EFFECT OF SACRIFICIAL ZINC ANODES ON CURRENT DENSITY FOR EFFECTIVE CATHODIC PROTECTION OF RCC

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1 EFFECT OF SACRIFICIAL ZINC ANODES ON CURRENT DENSITY FOR EFFECTIVE CATHODIC PROTECTION OF RCC Krishna Shinde 1, R. K. Shrivastava 2 and R. D. Angal 3 1 Department of Mechanical Engineering, Govt. College of Engineering, Aurangabad, Maharashtra, India 2 Department of Mechanical Engineering, Govt. Engineering College, Karad, Sangli, Maharashtra, India 3 Emeritus Prof. Indian Institute of Technology, Powai (IIT-B), Mumbai, Maharashtra, India ABSTRACT This work reports recent results of sacrificial zinc strip anodes performance and its effects on current density variations in the effective protection of reinforced concrete structures. This work is carried out at a location situated in Navi Mumbai. This paper reports the results of 18 months study conducted on reinforced cement concrete specimens of different additive compositions of aggregates, attached with zinc anodes electrically, immersed in electrolyte solution and exposed to normal coastal atmosphere. The performance and effect of zinc sacrificial anodes was recorded regularly and evaluated by measuring Current in Shunt µa (voltage drop in mv) and a detailed discussion of the effects on the current density is included in this report. Keywords: zinc anode, cathodic protection (CP), RCC (reinforced cement concrete), current density, steel rebar, corrosion. 1. INTRODUCTION The corrosion of rebar in reinforcement is the main cause of reinforced concrete structure deterioration especially those located in industrial and aggressive marine environment with the presence of chlorides.[1] Although steel rebar embedded in concrete of good quality is always protected by high alkalinity of pore water, which by the presence of oxygen in turn passivates the steel. The alkalinity is lost due to carbonation of the concrete and chloride ions penetration to the steel which can destroy the passive film [2-5]. Generally, the reinforced concrete structures are maintenance free and durable for its whole design life of approximately more than 60 years. [6-8] Moreover the corrosion of RCC exposed to rough marine and industrial environment affects the life of RCC and is reported as a serious problem today. Structures like bridges, buildings and other RCC structures are being damaged severely due to corrosion of rebar steel within as short as years of period [9, 10]. To combat with such deterioration, cathodic protection is the only way and zinc is considered as one of the most evaluated and practically used material for sacrificial anodes in RCC. The sacrificial zinc anodes are identified and tested as established form of cathodic protection system which are located and placed on RCC surface and electrically connected to the reinforcing steel bars for effective and efficient performance. This study used a unique experimentation method to investigate and report the effect of fluctuations of various aspects of industrial and marine environment on Current Density measurement for effective protection of RCC. The results obtained are discussed with respect to practical implications. 2. MATERIALS AND METHODS 2.1. RCC Block preparation The RCC blocks are specifically designed for the experimentation taking into consideration the literature review and practical experience of the experts in the field. The RCC blocks dimensions mm was cast in February M25 grade cement, sand and aggregate in the proportion of 1:3:3 respectively is used for casting with potable water and cement ratio of 0.5. Figure-1. Concrete block preparation. 4069

2 2.3 Zinc strip anode preparation For this experiment, required strip size was specifically designed and casted by taking in consideration the area of steel rods and zinc-steel area ratio as 1:7. The calculated strip size worked out to be mm. Anodes were casted with 1mm steel wire for electrical connectivity and wire later coated for experimental purpose. These anodes were carefully weighed and centrally pasted along the length of RCC block, side surface by Alcofine quick setting cement and later coated. Figure-2. Rebar frame embeded in concrete. The specially made mild steel rebar frame of 3 bars of 350,350 and 425mm are used, with diameter of 10mm each. These bars are electrically interconnected by welding 2 ribs of 75mm length for spacing. The frame is designed in such a way that 425mm bar protrudes outside the RCC block for the convenience of electrical connectivity for further experimentation. For further electrical connectivity and convenience of taking readings of voltage drop the MS wire was welded at 9 strategic locations and taken out of the RCC blocks for further experimentation. The ribs and connecting wires with welded joints in RCC blocks were coated with epoxy coating IPNet-RB which is anti-corrosive so as to prevented from corrosion. The RCC blocks were placed in a tray, immersed in 2% NaCl water or electrolyte water with 0.15molar Sodium Sulphate for the corrosion experiment. 2.2 Design of rebar castings of RCC In the experimental program, comparative study to be made on total 10 types of specimen made of: a) Control specimen with rusted ribbed rebar without inhibitor - 2 nos. (RC-1, RC-2) b) Migratory type of corrosion inhibitor admixed with rusted ribbed rebar - 3 nos. (RI-1, RI-2, RI-3) c) Migratory type of corrosion inhibitor admixed with cleaned & pickled ribbed rebar - 3 nos. (PI-1, PI-2, PI-3) d) Control specimen with cleaned & pickled ribbed rebar without inhibitor - 2 nos. (PC-1, PC-2) As per the aforesaid design of rebar the RCC blocks are cast into the moulds and cured for the standard time span of 28 days and this experiment was carried out on RCC blocks excluding control specimens. Figure-3. Zinc strip anodes. 2.4 Cathode preparation Steel rebar frame enclosed in the concrete block acts as a cathode in our experiment. One end of the frame was specifically kept long which protrudes outside the concrete block was connected with 1mm diameter MS wire by means of welding. To protect the outer protruding end of the cathode and connected wire from corrosion, the protruding end along with the welded part of the MS wire was coated and covered with polymer and the Gold bond epoxy, KCPL and M-seal and remaining part of the MS wire is also epoxy coated and covered with silicon tube so as to prevent them from electrolyte contact and atmospheric corrosion. The ends of these wires are soldered with banana clip to facilitate various electrical measurements and also for their easy connection with cathode. 2.5 Measurement methods and circuitry The corrosion experiment was started in April For faster and accelerated corrosion it was designed and decided to immerse the concrete blocks into electrolyte solution for full time for initial one month period of time and thereafter wet and dry cycle for each alternate day. It means concrete blocks were kept submerged for a period of 24 hours and then taken out and dried for 24 hours before further subsequent immersion. 2.6 Measurement of galvanic current (ZRA) The Galvanic current flowing in the circuit (Zn to Steel) is to be measured by Zero Resistance Ammeter (ZRA) or by adding a resistor in shunt and voltage drop by direct current meter. 4070

