Ground Anchors (Soil Nails)
Corrosion: Causes - Prevention and Protection Methods


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1.         Corrosion

1.1             General

 

Corrosion is the term used to designate the deterioration of a metal by chemical or electrochemical reaction with its environment. Most commonly used metals are produced by extraction from their oxides. Therefore, the refined metal is in a thermodynamically less stable state than that of its natural oxide form and under appropriate conditions will revert to oxides, i.e. it corrodes. When corrosion inhibiting constraints are lacking the metal will react with oxygen and water to form oxides and/or hydroxides. The process by which corrosion occurs is generally recognized to be electrochemical in nature, i.e. a galvanic cell is developed. The term galvanic corrosion is used, in a broad sense, to denote corrosion occurring from dissimilar adjacent surface conditions of a metal, differences in oxygen concentrations or differences in environmental conditions.

 

1.2       Aggressivity of the Environment

Test and/or field observations are used to classify the aggressivity of the environment.

Ground shall be considered aggressive if it has one or more of the following, a pH value less than 4.5, a resistivity less than 2000 ohm-cm, sulfides present, stray currents present or has caused chemical attack to other buried concrete structures.  In addition, aggressive atmospheric conditions need to be considered.

 

If the aggressivity of the ground has not been determined by testing, then aggressive conditions are assumed in:

·        Acid mine or industrial waste.

 

Aggressivity or the ground is influenced by:

 

·        Water and air permeability of the ground,

 

1.3       Basic electrochemical corrosion cell

 

Three basic constituent parts form an electrolytic cell: an anode, a cathode and an electrolyte. To form a corrosion cell the anode and the cathode must be connected metallically, i.e. electrons can move between anod and cathode. The term anode is used to denote the location at which the metal corrodes. At this location the metal atom gives up electrons in a reaction with the corroding medium. The free electrons are consumed at the cathode by oxygen reduction.  These two reactions are coupled by a migration of ions into the electrolyte at the anode and discharged at the cathode. Further, corrosion cannot occur without a potential difference producing a current flow between the anode and the cathode (corrosion current).

This potential difference may occur e.g. through differential oxygen concentration (different aeration) of the concrete surrounding the steel. Parts of the steel in a well aerated concrete form cathodic areas and those portions of low oxygen concentration form anodic areas.

 

1.4       Steel in concrete

 

The concrete/cementitious mortar provides the steel with a physical barrier and a chemical protection. Due to this physical barrier the access of corrosion promoting agents (carbon dioxide, oxygen and humidity from the air, chlorides from deicing salt or other agents coming into contact with the concrete) will be slowed down. The efficiency of this physical barrier depends on the permeability of the concrete and the thickness of the concrete cover. The influence of cracks can be neglected, until the crack width becomes greater than some empirical values. The chemical protection function of concrete is due to its characteristic alkalinity and the property of steel to create a tight nonconducting oxide film on its surface in an alkaline environment. This phenomenon is known as passivation because the steel remains passive against its environment. As long as the pH-value is within a range of 12,5 to 14 and the Cl--ion content is below a certain threshold level, the steel surface is passivated, no corrosion can occur.

 

Differences in the physical characteristics of the concrete (e.g. permeability) and/or its environmental conditions (aeration, humidity) and chemical changes in the concrete (carbonation, access of chlorides) can lead to changes of the steel surface such that the passive film on it will be destroyed. When this happens, iron ions can escape the iron cristal-lattice and go into solution with the moisture in concrete: the steel will corrode.

 

1.5       Factors affecting cell activity

 

The rate of corrosion is related to the ratio of the anodic region to the cathodic region. Increasing this ratio by decreasing the cathodic area results in a reduced current flow which denotes a decrease in the corrosion rate. Increasing the cathodic area (or decreasing the anodic area) decreases this ratio resulting in an increased corrosion rate.

 

When passivation occurs it reduces the corrosion rate to a very low level or completely halt the corrosion process. Obviously, when ions are present that destroy the passivation film the corrosion rate will be greatly accelerated.

 

The electrical conductivity of the electrolyte (the concrete) is an important parameter in the rate of corrosion, the higher the conductivity the grater the rate of corrosion. Conductivity is a function of temperature, moisture and ionic content, since the corrosion current flows through the electrolyte by ionic conduction.

 

1.6       Different corrosion phenomena

 

Categorization by the manner in which the steel is affected is discussed in this section.

 

1.6.1   Uniform corrosion

If an approximately uniform surface attack of steel occurs, it implies that no discrete anodic and cathodic sites exist.  The corrosion products form a continuous film and thus may retard further corrosion. This type of corrosion can develop when unprotected steel is exposed to the environment, as during shipping, storage or prior to concreting/grouting.

 

1.6.2   Localized corrosion/Pitting corrosion

Due to nonhomogeneity of the metal surface and/or the corrosive environment separate electrochemical cells may develop and localized corrosion can occur. As the ratio of anodic area to cathodic area is very small, a locally high corrosion rate occurs which may lead to a sudden brittle failure after a negligible loss of material. Localized corrosion occurs at locations where the metal surface passivation has been destroyed or damaged. In the presence of aggressive ions, such as chlorides, a pitting mechanism may occur.

