Introduction to Time-Dependent Behavior in Prestressing Steel

Prestressing tendons and bars are the backbone of many modern structures, from long-span bridges to high-rise parking garages. By placing the steel under a permanent compressive force, engineers can counteract the tensile stresses that concrete alone cannot resist. Over decades of service, however, two distinct but interrelated time-dependent phenomena—creep and fatigue—can degrade the prestressing steel. While both can lead to loss of prestress force, crack initiation, or sudden rupture, they arise from different loading conditions and microstructural mechanisms. A deep understanding of creep and fatigue is essential for designing safer structures, planning meaningful inspection intervals, and extending service life. This article explores the physical origins, influencing factors, measurement methods, and mitigation strategies for creep and fatigue in prestressing steel components.

What Is Creep in Prestressing Steel?

Creep is the time-dependent, irreversible deformation of a material under constant or slowly varying stress below the yield strength. For prestressing steel, creep manifests as a gradual elongation of the tendon. This elongation reduces the effective prestress force (the so-called prestress loss), which in turn can increase tensile stresses in the concrete, widen cracks, and reduce structural stiffness. Creep in steel is most pronounced at elevated temperatures (above about 0.3 to 0.4 of the melting point in Kelvin), but for typical bridge environments (0–40 °C) creep still occurs at a slower but measurable rate.

The creep strain is typically described in three stages:

  • Primary (transient) creep: The strain rate decreases rapidly with time as dislocations move and rearrange.
  • Secondary (steady-state) creep: A constant strain rate prevails, governed by diffusion-controlled dislocation climb.
  • Tertiary creep: The strain rate accelerates due to necking, micro-void coalescence, or grain boundary cavitation, leading to rupture.

Under typical prestress levels (0.6 to 0.8 times the ultimate tensile strength), the steel operates mostly in the primary and early secondary creep regimes, with negligible tertiary creep unless temperatures become elevated.

Factors That Influence Creep in Prestressing Steel

The creep rate depends on several key parameters:

  • Stress level: Creep strain increases nonlinearly with applied stress. Higher initial prestress accelerates creep losses.
  • Temperature: Even modest temperature rises (e.g., solar heating of bridge tendons) can double or triple the creep rate. For every 10–15 °C increase, the creep rate can rise by a factor of two to three.
  • Steel composition and microstructure: Quenched and tempered high-strength steels (e.g., ASTM A416) have finer grain structures that improve creep resistance compared to normalized grades. Alloying elements such as chromium, vanadium, and molybdenum help pin dislocations.
  • Stress relaxation: While creep is strain-driven, stress relaxation is the complementary phenomenon where stress decays under constant strain. Prestressing systems experience both concurrently; standards often treat them together as a combined prestress loss.

International design codes (ACI 318, AASHTO LRFD) provide empirical formulas to estimate creep losses based on initial stress-to-strength ratio and ambient temperature. For example, the PCI (Precast/Prestressed Concrete Institute) design handbook recommends a creep coefficient that accounts for temperature and stress level. PCI Design Handbook.

Understanding Fatigue in Prestressing Steel

Fatigue is the progressive, localized structural damage that occurs when a material is subjected to cyclic loading. In prestressing steel, fatigue damage manifests as the initiation and slow growth of one or more cracks, which eventually may propagate to a critical size and cause a sudden, brittle fracture. Fatigue is a leading cause of failure in tendons of bridges and parking structures that experience fluctuating live loads, wind, and vibration.

The fatigue process is divided into three stages:

  • Crack initiation: At points of stress concentration (e.g., surface defects, corrosion pits, threading of anchorages), microscopically small cracks form after a certain number of cycles.
  • Crack propagation: The crack grows incrementally with each load cycle. The growth rate can be described by Paris’ law: da/dN = C (ΔK)^m, where ΔK is the stress intensity range and C, m are material constants.
  • Final fracture: Once the crack reaches a critical length, the remaining cross-section can no longer carry the peak load, and the tendon ruptures.

Fatigue Loads in Prestressed Structures

Typical fatigue loads include:

  • Traffic-induced vibrations and live loads on bridges (axle loads, moving vehicles).
  • Wind-induced oscillations on cable-stayed or suspension bridges.
  • Thermal cycling due to daily solar radiation.
  • Machine-induced vibrations in industrial structures.

