High-Strength Steel Reinforcement (ASTM A1035/A1035M ...

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High-Strength Steel Reinforcement (ASTM A1035/A1035M Grade 690): State-of-

the-Art Review

Article  in  Journal of Structural Engineering · June 2020

DOI: 10.1061/(ASCE)ST.1943-541X.0002720

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State-of-the-Art Review

High-Strength Steel Reinforcement(ASTM A1035/A1035M Grade 690): State-of-the-Art Review

Saif Aldabagh, A.M.ASCE1; and M. Shahria Alam, M.ASCE2

Abstract: High-strength steel (HSS) and ASTM A1035 Grade 690, in particular, have been gaining popularity in the last two decades due toits considerably higher strength and corrosion resistance compared to conventional steel, i.e. ASTM A615 Grade 420. The stress-strainresponse of HSS is different from that of conventional Grade 420 steel because it lacks a well-defined yield point and yield plateau.Consequently, extensive research studies have been carried out to evaluate the performance of HSS reinforcement in structural concreteto examine the adequacy of current design provisions when a design yield strength of up to 690 MPa is used. In order to accommodateHSS reinforcement, the American Concrete Institute (ACI) released Design Guide for the Use of ASTM A1035/A1035M Grade 100 (690)Steel Bars for Structural Concrete (ACI 439.6R), whereas the AASHTO LRFD Bridge Design Specifications initiated the NationalCooperative Highway Research Program (NCHRP) Project 12-77 to evaluate the provisions relevant to the use of high-strength reinforcingsteel and other grades of reinforcing steel having no discernable yield plateau. This manuscript provides a synthesis of these two reports in asystematic way along with the other relevant research works. In addition, a comprehensive comparison between the requirements of ACI439.6R, AASHTO, and ACI 318, relevant to the use of HSS reinforcement, is presented. Specified yield strengths for HSS, when used indifferent applications set by the three codes, are provided in a summary table. The provided table is intended not only for comparisonpurposes but also to facilitate the use of HSS reinforcement by designers because it contains all of the important and most recent relevantclauses in the three codes. Lastly, future research recommendations are proposed to revise and increase the specified yield strength of HSS incertain applications to enable designers to take full advantage of the potential benefits of HSS reinforcements. DOI: 10.1061/(ASCE)ST.1943-541X.0002720. © 2020 American Society of Civil Engineers.

Author keywords: High-strength steel; Production methods; Corrosion; Mechanical properties; Flexure; Shear; Bond strength; Seismicapplications.

Introduction

As technology advances, new types of alloys are being introducedby steel reinforcement manufactures to produce reinforcing bars ex-hibiting certain desirable characteristics. This is normally achievedby altering the chemical composition whose effect on the propertiesof the alloy is well understood. High-strength steel (HSS) reinforce-ment offering higher strength and enhanced corrosion resistance isone of the common alternatives to conventional steel in the concreteconstruction industry. Steel reinforcing bars are generally classifiedas HSS bars when exhibiting a yield strength of more than 400MPa,i.e. the yield strength of conventional steel [ACI 439.6R (ACI2019)]. Such types of reinforcement are being produced in severalcountries and are designated based on the material standards adoptedby each country. Examples include ASTMA706 Grade 550 (ASTM2016) and ASTM A1035 Grade 690 and Grade 830 (ASTM 2019)in the United States and Canada; USD685A and USD685B in Japan[not yet recognized by the Japanese Industrial Standard (JIS)];AS/NZS 4671 Grade 500E and 500N in Australia and New Zealand[AS/NZS 4671 (AS/NZS 2001)]; and HRB500 and HRBF500 inChina. Although all of these types are characterized with yield

strength greater than 400 MPa, their mechanical properties arenot identical. The stress-strain response varies among the differenttypes. The stress-strain curve of the ASTM A1035 reinforcementlacks a well-defined yield plateau, hence establishing its yieldstrength requires further attention when compared to other typesof HSS reinforcement, as will be illustrated in subsequent sectionsof this study. ASTM A1035 Grade 690 steel reinforcement, whichis the most common type of HSS in the United States and Canada,is the focus of this manuscript. In this manuscript, HSS refers toASTM A1035 Grade 690 and conventional steel refers to ASTMA615 Grade 420 unless otherwise noted. Practicing structural en-gineers in the construction industry are facing different challenges,such as the congestion of reinforcing steel in concrete sections pos-sibly due to stringent architectural requirements and the corrosionof steel reinforcing bars in concrete members due to the corrosivesurrounding environment. HSS, with its enhanced strength and cor-rosion resistance, is an effective tool to address these challenges.Clearly, higher yield strength relieves the rebar congestion byreducing the amount of required reinforcement. Due to this reduc-tion, the placement productivity of reinforcement and concreteworkability is improved, thereby promoting a reinforced concreteof higher quality. Besides improving concrete quality, the reductionin the amount of reinforcement leads to bringing down the overallcost through savings associated with labor cost, which is a functionof the material weight. In addition, HSS reinforcement with its in-herent corrosion resistance property promotes enhanced long-termdurability in concrete structures exposed to severe environmentalconditions. Thomas et al. (2013) assessed the impact of usingASTM 1035 Grade 690 versus ASTM A615 Grade 420 steelreinforcement on the construction productivity by developing

1Graduare Research Assistant, School of Engineering, Univ. of BritishColumbia, Kelowna, BC, Canada V1V 1V7.

2Professor, School of Engineering, Univ. of British Columbia, Kelowna,BC, Canada V1V 1V7 (corresponding author). ORCID: https://orcid.org/0000-0002-9092-1473. Email: [email protected]

Note. This manuscript was published online on June 2, 2020. Discus-sion period open until November 2, 2020; separate discussions must besubmitted for individual papers. This paper is part of the Journal of Struc-tural Engineering, © ASCE, ISSN 0733-9445.

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two different designs of a hospital using the two types of reinforce-ment. HSS reinforcement was found to be effective in reducing theweight of reinforcement in beams but not in slabs, posttensionedgirders, and columns. This was due to certain code limitations re-lated to a minimum reinforcement requirement preventing a signifi-cant reduction in the required steel area. The study recommendedreplacing conventional steel with HSS in expensive labor marketswhere weight reduction in reinforcement can lead to substantialsavings in the labor cost. Although, at the time of the study, theunit material cost of HSS reinforcement was two times that ofASTM A615 reinforcement, it is anticipated that the price ofASTM A1035 reinforcement will decrease in the future as moremanufacturers start to produce it. In Virginia, as part of the Inno-vative Bridge Research and Construction Program (IBRCP), twoseparate bridge decks were built to compare the placement costsof epoxy-coated reinforcing (ECR) steel and HSS (Sharp andMoruza 2009). Crack sealing operation was required for the bridgedeck reinforced with ECR, which resulted in a significant increasein the unit cost of ECR, making it less cost-effective compared toHSS. Shen et al. (2019) implemented a Monte Carlo simulation toquantify the probabilistic life-cycle cost of bridge decks reinforcedwith conventional steel and HSS. HSS was found to have lowerlifecycle costs in bridge decks compared to conventional steel.Over the last two decades, several state departments of transporta-tion (DOTs) across the United States, such as Virginia (VDOT),Michigan (MDOT), and Utah (UDOT), among others, have beenstudying the applicability of replacing ECR with HSS to achievea service life of 75 years. HSS with its superior corrosion resistancecompared to ECR allows for extending the useful life of highwaystructures. Demonstration projects, with some involving the con-struction of full-scale bridge decks, indicated that, with properdetailing, bridge decks reinforced with HSS are characterized withsatisfactory in-service performance, lower lifecycle costs, andan extended anticipated service life compared to bridge decksreinforced with conventional steel (Kahl 2007; Barr and Wixom

