BENEFITS OF USE OF HIGH (YIELD) STRENGTH DEFORMED BARS IN R. C. C. STRUCTURES 1. Following materials are used in the R. C. C. structures:- (1) Cement(2) Steel(3) Sand(4) Aggregates(5) Water 2. Different types of steel bars used in R. C. C. works are mentioned below: 1) Mild Steel plain bars (Generally M. S. round bars are used) 2) Medium Tensile Steel bars 3) High yield strength deformed bars (H. Y. S. D. bars) 1. Mild Steel Bars (M. S. Bars) In past, mild steel was available in two different categories.
. M. S. Plain Bars:- M. S. plain bars of round section confirm to I S 432 – Part – I, 1982 (Grade-I) and (Grade-II) is most commonly used and easily available in the market. It’s diameter denoted by O. The permissible tensile strength of the mild steel confirm to grade-I of I S 432 – Part-I, 1982 is a) 140 N/mm? for bars up to and including 20 mm O b) 130 N/mm? for bars over 20 mm O The permissible tensile strength for mild steel confirming to grade-II of I S 432 – Part-I, 1982 is @ 90 % of grade-I bars.
Hence if the design details have already been worked out on the basis of Mild Steel confirming to grade-I of I S 432 – Part-I, 1982 and it happens to use grade-II of Mild Steel than the area of reinforcement shall be increased by 10 % of the required for grade-I steel. More over I S 432 – Part-I, 1982 does not recommend the grade-II M. S. bars for use in structures located in earthquake zone subjected to severely damage and structures subjected to dynamic loading (other than wind loading) 2. M. S. Deformed Bar (or Deformed Mild Steel Bars):- In past deformed M. S. ars were also available in the market confirm to I S 1139 – 1966. Deformed bars are furnished with lugs, ribs or other surface deformation. The technical specification of such deformed bars are mentioned below: (a) Permissible tensile stress : -140 N/mm? for bars up to and including 20 mm O 130 N/mm? for bars over 20 mm O` (b) Yield stress : -260 N/mm? for bars up to and including 20 mm O 240 N/mm? for bars over 20 mm O (c) Ultimate tensile stress : -420 N/mm? or all size diameter of bars Since the basic materials for plain mild steel bars and deformed mild steel bars is the same, hence the specifications for deformed mild steel bars has to be as far as possible in line with the specification for plain mild steel bars. The above-mentioned M. S. deformed bars were known as Fe 250 bars in the market, where figure followed the symbol Fe indicates the yield stress in N/mm?. But, after publishing I S 1786 – 1985 which supercedes I S 1139 – 1966 and as per the clause 0. 3 of I S 1786 – 1985. Fe 250 strength grade has been deleted.
Therefore material of Fe 250 grade i. e. deformed M. S. bars should not be used for Government Works. 2. Medium Tensile Steel:- This type of material is not commonly used in general R. C. C. work. Hence only elementary information are collected and mentioned here. 2. 2. 1(I)Medium Tensile Plain Bars (of round section) It should confirm I S 432 – Part-I, 1982 (a) Permissible tensile stress :-Half the guaranteed yield stress to maximum of 190 N/mm? 2. 2. 2Medium Tensile Deformed Bars:- In past, deformed medium tensile steel bars were also available in the market confirm to I S 1139 – 1966.
