In pursuit of durable marine structures David O. Koteng’ Associate Professor Department of Civil & Construction Engineering, The Technical University of Kenya..

1. Introduction Marine structures play an important role in the exploitation of the vast resources of the blue ocean. However, very often they suffer rapid deterioration due to the unfavourable conditions for most construction materials in a marine environment. When constructed of reinforced concrete, the primary cause of deterioration is rebar corrosion, the rusting of the imbedded steel. This results in heavy maintenance costs and/or early replacement of affected structures. This paper looks at the possible mitigation measures to extend the life of marine structures and provide sustainable development through the rational use of construction materials and reduced life cycle costs.

1. Introduction Typical rebar corrosion in concrete.

2. The role of materials in concrete performance 2.1 Cement – 3 types are readily available in the local market, made to KS EAS 18. CEM I produced from pure clinker and made up primarily of C3S, C2S, C3A, C4AF, and CaSO4. In the presence of water: C3A/C4AF + CaSO4 + H2O → non cementitious ettringite. C3S/C2S + H2O → C-S-H + Ca(OH)2 (non-cementitious) Excess C3A + H2O → C-A-H. Ca(OH)2 is leachable, participates in carbonation shrinkage and AAR. C3A forms disruptive ettringite or destroys cement paste in reactions with sulphates. CEM II/A-LL produced from clinker + CaCO3 (largely inert filler). Composition and hydration largely same as CEM I but has leachable CaCO3. CEM II/B-P & CEM IV/B-P produced from clinker + natural pozzolana. Less amounts of C3S, C2S, C3A, C4AF + SiO2 and Al2O3 from pozzolana. CEM I reactions followed by pozzolanic reactions: Ca(OH)2 + SiO2 → C-S-H Ca(OH)2 + Al2O3 → C-A-H Ca(OH)2 + SiO2 + Al2O3 → C-A-S-H

2. The role of materials in concrete performance 2.2 Aggregates (Fine/Coarse) Some contain reactive silica and/or carbonates which react with Ca(OH)2 in concrete in the presence of water to form large disruptive compounds. Sea organisms appear to have a preference for limestone aggregates in which they burrow and create tunnels (Oud, 1986). 2.3 Water – should be free from harmful salts such as sulphates, chlorides and acids. Alkali aggregate reaction in Kamburu Dam. (Watson, 1985.)

3. Mechanism of rebar corrosion. Causes - Ingress of CO2, SO2, etc. lowers the pH < 9. e.g. CO2 + H2O + Ca(OH)2 → CaCO3 + 2H2O - High level of Cl- destroys the oxide layer on rebar. e.g. Fe2O3.2H2O + 4Cl- + 6H+ → 2Fe2+ + 4Cl- + 5H2O Pourbaix (E vs pH) diagram for steel.

3. Mechanism of rebar corrosion. Process - Rebar corrosion is an electrochemical process and anodic and cathodic areas are formed in the rebar or rebar cage. concrete rebar Anodic reactions Fe = Fe2+ + 2e– Fe2+ + 2OH- = Fe(OH)2 4Fe(OH)2 + O2 = 2(Fe2O3·2H2O) Cathodic reactions 2H2O + O2 + 2e- = 4OH- 2H+ + 2e- = 2H2

4. Mitigation measures. 4.1 Using a coating on the surface of concrete. Stops ingress of Cl-, CO2, etc which initiate corrosion. Stops continuous supply of H2O and O2 which sustain corrosion. Sealants include linseed oil, epoxy, silane, siloxane, and methacrylate. (Keppler, Darwin, & Locke, 2000) Nanokote, West Leederville, WA 6901

4. Mitigation measures. 4.2 Using corrosion inhibitors. These are chemicals which attach to the surface of rebar and passivate it, usually admixed in concrete at production. Anodic inhibitors attach to anodic areas and include chromates, nitrites, molybdates, alkali phosphates, and carbonates. Cathodic inhibitors attach to cathodic areas and consist of zinc, salts of antimony, magnesium, manganese, and nickel. Organic inhibitors attach to both anodic and cathodic areas and include amines, esters, and sulphonates. (Keppler, Darwin, & Locke, 2000) Migrating corrosion inhibitors (MCI) can be organic or inorganic compounds and can be applied on the surface of hardened concrete and develop high vapour pressure which diffuses into concrete and attach on rebar (Bavarian & Reiner, 2002).

4. Mitigation measures. 4.3 Using coated rebars. Epoxy coated rebars Peak Products, Canada (2018) Zinc coated rebars Conview Victoria, Balwyn East, VIC 3103. Stainless steel coated rebars NX Infrastructure London, UK. 4.4 Using sacrificial anodes. Vector Corrosion Technologies, Cradley Heath, UK.

4. Mitigation measures. 4.5 Using impressed current. The principle of corrosion cathodic protection, V&C Austria. Pourbaix (E vs pH) diagram for steel. 4.6 Using stainless steel rebars. Ordinary stainless steel corrodes in Cl- environment. Special stainless steels have been developed for application in marine environment (Keppler, Darwin, & Locke, 2000). Stainless steel rebar Devorian Metals, Cornwall, UK.

4. Mitigation measures. 4.7 Using non-ferrous rebars. CFRP rebars Dongguan Juli FRP Products Co. Guangdong, China. 4.8 Using electro-chemical chloride extraction. Schematic representation of the chloride extraction process The Concrete Society, Surrey, UK

4. Mitigation measures. 4.9 Using high performance concrete. The process may take 4 to 8 weeks to conclusion. (Keppler, Darwwin, & Locke, 2000), (Sharp & Sharp, 2016). 4. Mitigation measures. 4.9 Using high performance concrete. High performance concrete (HPC) is characterized by dry pore structure, high density and low permeability, among other attributes. Incorporates pozzolanic admixtures to consume free Ca(OH)2 therefore less risk of of leaching, AAR, and carbonation shrinkage cracking. No water to transport reactants, participate in corrosion reactions, or act as electrolyte to transport OH- from cathode to anode. High density increases resistance to movement of OH- from cathode to anode and prevents ingress of contaminants and reactants from external sources.

The process may take 4 to 8 weeks to conclusion The process may take 4 to 8 weeks to conclusion. (Keppler, Darwwin, & Locke, 2000), (Sharp & Sharp, 2016). 5. Conclusions. Rebar corrosion can be mitigated by a suitable combination of the methods discussed above. HPC provides a first line of defense as cover to the rebar, but cracks are inevitable unless prestressed. A suitable coating seals openings on the concrete surface. Additional internal protection to the rebar completes the mitigation process. Cost considerations will influence the choice of the mitigation strategy. The use of pozzolanic cements, CEM II/B-P and CEM IV/B-P offers good prospects for providing HPC. Challenges in concrete mix design and production of HPC must be addressed through deliberate research.

References 1. Watson, G. M. (1985). Rare Concrete Problem at Kamburu Investigated. Kenya Engineer, May/June, 33-37. 2. Oud, H. J. (1986). Concrete in Hot Countries. Hertogenbosch: STUVO. 3. Keppler, J. L., Darwin, D., & Locke, C. E. (2000). Evaluation of Corrosion Protection Methods for Reinforced Concrete Highway Structures. Lawrence: University of Kansas Center for Research. 4. Bavarian, B. & Reiner, L. (2002). Corrosion Protection of Steel Rebar in Concrete Using Migrating Corrosion Inhibitors, MCI 2021 & 2022. Northridge: The Cortec Corporation.