Loss due to relaxation of strands is categorized as a long-term loss of prestress in a prestressed concrete member.

Relaxation is the phenomenon where the initial stress in the steel strands gradually reduces over time. This occurs due to several factors such as creep, shrinkage, and relaxation of the concrete.

1. Creep:
- Creep is the time-dependent deformation of concrete under sustained loading.
- It is caused by the viscous flow of the cement paste and the reorientation of the hydrated cement particles.
- This phenomenon leads to a gradual reduction in the stress transferred to the steel strands, resulting in the loss of prestress.

2. Shrinkage:
- Shrinkage refers to the reduction in volume of the concrete due to the evaporation of moisture.
- As the concrete dries and loses moisture, it undergoes shrinkage, which causes a decrease in the stress in the steel strands.
- This shrinkage-induced stress relaxation contributes to the long-term loss of prestress.

3. Relaxation of Strands:
- Steel strands used in prestressed concrete members exhibit relaxation behavior.
- When initially stressed, the steel strands experience an immediate loss of stress due to elastic deformation.
- However, over time, the steel strands continue to relax, resulting in a gradual loss of prestress.
- This relaxation occurs due to the redistribution of internal stresses within the steel strands.
- The relaxation process is influenced by the material properties of the steel, such as its composition and manufacturing process.

Other options mentioned in the question are not categorized as long-term losses of prestress:

a) Loss due to elastic shortening:
- Elastic shortening refers to the immediate reduction in length of the prestressed member when it is subjected to a prestressing force.
- This is a temporary and immediate loss of prestress that occurs during the prestressing operation, but it does not contribute to the long-term loss of prestress.

b) Loss due to friction:
- Frictional losses occur due to the resistance encountered by the steel strands as they pass through the ducts or anchorages during the prestressing operation.
- Frictional losses are considered as immediate and temporary losses of prestress and do not contribute significantly to the long-term loss.

d) Loss due to anchorage slip:
- Anchorage slip refers to the relative movement between the prestressing steel and the anchorage system.
- This slip can occur during the stressing operation or due to external loads.
- Similar to frictional losses, anchorage slip is considered a temporary and immediate loss of prestress and does not contribute significantly to the long-term loss.