Prestressed concrete is made by introducing internal forces (prestress) into the concrete before or during its use, which helps counteract the stresses it will experience from loads. One common method is called pretensioning.
Understanding Pretensioning
The pretensioning method involves applying tension to the reinforcing steel before the concrete is poured. This process sets up the crucial internal stress that defines prestressed concrete.
Here's a breakdown of the pretensioning process, based on common practices:
- Setting Up the Tendons: First, the prestressing tendons (wires, strands) are stretched to a predetermined tension and anchored to fixed bulkheads or molds. These tendons are high-strength steel cables or bars.
- Pouring the Concrete: The concrete is then poured around the taut tendons within the mold.
- Curing: The concrete is allowed to cure and harden.
- Releasing the Tension: Upon hardening, the tendons are released from their anchorages.
When the tension is released, the tendons try to shorten but are bonded to the now-hardened concrete. This bond transfers the tension from the tendons to the concrete, compressing it. This pre-compression is the "prestress" that makes the concrete stronger and more durable under load.
Why Use Prestressed Concrete?
Prestressed concrete offers several advantages:
- Increased Load Capacity: It can carry heavier loads over longer spans compared to conventional reinforced concrete.
- Reduced Cracking: The pre-compression helps prevent tension cracks under service loads.
- More Efficient Material Use: Often requires less concrete and steel for the same structural performance.
- Improved Durability: Less cracking leads to better resistance against water and corrosion.
Pretensioning is typically done in a precasting plant, allowing for controlled conditions and efficient production of elements like beams, slabs, and piles.