2009; Salomon and Moen 2017). Rebars conforming to ASTMA1035 are listed as an alternative corrosion-resistant reinforcementin bridge design manuals of 11 DOTs in the United States. TheseDOTs are Washington, Oregon, California, Montana, Idaho, Texas,Mississippi, Florida, Virginia, Maryland, and Maine DOTs. Usingdeicing salt, such as sodium chloride (NaCl), causes significantdamage to concrete and corrosion of reinforcement in bridge decks.Fig. 1 provides a map of the contiguous United States showing theaverage amount of road salt sold to each state annually averagedover a three-winter period and normalized to the area of each state(Panno et al. 2005). In states where the corrosion of a bridge deckreinforcement could be critical, i.e. the amount of used road saltexceeds 500 kg=km2=year, only Maine, Maryland, and VirginiaDOTs have considered ASTM A1035 as an alternative corrosionresistant steel. In order to allow designers to make use of the po-tential benefits of HSS, design codes, particularly in the UnitedStates, have been evolving to provide guidelines to design concretestructures reinforced with HSS reinforcement. In 2010, the AmericanConcrete Institute (ACI) published the Design Guide for the Useof ASTM A1035/A1035M Grade 100 (690) Steel Bars for Struc-tural Concrete [ITG-6R-10 (ACI 2010)]. The recommendationsprovided in that guide were mainly addressing those requirementsof ACI 318 that limit the efficient use of the HSS in structuralconcrete. Just recently, ACI Committee 439 released ACI 439.6R,which is an updated version of the ACI ITG-6R, hence supersed-ing it [ACI 439.6R (ACI 2019)]. In addition, in 2007, the Na-tional Cooperative Highway Research Program (NCHRP)project 12-77 was initiated to evaluate and propose appropriateguidelines to the AASHTO LRFD Bridge Design Specificationsto design concrete structures reinforced with ASTM A1035reinforcement as well as other types of steel reinforcement lack-ing a discernable yield plateau (Shahrooz et al. 2011). The projectrecommendations were first incorporated into the 2013 interimrevisions of the AASHTO LRFD Bridge Design Specificationsand are currently presented in the eighth edition of the AASHTO

Fig. 1. Map of the contiguous US showing the average amount of road salt sold to each state annually averaged over a three-winter period andnormalized to the area of each state. [From Panno et al. (2005). Copyright © 2005 University of Illinois Board of Trustees. Used with permission ofthe Illinois State Geological Survey.]

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LRFD Bridge Design Specifications (AASHTO 2017). In addition,some DOTs released ASTM A0135 rebar-specific documents, suchas the Structural Design Guidelines for Concrete Bridge Decks Re-inforced with Corrosion-Resistant Reinforcing Bars, which was re-leased by VDOT in 2014 (Salomon and Moen 2014). On thecontrary, design codes in Canada still lag behind the US standardswhere the CSA A23.3-14 and the CSA S6-14 do not provide guide-lines specifically addressing HSS reinforcement (CSA 2014a, b).These design codes do not prevent the use of HSS reinforcementbut impose strict limitations on the value of the design strength ofreinforcement, consequently making it less efficient. However, de-spite the current strength limitations in Canada, to make use of thesuperior corrosion resistance of HSS, the Ministry of Transporta-tion in Alberta has recently updated its Bridge Structure DesignCriteria (BSDC) Version 7.0 to reflect that ASTMA1035 reinforce-ment or solid stainless steel are the only two acceptable types ofsteel to be used in bridges with Class 2 exposure (BSDC 2018).Similarly, the Ministry of Transportation and Infrastructure inBritish Columbia has introduced the ASTM A1035 reinforcementinto its bridge standards and procedures manual as alternativecorrosion-resistant reinforcing steel (BCMoTI 2016). The imposedlimitations on design strength are primarily arising from the uncer-tainty associated with the behavior of members reinforced withHSS rebars when subjected to different types of loads. This paperattempts to compile and present all the knowledge generated fromexperimental and analytical research programs performed on HSSreinforcement. First, an overview of the common methods adoptedby steel manufactures to produce HSS reinforcement is provided.Then, the main characteristics of different grades of HSS reinforce-ment from a material prospective are presented. After that, the per-formance of HSS as corrosion resistant reinforcement, along withseveral serviceability considerations are explored. The behavior ofbeams, slabs, and columns reinforced with HSS rebars when sub-jected to shear, bending moment, and/or axial load is evaluated.The relevant clauses (if found) in available design guides are alsodiscussed. The bond characteristics between HSS reinforcementand concrete, and the applicability of the current splice and devel-opment length related design procedures to HSS rebars, are exam-ined. The seismic performance of concrete structures reinforcedwith HSS rebars is evaluated. Lastly, existing challenges limitingthe applications of HSS reinforcement as well as future recommen-dations allowing for more efficient use of such reinforcement areprovided.

Production of High-Strength Steel

Three common methods are generally used to produce HSS,namely: (1) cold working, (2) microalloying, and (3) quenchingand tempering. These three methods are described next. Cold work-ing, or so-called cold rolling, is considered one of the earliest meth-ods developed to produce HSS rebars (Caifu 2010). This method isnormally performed at temperature levels below the steel recrystal-lization temperature. At the recrystallization temperature, disloca-tion and movement of particles are generated, hence forming acrystallographic defect or irregularity within the crystal structure.The formation of these defects strongly influences the yieldstrength and ductility of the reinforcement. The final product usingthis production method tends to be harder than conventional steelbut lacks a clear yielding plateau. In addition, with this method, theductility is adversely affected, and therefore, it is not recommendedto adopt the cold working method to produce steel reinforcementused in earthquake-resistant concrete structures (NIST 2014). Inmicroalloying, small percentages (ranging from 0.02% to 0.15%)

of titanium (Ti), niobium (Nb), or vanadium (V) are added to pro-duce the HSS (Caifu 2010). This method utilizes two mechanisms,which are fine-grain strengthening and precipitation strengthening.In fine-grain strengthening, the thermomechanical process isadopted to form very fine grains in the steel product through thepinning of grain boundaries, i.e. the area between the grains.According to the Hall-Petch relationship, reducing the grain sizeincreases the strength of the steel, hence maintaining a fine grainsize is essential to produce steel of a higher yield strength (Pandeand Cooper 2009). After the fine-grain strengthening, the precipi-tation strengthening is initiated when the intermetallic carbides aredispersed through the grains, and therefore pinning line defects(dislocations) are formed, causing more increase in the yieldstrength. Titanium microalloying is very effective in precipitationstrengthening, but the titanium tends to react and combine withoxygen, sulfur, and nitrogen, making it difficult to control the prop-erties of the strengthened product. Niobium microalloying requiresrelatively low temperature and high deformation at the end of pro-duction, making it unsuitable to produce HSS, which normally in-volves high rolling temperatures and less deformation (Gervas’evet al. 2019). This method is commonly used in the production ofsteel sheets and strips. Vanadium microalloying is widely used inthe production of high-strength weldable reinforcement. Theincrease in yield strength in vanadium microalloying is primarilydue to the precipitation of carbides and nitrides (Hamed et al.2018). In this process, only 35.5% of the vanadium forms carbidesand nitrides, while 56.3% of the vanadium does not contribute tothe yield strength because it turns into a solid solution dissolved inthe matrix. A higher percentage (up to 70%) of vanadium formingcarbides and nitrides can be achieved by the addition of nitrogen(Caifu 2010). The negative impact of stain aging on the steel prop-erties is eliminated due to the ability of vanadium to pin the solublenitrogen. Quenching and tempering is a two-stage heat-treatmentprocess used to produce steel reinforcement with higher strengthand toughness. The first step is quenching, which involves the rapidcooling of metals in water, oil, forced air, or inert gases after heat-ing them to the austenitic phase, i.e. when the solid steel starts tocrystalize. The second step is tempering in which the quenchedsteel is heated to modify the microstructure leading to a decreasein toughness and an increase in ductility (Yan et al. 2015).

Mechanical Properties of ASTM A1035Reinforcement

Typical stress-strain relationships of ASTM A1035 Grade 690reinforcing steel, along with that of ASTM A615 Grade 420, arepresented in Fig. 2 for comparison. ASTM A1035 reinforcing steelexhibits higher tensile strength, yet it lacks a well-defined yieldpoint and yield plateau. The stress-strain curve of HSS is charac-terized by a linear portion up to a proportional limit (at stress from420 to 550 MPa), followed by a nonlinear relationship up to a ten-sile strength of 1,125 MPa. The strain at maximum tensile strengthranges from 0.04 to 0.06. The elongation in the gauge length of200 mm across the fracture for the bar ranges from 0.08 to 0.13[ACI 439.6R (ACI 2019)]. Others have reported elongation in200 mm across the fracture in the range of 0.08–0.10 (WJE2008). The reason for the noticeable variation in the rupture strainamong different studies is the inaccuracy associated with the straingauge and extensometer measurements when capturing the ultimatebehavior (Shahrooz et al. 2011). By comparison, the elongation inthe 200 mm gauge length across the fracture for ASTM A615Grade 420 ranges from 0.09 to 0.12 and that for ASTM A706Grade 420 ranges from 0.14 to 0.20 [ACI 439.6R (ACI 2019)].

