P K Maini, D Karmakar, AK Bhakat RDCIS, SAIL, Ranchi

Conventional Thermo Mechanically Treated (TMT) steel reinforcement bars are being used in Indian underground coalmines and tunnel construction as rock bolts for roof support system Steel with special attributes like High strength, good toughness etc are being developed for special application Critical safety aspect of such applications and to accrue the benefits of material with higher strength to weight ratio, development of higher strength TMT bars was started by SAIL Estimated annual demand for rock bolt TMT in CIL alone is around T Introduction

Rock Bolt grade TMT Rock bolt 3

4 Roof Bolt Assembly

 Underground Coal Mines  Tunneling  Hydroelectric Power  Roadways & Railways Embankment  Soil Anchoring Applications 5

Rolling of TMT Rebar Finish Rolling 1 st Cooling Pipe 2 nd Cooling Pipe 3 rd Cooling Pipe Header Pressure Cooling Bay Valve Process Parameters Entry Temperature Rolling Speed Equalization Temperature Core Surface RHT~ 1260  10C FRT~ 990  10C Temperature Time In-line Quenching Ar 3 Ms Bs 6

Microstructure of a TMT Rebar Core: Polygonal FerriteRim: Tempered Martensite 7

 Development of high strength bolt material Yield strength  600 MPa UTS > 1.1 x Yield Strength System stiffness > 70 kN/m  Field Demonstration Objectives 8

BS 7861, Pt.1: 1996 (amended 2004)  YS MPa  UTS – 1.2 times YS.  Elongation - at maximum load 8% min. & overall 18%  System stiffness > 70 kN/mm  Bolt diameter - 22mm Standards For Rock Bolt IS 13517:1992 & IS 13219:1992 (under revision)  Bolt material: IS 1786: 1985 (Tor & TMT rebar)  Bolt diameter: 20, 22 and 24mm 9

 Alloy Design  Rolling Parameters Finish Rolling Temperature Rolling Speed Cooling Rate Equalization Temperature  Rib Pattern Processing of Rock Bolt TMT 10

Target Chemical Composition GradeCMnSiSP Rockbolt TMT~0.20~1.1~ 0.1<0.045 Processing (Cont..) Rolling Parameters Soaking Temp Water Volume580 m3/hr (~at 9.0 kg/cm2) Mill Speed6m/sec Eqlisation Temp500 11

12 Microstructure

Analysis Mechanical Properties Yield Strength (MPa) min. Tensile Strength (MPa) min. Elongation (%) min. Targeted Achieved (Typical) CMnSiSP % <0.04 Chemical composition 13

 Transverse rib angle increased  Rib height and length increased to improve mean projected area which ensured better bond strength & system stiffness Ordinary TMTRockbolt TMT Rib Pattern 14

Surface rib profile of Roof bolt TMT bars (20 mm bars ) GradeLtrDtrWtrDlrWlrSpacingAngle Ordinary TMT Roof bolts

Surface Profile Mean Projected Area Mean Projected Area, mm 2 / mm 16

Testing under progress at NTH, Kolkata System Stiffness = Load / extension (Corresponding to 50 & 150 kN) = (150-50) / ( ) = kN/mm Stiffness Testing 17

Rib Profile Features New rib patternExisting TMT BarRemarks Transverse Rib Height Average height more andUniform along the length Average height less & Not uniform along the length Mean Projected area increases Transverse Rib Length No gap between longitudinal ribs Gap exists between longitudinal ribs Uniform stress distribution Transverse Rib Inclination to axis More (~ 70 o )Less (~ 50 o )Friction force enhances Transverse Rib Spacing Less (8-9mm)More (9-10mm)Mean Projected area increases Longitudinal Rib Height Less (~1mm)More (3mm)Does not contribute in bond strength Factors : Bond Strength 18

Bond Properties ProductMaximum Load (kN) Bond Strength* (kN) System Stiffness** (kN/mm) Ordinary TMT ~ 55 SAIL Rock Bolt – 84 Specified # BS min. * Bond Strength = Load at which slope of the graph falls below 20kN/mm ** System Stiffness = Load / Slip (between 50 –150 kN/mm) # As per BS 7861 on 22mm diameter rock bolt

By thread rolling process material is rearranged to create the desired form Cut thread Rolled thread Importance of Thread Rolling Softer Core Zone Macrostructure Softer Core Zone

Roll threading of Rock Bolts Thread Rolling Process

Thread Test View of thread test at NTH, Kolkata Rock Bolt type (20mm) Ultimate load (kN) RolledCut Ordinary~130~110 SAIL RB 1> 200~ 135

Site trials Selection of underground coalmine Rolling of high strength steel roof bolt rebars in 20mm diameters Fabrication of Cold Roll threaded bolts Installation of roof bolts at selected mines Performance Evaluation of bolts in terms of its Load vs displacements 23

IDENTIFICATION OF SITE Jhanjra Underground Mine of ECL was selected for the trials as Jhanjra area has well diversified roof panels which are in operations It was suggested that the 20 mm Bolts will be fixed with the help of Quad bolter with resin grout in the continuous miner panel

Trials with 20 mm bolts 20 mm roof bolts were installed at Panel No 45 Rise and 21 Level Junction. Length of Bolt insertion was varied from 900mm to Full grout i.e 1650 mm with the help of Qudbolter The usual size of roof bolts for this panel was 22 mm

Results (20 mm Bolts) At NTH, 20 mm threaded bolts could sustain 16.8 T of loading In mine, 20 mm bolts were tested in Resin grout application in place of 22 mm bolts 26

Performance of 20 mm Roof Bolt Bolts could withstand 17 T of loading and deflections were linear in permissible range (within 5 mm) 27

Branding Rock Bolt grade TMT is being marketed under brand name “SAILTMT R” in Fe 500 and Fe 600 categories

Cost Analysis Estimated cost for 22 mm bolts Cost of steel bolt bar= Rs 286 cost of fabrication = Rs 50 Total cost= Rs 336 Estimated cost for 20 mm bolts Cost of steel bolt bar= Rs 232 Cost of fabrication = Rs 45 Total cost= Rs 277 % Savings in material cost/bolt = 18.8% % Savings in total cost /bolt = 17.5%

Inclusion of Fe 600 in IS:

Through extensive R&D inputs, SAIL has developed steel for special applications like roof bolts. SAIL is able to match the requirements of mining / construction segments through innovative and cost effective products. Successful Tests and site trials have resulted in enhanced customer confidence for SAIL products. Saving of around 15 % is possible in case of substitution of ordinary TMT to High Strength (600MPa) TMT rebars. Conclusions 31

There’s a little bit of SAIL in everybody’s life “ THANK YOU ” 32