Post tension cables are used in concrete construction to add strength to thin and long slabs. This allows more span lengths between support columns. The cables have steel wires within plastic sheath. They are tensioned after pouring concrete,hence the name post-tension cables. Post-tension ‘tendon’ is a term used to define the complete assembly comprising of the sheath, pre-stressing strand, anchorages, corrosion-inhibiting coating or any grout around the steel wire or bar.
Post-tensioning involves reinforcing concrete using high-strength steel strands. The steel strands are commonly called ‘tendons’. It is used in a wide range of concrete structures including:
• Apartment buildings
• Office buildings
• Water tanks
It allows construction of structures that would otherwise be impossible.
Contractors need to locate post-tension cables when repairing structures. It is important to avoid hitting the cables when cutting or drilling for renovation. These cables have very high tension and cutting them can also compromise the structure’s integrity.
Today, post-tensioning cables are used in most aspects of construction. When it comes to buildings, it allows structures and features like fewer beams, longer clear spans, more slender elements, and thinner slabs. This further helps in reducing the amount of concrete required for construction. It can also help in reducing overall building height without affecting the floor height.
Using post-tension cables thus helps in significantly reducing the building’s weight. There will be lesser load on the foundation. Lower building weight and height can further contribute to more savings in terms of facade and mechanical system costs.
When there is permanent stress and elongation in the post-tension cables, a compressive force is created on the concrete. This compression helps in countering the tensile forces due to load. Thus, the load-carrying capacity of your concrete structure increases significantly when this system is used.
Post-tension cables also allow the slabs and beams to be continuous. Thus, a single beam can be run from one end to other instead of having to place a beam between all the columns. Thus, this unique system not only helps in increasing the structure’s strength, it also increases overall efficiency.
Post tensioning is of two main types – unbonded and bonded. In unbonded post tensioning, there is no bond between the pre-stressing steel and the concrete surrounding it anywhere except at the anchorages. Monostrand tendons are the most commonly used unbonded post-tension tendons. They are used within beams and slabs. The tendons have multiple strands coated with corrosion-inhibiting grease in an encasing of extruded plastic sheath.
Unbonded tendons are usually prefabricated at the plant before moving onsite for installation, especially for slab-on-ground and building construction. The tendons are laid out based on the installation drawings and then concrete is poured to the desired strength. Tension is then applied on the tendons and they are anchored. The anchorages prevent the tendons from returning to the original length.
Thus, post-tension cables add more strength to your buildings, help save concrete, reduce weight, and increase efficiency.