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The Use of Steel Deformed Bars in Reinforced Concrete in Pakistan

In reinforced concrete designed it is generally assumed that the strain in the steel deformed bars reinforcement is equal to the strain in the concrete immediately surrounding it. This is true in zones of compression but, in the parts subject to stress, the working stress in the steel deformed bars is usually highly enough for the strain to be more than the surrounding concrete can resist without cracking. The size of the cracks shaped in this way and their spacing along the reinforcement depends primarily on the bond but is also affected by the damage in the steel, the tensile strength of the concrete and its modulus of elasticity. 

Types of Steel Deformed Bars Used In Concrete Structures

1.      Hot Rolled Steel Deformed Bars: This is the most common type of reinforcement for regular RCC constructions. Hot rolling is done in the steel mills which involves giving it deformations on the outside i.e. ribs so that it can form bond with material. The stress - strain curve shows a separate yield point followed by a plastic stage in which strain increases without increase in stress. This is followed by a strain hardening stage. Steel Deformed Bar Grade 40 means minimum yield strength of 40,000 pounds per square inch and conforms to ASTM A-615 performance standards.

2.      Mild Steel bars in Pakistan: These are plain steel bars and have no ribs on them. These are used in small projects where economy is the real concern. As plain bars cannot bind very well with concrete hence hooks have to be provided at the ends. In this type of steel too stress - strain curve shows a distinct yield point followed by a plastic stage in which strain increases without increase in stress. This is followed by a strain hardening stage. Plastic stage in Mild Steel Bars in Pakistan is even more definite than Hot Rolled Steel Deformed Bars.

3.      Cold Twisted Bars Steel Reinforcement: When hot rolled steel bar undertakes process of Cold twisted Bars working, Cold twisted Bars worked reinforcement is produced. Cold working occupies twisting or drawing the steel bars at room temperature. This efficiently eliminates the Plastic Stage in the Stress-Strain curve, although it gives more control over the size and acceptances of steel bars. Due to removal of plastic stage it has lower ductility than Hot Rolled steel deformed bars. Its use is specific to projects where low acceptances and straightness are a main concern. The stress – strain curve does not show a distinct yield point as plastic stage is completely eliminated. Yield point is resolute by drawing a line parallel to the Tangent Modulus. Yield stress is the point where this line interconnects the stress – strain curve. This is known as 0.2% proof stress. If yield stress is resolute at 0.1% strain it is called 0.1% proof stress.

4.      Prestressing Steel: Prestressing steel is used in the form of bars or tendons which are made up of multiple strands, however, tendons / strands are more frequently used as these can be laid in various profiles, which is a primary requisite of prestressing steel. Prestressing strands are, in turn, made up of multiple wires (typical 2, 3 or 7 wire strands). Typical seven wire strand consists of six wires spun around the seventh wire which has a slightly larger diameter, thus forming a helical strand. These wires are cold twisted bars drawn and have very high tensile ultimate strength. Their high tensile strength makes it possible to efficiently prestress concrete even after undergoing short term and long term losses. These are used in prestressed concrete in bridges or prestressed slabs in constructions. Prestressing steel is also available as non-bonded strands encased in PVC sheath. It is used in Post-Tensioning of members. Prestressing strands are also available as Low Relaxation Strands which show low relaxation losses after prestressing. These are typically used in prestressing members with huge spans. Due to the method of cold drawing, which is similar in effect to cold working; plastic stage in this type of steel is eliminated.

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Model of Power Earthquake Resistant Steel Bars G 72

Model of Power Earthquake Resistant Steel Bars Playing A Key Role In Earthquake Resistant Construction Strategies

The key differentiating factor according to industry experts was the use of high quality earthquake resistant steel bars in Pakistan. Earthquake resistant steel bars used in Pakistan need to adhere to certain minimum requirements defined in 1786-2008. With Pakistan falling under Seismic Zones III, IV and V, there has been increasing focus on earthquake resistant construction in the country. The intense competition among earthquake resistant steel bars suppliers can be said to have contributed positively in increasing awareness about the various advantages, including earthquake resistant uniqueness that earthquake resistant steel bars provide in the Pakistan market. Engineers are often quoted stating that earthquakes don't kill but it is badly built buildings that do.

Seismic Performance Factor

Good quality earthquake resistant steel bars are known for their superior weldability, high strength, workability, elongation and ductility. The last two are of crucial importance when it comes to offering earthquake resistant properties. The high energy that is generated on a structure during earthquakes needs a yielding material to absorb this force without breaking, which is the case with brittle material.

