Quick Answer
Quick Answer
IS 1786 is the Bureau of Indian Standards specification for high-strength deformed (TMT) steel bars for reinforced concrete construction. It covers seven grades from Fe 415 to Fe 600, with D-grade variants imposing stricter carbon and CEV limits for superior ductility and weldability. BIS certification is mandatory for all IS 1786 rebar sold in India.
IS 1786 is published by the Bureau of Indian Standards (BIS) and governs Thermo-Mechanically Treated (TMT) and cold-worked deformed steel bars used as reinforcement in concrete structures. The current edition is IS 1786:2008 (with subsequent amendments). Grades are designated by the prefix "Fe" followed by the minimum 0.2% proof stress (yield strength) in MPa. The suffix "D" (Ductility) denotes enhanced ductility grades with stricter chemical limits — mandatory for earthquake-resistant construction and high-ductility moment frames. IS 1786 is under mandatory BIS certification (Schedule II), so all rebar must carry the ISI mark.
Scope and Applicability
IS 1786 covers:
- Thermo-mechanically treated (TMT) deformed bars
- Cold-worked deformed bars (less common in current market)
- Nominal diameter range: 6 mm to 50 mm
- Applications: reinforced concrete (RC) buildings, bridges, dams, industrial structures, seismic-resistant structures (D grades mandatory)
The standard does not cover plain round bars (IS 432 Part 1), mild steel bars, or prestressing wires and strands (IS 1785, IS 6003).
Grade Coverage
| Grade | Yield Strength Basis | Ductility Class | Primary Use |
|---|---|---|---|
| Fe 415 | 0.2% proof stress ≥ 415 MPa | Standard | General RC construction |
| Fe 415D | 0.2% proof stress ≥ 415 MPa | High ductility | Seismic zones, moment frames |
| Fe 500 | 0.2% proof stress ≥ 500 MPa | Standard | Most common grade in India |
| Fe 500D | 0.2% proof stress ≥ 500 MPa | High ductility | Seismic construction (preferred) |
| Fe 550 | 0.2% proof stress ≥ 550 MPa | Standard | Heavy RC structures |
| Fe 550D | 0.2% proof stress ≥ 550 MPa | High ductility | High-ductility heavy structures |
| Fe 600 | 0.2% proof stress ≥ 600 MPa | Standard | Special high-strength applications |
Chemical Composition Requirements
Ladle analysis limits. All values are weight percent maximum.
| Grade | C max | S max | P max | S+P max | CEV max |
|---|---|---|---|---|---|
| Fe 415 | 0.30 | 0.060 | 0.060 | 0.110 | 0.53 |
| Fe 415D | 0.25 | 0.045 | 0.045 | 0.085 | 0.42 |
| Fe 500 | 0.30 | 0.055 | 0.055 | 0.105 | 0.55 |
| Fe 500D | 0.25 | 0.040 | 0.040 | 0.075 | 0.42 |
| Fe 550 | 0.30 | 0.055 | 0.055 | 0.105 | 0.55 |
| Fe 550D | 0.25 | 0.040 | 0.040 | 0.075 | 0.42 |
| Fe 600 | 0.30 | 0.055 | 0.055 | 0.105 | 0.57 |
CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
Key observation: D grades cap carbon at 0.25% versus 0.30% for standard grades, and set a much tighter CEV (0.42 vs 0.53–0.57). This directly improves weldability and reduces risk of brittle fracture under seismic loading.
