Post-tensioning, or PT, has become increasingly popular over the past few years or so as the technology has been perfected. Better materials and construction methods has made this system a preferred one. Post tensioning works are finding it’s greater importance in overcoming the difficulties in progressing the construction works in the traffic areas. PT works are predominantly used in the Metro rail constructions in all the cities that are currently in progress and some cities it is extended in phases. One such group of Post tensioning experts is Utracon. Utracon’s India Operation was started by 2004 and more than 2500 over projects with 250 Million Sq.ft PT Slab area had been completed. UTRACON INDIA’S PRESENCE IN ALL THE 2/3 TIER CITIES OF INDIA, DESERVES SPECIAL MENTION FOR THEIR SERVICES.
Utracon is operating throughout India and its track record encompasses projects from private to public sectors, and completed sky scrappers and massive civil engineering projects are testimonials to its strive to be a leading and the most TRUSTED PROJECT PARTNER.
Analysed below is the process of Post tensioning system used by Utrcacon.
Post Tensioning Overview
Prestress Concrete: – Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from external loads are counteracted to a desired degree. In reinforced concrete member, the prestress is commonly introduced by tensioning the steel reinforcement.
The application of permanent compressive stress to a material like concrete, which is strong in compression but weak in tension, increases the apparent tensile strength of that material, because the subsequent application of tensile stress must first nullify compressive prestress.
Pre – Tensioning: – A method of prestressing concrete in which the tendons are tensioned before concreting. Pre-tensioning is done by stressing a wire, strand or bar, PRIOR to concreting. As these stressed elements are embedded and bon-ded in the concrete, no anchorages are re-quired to hold back the tension forces in it.
Pre-tensioning is predominantly done in precast yards. End blocks supported by piles are required to be erected to hold back these prestressing forces.
Post – Tensioning: – A method of prestressing concrete in the prestessing steel is tensioned against the hardened concrete. For post-tensioning, wire, strand or bar is installed and stressed to its intended forces AFTER concreting has taken place. Anchorages and wedges are used to hold back the forces.
Strand: – Strand is manufactured from 7 individual cold-drawn wires, outer wires helically wound around one center wire (king wire) as per IS code (IS14268:1995). Strand coatings do not affect the anchorage’s capacity or efficiency. For improved corrosion protection we offer systems using galvanized iron ducts. The strand should be stored in clean and dry place. There are two different types of strand i.e. 0.5” and 0.6”.
Sheathing: – The sheathing for bonded tendons shall be galvanized steel tube made from galvanized steel strip of 0.25 0.3mm thick. It is mortar tight to prevent entrance of cement grout during concreting.
The duct shall be sufficient strong to retain shape and to resist damage during construction.
HDPE: – The material for the duct shall be high density polyethylene with more than 2% carbon black to provide resistance to ultraviolet degradation. The thickness of the wall shall be 2.3±0.3mm as manufactured and 1.5mm after loss in the compression test for duct size up to 160mm OD. The duct shall be corrugated on both sides. The duct shall transmit full tendon strength from the tendon to the surrounding concrete over a length not greater than 40 duct diameter.
Anchorages: – The prestressing force is transferred to the concrete at the anchorage. Anchorage where the stressing take places are called active anchorage, and the other are called passive anchorage. Sometimes stressing is made from both ends of a tendon to reduce friction losses. The anchorage shall be safe and secure against both dynamic and static loads as well as impact.
The anchoring device shall be strong enough to resist in all respect a force equal to at least breaking strength of the prestressing tendons.
Posttensioning Working Sequence
- Completion of formwork as per shop drawing.
- Check level difference; pour strip location, slab thickness and drop panel depth before erection of bottom reinforcement.
- Completion of bottom reinforcement with appropriate Dia of bar and with equal spacing.
- Fixing the block out to casting and wrapping it with OPP tape.
- Marking of tendon in both live and dead end with marker.
- Anchorage should be fixed to the mark at end form work by supporting bars.
- Laying of GI duct is under progress.
- Fixing of duct to end anchorage with the help of vent coupler and it should be wrapped with OPP tape to avoid entry of foreign particles.
- De coiling of HT Strand and cutting the strand for required size of the tendon.
- Inserting the strands to the duct.
- Check C.G of the casting.
- Bursting reinforcement should be fixed to position at end anchorage
- Profile chair should be fixed to the tendon as per shop drawing.
- Fixing of grout hose in both live and dead end. If the span is more than 35m intermediate vent hose should be provided.