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The initial modulus of elasticity is 200 GPa, which decreasesslowly as the stress approaches the proportional limit and decreasesmore rapidly thereafter as evident in Fig. 2. Due to the absence of aclearly defined yield plateau, the yield strength can be establishedby a number of methods: (1) 0.35% extension under load (EUL)method; (2) 0.5% EUL method; and (3) 0.2% offset method beingthe most common. In the 0.2% offset method, the yield strength isthe stress on the engineering stress-strain curve at its intersectionwith a line having a slope equal to the modulus of elasticity ofsteel and starting from a strain of 0.2%. In the 0.35% and 0.5%EUL methods, the yield strength is the stress corresponding toa strain of 0.0035 and 0.005, respectively. Fig. 3 illustrates howthese methods are used to establish the yield strength of ASTMA1035 reinforcing steel. Yield strength values determined basedon the 0.2% offset method are characterized with the most variabil-ity (COV ¼ 10.3%), while those determined based on absolutestrain approaches are found to be consistent at each strain levelconsidered (COV ¼ 7%). According to the NCHRP Report 679,regardless of the method used to determine the yield strength, thecondition that fu > 1.25fy is always stratified (Shahrooz et al.2011). The ASTM A1035/A1035M standard specifies two mini-mum yield strength levels based on the 0.2% offset method, whichare 690 and 830 MPa. Rebars meeting the minimum yield strengthrequirement of 690 and 830 are designated as Grade 690 and Grade830, respectively (ASTM 2019). The measured yield strength ofASTM A1035 Grade 690 steel, based on the 0.2% offset method,normally exceeds 830 MPa, i.e. a minimum yield strength forGrade 830, hence steel manufacturers are mainly focusing on pro-ducing only Grade 690 (WJE 2008). Two functions have been

proposed to accurately capture the stress-strain relationship ofASTM A1035 reinforcing steel, which are the Ramberg-Osgood(R-O) function (Ramberg and Osgood 1943) and the PCI stress-strain equation for prestressing steel. The constants in both equa-tions were calibrated to closely match the measured stress-strainresponse of the ASTM A1035 reinforcing streel (Shahrooz et al.2014; Mast et al. 2008). The final calibrated version of the PCIstress-strain equation to predict the stress in steel (fs) beyond theproportional limit is as follows:

fs ¼�1172 − 2.379

εs þ 0.00104

�ð1Þ

where εs = corresponding reinforcing steel tensile strain rangingfrom 0.00241 to 0.060. The final version of the R-O function,which was calibrated using US customary units to fit the experi-mentally measured stress-strain response of #8 bar size, is givenin Eq. (2) (Shahrooz et al. 2014)

fs ¼ 29000εs

�0.0145þ 1 − 0.0145

½1þ ð200εsÞ2.4�1=2.4�

≤ fpu ð2Þ

where fs = stress in the steel in ksi; εs = corresponding strain;and fpu = ultimate strength in ksi. DeJong et al. (2006) reportedthat the fatigue performance of HSS is superior to that of conven-tional Grade 420 steel. Their tests demonstrated that HSS rebarsexhibit a fatigue strength (at N ¼ 1 million cycles) of 310 MPa,while that of Grade 420 steel is 165 MPa. Extending the applicabil-ity of fatigue or endurance limit of conventional steel found in theAASHTO LRFD Bridge Design Specifications to reinforcing steelfor higher-strength steel yields conservative results (Soltani et al.2012). Fatigue considerations are expected to have little to no effecton the design of concrete structures reinforced with steel havingfy ≤ 690MPa (Soltani et al. 2012).

Corrosion Performance of ASTM A1035 ReinforcingSteel

ASTM A1035 reinforcing steel is a microcomposite Fe-C-Cr-Mnalloy furnished to three different chemical compositions, desig-nated as Alloy Type CL, CM, and CS. The chromium contentranges from 2% to 10.9% in these alloys, with an average value of9% (ASTM 2019). The 9% chromium content is too low for theASTM A1035 reinforcing steel to be classified as stainless steel(Cr > 10.5%) but sufficiently high to generate corrosion resistancesuperior to that of conventional steel, i.e. ASTM A615 or A706(Shahrooz et al. 2011). A significant amount of research has beencarried out to evaluate the corrosion performance of the ASTMA1035 steel reinforcement and compare it to that of other commonalternatives, such as ASTM A615 steel, LN316 stainless steel,epoxy-coated conventional steel, and ASTM A767 (galvanized)steel. Table 1 provides a summary of the selected research worksquantifying the corrosion performance of these types of reinforce-ment (the reader may refer to the cited work for further details). Itis evident from Table 1 that ASTM A1035 reinforcement exhibit2–10 times more corrosion resistance than the ASTM A615steel. This is attributed to the formation of a dense chromiumoxyhydroxide-based film, which is more corrosion-resistant thanthe passive layer formed on carbon steel. ASTM A1035 steel ex-hibits a lower corrosion resistance performance than the epoxy-coated A615 steel. Darwin et al. (2002) reported that bridge decksincorporating ASTM A1035 reinforcement are less cost-effectivethan the decks reinforced with epoxy-coated steel. However, theirconclusion was based on a life-cycle cost analysis that did not

Fig. 2. Stress-strain curves for ASTM A1035 Grade 690 and ASTMA615 Grade 420 reinforcing steel.

Fig. 3. Different methods to establish the yield strength of ASTMA1035 reinforcing steel.

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consider material savings resulting from the use of ASTM A1035reinforcement, possibly due to design code restrictions on thedesign yield strength of ASTM A1035 reinforcement at that time.In addition, ASTM A1035 steel was found to be substantiallyless corrosion-resistant than LN316 stainless steel. Ji et al. (2005)reported that bridge decks containing LN316 stainless steel arecharacterized by lower total life-cycle cost when compared toASTM A1035 and A615 epoxy-coated steel. However, it is well-established that, due to its high initial cost, utilizing LN316 stain-less steel can only be justified in parts of the structure exposedto chlorides or other severe environments. ASTM A1035 steelreinforcement offers an intermediate corrosion performance, be-tween that of ASTM A615 and LN316 stainless steel, at a costcheaper than that of LN316 stainless steel, making it more suitablefor a wider range of structural applications when compared to theother corrosion-resistant reinforcement alternatives.

ASTM A1035 Reinforcing Steel in Flexural Members

Longitudinal Reinforcement

The use of HSS bars as an alternative to ASTM A615 Grade420 bars in flexural members would not be effective in reducingreinforcement quantities or possibly member cross-section sizesif there are limitations on the design yield strength. Hence, Mastet al. (2008) and Shahrooz et al. (2014) evaluated ACI 318 andAASHTO provisions related to the use of high-strength reinforcingsteel in flexural members, respectively. Prior to and including theACI 318 (ACI 2011), reinforcement yield strength used for the de-sign (defined as the stress corresponding to the strain of 0.0035)must not exceed 552 MPa [ACI 318 (ACI 2011)]. Similarly, priorto and including the AASHTO (2012) edition of the AASHTOLRFD Bridge Design Specifications, adopting a design yieldstrength value greater than 517 MPa was not permitted (AASHTO2012). These limits prevented designers from taking full advantageof HSS reinforcement in flexural members. Mast et al. (2008)carried out a numerical analysis on a rectangular singly reinforced