The fatigue strength of prestressing steel is determined by the endurance limit—the stress amplitude below which the steel can theoretically withstand an infinite number of cycles. For high-strength prestressing strands, the endurance limit is typically about 10–20% of the ultimate tensile strength (UTS) at 2 × 10⁶ cycles in air, but it decreases significantly in corrosive environments (corrosion fatigue).

Fatigue vs. Stress-Corrosion Cracking

It is important to distinguish fatigue from stress-corrosion cracking (SCC). While both involve cracking under tensile stress, SCC is driven by a specific corrosive environment and static stress, rather than cyclic loading. However, in practice, the two can interact: pre-existing corrosion pits act as stress raisers that greatly reduce fatigue life—a phenomenon known as corrosion fatigue. NIST research on corrosion fatigue of high-strength steel wires provides valuable data on this interaction.

Key Differences Between Creep and Fatigue

Although both are time-dependent failure mechanisms, creep and fatigue differ fundamentally in their causes and manifestations:

Aspect Creep Fatigue
Loading type Constant or slowly varying stress Cyclic (repeated) stress
Deformation Gradual, permanent elongation (strain increase) Localized crack growth; minimal overall deformation until fracture
Time to failure Often long-term (months to decades) except at high temperatures Can occur in a few thousand cycles (e.g., high-amplitude loading) or many millions
Temperature sensitivity Highly sensitive; rate increases exponentially with temperature Less sensitive, but elevated temperature can accelerate
Failure mode Necking, cavitation, ductile rupture (if tertiary stage reached) Brittle fracture at a localized crack
Primary mitigation Reduce sustained stress, use creep-resistant alloys, control temperature Minimize stress ranges, avoid stress concentrations, protect from corrosion

In prestressing applications, the two phenomena can act simultaneously, especially in bridges where sustained dead load (producing creep) is combined with repeated live loads (producing fatigue). Field studies indicate that creep losses can reduce the mean stress on tendons, which somewhat lowers the stress amplitude and may actually extend fatigue life—but this trade-off is complex and must be evaluated on a case-by-case basis. FHWA report on prestressed concrete bridge performance discusses such interactions.

Impact on Structural Integrity and Service Life

Both creep and fatigue have direct consequences on the safety and performance of prestressed concrete structures:

  • Loss of prestress: Creep combined with steel relaxation can reduce the effective prestress by 10–20% over the design life. This leads to wider cracks, increased deflection, and reduced shear strength.
  • Increased risk of corrosion: Wider cracks from prestress loss allow moisture and chlorides to reach the steel, accelerating pitting and corrosion fatigue.
  • Sudden, catastrophic failure: Fatigue fractures typically occur without warning, as cracks grow internally until the remaining net section can no longer carry the peak load. A single broken strand can overload adjacent strands, causing a cascading failure.
  • Reduced ductility: Creep can cause grain boundary cavitation, embrittling the steel and reducing its ability to deform before fracture.

Case studies illustrate these risks. The 1990 collapse of the Mjøsa Bridge (Norway) was attributed to fatigue of prestressing tendons combined with corrosion. Investigations revealed that inadequate drainage and deicing salt created a corrosive environment that drastically shortened the fatigue life of the strands. More recently, several post-tensioned concrete bridges in the United States have been retrofitted after inspections found broken wires in unbonded tendons, attributed to a combination of corrosion fatigue and hydrogen embrittlement.

Testing and Measurement Methods

Reliable assessment of creep and fatigue requires specialized testing both in laboratory and field settings.

Creep Testing

Standard creep tests (e.g., ASTM E139) involve applying a constant tensile load to a steel specimen at a controlled temperature while continuously monitoring elongation. For prestressing strands, manufacturers routinely test stress relaxation (the related phenomenon) by loading a strand to a target force and then holding it at constant length while measuring the force decay over 1000 hours. The results are extrapolated to predict 50-year losses. Because creep is highly temperature-dependent, tests are often conducted at 20 °C, 30 °C, and 40 °C to develop activation energy models.

Fatigue Testing

Fatigue tests (ASTM E466, E468) subject the steel to a sinusoidal stress cycle (usually between a minimum and maximum load) at a frequency of 1–20 Hz. For prestressing strands, tests are performed on 7-wire strands with anchorages to replicate real load-transfer conditions. The number of cycles to failure is recorded for different stress amplitudes, producing an S-N (stress-life) curve. Because environmental conditions play a major role, accelerated corrosion-fatigue tests (e.g., in salt spray or under cathodic protection) are increasingly common. ASTM E466 standard practice for conducting force controlled constant amplitude axial fatigue tests offers a widely used protocol.