The technical specifications of such deformed bars are mentioned below: (a) Permissible tensile stress : -Half the guaranteed yield stress to maximum of 190 N/mm? (b) Yield stress : -360 N/mm? for bars up to and including 16 mm O 350 N/mm? for bars>16 mm O and incl. 32 mm O 340 N/mm? for bars >32 mm O (average 350 N/mm? is considered) (c) Ultimate tensile stress : – 550 N/mm? for all size diameter of bars Since the basic materials or plain medium tensile steel bars and deformed medium tensile steel bars is the same, hence the specifications for deformed medium tensile steel bars has to be as far as possible in line with the specification for plain medium tensile steel bars. The above mentioned medium tensile deformed steel bars were known as Fe 350 bars in the market, where figure follow the symbol Fe indicates the yield stress in N/mm?. After publishing the I S 1786 – 1985 which supersedes I S 1139 – 1966 and as per the clause 0. 3 of I S 1786 – 1985, Fe 350 strength grade bars been deleted. Therefore material of Fe 350 grade i. . deformed medium tensile deformed bars should not be used for Government Works. 3. High Yield Strength Deformed Bars (H. Y. S. D. of H. S. D Bars):- High yield strength deformed bars furnished with lugs, ribs or other form of surface deformations for the purpose of increasing their bond strength with concrete. They are available in the form of:- 1) Cold Twisted Deformed Bars (C. T. D. bars) or Ribbed Tor Steel 2) Thermo Mechanical Treated Deformed bars (T. M. T. bars) HYSD bars possess following characteristics:- 1) Considerably increased yield stress. ) Increased tensile strength (Ultimate tensile stress) 3) Increase Bond strength @ 40 % more than of M. S. round bars. Because of increased bond strength, such bars do not need “End Hooks” and require “reduced length for overlap” etc. thereby effecting reduction in the cost of reinforcement and labours. HYSD bars having yield stress (proof stress) corresponding to 0. 2 % strain of original gauge length of test specimen equal to 415 N/mm? and permissible tensile stress equal to 230 N/mm? are extensively used in the R. C. C. design works. These bars are also commonly known as Fe 415 HYSD bars.
In case of structures requiring use of still higher tensile strength bars, another form of HYSD bars known as Fe 500 bars are used. These bars have a yield stress (proof stress) corresponding to 0. 2 % strain of original gauge length of test specimen equal to 500 N/mm? and have permissible tensile stress equal to 275 N/?. 1. In past two different I S codes were applicable to deformed bars and HYSD bars. (1) I S 1139 – 1966 :-Specifications for hot rolled mild steel, medium tensile steel and high yield strength steel deformed bars for concrete reinforcement (2)
I S 1786 – 1979 :-Specifications for cold worked steel high strength deformed bars for concrete reinforcement 2. 3. 2As the time passes, technological advancement take place. To take the advantage of the technological changes, the specifications of above two I S have been merged and new I S 1786 – 1985 was published with modified designation and the title. I S 1786 – 1985 :-Specification for high strength deformed steel bars and wires for concrete reinforcement As per para 0. 2. of I S 1786 – 1985, HYSD bars are manufactured by, 1) Micro alloying with Nb, V, Ti, B, Mn, Cr, Mo, Cu etc. in combination with one another or individually. 2) Thermo Mechanical Treatment (T. M. T. process) With these two process higher strength value could be achieved at low carbon levels even in larger diameter deformed bars. As mentioned in para 0. 3 of I S 1786 – 1985, the requirements of chemical composition for deformed bars have been modified, a new strength grade Fe 550 has been introduced and Fe 250 and Fe 350 strength grade have been deleted.
Hence, mild steel deformed Fe 250 and medium tensile steel deformed bars Fe 350 confirming to I S 1139 – 1966 should not be used in Government works. I S 1786 – 1985 covers the requirements of deformed steel bars and wires for use as reinforcement in concrete in the following three strength grades. (a)Fe 415 (b)Fe 500 (c)Fe 550 The figures following the symbol Fe indicates the yield stress in N/mm? (proof stress corresponding to stress at which 0. 2 % elongation of original gauge length of test specimen take place) The chemical composition of each grade of steel is mentioned in the para 3. of I S 1786 – 1985 are as below:- Constituent Percent Maximum______ Fe 415Fe 500Fe 550 Carbon0. 300. 300. 30 Sulphur0. 0600. 0550. 055 Phosphorus0. 0600. 0550. 050 Sulphur and Phosphorus0. 110. 1050. 10 Note 1:- For guaranteed weldability, the Carbon Equivalent using the formula, CE=C + (Mn/6) + (Cr + Mo + V)/5 + (Ni + Cu)/15 should be not more than 0. 53 percent, when micro alloys / low alloys are used. When micro alloys are not used, carbon equivalent using the formula, CE=C + (Mn/6) shall be not more than 0. 42 percent.
