Material Property Terms
Material Property Terms
Deformations | Modulus of Elasticity | Stress and Strain in Reinforcing Steel |
Yield Stress of Steel | Yield Strain of Bars | Yield Stress of Cold Rolled Bar |
Tensile Strength of Steel | Stress and Strain in Reinforced Concrete | Ductility |
Uniform Elongation | Strain Ageing | Chemical Composition | Carbon |
Other Elements | Carbon Equivalence
Deformations
Deformations appear as a raised pattern on the surface of the bar. The overall cross-section should be as circular as possible to facilitate uniform straightening and bending. Deforming the surface is the final rolling operation.
The surface pattern consists of transverse deformations and longitudinal ribs. Only the deformation contributes to the anchorage of a bar. The deformation pattern allows considerable scope for steel makers to use additional ribs forproduct and mill identification.
When considering the cross-sectional area or the mass per metre of a bar or wire, the deformation is regarded as a redistribution of the material and not as an appendage.
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).
Example 1
• If a force of 60 kilonewtons (kN) is applied to an N16 bar of area 200
mm2, the stress in that bar is:
= 60,000/200
= 300 newtons per mm2, 300 megapascals or 300 MPa
• If the same force is applied to an N32 bar of area 800 mm2, the stress is
= 60,000/800
= 75 MPa, a lower value because of the larger area
• Conversely, for the same stress of 300 MPa, the N32 bar would be carrying a load
= 300 x 800 newtons
= 240 kN
Strain is a measure of the amount by which a tensile force will stretch the bar. Strain is expressed in the units of ‘mm/mm’, or ‘percentage strain’ based on the original gauge length.
Example 2
Using our N16 example from above, the relationship between stress and strain is:
• Strain
= the stress divided by Young’s Modulus
= 300/200,000
= 0.0015 mm/mm
= 0.15% of the gauge length
Using the N16 example again with a gauge length of 5 bar diameters, we have:
• Extension under load
= 0.0015 x 5 x16 mm
= 0.12 mm at a stress of 300 MPa
Example 3
For the same N32 bar at a stress of 75 MPa:
• Strain
= 75/200,000
= 0.0004 mm/mm
A lower stress in the bar means smaller strain and thus narrower crack widths in the reinforced concrete element, if they occur.
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 bar 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.
In fact, if the bar is unloaded part way through the test, below the yield point, the bar will return to its original length. This is why it is called elastic behaviour. The yield stress measured with a second test will be at least as high as it was during the first test.
The characteristic yield stress specified in Australian Standards determines the Grade of the steel. Grade D500N bars must have a characteristic yield stress not less than 500 MPa; Grade D250N and R250N bars must have a characteristic yield stress not less than 250 MPa.
To illustrate the connection between stress, strain and yield stress, there is also a ‘yield strain’ calculated as follows:
Yield Strain of Bars
= yield stress/modulus of elasticity
= 500/200,000
= 0.0025 mm/mm
= 0.250% of the gauge length
There are three properties that relate to each other in the elastic range:
Stress = Strain x Young’s Modulus
1. Young’s Modulus
= 200,000 MPa for all steels
2. Yield stress for Grade D500N
= 500 MPa, and the calculated yield strain is
= 0.0025 mm/mm
= 0.250% of the gauge length
3. Yield stress for Grade R250N and D250N
= 250 MPa, and the calculated yield strain is
= 0.00125 mm/mm
= 0.125% of the gauge length
Yield Stress of Cold Rolled Bar
Cold rolled bar does not exhibit a true yield point; there is no point during a stress-strain test where true yielding is visible.
AS/NZS 4671 allows the 0.2% proof stress to be used as the yield stress when there is no observable yield point.
Tensile Strength of Steel
This is the maximum stress which the steel can carry. In the past, this strength was called the ‘ultimate tensile strength’. It is not used directly in reinforced concrete design, however the ratio of tensile strength to yield stress is important to ensure a ductile failure mechanism.
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.