concrete section having different reinforcement ratios to assess theadequacy of using a proposed idealized elastic-perfectly plasticstress-strain curve (similar to that used for ASTM A615 Grade420) for ASTM A1035 Grade 690 steel in a section analysis sat-isfying equilibrium and strain compatibility. The idealized elastic-plastic stress-strain relationship consists of an elastic portion witha modulus of elasticity of 200 GPa followed by a perfectly plasticbehavior when fy ¼ 690 MPa. Mast et al. (2008) established theappropriate tension-controlled and compression-controlled strainlimits for the proposed simplified material model. Tensile strains inthe reinforcement closest to the bottom surface greater than 0.009 arerequired to ensure tension-controlled behavior. Compression-controlled behavior occurs when these tensile strains are less than0.004 (Mast et al. 2008). Simple beams reinforced with ASTMA1035 Grade 690 reinforcing steel and designed at a tension-controlled strain limit of 0.009 exhibit comparable ductility behav-ior to those of the beams reinforced with ASTM 615 Grade 420 ata tension-controlled strain limit of 0.005 (Mast et al. 2008). Onthe other hand, Shahrooz et al. (2014) carried out a similar analysisbut considered sections having different concrete compressivestrengths and tension and compression longitudinal reinforcementratios. Their recommendations were very similar to that of Mastet al. (2008) except for the following: (1) the yield point in theidealized elastic-perfectly plastic stress-strain curve is taken asthe stress corresponding to strain equal to 0.0035 or 0.005 ratherthan 690 MPa (fixed value); and (2) the tension-controlled strainlimit is 0.008 instead of 0.009. The flexural strength reductionfactor ϕ versus extreme tensile strain, εt, relationships proposedby Mast et al. (2008) and Shahrooz et al. (2014) are presentedand compared against that of ASTM A615 Grade 420 in Fig. 4.

The recommendations of Mast et al. (2008) on computingthe flexural resistance of concrete beams reinforced with ASTMA1035 were incorporated into the ACI ITG-6R (ACI 2010) (cur-rently referred to as ACI 439.6R). Despite that, ACI 318 still doesnot permit the use of ASTMA1035 steel as a longitudinal reinforce-ment in flexural members [ACI 318 (ACI 2014)]. Shahrooz et al.’s(2014) recommendations are currently presented in the eighthedition of the AASHTO LRFD Bridge Design Specifications,

Table 1. Quantitative measures of corrosion performance

Performancemeasure Test Units

ASTMA615

ASTMA1035

316LNstainless

Epoxy-coatedA615

GalvanizedA767 Citation

Time to corrosioninitiation

Accelerated corrosion Days 92 245 >1,082 — — Clemena andVirmani (2004)

Time to corrosioninitiation

Rapid macrocell Weeks — 21.4 — — — Farshadfar (2017)

Time to corrosioninitiation

Modified version of ASTM G109 Years 2.3 15 — — 4.8 Darwin et al.(2007)

Critical chloridethreshold

lb=yd3 1.63 6.34 19.14 — 2.57

Critical chloridethreshold

Accelerated chloride threshold (ACT) kg=m3 0.5 4.6 10.8 — — Trejo and Pillai(2004)

Critical chloridethreshold

Tests include rapid macrocell tests,corrosion potential tests, bench-scaletests (the southern exposure andcracked beam tests), and two modifiedversions of the southern exposure test

kg=m3 0.91–1.22 3.70–4.07 — — — Ji et al. (2005)

Weight loss Accelerated corrosion (after 56 weeks) % 19.21 2.43 — — — Seliem (2007)Corrosion rate Pore solution (after 26 weeks) mA=m2 43–50 10 — — — Fahim et al.

(2019)Cracked-beam test (after 420 days) μm=year 4–8 2.3 — — —Corrosion rate Macrocell μm=year 35.64 12 — 4.2 — Gong et al.

(2002)Bench-scale (southern exposure test) 5.6 1.56 — 0.31 —Bench-scale (cracked-beam test) 4.84 2.7 — 0.92 —

Corrosion rate ASTM B117 μm=year 914.7 625 1.2 — 1,190.2 WJE (2006)

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which permits the use of ASTM A1035 Grade 690 in flexuralelements in Seismic zone 1 (AASHTO 2017). According toAASHTO (2017), the value of fy used in the design of concretebeams reinforced with HSS is equal to the specified minimum yieldstrength defined in the material standards. Consequently, for ASTMA1035 Grade 690, a design fy of 690 MPa is appropriate becauseit is the specified minimum yield strength in the ASTM A1035(ASTM 2019) for ASTM A1035 Grade 690. Both the CSAA23.3-14 and CSA S6-14 do not permit the use of ASTM A1035reinforcing bars and limit the design yield strength value to500 MPa for other applicable types (such as CSA G30.18 Grade500) of deformed reinforcement with a yield strength greater than400 MPa (CSA 2014a, b). To prevent compression-controlledfailure in flexural members reinforced with ASTM A1035 bars,Mast et al. (2008) suggested using compression reinforcementwith fy ¼ 550 MPa. Aldabagh et al. (2018) demonstrated thatcompression reinforcement had little to no effect in controllingthe concrete compression failure of concrete beams reinforced withASTM A1035 Grade 690 bars. In addition, they reported that fiber-reinforced concrete is more effective in enhancing the flexural char-acteristics of such beams than compression steel.

A minimum amount of longitudinal reinforcement computed inaccordance with ACI 318 requirements using fy ¼ 690 MPa mustbe provided at every section where tension reinforcement isrequired [ACI 439.6R (ACI 2019)]. Puranam (2018) confirmed thatreducing the minimum longitudinal reinforcement ratio in slabsreinforced with HSS in inverse proportion to the increase in theyield strength is acceptable. Experimental tests of one-way slabsreinforced with HSS having a longitudinal reinforcement ratioas small as 0.09% exhibited a rotational capacity greater than4% (Puranam 2018). Moment redistribution is inapplicable tomembers containing ASTM A1035 bars until further research databecomes available [ACI 439.6R (ACI 2019)]. However, Puranam(2018) reported that beams designed to have a target tensile strainof 0.005 exhibited sufficient rotational capacity for moment redis-tribution (Puranam 2018).

Transverse Reinforcement

The performance of ASTM A1035 steel as transverse reinforce-ment in flexural members was first evaluated by Sumpter et al.(2009). The main two variables considered in their work were

the stirrup spacing (152, 102, and 76.2 mm) and the type of lon-gitudinal and transverse steel reinforcement (ASTM A1035 andASTMA615). Their main focus was the shear behavior under over-load conditions in which the steel is subjected to high stress levels.Direct replacement of ASTM A615 stirrups with ASTM A1035stirrups resulted in a small increase in the shear-carrying capacityand improved serviceability in terms of crack distribution and crackwidth. They demonstrated that with fy ¼ 552 MPa, the use of ACI,CSA, and AASHTO design codes leads to conservative estimatesof the shear strength, with the CSA being the most accurate. Be-cause the failure of the test beams was governed by concrete crush-ing, the maximum stress developed in ASTM A1035 stirrups was552 MPa. It was recommended to limit the design yield strength to552 MPa to ensure the conservativeness of shear strength predic-tions (Sumpter et al. 2009). In 2011, Munikrishna et al. (2011) as-sessed the shear behavior of large-sized concrete beams reinforcedwith ASTM A1035 Grade 690 steel and designed to induce stressesof 550 and 690 MPa in the stirrups. They reported that it is feasibleto reduce the transverse reinforcement ratio in concrete beams byusing ASTM A1035 steel while maintaining shear strength similarto that of beams with a higher transverse reinforcement ratio butreinforced with ASTM A615 Grade 420 steel. At ultimate, priorto the crushing of the concrete strut, the measured strains in theASTM A1035 stirrups were equal to or greater than 0.0035, whichcorrespond to the stress of 690 MPa. Using a design yield strengthvalue of 690 MPa to predict the shear strength of concrete beamstransversely reinforced with ASTM A1035 steel was found to beacceptable if 135° hooks are provided (Munikrishna et al. 2011).The measured shear crack widths were within limits in the two re-search programs (Sumpter et al. 2009; Munikrishna et al. 2011).Shahrooz et al. (2017) conducted full-scale testing to examinethe performance of ASTM A1035 steel stirrups in flexural mem-bers. Their findings were in conformity with Munikrishna et al.(2011). They recommended modifying AASHTO provisions topermit using fy ¼ 690 MPa when computing the shear strengthif ASTM A1035 Grade 690 steel stirrups are used (Shahroozet al. 2017). The ACI 439.6R (ACI 2019) considers Sections11.1, 11.2, 11.4, and 11.5 of ACI 318 (ACI 2008) to be applicablewhen designing for shear and torsion in flexural members rein-forced with ASTM A1035 bars [ACI 439.6R (ACI 2019)]. TheACI 439.6R (ACI 2019) permits using fy ¼ 552 MPa in ACI318-08, in Eqs. (11-2), (11-3), and (11-15), to compute the shearcapacity based on the recommendations of Sumpter et al. (2009).However, spacing and minimum quantity requirements of ACI 318,sections 11.4.5 and 11.4.6, must be satisfied. If shear cracking isa critical design consideration, fy in the preceding equations mustbe limited to 410 MPa. In addition, due to the lack of research data,the value of fy must be limited to 410 MPa in stirrups designedfor torsion [ACI 439.6R (ACI 2019)]. Shahrooz et al.’s (2017)recommendations were incorporated into Articles 5.7.2.5, 5.7.2.6,5.7.2.7, and 5.7.3.3 of the AASHTO (2017) edition of theAASHTO LRFD Bridge Design Specifications. In these articles,which are related to the design of transverse reinforcement inflexural members, a design fy of 690 MPa can be used whenASTM A1035 steel is used.