Note 2:- Additional micro alloying element is not mandatory for any of the above grades. When strengthening element like Nb, V, B and Ti are used individually or in combination, the total contents shall not exceed 0. 30 percent; in such a case manufacturer shall supply the purchaser or his authorized representative a certificate stating that the total contents of the strengthening elements in the steel do not exceed the specified limit. Note 3:-Low alloy steel may also be produced by adding alloying element like Cr, Cu, Ni, and P either individually or in combination, to improve allied product properties.
However, the total content of these element shall not be less than 0. 50 percent. In such case, manufacturer shall supply the purchaser of his authorized representative a test certificate stating the individual contents of all the alloying elements. In such low alloy steel when phosphorus is used, it shall not exceed 0. 12 percent and when used beyond limit prescribed above, the carbon shall be restricted to a maximum of 0. 15 percent; and in such case the restriction to maximum content of Sulphur and phosphorus shall not apply.
It should be noted that there is a danger of pitting and crevice corrosion when weathering steel that is, those with chemical composition confirming to I S 11587 – 1986 ‘Specification for structural weather resistant steel’ are embedded in chloride contaminated concrete. 4. Corrosion Resistant Steel (CRS bars):- Every Steel product has its own inherent corrosion resistant properties depending upon its chemical composition. When freshly cast concrete (pH = 12 -15) is highly alkaline material and the embedded steel should be protected by passive oxide layer.
Gradually, carbonation and chlorination break down the natural passivating protection of steel. In freshly concrete about 7500 to 8000 ppm chloride ions start corrosion. However when alkalinity reduces, the chloride level needed to start corrosion is lowered to below 100 ppm. Cold Twisted Deformed bars has though better yield strength, the additional operation of twisting is involved during the manufacturing process and because of this the bars are subjected to torsional stresses and become danger of formation of pitting and crevice at the surface increase tremendously.
This kind of CTD bars when embedded in chloride contaminated concrete they become less corrosion resistant. To over come the problems of CTD bars mentioned above, the production of CRS bars are route through the process of TMT (Thermo Mechanical Treatment) by altering and addition in manufacturing process and using perfect chemical composition for the production of CRS bars. The finished CRS bar has a microstructure comprising a tough outer layer of “Tempered Martensite” and a ductile core of “Ferrite – Pearlite”. The copper plugs pores in rust.
Phosphorus act as an inhibitor as P2O5 . The oxide layer formed is dense and with very little pores/cracks and thus cuts down the ingress of oxygen, Sulphur dioxide, chlorides etc. through oxide-metal interface. Further reducing chromium content in steel has improved the pitting resistance. 1. Comparison of CRS (route through TMT) and CTD bars:- The micro structure resulting from the TMT treatment leads to higher corrosion resistance compared to conventional CTD Mild Steel or CTD HYSD rebar on account of : ?
Being free from torsional stresses ? Presence of self tempered lathe martensitic layer on surface known to inhibit corrosion attack ? Homogeneous distribution of corrosion resistant elements from core to surface The CRS (route through TMT) bars only made out from basic raw materials of Iron Ore and should not be made out from Ingot/Plates or re rolled or re mould of scrap. Some of the prominent steel manufacturer have their own production for CRS bar viz. ) TATA has “CRS500” (as they produce CRS bar in only Fe 500 grade presently), 2) SAIL has “TMT HCR”, 3) VIZAG has VSP steel TMT Re-bars as “HSCRM” via Tempcore Process etc. The physical and mechanical properties confirm to equivalent grade of TMT steel in their respective category. However the chemistry is different with addition of corrosion resistant element like copper, chromium, phosphrous etc. The corrosion resistant steel is better compared to normal TMT. The chemical composition of TATA TISCON TMT CRS bar are given below in % for knowledge purpose and not for the advertisement of this company.
