Concrete electrical conductivity

The absence of an electron from a covalent bond leaves a hole and the neighboring valence electron can vacate its covalent bond to fill the hole, thereby creating a hole in a new location. The new hole can, in turn, be filled by a valence electron from another covalent bond, and so on. Hence, a mechanism is estabUshed for electrical conduction that involves the motion of valence electrons but not free electrons. Although a hole is a conceptual artifact, it can be described as a concrete physical entity to keep track of the motion of the valence electrons. Because holes and electrons move in opposite directions under the influence of an electric field, a hole has the same magnitude of charge as an electron but is opposite in sign. 

Depending on their structure, the polymers containing heterocycles have various applications. For example, poly(furfuryl alcohol) is used in composite materials with fillers such as sand and concrete, in copolymers with formaldehyde, etc. Some of the polymers from this group have special properties such as good electrical conductivity (after appropriate doping). Among these polymers are poly(thiophene-2,5-diyl) and particularly polypyrrole, CAS 109-97-7, (usually in carbon black doped with an organic acid anion). The structure of this polymer is shown below ... 

As was remarked as early as chap. 1, many of the properties of materials are not intrinsic in the sense that they are highly dependent upon the structure, purity and history of the material in question. The electrical conductivity is one of the most striking examples of this truth. Nowhere is this more evident than in considering the role of doping in semiconductors. This is hinted at in fig. 7.2 where it is seen that the conductivity of Si ranges over more than 6 orders of magnitude as the concentration of impurities is varied. In fig. 7.3 this effect is illustrated concretely with the variation in resistivity of Si as a function of the concentration of impurities. 

Anode system. The anode system, which consists of the anode material plus its overlay, must supply the required current for the anticipated service life and distribute it to the reinforcement that needs to be protected. Anode materials and current density aspects have been dealt with in a previous section. The general requirements of an anode system are it has to adhere to the concrete surface it should be suitable for appHcation to the surface needing protection (top, bottom, horizontal, vertical, flat, curved), it should be durable and have low installation cost it should produce acceptable weight addition and change of the appearance and dimensions of the structure. If an overlay is used, it should have durable bond to the substrate concrete, sufficient mechanical strength and electrical characteristics equal to those of base concrete (ionic conductivity). 

As stated in Chapter 2, corrosion proceeds by the formation of anodes and cathodes (Figures 2.1 and 2.2). In the case of chloride attack they are often well separated with areas of rusting separated by areas of clean steel. This is known as the macrocell phenomenon. Chloride induced corrosion is particularly prone to macrocell formation as a high level of water is usually present to carry the chloride into the concrete and because chlorides in concrete are hygroscopic (i.e. they absorb and retain moisture). The presence of water in the pores increases the electrical conductivity of the concrete. The higher conductivity allows the separation of anode and cathode as the ions can move through the water filled (or water lined) pores. 

Conductivity must be low enough to allow current to pass into the steel. A resistivity of 15 kQ-cm at 28 days under saturated conditions has been specified by the United Kingdom DoT based on successful use of materials of this resistivity on the Midland Links. There is some disagreement about this specification but it is the only quantitative information available. In the United States some applicators use salt in the concrete to even out the resistivity with the parent concrete. This practice is rarely accepted elsewhere. BSEN 12696 (2000) states that the concrete repair material shall be within 50-200% of the nominal parent concrete electrical resistivity. 

CEMENTS FOR APPLICATIONS IN WfflCH ELECTRICAL CONDUCTIVITY OF CONCRETE IS REQUIRED... 

Applications of electrically conductive concrete include conductive floors and cathodic protection of reinforced bridge decks. 

Effective shielding capability may be achieved by incorporating electrically conductive materials, especially carbon fibers or carbon filaments, into the concrete mix. Owing to... 

Solotov, A. et al. (1997) Properties of electrically conductive concretes (in Russian). Izvestiya Vysshikh Uchebnykh Zavednii, Stroitel stvo (6), 38-44 [ref. CA 127/247533]. 

Tamas, F.D. (1982) Electrical conductivity of cement pastes. Cement and Concrete Research 12,115-120. 

Tumidajski, P.J. (1996) Electrical conductivity of Portland cement mortars. Cement and Concrete Research 26,529-534. 

The electrical conductivity and resistivity of concrete is indirectly related to the degree of resistance offered by concrete to the passage of corrosive species to the steel reinforcement The concrete resistivity is very high (>4KS2) and it can be best measured by the EIS technique (Electrochemical Impedance Spectroscopy) described earlier. ASTM Test C1202-94 can be used to determine concrete resistivity (Fig. 12.37). 

Chlorides have probably received the most study in relation to their effect on corrosion. Like other ions, they increase the electrical conductivity of the water, so that the flow of corrosion currents will be facilitated. They also reduce the effectiveness of natural protective films, which may be permeable to small ions. Nitrate is very similar to chloride in its effects but is usually present in much smaller concentrations. Sulfate in general appears to behave very similarly, at least on carbon steel materials. In practice, high-sulfate waters may attack concrete, and the performance of some inhibitors appears to be adversely affected by the presence of sulfate. Sulfates have also a special role in bacterial corrosion under anaerobic conditions. 

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