 

1.6.3   Crevice corrosion

The mechanism of crevice corrosion is similar to that of pitting corrosion. Crevices can originate from rolling defects or at close contact of the steel to an other impervious body, e.g. under nuts. Here in the poorly aerated region anodic processes occur resulting in a brittle type of failure.

 

 

 

1.6.4   Stress corrosion cracking (SCC)

SCC is a type of locally concentrated corrosion defined as cracking that may result from the combined action of corrosion and static tensile stress, which may be either residual or externally applied. SCC occurs mainly in alloys where a passivated oxide film on the surface has developed. The attack of the corrosion agent (mostly chloride ions) concentrate on certain sensitive regions of the steel such as cracks or spalls.

 

1.6.5   Hydrogen embrittlement

Cracking due to hydrogen embrittlement of steel under stress occurs when atomic hydrogen diffuses through the metal lattice, where they recombine to hydrogen molecules producing an internal pressure in the metal. Absorption of atomic hydrogen by the prestressing steel usually occurs by cathodic charges, which develop in a corrosive environment when steel is electrically coupled to more anodic metal,  e.g., zinc coating. The atomic hydrogen may be formed by the corrosion process itself or as a result of some manufacturing operations, e.g., pickling or welding. Cracking of the steel can either be a direct consequence of the tensile stresses developed by the hydrogen molecules within the metal lattice during their formation from two hydrogen atoms or in combination with tensile stresses. It should be mentioned that different steel types are differently susceptible to hydrogen embrittlement.

 

1.6.6   Corrosion fatigue

Corrosion fatigue is the result of the combined effect of corrosion and cyclic stresses. Cyclic stresses may cause more rapid failures when accompanied by corrosion than static stresses of the same magnitude. As both, fatigue and corrosion can reduce the life of prestressing steel, their combined effect is the most detrimental when the life under the influence of the cyclic stresses is comparable to that under corrosive influences.

 

1.6.7   Fretting corrosion

Fretting is a surface wear phenomenon occuring when two contacting surfaces are subjected to oscillating relative motion of small amplitude. Fretting corrosion is a form of fretting in which chemical reaction predominates. Fretting corrosion may occur between a prestressing wire and the metal duct/sheathing or between wires in a strand, especially where the direction of the prestressing tendon changes.

 

1.6.8   Stray current corrosion

Structures which may be affected from stray current corrosion are those associated with electric substations, electrified rail or tramway systems, or which are in contact with structures where a very large amount of welding occurs. Alternating current electricity is much less likely to cause severe corrosion unless it is of low frequency (approx. 17 Hz). The development of stray current can be suppressed with a proper electrical insulation of the structure or a good metallic contact of the steel components of the structure with the source of the stray current.

 

1.6.9   Corrosion in soil. Microbiological corrosion

Soil as a corrosive medium can be regarded as a porous substance consisting of more or less solid, partly colloidal, soluble and hygroscopic constituents and living organisms. The pores of the soil contain air and water. Above the ground-water level the finest capillary tubes are filled with water whereas the greater pores contain air. For corrosion in soils a certain moisture is necessary and, generally oxygen. Nevertheless, steel can corrode under anaerobic conditions as well. The most common form of microbiological attack comes from the metabolic processes of sulfate reducing bacteria (SRB). The waste product of SRB metabolism are sulfide ions, which react with the metal allowing dissolution of the anodic region of the corrosion cell to result in metal sulfide.

 

 

2          Corrosion protection - possible methods

 

2.1       General

Corrosion protection can be achieved when one or more reactions of the corrosion cell are prevented: the anodic, the cathodic or the electrolytic. These mean, that either no humidity and/or no oxygen and/or no carbon dioxide and/or no chloride ions may arrive at the steel surface.

 

Objective of the engineer: Redundant, reliable and durable corrosion protection system

 

2.2       Requirements of corrosion protection

 

The corrosion protection elements and systems must meet some fundamental requirements:

The grout encapsulation protection system is an excellent example of a good combination: the plastic sheathing is a good humidity and oxygen barrier, i.e. proper passive protection, the cementitious grout provides alkaline, i.e. active protection, both, sheathing and grout provide also mechanical protection to the steel.

