Concrete resistance

The reason for the use of zinc as a power-impressed rather than a sacrificial anode is that the high concrete resistivity limits the current output, and a higher driving voltage than that provided by the e.m.f. between zinc and steel in concrete is used to provide the necessary current output. No cementitious overlay is required, although it may be advisable to paint the top surface of the sprayed zinc to prevent atmospheric corrosion of the zinc anode. 

Air content of freshly mixed concrete by the pressure method Air content of freshly mixed concrete by the volumetric method Unit weight, yield, and air content of concrete Specific gravity, absorption, and voids in hardened concrete Resistance of concrete to rapid freezing and thawing Scaling resistance of concrete surfaces exposed to deicing chemicals... 

Sulphur concretes resistant to moisture show good durability in the presence of various acid and salt solutions which are normally aggressive to Portland cement concretes (4, J>, 6). Care must nevertheless be exercised with the aggregates to be used. 

Emulsion-based coating elastomeric nature ability to bridge smaller cracks in concrete resistance to carbonation, sulphates and ingress of chloride ions concrete and masonry structures... 

This test method was proposed by Senbetta16. Its original purpose was to evaluate the impact resistance of floor materials in terms of the capacity to resist surface disintegration due to repetitive low level impact. Instead of floor materials, this test was employed to measure the influence of plastic fillers on the concrete resistance to repetitive low impact. The specimens which were used in the test were nine 2-in. (50-mm) concrete cubes made with mix No. 2. The test uses the Los Angeles (LA) abrasion machine, which is traditionally utihzed to test aggregates according to ASTM C 131 and C 535 specifications. 

Polyester polymer concrete resists oxidizers, acids, oils, and petroleum products, but there is not enough resistance to alkaline solutions and water. The strength of polyester PC in water decreases faster than in solutions of inorganic salts and some acids therefore, resistance in acid solutions can simultaneously serve as an estimate of water resistance. For example, the flexural strength of the PC, immersed in a 10% solution of sulfuric acid or 10% sodium chloride solution is reduced by 30% after 80 days of immersion. 

A further example, which confirms the necessity of evaluating the resistivity of the medium very carefully, concerns the corrosion of rebars in reinforced concrete. In this caae the intensity of the current flowing between the anodic and cathodic zones of a macrocell depends on the resistivity of the concrete and the extent of the region involved. To determine the concrete resistivity various methods have been developed, which can be applied in the laboratory [14] as well as in the field [15]. It should be noted, however, that in the latter case most researchers have pursued the approach suggested by Wenner [16] for the evaluation of the resistivity of soils. The contribution of the ohmic drop to the electrode overvoltage cannot be neglected when the values of the corrosion rate of the rebars are appreciable, even if the current intensity is small within a given polarization potential interval, because under such conditions the interpretation of experimental results could be completely distorted. 

A. Mazzoni, E. Nolan, H. Wojtas, Reinforcement corrosion and concrete resistivity - State of the art, laboratory and field results , Proc. Int. Conf. on Corrosion and Corrosion Protection of Steel in Concrete,... 

W. Sand, E. Bock, D. C. White, Biotest system for rapid evaluation of concrete resistance to sulfur-oxidising bacteria . Materials Performance, 1987, 26, 14-17. 

The influence of temperature and humidity affect the corrosion rate through their influence on the electrochemical reactions at the steel/concrete interface and through their influence on ion transport between anodes and cathodes. Although the mechanisms are not fully understood, it appears that the concrete resistivity (or conductivity) is strongly related to the corrosion rate at moderate or low temperature [35-38]. Variation in resistivity due to variation of humidity (at constant temperature) caused an inversely proportional variation of corrosion rate in carbonated mortar and concrete with low amounts of chloride or without chloride. Variation of temperature (at constant humidity) caused a similarly varying corrosion rate. 

This decrease may not occur in the case of structures subjected to wetting/ drying cycles or in conditions where oxygen consumed at the surface of the passive steel is replaced. This may happen in hollow piles of offshore structures, as depicted in Figure 8.2. Similar conditions may arise in tuimels buried or submersed in chloride-containing environments. Rebars on the inside of hoUow (air-filled) structures may be effective cathodes with noble potentials. They increase the potential of rebars closer to the seawater side of the cross section, stimulating corrosion initiation at lower chloride contents than without additional cathodic effects. Subsequently they may increase the corrosion rate at the anodes by consuming the electrons produced. The final corrosion rate will be a function of the ratio between anodic and cathodic areas, which is influenced by the concrete resistivity. 

The best protection against stray current is, therefore, provided by concrete. Those methods that can improve the resistance of concrete to carbonation or chloride contamination, which are illustrated in Chapters 11 and 12, are also beneficial with regard to stray-current-induced corrosion. It should be observed that this may not be the same for preventative techniques, since conditions leading to corrosion initiation due to stray current are different, in terms of potential, from those leading to corrosion initiation due to carbonation or chloride contamination. For instance, the use of stainless steel or galvanized-steel bars, which improves the resistance to pitting corrosion in chloride-contaminated concrete (Chapter 15), does not substantially improve the resistance to stray current in chloride-free and non-carbonated concrete [4]. In any case, a high concrete resistivity will reduce the current flow due to stray current. 

Temperature changes have important effects on concrete resistivity. A higher temperature causes the resistivity to decrease and vice versa (for a constant relative humidity). This is caused by changes in the ion mobility in the pore solution and by changes in the ion-solid interaction in the cement paste. As a first approach an Arrhenius equation can be used to describe the effect of temperature on conductivity (inverse of resistivity) ... 

