Corrosion and Its Effects on Anchorage Tendons or Reinforcements

To ensure the long-term durability and longevity of anchorage tendons (or reinforcements), protecting the metallic components of the tendon against corrosion is essential. Corrosion protection of anchorage tendons typically involves physical protective layers—either single-layer or multi-layer—that shield the tendon from the surrounding corrosive environment. These protective layers include covers, anti-corrosion compounds, sheaths, encapsulation, epoxy coatings, and grout. The selection of the physical protection type depends on factors such as the design life of the structure (temporary or permanent), the corrosiveness of the environment, the consequences of anchorage system failure, and the additional costs associated with higher levels of corrosion protection.

Mechanism of Metal Corrosion

Corrosion is an electrochemical reaction involving the base metal, oxygen, and water. During this process, the metal changes into its natural oxidized state. In anchorage tendons, corrosion is especially common in steel tendons that have been improperly stored or handled at the construction site. Galvanic corrosion—where metal ions migrate due to electric currents in an electrolyte—is less common but can occur. This phenomenon may take place under the following conditions:

  • Transfer within the tendon itself—from one tendon end to another.

  • Proximity of the tendon steel to another steel object.

  • Placement of the tendon in aerated soils (above groundwater level, engineered fills, and sands) or in non-aerated soils (below groundwater level, clayey soils).

This problem is more likely if significant fluctuations occur in the ground conditions along the tendon length, particularly changes in soil pH and electrical resistivity. The potential for metal degradation due to corrosion is very high in environments such as:

  • Soils near the groundwater table

  • Acidic soils with low pH

  • Soils with high concentrations of aggressive ions such as chlorides or sulfides

  • Soils with leakage electrical currents

Types of Corrosion for Prestressing Steel Tendons

Corrosion of prestressing steel can be classified into six main types:

  1. General corrosion

  2. Localized or pitting corrosion

  3. Stress corrosion cracking or hydrogen embrittlement

  4. Fatigue corrosion

  5. Corrosion due to stray electrical currents

  6. Microbial and chemical attack

The last three types typically occur under special loading conditions or particular soil environments.

Typically, unprotected prestressing steel undergoes general corrosion during storage and handling phases. General corrosion involves a thin, uniform layer of rust and oxide on the tendon surface. It is often observed on steel tendons exposed to open air at the site during storage. If the exposure time is short or sufficient protection measures are in place, general corrosion affects only a very thin surface layer and is generally not considered serious damage. A site engineer can easily remove light surface rust by wiping with a cloth and inspect for cracks or pits. Tendons with only light rust can be installed in boreholes without additional cleaning.

Localized Corrosion

Localized corrosion appears as pits or cracks in one or more specific areas on unprotected steel. In highly aggressive soils, unprotected steel may develop severe pits and cracks within weeks of exposure. Complete encapsulation of tendons in aggressive soils is critical to prevent localized corrosion.

Stress Corrosion Cracking

Hydrogen embrittlement (stress corrosion cracking) occurs in high-strength steel elements, manifesting as cracks originating at pits. During the progression of stress corrosion cracking, tensile stresses concentrate around the pits, causing cracks to propagate deeper into the metal beneath the rust. Over time, these cracks may extend significantly, potentially leading to tendon rupture. The presence of pits or cracks on tendon surfaces is sufficient reason for rejecting the tendon.

Fatigue Corrosion

Fatigue corrosion arises under cyclic loading and leads to crack propagation in steel tendons. This type of corrosion is relatively uncommon since most anchorage tendons are not subjected to severe cyclic loads.

Corrosion Due to Stray Currents

Corrosion caused by stray electrical currents leads to pitting on prestressing steel exposed to prolonged stray current exposure. Such currents may originate from electrical systems like railways, power transmission lines, or welding activities. Stray current corrosion is particularly destructive in marine or wet environments. Electrical sources more than 30 to 60 meters from the tendons typically do not cause significant stray current corrosion (FHWA-SA-96-072, 1995). Protection from stray current corrosion usually involves electrically insulating the steel using non-conductive materials such as plastic coatings.

Microbial and Chemical Attack

Microbial corrosion causes pitting in unprotected prestressing steel and is common in swampy soils and sulfate-rich clays below the groundwater table. Such soils are highly corrosive, and the use of fully encapsulated tendons in these environments is mandatory. Further discussions on the effects of corrosion on prestressing steel can be found in FHWA-RD-82-047 (1982).