The development of architectural design elements forces the construction industry to change and develop. We prepared a study on Prestressed Concrete, a type of concrete that enables wide openings and special design works, as in the architectural concrete type in our previous series.
Definition of Prestressed Concrete
The definition of prestressed concrete according to several regulations that apply in the world of construction is as follows:
According to the ACI (American Concrete Institute), prestressed concrete is concrete that experiences internal stresses with a magnitude and distribution in such a way that it can compensate to a certain extent the stress that occurs due to external loads.
In another definition, according to the Consensus Concrete Guidelines draft 1998, prestressed concrete is reinforced concrete that has been given internal compressive stress to reduce the potential internal tensile stress due to workload.
Prestressed concrete can also be defined as reinforced concrete where the tensile stress is under certain loading conditions with such a value and distribution to a safe limit by applying a permanent compressive force, and the prestressed steel used for this purpose is pulled before the concrete has hardened (prestressed) or after the concrete has hardened. (post-pull).
The main difference between reinforced concrete and prestressed concrete is that reinforced concrete combines concrete and steel reinforcement by joining and allowing them to work together as desired. Meanwhile, prestressed concrete combines high-strength concrete and high-strength steel in “active” methods.
The manufacture of prestressed concrete is achieved by pulling the steel and holding it to the concrete, thus subjecting the concrete to a stressed state. This active combination results in better behavior of the two ingredients.
Steel is a material known for its toughness and toughness. This allows the steel to be made to work with high tensile strength by prestressing. Meanwhile, concrete is a brittle material and its ability to withstand tension is improved by applying pressure, while its ability to withstand pressure is not reduced. So prestressed concrete is the ideal combination of two modern high strength construction materials.
Principles and Methods of Stress Concrete Work
There are 2 types of methods of giving concentric force to prestressed concrete, namely:
Pre-tensioned Prestressed Concrete
In this method, the tendon is tightened with the help of auxiliaries before the concrete cast. The concentric force is maintained until the concrete is hard enough. Once the concrete is hard enough, the tendons are cut and the prestressing force is transferred to the concrete through the attachment. In mass manufacturing, this method is very suitable.
Prestressed steel is pre-tensioned to independent anchoring before casting of the surrounding concrete. The term pre-attraction means the application of prestressed steel, not to the beam. Precast application is usually carried out at the precast concrete manufacturing site.
Post-tensioned Prestressed Concrete
In this method, the tendon withdrew after the concrete cast. Before casting is carried out, a sleeve is attached to the groove of the tendon. After the concrete is finished, the tendons are inserted into the concrete through the tendon shawls that were previously installed during casting. Withdrawal is carried out after the concrete reaches the desired strength according to the calculation. After the withdrawal is made, the sleeve is filled with grouting material.
The principles of this post-pull are briefly as follows:
Stage 1: Prepare a formwork complete with holes for the tendon ducts, which are curved according to the moment plane of the beam, after which the concrete is cast.
Stage 2: After the concrete has been cast and can bear its own weight, the prestressed tendon or cable is inserted into the tendon duct, then it is pulled to get the prestressed force. The method of pre-stressing is by tying one armature, then the other end of the armature is pulled (pulled from one side). However, some are pulled on both sides and then held up simultaneously. After being transported, grouting is carried out on the armature hole earlier.
Stage 3: After being anchored, the concrete block becomes compressed, so the concentric forces have been transferred to the concrete. Because the tendon is curved, due to the concentric force the tendon gives an even load to the beam in an upward direction, as a result, the shape of the beam bends upwards.
To facilitate transportation from the factory to the site, usually, the pre-stressed concrete is made with a post-tension system. This is carried out segmentally (the beam is divided into several parts, for example, the sections are made with a length of 1 to 3 m).
Unlike conventional concrete, prestressed concrete undergoes several stages of loading. At each loading stage, checks must be made on the condition of the compressed and tensile fibers from each section. At this stage, different allowable stresses apply according to the conditions of the concrete and tendons. There are two stages of loading on prestressed concrete, namely transfer, and service.
