 and wobble factor as 0.001 rod/m. Anchorage device can transmit a force not less than the ultimate tensile strength of the tendon without overstressing the concrete. Cubes of nominal size 150 mm were cast for determining the concrete strength at transfer. These Cubes were stored under the similar conditions as the concrete they represented. Post-tensioning was carried out after concrete attained a strength of 25 N/mm2. Grout for filling pre-stressing ducts composed of cement, water and additive to reduce shrinkage and bleeding was used. The grout was injected into each duct. Continuous steady flow of grout was maintained until the duct got filled by pouring from all vents and from the far end until all entrapped water and air got expelled. The vents were thereafter closed as required to ensure complete filling of the duct.
Before casting, the contractor was asked to submit the design and shop drawings for the pre-stressing system. The design was based on Presscrete Post-Tensioning System (PPS). Concrete in one-member panel was placed in one operation. Bottom reinforcement was laid as per the design/drawing after the formwork was ready. The cover was maintained as per specifications. Bursting steel for slab casting was laid before the ducts were joined for the casting. Galvanised flat duct was laid, and strands inserted into the flat ducts according to the drawings. Bar chairs were placed, and profile adjusted as per design. Grout vent with hose was fixed at both ends of the tendons and at the mid spans of the tendons for tendons exceeding 25 m length.
Care was taken that the pre-stressing tendons do not get displaced or damaged prior to casting and during casting. It was also ensured that discharge of concrete was not directed onto the pre-stressing tendons. Vibrating was avoided for concrete from contacting the pre-stressing tendons. Proper compaction of the concrete in anchorage areas was ensured due to high local stresses in these areas and care taken that all the grout hoses remain exposed and protruding from the concrete surface.
After concreting, the carpenter removed the vertical sides of end formwork. All the stressing recesses were cleared, and pre-stressing barrels and wedges fixed to all pre-stressing casting. When concrete reached the transfer strength as per the shop drawings and confirmed from the results of cube tests, stressing of cables was carried out. All cables were tensioned to the required jacking force and monitored through the pressure gauges.
Excess cable length was cut after the stressing and pre-stressing tendons flushed with clean water. Grout was mixed according to the design mix for at least two minutes until a colloidal consistency produced. The water was put into the container first and then cement added slowly. When the cement and water got thoroughly mixed, the non-shrink additives were added to the mix. The grout was then injected into the pre-stressing tendons. Grouting was carried out at one end of the duct until clear grout flowed out from the other end. The grout hose at the both ends was then sealed. Grouting works were commenced when all scaffolds at a storey where grouting works were to be carried out, were removed.
Bonded post-tensioning system was used in the present case. The tendons were laid in sheathing. Casting was done by keeping one end fixed called dead end. After casting when concrete attained the desired strength, stressing was done. The tendons were laid in the slab according to the profiles before pouring the concrete. After the strands got locked within the anchorage by the wedge, they were individually stressed with hydraulic jacks. The ducts were then filled with cement-based grout for bonding the strands to the concrete through the duct all along the length of the tendon.
Since the cables/tendons in the slab are stretched to high tensile strength, it is extremely important that the slab is not drilled, cut, chiselled or disturb in a way to expose the tendons.
Pre-Stressed Concrete versus RCC
The advantages and limitations of pre-stressed concrete over RCC are presented below:
- Reduction in concrete due to thinner concrete members.
- Reduction in rebar in floor elements.
- Reduction in dead load resulting into saving in concrete and reinforcement of structural members including foundation.
- Saving in building cladding, vertical mechanical/service elements, rebar and concrete in shear walls and other materials due to lesser sizes.
- Potential pour cycle of 3-4 days.
- Reduced re-shoring.
- Better coordination with embeds and MEP openings.
- Improved seismic behaviour.
- Reduced deflection and vibration.
- Improved crack control and water proofing properties, especially beneficial for parking garages and balconies.
- Longer spans and fewer columns giving greater flexibility in floor layouts in office/residential buildings and better lighting in parking garages which enhances personal safety.
Reduced Lifetime Cost
- Lower maintenance and lifecycle cost.
- Reduced building height resulting to higher headroom and energy savings.
- Potential to conform to green building norms.
- Pre-stressed concrete design is more suitable for long span structures and those carrying heavy loads. Pre-stressed structures are slender and thus yield to more clearance. They do not crack under working loads and dead loads, and the deflection is reduced due to cambering effect of pre-stress.
- Though both reinforced concrete and pre-stressed concrete will be safe when designed for the conditions but in pre-stressed concrete, there is partial testing of both steel and concrete during prestressing operations and when materials can stand pre-stressing, they are likely to possess sufficient strength for the designed service loads. The resistance to corrosion is better in pre-stressed concrete than in reinforced concrete due to denser concrete and non-existence of cracks. Pre-stressed concrete members require higher order of care in design, construction and erection than reinforced concrete.
- Pre-stressed concrete is economical in certain cases when same unit is repeated many times, under heavy loading of structures, and long spans.
- Pre-stressed members are likely to be less prone to corrosion and as such better suited in coastal areas and in the places where atmospheric conditions are adverse.
Pre-stressed members cannot be drilled during their service life else are likely to get damaged, which may cause failure of the structure. In India, the post-tensioning is a specialised job and is done by only a few agencies having expertise in post-tensioning system. Due to involvement of specialised agencies, the specifications are followed as per the design standards and quality maintained during all the operations. Due to better quality control, members with very low porosity are produced thereby having longer service life than conventional RCC members.
Pre-stressed construction technology has been widely used in the bridges but is gaining popularity in multi-storeyed buildings particularly having large spans and to gain more headroom. Also, pre-stressed construction technology helps in preventing corrosion and produce faster and quality construction. Due to adoption of such construction technology, more units can be planned within same height for reducing cost per unit of floor area. In Mumbai, pre-stressed construction technology is being adopted widely by the builders and government organisations. CPWD is also taking up such construction in Mumbai, details of such a building constructed for CBI have been presented.
2. Post-tensioned in buildings (1992) published by VSL International Ltd., authored by Franz A. Zahn, and Hans R. Ganz, from https://www.vsl.net/sites/ default/files/vsl/datasheet/PT_Buildings.pdf
3. Report on “Design by Anu and approved by Er. Muthu” from Utracon Structural system Pvt Ltd (2012). Post-tensioning Slab in Construction of Office Complex for CBI at BKC, Mumbai.
4. Soni, K M and Kataria, Bharat (2014). Durability of Concrete: A Case Study of an Office Building with Post-Tensioned Slab. Proc. 2nd International Congress on Durability of Concrete, New Delhi.
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