What provides earthquake resistant TMT steel bars with this ductility and elongation characteristics is their structure. Earthquake resistant steel bars feature a combined metallurgical structure, with a strong external martensite surface and a soft ferrite-pearlite core. This feature remains the same whichever way they are manufactured, either through the 'Thermax' or through the 'Tempcore' processes. Earthquake resistance is incorporated in any structure by the action of the core allowing mild tilting and the external surface bringing it back to its original position during seismic movement. Along with this ductility, the elongation factor is also critical since it accommodates the inelastic strains better during a cyclic loading situation like earthquakes and repeated aftershocks.

Rib formation is another factor that needs to be taken into consideration whenever dealing with earthquake resistant steel bars. This is because the rib formation is what dictates reinforcement to cement. Concrete while having high compressive strength displays low tensile strength and it therefore becomes necessary to reinforce it with steel in order to achieve the necessary tensile strength. Specially designed notching machines are used by manufacturers for rib formation on a steel bar. The machines are computer controlled and ensure factors such as even height, rib distance and length of ribs.

G 72 Steel Deformed Bars and the Ductility Factor

When it comes to specifications G 72 evolved as the standard for TMT steel bars when they were initially introduced. These days they have been replaced by G 72. Lower levels of sulphur and phosphorus are another feature of re-bars made using steel with the G 72. BIS specifications stipulate that the sulphur (S) content in percentage be less than 0.040 and phosphorous (P) 0.040, with the percentage of sulphur and phosphorous put together not exceeding the figure of 0.075 in the case of Steel deformed bars G 72.

Corrosion Fighting Properties

Corrosion resistance is another key factor that is increasingly influencing purchase decision of earthquake resistant steel bars. Specially manufactured steel bars denoting High Corrosion Resistant Marine (HCRM) can help withstand corrosion. The corrosion fighting properties in these steel bars is done through the incorporation of Nickel, Copper and Chromium. While these steel bars cost higher than regular bars, there is increasing awareness these days that with their long lasting nature, the lifecycle cost of the product works out lesser than ordinary steel bars. HCRM steel bars are being widely used, especially near coastal areas where the presence of corrosion causing elements is higher, for the construction of bridges and high rises.

Important Component

Stirrups, which are closed loops of reinforcement steel bars, are also of crucial importance to protect structures from seismic activity. There has been growing awareness about stirrup bending in the Pakistan market in recent times. These days most of the leading manufacturers and suppliers of earthquake resistant steel bars offer stirrup shaping services as part of their value added services. Stirrups in RC beams prevent the buckling of compressed longitudinal bars due to flexure. They also help by resisting vertical shear cracks, along with preventing the bulging of concrete outwards due to flexure.

Both the ends of the vertical stirrups are to be bent into a 1350 hook. The ends need to be sufficiently beyond the hook for this will ensure that the stirrup will not open out in the case of a seismic event. The hooks play a crucial role during intense seismic activity, when concrete spalling occurs, thus preventing buckling of concrete, as well as vertical reinforcement steel bars.

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Common Definitions Connected With Reinforcing Steel Deformed Bars

As reinforcing steel deformed bars are rods of steels provided with lugs, ribs or deformation on the surface of bar, these bars minimize slippage in concrete and increases the bond between the two materials. Steel deformed bars have more tensile stresses than that of mild steel plain bars. These bars can be used without end hooks. The deformation should be spaced along the bar at substantially uniform distances.

Reinforcing steel deformed bar – It is a steel product with a circular or practically circular cross section which is suitable for the reinforcement of concrete.

Ribbed steel reinforcing bar – It is a steel deformed bar with at least two rows of transverse ribs, which are uniformly distributed over the entire length.

Nominal cross sectional area – It is the cross sectional area equivalent to the area of a circular plain bar of the same nominal diameter, d (i.e. ?d²/4).

Longitudinal rib – It is a uniform and continuous protrusion parallel to the axis of the reinforcing deformed steel bar.

Transverse rib – It is a rib on the surface of the reinforcement steel deformed bar other than a longitudinal rib.

Transverse rib flank inclination angle – It is the angle of the rib flank measured perpendicular to the longitudinal axis of the rib.

Transverse rib inclination angle – It is the angle between the axis of the transverse rib and the longitudinal axis of the steel reinforcement deformed bar.

Rib height - It is the distance from the highest point of the rib (transverse or longitudinal) to the surface of the core, to be measured normal to the axis of the reinforcement steel deformed bar.

Rib spacing – It is the distance between the centers of two consecutive transverse ribs measured parallel to the axis of the reinforcement deformed  steel bar.

Relative rib area – It is the area of the projection of all ribs on a plane perpendicular to the longitudinal axis of the reinforcement steel deformed bar divided by the rib spacing and the nominal circumference.

Standard property – It is the property which is determined as part of the routine inspection and test requirements.

Special property – It is the property which is not determined as part of the routine inspection and test requirements. (e.g. fatigue properties).