What is the difference between reinforced concrete and prestressed concrete?
Both reinforced concrete and prestressed concrete have steel bars as their reinforcement. The main function of these reinforcement is to strengthen the concrete weaken as it undergoes tensile stress.
So how does one differs from the other and what is the best condition to use prestressed concrete?
1. What is reinforced concrete?
Reinforced concrete or RC is a composite material used in general construction in which the low tensile strength and ductility of the concrete are fortified by the addition of reinforcing steel bars having higher tensile strength and ductility. On the construction process, prefabricated steel cages or steel bars are wired together and placed inside a formwork. Concrete is then poured into the formwork and vibrated using a vibrator to remove air voids on the fresh concrete and to ensure bond between the rebar and the concrete.
Figure 1: Rectangular concrete beam, with steel rebar is an example of a reinforced concrete member
2. The implementations of reinforced concrete
In modern day construction, Reinforced Concrete is widely used due to its workability and availability of its raw materials, mostly as main members of a particular structure such as columns, piers, piles, beams, slabs and footings for buildings, houses, dams, bridges and other similar structures. Reinforced concrete can easily be designed and configured to shapes that conform to architectural demands which would take time and cost more when using materials such as timber or steel. Typical uses for reinforced concrete also are highway paving and sidewalks and other solid concrete form where most of the pressure will be compression. Reinforcing the concrete with steel bars adds the tensile strength to allow a concrete to bend or flex without breaking apart.
1. What is prestressed concrete
Simply putting it as concrete formed under stress. Reinforcement bars are placed in a form and stressed or stretched with forces on each end of bars pulling them. When concrete is poured around the stressing bonds before pressure is released, it creates tension; when it is released the steel's tendency to try to resume its original shape adds a compressive force to the concrete laterally, giving it strength to span distances.
2. The implementations of prestressed concrete
Prestressing is used mainly to make beams and piers in such construction as highway overpasses and commercial buildings. It enables a concrete beam to support weight between piers on either side; without such reinforcing, concrete's lack of tensile strength would cause it to collapse without support in the middle.
Here are three major implementations of prestressed concrete:
- Pre-Tensioned Concrete: In this form, concrete is cast around steel bars or cables under tension. The concrete naturally bonds to these “tendons” while it cures. Compression by static friction transfers the tension to the concrete once it is released. Subsequently, any tension on the concrete transfers readily to the tendons. Pre-tensioned concrete elements are common in beams, lintels, and floor slabs.
- Bonded Post-Tensioned Concrete: In this form, compression is applied in situ during curing. A duct of aluminum, plastic, or steel is used in casting and follows the area where tension would occur in the concrete. Tendons are pushed through the duct, then tensioned via hydraulic jack after hardening. Once tendons’ stretching meets design specifications, they are wedged in place and the duct is grouted.
- Unbonded Post-Tensioned Concrete: Here, the individual tendons retain freedom of movement relative to the concrete. Tendons are prepared with a coating of lithium-based grease, then given a plastic-based “shell” formed through extrusion. The steel cables are tensioned against anchors placed in the slab’s perimeter. This design provides the ability to de-stress the embedded tendons prior to repair.
Figure 2: Simple diagram of a prestressed concrete.
SkyCiv Reinforced Concrete Design Software
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