According to the electrochemical nature of the corrosion process and the macrocell-corrosion model (Chapter 8), a relationship may be expected between the concrete resistivity and the corrosion rate of the reinforcement after depassivation. Using a simplified approach, the corrosion rate of steel in concrete should be inversely proportional to the resistivity. This was confirmed in a general sense [24, 25], although the relationship is not universal rather it depends on the concrete composition [21]. 

Measurements at the concrete surface. All methods for on-site measurement of concrete resistivity involve at least two electrodes (of which one may be a reinforcing bar). A voltage is superimposed between the electrodes and the resulting current is measured, the ratio gives a resistance (measured in O). The resistivity is obtained by multiplying the measured resistance by a geometrical conversion factor, the cell constant. This approach is valid only for a homogeneous material. 

Interpretation. The results of concrete resistivity measurements can be used for a quantitative or qualitative interpretation. Resistivity data measured on a structure and corrected for the temperature effect can be compared to reference data of similar concrete types (Table 2.4). Usually additional information is necessary. If, for example, a wet structure made with OPC has a mean resistivity value of 50 fl m, it means that the water-to-cement ratio and the porosity must be quite high. Consequently, the corrosion rate after depassivation will be high. 

Reinforcement corrosion and concrete resistivity - State of the art, laboratory and field results , Proc. 

T. Valente, Laboratory investigation of concrete resistivity and corrosion rate of reinforcement in atmospheric conditions , Proc. Rilem/... 

Repair materials. Repair materials to be used in areas where current is flowing should have similar electrical resistivity as the parent concrete in order not to disturb uniform current flow. Repair mortars should be tested for resistivity for several months to obtain mature values, preferably in a cHmate that simulates the exposure of the structure [7,8]. A difference of a factor two between mortar and concrete resistivity is considered acceptable [13]. It appears that most cementitious... 

Because of the relatively high resistivity of atmospherically exposed concrete substructures, most anodes utilize impressed current to achieve the necessary driving voltages to supply the current required for corrosion control. However, an exception to this is the use of sacrificial zinc anodes for CP of coastal bridges in Florida, which have a relatively low concrete resistance. However, studies continue to examine the use of sacrificial anodes because of the benefit of its low maintenance compared to impressed-current CP systems. Two of these studies are the following ... 

The splash/tidal zone of bridges and walls represent cylindrical columns immersed in water. These structures involve entry in two dimensions. Chloride ions enter concrete by adsorption at the surface, which is given by an empirical equation. The effective chloride diffusion coefficient is derived from concrete permeability, water/cement ratio, and concrete resistivity. When concentration reaches a critical value in the vicinity of steel, corrosion begins. As shown in Fig. 12.6, a boundary layer exists adjacent to the concrete... 

In both cases, the critical transport parameter is the chloride diffusion coefficient. The diffusion coefficient varies with water to cement ratio and time. In this model, the effective chloride diffusion coefficient is derived from concrete permeabihty, water/ cement ratio, and concrete resistivity. 

W. Morris, A. Vico, M. Vazquez, Chloride induced corrosion of reinforcing steel evaluated by concrete resistivity measurements, Electrochim. Acta 49 (2009) 4447-4453. 

K. Hombostel, C.K. Larsen, M.R. Geiker, Relationship between concrete resistivity and corrosion rate-A hterature review, Cem. Concr. Compos. 39 (2013) 60—72. 

Resistivity Concrete resistivity/ corrosion risk General/specialist 1 reading in 20 s 10 to 20 per day. 

The four probe resistivity meter or Wenner Probe was developed for measuring soil resistivity (ASTM G57). Specialized modifications of the Wenner probe are frequently used for measurement of concrete resistivity on site. The measurement can be used to indicate the possible corrosion activity if steel is depassivated. Proprietary version of the system are shown in Figure 4.12. Most systems uses four probes. The outer probes pass a current through the concrete while the inner probes detect the voltage difference. This approach eliminates any effects due to surface contact resistances. 

The electrical resistivity is an indication of the amount of moisture in the pores, and the size and tortuosity of the pore system. Resistivity is strongly affected by concrete quality, that is, cement content, water/cement ratio, curing and additives used. The chloride level does not strongly affect resistivity as there are plenty of ions dissolved in the pore water already and a few more chloride ions here or there does not make a big difference. However, chlorides in concrete can be hygroscopic, that is, they will encourage the concrete to retain water. This is why chlorides are often accused of reducing concrete resistivity. 

Concrete resistivity of the area around the sensor is obtained by the formula ... 

There is a frequent temptation to multiply the resistivity by the reference electrode potential and present this as the corrosion rate. This is incorrect. The corrosion rate is a function of the interfacial resistance between the steel and the concrete, not the bulk concrete resistivity. The potential measured by a reference electrode on the concrete surface is not the potential at the steel surface that drives the corrosion cell. Correlations between resistivity, reference electrode potential and corrosion rate may be found in similar samples in similar conditions in the laboratory but in the variability of the real world any correlation is fortuitous. 

Chapter 4 dealt with the different test methods available for assessing corrosion related properties of reinforced concrete. Most of the techniques are applied with hand-held battery operated equipment and a one off reading is taken. In some cases such as cover meter measurements that reading will not change. In others, such as concrete resistivity, corrosion rate or corrosion potential (reference electrode potential) it will change as aggressive agents in the environment move into the concrete and lead to or accelerate corrosion. 

Galvanic cathodic protection systems have been used extensively since the early 1990s in Florida on prestressed concrete bridge support piles in the sea. One of the reasons the galvanic system is used there is because concrete resistivity is low due to the marine exposure conditions. The Florida systems frequently incorporate a distributed anode of zinc fixed on the atmospherically exposed concrete and bulk zinc anodes in the water which pass current through the low resistance sea water to protect the submerged area as shown in Figure 7.4. 

---