3 The schematic diagram of the circuit is as shown in Figure-4 below. Figure-5. Experimental set up. Figure-4. Schematic diagram: Current measurement (between anode to cathode at various points) by voltage drop method through 10Ω shunt resistance. Figure-6. Current in shunt measurement. The current in shunt (voltage drop) readings were taken every week over the period of 18 months and in all 4320 readings were recorded in tabular form for further experimental analysis. 3. RESULTS AND DISCUSSIONS For finding current in shunt readings, voltage drop is measured as per the circuitry by DCM (Direct Current Meter) and reading hence obtained and spread over 72 weeks are shown in graphical form in Graph

4 Current in shunt (voltage drop) with direct current meter Anode Voltage Drop PI-1 PI-2 RI-1 RI-2 PC-1 RC Time (Weeks) Figure-7. Graph: Current in shunt. From the graph, it is observed that at the beginning, voltage readings obtained for all specimens were of higher order, except PC-1, which picked up later and averaged out after week 30.Other specimen though showing initial high voltage readings, then fluctuating marginally below average, later averaged out from week 30. This is because of initial variation needed to stabilize the CP system. Initially, before the CP system gets activated fully, the voltage readings were higher resulting in higher corrosion currents resulting in higher current density making the system more prone to rebar corrosion. As CP system starts operating, the voltage readings drop drastically and as system stabilizes, the voltage readings start to average out and get fully averaged out after 30 weeks Current density calculations Sample code Avg. Volts=V (mv) Table-1. Current density calculations. Avg. Current (µa) I corr =V/R 1000 R=10Ω Current Density (µa/cm 2 ) CD=I corr /Area Area=πdl=π =312.58cm 2 l= =995mm PI PI RI RI PC RC CONCLUSIONS a) From the readings of current in shunt (voltage drop) recorded in tabular form as stated above, the average voltage is obtained lies between 4.468mV to 5.482mV. Total area of cathode is cm 2 from which current density is calculated which lies between 1.42µA/cm 2 to 1.75µA/cm 2 as per Table

5 b) From this observation it shows that when current density for a concrete with certain ph when lies between 1 to 2µA /cm 2 it is in passive / protection zone. When current density for certain ph raises above 2µA /cm 2 or higher then it leads to corrosion zone to severe corrosion zone, for higher values. c) In our case current density lies between 1.42 to 1.75µA /cm 2 range, so protection is there and hence it indicates that protection is provided by anodes. Also life of anode increases with lesser current density. REFERENCES [1] P. Venkatesan, N. Palaniswamy, K. Rajagopal Corrosion performance of coated reinforcing bars embedded in concrete and exposed to natural marine environment. Elsevier, Progress in Organic Coatings. 56: [2] Z.P. Bazanth Physical Model for Steel Corrosion in Concrete sea structures-theory, J. Str. Div. ASCE 105 (ST6): [3] ACI Committee 222. Corrosion of metals in Concrete ACIR-85. American concrete Institute Detroit MI. [4] RILEM Technical Committee 60 CSC. Corrosion of Steel in Concrete, State of the art report. [5] C.L. Page, K.W.J. Treadaway, P.B. Bamforth (Eds.) Corrosion of Reinforcement in Concrete. Society of Chemical Industry London. [6] F.E. Turnearsure, E.R. Maurer Principles of Reinforced Concrete Constructions. John Wiley & Sons New York. [7] Building Research Establishment. Durability of Steel in Concrete. Part I. Mechanism of protection and corrosion. BRE Digest. 263: 1-8. [8] F.M. Lea Chemistry of Cement & Concrete. Edward Arnold Publishers Ltd. London. [9] Proceedings of International Congress of Navigation London. 1923: Venice 1931, Lisbon. [10] Seminar on Pile Foundations. Corrosion detailing and ground Anchors Report. IABSE Madras. 4073