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Corrosion Protection Systems for Anchor Tendons in Soil Nailing and Anchored Walls

Corrosion protection systems provide one or more impermeable protective layers around tendons or anchors to safeguard them against corrosion. The corrosion protection must meet the following criteria:

• Ensure the service life of the tendon matches or exceeds the predicted minimum service life of the soil nailing or anchored system, considering corrosion-induced failure.

• Avoid introducing any harmful or incompatible agents that reduce the capacity of the tendons.

• Allow unrestricted tendon movement in the unbonded length so that all load transfers through the bonded length.

• Use chemically stable materials that do not react with adjacent components.

• Require minimal or no repair or replacement during the service life, except in rare cases.

• Be sufficiently strong and flexible to tolerate deformations during loading and tendon prestressing.

• Be durable enough to resist damage during construction, transportation, storage, and installation.

In 2001, a research project titled “Evaluation of Post-Tensioned Metals in Geotechnical Projects” provided methods to assess corrosive soils, estimate residual service life, and predict design life in new projects.

Corrosion Protection System Design

Corrosion protection systems are designed to protect steel components of anchored or soil-nailed systems. The three main corrosion-prone sections of tendons include:

• The anchor element

• The unbonded length

• The bonded length

The corrosion protection system consists of components that together create a durable, non-brittle shield for each tendon section and their connections. Steel components include the anchor head, bearing plate, sheath, prestressing steel, and couplers (if used). The corrosion protection components include:

• For the anchor element: concrete cover, sheath, and anti-corrosion compounds

• For the unbonded length: grout and a sheath filled with corrosion-inhibiting compounds (grease and special wax) or grout

• For the bonded length: grout and encapsulating coating with spacers or epoxy coating

These components are used for both strand and bar tendons.

Components and Requirements for Corrosion Protection

• Anchor Cover: Protects the anchor head and exposed prestressing steel from corrosion and physical damage, made of plastic or stainless steel.

• Sheath: Protects the prestressing steel in the unbonded length, typically made from stainless steel or PVC pipes.

• Anti-Corrosion Compounds: Grease and wax-based soft compounds protecting steel components, maintaining flexibility.

• Grout: Protects prestressing steel in bonded and unbonded lengths, made of cement or polyester resin (polyester resin grout is generally not preferred due to porosity).

• Sheath (Sheath): Plastic smooth or corrugated tubes filled with anti-corrosion compounds, typically pulled-on or extruded.

• Heat Shrink Tubing: Used mostly for protecting prestressing bar couplers and sometimes as sheaths for bar tendons.

• Encapsulation: Tubes or corrugated sleeves protecting prestressing steel in the bonded length.

• Spacers: Steel or plastic components that maintain tendon position inside the borehole to ensure minimum grout coverage.

Corrosion Protection Classes

Corrosion protection is divided into three classes:

• Class I

• Class II

• No Corrosion Protection (see Table 1)

For corrosive soil conditions or when multiple protection layers are required, Classes I and II are applied. Class I usually involves encapsulated tendons with several protection layers, whereas Class II applies fewer layers. For temporary projects with non-corrosive soil, corrosion protection may be omitted.

Table 1: Corrosion Protection Requirements (modified after PTI, 1996)