Post Tensioning
Post Tensioning is a great tool for builders, a delight for developers and is also kind on the environment. It allows construction of structures with any shape and reduces environmental impacts, costs, construction time, and materials. Since the application for the first time, Post Tensioning technology has experienced outstanding advances – especially in the terms of quality assurance,performance and durability as well as corrosion protection
Post Tensioning applications include office floors, residential and commercial buildings, parking structures, warehouses, bridges, and rock/soil anchors. In many cases, Post Tensioning allows construction that would otherwise be impossible due to either site constraints or architectural requirements
Bonded Post Tensioning is a procedure used to reinforce (strengthen) concrete with high strength steel bars, typically referred to as tendons. In this concrete is cast around plastic, steel, or aluminum curved ducts in the area where tension would occur. A set of tendons are passed through the ducts and the concrete is poured in. Once the concrete hardens, the tendons are tensioned by the hydraulic jacks against the concrete itself. When the tendons are stretched sufficiently, as per the design specifications, they are wedged in position to maintain tensions after the jacks are removed, transferring pressure to the concrete.
Benefits of Bonded Post Tensioning over Unbounded Post Tensioning
- Substantial reduction in accustomed reinforcement requirements as tendons cannot distress in pileup.
- Tendons can be effortlessly “woven” allowing a more competent design approach
- No long term maintenance issues with the integrity of the anchor / dead end
- Improves the structural performance of concrete members by reducing tensile stress in the outer concrete fibers
- A sophisticated yet adaptable and versatile engineering technique
- Higher ultimate strength due to bond generated between the strands and concrete
Benefits of Post Tensioned Slabs over Conventional Slabs
Flexibility
Internal layout flexibility is greatly increased, making it much easier to place or reposition partitions.
Time saving
Framework stripping and fewer reinforcing elements results in rapid construction than R.C.C. Construction.
Economical
Reduced footing thickness, fewer support columns, lighter structures and thinner slabs resulting in less material costs and reduction in excavation and maintenance cost.
Sturdy
Increased crack resistance under load, temperature variations and concrete shrinkage due to highly compressive characteristics of prestressed concrete structure.
Better water tightness
Increase in water seepage due to flatter surface and limited deflection in the concrete structure
Multi-strand Post Tensioning is extensively used in construction of bridges, transportation structures and is also successfully applied for commercial building constructions.The KGN multi-strand system is symbolized by standardized tendons units of up to fifty five 13-mm (0.5)” or 15-mm (0.6)” diameter strands with a wide selections of anchorage types. Ducts are either steel or plastic and are fully bonded with cement or other high-performance grout.
Manufacturing is done on site or in factory to determine tendon lengths in advance. Simultaneous stressing of all strands is done with individual locking of each strand at the anchorage point. Stressing is carried out in number of phases with simple and reliable equipment for installation, stressing and grouting.Additionally, as the strands are stressed simultaneously, less labour is required ultimately resulting in cost savings.
Design advantages of multi-strand post tensioning systems in large structures include
- Increased span lengths
- Higher load-carrying capacity
- Reduced deflection
Benefits of KGN’s bonded slab Post Tensioning include:
- Enhanced control over cracks and deflections
- Reduction of mild reinforcing steel minimizes the primary sources of corrosion and significantly reduces life cycle maintenance costs.
- KGN supports thin post tensioning slab design – reducing overall building height which lowers costs for foundations, frames as well as facade/cladding components.
- As bonded tendons can be cut with only localized loss of capacity, KGN system successfully facilitates future building modifications.
This custom of KGN’s bonded post tensioning slab system has been adopted in many prestigious buildings. This approach is more economical than other systems, especially in fast construction cycles. The less material handling on site and a reduced labour force minimizes site congestions. Most importantly, KGN s specialized and highly efficient teams believe in providing high quality service to its clients
The KGN’s Post Tensioning system is extensively channelized in the construction of slabs on grade.We adopt a versatile method to produce slabs in commercial and industrial structures by using the principles of post tensioning to compensate excessive tensile stress.
