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Challenges of Rebar Testing

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The presentation is an overview of recent changes and challenges facing rebar manufacturers, suppliers, and consumers.

Published in: Engineering, Technology, Business
  • I'm pre-kindergarten when it comes to material testing. Where I am, there had been certain proposals to subject ASTM A-706 and A-615 rebars to cyclic testing to assess their potential performance during design-level eaarthquakes. I know that rebar couplers must be subjectt to cyclic load testing. This is most likely because being a REBAR JOINT, they need to be tested for repeated stress reversals. My old school mentality was normally that tensile and bend test was already sufficient for rebars, as a material. What is the paramount and specific significance of subjecting rebars (say QT/TMT) to cyclic testing. I was of the impression thatt it is more significantt to test the girder-column joint (concrete, steel and all) as a whole specimen, not the rebar -- unless you have couplers or ACI-s[pecified strong connections.
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Challenges of Rebar Testing

  1. 1. Instron® presents an overview of changes and challenges facing rebar manufacturers, suppliers, and consumers. Challenges of Rebar Testing
  2. 2. 2 Why Now? • Construction average annual growth rate projections to 2025 *Qatar From Global Construction 2025 : “Over the next 7 years the global construction industry is on pace to grow almost 5 trillion dollars from $7.2 trillion to over $12 trillion, while construction output will grow 70% by 2025.”
  3. 3. 3 What are Implications for Rebar? • Expect increased volumes • Potential for higher strengths and larger diameters • Increased use of stainless or coated grades • Mechanical coupler requirements
  4. 4. 4 How Will This Impact Your Testing? • Are additional testing systems required to meet increased testing volumes? • Will greater force capacity be necessary to accommodate increased strengths and/or diameters? • Are current gripping solutions effective for stainless or coated grades? • What demands will you have for testing mechanical splices? Is your testing program prepared for these expected changes?
  5. 5. 5 Steel Rebar Applications: Overview • Embedded in concrete to improve tensile strength • Deformations on bar provide for better adherence to the concrete • Thermal expansion properties similar to concrete • Structural shapes can be created by bending rebar Applications influence the mechanical testing and specification requirements
  6. 6. 6 Product Standards Linked to Testing Standards: ISO 6892-1 2009 ASTM E8M:2013 ASTM A370 Standard Test Methods and Definitions for Mechanical Testing of Steel Products ASTM A615 Standard specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement ISO 15630-1 Steel for the reinforcement and pre-stressing of concrete – Test Methods Part 1: Reinforcing bars, wire rod and wire BS 4449 Specification for Carbon steel bars for the reinforcement of concrete GB 1499 Hot rolled ribbed steel bars for the reinforcement of concrete JIS G3112 Steel bars for concrete reinforcement AC133 Mechanical Coupler Test Common Rebar and Testing Standards
  7. 7. 7 Rebar Testing Challenges • Violent Tensile Failures • Irregular and/or Bent Specimens • Repeatable/Accurate Tensile Results • Strain and Elongation Measurement • Cyclic Testing for Couplers • Bend Testing
  8. 8. 8 Challenge: Irregular Surface Rebar’s irregular surface geometry and scaling during elongation present several gripping challenges
  9. 9. 9 Gripping Irregular Surface • Application-specific jaw faces effectively prevent slippage and jaw breaks while still allowing for safe and easy removal of broken test pieces • Internal components are shielded from falling scale produced during rebar elongation—reduces system maintenance costs and possible down time
  10. 10. 10 Challenge: Bent Specimens Rebar specimens are often cut from coiled material and must be straightened prior to tensile testing. As a result, specimens are not always perfectly straight.
  11. 11. 11 Bent Specimens: Wedge Grip Solutions • Able to accommodate specimens with slight or minor bends • Effective and economic approach for testing most common rebar • Clamping force is directly proportional to the amount of tensile load applied
  12. 12. 12 Bent Specimens: Side-Acting Grip Solutions • Dual, side-acting grips are able to accommodate specimens with moderate to severe bends • Synchronizer can overcome side-loads while still maintaining axial alignment • No need to reset (re-center) grip jaws between tests • Clamping force is adjustable helping reduce jaw breaks
  13. 13. 13 Challenge: Violent Failures Rebar specimens exhibit violent failures with significant recoil (over 60G of acceleration), which can lead to increased system wear.
  14. 14. 14 Absorbing High-Energy Release • Robust hydraulic grips and load frames effectively absorb released energy preventing unwanted damage to the testing system • Standard system capacities ranging from 300–3500 kN (67,500–800,000 lbf) easily accommodate any size and grade of rebar
