Steel Reinforcement Bar (Rebar) – A Tensile Testing Guide
Steel Reinforcement Bar (Rebar) – A Tensile Testing GuideIntroductionSteel reinforcing bar, or rebar, is embedded in concrete toimprove the overall strength of the concrete that surroundsit. Material products standards exist to help ensure thatrebar produced throughout the world exhibits the samephysical, chemical, and mechanical properties regardlessof the source. Proper mechanical testing is then necessaryfor determining if the rebar meets its publishedspecifications, ensuring the quality of the product.Mechanical testing requirements for rebar can vary, buttypically fall into the following basic test categories: Tensile Bend Compression FatigueOther related product testing, such as slip testing ofmechanical splices (couplers), may also be required. Thisdocument will focus primarily on the very common, yetsometimes challenging, tensile test.Tensile Testing and StandardsAt a global level, technical committees governed by theInternational Organization for Standardization (ISO) developproduct and testing standards for reinforcement barproducts. In addition to specifying properties, such as theminimum upper yield strength (Reh), Rm/Reh ratio, andelongation values for ribbed steel bar products, ISO productstandards, such as ISO 6935-2, also specify how thetensile properties are to be measured. Unique testingrequirements for the product are included directly in thestandard and additional reference is made to ISO 15630-1,which focuses specifically on test methods for similarproducts. ISO 15630-1 provides further references to themore general metals tensile testing standard, ISO 6892-1,where applicable.ISOASTMRebar ProductStandard6935-2A615Rebar TestingStandard15630-1A370Metals TensileTest Standard6892-1E8On a regional level, many countries also have localstandards organizations that may have existed even beforethe global ISO committees were formed. They oftenmaintain their own product and testing standards or canelect to adopt the global ISO standards where appropriate.For example, in the US ASTM has long-developed productand testing standards for reinforcement bar. Productstandards, such as ASTM A615, A706, A955, and A996,provide minimum product specifications and also includeunique testing details for determining the tensileproperties. Reference may also be made to additionaltesting requirements found in ASTM A370. This steeltesting standard covers the mechanical testing of steelproducts. It then includes further reference to the primarymetals tensile testing standard, ASTM E8.Regardless of the governing body, the information providedin most global and local standards is quite detailed andintended to help the user understand the following basictesting requirements: Equipment required Associated terminology and symbols Specimen preparation Testing procedures or methods Calculations or results to be determinedEven though standards provide these thorough details,some aspects may still be left to the user’s interpretation,which can often lead to variations in how the testing isperformed. Additionally, if a lab is testing product to avariety of global or local standards, it can be challenging tofully understand and capture the subtle differences interminology and methodologies found in the differentstandards.This document is meant to act as a supplement to rebarproduct and testing standards and will attempt to providefurther explanation in areas that are commonlymisinterpreted or misunderstood by users. The content isintended to be general and summary in nature so it can beapplied regardless of what test standard is being followed.Table 1 – Examples of common rebar product and testing standardswww.instron.com Page 1 of 9
Specimen Deformations and ScalingEquipment ConsiderationsAccommodating Bent SpecimensAs the standards indicate, it is necessary to straightenrebar specimens prior to tensile testing. As a result, manytest pieces may still have a slight bend or non-linearity overtheir length. Therefore, it is best if the load frame and gripsare able to accommodate slightly bent specimens.Fig. 2 - Uncoiled rebar exhibiting slight bends over lengthGrips that mechanically clamp on center are recommendedin order to maintain axial alignment of the specimen.Hydraulic, side-acting grips, such as the Instron DuraSync design are best for addressing bent specimensbecause the mechanical balancing (synchronizing) betweenthe 2 sides allows them to always clamp on center evenwhen side loads from bent specimens are acting againstthe jaws closing. This helps improve alignment andeliminates the need to “reset” the grips between tests.Grip jaws (faces) must accommodate the deformations andscale that is common on the surface of rebar specimens.Buildup of scale in the teeth of the jaws can lead tospecimen slippage. Tooth patterns that are too aggressivecan cause premature specimen failures and may alsoprevent the specimen halves from being easily removedafter the test. Therefore, tooth profiles should allow scale tofall away naturally or be easily brushed away between tests.They should also alleviate the chance of failures that arecaused by the grips. If the broken specimen halves remainstuck in the jaw faces, the operator must dislodge themthrough use of a hammer or other means. This can reduceefficiency and add to operator fatigue and frustration.The mechanical functions of the grips should also beprotected against the falling scale. If scale is allowed to getbetween moving parts, critical surfaces can be galled andlead to poor performance or grip failure. It is important toregularly remove scale from the testing equipment to helpprevent unnecessary wear and tear.