INTRODUCTION
• It is widely appreciated that steel reinforcement plays a critical role in influencing the structural behaviour of RCC members .
• Strong in tension and at the same time, ductile enough to be shaped or bent.
• Thermal expansion of both steel (0.0000117/0C) and concrete are approximately same.
• Bonds well with concrete (When concrete sets and hardens, its shrinks causing the steel embodied in it to be held firmly and thus preventing any relative movement between the two material)
• Most engineers tend to take the properties of reinforcement for granted and more often than not remain ignorant about the influence of the metallurgical and production characteristics on the mechanical properties of steel.
• Given the liberalised import regime and the large-scale conversion of imported scrap in the form of scrap rails, automobile scrap, defence scrap, scrap from ship breaking, etc of unknown or unsuitable quality, into steel rebars, possibly under less-than controlled conditions, raises apprehensions about the quality and reliability of steel rebars used in the construction industry.
• To be able to take informed decisions about the quality of steel rebars in a holistic manner, it is imperative that engineers are conversant with the steel characteristics that have a bearing on the mechanical properties of the rebars
Introduction to Iron and Steel
• Iron: Iron is an element and can be pure.
• Cast iron: Iron that contains about as much carbon as it can hold which is
• about 4%.
• Wrought iron: Iron that contains glassy inclusions.
• Steel: Iron with a bit of carbon in it, generally less than 1%.
• Pig iron: Raw iron, the immediate product of smelting iron ore with coke and limestone in a blast furnace. Pig iron has a very high carbon content, typically
• 4-5%, which makes it very brittle and not very useful directly as a material.
STEEL PRODUCTION Materials used to produce pig iron
• Coal—as coke—used to supply carbon. In a blast furnace, ore is heated in the presence of Carbon—this allows oxygen in the ore to react with carbon to form gases.
• Limestone—Helps to remove impurities
• Iron ore—Processed ore at the start of the process has about 65% iron.
Impurities (slag) float on the top of melt.
1.0 ton of steel requires about 3.2 tons of raw materials to produce
• Rebars are rolled from billets
• Manufacture of billets are carried by Open hearth, electric, duplex or a combination of these process
• Billets are produced either from iron ore through the blast furnace converter route, or by melting scraps and refining the same in the furnace (re-rollables).
• Ribs are indented on the surface of the deformed bars during process of rolling. (HYSD bars have ribs on the surface and this increases the bond strength at least by 40% )
• Plain Mild steel of grade Fe-250 were widely used till 1967.
• Square twisted bars (deformed bars) were first introduced in India in 1965.
• Cold Twisted deformed Bar (CTD) – produced by cold working process, basically a mechanical process.
• It involves stretching twisting of mild steel, beyond the yield plateau and subsequently releasing the load.
• Yield strength in the range of 405 MPa
• High Strength Deformed Bar
Reinforcement Steel: TYPES
deformed bar
plain round bar
Influence of different chemical ingredients in steel on properties of rebars.
CARBON Hardness, strength, weldability and brittleness
High carbon content contributes to the tensile strength of steel. Lower carbon content less than 0.1% will reduce strength. High carbon content of 0.3% and above makes the steel bar unweldable and brittle.
SULPHUR Present as an impurity which increases its brittleness.
Presence of sulphur should be limited as per IS 1786.Higher sulphur makes bars brittle.
PHOSPOROUS Present as an impurity which increases strength & brittleness.
High phosphorous content contributes to the increase in strength and corrosion resistance properties but brings brittleness.
CHEMICAL PROPERTIES CHEMICALS CONTROLLING PROPERTY
ACTUAL EFFECT
Reinforcement Steel: STEEL RECEIPT AT SITE
• Test certificate will confirm the compliance of quality of supplied materials as per the requirement of relevant Indian standard code.
• Steel bundle should have tag giving detail of the Batch/lot detail
Reinforcement Steel: FIELD TEST
Rolling Margin – Weight per meter For any steel reinforcement bar, weight per running meter is equal to d2/162 Kg, where d is diameter of the bar in mm.
For example, 10 mm diameter bar will weigh 10x10/162 = 0.617 Kg/m. Tolerances on nominal mass
TERMS RELATING TO STEEL DETAILING Bend Test • Specimen Shall be doubled over the mandrel by continuous pressure until sides are parallel. • If there is no rupture or crack visible, the specimen shall be considered to have passed the test.