C MnSPSiCRECE 0. 15 1. 00. 040. 100. 450. 500. 42 max maxmaxmaxmaxmaxmax CRE=Corrosion Resistant Element =Cu + P + Cr CE=Carbon Equivalent The above CRS bars confirm to I S 1786 – 1985 re affirmed 1990, Third Reprint April 1992 (up to date 2003) 5. Galvanized Rebars:- The application of reinforcement in the coastal area, one has also to consider about galvanized rebar which have much superior corrosion resistant properties and used extensively abroad for such application. As far as this note is concerned, details regarding galvanized rebar is out of scope. . Fusion Bonded Epoxy Coated Bars:- I S 13620 – 1993 The bars confirm to I S 1786 mentioned above is used with protective epoxy coating applied by electrostatic spray method and develop CRS properties in the reinforcement. Details regarding epoxy coated reinforcement material are given below. Reinforcement embedded in concrete exposed to damp saline condition on long term near coastal area particularly contaminated by chloride is of interest. When the level of chloride exceed 0. 95 %, pitting occur more deeply and resulting in weight loss of reinforcement.
Due to lack of protective film being developed, steel is subjected to pitting and crevice corrosion in the presence of chloride. The coating material of inorganic compound epoxy type having proper chemical resistant capacity, as specified in I S 13620 – 1993 is applied by electrostatic spray method on abrasive blast clean to near white metal steel reinforcement. The epoxy powder may be applied either a hot or cold bar. The epoxy coated bar shall be given a thermal treatment of epoxy resin which will provide a fully cured finished coating. Coating thickness shall be 0. 1 mm to 0. 3 mm after curing.
Fusion Bonded Epoxy Coated reinforcement bars should be tested for 1) Continuity of coating, 2) Adhesion of coating, 3) Resistance to applied voltage, 4) Bond strength to concrete, 5) Abrasion resistant, 6) Impact test, 7) Hardness test etc. The I S code No. 13620 is under revision. Salinity Map of India is available. Generally in 20 km or nearer coastal area the foundation as well as super structures are to be made of Fusion Bonded Epoxy Coated bars. The rest of the area where the soil has it’s inhibit of saline properties the foundation should be done using the epoxy coated bars. 7.
Measurement of Diameter of Deformed Bar:- As a deformed bar is provided with lugs, ribs or deformation on the surface, it’s diameter cannot be measured directly with the conventional measuring instruments. As per para 2. 5 of I S 1786 – 1985, the diameter of plain round bar/wire having the same mass per meter length as the deformed bar/wire. Hence, the “Nominal Diameter” of deformed bar is determined by weighing the specimen of measured length (generally on meter long specimen) then derive it’s cross section area and nominal diameter as per table – 1 of I S 1786 – 1985. The diameter of a deformed bar is denoted by a sign tor
Ready Reckner Formulae:- (for field purpose only) The diameter of deformed bar is approximately determined by, a) Gross cross sectional are in mm? = w/0. 00785 L (I S 1786 5. 3. 1) where, w = mass in kg weighed to a precision of ± 0. 5 percent, and L = length in m measured to a precision of ± 0. 5 percent. (b)Diameter = 12. 7 vm/l in mm where, m = mass of specimen in gram (g) l = length of the specimen in mm 3. Technical specification of mild steel plain bars, medium tensile steel plain bars and high yield strength deformed bars are summarized below:- |Sr.
