To prevent premature cracking in reinforced concrete structures and fully utilize high-strength steel bars and concrete, prestressing is applied before the structural components bear loads. This process reduces or offsets the tensile stress in the concrete caused by external loads, often placing the components under compression and resulting in prestressed concrete.
Factors Causing Prestress Loss
1. Construction Equipment
Insufficient strength and stiffness in the pedestal can lead to poor stability, causing deformation, overturning, or sliding, which results in prestress loss. Additionally, fixtures with weak self-locking and anchoring capabilities, along with taper pins that have lower strength and hardness than the prestressed reinforcement, contribute to prestress reduction.
2. Concrete Materials
If the prestress exceeds the concrete’s compressive strength, it can crush the concrete, causing prestress loss. Concrete’s inherent shrinkage and creep can also lead to excessive prestress reduction. Improper coarse aggregate size, low-strength steel bars or wires, poor plasticity, and insufficient surface roughness of the steel reinforcement all contribute to prestress loss.
3. Tension Control Stress
The value of the tension control stress directly impacts prestressed concrete performance. If this value is too low, the resulting prestress after losses will be insufficient to effectively enhance crack resistance and stiffness in the concrete components.
4. Temperature Difference
Temperature differences between the prestressed steel bars and tensioning equipment during concrete heating and curing cause prestress loss. Initially, steel bars and the pedestal are at the same temperature (t1). As temperature rises to t2 during curing, steel bars not yet bonded to concrete expand freely. Once bonded, steel bars and concrete expand and contract together. The stress reduction caused by temperature changes during curing cannot be recovered, resulting in temperature-induced prestress loss.
5. Time
Over time, prestress loss occurs due to concrete creep, shrinkage, and steel tendon relaxation. Time-dependent deformations are calculated for each construction phase to estimate creep and shrinkage effects. Relaxation refers to the gradual decrease in tensile stress on steel tendons under constant strain. The extent of relaxation loss depends on initial stress, elapsed time, and material properties.
6. Elastic Deformation of Concrete
Similar to time-dependent effects, elastic deformation of concrete contributes to prestress loss. Concrete creep and shrinkage, combined with steel relaxation, lead to gradual stress reduction, which can be tracked and predicted through analysis and charts.
7. Friction Between Prestressed Steel Bars and Duct Walls
Friction arises from bends and deviations in the duct path. When tensioning curved steel bars, normal stress between the bars and duct walls generates frictional resistance. Unevenness or misalignment of ducts during construction increases this friction, contributing to prestress loss.
Methods to Reduce Prestress Loss
1. Use High-Strength Pedestals
Selecting pedestals with high strength, rigidity, and stability minimizes deformation, sliding, and overturning. Fixtures should have strong self-locking capabilities, with taper pins harder than the prestressing reinforcement. Minimizing the use of shims is recommended since each additional shim can increase anchor deformation and steel shrinkage by about 1mm. Extending pedestal length where possible also helps.
2. Choose High-Strength Concrete
High-strength concrete improves the bond between steel bars and concrete, especially in pre-tensioned components. Post-tensioning enhances the local compressive capacity at anchorage points. Using high-grade cement to reduce cement content and water-cement ratio, along with well-graded aggregates and thorough concrete vibration, enhances concrete compactness, thus reducing shrinkage and creep.
3. Use Coarse Aggregates with Larger Particle Size and Rough Surfaces
Within acceptable limits, larger and rougher coarse aggregates improve bonding between concrete and steel reinforcement. The prestressing level depends on the tension in the steel bars. However, overstressing high-strength steel bars or wires may cause fractures due to insufficient strength, leading to prestress loss.
4. Use Steel Bars or Wires with Good Plasticity
Steel bars or wires with good plasticity enhance tensile stress and preload forces due to greater shrinkage after tension removal. For high-strength steel bars, surface treatments such as scratches or indentations improve bonding. For ordinary steel bars, deformed bars are preferable to increase friction between concrete and steel.
5. Construction Practices
To reduce prestress loss from relaxation and other factors during construction, over-tensioning is commonly applied. Tensioning stress is first controlled between 1.05σ and 1.1σ, maintained for 2-5 minutes, then unloaded to the design stress σ. Using hot-rolled steel bars rather than carbon steel wires helps minimize relaxation losses. For longer elements, tensioning at both ends reduces frictional losses.
6. Use Larger Diameter Annular Components
In components with bends, ensure prestressed steel wires have smooth surfaces and apply grease if needed to lower friction. Ducts should follow specifications strictly to avoid surface unevenness. Compression by prestressed tendons reduces annular component diameter, which decreases tensile stress and causes prestress loss. Using larger diameter annular components helps mitigate this effect.
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