Corrosion of concrete electric poles

Concrete electric poles have been used extensively for supporting heavy transmission lines for several years. In this article, we will cover the types of concrete poles, their requirements, their corrosion, and their inspection techniques.

Source

Types of concrete poles

There are two types of concrete poles:

  • PCC: PCC stands for Plain Cement Concrete. This type of PCC pole has an entire concrete structure without any metallic reinforcement. The preparation comprises of inserting a high-tensile steel wire in a mould and stretching it to a predetermined extent. Another wire, which is galvanised steel wire, is introduced as the earth wire in the mold. The mould is then filled with concrete mix, which is then compressed. Such poles are also called pre-stressed cement concrete poles (PSC) as the compression leads to a stress generation in the concrete mix.
  • RCC: RCC is short for Reinforced cement concrete. As the name suggests, this type of concrete pole is reinforced with TMT steel bars for greater strength and endurance. In the production of RCC poles, a structure of TMT bars is first assembled and then embedded in pol-shaped concrete slabs.

Specifications

Specifications for PSC electric poles

  • Overall length of pole
  • Working load
  • Bottom depth
  • Top depth
  • Breadth
  • Depth of planting
  • Factor of safety
  • Diameter of HT pre-stressing wire
  • Minimum tensile strength of HT pre-stressing wire
  • Number of HT wire
  • Concrete grade
  • Ultimate load
  • Cube crushing strength of concrete at 28 Days
  • Allowable max compressive strength in concrete at working load

Specifications for RCC electric poles

  • Overall length of poles
  • Cement grade and composition
  • Composition and grade of aggregates
  • Mechanical properties of reinforcing bars and wires
  • Surface condition and preparation of reinforcing bars
  • Composition of admixture
  • Quality and composition of water used for concrete preparation
  • Maximum wind pressure
  • Depth of planting
  • Transverse strength at failure
  • Welding and lapping of reinforcement
  • Earthing wire installation

Corrosion of PSC and RCC poles

  • Spalling of concrete cover
  • Uniform or localized corrosion of metallic wires and bars due to ingress of moisture and oxygen through the concrete pores
  • Soil corrosion and concrete degradation in the areas buried under the earth due to ingress of chlorides, sulphates, nitrates, and moisture from the soil.
  • Saturation of concrete at the soil/atmosphere interface due to capillary absorption of groundwater due to wicking effect
  • Wind erosion of the concrete at greater heights.
  • Crevice corrosion of wires and poles at the inlet and outlet points of wires inserted in the poles.
  • Uniform corrosion of bare wires
  • Pitting corrosion at areas left bare due to insufficient or damaged coating
  • Stress corrosion cracking of the reinforcing bars or embedded wires due to local stress concentrations in the concrete

Inspection and testing

Here are a few typical inspection measures and tests conducted to assess the integrity of the concrete electric poles:

Wrap up

Concrete poles have been used extensively for transmission lines and utilities. While the basic specifications are common, there may be variations in certain parameters with respect to the location and climate.

Hence, a comprehensive view of the requirements and testing conducted globally will help assess the corrosion causes and select the most suitable inspection measures to nip the corrosion in the bud.

References

  • https://www.cracindia.in/admin/uploads/IS-785.pdf
  • https://www.wbsedcl.in/irj/go/km/docs/internet/webpage/techspec/Technical_Spec_of_Pole.pdf
  • https://www.materialsperformance.com/articles/material-selection-design/2019/01/control-of-environmental-degradation-of-concrete-power-poles
  • https://www.materialsperformance.com/articles/corrosion-basics/2020/11/bend-test-of-concrete-power-poles
  • https://www.irjet.net/archives/V8/i5/IRJET-V8I5749.pdf