Mechanical Properties
| Grade | 0.2% Proof Stress min (MPa) | UTS min (MPa) | UTS/YS Ratio min | Elongation min % (GL = 5.65√A) | Total Elongation at Max Force min % |
|---|---|---|---|---|---|
| Fe 415 | 415 | 485 | — | 14.5 | — |
| Fe 415D | 415 | 500 | 1.12 | 18.0 | — |
| Fe 500 | 500 | 545 | — | 12.0 | — |
| Fe 500D | 500 | 565 | 1.08 | 16.0 | — |
| Fe 550 | 550 | 585 | — | 10.0 | — |
| Fe 550D | 550 | 600 | 1.06 | 14.5 | — |
| Fe 600 | 600 | 660 | — | 10.0 | — |
Notes:
- Tensile test per IS 1608 using 5.65√A gauge length
- The UTS/YS (fu/fy) ratio minimum for D grades ensures adequate strain hardening reserve — critical for seismic energy dissipation
- Yield strength is determined as 0.2% offset proof stress; upper yield point may be used if it appears clearly
Bend and Rebend Test Requirements
Bend Test
Bars must be capable of being bent through 180° without cracking. Mandrel diameter as a multiple of bar nominal diameter (d):
| Nominal Bar Diameter | Mandrel Diameter (Fe 415 / Fe 415D) | Mandrel Diameter (Fe 500 / Fe 500D) | Mandrel Diameter (Fe 550 / Fe 550D) | Mandrel Diameter (Fe 600) |
|---|---|---|---|---|
| Up to 10 mm | 2d | 3d | 4d | 5d |
| > 10 mm, ≤ 16 mm | 3d | 4d | 5d | 6d |
| > 16 mm, ≤ 28 mm | 4d | 5d | 6d | 7d |
| > 28 mm | 5d | 6d | 7d | 8d |
Rebend Test
The rebend test (IS 1786 Clause 9) assesses susceptibility to strain-ageing embrittlement:
- Bend bar to 45° around the appropriate mandrel
- Age the bent bar at 100 °C for 30 minutes
- Rebend back to 22.5° (returning towards original position)
- No crack or fracture on the tension surface = satisfactory
The rebend test is compulsory for all grades and diameters.
Weight per Metre — Standard Bar Diameters
Theoretical mass based on nominal cross-sectional area. Tolerance on mass per metre: −4% to +4% for individual bars, −2.5% to +2.5% for batch average.
| Nominal Diameter (mm) | Cross-Sectional Area (mm²) | Mass per metre (kg/m) |
|---|---|---|
| 6 | 28.27 | 0.222 |
| 8 | 50.27 | 0.395 |
| 10 | 78.54 | 0.617 |
| 12 | 113.10 | 0.888 |
| 16 | 201.06 | 1.579 |
| 20 | 314.16 | 2.466 |
| 25 | 490.87 | 3.854 |
| 28 | 615.75 | 4.834 |
| 32 | 804.25 | 6.313 |
| 36 | 1017.88 | 7.990 |
| 40 | 1256.64 | 9.865 |
| 50 | 1963.50 | 15.413 |
Additional Tests and Requirements
Rib Geometry
Deformed bars must have transverse ribs and at least one longitudinal rib. Rib height, spacing, and inclination are specified in IS 1786 Annex A. The relative rib area (fR) must be ≥ 0.052 for diameters ≤ 10 mm and ≥ 0.056 for larger diameters.
Chemical Analysis Frequency
- One ladle analysis per heat (cast)
- Product (check) analysis on one sample per lot of 50 tonnes or part thereof
- Product analysis tolerance: +0.02% on C, +0.005% on S and P
Surface Condition
Bars must be free of harmful surface defects (seams, laps, cracks). Light surface rust is acceptable. Loose mill scale that may reduce bond with concrete is not acceptable.
BIS Mandatory Certification
IS 1786 is listed under Schedule II of the BIS (Conformity Assessment) Regulations:
- All TMT rebar sold in India must carry the ISI Mark with the producing mill's BIS licence number
- MTCs must state the BIS licence number, heat number, grade, and nominal diameter
- Bar bundles must be tagged with the licence number and grade
- Imported rebar must also hold BIS certification or Type Approval
Cross-Standard Equivalents
| IS 1786 Grade | ASTM Equivalent | BS / EN Equivalent | Remarks |
|---|---|---|---|
| Fe 415 | A615 Gr 60 (approximate) | B500A (BS 4449) | Fy 415 MPa vs 420 MPa for A615 Gr 60 |
| Fe 500 | A615 Gr 75 (approximate) | B500B (BS 4449) | Most common rebar grade in India |
| Fe 500D | A706 Gr 60 | B500C (BS 4449) | Enhanced ductility — seismic use |
| Fe 550 | A615 Gr 80 | — | High strength; weldability requires care |
| Fe 600 | A1035 Gr 100 (approximate) | — | Ultra-high strength; limited use |
Equivalences are approximate. IS 1786 and ASTM A706 are the closest pair for seismic applications due to similar CEV and UTS/YS ratio requirements.