In lightly reinforced elements (with ρ less than 1%) containinglongitudinal ASTM A1035 rebars, the use of ACI 318 provisions tocompute the shear strength provided by the concrete leads to unsafepredictions [ACI 439.6R (ACI 2019)]. Consequently, ACI 439.6Rrequires that all lightly reinforced beams should contain a minimumshear reinforcement. Desalegne and Lubell (2011) proposed a sim-plification to the Hoult et al. (2008) model to predict the shearstrength provided by the concrete, Vc, in lightly reinforced flexuralmembers (beams and one-way slabs) containing only longitudinal

Fig. 4. Comparison of strain limits for ASTM A1035 Grade 690 andASTM A615 Grade 420 [ACI 318 (ACI 2014); Shahrooz et al. 2014;Mast et al. 2008].

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ASTM A1035 rebars (without shear reinforcement) and subjectedto an insignificant axial load, as follows:

Vc ¼�

154

1000þ 2.1d

� ffiffiffiffiffif 0c

pbwd ð3Þ

where d = effective depth of longitudinal tension reinforcement;bw = web width; and f 0

c = concrete compressive strength. The pro-posed simplified model assumes a longitudinal reinforcement strainof 0.0042, which corresponds to a stress of 690 MPa.

The punching shear behavior of two-way slabs longitudinallyreinforced with ASTM A1035 rebars was evaluated by Yang et al.(2011). Direct replacement of conventional steel bars with ASTMA1035 rebars resulted in a 27% increase in the punching shearresistance. Two-way slabs longitudinally reinforced with ASTMA1035 steel exhibited similar punching shear resistance, but lowerstiffness and larger cracks, when compared to their ASTM A615-bar reinforced counterparts with a higher longitudinal reinforce-ment ratio. The use of ACI 318 (ACI 2008) and CSA A23.3(CSA 2004) leads to conservative estimates of the punching shearresistance of two-way slabs reinforced with ASTM A1035 bars(Yang et al. 2011).

Utilizing ASTM A1035 Grade 690 rebars to resist the shear-friction mechanism, which is generated when shear is transferredacross a concrete interface subject to Mode II (sliding mode) dis-placement, is not recommended. Harries et al. (2012b) tested push-off specimens simulating the connection between an AASHTOgirder and a slab to assess the performance of shear-friction inter-faces containing ASTM A1035 Grade 690. It was observed thatlarge values of crack openings on the order of 2.5 mm were re-quired to generate sufficient strains, and consequently stresses, inthe ASTM A1035 rebars to fully engage them. For this reason, itwas found that reducing the amount of reinforcement in inverseproportions to the increase in the yield strength at the shear inter-face is inapplicable. Based on the findings of Harries et al. (2012b),AASHTO (2017) limits the design yield strength of ASTM A1035steel to 410 MPa in shear-friction interfaces.

Serviceability Considerations

DeflectionA fundamental issue associated with the use of the ASTM A1035steel reinforcement is that these reinforcing bars will experiencehigher stresses and, consequently, strains compared to those ofconventional bars (ASTM A615/A615M) at service load levels.Concrete members reinforced with HSS and designed to have acomparable strength relative to those reinforced with conventionalsteel (ASTM A615/A615M) are likely characterized by a reducedamount of longitudinal reinforcement. This reduction in the amountof reinforcement, while maintaining the same nominal dimensions,induces higher stresses and strains in ASTM A1035 steel whencompared to ASTM A615/A615M steel. The reinforcement stressat service load conditions, fs, is normally approximated to beon the order of 0.6fy. Therefore, longitudinal reinforcing barstresses of 248, 414, and 496 MPa at the service load would beappropriate for steel reinforcement having a yield strength of414, 690, and 827 MPa, respectively. These noticeable differencesin reinforcing bar stresses between conventional steel and differentgrades of HSS at service load levels greatly impact deflection cal-culations and crack control parameters. In one-way members, theACI 439.6R adopts similar deflection control provisions to thatin the ACI 318 [ACI 318 (ACI 2014)]. These provisions suggesttwo methods to control the deflection at the service load level:(1) implicitly control deflection through minimum thicknesses;

and (2) direct deflection calculations. In the first approach, Table9.5(a) in ACI 318, depending on the member type (for example,solid one-way slab or beam) and support configuration (for exam-ple, simply supported or continuous), gives minimum memberthickness h for span L [ACI 318 (ACI 2014)]. This approach isnot applicable to members supporting or attached to partitionsand other construction likely to be damaged by large deflections.Footnote (b) of Table 9.5(a) requires that the expressions pre-sented in the table be multiplied by an adjustment coefficient(0.4þ fy=690) to increase h when reinforcing steel with yieldstrength other than 414 MPa and ranges between 280 and 550 MPaare used. However, although this range does not cover the yieldstrength of ASTM A1035 Grade 690 steel (with fy ¼ 690 MPa),the ACI 439.6R still confirms the applicability of the multiplier tomembers reinforced with ASTM A1035 Grade 690 steel based onthe findings of Mast (2006). When the yield strength value of690 MPa is used, the adjustment coefficient will be 1.4. Mast(2006) reported that the deflection of members reinforced withASTM A1035 Grade 690 steel at the service load is 1.4 timesof those reinforced with ASTM A615 Grade 414 MPa (Mast2006). Puranam (2018) reported that the extrapolation of the ex-pressions presented in Table 9.5(a) to members reinforced withHSS reinforcement would lead to acceptable immediate andlong-term deflections.

In the other method, direct deflection calculations are based onan effective moment of inertia Ie, which accounts for variablecracks at different sections along the member length. The Ie pro-vides a transition between the upper and lower bounds of the grossmoment of inertia Ig and cracked moment of inertia Icr as a func-tion of the ratio Mcr=Ma, where Mcr is the cracking moment,and Ma is the maximum service moment. Both ACI (2014) andAASHTO (2017) prescribe Branson’s (1977) equation to computeIe of the cracked concrete section, as follows:

Ie ¼�Mcr

Ma

�3

Ig þ�1 −

�Mcr

Ma

�3�Icr ≤ Ig ð4Þ

However, Bischoff (2005) proposed a modified version ofBranson’s equation considering the effect of tension stiffeningespecially for soft sections having a low reinforcement ratio, asfollows:

Ie ¼Icr

1 − ð1 − IcrIgÞðMcr

MaÞ2 ≤ Ig ð5Þ

Although the ACI 439.6R and the NCHRP report provide thetwo equations to compute Ie, it is generally recommended to useBischoff’s equation because members reinforced with HSS are nor-mally characterized by sections having a low reinforcement ratio[ACI 439.6R (ACI 2019); Shahrooz et al. 2011]. Additionally,Bischoff’s approach is based on fundamental mechanics, whereasBranson’s formulation is empirical and calibrated for conventionalsteel, hence the former is more suitable for any type of elasticreinforcing material, such as HSS. Soltani (2010) reported that forspecimens reinforced with HSS that have a low reinforcement ratio(0.007), short-term deflections were underestimated when Branson’sequation was used. Similarly, Puranam (2018) reported that thedeflection estimation based on Ie using Branson’s equation wassmaller than those obtained from Bischoff’s equation and the actualmeasured values for elements having low reinforcement ratios.

