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2017SummerShotcreteEMag

| SUSTAINABILITY In an electrochemical cell (chemical bridge), one area of the steel becomes the “anode” and another area becomes the “cathode,” and the pore water is the electrolyte. The positively charged ferrous ions at the anode pass into the pore solution, and the negatively charged free electrons absorbed by the pore solution, combining with water and oxygen to form hydroxyl ions (OH–). Hydroxyl ions combine with iron (II, III) ions (ferrous, ferric) to form hydrated iron (II/ III) oxide - Fe(OH)3, or “rust” (nomenclature for rust written for understanding convenience). CHEMICAL CAUSES OF METAL REINFORCEMENT CORROSION There are many chemicals and environments that can facilitate mechanisms involve carbonation and chloride ingress. Carbonation Carbonation of the cement binder occurs due to carbon dioxide (CO2) in the atmosphere, or contact with bicarbonate ions (HCO3 Fig. 4: Macro void space (air pocket) around reinforcement and bar termination bent upward Fig. 5: Structural crack created pathway allowing chemical bridge to initiate corrosion 56 pass through the steel to the cathode. They are then corrosion.10 Two of the most common deterioration –) in a water-contact environment. Over time, the compound calcium hydroxide Ca(OH)2, which comprises between 15 and 22% of a fully hydrated cement, can carbonate to form calcium carbonate (CaCO3). Because Ca(OH)2 is 11.2 pH and CaCO3 is 9.0 pH, this chemical reaction causes the pH of the overall cement binder to drop, starting from the outer surface and progressing inwards. As CO2 or HCO3 – penetrates the cement binder system, the predominant hydroxide (OH–) alkalinity species within the pore solution reacts to form the carbonate ion (CO3). This reaction forms a CaCO3 precipitate (or solid solution) and pure water. This in turn reduces the oxygen content and lowers the pH of the pore solution, causing the loss of the iron oxide passivation layer and facilitating the electrochemical reaction that deteriorates the steel to rust. Chlorides Chlorides present a double threat for corrosion. Chlorides are an excellent electrolyte or conductor. Once a short circuit or chemical bridge is created, chlorides increase the corrosion reaction rate. Chlorides also concentrate at the corrosion pitting sites and form ferric chloride (FeCl2) “rust.” Ferric chloride enters a secondary reaction FeCl2 + 2H2O = Fe(OH)2 + 2HCl, which further lowers the pH of the pore solution at the corrosion pitting sites, and accelerates localized corrosion.11 This double threat makes the prevention or significant slowing of chloride ingress a priority for many environments, to achieve the anticipated service life of the structure. CORROSION PROTECTION To achieve corrosion protection of the embedded metal reinforcement, as well as the anticipated service life of the shotcrete structure, each of the aforementioned aspects for a proper shotcrete installation are critical.* This also includes sound engineering and design practices, specifying the necessary metal reinforcement and its proper installation, as well as the adequate embedment or “coverage” Fig. 6: Insufficient coverage over reinforcement. Reinforcement bowed outward Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
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