Page 68

2015WinterShotcreteEMag

Technical Tip Use of Cathodic Protection in a Shotcrete System By Jesse Osborne Uor transported with a concrete pump or and the flow of ions through an electrolyte.Corrosion is an electrochemical processinvolving the flow of electrons through metal-sed successfully for over 100 years, shotcrete, or gunite, is concrete that is conveyed pneumatic gun and sprayed into place. While the There are four necessary components to a cor- method has remained relatively constant, the rosion cell: an anode, a cathode, a metallic path, equipment, materials, and applications are con- and an electrolyte. In a corrosion cell, the most tinuously improved and expanded. Today, con- active metal will act as the anode with the most crete can be applied through the use of robotics noble metal acting as the cathode. Corrosion in areas formerly considered inaccessible or typically occurs at the anode. Because of incon- unsafe, shotcrete can be batched as self-consoli- sistencies in the purity of structural steel, it is dating concrete (SCC), and materials containing likely to have microscopic corrosion cells within macrofibers can be placed thinner while still a single piece of steel. The corrosion cell will maintaining structural integrity. Along with the look like this: continuous improvement, there are also some Anode = parts of the structural steel occurrences that have plagued shotcrete for years. Cathode = parts of the structural steel One of the chief issues is corrosion of steel Metallic path = the steel itself reinforcement. The use of synthetic fibers has Electrolyte = shotcrete reduced the dependency on steel; however, steel Cathodic protection works by introducing a cannot be easily replaced in all applications. more active metal (zinc) to the electrolyte (in this When steel reinforcement is used, it is a good case, shotcrete) and connecting it metallically to idea to protect it with the use of one of the avail- the structural steel using the tie wires that are able corrosion control methods currently avail- attached to the anode. Because zinc is a more able. There are various methods of protection active metal than steel, all of the steel becomes available to the concrete industry. Calcium nitrite the cathode. So the alternative (more active) cor- has been available for many years and has a rosion cell will now look like this: proven track record of success. The mechanism Anode = sacrificial element for this method of protection essentially slows Cathode = structural steel the ingress of chloride and “binds” it up through Metallic path = galvanized steel tie wires a process referred to as ion exchange before it Electrolyte = shotcrete reaches steel reinforcement. Cathodic protection Cathodic protection dates back almost 200 years is also used extensively in the concrete repair and can be traced to British naval vessels, where segments, as well as in new construction. it was used to slow the corrosion of the steel hulls Cathodic protection involves the use of zinc on ships. Its use in concrete repair and protections anodes. This method of corrosion protection has has been common for the last 30 to 40 years. been available for some time, but its use in shot- These anodes are manually attached to steel crete is comparatively new. reinforcement before the concrete or shotcrete is Under normal conditions, steel in concrete is placed. While these anodes eventually require essentially protected. There is a passive layer replacement, they will extend the expected repair that protects the steel in concrete due to the high interval by two to three times, making them a pH of portland cement. However, when chlorides worthy investment. or CO2 eventually reach the steel, the passive The concept of cathodic protection in concrete protective layer breaks down and corrosion is and concrete repair is relatively straightforward initiated by the creation of several microscopic and widely accepted. The mechanism by which corrosion cells. it works (explained previously) is fairly simple. 66 Shotcrete • Winter 2015


2015WinterShotcreteEMag
To see the actual publication please follow the link above