“Resetting” is typically associated with hydraulicallysynchronized grip designs that cannot clamp on centerwhen specimen side loads exist. Failure to reset thesetypes of grips between tests can allow for misalignmentbetween the upper and lower grips.Fig. 4 - Abundant scale accumulation on lower grip after one testFig. 3 - DuraSync side-acting grips clamping rebar specimen on centerwww.instron.com Page 2 of 9
Violent Specimen FailuresBecause rebar specimens release a lot of stored energyduring tensile failure, the testing system must be able towithstand the shock that results from the specimen recoil.The grips are impacted the most and must be robustenough to absorb the energy and still hold the brokenspecimen halves so they do not eject from the testingframe. Flying specimen pieces could become a safetyhazard to the operator and result in damage to theequipment. For all of these reasons, hydraulically actuatedgrips (wedge or side-acting) are recommended.Fig. 5 - #18 (57 mm) bar separation (recoil) after failurebars. Depending on the deformations, they can be attachedto the flat surfaces in between deformations or on alongitudinal rib if one exists.The most common extensometers used in rebar testing aremanual clip-on style instruments that are attached directlyto the rebar by the operator prior to running the test. If theinstrument is not designed to remain on through failure, itmust be manually removed by the operator after yieldingoccurs, but before the specimen fails. Manual instrumentsthat are designed to withstand specimen failure offeradvantages, but will likely experience faster wear of theknife edges if frequently used through failure.Fig. 6 - Manual, clip-on style rebar extensometerExtensometersExtensometers are not always required when testing rebar.If a distinct Yield Point (Upper Yield - Reh) is visible, theyield strength can be determined without an extensometerby reporting the Stress value at this point. Elongation afterFracture (ASTM and ISO) and the Total Elongation atMaximum Force (ISO) can both be determined manuallyafter the test from marks placed on the specimen surface.However, there are many times when an extensometermust be used in order to calculate results such as OffsetYield (Rp 0.2) or when determining elongation valuesautomatically from an extensometer instead of manuallyfrom specimen marks. In these cases, extensometers usedtypically have large gauge lengths compared to those usedon machined metals specimens. They must also be robustenough to withstand scale falling on them during testingand be able to attach to the uneven surface of deformedMost manual instruments are also designed to have a fixedgauge length. When testing many sizes of rebar with varyinggauge lengths, it is necessary to have severalextensometers that have unique gauge lengths. There aresome manual instruments on the market that can beconfigured for several different gauge lengths, allowing asingle instrument to cover most common requirements.Such devices will require the operator to manuallyconfigure the instrument properly between tests requiring adifferent gauge length.Automatic contacting instruments, such as the Instron AutoX750 offer several advantages over manual devices.Automatic removal and attachment allow the operator tostay out of the test space, eliminating any risks associatedwith specimen failures. The gauge length is setautomatically from software inputs and is infinitelyadjustable over the entire travel of the instrument, allowinga single instrument to cover all specimen requirements. Itwww.instron.com Page 3 of 9
can also be left on through failure if desired. Automaticinstruments are likely the best solution if automaticrecording of elongation measurements is required. This willbe addressed further in the results section.PretestDuring the pretest stage, the machine is made ready fortesting. The proper grips are installed and test openingadjustments are made. Prior to installing the specimen, theforce (load) measurement should be set to zero. Once thespecimen is loaded into the system, the force should NOTundergo any further “zeroing” as this will affect the testresults. If using a manual extensometer for measuringstrain, it should be attached to the specimen making sureto properly set the knife edges at the instrument’s gaugelength. The strain measurement should then be set to zeroprior to loading the specimen.PreloadingThe preloading stage is used to apply a minimal preload( 5% of expected yield strength) to the specimen in order toproperly seat it in the grips and to also aid in pulling thespecimen straight prior to testing. A plot of stress or forceversus crosshead or