- Proper packing of casting to avoid entry of foreign particle.
- Pouring of concrete (M-35 Grade).
- Proper vibration of the member @ face of the anchorage.
- Concrete should attain minimum strength of 28N/mm2
- Removal of block out.
- Fixing of wedge Plate and wedges.
- 25% of initial load for slack removal.
- Mark each strand with help of the marker
- Stress the strand for final loading as per stressing calculation report.
- Ensure Protruding strand length is sufficient for stressing (350 – 400mm)
- Stress of tendon is done by (T-25 Jack) as it can stress only one strand at a time.
- Elongation for each strand should be checked after 24hrs.
- Multi jack should be hoisted with chain pulley.
- All strands should be inserted to jack guide pipe.
- Ensure the strand is locked to jack pulling head.
- 25% of initial load for slack removal.
- Record the strand elongation for every incremental of 10 Mpa.
- Release the pressure when the jack has reached desired load.
- Mix the grout as per grout mix.
- The mix is prepared with W/C ratio of 0.4 0.45 as per site condition with additive of 0.2% (Weight of the cement)
- Inject the grout from tendon inlet hose.
- As the grout flows out of the intermediate vent, lock these vents in the direction of grout.
- Lock the outlet hose when the grout flows out.
- Check the grout pumped out has the same consistency as its pumped in.
- Lock the inlet hose when the required pressure is achieved (3-5 bars).
Advantages & Applications
For long span & heavy loading structures:
P.T. has been proven to be economical for normal commercial buildings and to a certain extent, residential buildings, with spans greater than 7.0m. For a typical structure with floor loading of 5.0 kN/m2 and with spans of approximately 10.0m, anticipated savings as compared to conventional RCC design:
Greater Headroom – Due to the more efficient design concept of post-tensioning, PT structures are generally thinner as compared its RCC counterpart. E.g. for a typical residential project, RCC flat slab may be of 250mm thick, whereas PT slab of 200mm thick is sufficient.
Neat & Simplified Layout – For large spans of more than 10m, RCC design would generally adopt a ribbed beams framing system. This could be substituted with PT flat slab or banded slab system, which are very friendly for construction and fixing of services.
Speedier Construction – Due to simplified structural layout of floor and reduced materials used, construction can be carried out faster with less material wastages.
Time saving is derived mainly from the following
Reduced Steel Quantity – Steel laying is a very labour intensive activity, and by reducing the quantity of steel in PT design, the slab can be ready for concreting earlier.
Generally PT flat slab requires approximately 20 to 35kg/m3 of steel compared to that of RCC slab’s 80 to 120 kg/m3.
Earlier Stripping of Shuttering – Post-tensioned slab would be ready for stressing operation when concrete strength achieves 28 N/mm2.
For a concrete grade of 35 N/mm2, this normally happens on the 5th day after concreting.
Upon completion of stressing, all the slab shuttering can be removed and recycled for other slab’s concreting.
This earlier stripping of shuttering would mean that the builder would need less shuttering materials and the finishing work at the lower floors can be carried out earlier.
Improved Deflection Criteria:
- The presence of prestressing helps a PT floor slab to “arch” slightly after stressing of tendons.
- This will help to reduce the overall downward deflection of floor slab and in turn help to alleviate the long term deflection problems, especially for long span structures.
- Long term deflection, if left unchecked, would result in the cracking up of stiff non-structural members, e.g. brick walls, floor tiles, etc., in the long term (i.e. 5 to 10 years after construction).
Economical & Superior Design:
Post-tensioning offers savings to clients due to its superior structural behavior and other advantages.
Proposals have been made and accepted by local builders and developers in India as time and again the post-tensioning solution out performed the RCC scheme both in terms of costs, structural depths and other benefits to the future performance of the building.
- Earlier Stripping of Shuttering.
- Flexibility in providing service ducts and false ceilings.
- Larger column free space.
- Reduction in Dead load due to Thinner slab which will make cost savings
- Less no of work force engaged for Post tensioning works than RCC slabs
- Reduction in reinforcement quantity.
- Post-tensioning can thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors reducing the foundation load and can be a major advantage in seismic areas.
- Reduces occurrence of cracks.
- Freezing & thawing durability is higher than non prestressed concrete.
- Post-tensioning is the system of choice for parking structures since it allows a high degree of flexibility in the column layout, span lengths and ramp configurations
- Greater Flexibility for penetrations and openings in floors, both pre and P/ost Construction.