Novel Technique of Retrofitting of Corrosion Damaged RCC Foundations of Transmission Towers Exposed To Marine Environment

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    Corrosion of steel reinforcing in concrete is one of the greatest threats to the durability of reinforced concrete. It is the single most expensive corrosion related problem, and affects the integrity of thousands of reinforced concrete structures. Structures located in aggressive environments, such as exposure to seawater salts are highly susceptible to corrosion activity.

    In a tropical country like India, where approximately 80% of the annual rainfall takes place in the two monsoon months, rusting related problems are very common, especially for infrastructural and industrial structures. India also has a very long coastline where marine weather prevails.

    The repair of these structures with like materials is often difficult, expensive, hazardous and disruptive to the operations. The removal and transportation of large amounts of concrete and masonry material causes concentrations of weight, dust, excessive noise, and requires long periods of time to gain strength before the building can be re-opened for service.

    A recent solution for repairing damage due to corrosion in reinforced concrete is to use fiber reinforced plastic (FRP) composite wrap along with modern corrosion inhibition techniques. In this two prong technique, modern corrosion inhibitors lowers the rate of corrosion. And FRP composite in addition to strengthening a concrete member, provides a barrier to protect the concrete from an aggressive environment.

    Figure 1: Location of tower foundationsThe present case study offers the solution for RCC transmission line foundations for a leading electricity supply company in India. This transmission line is located along west cost of Mumbai (Figure 1.) and passes through land, sea and creek areas. Due to aggressive marine exposer reinforcement inside reported major corrosion leading to cracking in structure, endangering the stability of transmission towers. Due to repeated repairs the client was looking for long term repair solution. A novel technique was evolved and executed in difficult locations. The solution has worked successfully and performing without any problem for the last seven years in aggressive marine environment.

    Mechanisms of Corrosion and its Effect at Site

    Corrosion is defined as the destructive result of chemical reactions between a metal or metal alloy and its environment. The process involves a transfer of an electronic charge in an aqueous solution (Jones 1996). In steel, corrosion results in the metallic iron being converted to the voluminous corrosion product ferric oxide (Mailvaganam 1992). Reinforcing steel is susceptible to uniform corrosion and pitting that may be caused by several different mechanisms. New concrete typically has a pH of 12.5 or higher. The high alkalinity passivates the steel by forming a protective film of ferric oxides. This film prevents the reinforcing steel from corroding. However, certain factors can increase the likelihood of the passive film being broken down. These factors include development of cracks in the concrete that extend to the steel, high permeability and/or high porosity of the concrete, inadequate concrete cover over the steel, or high levels of chlorides (Perkins 1997).

    Once the passive film breaks down, an electrochemical reaction takes place. This reaction may be separated into two partial or half-cell electrochemical reactions. The first reaction is the oxidation reaction or anodic reaction [1]. The second reaction is the reduction reaction or cathodic reaction [2]. The two half-cell reactions are expressed below:

    – The anodic reaction: Fe ? Fe2+ + 2e-….1
    – The cathodic reaction: 2e- + H2O + ½ O2 ? 2OH-…….2

    This leads to the development of ferric oxide or ‘rust.’ The formation of ferric oxide is expressed in the following equations (Broomfield 1997):

    – Ferrous hydroxide: Fe2+ + 2OH- ? Fe(OH)2…….3
    – Ferric hydroxide: 4Fe(OH)2 + O2 + H2O ? 4Fe(OH)3……4
    – Hydrated ferric oxide: 2Fe(OH)3 ? Fe2O3H2O…………5Figure 2: Corrosion Process

    The passivated area becomes the anode, and the remaining steel becomes the cathode. The moist concrete acts as an electrolyte, allowing the electrons to move from the anode to the cathode (Allen, Edwards, and Shaw 1993). The corrosion process is illustrated in Figure 2.

    Figure 3: Damages to RCC foundation because of marine environmentWhen iron oxides are formed, they have a volume that is between two to ten times greater than the volume of the steel that it replaced. This volume increase places considerable tensile stresses on the concrete, which leads to cracking of the concrete (Broomfield 1997). After cracks have formed, the concrete may begin to separate at the level of the reinforcing along a plane parallel to the concrete surface. This separation is referred to as delamination. As the corrosion process continues, pieces of concrete may eventually break away or “spall” off. This process is illustrated in actual condition at site in Figure 3.