  15. 15. 15 Challenge: Measuring Strain When an extensometer is used: • Long gauge lengths required • Must attach to uneven surface of rebar without slipping • Remain attached through maximum force or failure for automatic elongation results (optional) Manual Clip-on Extensometer Automatic Contacting Extensometer
  16. 16. 16 Suitable Strain Measurement - Manual Manual Clip-on Extensometer • Securely clamps to the irregular specimen, reducing strain errors resulting from slippage or undesired movement on uneven surface • Instruments are uniquely identified by testing software preventing accidental use of the incorrect extensometer • Some models are designed to remain on through fracture
  17. 17. 17 Suitable Strain Measurement - Automatic Automatic Contacting Extensometer (No. 18 bar shown) • Adjustable gauge length accommodates a wide range of specimen diameters and gauge lengths with one instrument • Automatic clamping and release improves operator safety and reduces test result variability between operators • Can remain attached through peak load or failure allowing for automatic recording of elongation results
  18. 18. 18 Automatic Elongation Measurement Test standards often allow for the automatic measurement of elongation results from strain data if the testing system is capable Using an extensometer that remains attached through peak load, or even fracture, will reduce or eliminate the need for manually marking the specimen and allows the operator to focus on more value-added activities
  19. 19. 19 Integrate Manual Elongation • Consider digitally interfaced specimen measuring tools • Push button data transfer • Saves time • Reduces manual input errors
  20. 20. 20 Challenge: Tensile Test Results Historically, many of these results have required manual identification, measurement, or calculation. ISO ASTM Yield Point (Distinct) Upper Yield Strength (ReH) Yield Point (Drop of Beam or Halt of Pointer) Yield Strength (Offset Method) 0.2% Proof Strength, non- proportional elongation (Rp 0.2) Yield Strength (0.2% Offset) Maximum Stress Tensile Strength (Rm) Tensile Strength Ratio of Tensile Strength/Yield Strength Rm/ReH Not Required Strain at Maximum Force % Total Elongation at Maximum Force (Agt) Not Required Elongation after Fracture % Elongation After Fracture (A or A5) Elongation Common Rebar Tensile Results
  21. 21. 21 Automatic Capture of Test Results Today, all calculation results can be performed automatically, saving time and reducing operator and system variability
  22. 22. 22 Test Method Setup: CAUTIONS! Software Stored Test Parameters • Securely store critical test settings • Better repeatability/reproducibility • Operators can quickly run same test + Proper Test Method Setup • Improve accuracy • Ensure standards compliance • Reduce risk of incorrect results! Improper Setup for Slope Proper Setup for Slope Great repeatability, but poor accuracy!
  23. 23. 23 Common Setup Errors That Affect Results • Defining the modulus (slope) region incorrectly • Yield strength is incorrect (high) • Addressed in previous slide example • Improperly zeroing or taring force data • Zeroing after load is applied to the specimen • Peak Load and Tensile Strength incorrect (low) • Running tests in load or stress control during yielding • Can “hide” yield point behavior • Yield Point (Drop of Beam) does not calculate VERIFY SETUP and STANDARDS COMPLIANCE! DON’T RISK BAD PRODUCT IN THE FIELD!
  24. 24. 24 Challenge: Bend Testing • Rebar is often bent for use in concrete applications • Test standards require a bend test to determine if cracks form during bend • Some standards also require rebend test
  25. 25. 25 Bend Testing Rebar Effectively System testing stroke is long enough to perform entire bend on largest diameters eliminating the need for a separate bend tester Dual test space systems allow bend fixtures to remain in the machine during tensile testing, which reduces setup time and operator fatigue from moving heavy fixtures. Upper fixture can be changed to perform rebend test
  26. 26. 26 Challenge: Coupler Testing • Testing mechanical splices, requires cycle testing 2 pieces of rebar joined by a coupler while measuring movement across the splice • Tests can be tension-tension cycling only or demand tension-compression cycling like the requirements of AC133, CS2, ISO 15835 or ASTM A1034 Example test stages from AC133:
  27. 27. 27 Cycle Testing Mechanical Couplers • High-pressure, side-acting grips provide a positive clamping force allowing for tensile and compressive loading • Extensometers measure elongation across the splice and also on the rebar pieces that are joined together • Test software records strain at yield on first cycle and automatically determines strain targets for remaining cycles
  28. 28. 28 Rebar Testing Challenges: Overcome Understanding these challenges and their potential impacts is the first step towards improving your rebar testing program • Irregular Specimens • Violent Failures • Strain and Elongation • Repeatable Results • Bend Testing • Testing Couplers
  29. 29. Are you aware of any of these challenges in your testing program? Is your quality program taking risks if they are not addressed? Visit Instron.com for more on rebar testing solutions

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