Post-Tensioning Systems
Unbonded Systems
When building a concrete floor slab with post tension system a deck is required as a form for the strand wires and concrete. This large deck shown has a board form finish inlayed on top of the plywood decking so that when the concrete is poured it will mirror the surface. The strand wires are brought onto the site in a coiled forms. First the workers mark the locations of the monoanchors that each strand wire will require to be jointed and then they fix the Anchors at the end of the forms in their planned places. The anchors chosen are encapsulated so that they have an additional advantage of waterproofing to protect the strand wires from corrosion. The strand wires themselves are greased inside a plastic hose that will allow them to be elongated after pouring the concrete.
The next step after the anchors have been fixed around the edges of the formwork, the coils are unwound and are inserted inside the anchors in preparation for the proper locations. In the shown slab the strand wires are placed in a basket weave and must be located according to a certain profile so that they pass through the designed levels correctly over top one another. In addition this slab is particularly more difficult than other slabs because the thickness varies and the strand wires are located so that they curve in rise in the correct locations following the shop drawing by the designer.
The Post Tensioning workers use many sized bar chairs that locate the strand cables at there correct profiles. The strand cables level usually reached the highest level in the form over the positions of the supports and lowest level between the supports so the tension force on the cables pulls pressing up the slab in the middle of the spans. The strand mono anchors are also separately located at the end of slabs but are gathered as they leave the edges on there path to the opposite edge..
The final profile of the post tensioning looks like a complex network that includes reinforcement of the top mesh over the column positions. Now the deck is ready for the electromechanical work to place all of the electrical pipes through the network of post tension strand wires before pouring the concrete. Once the concrete is poured the slab will begin to wet and after it has reach the required strength the cables can be stressed. Each cable is stressed separately with around 30,000 lbs. of force.
The Post Tensioning Wedges are fixed together around the strand and inside the cone openings at the slab end. If you see the wedges you can notice that they have teeth that allow the steel parts of the wedge to hold the strand. The teeth are angled in a way that the strand can easily be moved outside the anchor but when the strand is released the teeth grip strand and the wedge shaped part is resisted strongly against the anchor. The excess wire now can be cut off and the stressing force is now completely applied onto the slab. The force is very strong against the concrete slab that there is tiny shrinking in the slab as well as lift provided by the strands.
The Post Tensioning workers use many sized bar chairs that locate the strand cables at there correct profiles. The strand cables level usually reached the highest level in the form over the positions of the supports and lowest level between the supports so the tension force on the cables pulls pressing up the slab in the middle of the spans. The strand mono anchors are also separately located at the end of slabs but are gathered as they leave the edges on there path to the opposite edge..
The final profile of the post tensioning looks like a complex network that includes reinforcement of the top mesh over the column positions. Now the deck is ready for the electromechanical work to place all of the electrical pipes through the network of post tension strand wires before pouring the concrete. Once the concrete is poured the slab will begin to wet and after it has reach the required strength the cables can be stressed. Each cable is stressed separately with around 30,000 lbs. of force.
The Post Tensioning Wedges are fixed together around the strand and inside the cone openings at the slab end. If you see the wedges you can notice that they have teeth that allow the steel parts of the wedge to hold the strand. The teeth are angled in a way that the strand can easily be moved outside the anchor but when the strand is released the teeth grip strand and the wedge shaped part is resisted strongly against the anchor. The excess wire now can be cut off and the stressing force is now completely applied onto the slab. The force is very strong against the concrete slab that there is tiny shrinking in the slab as well as lift provided by the strands.
Post-Tensioned Concrete
The advantages for using United Post Tensioning Systems in concrete are countless, but the most important is thinner clear spans that would not be achieved by conventional slabs.
United Post-tensioning Systems supplies a method to become stronger concrete by derive the power of the concrete in condense and conquer its fragility to withstand the tension. United Systems implement a compress force to the construction through the pressure of high strength strand wire with generalized anchorage assemblies. The adaptability of the fiber is a significant benefit, permit it to be outline through the concrete component , offset a piece of the administer loads to provide an brilliant dynamic contraction.
Post-Tensioned Concrete Slabs
United slabs presents much power over conventional reinforced concrete and help overcome some of the limits faced by designers, construction companies.
Thinner Slabs
Thinner slabs increase flooring height and can even develop extra floors within the same identical structure elevation. Extra cost reduction can be done on the expense of reducing walls height and finishing.
Longer Spans
The longer spans gain with United slabs decrease the slab supports , giving greater manageability of design and increase open area.
Crack Resistance
The decrease cracking of Unites slabs makes them perfect for open soffit usages.
Flexibility
Able to deal with challenging or hard math, follow sporadic column installation and curved edges, United slabs allow engineers more options to create more usable construction.
Reduced Material Usage
When analyze to strengthened concrete, United Slabs need quantities of concrete and support, in some territory by as much as 30%, producing operative cost savings.
Lower CO2 Emissions
The incline CO2 emissions accompanying from diminished quantities of concrete and steel provide environmental reward.
Faster Construction
Minimized quantities of steel within the slabs speed up assembly slot by minimizing installation time. The stressing of the post-tensioning system at early concrete stages allows a quicker shoring and re-shoring time interval.
The strand wire at the edge of the slab is colored with spray paint and will be used for measuring the elongation when the wire is been stressed. Each wire is has a certain elongation on the shop drawing to have a specific amount of force that will be measured by a supervisor for approval.
The hydraulic stressing jack is inserted into the strand and fix's the wedges as it holds down around the strand and pulls pushing the wedges towered the anchor and the strand outside. The Jack is operated by a hydraulic pump and is controlled by the operator until it reaches the required stressing force. The jack then returns to its beginning position and the second operator moves to the next anchor.
After the wires are tensioned and checked for correct elongation length and approved at approximately 5" per 720" strand length. The cable is then cut off just after the wedge so a plastic encapsulating cap packed with grease can be fixed over the leftover cable and wedge. The circular pocket at the edge is grouted with corrosion inhibitor. Finally a bituminous layer of waterproofing is painted over the slab to ensure protection.
Grouted Systems
United grouted system is used mainly in slabs and transversally in bridges decks. It is also used in beams with planted columns , control structures and other civil appliance.
The systems attach bare wires which dash through a steel or plastic smooth oblong duct. The wires are stressed independently using Monostrand Jacks and the tube filled with cementitious adhesive. The exceptional design of the United system grants the wires to be planted in two layers within the Anchor head accumulation and secure decreased centre-to-centre height between the anchors.
Why a Grouted system is preferred ?
This is another question that arises. Why do we use bonded tendons? Well there are anumber 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.