Short Answer:

In prestressed concrete, various types of losses reduce the initial tension applied to the tendons over time. These are called prestress losses. The main types include elastic shortening, creep of concrete, shrinkage of concrete, relaxation of steel, and anchorage slip.

Each of these losses affects the effectiveness of prestressing and must be considered in design. If not properly accounted for, they can reduce the strength and durability of the structure, leading to inefficient performance or early failure in service.

Detailed Explanation:

Types of losses in prestressed concrete

Prestressed concrete works by applying pre-compressive stress to concrete to balance the tensile stress from external loads. However, the initial tension applied to steel tendons reduces over time due to various physical and chemical changes in both concrete and steel. These reductions are called losses of prestress, and they affect the final behavior of the structure. Understanding these losses is important to ensure safe and efficient design.

There are two broad categories of prestress losses:

1. Immediate losses – occur during or just after prestressing.
2. Time-dependent losses – happen gradually over the life of the structure.

Let’s look at each type in detail.

1. Elastic Shortening of Concrete

When prestress is applied, concrete shortens slightly due to compression. As the concrete shortens, the steel inside also shortens, reducing the tension in tendons. This is called elastic shortening loss, and it is more significant in pre-tensioned members than in post-tensioned ones.

2. Creep of Concrete

Creep is the slow, continuous deformation of concrete under sustained load. Over time, this causes additional shortening of concrete, which leads to a reduction in prestress force in the tendons. Creep depends on load level, age of concrete at prestressing, and environmental conditions.

3. Shrinkage of Concrete

Shrinkage happens when moisture evaporates from concrete. As the concrete shrinks, it tries to pull the tendons along, causing tension loss in the steel. This occurs regardless of whether the member is loaded or not. It is affected by cement content, humidity, and curing methods.

4. Relaxation of Steel

Even if the tendon length is held constant, steel gradually loses stress over time, especially at high-stress levels. This is called relaxation loss. It depends on the type of steel used, its stress level, and temperature. Modern prestressing steels are designed to minimize this effect.

5. Anchorage Slip

In post-tensioned concrete, when tendons are anchored at their ends, a small amount of slip occurs at the anchorage point. This causes a drop in tension. Anchorage loss is not present in pre-tensioned systems, as the bond develops before release.

Other Minor Losses

Some additional losses may occur due to:

• Friction loss in curved ducts in post-tensioning.
• Temperature variations causing expansion or contraction.

Conclusion:

Losses in prestressed concrete are inevitable and must be calculated accurately during design to ensure that the remaining prestress is sufficient for the structure to function safely. The main types of losses include elastic shortening, creep, shrinkage, relaxation of steel, and anchorage slip. Each of these affects the final stress level in tendons and, if not considered, may reduce performance or safety.