Builders in the construction industry use post-tensioning systems to strengthen their construction projects. It can also apply to post-tension slabs, which are essential for producing concrete slabs that withstand for years and resist strong weather conditions.
We know that concrete has good strength to bear heavy loads, and when combined with high-strength steel tendons, it becomes even stronger, efficiently supporting higher loads. In this article, we will explore the components, construction techniques, advantages, and disadvantages of the post-tension slab in detail.
What is a Post-Tension Slab?
While experimenting with various methods of reinforcing and strengthening concrete slabs in the early 90s, the prestressing method of concrete gained attention from construction professionals. It became widely used in their projects due to its durability and load-bearing capacity.
These post-tensioning slabs are widely used to reinforce concrete slab foundations. It contains steel cables or tendons that crisscross across the centre of the slab and are subjected to high tension, producing high strength after the concrete has set. When the poured concrete dries, the steel tendons provide stress and tighten through their cables for strengthening concrete slabs.
Uses of Post-Tension Slabs
As time passes, the stability of the concrete slabs may weaken. Climate changes, particularly high temperatures, readily affect them. The concrete slabs may expand and contract due to the unstable soil. Normal concrete slabs cannot withstand these conditions.
So construction professionals choose these post-tension slabs to avoid cracking, contraction, and expansion of the slabs and to strengthen the foundation. To withstand any natural calamities, these post-tension slabs provide additional support. The steel and tendons in the concrete slabs can bear heavy loads and provide a strong, solid foundation or roof and flooring systems. They can be used in bridges, walls, multi-story buildings, elevated slabs, foundations, and columns.
Working Principles of Post-Tensioning
When a heavy load is brought up on a structure, the concrete slab undergoes stress, leading to cracks and changes in its original shape and size. This can be reduced by introducing post-tension slabs in the construction process.
Generally, concrete is good in compression but weak under tension, while steel is good under tension. The post tensioning process subjects the concrete to compression and steel to tension, before the actual usage of the slab in the structure. When tendons are subjected to tension, the concrete compresses initially. This finally results in a slab with extremely high strength, which can readily resist excess compressive and tensile stress.
Components of Post-Tensioned Slab
The post-tension slab is a combination of traditional concrete slabs with steel tendons inside it. The main components of a post-tensioned slab are ducts, tendons, and anchors.
- Ducts
The ducts are small weightless steel pipes having flexible couplings transferring high torque. The standard lengths of these ducts are 5 m and 6 m. They can be connected with an external screw coupling and sealed with PE tape. They are resistant to water and excessive strain. Steel tendons are placed in these ducts during the concreting process.
- Tendons
Tendons are a vital component of the post-tensioned slab. It is made of one or more pieces of prestressing steel with a protective coating. These tendons can be housed in ducts for further use. The standard size of the tendons ranges from 0.50 to 0.60 inches.
- Anchors
Anchors are used to connect or anchor two tendons into the concrete. The ultimate function of the anchor is to transfer the stress into the concrete to support the strengthening process.
Construction Techniques for Post-Tension Slabs
Planning and design
The planning and designing of post-tension slabs is a tedious process that includes safety and compliance. To define the layout of the tendon, you should consider the slab’s requirements, dimensions, and use. To ensure the strength and durability of the tendon, selecting the right concrete mix is crucial.
Installation of Tendons
As per the specified details in the designing process, tendons are installed. The tendons are covered with an outer layer generally made of plastic (ducts), allowing them to move and prevent corrosion. These can also avoid contact with the concrete.
Pouring the concrete
As the tendons are placed at the preferred locations, the high-quality concrete mixture is poured into them. The concrete can be set for curing until it reaches the required strength.
Tensioning
The tensioning process begins by stretching the anchored tendons at the edge of the slabs using hydraulic jacks at the right force.
Final touch
Once the tendons are properly tensioned, the slab can be finished with sealants, and other specific materials as per the project’s requirements. The sealants and flooring materials are used to create a smooth and long-lasting surface.
Construction of Post-Tensioned Slab
- It requires skilled labourers and supervisors for the installation of post-tensioned slabs.
- An engineer must design and lay down the tendons, which are hosed into a plastic or steel duct that is water-resistant.
- The poured concrete mixture may take around 20-25 days to cure and gain 75% strength. After this, the anchored tendons are stretched and coupled using hydraulic jacks.
- 80% of force is applied on a tendon to stretch and compress the concrete.
- The non-used tendons at one end are bound and put into the anchor pocket.
Advantages of Post-Tensioned Slab
- It supports an architect to work on his floor designs with thin slabs and columnless spaces.
- The thinner concrete slabs support higher floor-to-floor height, which can be used to create additional floors.
- The concrete and reinforcement are reduced by up to 20% to 30%.
- It reduces crack risks and is low-cost maintenance.
- It provides significant benefits for the developers, architects, engineers, and contractors.
Disadvantages of Post-Tensioned Slab
- Additional costs are required for the installation of post-tensioned slabs.
- Once it is finished, it can’t be remodelled. It needs much attention from planning to finishing.
- At the initial stage, cracks in concrete can be unavoidable.
Conclusion
In conclusion, post-tensioned slabs are efficiently processed to bear heavy loads and withstand natural calamities. It supports the construction professionals to work as specified in the design process. The efficiency and strength of post-tensioning slabs can withstand the test of time.