    Figure 4: Site-conditionsSite Challenges

    As the location of towers were along the coastline and in creek with marshy land there was no conventional means of transport, entire material and man power needs to be shifted by small boats or manually. One had to time the repair activity as per the high tide and low tide which left contractor for effective working time of 3 to 4 hours per day. There was no fresh water and electricity available at site. All these situation are shown in figure no. 4. This had to be arranged by carrying special boat with fresh water tank and generator. Also few Figure 5locations were designated forest area so one had to take special care of environmental issues. Considering these difficulties one had to device appropriate methodology of durable, efficient and fast repair method. Conventional method don’t qualify these requirement so it was decided to use modern composite methods by FRP wrapping as it is durable, light in weight so transportation is not a problem and quick to implement.

    Repair Methodology:

    Figure 5: Cleaning of concrete surface by grinding and removing existing coatings1-Chipping and Surface Cleaning by grinder

    Chip off loose and infected concrete from surface of existing structural elements with help of chisel and hammer till sound concrete is encountered. Remove laitance, oil etc. present on concrete sur-face by grinding/ sand blasting. De-scale the surface of exposed reinforcement with help of brush to remove the rust scales. (Figure 5)

    2- Treatment Corrosion Damaged Reinforcement

    (i) Application of rust converting alkaline primer: Thoroughly clean the corroded reinforcement/ steel rebar by wire brush or rotary grinder. Remove all the cor-rosion scales present on the bare and reach up to sound steel. Apply of rust converting alkaline primer on corrosion affected steel bars after removing all the scales. It is alkaline in nature and convert both hematite & magnetite compounds in to stable compounds. The material shall pass minimum 400 alternate immersion cycles of 2 minutes in 3.5% NaCl solution at room temperature.

    (ii) Application of two coats of IPNet- RB anticorrosive epoxy coating on steel rebars: Application of primer shall be followed with application of two coats of IPNet- RB (confirming to CBRI requirement) anticorrosive epoxy coating for bar protection against future corrosion. Coating is for old a well as newly provided steel. This system (Interpenetrating poly-mer network system for rebars: IPNet-RB) once applied on steel shall provide extended protection against future carbonation and chloride attack.

    Figure 6: Coatings of IPN and penetrating type corrosion inhibitors(iii) Application of concrete penetrating corrosion inhibitor on concrete surface: Carry out application of ‘Bi-polar migratory corrosion inhibitor on concrete surface by brush in two coats. This inhibitor has migratory kind of property which per-mits the material to migrate to a virtual extent of 60 mm, through pores of con-crete, inhibiting the corrosion and de-passivating the Electro-chemical reaction. It has property to attack anode as well as cathode, which is purely alkaline in nature. Grout the corrosion inhibitor in case of excessively damaged RC sections by drilling 50 to 75 mm deep holes at the spacing of 350mm c/c with the dosing of 100ml per hole in concrete body. Figure 6

    3- Making Up of Lost Steel Area Due to Corrosion by Additional Steel

    Makeup lost steel area due to corrosion by providing additional steel reinforcement. The steel shall confirm to IS 1786 grade Fe 415/ Fe 500. Anchor the steel rebar in sound concrete body up to desired depth by structural GRADE adhe-sive. Fixing of rebars is to be with pre left binding wires with existing steel at regular grid after aligning concrete profile with new mortar up to existing steel face.

    4- Making Up Lost Strength of Core Concrete by Low Viscosity Monomer

    Make up of lost strength of core concrete shall be with grouting of low viscosity (2-5cps- as per ASTM-D-2196) monomer. This is a low viscous high mole-cular weight thermosetting polymer. Due to its low viscosity it effectively fills up all micro-cracks and voids up to full depth of concrete. Beside enhancing existing bin-ding matrix this shall also enhance in ductility property of elements.