AS3600-2009 is based on steel strengths of up to 500 MPa. This determines all the ‘deemed to comply’ rules such as cog lengths, transverse-wire overlaps for fabric, and the requirements for minimum areas of reinforcement. Plastic and drying shrinkage are two other causes of stress in concrete.
For Australian concretes, the shrinkage strain ranges from 0.0005 mm/mm to 0.0012 mm/mm. This range is close to the yield strain of Grade 250 bars (0.00125 mm/mm). AS3600-2009 contains rules for control of cracks caused by shrinkage and flexure for bars and fabric up to Grade 500 (yield strain 0.0025 mm/mm).
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.
AS/NZS 4671 has introduced three ductility grades for reinforcing steel and two ductility measures. AS3600-2009 has also retained the ductility control of a reduced strength reduction factor for bending members with a ku > 0.4, that is for bending members with excessive tensile steel.
The three ductility grades are Low (L), Normal (N) and Earthquake/Seismic (E). The measures for ductility are Uniform Elongation and the Tensile Strength / Yield Stress Ratio. E Grade material is specifically for use in New Zealand and is not available in Australia.
The Uniform Elongation provides a measure of the ability of the reinforcement to deform, both elastically and plastically, before reaching its maximum strength.
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. Elongations greater than 1% are measured from the crossheads of the tension testing machine. Uniform elongation can be measured manually by marking a bar at 1 mm intervals prior to tensioning. The bar is then loaded in tension until failure. An elongation measurement (L) is obtained by measuring the length of the bar at a distance of 50 mm from the break for a length between 100 marks (that is, 100 mm length prior to tensioning). The uniform elongation for manual testing is obtained from the formula:
Agt = ((L-100)/100 + (Tensile Strength)/200,000) x 100%
The first part of the equation measures the plastic deformation of the bar away from the zone affected by necking. The second term measures the elastic deformation. At failure the bar shortens as elastic strain is relaxed, hence the elastic deformation must be added back onto the permanent plastic deformation to obtain the total elongation at maximum stress. Uniform elongation is a measure of the ductility of the steel.
A steel with high uniform elongation (greater than 5%) is considered ductile; low ductility (under 5%) is considered to be a sign of brittleness. Uniform elongation is not required directly for design purposes, however, its value is important when specifying and checking the properties of a steel. Design methods requiring high rotation, such as moment redistribution and plastic hinge design, should not use low ductility steels.
AS/NZS 4671 gives minimum values for the Uniform Elongation (Agt) for the different reinforcing steels.
| 250N | 500L | 500N | |
|---|---|---|---|
| Agt | 5.0% | 1.5% | 5.0% |
Strain Ageing
When normal mill steels such as plate, wire, and plain or deformed bars are bent or otherwise reshaped, the steel becomes less ductile with time. There are many reasons, but the main cause seems to be change in crystal structure and the effects of the chemical composition.
Strain ageing causes problems when:
- The steel is bent around a small pin
- Bent material is galvanised
- A weld is located close to (within 3 db) or at the bend.
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). AS/NZS 4671 allows both a cast analysis and a product analysis.
Carbon
Carbon turns iron into steel. The Carbon content of steel is limited because as the carbon content increases, the ductility of the steel decreases.
Other Elements
- Manganese increases the strength of steel up to a certain point
- Nitrogen, Phosphorous, Silicon and Sulphur can be deleterious
- Micro-alloying and grain-refining elements, such as Aluminium, Niobium, Titanium and Vanadium, can be used to increase the strength but they can affect other properties, sometimes not to the best advantage of the steel
- Residual elements such as Copper, Nickel, Chromium and Molybdenum can occur in steel. if they are present in any scrap used in steel making. Up to a certain limit they may be considered as incidental and not detrimental to the product.
Carbon Equivalence (CE)
This term is regarded as a measure of the weldability of a steel. It is derived from a formula that allows for the influence of Carbon, Manganese, Chromium, Molybdenum, Vanadium, Nickel and Copper. The Australian formula for CE is:
C+Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu) / 15
When the CE exceeds 0.45, the steel cannot be welded.
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