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2017SummerShotcreteEMag

QUALITY OF SHOTCRETE As shown previously, it is somewhat straightforward to design and fabricate a good shotcrete pumping system and equipment to produce durable shotcrete. It is also somewhat straightforward to design and proportion shotcrete materials to be durable. However, qualifying the nozzleman’s consistency of workmanship throughout a shoot remains elusive. Currently, the combination of compression testing and the ASTM C1202 RCP Test Method (or similar testing to establish shotcrete’s apparent interconnectivity) are used as a rapid method of qualifying a shotcrete, and by default the nozzleman’s workmanship. Knowing the overall density and interconnectivity of shotcrete reveals the rate of moisture/ water ingress that facilitates most deterioration mechanisms. Ultimately, each of the aforementioned aspects of shotcreting determine shotcrete durability. METAL REINFORCEMENT PASSIVATION When steel is manufactured from natural iron sources, the iron is in a somewhat low energy state. An addition of energy is transferred to the iron during the melting, refining, and shaping processes during the fabrication of steel. The bound energy of pure iron (or corrosion potential) within reinforcement is higher, for example, than the iron compounds within the cement binder. This energy allows steel to be molded or stamped into various shapes and to be malleable and ductile, yet retain its shape. However, it is thermodynamically unstable for most placement conditions. Furthermore, this induced energy is not consistent across the surface of the steel. This increased energy, and the uneven energy fluctuations, would be quickly shortcircuited without some type of passivation layer added to the surface of the steel during fabrication. Without a passivation coating, the resulting instability would create an energy release, allowing an electrochemical corrosion reaction to proceed. As a result, all steel reinforcement for use in construction in the United States has a passivation coating (usually an electroplated coat) applied during the fabrication of the steel. Passivation: A condition whereby metal is in a nonreactive or “dormant” state. For metal reinforcement embedded in shotcrete, the highly alkaline environment of the cementitious binder quickly forms an iron oxide layer through a reaction with the higher energy iron at the surface of the steel. This passivation layer protects the remainder of the higher-energy iron within the steel from entering reaction (refer to Fig. 1). In other words, within a few days of being embedded in the hardened cement paste, some of the outer-surface iron reacts to form a stable layer of iron oxide, creating an additional passivation layer necessary to prevent further reaction or corrosion. This new iron oxide passivation layer is stable against the highly alkaline environment of the cement on one side, and against the higher-energy iron within the steel on the other side; therefore, it serves as an “insulator” against further corrosion. A demonstration of the reaction creating an iron oxide passivation layer can be seen in Fig. 2 and 3. It is a very thin layer, yet a highly protective formation on the surface of the metal. METAL REINFORCEMENT CORROSION If the electro-plated passivation coating applied during fabrication of the steel, as well as the iron oxide passivation layer which formed after embedment in cement are compromised, then a short circuit can be created between two exposed areas of differing energy levels. If an electrochemical cell (or chemical “bridge”) can be established, a corrosion reaction will proceed. Fig. 1: Passivation (iron oxide) layer Fig. 2: Metal plate having one end that was ground down (shiny section) to expose virgin steel Fig. 3: Same metal plate after being halfway submersed in a high-alkaline (high-pH) water for 24 hours showing the formation of an iron oxide passivation layer (grayish-black) www.shotcrete.org Summer 2017 | Shotcrete 55


2017SummerShotcreteEMag
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