How does pre-tensioned and post-tensioned concrete work?
Inside the concrete slab or beam, we put reinforcement bars to make the structure more ductile and the max force for failure due to bending will be increased. In Pre-tensioning the tendons (reinforcement bars) are subjected to high pressure to bend upward (location is at the maximum deflection of any given structure) before the concreting. This force is maintained until the concrete attains it structural strength. As a result, the beam or the slab bends downward, so when the loads are imposed and if the structural deflects, the structure will be at the squared level because the pre tensioned structure compensates to any deflection due to load. In Post tensioning, its the other way around, we lay tendons with ducts at the at site, we pour concrete and we apply force on the tendons after 7 days or after the conrete achieves 85% of its strength. With these, we tend to reduce the thickness of the structure since the postensioned tendons will carry the imposed load on the structure.
