Challenges of Rebar Testing
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This document discusses challenges facing rebar manufacturers and testing programs due to expected increases in construction volumes and changes in rebar specifications. It identifies challenges such as testing irregularly shaped rebar, absorbing high energy failures, accurately measuring strain and elongation, ensuring repeatable tensile test results, performing bend tests, and cycle testing mechanical coupliers. It emphasizes that testing programs should be prepared to address these challenges to avoid risks to product quality as construction and rebar standards evolve.
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Challenges of Rebar Testing
- 1. Instron® presents an overview of changes and challenges facing rebar manufacturers, suppliers, and consumers. Challenges of Rebar Testing
- 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 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 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 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 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 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 Challenge: Irregular Surface Rebar’s irregular surface geometry and scaling during elongation present several gripping challenges
- 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 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 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 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 Challenge: Violent Failures Rebar specimens exhibit violent failures with significant recoil (over 60G of acceleration), which can lead to increased system wear.
- 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 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 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 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 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 Integrate Manual Elongation • Consider digitally interfaced specimen measuring tools • Push button data transfer • Saves time • Reduces manual input errors
- 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 Automatic Capture of Test Results Today, all calculation results can be performed automatically, saving time and reducing operator and system variability
- 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 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 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 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 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 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 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. 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
Editor's Notes
- These growth rate projections will mean New and repairs on bridges, roadways, dams, tunnels, mass transit systems, water and sewage systems, airports, hotels, stadiums, shopping centers…Growth regions include Emerging Market regions Russia, China, India as well as US, Middle East, Canada, Australia, Africa and moreInfrastructure and housing sectors to show rapid growthParts of Western Europe show the least potential for growth.According to “Global Construction 2020” reports, Construction in the key emerging markets and cities of Asia Pacific is expected to more than double over the next decade to 2020, providing significant growth opportunity for all parts of the construction industry.Housing is the largest global sector for construction, whilst the global infrastructure market is expected to show the highest growth in the next decade. Global sports events, nuclear and renewable energy and other key sectors, helping to drive and shape demand, will be highlighted, along with the key challenges the industry faces in dealing with the environment and climate changeGlobal Construction 2025 launched on 1 July 2013. This new report just released forecasts the volume of construction output will grow by more than 70% to $15 trillion worldwide by 2025.The global construction industry is one of the largest in the world contributing to 13% of the global GDP and there seems to be no signs of slowing down. According to the Global Construction 2020 report by Global Construction Perspectives and Oxford Economics, 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.Over half of the industry’s $4.8 trillion increase will come from the world’s (eventual) top three construction markets; China, United States and India.
- Increased throughput and load capacity?
- Increased throughput and load capacity?
- Allow it to expand and contract without compromising the structureBend without signs of crackingOxidation of bar prevented by being fully encased in concrete. Exposure of bar to air would allow for oxidation and degradation (swelling and scaling).
- Various rebar product standards link back to one or more international testing standards.The goal behind this is so the testing procedures can be standardised, this also prevents the duplication of effort.Guidelines for Rebar testing are widely available in many common testing and product standardsCompliance with standards helps ensure product quality and a better reputation in the marketplaceCommon Rebar Test Standards include:ISO 15630-1ASTM A615, A370 (Annex A9)GB 1499BS 4449, BS EN 100080JIS G3112
- The teeth of the grip jaws initially clamp onto the ridges of the specimenCan prevent specimen engagement and lead to slippageAggressive tooth patterns or too much clamping force can cause premature specimen failure Surface scale can build up in teeth of jaws and lead to slippage. Scale can also get into mechancial parts of some grip designs and cause premature damage.
- Course tooth patterns are typically 3 to 8 teeth per cm (8 to 20 teeth per inch)Groove cut in bottom of vee allows for rebar to be easily removed after failure.Relief machined at leading edge of jaw face to prevent grip breaks.Teeth patterns that are too aggressive can cause jaw breaks.Grips designed so single set of jaw faces accept a wide range of specimen diameters. User does not have to change jaws frequently.
- The teeth of the grip jaws initially clamp onto the ridges of the specimenCan prevent specimen engagement and lead to slippageAggressive tooth patterns or too much clamping force can cause premature specimen failure Surface scale can build up in teeth of jaws and lead to slippage. Scale can also get into mechancial parts of some grip designs and cause premature damage.
- The teeth of the grip jaws initially clamp onto the ridges of the specimenCan prevent specimen engagement and lead to slippageAggressive tooth patterns or too much clamping force can cause premature specimen failure Surface scale can build up in teeth of jaws and lead to slippage. Scale can also get into mechancial parts of some grip designs and cause premature damage.Content on slide applies to either the IPG standard wedge grips or DuraSync grips. DuraSync grips are a bit better at dealing with severely bent specimens.
- Standard rebar sizes require test system capacities up to 2,000 kN (400,000 lbf).Larger, higher-grade bars require capacities up to 4,000 kN (800,000 lbf).
- Grip actuation and load frame actuation are sealed form dirt and debris and are able to absorb a lot of the energy that is released at specimen failure.EM systems do not stand up to the repetitive shock to ball screws. Similar length specimens means shock loads are always going to impact same region of ball screws on EM systems.Robust, Internal Grip actuation absorbs energy at failure.Dual Test Space systems (DX and HDX models) feature Integrated Backlash Eliminators that prevent damage to screw columns
- AutoX750 – Automatic ExtensometerAutomatic setting of gauge lengthAutomatic attach and release After Agt (Extension at Max Load)Software commands instrument to auto release after Max ForceAutomatic recording of Percentage Elongation at Max Force (Agt)Can remain on through fracture for most sizes and systemsAutomatic recording of Percentage Elongation After Fracture (A)Accuracies are suitable for all yield and elongation test resultsAdd Reference to Video of AutoX750 remaining attached through failure (SharePoint??)
- AutoX750 – Automatic ExtensometerAutomatic setting of gauge lengthAutomatic attach and release After Agt (Extension at Max Load)Software commands instrument to auto release after Max ForceAutomatic recording of Percentage Elongation at Max Force (Agt)Can remain on through fracture for most sizes and systemsAutomatic recording of Percentage Elongation After Fracture (A)Accuracies are suitable for all yield and elongation test resultsAdd Reference to Video of AutoX750 remaining attached through failure (SharePoint??)
- Many extensometers need removed prior to failure, so measurements must be done manually.NOTE: Round robin testing may still require the manual method to be used for Elongation after Fracture (Even if you have Automatic capability). The rest of the time, the automatic method could still be used.
- In some cases, the manual method for elongation after fracture is still required for referee testing.The customers of rebar suppliers may also demand the results to be taken manually. In either case, we can supply gauge marking tools and specimen measuring tools as required.
- Bend fixtures can be large and require longer test stroke than tensile test requirements.
- Bend fixtures include multiple loading nose diameters to match rebar diameters. The larger the bar diameter, the larger the radius of the loading nose and the longer the required test stroke.NOTE: Single test space systems like KPX or LX models will require removal of the fixture when tensile testing. In these cases, it may be more cost-effective to have a separate bend testing system.NOTE: Smaller capacity DX models have 150mm (6 inches) of stroke and may be limited to bend testing smaller diameter bars only.
- On the job site, rebar lengths are often spliced together using mechanical couplers.This test is particular popular in earthquake prone regions.
- After cycling, the specimen is pulled to failure. Failures are expected in the rebar, not the coupler.