    Figure 7: Pressure grouting of monomerGrouting Process: (Figure-7)

    – Drilling of holes 12mm dia and about 50mm deep into concrete along the cracks or in honey combed and deteriorated areas, fix the perforated nozzles and seal the sides with Epoxy sealant.
    – Epoxy sealant: This is a non-shrink sealant. It is built in resilience to absorb impact and movements in joints.
    – Inject very low viscous injection resin into pre- drilled nozzles at a pressure of 2 to 4kg/cm2 using
    Compressed air and injecting gun. Seal the nozzles with epoxy after injection is completed.

    5-Sectional Reconstruction for Excessively Damaged Concrete

    (i) Bonding Coat: Bond between new and old concrete is important aspect for effective participation of total cross Sectional area of concrete. Selection of type of bond coat is based on, type of stresses bond strata is expected to go and prevailing area where application is to be carried.
    After the various pre-treatment apply liberal quantity of bond coat on cleaned concrete surfaces as per the detailed manufacture’s procedure. Ensure that the application of new concreting is carried out during the pot life of material.
    Figure 8: Sectional reconstruction by micro concrete(ii) Making up lost – section with Micro-concreteor latex modified mortar (Figure 8)
    Makeup mortar is based on type of structural element and its location. For replacing the carbonated part of concrete and repairing the damaged surface of concrete using Thixotropic Modified mortaris recommended. If extend of damages are high use of micro concrete was re-commended and for minor damages readymade hand-pack mortar were used.

    6-Strengthening With Non-Metallic Fibers Reinforced Composite

    Procedure for wrapping with Carbon fiber wraps

    Sanrachana 16    Sanrachana 17    Sanrachana 18

    – Surface preparation: Grind/ Sandblast repaired concrete substrate for cleaning rounding sharp edges to min 20-25 mm radius.
    Sanrachana 20– Profiling: Apply compatible primer on prepared substrate, Fill holes and uneven surface with thixotropic putty.
    – Wrapping: Apply first coat of fiber compatible saturant, cut the fabric to size, wrap the fiber sheet to structural element at desired orientation using tamping roller to avoid any air voids.
    – Finishing and sand pasting and Plastering: Applying second coat of saturant after min. 12 hrs, rectify air voids if any, paste the sand to make surface rough to receive plaster.
    Application of protective coating of aliphatic acrylic paint. Entire sequence of operation is presented in figure 9.

    Figure 10: UCR masonry for protection against scouring7. Protection against scouring by UCR Masonry (Figure 10)

    Due to continuous tidal activity scouring of foundation was observed. It had also damaged concrete at few location. To protect the concrete and the foundation from further scouring UCR masonry was provided near the bottom of foundation to dissipate tidal forces.

    Concluding Remarks:

    Two prong approach of modern corrosion inhibitor to reduce the rate of corrosion inside concrete and application of FRP wrap to protect and strengthen the RCC structures in aggressive environments can be successfully applied. Total 30 RCC foundations were successfully treated in duration of 9 months. The job was undertaken 6 years back and is closely monitored by client. Where conventional repairs were not durable and required to attend every year. With modern methods no damages were reported in last 6 years and system is performing satisfactorily.

    References:

    – Mailvaganam, Noel P., ed. (1992). Repair and Protection of Concrete Structures, Institute for Research in Construction, National Research Council of Canada, CRC Press, Inc., Ottawa, Ontario.
    – Perkins, P.H. (1997). Repair, Protection and Waterproofing of Concrete Struc-tures, E.& F.N. Spon, London, UK.
    – Broomfield, John P. (1997). Corrosion of Steel in Concrete, E.& F.N. Spon, London, UK.
    – Allen, R.T.L., S.C. Edwards, and J.D.N. Shaw, ed. (1993). The Repair of Con-crete Structures, Second Edition, Blackie Academic & Professional, Glasgow, UK.
    – E. W. Berver, J. O. Jirsa, D. W. Fowler, H. G. Wheat, and T. Moon (2001) Re-port No.FWHA/TX-03/1774-2, Effects of Wrapping Chloride Contaminated Concrete with Fiber Reinforced Plastics, Center for Transportation Research, The University of Texas at Austin, 3208 Red River, Suite 200, Austin, TX 78705-2650

    Acknowledgment:

    Entire site team of Engineers and supervisors of Sarachana for their hard work for timely completion of the project.

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