MTC Verification Checklist
When verifying a mill test certificate for IS 1786 rebar, confirm:
- Standard and grade clearly stated (e.g., IS 1786:2008 Fe 500D)
- BIS licence number of the producing mill is present
- Heat number traceable to bar bundle tags
- Chemical analysis: C, S, P, S+P, and CEV within grade limits
- 0.2% proof stress meets the grade minimum for the declared diameter
- UTS meets grade minimum; UTS/YS ratio ≥ minimum (for D grades)
- Elongation (% on 5.65√A) meets grade minimum
- Bend test result stated (pass/satisfactory)
- Rebend test result stated (pass/satisfactory)
- Mass per metre tolerance within −4%/+4% on individual and −2.5%/+2.5% on batch average
- Tensile test standard IS 1608 cited; test machine calibration current
Frequently Asked Questions
What is the difference between Fe 500 and Fe 500D?
Fe 500D is the high-ductility variant of Fe 500. Both have the same minimum yield strength (500 MPa) and minimum UTS (565 MPa for D vs 545 MPa standard), but Fe 500D imposes stricter limits on carbon (0.25% max vs 0.30%), sulphur and phosphorus (0.040% vs 0.055% each), and total S+P (0.075% vs 0.105%). Fe 500D also requires a minimum UTS/YS ratio of 1.08, ensuring adequate strain hardening for seismic energy dissipation. For construction in earthquake zones III, IV, and V, IS 13920 recommends Fe 500D or equivalent ductile grades.
Why does the rebend test matter for TMT rebar?
The rebend test detects susceptibility to strain-ageing embrittlement — a condition where bars that were bent during erection and then exposed to elevated temperatures (welding heat, summer heat in tropical climates) become brittle at the bent location. Strain ageing causes carbon and nitrogen atoms to migrate to dislocations, pinning them and raising the effective yield point while drastically reducing ductility. A bar passing the rebend test has demonstrated resistance to this failure mode.
Can IS 1786 rebar be welded?
Yes, but with grade-dependent requirements. D grades (Fe 415D, Fe 500D, Fe 550D) with CEV ≤ 0.42 can typically be welded without preheat for diameters up to 32 mm using matching or under-matching electrodes. Standard grades with CEV up to 0.55–0.57 require preheat calculation per IS 9595 or AWS D1.4 for diameters above 20 mm. Fe 600 with CEV up to 0.57 and higher carbon demands careful procedure qualification. Mechanical splices (couplers) are preferred over welding for high-grade bars in seismic applications.
How do I identify the grade of TMT rebar from the bar itself?
IS 1786 requires grade identification marks rolled into the ribs: the number of longitudinal lines indicates the grade (Fe 415 = one line, Fe 500 = two lines, Fe 550 = three lines, Fe 600 = four lines). D grades add an additional short transverse rib mark or a "D" rollmark depending on the mill. The mill's identifier mark and BIS licence number are also rolled in at intervals. Physical marks should match the MTC and bundle tags.
What is the batch/lot definition for mechanical testing under IS 1786?
A lot is defined as all bars of the same grade, same nominal diameter, and same heat (cast) presented for inspection at one time. The minimum frequency is one tensile test, one bend test, and one rebend test per 50 tonnes or part thereof of each lot. For continuous cast heats, the standard allows composite sampling procedures agreed between the supplier and purchaser.
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