ELECTRICAL RESISTANCE OF BLAST FURNACE SLAG CEMENT AND ORDINARY PORTLAND CEMENT CONCRETES

Abstract

Corrosion propagation after depassivation of steel reinforcement in concrete caused by chloride penetration or carbonation may be governed by anodic, cathodic and/or electrolytic (resistance) properties of the corrosion cell. This paper focuses on the electrical resistance. Besides for corrosion propagation concrete resistivity may be of interest for the application of electrochemical techniques for assessment of the corrosion risk of a particular structure (potential and resistance mapping) or for corrosion protection (cathodic protection, realkalinisation, chloride-removal). Electrical resistances have been measured for about two years of concrete made using Blast Furnace Slag Cement (70% slag) (BFSC) and OPC. The influence of the water cement ratio and the humidity of the environment was studied. Water cement ratios were 0.45, 0.55 and 0.65. Environments include 50%, 65%, 80%, 90% (later transferred to fog room) and immersion in tap water, all at 20C. Specimens were 150mm cubes, electrodes were embedded brass rod electrodes at 10, 15, 20 and 50mm cover. Calibration using salt solutions allowed calculations of resistivity (specific resistance) values. Results of measurements during exposure up to two years are reported here and discussed briefly. General trends are: - BFSC concrete has a much higher resistivity than OPC concrete; - resistivity increase due to drying out is slow but steady; - resistivity increases going from wet to dryer environment. A tentative interpretation in terms of corrosion rate is given. The higher resistivity of slag cement concrete may significantly decrease the corrosion rate. There is evidence suggesting a higher resistivity correlates to a lower chloride penetration rate. In conclusion, the significantly higher resistivity of blast furnace slag cement concrete compared to portland cement concrete may improve the life time of concrete structures by increasing both corrosion initiation and propagation times. (A) For the covering abstract of the conference see IRRD 853281.