The loading of any structural element induces stresses in both compression and traction. The stability of constructions requires that these constraints never exceed the capacity of the materials. Various techniques have been developed to overcome this difficulty.
A first technique is the reinforced concrete where steel bars are placed in the formwork before concreting. They adhere to concrete and form a composite material with it. Since concrete has no tensile strength, rebar provides this essential strength for the load transfer. The bars are passive; they are only solicited when the structure is loaded. Concrete alone does not offer the required flexibility. Therefore, reinforcing bars - a reinforcement - are often incorporated into concrete to limit the extent of cracks. However, this reinforcement provides only passive reinforcement, it means, it does not transfer any load or provide additional resistance before the concrete cracks.
Prestressed Concrete
The second technique is the prestressed concrete. As in reinforced concrete, steel is associated with concrete, but, in this case, it is tensioned before loading the structure. Prestressing is therefore a technique which consists in inducing favorable internal forces. These are designed to reduce the weakness of concrete in tension. The design of reinforced concrete prestressed elements often includes the use of passive steel bars in parallel to the prestressed steel. Prestressing is most often carried out using tensioned cables embedded in concrete. Steel is not necessarily in direct contact with concrete.
Two types of prestressed concrete can be distinguished depending on when the steel is tensioned. The cables can be tensioned before concreting the element - pre-tension - or after - post-tension.
Pre-tensioning
This method consists of placing "cables" or steel strands which are stretched between two prestressing benches which serve as anchors. By definition, a strand is generally a cable made up of 7 individual steel wires: 6 wires coiled in a long pitch around a central wire. Each strand is individually anchored and tensioned by a single-strand anchorage until it is removed from the form. It passes through a formwork where concrete is poured in place in contact with them. After curing the concrete and achieving the minimum compressive strength, the strands are cut at each end between the formwork and the bench. At this point, the steels transfer part of their tension in compression to the concrete: compression of the concrete balances the tension in the strands. The tensile force is then transferred by adhesion to hardened concrete. A certain desired deflection of pre-tension at the level of the element under tension is often observable after the transfer. The elements are then stripped and stored ready to be transported. Pre-tensioning is carried out in specialized workshops equipped with tensioning benches. This technique is used in prefabrication and allows the production of beams, posts, prestressed slabs ...
Post-tensioning
Post-tensioning is an operation carried out on site which consists in putting strands in tension after the concrete has been poured and has reached a minimum compressive strength. The fresh concrete is poured in place in a formwork including reinforcement where specially designed ducts and anchors are installed embedded in concrete. Once the element is in place or the formwork has been removed, the strands can be threaded inside the duct and tensioned using a jack. After removing the jack, the strands transfer part of their tension in compression to the concrete: the compression of the concrete balances the tension. The layout of the duct is not necessarily straight; they can be curved to transfer forces into predefined areas. This system therefore offers active reinforcement. Post-tensioning places the concrete structure under compression in areas where the loads cause tensile stresses. It applies a compressive stress to the materials, which compensates for the tensile stresses that the concrete could undergo under the loading of the element. A certain desired deflection at the level of the element under tension is then observable after the transfer. Unlike pre-tensioning, prestressing steel is installed and tensioned on site at the final position of the element.
Post-tensioning offers the possibility of constructing the concrete elements at any desired location including its final position in the structure. The transport constraints of the elements must however be considered. Post-tensioning is therefore an essential system for the construction of segmental structures. This method is generally used for large civil engineering works. Post-tensioning is implemented using strands or bars of high-strength steel. We call a tendon, the entire post-tensioning system which includes the group of one or more strands or a bar threaded into a duct anchored at both ends by anchoring devices and which is injected or not to fill the empty space remaining after tensioning.
A wide variety of post-tensioning systems are available to suit several applications. We can characterize its tendons according to 3 main categories:
Internal Bonded Tendon
System composed of several strands which are placed inside a plastic or metal sheath injected by a cementitious grout and which is embedded in a concrete element. After the tensioning, the sheath is injected with a cementitious grout which cures and hardens in order to adhere the tendon to the concrete which surrounds it. The cure for grout and hardening allows the tendon to behave as an integral system without any relative movement between the steel strands and the concrete element. The grout provides a physical barrier to water and contaminants while providing an alkaline environment that protects the prestressed steel from corrosion. This type of post-tensioning system is often used for engineering structures and for heavily stressed beams in buildings. Flat tendon systems with oval ducts can also be an excellent choice for thin slabs and bridge deck.
Internal Unbonded Tendon
System confined to a concrete element whose prestressing steel does not adhere to the concrete surrounding it, except at anchors. The system typically consists of a single strand which is coated with a corrosion inhibitor like grease or wax and which is protected by an extruded plastic sheath. This type is used for structural slabs of buildings or parking lots and slabs on the ground. Increasingly, it is integrated into infrastructure engineering projects.
External Unbonded Tendon
System which is installed outside the structural element with the exception of the anchors and at the point of deviation. Tendons are usually installed in smooth sheaths which can be injected with a cementitious grout or liquid wax. The external tendons allow a relative movement between the tendons and the structural element to which they are attached and that is why they are considered in practice as Unbonded. They are typically used for the erection of prefabricated segments and cable stayed bridge. This type of post-tensioning allows access for maintenance and replacement of the strands, and thus represents the best solution for the improvement and rehabilitation of engineering structures.
Advantages of post-tensioning
More flexibility in design: length and free span of the elements
Allow faster construction by using prefabricated segments
Reduction of costs of building materials by optimizing the dimensions of the elements of the structure
Possibility of increasing the load of the structural element later
Ability to control when and where force is applied to the structure
Durability: controls and minimizes cracking of the concrete
Reduction of maintenance costs over the life of the structure
To learn more about prestressing, our team offers a full range of services to help you in your different projects. Contact our experts at: info@force-s.com
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