Rebend Test
• The test piece shall be bent to an included angle of 1350 using mandrel of appropriate dia. • The piece shall be aged by keeping in boiling water for 30mins & allowed to cool. • The piece shall then be rebend to have an included angle of 157 ½ 0 • If there is no rupture or crack visible, the specimen shall be considered to have passed the test
Transverse reinforcement:
• Transverse reinforcements are very important. They not only take care of structural requirements but also help main reinforcements to remain in desired position. They play a very significant role while abrupt changes or reversal of stresses like earthquake etc.
They should be closely spaced as per the drawing and properly tied to the main/longitudinal reinforcement.
Lap length or development length :
• Lap length is the length overlap of bars tied to extend the reinforcement length.. Lap length about 50 times the diameter of the bar is considered safe. Laps of neighboring bar lengths should be staggered and should not be provided at one level/line. At one cross section, a maximum of 50% bars should be lapped. In case, required lap length is not available at junction because of space and other constraints, bars can be joined with couplers or welded (with correct choice of method of welding).
Anchorage length:
This is the additional length of steel of one structure required to be inserted in other at the junction. For example, main bars of beam in column at beam column junction, column bars in footing etc. The length requirement is similar to the lap length or as per the design instructions.
Development length:
Development length is defined as minimum length of bar in which the bar stress can increase from zero to the yield strength. If the distance is less than the development length the bar will pull out the concrete. The development length is a function of yield stress, bar diameter, average bond stress at surrounding concrete. (bond consist of adhesion due to the colloidal action in cement & also due to friction between steel & Concrete.)
Bent up bars:
it is actually economization of material. normally, for fixed ended beams positive (sagging) moments are present at the mid span and negative (hogging) moments at the supports .so bottom steel is required at the mid span and top steel resists negative moments at the supports. a bent-up is provided to take the steel on the top at supports, as bottom steel is not normally required at the supports
BAR BENDING SCHEDULE
Bar-bending-schedule is the schedule of reinforcement bars prepared in advance before cutting and bending of rebars. This schedule contains all details like size, shape , number of bars and dimension of rebars to be cut.
•Generally Engineers don't consider increase in steel length (bend allowance) for each bend to arrive at cutting length
•Typically 2d for 12mm bar dia & 1d for higher dia should be considered for every bend.
(Fixing & Reconciliation is highly affected if above allowance is not considered)
Work Procedure For Fixing Reinforcement
• Ensure reinforcement brought to site is from approved /tested lot (Quality Control Department should ensure)
• Reinforcement brought to the site, should be clean and free from rust, mud, oil, grease paint or any foreign deleterious material present on the surface
• Bar bending schedule to be made as per Good For Construction (GFC) drawing and duly checked by Project Manager, after his approval cutting and bending should begin
• Formwork to receive the reinforcement should be clean & free from debris
• Ensure that the size of bar, spacing between bars, correct bends, proper laps, curtailment of bars, tying of binding wire, enough chairs kept in proper position and cover blocks are all in order.
• Dowel Bars should not be inserted into placed concrete.
• Reinforcement should be placed and tied in such a way that concrete placement be possible without segregation of the mix. Re-bar spacing to be adjusted (temporary) to ease insertion of needle.
• Numbers of laps and chairs to be recorded on BBS as per actual placed at site.
• Use of mechanical splicing should be approved by RCC consultant. They should be provided as far as possible away from the section of maximum stress.
General Points/Precautions
1. Care shall be taken to see that starter is not damaged while tying column reinforcement. Normally column reinforcement fixing is started on next day of column starter casting. Or if required first tie column reinforcement and than cast starter by lifting few bottom rings to facilitate concreting of starter
2. Top ring shall not be half inside the concrete and half outside the concrete as seen in picture. It shall be 40mm below top of concrete. Same to be ensured for column starter
3. All hooks or stirrups of RCC member to be bend to 45 degree, fitter can use tool shown in picture
4. No binding wire knots shall protrude in cover zone
5. General Consumption of Binding wire should be 9 to 12 Kg/MT – Residential Building/Podium (regular slab)
6. Provide stirrups about 600mm above shuttering top so that needle vibrator can be inserted without any kind of obstructions
7. General Consumption of Binding wire should be 9 to 12 Kg/MT – Residential Building/Podium (regular slab)
8.Do not tie stirrups above concreting level, since they will obstruct flow of concrete.
CONCLUSIONS
• Steel reinforcements are like a skeleton in human body. Plain concrete without steel or any other reinforcement is strong in compression but weak in tension. Steel is one of the best forms of reinforcements, to take care of those stresses and to strengthen concrete to bear all kinds of loads.
• To be able to take informed decisions about the quality of steel rebars in a holistic manner, it is imperative that we are conversant with the steel characteristics that have a bearing on the mechanical properties of the rebars
• Understanding reinforcement steel in totality help us exploits its various characteristics, for delivering a duarble end product.
