Corrosion of Carbon Steels...

Corrosion of Carbon Steels Carbon (C) steel is the most widely used engineering material. Despite its relatively limited corrosion resistance, C steels are used in large tonnages in marine applications, power plants (both nuclear based power and fossil fuel based power), metal processing equipment, power transmission, transportation, chemical processing, petroleum production and refining, pipelines, mining, and construction etc. The annual cost of metallic corrosion to the total economy of a country is very high. Because C steels represent the largest single class of alloys in use, both in terms of tonnage and total cost, it is easy to understand that the corrosion of C steels is a problem of enormous practical importance. This, of course, is the reason for the existence of entire industries devoted to providing protective systems for iron and steel. Because of corrosion, the design aspects are also very important. Indeed, design changes are often the most efficient manner of dealing with a particular corrosion problem. C steels also often called mild steels have limited alloy content, usually less than 2 % of the total of all the additions. These levels of additions do not generally produce any remarkable changes in general corrosion behaviour. One possible exception to this statement is the weathering steels, in which small additions of copper (Cu), chromium (Cr), nickel (Ni), and/or phosphorus (P) produce significant reductions in corrosion rate in certain environments. At the levels present in the C steels, the usual impurities have no significant effect on corrosion rate in the atmosphere, neutral waters, or soils. Only in the case of acid attack is an effect observed. In this latter case, the presence of P and sulphur (S) markedly increase the rate of attack. Indeed, in acid systems, the pure irons appear to show the...

Steel as a Material for Bridges...

Steel as a Material for Bridges Steel is widely used around the globe for the construction of bridges of sizes ranging from from the very large to the very small. It is a versatile and effective material that provides efficient and sustainable solutions. Steel has long been recognized as the economic option for a range of bridges. It dominates the bridge constructions for long span bridges, railway bridges, footbridges, and medium span highway bridges. It is now increasingly the choice for shorter span highway structures as well. Early bridges were made of stone, wood and concrete. The arrival of the steam train in the mid-18th century ushered in a new era in bridge design. A stronger material was needed as bridges were required to carry heavier loads over longer spans. Iron was first used to bridge the ‘Tees’ river in England in 1741. By the 1880s, steel had become a material of choice. Amongst bridge materials steel has the highest and most favorable strength qualities, and it is therefore suitable for the most daring bridges with the longest spans. Normal building steel has compressive and tensile strengths of 370 N/sq mm, about ten times the compressive strength of a medium concrete and a hundred times its tensile strength. A special merit of steel is its ductility due to which it deforms considerably before it breaks, because it begins to yield above a certain stress level. Steel is an ideal material for bridges. It is an essential part of modern bridges because it is strong, can flex without fracturing and has a long life, even in the harshest conditions. It can be used to build bridges of any length because of its durability and ease of manufacture and maintenance. New grades of steel increase the...

Concrete and Reinforced Concrete...

Concrete and Reinforced Concrete Concrete is a composite building material made from a mixture of sand, gravel, crushed rock, or other aggregates (coarse and fine) held together in a stone like mass with a binder such as cement and water. The stone like mass is formed due to the hydration of cement and eventually due to its hardening. Sometimes one or more admixtures (plasticizers, super plasticizers, accelerators, retarders, pazolonic materials, air entertaining agents, fibers, polymers and silica furies) are added to change certain characteristics of the concrete such as its workability, durability, and time of hardening. Hardened concrete has a high compressive strength and a very low tensile strength. Concrete is one of the most popular materials for buildings because it has high compressive strength, flexibility in its form and it is widely available. The history of concrete usage dates back for over a thousand years. Contemporary cement concrete has been used since the early nineteenth century with the development of Portland cement. Despite the high compressive strength, concrete has limited tensile strength, only around 10 % of its compressive strength and zero strength after cracks develop. In the late nineteenth century, reinforcement materials, such as iron or steel rods, began to be used to increase the tensile strength of concrete. Today concrete is generally strengthened using steel bars known as reinforcement bars (rebars in short) in the tension zone. Such elements are known as ‘reinforced concrete’.  In the reinforced concrete, concrete and steel deform together and hence ribbed reinforcing bars are used for increasing the capacity to resist bond stresses. Reinforced concrete can be moulded to any complex shape using suitable form work. It has high durability, better appearance, fire resistance and is economical. It is a combination of concrete and steel wherein...

Structural Steel – Preferred Material for Construction...

Structural Steel – Preferred Material for Construction Structural steel is not just a material which has only the technical competence. It has many other qualities that make it the preferred material for architects, designers and engineers. It is economical and provides great mechanical functionality; it permits the design of structures which are graceful, light and airy; it streamlines construction site processes; and it offers rapid execution. A major advantage, however, is the infinite freedom for creation which it provides to the architects, designers and engineers. The combination of structural steel with different materials lends themselves to rich and varied types of construction. When combined with glass, structural steel makes fabulous use of light and space. Structural steel is the material ‘par excellence’ when it comes to inventing new structures and forms. All solutions are possible, from the very simplest to the most challenging. Structural steel can be used for small buildings as well as large structures, for routine construction projects, and those subject to complex urban constraints. No other material is used to make structures which are so thin, light and airy. Forms can be created using different structural effects and envelopes with pure or finely sculpted curves. Architects, designers and engineers can give free reign to their imagination and creativity with structural steels. Structural steel is a standard construction material made from specific steel grades and is available in standard cross sectional shapes. This steel exhibits desirable physical properties such as strength, uniformity of properties, light weight and ease of use etc. This makes it one of the most versatile structural materials in use. Major applications for this steel is in high rise and tall multi-storey buildings, industrial buildings, towers, tunnels, bridges, road barriers, and industrial structures etc. Within the overall architectural concept,...

Galvanized Steel Reinforcement Bars...

Galvanized Steel Reinforcement Bars  Galvanized steel reinforcement bars (also called galvanized steel rebars) are the normal reinforcement steel bars which are coated with a protective layer of zinc (Zn) metal. Zn coating is usually carried out by hot dip galvanizing process. The Zn coating serves as a barrier to the corrosive environment which the rebars are exposed to when embedded in concrete. In addition to the barrier protection, Zn also provides cathodic protection where Zn corrodes preferentially when in contact with unprotected steel. This means that in case of any gap in Zn coating the surface of bare steel is protected by the surrounding Zn. The reaction between steel and molten Zn produces a coating on the steel made up of a series of iron -zinc alloy layers (gamma, delta and zeta) which grow from the steel-zinc interface with a layer of essentially pure Zn (eta) at the outer surface. What distinguishes galvanizing from other types of coatings is that the coating is metallurgically bonded to the steel. It actually becomes an integral part of the steel, as compared to the paints and epoxy coatings which are simply attached to the steel surface by physical bonding. The alloy layers in the coating are harder than the base steel resulting in a coating that is not only firmly adhered to the steel but is tough and hard and can resist abrasion and fairly heavy handling. It also allows the galvanized rebar to be handled, transported and fabricated in the same manner as ordinary steel. A typical galvanized coating structure is shown in Fig 1.  Fig 1 Galvanized coating structure  The first regular use of galvanized coating was done in USA during 1930s. Since then, and especially during the last 25 -30 years, it is being...