In the field of civil engineering, pre-stressed concrete is a crucial material used to enhance the performance of concrete structures. The process involves applying a compressive force to concrete before it is subjected to external loads. This compressive force, or “pre-stress,” improves the material’s ability to resist tensile forces and reduces cracking, enhancing the durability and strength of the concrete. However, pre-stress is not a permanent phenomenon. Over time, the pre-stress in concrete structures diminishes, which can affect their performance. This reduction in pre-stress is known as the loss of pre-stress.
Loss of pre-stress is a gradual process that occurs due to a variety of factors. These factors can be broadly categorized into losses that occur during the tensioning process, at the anchoring stage, and those that happen subsequently over time due to material behavior. Understanding the causes and types of pre-stress loss is essential for engineers to design more effective pre-stressed concrete structures.
Types of Pre-Stress Losses
1. Loss During the Tensioning Process
The tensioning process is where the initial pre-stress is applied to the concrete member. During this phase, friction between the tendons (the steel wires or strands) and the ducts (tubes in which the tendons are placed) plays a significant role in reducing the effective pre-stress that is transferred to the concrete. This friction occurs in the jacking and anchoring systems, as well as along the duct walls, particularly where the tendons fan out at the anchorage points.
A. Frictional Losses
Friction in the system results in the actual stress in the tendons being less than the value indicated by the pressure gauge. Friction can arise in various places during the tensioning process, such as in the jack itself or where the tendons come into contact with the walls of the ducts. This reduces the efficiency of the pre-stressing force, as less stress is applied to the concrete than originally intended.
B. Types of Frictional Losses
- Length Effect: In straight tendons, imperfections in the duct (such as slight bends or roughness) can cause the tendon to touch the duct or the concrete, leading to frictional losses. This effect, sometimes referred to as the “wobbling” or “wave” effect, reduces the amount of pre-stress transferred to the concrete.
- Curvature Effect: For curved ducts, the loss of pre-stress depends on the radius of curvature of the duct and the friction between the tendon and the duct surface. Sharp bends or tighter curves create more resistance, which leads to higher frictional losses. The tighter the curvature, the greater the frictional force, and thus the greater the pre-stress loss.
2. Loss at the Anchoring Stage
Once the tendons are tensioned, they are anchored at the ends of the concrete member. However, the anchoring fixtures themselves are not perfectly rigid and may stretch slightly during the tensioning process. This slight elongation results in a reduction of the pre-stress in the tendons.
In some cases, friction wedges used to hold the tendons in place may slip slightly, further contributing to the loss of pre-stress. This loss is typically small but important, and engineers account for it during the design phase. They often compensate for this loss by elongating the tendons slightly during the tensioning process.
3. Subsequent Losses After Initial Tensioning
In addition to losses that occur during the tensioning and anchoring stages, further losses occur over time as the concrete and steel undergo changes due to environmental factors and material properties. These losses are generally slower but more significant over the long term.
A. Loss Due to Shrinkage of Concrete
Concrete undergoes shrinkage over time due to chemical changes and the drying process. This shrinkage occurs as water evaporates from the concrete and the material contracts. The amount of shrinkage depends on the moisture conditions and the time elapsed since the concrete was poured, rather than the loads or stresses applied to the concrete. As the concrete shrinks, it can cause a reduction in the initial pre-stress.
To minimize shrinkage, engineers may reduce the water-cement ratio and optimize the cement content during mixing. Proper curing of the concrete is also essential to control shrinkage and maintain the pre-stress in the structure.
B. Loss Due to Creep of Concrete
Creep is the slow, continuous deformation of concrete under sustained stress. Unlike shrinkage, which is a volumetric change, creep refers to the gradual elongation or deformation of concrete under constant load. Over time, the concrete will deform more as the pre-stress is maintained, leading to a reduction in the effective pre-stress. The amount of creep depends on factors like the age of the concrete, the magnitude of the applied stress, and the type of concrete mix used.
Creep is a significant factor in long-term pre-stress loss, as it leads to additional deformation that can further reduce the initial pre-stress.
C. Loss Due to Elastic Shortening of Concrete
When pre-stress is applied to the concrete, the concrete compresses and shortens, which in turn shortens the steel tendons. This shortening of the concrete is known as elastic shortening. There are different behaviors depending on the type of pre-stressing used:
- Pre-tensioned Members: In pre-tensioned systems, the tendons are stressed before the concrete is placed. When the tendons transfer their pre-stress to the concrete, the concrete will shorten, leading to a reduction in the initial pre-stress in the tendons.
- Post-tensioned Members: In post-tensioned systems, the tendons are stressed after the concrete has set. As the tendon is jacked against the concrete, the concrete shortens, but this only happens once, and no further shortening can occur once the tendon is fully tightened.
4. Loss Due to Creep of Steel (Stress Relaxation)
Steel tendons also experience a phenomenon known as stress relaxation, where the stress in the steel tends to decrease over time, even if the strain remains constant. This happens as the steel tends to undergo a slight plastic deformation under constant tension. The magnitude of this loss depends on factors such as the type of steel used, the environmental conditions, and the amount of pre-stress applied.
The total loss of pre-stress in a concrete structure depends on a combination of the above factors, including the properties of the concrete and steel, the curing conditions, and the magnitude of the pre-stress.
Summary of Pre-Stress Losses
To summarize, pre-stress losses in concrete can be attributed to a combination of frictional losses during the tensioning process, losses at the anchoring stage, and subsequent losses due to material behaviors such as shrinkage, creep, and stress relaxation. The first three types of losses—those occurring during tensioning and anchoring—result in a reduction in the length of the concrete, which leads to a decrease in the initial elongation of the steel tendons.
- Pre-tensioned members experience the greatest loss of pre-stress due to elastic shortening, as the concrete shortens when the pre-stress is transferred.
- Post-tensioned members generally experience less loss of pre-stress from elastic shortening, although there may still be losses as tendons are progressively stressed.
Understanding these various forms of pre-stress loss is crucial for engineers, as it allows them to design concrete structures that maintain their strength and stability over time. Proper consideration of these factors ensures that pre-stressed concrete members continue to perform effectively, even as the pre-stress diminishes gradually over the life of the structure.