Resistivity measurements, reinforced concrete

Figure 8-21. Electrode arrangement in measuring polarization resistance on reinforced concrete structures (schematic), /fg reference electrode, CE counter electrode, WE working electrode (rebars). The diameter of the counter electrode is L, the length of current spread-out...

The cathodic protection of reinforcing steel and stray current protection measures assume an extended electrical continuity through the reinforcing steel. This is mostly the case with rod-reinforced concrete structures however it should be verified by resistance measurements of the reinforcing network. To accomplish this, measuring cables should be connected to the reinforcing steel after removal of the concrete at different points widely separated from each other. To avoid contact resistances, the steel must be completely cleaned of rust at the contact points. 

S. Fiore, R. B. Polder, R. Cigna, Evaluation of the concrete corrosivity by means of resistivity measurements , Proc. Fourth Int. Symp. on Corrosion of Reinforcement in Concrete Construction, C. L. Page, P. B. Bamforth, J. W. Figg (Eds.), Society of Chemical Industry, Cambridge, UK, 1-4 July, 273-282, 1996. 

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. 

C. Andrade, ). A. Gonzales, Quantitative measurements of corrosion rate of reinforcing steels embedded in concrete using polarization resistance measurement . Materials and Corrosion, 1978, 29, 515-519. 

F. Hunkeler, Monitoring of repaired reinforced-concrete stmctures by means of resistivity measurements , in Repair of Concrete Structures, Svolvaer,... 

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

An alternative approach using a single electrode on the surface and the rebar network can be used to measure the resistivity of the concrete cover. This is available as part of a corrosion rate measuring device (see Section 4.12 and Figure 4.13) and uses the reinforcement cage as one electrode and a small surface probe as the other electrode. The advantage of this approach is that it measures the resistivity of the cover concrete only. The disadvantage is that it suffers from contact resistance problems. 

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. 

Naish, C.C., Marker, A. and Carney, R.F.A. (1990). Concrete Inspection Interpretation of Potential and Resistivity Measurements. Corrosion of Reinforcement in Concrete, International Symposium. Wishaw, Warwickshire, UK. 

When concretes with the same moisture content are compared, resistivity is higher for the lower wIc ratio, for higher levels of hydration and for the use of additional components like fly ash, SF, etc. Resistivity increases with age and hardening of concrete because the pore system becomes less permeable there is an inverse correlation between concrete resistivity and the chloride diffusion rate. This fact is exploited for the measurement of concrete resistivity in view of assessing its susceptibility to chloride penetration and, consequently, to corrosion of reinforcement for both its initiation and rate of progress. The resistivity of external concrete cover is the most important and is expressed by the formula ... 

Reinforced concrete cast in situ suspended floor slabs can give reasonable gas resistance, if they are detailed correctly with anti-crack reinforcement. They also need to be sealed at joints (possibly with water bars a design measure to ensure joints are watertight) and sealed at all service penetrations with water bars. Evidence from Building Research Establishmenf (BRE) research has shown that reinforced concrete rafts with a venting layer below give the best level of protection to gas ingress. 

Points for inspection of gas protection measures (gas-resistant membrane on a cast in situ reinforced concrete suspended floor slab (provided by Cooper Associates (Nik Reynolds and Ben Hill), prepared for Liverpool City Council)... 

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). 

Shah, S.P. and Gopalaratnam, V.S., Impact resistance measurements for fibre cement composites. In Developments in Fibre Reinforced Cement and Concrete. RILEM Sympositim, Sheffield, 1986, vol. 1, pp. 267-276. 

Direct measurement of the effect of aggressive reagents on concrete durability appears to be confined to sea water and sulfate attack, where in both areas it is recognized that the lower the water-cement ratio, the greater will be the resistance to attack and the use of a water-reducing admixture will be obviously helpful. This is confirmed by work carried out in Holland [95] and Japan [84] and a general conclusion is that a reduction in the water- cement ratio from 0.5 to 0.40, would allow a reduction in thickness of cover of the reinforcement by about 50%. 

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