Eliminate Joints
Joints that are reinforced concrete slabs have long been a reason for costs and delays because of the constant maintenance. The KGN post–tensioning concrete slab on grade is thinner as compared to traditionally reinforced slabs.It saves construction time and cost, because smaller amounts of concrete are laid in larger areas per day, often round about 2,500 square meters.
Supports any load
The KGN’s post-tensioning slab on grade can be designed and constructed to accommodate any load requirement.
The joint free post-tensioning slab on grade technology is used mainly in high load structures like industrial floors and pavements, storage buildings, logistic parks, etc.
Crack control
Post-tensioning slab on grade effectively reduces the risk of cracks than reinforced slabs because of the compressive forces that are applied by the post tensioning cables.
Fast-track construction
Post-tensioning slabs can be constructed much faster than conventional reinforced slabs. Time is also saved by using fewer joints and narrower footings with less excavation and less concrete.
Reduced maintenance
The use of fewer – or even no – joints significantly reduces the need for future maintenance. Studies have shown that the major costs over the life of a structure involve joint maintenance. With a post-tensioning slab, the costs of joint and crack repair are reduced as too is the damage to plant caused by the breakdown of joints.
Flexible slabs
Thinner slab isgenerally more flexible. Large overload on the slab may cause cracking;however thin pre-stressedslab returns to its previous state when the load is removed. In contrast, with a conventional reinforced slab a weak point remains in the substructure, leading to deterioration.
Flatter slabs
Fewer joints and greater spacing between them greatly reduces the risk of slab curling.
Deflection control
Soil expands when wet and shrinks when dry. This can cause movements and cracking in the foundation structure. post tensioning increases the slab’s tensile capacities, making it more resistant to problematic soils.
Ground Anchor
KGN designs, fabricates and installs two types of ground anchors – strand and bar anchors. The type of anchors used depends on if it is rock soil, for temporary or permanent use, whether or not it is to be tensioned, and whether or not permanent corrosion protection is required.
KGN offers all of these alternatives and can support a full anchor material supply service (anchors and accessories) with back-up including design services, advice consultancy, testing, installation, and tensioning and site supervision.
The construction of the KGN strand anchor depends on the type of rock or soil, the design, the corrosiveness of the environment, the presence of stray electrical currents and the intended service life. While temporary ground anchors require less or no corrosion protection, permanent ground anchors (with a service life exceeding two or three years) need to have a comprehensive permanent corrosion protection system.
The anchorage is the combined system of anchor head, bearing plate, and trumpet that is capable of transmitting the prestressing force from the prestressing steel (bar or stand) to the ground surface or the supported structures. The unbounded length is that portion of the prestressing steel that is free to elongate elastically and transfer the resisting force bond length to the structures. A bond breaker is a month plastic sleeve that is placed over the tendon in the unbounded length to avoid the prestressing steel from bonding to the surrounding grout. It enables the prestressing steel in the unbounded length to elongate without obstruction during testing and stressing and leaves the prestressing steel unbounded after lock –off
The tendon bond length is that length of the prestressing steel that is bonded to the grout and is capable of transmitting the applied tensile load into the ground. The anchor bond length should be located behind the critical failure surface.
The basic components of a grouted ground anchor include the:
- Anchorage;
- Free stressing (unbounded) length and
- Bond length
The anchorage is the combined system of anchor head, bearing plate, and trumpet that is capable of transmitting the prestressing force from the prestressing steel (bar or strand) to the ground surface of the supported structure. The unbounded length is that portion of the prestressing steel that is free to elongate elastically and transfer the resisting force from the bond length to the structure.
A bond breaker is a smooth plastic sleeve that is placed over the tendon in the unbounded length to prevent the prestressing steel from bonding to the surrounding grout. It enable the prestressing steel in the unbounded length to elongate without obstruction during and stressing and leaves the prestressing steel unbounded after lock-off.
The tendon bond length is that of the presterssing steel that is bonded and is capable of transmitting the applied tensile load into the ground the anchor bond length should be located behind the critical failure surface.