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reinforced concrete design

Reinforced concrete

Reinforced concrete (RC) is a versatile composite and one of the most widely used materials in modern construction. Concrete is a relatively brittle material that is strong under compression but less so in tension. Plain, unreinforced concrete is unsuitable for many structures as it is relatively poor at withstanding stresses induced by vibrations, wind loading, and so on.

To increase its overall strength, steel rods, wires, mesh or cables can be embedded in concrete before it sets. This reinforcement, often known as rebar, resists tensile forces. By forming a strong bond together, the two materials can resist a variety of applied forces, effectively acting as a single structural element.

Whilst concrete has been used as a construction material since Roman times, the use of reinforcement, in the form of iron, was only introduced in the 1850s by French industrialist François Coignet, and it was not until the 1880s that German civil engineer G. A. Wayss used steel as reinforcement.

Reinforced concrete can be precast or cast-in-place (in situ) concrete, and is used in a wide range of applications such as; slab, wall, beam, column, foundation, and frame construction. Reinforcement is generally placed in areas of the concrete that are likely to be subject to tension, such as the lower portion of beams. It is usual for there to be a minimum of 50 mm cover, both above and below the steel reinforcement, to resist spalling and corrosion which can lead to structural instability.

Many types of non-steel reinforcement can be used, predominately as a means of controlling cracking. Fiber-reinforced concrete is a concrete mix that contains short discrete fibers that are distributed uniformly throughout the material. Fibers can be made of glass, polypropylene, synthetic and natural materials, as well as steel.

Prestressed concrete allows for predetermined, engineering stresses to be placed in concrete members to counteract the stresses that occur when they are subject to loading. In ordinary reinforced concrete, stresses are carried by the steel reinforcement, whereas prestressed concrete supports the load by induced stresses throughout the entire structural element.

This makes it more resistant to shock and vibration than ordinary concrete, and able to form long, thin structures with much smaller sectional areas to support equivalent loads. Pre-stressing may be achieved by pre-tensioning or post-tensioning.
Reinforced concrete is extremely durable and requires little maintenance. It has good thermal mass and is inherently fire resistant. Rebar is generally made from 100% recycled scrap, and at the demolition stage, the concrete and rebar are capable of being separated so that the steel can be recycled.

However, concrete has a relatively high embodied energy, resulting from its extraction, manufacture, and transportation. Waste materials can be included within the concrete mix such as RCA (Recycled Crushed Aggregate), GGBS (Ground Granulated Blast-Furnace Slag) and PFA (Pulverised Fuel Ash), however, issues such as moisture content and material variability may make its recycling unviable.

Advantages and disadvantages of reinforced concrete

Reinforced Concrete is a structural material, is widely used in many types of structures. It is competitive with steel if economically designed and executed.

Advantages of reinforced concrete:

  • It has a relatively high compressive strength
  • It has better resistance to fire than steel
  • It has a long service life with low maintenance cost
  • In some types of structures, such as dams, piers, and footings, it is the most economical structural material
  • It can be cast to take the shape required, making it widely used in pre-cast structural components
  • It yields rigid members with minimum apparent deflection
  • Yield strength of steel is about 15 times the compressive strength of structural concrete and well over 100 times its tensile strength
  • By using steel, cross-sectional dimensions of structural members can be reduced e.g in lower floor columns

Disadvantages of reinforced concrete:

  • It needs mixing, casting and curing, all of which affect the final strength of concrete
  • The cost of the forms used to cast concrete is relatively high
  • It has low compressive strength as compared to steel (the ratio is about 1:10 depending on material) which leads to large sections in columns/beams of multistory buildings Cracks develop in concrete due to shrinkage and the application of live loads

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