actuator displacement will typicallyshow significant displacement for a minimal increase inload due to the grips and load string pulling tight (taking upsystem compliance). If a preload is not applied and anextensometer is being used, many rebar specimens willshow negative strain at the beginning of the test as thespecimen straightens. Because of this and/or systemcompliance, the data during the preloading portion of thetest is often ignored or not recorded on the Stress-Straingraph.Fig. 7 - AutoX750 testing #11 (36mm) Rebar on a 1500KPXTesting Speeds and ControlOne of the more challenging aspects of complying with teststandards is determining how to properly and efficientlyexecute the tensile test. Despite standards providingspecific details for allowable test speeds and control modesfor the different stages of the test, it can still be difficult toperform the test properly. This may relate to both standardinterpretation challenges and the limitations of the testequipment.Details that influence test control and speeds can be foundscattered throughout various sections of test standards. Itmay also be necessary to reference more than onestandard in order to have all the required test setupinformation. This can make it very difficult to fullyunderstand all aspects of the test sequence and how tomake it work on a given testing system.For rebar tensile testing, it is helpful to break down thetensile test into the separate stages of the test. This appliesregardless of which test standard is being followed.The 5 basic regions are: Pretest Preload Elastic Region Yielding Plastic RegionOn servo-controlled systems, preloading is usually doneslowly using crosshead or actuator displacement feedbackfor controlling the test speed. Controlling preloading fromload, stress, or strain feedback is not recommended as itcould lead to undesirable and rapid acceleration until thespecimen is pulled tight in the grips.Depending on the amount of system compliance or slackthat was taken up (reduced) during the preload, it may benecessary or desirable to zero the strain measurement atthe end of preloading. However, caution must be taken soas to not adversely affect the overall strain measurement.In either case, test results that rely on strain from theextensometer should be adjusted so any non-linearbehavior at the very beginning of the test curve does notadversely affect any test results. This is addressed underthe Linear Slope section of results later in this document.www.instron.com Page 4 of 9
Elastic Region (before Yielding)The elastic region or straight line portion of the test as seenon the Stress-Strain plot can often exhibit some non-linearbehavior initially due to further straightening of the rebarspecimen. If using an extensometer, this may show up asslightly negative strain at the beginning of the test and isgenerally considered normal for rebar.Depending on the standard being followed, a variety of testcontrol and target speeds are allowed during the elasticregion and until the onset of yielding. The control andassociated rate used may depend on the equipmentlimitations or specific product being tested.When running the tests on servo-controlled systems, it isimportant to keep the following scenarios in mind. If usingcrosshead or actuator displacement control it is generallyacceptable to use the same control and speed throughboth the elastic and yielding portions of the test. However,if stress or strain feedback control is used, the test mustswitch to crosshead or actuator displacement control justprior to or at the onset of yielding.YieldingOnce yielding begins, many rebar grades exhibit a definedyield point that is seen as an abrupt bend in the StressStrain test curve. It is then followed by a period of specimenelongation with little to no increase in force. Because ofthis, servo-controlled systems must be controlled usingcrosshead or actuator displacement feedback in order tomaintain a constant rate of travel throughout yielding. It isvery important to note that using stress control duringyielding will cause the test to accelerate excessively, whichis in direct violation of the standards. This can also causethe yield point (upper yield) to be masked or smoothed andcause yield strength results to be higher than expected.Likewise, strain control from an extensometer can alsobecome erratic during yielding and, is therefore, notrecommended when testing rebar.Fig. 8 – Elastic and Yielding regions of a rebar Stress-Strain curvePlastic Region (after Yielding)As the standards clearly define, it is acceptable for the testspeed to be increased after yielding has completed. Forservo-controlled machines, the best way to control the testduring this final region is from crosshead or actuatordisplacement feedback (same as yielding). However, thespeed used can be increased according to the standardbeing followed. This allows for the test to complete in ashorter period of time while still producing acceptable andrepeatable results.ISO 6892-1:2009ASTM A370-14ELASTIC REGION(crossheadseparation rate)0.00025/sec * Lc0.0625 in/min * GLELASTIC REGION(strain rate)0.00025/secNot SpecifiedELASTIC REGION(stress rate)6 – 60 MPa/sec10-100 ksi/minYIELDING(crossheadseparation rate)0.00025/sec * Lc0.0625 in/min * GLPLASTIC REGION(crossheadseparation rate)0.0067/sec * Lc0.5 in/min * GLTable 2 – Target test rates for rebar test regionswww.instron.com Page 5 of 9
