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Section 2.1
Overview
Last Revised: 04/07/2018
Reinforced Concrete structural components are composite members made from concrete and steel. Figure 2.1.1 shows steel embedded in concrete. As with all composite elements the mechanics used to determine stress and strength is a bit more complex than required by homogeneous materials. In order to perform the required analysis, it is necessary to understand the materials properties of each of the materials used in the composite and their interaction with each other.
The union of steel and concrete takes advantage of the strengths of each.
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Figure 2.1.1 |
Concrete's general properties include:
- Concrete is a brittle material which is great in compression. The lack of ductility is problematic in most structural applications so means need to be found to improve the ductility of structural elements made from concrete.
- Concrete is virtually useless in tension due to the unreliability of tensile strength and the brittle nature of tensile fracture.
- Concrete is relatively inexpensive to produce. It can also be produced almost anywhere the basic raw materials are available.
- Concrete has the ability to be formed into an almost infinite variety of shapes, making it very attractive for a variety of applications which other materials are not suitable for. The downside of this ability is that creating the forms is time and labor intensive.
- Concrete structures tend to be heavier than most. Excess weight can often be a disadvantage in many applications, however there are times when the weight is needed.
- Concrete can be mixed in such a way as to minimize or eliminate problems related to corrosion, fire, and environmental degradation.
Steel's general properties include:
- Steel is a ductile material with great compressive and tensile properties.
- While steel has a density much higher than concrete, much less of it is needed in any given application, making for generally lighter structures.
- As steel components use much less material, these components must be carefully designed to prevent local and general buckling.
- Steel is relatively expensive to produce and is not generally locally produced.
- Steel components are generally fabricated off site making on site erection very quick.
- Steel is very susceptible to corrosion. Great effort is often required to protect steel from corrosion.
- Steels does not perform well in fires.
The union of concrete and steel to make reinforced concrete creates a "composite material" which utilizes many of the strengths of each material. The addition of steel makes it possible to add ductility to an otherwise brittle concrete structural element. The concrete acts as a coating to protect steel from corrosion, fire, and environmental degradation. The concrete can also provide lateral support for the steel, reducing or eliminating local buckling concerns, thus allowing for significantly less steel than would otherwise be required.
Even with these great possibilities, the advantages would not be realized if the two materials could not work together. The good news is that steel and concrete bond well together and they have similar coefficients of thermal expansion.
Bonding is important because these materials must structurally interact as a composite material. Complete bonding is required to cause strains to be compatible. The compatibility of strain is fundamental concept used in the mechanics necessary to determining the strength of reinforced concrete structural elements.
Having similar coefficients of thermal expansion is important to bonding and composite behavior. Having similar coefficients of thermal expansion allows reinforced concrete components to undergo thermal changes without causing significant stresses to develop in component materials. If such stresses were to develop, the structural effectiveness of the components would be lessened, not to mention increasing the complexity of the mechanics used to determine the strength of structural elements.
In order to understand the composite material, you need an understanding of the two basic material from which it is made. The next two sections cover the basic properties of each and background behind the typical materials which are available.
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