Range of electric conductivity of concrete:
Every material has more or less conductivity. When conductivity is very low in any material, we termed it as an insulator. But no generalized comment can be made about concrete, as its electrical conductivity depends on many factors.
• Moist concrete acts basically as an electrolyte that has resistivity of ≤100 Ω-m, can be said as semi-conductors.
• Air-dried specimen was found to have resistivity of the order of 104 Ω-m.
• But an oven-dry specimen of concrete has resistivity of around 109 Ω-m, this value lies within the range of good insulator.
Source of electric resistance
Presence of chlorides
Immersion in sea water
When concrete is immerse in sea water and subjected to wetting and drying cycles, a thin layer of calcium carbonate and magnesium hydroxide are formed on surface. This layer can greatly increase resistivity of concrete. When this layer is taken out, the natural resistivity of concrete (when stored in fresh water) will be restored.
Application of admixture
When electric conductivity of concrete is expected, fine aggregate can be replaced by conductive fine aggregate. Grinding nearly pure crystal of carbon (which is available as proprietary product). When resistivity lies between 0.005 to 0.2 Ω-m; the important properties of concrete including compressive strength are not found to be altered significantly.Voltage and temperature effect:
Resistivity of concrete reduces with an increase in ambient temperature. Regarding voltage of the system, resistivity increases with increase in voltage. Figure 2 describes this relation; test conducted on an oven-dry specimen dried in desiccator not permitting absorption of moisture.
|Figure 2:Relation between resistivity and voltage applied on concrete (1:2:4 and W/C=0.049) oven-dried and cooled in the presence of desiccants|
Type of current
The maximum reference provided so far in the above sections are based on alternating current (a.c). The resistivity of concrete to direct current (d.c) may not be the same as It is influence by polarization effect. At 50 Hz, resistivity of concrete to a.c and d.c almost same. Generally a concrete member that is matured in air, impedance of it is approximately equal to d.c resistivity. It is interpreted by Hammond & Robson to show that resistance in concrete is so much smaller than capacitive reactance of it that only former contributes to impedance significantly and power factor is close to unit. Typical data sheet of alternating current is shown in Table 1.
|Table 1: Typical Values of Electrical Properties of Concrete|
|Table 1: Typical Values of Electrical Properties of Concrete..Contd.|
Two forms of water present in concrete; in early stage there have evaporable water which is removed throughout the process of gaining maturity and the next one that has to be considered in electrical resistivity is gel water. Resistivity of concrete is increased significantly with the removal of evaporable water. Electric current uses electrolytic means to conduct through concrete having moisture. But when capillaries are not continuous, electric current passes through gel water. For a particular concrete of defined mix design, drying out in air decrease electric conductivity at the surface zone. As an example Tritthart and Geymayer reported that resistivity of a concrete having water/cement ration of 0.50 increase eleven fold which was found even more at higher water/cement ratio.
Thus it is expected that increase of water of any amount and increase in content of ions exist in pore water, reduce resistivity of cement paste; actually a sharp decrease in resistivity is found with increase of water/cement ratio. Table 3 and Figure 2 have been presented below showing relation between resistivity and water/cement ratio for hydrated cement paste and for concrete. This relation can be established by the laws of conductivity of homogeneous materials.
Figure 3:Relation between electrical resistivity and water/cement ratio for concrete having maximum size of aggregate of 1.5 in (40 mm) and ordinary Portland cement (maturity 28 days)
In the usual mix design that are proposed for concrete construction considering both strength and durability requirements have not much options to vary water content for a selected aggregate grading and requirement of workability i.e. resistivity is now more dependent on cement content. The chemical composition and content of cement controls availability of ions in the evaporable water. . So the sources of ions in water may be cement and water itself. Thus cement type and content both have influence on resistivity. In normal construction practice, decrease in cement content in mix results resistivity of concrete to increase as for a constant water/cement ratio with lower cement content, a reduction in electrolytes are occurred to pass electric current.
|Table 2: Effect of Water-Cement Ratio and period of Moist Curing on Resistivity of Cement Paste|
Table 3 provides some idea about influence of cement content and type on resistivity of concrete; here it is found that resistivity of concrete having high-alumina cement is 10~15 times more than ordinary portland cement, both having same properties.
Electrical properties of concrete like resistivity or conductivity are important in some especial applications like railway sleeper or in some structures where concrete is designed to provide protection from divergent current. In railway signal systems, insufficient resistivity of concrete is not expected. These properties are also used in analyzing concrete properties in fresh and hardened state. Electrical resistance also have influence on on-going corrosion of steel embedded in concrete.
|Table 3: Comparison of dielectric strength of concrete|
Dielectric strength of any insulating material is the maximum electrical field which a material can counteract without failure i.e. sustain its insulation properties. It was observed that dielectric strength of concrete with Portland cement is significantly lower than that have high-alumina cement. Table 3 shown dielectric strength of concrete. It can be concluded from the table that dielectric strength of concrete is not affected by moisture content; an oven dry concrete has approximately same dielectric strength as compared to that is stored in air (having higher moisture content).