The transfer stage is the stage when the concrete begins to dry and the prestressed cable is drawn. At this time, usually, only the dead load of the structure works, namely the structure’s own weight plus the load of workers and tools. At this time the live load has not worked so that the working moment is minimum, while the working force is maximum because there is no loss of prestressing force.
Conditions of service(service) are the condition when prestressed concrete is used as a structural component. This condition is achieved after all the prestressed force losses are considered. At this time the external load is at its maximum, while the stress force is close to the minimum price.
Prestressed Concrete Material
Concrete is the result of mixing several materials in the form of cement, water, and aggregate. With the ratio of the weight of the mixture, namely 44% coarse aggregate, 31% fine aggregate, 18% cement, and 7% water. After 28 days, the concrete will reach the ideal strength which is called the characteristic compressive strength. The characteristic compressive strength is the stress that has exceeded 95% of the uniaxial compressive strength measurement taken from the standard stress test, namely with a 15×15 cm cube, or a cylinder with a diameter of 15 cm and a height of 30 cm. The concrete used in the manufacture of prestressed concrete is concrete that has high compressive strength with an FC value of at least 30 MPa.
Steel: steel materials commonly used in manufacturing practices are as follows.
- PC Wire, usually used for prestressed steel in prestressed concrete with a prestressed system.
- PC Strand wire, usually used for prestressed steel for post-pulling prestressed concrete.
- PC BAR wire, usually used for prestressed steel in prestressed concrete with a prestressed system.
- Ordinary reinforcement, namely reinforcement that can be used for conventional concrete such as plain iron and screw iron.
Prestressed Concrete Technology
Prestressed concrete is the application of the stresses that the element will encounter under load to the structural elements after engineering calculations. From a technical point of view, it is the combination of the high compressive strength of concrete with the high tensile strength of the steel building material.
In the working logic of classical reinforced concrete structures, most of the stresses are met by steel reinforcements. Concrete and steel carry stresses together in building systems, also known as post-tensioned concrete. The best examples of this feature are post-tensioned beams, prestressed columns, and so on. We can see it in the building elements. It makes the structure more resistant to unexpected shocks and loads.
Advantages of Prestressed Concrete
- Concrete compressive strength is used efficiently
- Steel building material is used efficiently
- Reduces the risk of corrosion by reducing tensile cracking
- Resistant to shear stress
- The same performance can be obtained with small sections compared to reinforced concrete
- Structure weight is reduced
- Can be composited
Disadvantages of Post-tensioned Concrete
- It is difficult to work with
- It must be manufactured by controlling
- Special alloy prestressed steels are expensive
- Its assembly requires attention and different construction equipment
- Construction cost is relatively expensive
Prestressed Concrete Technology has some advantages compared to reinforced concrete structures due to its properties. While it allows concrete and steel building materials to work together, as post-tensioned structural elements better meet equivalent loads, long structures with small cross-sectional areas make it possible to pass wide openings.
Properties of Prestressed Concrete
- It has high strength
- Tensile stress is low
- Its strength is quite high
- It is economical compared to conventional reinforced concrete.
- It is light compared to reinforced concrete
- Resistant to cutting forces
Due to the above reasons, its use is increasing in today’s construction industry. Post-tensioned concrete systems are used in most industrial buildings, commercial buildings and factories built recently.
Where to use?
Ardgermeli Concrete is in essence a “prestressed” form of loads applied by calculating these forces in advance to balance the tensile stresses created by external loads with the compressive stress that the concrete meets.
In reinforced concrete elements, this stress, that is, leaving it under stress, is generally applied to steel reinforcement. When we examine the usage areas of Prestressed Concrete, the following types of structures come to the fore;
- Foundation beams
- Railway sleepers
- Bridge structures
- Water tanks
- Airport etc. runway structures
- Tall buildings
- Factory buildings
Prestressed concrete is not preferred for columns and walls due to its material properties. When we examine the properties of the structures in which it is used, it is seen that there are structures with very high bending stresses. Additionally, it is preferred as a retaining wall in some buildings. The main reason why it is preferred in such structures is its ease of manufacture and economy.