Have you ever watched construction crews building a parking garage or skyscraper and wondered how they were able to raise and attach huge concrete panels like they were building blocks? You never see them actually fill concrete into a form to make the panels, so how do they do it? The trick is actually a construction system referred to as post tensioning and it has allowed for many serious miracles in urban construction since its inception.
What is Post Tensioning? Put simply, this is a technique for stressing anchorage, to make them stronger and much more immune to cracking and breaking under environmental conditions. Concrete, when created and formed, could be very strong in its compressed form yet it is still relatively weak in other places. By bonding the concrete block during creation with stainless-steel wires or rods, it improves its tensile strength, making the whole more durable to extreme changes.
The primary advantage to creating concrete construction panels in this way is it reduces the maintenance needed to keep your structure as a whole fully operational. Whether or not the finished product is a slab foundation or a nuclear containment wall, it can have the flexibility it requires to resist temperature and environmental changes, less material will likely be needed to cover the identical area, a lighter structure weight overall and a higher-level of structural integrity having a marked increase in deflection and vibration control. This technique has been a boon to those areas that are regularly plagued by earthquakes, and hurricanes.
How Will It Be Done? Tensioning strands of stainless wire, usually grouped into bundles, are stretched along the size of a vacant concrete form prior to pouring. As the concrete hardens, the strands are tightened, “stressing” them throughout the hardening concrete. The strands are anchored by adjustable chucks, which can be loosened or tightened during installing of the finished panel.
The two main methods widely used today in developing a post tensioned slab of concrete: wedge anchor. In the bonded method, a bonding grout is put into each channel by which the tensioning wires happen to be strung. This process increases the stability in the whole, further reducing maintenance costs in construction. Architecture designs that require greater flexibility, like those found in earthquake zones, will use the unbonded method, since they require greater flexibility, as opposed to additional reinforcement.
Typical steel strands found in post-tensioning have a tensile strength of 270,000 pounds/in². In comparison, an average non-prestressed bit of reinforcing features a tensile strength of 60,000 psi. Strands routinely have a diameter of 1/2 in., and are stressed to a force of 33,000 pounds. The stresses brought to the concrete cancel out the expected external loads the concrete is going to be exposed to.
In post-tensioning the steel is held in a duct, which stops the steel and concrete from binding after the concrete solidifies. The steel may then be stressed following the concrete sets. This is usually carried out in two stages. The first prestress applied is up to 50% from the final force and this is accomplished when the compressive strength reaches 12-15N/mm².
The second stage happens when the barrel and wedge anchor occurs and this is accomplished when the concrete meets its design strength. The strands and bars are tensioned by stretching all of them with a hydraulic jack. They are then fixed in place having an anchoring component, which supports the force inside them for that life of your building. Post-tensioning allows the engineer to have all of the benefits rwkhni using prestressed concrete whilst keeping the freedom of constructing on site.
Probably the most prominent markets for concrete which has been created through post tensioning represent nearly all installations that can be found in the typical city’s infrastructure. Airports, stadiums, parking garages, nuclear plants and roadways all incorporate these intricately created and durable slabs into their design. Within the private sector, constructions like hotels, universities, offices and condominiums also have taken benefit from this flexible substitute for traditional concrete installations. Need to know a little more about post tensioning processes and what types of forms can be produced for construction purposes?