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An overview of recent changes and challenges facing rebar manufacturers, suppliers, and consumers

Challenges of Rebar Testing

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Why Now?• Construction average annual growth rate projections to 2025

*QatarSource: Engineering News-Record

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Why is this Important?

• Construction growth implications for rebar:

• Increased volumes

• Increased strengths and sizes

• Increased use of stainless grades

• Mechanical coupler requirements

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How Does this Impact You?• Are additional testing systems required to meet increased testing

volumes?

• Will a larger testing system (load capacity) be necessary to accommodate increased strengths and/or sizes?

• Can current gripping solutions withstand stainless grades?

• Will you need to test mechanical couplers in the future?

• Is your testing program prepared for these expected changes?

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Steel Rebar Applications• Embedded in concrete to improve

tensile strength

• Deformations on bar for better adherence to the concrete

• Thermal expansion properties similar to concrete

• Structural shapes formed by bending rebar

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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

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Rebar Testing Challenges

• Violent Tensile Failures

• Irregular and Bent Specimens

• Automatic Tensile Results

• Strain and Elongation Measurement

• Cyclic Testing for Couplers

• Bend Testing

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Challenge: Irregular Surface

• Rebar’s irregular surface geometry and scaling during elongation present several gripping challenges

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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 – that reduces system maintenance costs and possible down time

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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.

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Gripping Bent Specimens

• Grips accommodate slightly bent specimens and will overcome small side-loads while still maintaining axial specimen alignment

• No need to reset (re-center) grip jaws between tests improves testing throughput

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Challenge: Violent Failures

• Rebar specimens exhibit violent failures with significant recoil (up to 60G of acceleration), which can lead to increased system wear.

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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

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Challenge: Measuring Strain

• Non-machined rebar specimens require extensometers with long gauge lengths

• Extensometers must be able to attach to the uneven surface of rebar and maintain contact during elongation

• Automatic elongation results require instrument to remain attached beyond maximum force or even failure

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Suitable Strain Measurement - Manual

• Securely clamps to the irregular specimen preventing strain errors due to slippage or undesired movement on uneven surface

• Instruments are uniquely identified by testing software that prevents accidental use of the incorrect instrument or gauge length

Manual Clip-On

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Suitable Strain Measurement - Automatic

• Adjustable gauge length accommodates a wide range of specimen diameters and gauge lengths with a single instrument

• Automatic clamping and release improves operator safety and reduces variability between operators

• Can remain attached through failure allowing for automatic recording of Ag, Agt, and A (Elongation after Fracture).

Automatic Contacting

Link to YouTube Video

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Automatic Elongation Measurement

• Test standards now 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 and fracture will reduce the need for manually marking the specimen and allows the operator to focus on more value-added activities

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Challenge: Automatic Test Results• Common test results for rebar include:

• ReH (Upper Yield, Halt of Force, Drop of Beam)

• Rp0.2 (0.2% Proof Strength, Offset Yield)

• Fm (Maximum Force, Peak Load)

• Rm (Tensile Strength, Ultimate Strength)

• Ag, Agt (Elongation at Maximum Force, Uniform Elongation)

• A (Percent Elongation after Fracture)

• Historically, many of these results have required manual identification (ReH), measurement (A), or calculation (Ag).

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Automatic Capture of Test Results• All calculation results can be automatically performed, saving

time and reducing operator and system variability

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Challenge: Bend Testing

• Rebar is frequently bent for use in concrete applications

• Test standards require a bend test to determine if cracks form on the outside of the bend

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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.

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Challenge: Coupler Testing• Seismic testing couplers for slippage, which is

comprised of 2 pieces of rebar joined by a coupler, requires cycling the specimen

• Tests can be tension-tension cycling only or demand through-zero cycling like the requirements of AC133, CS2, or ISO 15835

Example test stages from AC133:

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Cycle Testing Mechanical Couplers

• High-pressure, side-acting grips provide a positive clamping force to allow loading in both the tensile and compressive directions

• Testing software allows strain at yield stress to be recorded on initial cycle so it can automatically determine reversal points for remaining test cycles

Does this sound familiar?

Do you see the same challenges in your business?

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