Designers can choose to use either the implicit deflection pro-visions or carryout direct deflection calculations when checkingdeflections; however, they are encouraged to consider the latter ap-proach, especially for detailed design. Desalegne and Lubell (2012)

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recommend the use of direct deflection calculations instead ofimplicit deflection provisions when checking deflections of slabslongitudinally reinforced with ASTM A1035 steel because theimplicit deflection provisions can lead to uneconomic estimatesof the minimum required slab thickness. The use of implicit deflec-tion provisions is not permitted in lightly reinforced members[ACI 439.6R (ACI 2019)].

Crack ControlOne of the main factors affecting the crack width in normal sizebeams and one-way slabs is the strain in the tension steel. As men-tioned previously, the strains in the tension steel are expected to begreater in flexural members reinforced with HSS than those in con-ventional steel, consequently generating wider cracks. ACI 439.6R(ACI 2019) considers equation Eq. (10-4) of ACI 318 (ACI 2008)for crack control to be applicable to flexural members reinforcedwith HSS. Eq. (10-4) of ACI 318, which is a modified versionof the Frosch equation (Frosch 1999), providing the maximum lon-gitudinal bar spacing s for adequate crack control, as follows:

s ≤ 15

�40000

fs

�− 2.5cc ≤ 12

�40000

fs

�ð6Þ

where cc = minimum concrete cover measured to the center ofreinforcing bar closest to the extreme tension face in inches;and fs = service load stress in the reinforcing bar in psi. AlthoughEq. (6) is only valid for service load steel stresses ranging from 170to 330 MPa (24,000–48,000 psi), ACI 439.6R (ACI 2019)permits taking fs as 460 MPa (67,000 psi), which correspondsto two-thirds of the specified minimum fy of HSS. A bilinear re-lationship is generated when the previous equation is used, as illus-trated in Fig. 5. It can be seen from the figure that when the fs of460 MPa is used, Eq. (6) gives conservative results when comparedto the case where fs is taken as 250 MPa. For example, 160 mmspacing s would be required for ASTM A1035 Grade 690reinforcement (i.e., with fs ¼ 460 MPa) to limit crack widths to0.41 mm at a 25-mm cover, whereas a 340-mm spacing wouldbe required to meet the same crack width criterion for ASTMA615 Grade 420 (i.e., with fs ¼ 250 MPa). The ACI 439.6R(ACI 2019) recommends using the preceding equation not onlyfor beams but also for members with minimal cover, such as slabsand walls because they are designed based on fs ¼ 460 MPa andfy ¼ 690 MPa.

On the other hand, the NCHRP Report 679 (Shahrooz et al.2011) recommended adopting AASHTO (2007) provisions for

crack control, which takes a similar form of the ACI equation, asfollows:

s ≤ 700γdβsfs

− 2dc ð7Þ

where dc = minimum concrete cover measured to the center ofreinforcing bar in inches; γd = coefficient to calibrate the requiredspacing depending on the exposure class; and fs = service loadstress in the reinforcing bar in ksi, and

β ¼ 1þ dc0.7ðh − dcÞ

ð8Þ

where h = depth of the concrete section in inches. The AASHTO(2017) de facto crack width limits for Class 1 and Class 2 exposureare 0.43 and 0.33 mm, respectively. Soltani et al. (2013) carried outa detailed analytical assessment of expected crack widths in con-crete prisms reinforced longitudinally with HSS rebars of differentsizes in which nonlinear stress transfer between rebars and sur-rounding concrete and nonlinear bar slip functions associated withthe stress transfer were considered. Except for the prism reinforcedwith the largest bar considered (#10), all other prisms were char-acterized with crack widths below 0.43 mm up to a reinforcing barstress of 496 MPa. Harries et al. (2012a) demonstrated throughflexural tests on concrete beams having longitudinal reinforcementratios ranging from 0.007 to 0.023 that the measured average crackwidths on beams reinforced with HSS remain below AASHTO defacto limits for Class 1 and Class 2 exposure up to a reinforcing barstress of 496 MPa. Aldabagh et al. (2018) reported that the crackwidths and the deep propagation of cracks in beams reinforcedwith ASTM A1035 reinforcement can be greatly restrained whenfiber-reinforced concrete is used. Nevertheless, it is evident fromthe previous discussion that the ACI and AASHTO crack controlprovisions exhibit sufficient inherent conservativeness to extendtheir applicability to concrete elements reinforced with HSS.

ASTM A1035 Reinforcing Steel in CompressionMembers: Longitudinal and TransverseReinforcement

Ward (2009) carried out a parametric study to determine whethercolumns reinforced with A1035 longitudinal and transversereinforcement will reveal any unexpected results compared tocolumns reinforced with commonly used A615 steel. The variablesin the parametric study included the following: (1) reinforcementgrade; (2) column type; (3) column size; (4) transverse reinforce-ment bar size; and (5) concrete compressive strength. The moment-curvature responses were analytically generated for an axial loadcorresponding to 0.1Agf 0

c, where Ag is the column gross sectionarea. The stress-strain response of the ASTM A1035 steel wasmodeled using the Ramberg-Osgood function described previouslyin this manuscript. Columns reinforced with ASTM A1035 steelhad a longitudinal reinforcement ratio of 2%, whereas those rein-forced with ASTM A615 had a longitudinal reinforcement ratioof 4%. Because of this, columns reinforced with ASTM A615were stiffer than the columns reinforced with ASTM A1035. Thisneeds to be considered when designing columns to resist seismicloading. A detailed discussion regarding the performance of ASTMA1035 in the seismic application is provided in the subsequent sec-tion of this manuscript. Other than reduced stiffness, no unusualor unexpected trends were reported in the response of columns re-inforced with ASTM A1035 when compared to the response ofthose reinforced with ASTM A615. As a result, Article 5.6.4.4in AASHTO (2017) permits using a design fy of 690 MPa in

Fig. 5. Bar spacing versus clear cover for crack control for reinforcingsteel with fs of 250 and 460 MPa. Note: 25 mm (1 in.); and 6.9 MPa(1 ksi).

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Eqs. (5.6.4.4-2) and (5.6.4.4-3) to compute the nominal axialresistance of columns reinforced with longitudinal ASTM A1035reinforcement and located in Seismic zone 1. On the other hand, theACI 439.6R (ACI 2019) imposes a limitation of 550 MPa on fy forthe longitudinal reinforcement when using Eqs. (10-1) and (10-2)of ACI 318-08 (ACI 2008) to compute the axial load capacity ofcolumns longitudinally reinforced with ASTM A1035 Grade 690[ACI 439.6R (ACI 2019)]. Steel strain corresponding to this stressis 0.0028, which is almost equal to the maximum usable concretecompression strain (0.003) assumed by the ACI 318. For memberssubjected to a combined axial load and moment, fy shall be takenas 690 MPa for the longitudinal rebars in tension, whereas fy shallbe taken as 550 MPa for longitudinal rebars in compression [ACI439.6R (ACI 2019)].

Another parametric study was carried out by Ward (2009) toassess the adequacy of AASHTO (2007) §5.7.4.6, Eq. 5.7.4.6-1,in determining the required volumetric ratio of spiral reinforce-ment in nonseismic applications. The use of fyh ¼ 690 MPa inAASHTO (2007) Eq. 5.7.4.6-1 was found to be justified onlyfor Seismic zone 1. For this reason, Article 5.6.4.6 in AASHTO(2017) permits using fyh ¼ 690 MPa in Eq. 5.6.4.6-1 (which isidentical with Eq. 5.7.4.6-1 in AASHTO 2007) to compute thevolumetric ratio of spiral reinforcement in compression memberswhose design is not controlled by seismic requirements. Similarly,ACI 439.6R (ACI 2019) permits using fy ¼ 690 MPa in Eq. (10-5)in Section 10.9.3 of ACI 318 to determine the required volumetricratio in compression members reinforced with ASTM A1035spirals. Also, when ASTM A1035 bars are used in columns astransverse reinforcement, minimum size and maximum tie spacingrequirements of section 7.10.5 of ACI 318 must be satisfied, irre-spective of the yield strength of reinforcement [ACI 439.6R (ACI2019)]. The previous discussion related to computing the shearcapacity in flexural members is applicable to compression membersdesigned to resist shear forces [ACI 439.6R (ACI 2019)].

