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.
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
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
Hinweis der Redaktion
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.