In a post-tensioned prestressed concrete beam, what are the two primary time-dependent losses of prestress force, and how do they fundamentally differ in their origin?

In a post-tensioned prestressed concrete beam, the two primary time-dependent losses of prestress force originating from the concrete itself are Creep of concrete and Shrinkage of concrete. Both phenomena lead to a reduction in the initial tension applied to the prestressing tendons over an extended period.

Creep of concrete is the long-term, time-dependent deformation or shortening of concrete under sustained compressive stress. Its fundamental origin lies in the viscous flow of the cement paste within the concrete, the gradual rearrangement of its internal microstructure, and the slow movement of adsorbed water under a constant load. When the concrete in the beam is subjected to the continuous compressive force from the prestressing tendons, it slowly deforms and shortens over months and years, beyond its initial elastic deformation. This gradual shortening of the concrete beam causes the prestressing tendons, which are embedded within or bonded to the concrete, to also shorten. This reduction in the length of the tendons directly lessens their overall strain, which in turn reduces the tensile stress they are carrying, thereby resulting in a loss of prestress force.

Shrinkage of concrete is the time-dependent volume reduction or shortening of concrete that occurs due to processes unrelated to applied mechanical stress. Its fundamental origin is primarily the loss of moisture through drying as water evaporates from the concrete’s pores, causing the cement paste to contract. Chemical reactions during the ongoing hydration of cement, known as autogenous shrinkage, also contribute to a slight volume reduction. This shortening occurs as the concrete cures and dries, regardless of whether it is under external load. As the concrete beam naturally shortens due to these internal drying and chemical processes, the prestressing tendons are again forced to conform to this reduced length. This forced shortening of the tendons reduces their strain and consequently diminishes the tensile stress they maintain, leading to a loss of prestress force.

The fundamental difference in their origin is that Creep is a stress-induced phenomenon, meaning it is a deformation that occurs and progresses specifically due to the presence of a sustained mechanical load on the concrete. It is a time-dependent response to stress. In contrast, Shrinkage is primarily a volume-change phenomenon driven by internal material properties like moisture content and chemical reactions, and it occurs independently of externally applied mechanical stress. While restrained shrinkage can induce internal stresses, the root cause of shrinkage itself is not the applied prestressing force but the material's inherent tendency to contract over time.