Modulus of Elasticity
This is often called ‘Young’s Modulus’ and is denoted by the notation Es. It is a measure of the constant relationship between stress and strain up to the elastic limit. For all reinforcement steels Es has a value of 200,000 MPa. The Modulus of Elasticity is the slope of the stress-strain graph prior to yielding of the steel.
Stress and Strain in reinforcing steel
Stress is a term that allows comparison between the strength of different sizes of the same material. Stress measures the force applied to a unit of area and is stated in megapascals (MPa).
Yield Stress of Steel
This is the property which determines the maximum usable strength of a
reinforced concrete member. The yield stress of steel is determined by stretching a sample (approximately 600 mm long) in a tensile-testing machine.
When a steel is tensioned, the amount by which the length increases (called
‘strain’) is directly proportional to the load (or ‘stress’) applied to the bar in the
elastic range. The ‘yield point’ of the steel is reached when strain is no longer
directly proportional to the stress applied to the bar. Beyond the yield point the
bar behaves plastically and is permanently deformed. With hot rolled bars, the ‘yield point’ is quite visible on the stress–strain curve.
Once the yield point is reached, the strain increases rapidly for a minor increase in the applied load. The stress level at yield is called the yield stress and the steel is said to have ‘yielded’. After yield, the strength of the bar increases due to strain hardening until the tensile strength is reached. After maximum tensile strength has been reached, the capacity of the bar reduces and necking is visible. Eventually the bar breaks.
Stress and Strain in Reinforced Concrete
Up to the point where the concrete starts to crack, the strains in the steel and concrete are equal but the stresses are not. The steel carries a much higher proportion of the applied load at a much higher stress – because it has a higher modulus of elasticity.
Ductility
Ductility is the ability of a structure to undergo large deformations and deflections when overloaded. If a structure cannot withstand large deformations and deflections when overloaded, then it is subject to brittle failure.
The three ductility grades are Low (L), Normal (N) and Earthquake/Seismic (E). The Tensile Strength / Yield Stress Ratio is a measure of the reinforcement’s ability to work harden when undergoing plastic deformation. This means the strength of the steel increases when it is loaded beyond its yield strength.
Uniform Elongation (Agt)
Uniform elongation is a strain measure. It is a measure of the maximum
amount by which a steel sample will stretch before it reaches maximum stress.
For strains up to 1% elongation, an extensometer is used.
Chemical Composition
The selection of the correct chemistry for any steel product is extremely important
because it can have a marked effect on the use of the final product. The most important elements in the composition of reinforcing steel are Iron (Fe) Carbon (C) and Manganese (Mn)
Carbon - turns iron into steel. The Carbon content of steel is limited because as the carbon content increases, the ductility of the steel decreases.
Manganese increases the strength of steel up to a certain point
Nitrogen, Phosphorous, Silicon and Sulphur can be deleterious
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