No. |Type of the material |Related I S |Permissible |Yield Stress |Ultimate | | | | |Tensile Stress |(minimum) |Tensile | | | | | | |Stress | | | | | | |(minimum) | |1 |Mild Steel Plain bars |432-1982 Part – I |140 N/mm? up to and |250 N/mm? |420 N/mm? | | |confirming to grade – I | |incl. 20mmdia. ) |(Average) | | | | | |130 N/mm? | | | | | | |> 20mm O | | | |2 |Mild Steel Plain bars |432 – 1982 Part – I |90 % of above value |90 % of above value |380 N/mm? | |confirming to grade – II | | | | | |3 |Medium Tensile Steel plain |432 – 1982 Part – I |Half the guaranteed yield |350 N/mm? (average) |550 N/mm? | | |bars | |stress subjected to max. | | | | | | |190 N/mm? | | | |4 |High Yield Strength |1786 – 1985 |230 N/mm? 415 N/mm? |485 N/mm? | | |deformed bars Fe 415 | | | | | |5 |High Yield Strength |1785 – 1985 |275 N/mm? |500 N/mm? |545 N/mm? | | |deformed bars Fe 500 | | | | | |6 |High Yield Strength |1785 – 1985 | |550 N/mm? |585 N/mm? | |deformed bars Fe 550 | | | | | 4. Comparison between M. S. Plain Bars of Round Section and HYSD Bar:- Sr. No. DescriptionM. S. Plain bar of HYSD bar confirmRemarks Round Section Confirming to I S 432 – 1982 Part-I Grade – I _____ 1Permissible Tensile Stress140 N/mm? (for bar Fe 415 – 230 N/mm? In deformed up to and incl. 20 Fe 500 – 275 N/mm? bars tensile mm O) stresses does 130 N/mm? (for barnot depend > 20 mm O)on bar dai. 2Permissible Bond StressFor Concrete Grade 40 % more than M S
M15 – 0. 6 N/mm? Plain round bars (Average) M20 – 0. 8 N/mm? (Average) 3Distribution ReinforcementFor Concrete GradeFor Concrete Grade M15 – 0. 15 % of M15 – 0. 12 % of cross sectional areacross sectional area 4AnchorageEnd hooks are End hooks are not requiredrequired 5Length of OverlapAs per Design Less than that of M S requirementsPlain round bars as bond strength increase 5. 0 CONCLUSION:- 1Mild Steel and Medium Tensile Steel which are to be used for R. C. C. works should be in the form of plain bars only (other round or square section). I S 432 – Part-I – 1982 does not recommended the Grade – II M. S. bar use in structures located in earthquakes zone subjected to severely damaged and structures subjected to dynamic loading (other than wind loading) 3I S 1786 – 1986 has deleted the strength grade Fe 250 and Fe 350. Hence Mild Steel Deformed Bars (Fe 250) and Medium Tensile Steel Deformed Bars (Fe 350) should not be used for the Government works. 4High Yield Strength Deformed Bars (HYSD bar) strictly confirming to only I S 1786 – 1985 should be used in important R.
C. C. works. The bars shall be made out of Iron Ore/Fresh Billets only and not out of scrap steel, if the same is used in re rolling mills. 5HYSD bars are available three different categories :- (1)Fe 415(2)Fe 500(3)Fe 550 6HYSD bars confirm to I S 1786 – 1985 are available in two different forms:- (1)Cold Twisted Deformed bars (CTD bars) or Ribbed Tor Steel (2)Thermo Mechanically Treated Deformed Bars (TMT Bars) Thus the TMT bars are the type of HYSD bars confirm to I S 1786 – 1985.
No separate I S code is prevailing for TMT bars. The Bars manufacturer produce the CRS bar in the form of TMT bar it self and not in the form of CTD. 7The Real HYSD bars are manufactured from Iron Ore/Billets only by the standard steel manufacturing companies. The re rolled CTD bars manufactured from M. S. Scrap only with impurities are available in plenty in the market. But actually they are not real HYSD bars. They are likely to be deformed M. S. bars. 8The main aim of this note is not to recommend any particular brand or manufacturer.
It is observed that HYSD bars when used as Main Steel / Distribution Steel in combination with high strength concrete i. e. M20 and above prove to be more economical due to high tensile strength resulted in to less consumption. Reference:- (1)I S 432 – 1982 Part-I (2)I S 1139 – 1966 (3)I S 1786 – 1979 (4)I S 1786 – 1985 (5)Treasure of R. C. C. Design – Dr. Shushil Kumar (6)Hand Book of Civil Engineering – P. N. Khanna (7)Literature of TATA Iron & Steel Co. / HY – Tuf Steels Pvt. Ltd. / VIZAG Steel Co. / SAIL product chart (8)Testing of Building Materials and Structures – N. Novogodsky
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