Concrete has low tensile capacity because it is weak in tension and can therefore easily cause structures to crack. In order to stop structures from cracking, the tensile stresses must be eliminated or reduced. The force applied to do this is called a prestressing force. Prestressing creates stress or strain in a structure to prevent cracking and enhance durability and fatigue endurance. There are various types of prestressing systems depending on the construction method, design and purpose.
External and Internal Prestressing
When external steel wires known as tendons are placed on the outer section of the region being stressed, then the technique is called external prestressing. The tendons lie outside or inside the hollow space of a box girder and forces are only transferred at the deviations or anchorage blocks. External prestressing requires a great deal of accuracy in planning, executing and maintenance; therefore, it is not very common. The technique is suited for strengthening buildings and constructing bridges. On the other hand, internal prestressing is achieved by placing the tendons within a structure. Prestressing applications use internal technique because it is easy to achieve a greater degree of accuracy.
Pre-Tensioning or Post-Tensioning
Pre-tensioning and post-tensioning are the two common techniques of prestressing.
In pre-tensioning, the tension is applied to the tendons before the casting of the concrete. The prestressed tendons are transferred to the hardened concrete through a bond. The tendons can be of individual wires or a seven-wire strand and are often used in precast beams.
Post-tensioning is when prestressing is done against the hardened concrete. When the concrete has been hardened, the prestressed tendons are passed through ducts cast into the concrete and locked with mechanical anchors. The tendon force is thereby transferred to the structure through the anchorage wedges.
Linear or Circular Prestressing
Linear prestressing is a type of prestressing used in straight, bent or flat structures. The technique can even be used on curved structures as long as the tendons don't go round in circles because they are linearly prestressed. For example, prestressing of poles, slabs and beams can be done using the linear prestressing technique. In contrast, curved or circular structures employ the circular prestressing technique. The tendons are wound in circles. Examples include pipes, silos and tanks.
Full or Partial Prestressing
The technique where no restrictions are placed on concrete tensile stresses under service conditions is called partial prestressing. The part under tension is reinforced using additional mild steel bars to control the cracks and hence the crack width is within allowable limit. Deviations from the set prestressing can cause deformation, cracking and fatigue on the structure; therefore, when using this technique during construction, you must always monitor the prestressing force carefully. On the other hand, full prestressing is a technique where no tensile stress is allowed in concrete under service loads. The area of prestressing steel is more than the area needed for safety at ultimate state, therefore leading to inefficient use of prestressing.