October_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 7 3 2 tion to steel bridges and structures. Mul- tiple coats and coating combinations provide enhanced protection. Three- coat systems with zinc-rich epoxy prim- er and polyurethane or epoxy-based top coat are the most commonly used method for steel bridges. While three- coat systems are effective, they require lengthy application times due to mul- tiple coats and long curing times. Also, other properties of multicoat systems, such as hardness, adhesion strength, and abrasion resistance are often medi- ocre, thus leading to the development of other coating systems. More ad- vanced systems include polyurea, ep- oxy mastic (EM), waterborne coatings (WBC), calcium sulfonate alkyd (CSA), glass flake-reinforced polyester (GFP), and siloxane (SLX). Table 1 shows the relative advantages and disadvantages of various coating systems. In 2011, the U.S. Federal Highway Administration (FHWA) published re- sults of an analysis of the efficiency of different single-coat systems compared with conventional multicoat systems in various types of environments [4] . The study concluded that single-coat systems, such as CSA and polyurea (with primer), had comparable perfor- mance to that of three-coat systems and could therefore replace them. Ta- ble 1 also shows that the properties of polyurea and CSA could allow replac- ing conventional coating systems. To- day’s methods are protective enough for the current environment, but in- creasing harshness of the environment with greater SLR requires development of new methods and technologies. For example, currently used coatings dry slowly and can take days to cure, which is undesirable in the case of frequent weather changes. Polyurea-based composite coat- ings potentially can replace existing coating systems, with their main ad- vantage being extremely fast cure times coupled with excellent corrosion and abrasion resistance [2,4] . Curing begins in less than a minute and can be com- plete within minutes depending on the reactivity of precursor materials. Due to its quick dry time and high adhesion strength (600 to 1000 psi) [4] , polyurea coating can be applied on any surface without significant surface cleaning and treatment. Existing commercial applications of polyurea include wa- terproofing, roofs, flooring, and vehi- cle bed liners, in addition to unexplored potential applications including: • Field repair coatings on existing steel and marine structures due to extremely fast cure cycle • Abrasion-resistant coatings on concrete pillars of bridges (in splash zone) and buildings to tackle the increased abrasion due to frequent storm surges and flooding events Development of polyurea-based multicoat composite coatings is pos- sible by combining the benefits of inhibitive pigments and sacrificial me- tallic powders in one coat and excel- lent mechanical properties of polyurea in another coat. While polyuria-based coatings have a slight economic disad- vantage compared with convention- al three-coat systems, the prospective benefits of innovative polyuria-based coatings can potentially overcome the cost. Corrosion protection of reinforced concrete structures . Greater corrosion and degradation of reinforced concrete infrastructure resulting from increas- ing sea level will require more pro- tection. The highly alkaline nature of concrete provides an intermediate level of corrosion protection to the steel re- bar. Concrete also serves as a physical barrier to the corrosive media and de- lays rebar corrosion. However, certain mechanisms undermine the protective cover provided by the concrete around the rebar. The alkalinity of hardened concrete is reduced by losing alkaline ions through leaching to water and by reacting with other ions present in the water or moist environment. Increas- ing CO 2 content in the atmosphere also reduces concrete alkalinity. When CO 2 dissolves in water through rain and oce- anic absorption, it forms carbonic acid, which can reduce concrete alkalinity. Also, corrosion of reinforcing steel oc- curs due to seepage of chloride ions often present in high amounts in sea water and the environment of coast- al areas. After rebar corrosion begins, corrosion products (rust) expand due to the volumetric addition of ions and occupy more space compared with the original steel. The expansion results in increased internal stresses and devel- ops cracks in the concrete structure, which subsequently can result in fail- ure of a structure or increased mobili- ty of deleterious corrosive media (e.g., moisture, oxygen, and chlorides) to the embedded rebar. Repair of cracked or chloride-contaminated concrete struc- tures is not suitable, as the structure tends to crack again. Traditionally, two methods used to protect reinforced concrete struc- tures from corrosion include galvanic protection and impressed current ca- thodic. The sacrificial-anode galvanic protection method works on the princi- ple of flow of electric current between two different metals due to the poten- tial difference between them. In a rein- forced concrete structure, one anode is the embedded steel rebar, and an- other anodic metal is externally used. Zinc, aluminum, and magnesium are the most commonly used metals in this Fig. 3 — (a) In-field application of zinc using thermal spray coating system, and (b) titanium mesh inlay to be used in concrete pillar [6] .