Bond Strength Characteristics of ASTM A1035Reinforcing Steel

Because of the higher tensile strength and the unique nonlinearstress-strain response of ASTM A1035 steel, bond characteristicsof ASTM A1035 reinforcing bars are different from those ofASTM A615 steel rebars. The equations for development andsplice lengths found in design codes, such as ACI 318 (ACI2014) and ACI 408R (ACI 2003), are empirical and were devel-oped based on research work performed using steel having amaximum yield strength of 555 MPa. The applicability of theseequations to determine the development and splice lengths ofASTM A1035 steel bars was initially evaluated by Seliem et al.(2009). Sixty-nine large-scale beam-splice specimens containingASTM A1035 steel rebars were tested. The results indicated thatsubstantially higher stress levels can be achieved in the rebars ifconfining transverse reinforcement is provided. The presence oftransverse reinforcement allowed for splitting cracks to be distrib-uted along the spliced rebars and thereby promoted higher strengthand greater deformation before failure. Stresses of up to 1,035 MPawere recorded in 25- and 36-mm spliced rebars with a transversereinforcement confinement. Section 12.2 of ACI 318 was found tobe applicable to calculate the development and splice lengthsof only ASTM A1035/A1035M confined rebars. Nevertheless,Seliem et al. (2009) proposed adopting a revised strength reductionfactor of 0.80, instead of 0.82, in the ACI 408R (ACI 2003)Eq. 4-11a, to reasonably estimate the development and splicelengths for both confined and unconfined spliced ASTM A1035

rebars (Seliem et al. 2009). Similar recommendations were pro-posed by Hassan et al. (2012) but with a strength reduction factorof 0.92. Alternatively, Hosny et al. (2012) proposed a generalsimplified equation for the bond strength of unconfined reinforcingbars applicable to ASTMA615/A615M, A706/A706M, and A1035/A1035M, as follows:

ld ¼ð0.042fsdbÞ2Cmin

ffiffiffiffiffif 0c

p ðMPa andmmÞ ð9Þ

This equation accounts for the specified stress in spliced bars,fs; nominal bar diameter db; concrete compressive strength f 0

c; andminimum concrete cover Cmin. The recommendations of Seliemet al. (2009) are presented in ACI 439.6R (ACI 2019) and are appli-cable to ASTM A1035 spliced rebars in tension. However, forASTM A1035 rebars in compression, with unavailable test data,the ACI 439.6R suggests using Section 12.3 of ACI 318 but witha specified yield strength limited to 550 MPa [ACI 439.6R (ACI2019)]. Article 5.10.8.2 of AASHTO (2017) permits the use of fyup to 690 MPa in Eq. 5.10.8.2.1 a-1 to determine the developmentlength of ASTM A1035 bars based on the findings of Harries et al.(2010). However, this is only applicable when the ASTM A1035spliced rebars are confined and the design concrete compressivestrength does not exceed 100 MPa.

Substituting fy of 690 MPa in the preceding equations wouldrequire long development or splice lengths. Because these lengthsmay be uneconomical or impractical, designers are encouraged toconsider mechanical splices (couplers) or spliced T-headed rebars[ACI 439.6R (ACI 2019)]. Mechanical splices used to connectASTM A1035 rebars must be capable of developing the actualtensile strength and actual elongation of the spliced rebars [ACI439.6R (ACI 2019)]. The reason for this stringent requirementis to ensure that adequate ductility is provided in the connectedbars. Excessive heating of the reinforcing bars when installingthe mechanical splices must be avoided so that it does not causeunfavorable alterations to the microstructure of the ASTM A1035steel. Reinforced concrete beams containing ASTM A1035 rebars,which are lap-spliced using mechanical couplers, exhibit ductileflexural failure (i.e., rebars yield in tension, and concrete crushesin compression) when transverse reinforcement is provided (Brenaet al. 2018). In the case of spliced T-headed rebars, although theACI 318 provides the requirements of developing such rebars,it is limited to reinforcing bars with fy not exceeding 410 MPa,thereby they cannot be extended to ASTM A1035 steel. Conse-quently, the ACI 439.6R (ACI 2019) requires that when T-headedrebars are used, experimental results showing the adequacy of suchmechanically attached heads need to be provided. Berna et al.(2018) demonstrated that transverse reinforcement within the lapsplice of T-headed ASTM A1035 bars promotes ductile flexuralfailure by changing the force-transfer mechanism between adjacentbars and increasing stresses along with the splice and memberdeformation capacity. From the previous discussion, it is evidentthat transverse confinement along the lap-splice region of ASTMA1035 rebars is crucial and must always be provided to avoid asudden and brittle failure irrespective of the type of splices.

The anchorage strength of ASTM A1035 hooked bars wasevaluated by Harries et al. (2010), Sperry et al. (2017), and Sperryet al. (2018). If adequate cover and confinement are provided, thedevelopment length of ASTM A1035 hooked bars prescribed byAASHTO (2007) Section 5.11.2.5 (2) was found to be adequateto develop tensile stress up to 860 MPa (Harries et al. 2010). De-velopment lengths of ASTM A1035 hooked bars calculated basedon Section 25.3 of ACI 318 are very conservative for No. 16 barsand become gradually less conservative with the increase in bar size

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and concrete compressive strength (Sperry et al. 2017). The changein bend angle (90° or 180°) and side concrete cover (between 65 and80 mm) had no impact on the anchorage strength of ASTM A1035hooked bars (Sperry et al. 2018). The anchorage strength of ASTMA1035 180° hooked rebars noticeably increases when confiningreinforcement with parallel or perpendicular orientation is provided.However, for ASTMA1035 90° hooked rebars, confining reinforce-ment oriented parallel to the development length is more effective inincreasing the anchorage strength than reinforcement orientedperpendicular to the development length (Sperry et al. 2018).

Seismic Performance of ASTM A1035 ReinforcingSteel

Several researchers have evaluated the seismic performance ofhigh-strength transverse reinforcement as column confinement(Muguruma and Watanabe 1990; Muguruma et al. 1991; Suganoet al. 1990; Budek et al. 2002). Concrete columns confined withhigh strength reinforcement (fyt up to and beyond 827 MPa) ex-hibited satisfactory performance when compared to those confinedwith conventional steel. Moreover, the seismic performance ofhigh-strength longitudinal reinforcement in columns was assessedby Stephan et al. (2008), Restrepo et al. (2006), Lepage et al.(2008), Rautenberg et al. (2013), Trejo et al. (2016), Barbosa et al.(2016), and Barcley and Kowalsky (2020). Columns reinforcedwith longitudinal high-strength steel rebars (Grade 550–830)had drift capacities ranging from 4% to 8%, indicating a satisfac-tory performance when compared to similar columns reinforcedwith conventional steel. However, columns reinforced longitudi-nally with ASTM A1035 Grade 690 steel were characterizedwith a limited ductility capacity and reduced energy dissipation.The lower energy dissipation is attributed to the reduction in thepostcracking stiffness (Rautenberg et al. 2013). Because columnsdesigned with ASTM A1035 Grade 690 steel contain less longitu-dinal reinforcement than columns designed with conventional steel,they are characterized with a smaller initial stiffness and, hence,greater yield displacements. This, along with a relatively smallultimate lateral deformation, leads to a limited ductility response(Restrepo et al. 2006). While reviewing the findings of the previousresearch work, it is evident that greater concerns related to ductilitycapacity and energy dissipation arise when ASTM A1035 Grade690 steel is incorporated as longitudinal rather than transverse(confining) steel in plastic hinge regions. In addition to the previousseismic applications, the performance of ASTM A1035 Grade690 in beam-column joints subjected to pure flexural monotonicand cyclic quasi-static loading protocols was evaluated by Ibarraand Bishaw (2016). The use of ASTM A1035 Grade 690 steelincreased the flexural capacity of the beam-column joints bymore than 60% but reduced the ductility and energy dissipationcapacity. In order to overcome the aforementioned deficienciesresulting from the use of ASTM A1035 Grade 690 in seismicapplications, Lepage et al. (2012) and Ibarra and Bishaw (2016)considered introducing steel fibers. The presence of these fibersincreased the peak strength and greatly reduced the cracking andspalling of concrete but had an insignificant impact on the energydissipation capacity. A number of studies (Rautenberg et al. 2013;Rautenberg and Pujol 2013; Billah and Alam 2013; Bishaw 2016)numerically evaluated the seismic performance of concrete col-umns reinforced with ASTM A1035 Grade 690 steel. Rautenberget al. (2013) and Rautenberg and Pujol (2013) reported that the roofdrifts of frame buildings having columns longitudinally reinforcedwith ASTM Grade 830 steel were not consistently larger than theroof drifts of buildings having columns longitudinally reinforced