Nondestructive Evaluation (NDE) in the Field

For in-service structures, direct measurement of creep and fatigue damage is challenging. Engineers rely on:

  • Acoustic emission (AE): Detecting the high-frequency sound of wire breaks or crack growth in real time during loading.
  • Magnetic flux leakage: Detecting areas of broken wires in unbonded tendons by scanning the duct with a magnet.
  • Strain gauges and fiber-optic sensors: Monitoring long-term creep strains in the concrete adjacent to tendons.
  • Impact-echo and ultrasonic tomography: Identifying voids in grouted ducts that can lead to corrosion and fatigue.

Mitigation Strategies in Design and Maintenance

Addressing creep and fatigue requires a multi-layered approach from material selection through long-term monitoring.

Material-Level Measures

  • Use steel with a proven low relaxation (LR) grade. ASTM A416 “low relaxation” strands undergo a thermo-mechanical treatment that reduces long-term losses by up to 60% compared to stress-relieved strands.
  • Specify fine-grained microalloyed steels (with vanadium, niobium, or titanium) for improved creep resistance.
  • Apply protective coatings (zinc galvanizing, epoxy coating, or corrosion-resistant alloys) to mitigate corrosion fatigue.

Design-Level Measures

  • Limit the maximum sustained stress to below 70% of UTS to stay within the primary creep regime.
  • Provide adequate concrete cover and drainage to prevent moisture accumulation at tendon anchorages.
  • Use parabolic or draped tendon profiles to reduce stress ranges at critical sections.
  • For fatigue-critical bridge members, design the live-load stress range to be below the constant-amplitude fatigue limit (CAFL) for the steel grade.
  • Detail anchorages and couplers to minimize stress concentrations; smooth transitions and shot-peened surfaces can improve fatigue life by a factor of 2–5.

Inspection and Maintenance

  • Implement a risk-based inspection program with intervals based on age, traffic volume, and environmental exposure.
  • Conduct periodic visual inspections for rust staining, cracking of concrete near anchorages, or exposed wires.
  • Use nondestructive testing methods as described above, especially in high-risk zones such as the underside of bridge decks and expansion joints.
  • When broken wires are found, the affected tendon should be detensioned and replaced if possible, or supplemented with external prestressing.
  • Maintain documentation of prestress losses over time through monitoring of jacking forces and periodic load testing.

Ongoing research aims to improve our understanding and management of creep and fatigue in prestressing steel:

  • Probabilistic life-cycle models: Advanced statistical models (e.g., Bayesian networks) combine creep and fatigue damage accumulation with corrosion progression to forecast remaining service life more accurately.
  • High-performance steels: New metallurgical routes, such as quenching and self-tempering (QST) and thermomechanical controlled processing (TMCP), produce steels with finer microstructures that resist both creep and fatigue.
  • Smart monitoring: Embedded fiber Bragg grating (FBG) sensors can measure strain and temperature continuously, allowing real-time assessment of prestress losses and early detection of anomalous behavior.
  • Digital twins: Structural digital twins integrate sensor data, inspection records, and material models to simulate the evolution of creep and fatigue damage, supporting condition-based maintenance.
  • Machine learning for S-N curves: Researchers are using neural networks trained on large databases of fatigue test results to predict fatigue lives for new steel compositions and environments. A review of machine learning applications in fatigue life prediction highlights the potential.

Conclusion

Creep and fatigue are two of the most significant long-term degradation mechanisms for prestressing steel components. Creep slowly erodes the precompression force, while fatigue can nucleate and grow cracks that lead to sudden fracture. Their effects are amplified by environmental factors such as temperature and corrosive agents. Engineers must account for both phenomena at every stage—material selection, design, construction, and maintenance—to ensure that prestressed structures remain safe and serviceable for their intended design life. By integrating advanced testing, monitoring, and modeling techniques, the industry can move toward more resilient infrastructure that withstands the test of time and repeated loading.

For further reading, refer to ACI 209.2R-08 on creep and shrinkage, AASHTO LRFD Bridge Design Specifications for fatigue design, and the Post-Tensioning Institute’s Guide Specification for Grouted Post-Tensioning for specific corrosion-fatigue mitigation details.