Results NomenclatureTest standards incorporate terms, result names, andsymbols to properly identify critical information soughtduring testing. It is very important to fully understand thisinformation in order to ensure standards compliance andproper results reporting. If testing to multiple standards, itis also necessary to understand the similarities anddifferences between these items. In some cases, standardsorganizations can use different terms or result names torefer to the same property. The following table shows a fewcommon examples of results that are found in ISO andASTM standards. You can see from the table where thereare similarities and also differences.ISOASTMYield Point(distinct)Upper Yield Strength(ReH)Yield Point(Drop of Beam or Haltof Pointer)Yield Strength(Offset Method)0.2% Proof Strength,non-proportionalelongation(Rp 0.2)Yield Strength(0.2% Offset)Maximum StressTensile Strength(Rm)Tensile StrengthRatio of TensileStrength/YieldStrengthRm/ReHNot RequiredStrain atMaximum Force% Total Elongation atMaximum Force(Agt)Not RequiredElongation afterFracture% Elongation AfterFracture(A or A5)Percent ElongationTable 3 - Common rebar tensile results for ISO and ASTMResults – No ExtensometerFor lower grade bars that exhibit a distinct yield point, it ispossible to perform the entire test without the use of anextensometer. The yield point can be determined from thestress-extension test curve by locating the first point atwhich stress drops while extension continues to increase.On older testing systems, the yield point can be determinedmanually from witnessing the momentary drop of the loadpointer and calculating the stress from this load value andthe nominal cross sectional area of the bar.Fig. 9 – Distinct Yield Point on a stress – extension curveIn the previous section that discussed test control, it waspointed out that the machine actuator or crosshead shouldnot accelerate during yielding. This can lead to the yieldpoint being “hidden” on the test curve as the data getssmoothed as a result of the acceleration. If using a servocontrolled system, make sure the test control is fromactuator or crosshead extension throughout yielding. Ifusing a manually controlled system, be sure to maintain aconstant rate of crosshead separation during yielding. Ifyou are not seeing an expected defined yield point, begin byexamining the test control used.If no extensometer is used, elongation results such asthose in the previous table must be determined manuallyfrom marks placed on the specimen prior to testing. As thestandards describe, the broken specimen halves are placedback together after the test and manual measurements aretaken from the marks found on each side of the specimenfracture. If a dispute arises over elongation results, themanual method is typically required for resolving suchconflicts.Overall, the manual method of testing is relatively simple,but relies heavily on the operator to properly record theyield point and manual elongation measurements. Eachadditional manual step in the process can lead to reducedrepeatability and reproducibility of results betweenoperators and systems. This can put results at risk fordispute and may require more frequent retesting.www.instron.com Page 6 of 9
Results – With ExtensometerMany higher grades of rebar do not exhibit a distinct yieldpoint. In these cases it is usually necessary to determinethe yield strength from the offset method. This requiresmeasuring strain with an extensometer and plotting aStress-Strain curve from which a 0.2% offset yield strength(Rp 0.2) can be determined.Most modern testing systems are capable of automaticallygenerating the yield strength. However, it is important toverify and validate the test method setup to make sure it isdelivering consistent and accurate yield strength results.The following areas should be of particular focus.Linear SlopeThe test standards describe various approaches for fitting aline to the linear portion of the test curve. This line is meantto represent the slope of the elastic region of the curve andcan intersect the strain axis somewhere other than theorigin due to grips seating and the load string pulling tightas described previously in th
ASTM. Rebar Product Standard 6935-2 A615 Rebar Testing Standard 15630-1 A370 Metals Tensile Test Standard 6892-1 E8 . Table 1 – Examples of common rebar product and testing standards . On a regional level, many countries also have local standards organizations that may have existed even
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440.6-08, Specification for Carbon and Glass Fiber-Reinforced Polymer Bar Materials for Concrete Reinforcement, has requirements for the tensile strength of rebar products. Neuvokas basalt rebar will meet or exceed the requirements for glass FRP rebar in all cases. This ACI documents lists the required strength for glass FRP rebar, #3 size, at 110
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