with twice as much conventional steel. Columns in high-rise build-ings incorporating high-strength concrete and reinforcement werefound to be less vulnerable to far-field ground motions than thoseincorporating normal strength materials (Billah and Alam 2013).On the contrary, Bishaw (2016) reported that, in moment-resisting frames, elements reinforced with high-strength steelexhibited reduced ductile characteristics and, hence, lower collapsecapacities when compared to elements with conventional steel. Thereason for the contradicting conclusions drawn from Billah andAlam (2013) and Bishaw (2016) could be due to the variabilityin adopted material models and selected ground motions in theirnumerical simulations.

The applicability of the ACI 439.6R (ACI 2019) recommenda-tions are limited to structures assigned to seismic design category(SDC) A, B, and C. For structures assigned to SDC D, E, or F, theapplication of the ACI 439.6R recommendations is limited to slabsystems, foundations, and other components, which are expected toremain elastic during a seismic event. The only exception is whenASTM A1035 steel is used as transverse reinforcement in columnsfor concrete confinement. In this case, the ACI 439.6R permits usingfyt ¼ 690 MPa for confinement but limits fyt to 410 MPa for shearstrength [ACI 439.6R (ACI 2019)]. Similarly, ACI 318 permits us-ing a yield strength of up to 690 MPa when ASTM A1035 Grade690 is used for confinement (ACI 2014). In AASHTO (2017), theNCHRP Report 679 recommendations are considered applicableonly for elements and connections in Seismic zone 1 (Shahroozet al. 2011). This means that, in Seismic zones 2, 3 and 4, the yieldstrength of ASTM A1035 Grade 690 reinforcement would be lim-ited to 500MPa in most applications. Russell et al. (2011) examinedthe applicability of the NCHRP Report 679 guidelines to bridgeslocated in Seismic zones 3 and 4. They recommended extendingthe applicability of the NCHRP Report 679 guidelines to capacityprotected members (such as footings, bent cap beams, joints, andsuperstructure) in Seismic zones 3 and 4 because these elementsare designed to remain essentially elastic. In addition, they providedseveral examples of bridges reinforced with ASTM A1035 Grade690 steel and constructed in Seismic zones 3 and 4 with documentedsatisfactory performance. Nevertheless, their recommendations werenot incorporated into AASHTO (2017).

Summary and Conclusions

In the last two decades, there has been a significant amount of re-search work carried out on ASTM A1035 steel bars to assess theirperformance in structural concrete. ASTM A1035 steel bars exhibithigher strength and corrosion resistance in comparison with con-ventional ASTM A615 Grade 420 steel. One of the most uniqueproperties of the ASTM A1035 Grade is the lack of a distinct yieldpoint and yielding plateau. The high-strength steel, and ASTMA1035 steel, in particular, are normally produced through coldworking, microalloying, or quenching and tempering, with micro-alloying being the most common. The ASTM A1035 steel exhibitssuperior corrosion resistance to that of conventional ASTM A615Grade 420 steel. ASTM A1035 steel can be 2–10 times morecorrosion-resistant than conventional steel depending on the typeof alloy, i.e. chromium content. Beams, slabs, and columns longi-tudinally and/or transversely reinforced with ASTM A1035 steeland designed with fy ¼ 690 MPa exhibit satisfactory performancein flexure and shear provided that these elements are not part of theseismic-force-resisting system. The use of ASTM A1035 steel forseismic applications is only justified in columns for concreteconfinement. Design codes, specifically in the United States, haveevolved accordingly to accommodate ASTM A1035 steel to allow

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designers to make efficient and safe use of their potential advan-tages. However, the degree to which these design codes haveaccommodated ASTM A1035 steel varies. Table 2 provides a com-parison between the requirements of ACI 439.6R (ACI 2019),AASHTO (2017), and ACI 318 (ACI 2014) related to the use ofASTM A1035 Grade 690 reinforcement. It is evident from Table 2that the ACI 439.R6 (ACI 2019) and AASHTO (2017) have madeconsiderable progress in accommodating ASTM A1035 steel whencompared to ACI 318, yet there are still other limitations. TheAASHTO (2017) requirements presented in Table 2 are onlyapplicable in Seismic zone 1. The authors find this limitation un-justifiable and must be revised particularly for capacity-protectedelements in Seismic zones 2, 3, and 4 because these members aredesigned to remain essentially elastic regardless of the seismiczone. The authors confirm that further systematic and comprehen-sive tests are required to justify the use of ASTM A1035 steel aslongitudinal reinforcement in the seismic application. In addition,other topics that could be explored in future research work includethe performance of ASTM A1035 steel to resist torsion and thebehavior of ASTM A1035 steel in compression splices. The speci-fied yield strength of ASTM A1035 steel in these applications isstill limited to 550 MPa due to the lack of research data (Table 2).With more revisions accommodating ASTM A1035 steel in designcodes and systematic and well-designed future research programs,designers, engineers, and practitioners would have a greater chanceof utilizing ASTM A1035 steel and making use of its potential ben-efits in various applications.

Acknowledgments

The financial contributions of the Mitacs Accelerate Grant andthe Green Construction Research and Training Centre (GCRTC)Scholarship at the Univ. of British Columbia (UBC) were criticalto conduct this research and are gratefully acknowledged.

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Table 2. Comparison between the requirements of ACI 439.6R, AASHTO (2017), and ACI 318 related to the use of ASTM A1035 Grade 690 reinforcement

Application Design requirements ACI 439.6R AASHTO (2017) ACI 318

Longitudinal reenforcementin flexural members

Section or article 4.2 5.4.3.1 and 5.4.3.3 N/ASpecified fy for flexural capacity 690 690Section or article 4.3 5.6.2.1Compression- and tension-controlled strainlimits

TC: 0.009 TC: 0.008CC: 0.004 CC: 0.004

Section or article 4.7 5.6.3.4Moment redistribution Not permitted Maximum 1,000εt%Section or article 4.8 5.6.3.5.2Specified fy in deflection calculation 690 N/ASection or article 4.9 5.6.7Specified fs for crack control 460 460Section or article 4.1 5.6.3.3Specified fy in minimum longitudinalreinforcement calculation

690 N/A

Stirrups to resist shear Section or article 4.11 5.7.3.3Specified fy for shear capacity 550 690Section or article 4.11 5.7.2.5 and 5.7.2.6Specified fy in minimum transversereinforcement and maximum spacing calculation

550 690

Stirrups to resist torsion Section or article 4.11 5.7.3.6.2Specified fy for torsion capacity 410 410

Longitudinal reinforcementin compression

Section or article 5.2 5.6.4.4 N/ASpecified fy 550 690

Spirals (nonseismic) Section or article 5.3 5.6.4.6 25.7.3.3Specified fy in spacing calculation 690 690 690

Spirals (seismic) Section or article N/A N/A 18.7.5.4Specified fy in spacing calculation 690

Splice in tension Section or article 10.2 5.10.8.2.1 N/ASpecified fy 690 690

Splice in compression Section or article 10.2 5.10.8.2.2Specified fy 550 410

Hooks in tension Section or article N/A 5.10.8.2.4Specified fy 690

Note: fy = design yield strength; fs = service load stress; TC = tensioned-controlled; CC = compression-controlled; and N/A = not available.

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