 

2.3       Corrosion protection methods

 

2.3.1   Hot dip galvanizing

Steel anchor components should be hot dip galvanized as per ASTM A-153(AASHTO M232). Zinc is a well known, common and relatively inexpensive coating material for iron and steel. Zinc acts as a sacrificial anode, i.e. it corrodes in a corrosive environment and lets the steel play the role of cathode. The high alkalinity of concrete and grout (pH > 12) dissolves the zinc to a certain extent, at pH < 12 a very low corrosion rate of zinc occurs due to the development of a passivation film on the zinc surface. This film will stabilize if atmospheric CO2 reaches the surface of zinc coating. This is the reason for the known durability of zinc coatings under open-air conditions. However, it should be remembered that the zinc coating is a sacrificial coating, hence its lifetime depends on the agressiveness of the environment and the thickness of the coating applied. Zinc coating is quite ineffective if Chloride- or Sulphate ions beyond a threshold level are present: local pitting corrosion occurs. When the zinc coating is damaged the cathodic partial reaction related to the corrosion of zinc (i.e. development of hydrogen-atoms) primarily occurs on the free steel surface due to the electrochemical characteristics of the zinc-iron „battery“. The hydrogen atoms can partially be absorbed by the steel, causing hydrogen-embrittlement of steel types which are susceptible to this phenomenon. The rate of hydrogen-development decreases during hardening of the fresh cement paste around the galvanized steel in a matter of hours due to the development of a passive film on the zinc coating. Thus, the risk of hydrogen-embrittlement vanishes within a few hours or days.

 

2.3.2. Fusion bonded epoxy coated-

Epoxy Coating shall conform to one of the following: ASTM A-934, ASTM A-775, or AASHTO No. M284.

 

2.3.3. Multiple corrosion protection with cement grout and corrugated plastic sheathing.

Steel is centered and fully encapsulated inside corrugated PVC or HDPE sheathing and annular space between bar and sheathing will be shop cement grouted.

The sheathing shall have sufficient strength to prevent damage during construction operations, shall be watertight, chemically stable without embrittlement softening, and nonreactive with concrete.

 

2.3.3.A. Corrugated plastic sheathing shall be either polyvinyl chloride (PVC) or high density polyethylene (HDPE).  The minimum sheathing wall thickness shall be 40 mils.

The ground anchor encapsulation shall be fabricated from material with the following properties:

 

Capable of transferring stresses from the grout surrounding the tendon to the grout in bond length

Able to accommodate movements during testing and after lock-off;

Resistant to chemical attack form aggressive environments. Grout or grease;

Resistant to aging by ultra-violent light;

Non-detrimental to the tendon;

Capable of withstanding abrasion, impact and bending during handling and installation and

Capable of resisting internal grouting pressures.

 

Steel bars are centered and fully encapsulated inside corrugated PVC or HDPE sheathing and annular space between bar and sheathing will be shop cement grouted. The corrugated plastic sheathing insures a minimum 5mm (0.2 inch) grout cover encapsulating the rebar steel either over the entire bond zone or in a specified outer length (often a one meter length, under the bearing plate in the active failure plane zone) to maximize corrosion protection in especially aggressive soil conditions.  The corrugated plastic sheathing provides a rigid mechanical lock between the factory-installed grout and the field-applied grout.

 

If steel bar couplers are used, they will be field installed with a double or multiple corrosion protection (DCP or MCP) system as per manufacturer instructions or as shown in the shop drawings.

 

Corrugated plastic sheathing shall be either polyvinyl chloride (PVC) or high density polyethylene (HDPE).  The minimum sheathing wall thickness shall be 40 mils. .  The material will conform to ASTM D-3350 polyethylene, Index No. 335520 C, Table 1, ASTM D-1248, and AASHTO No. M252 for HDPE or ASTM D-1784 Class 13464-B for PVC.  Diameter of the corrugated sheathing is as noted on the shop drawings.  

 

2.3.3.B. Cement grout inside the annular space between steel and corrugated sheathing is the most efficient element of corrosion protection. It must provides a proper alkalinity, low permeability, high resistivity, minimum to no shrinkage in both plastic and hardened states, proper fluidity, little or no segregation and no bleeding.

The grout mix used for rock and soil anchors shall be stable (bleed less than 2 percent), fluid and provide a strength of at least 21 Mpa (3000 psi) at time of stressing.  The type of cement that is selected for grout that will be in contact with the ground shall take into account the known or possible presence of aggressive substances.  Soil samples may be necessary to evaluate the aggressivity of the soil.

 

Grout cube testing is not normally required, but may be utilized to evaluate the time of anchor stressing and the quality of the grout mix.  Insufficient cube strength shall not be caused for rejecting a successfully tested anchor. A neat cement grout made with a W/C ratio of 0.4 to 0.45 by weight and Type I cement, mixed in a colloidal mixer, will normally satisfy these requirements.

If significant grout pressures are used in cohesionless soils, mix water will be squeezed out of the grout as it attempts to travel through the soil (pressure filtration).  This results in an in-place grout with a lower water cement ratio than for the grout that was initially injected.

For this reason, water cement ratios as high as 0.55 can be used in cohesionless soils, if the effective grout pressures exceed 0.4 Mpa (50 psi). In situ grout may be weakened, if the grout is cured at low temperatures or when the grout has been diluted with groundwater. 

 

3.         Conclusion

A proper protection of the ordinary reinforcing steel and the prestressing steel against corrosion is a great challange for the engineers:  both for designers and contractors. A lot of knowledge is available: it must only be properly applied. Advanced corrosion protection methods and advanced corrosion protected systems like double corrosion protection system can be applied to produce structures with the required  durability.