793 ACI Structural Journal/November-December 2015 ACI STRUCTURAL JOURNAL TECHNICAL PAPER This paper evaluates the performance of reinforced concrete (RC) fexural members reinforced with two different types of high- strength steels—Grade 100 A1035 and SD685—under monotonic loading. Test results indicate that design concepts of the current ACI Building Code can be used to evaluate the strength of speci- mens reinforced with either type of high-strength fexural reinforce- ment. With similar design parameters, specimens reinforced with high-strength fexural reinforcement exhibit equivalent ultimate displacement to those with conventional Grade 60 steel. Specimen behavior is greatly infuenced by the buckling of compression rein- forcement after spalling of cover concrete in the compression zone. The maximum spacing of transverse reinforcement (Grade 60) not exceeding 8d b is suggested to restrain either SD685 or A1035 high- strength longitudinal reinforcement against premature buckling in fexural members primarily subjected to gravity-type loading, where d b is the diameter of smallest compression reinforcement. Keywords: deformation capacity; fexural strength; high-strength steel. INTRODUCTION The demand of high-rise buildings in several urban areas has increased dramatically in recent years. Driven by economic advantages and improvement in seismic performance (Liel et al. 2011), reinforced concrete (RC) has become the favored construction material for high-rise buildings. The effort to maintain reasonable member sizes often results to heavy reinforcing bar congestion, which is always signifcantly challenging to handle during construc- tion, adversely affecting construction speed and quality. The use of high-strength steels has the potential to mitigate this issue (Aoyama 2001; Mast et al. 2008; Sumpter et al. 2009; Shahrooz et al. 2011, 2014; Harries et al. 2012). Different high-strength steels have been developed with distinct stress-strain characteristics. Despite the encouraging results concerning the use of high- strength steel, most of the existing research studies focus only on one type of steel at a time. Test results comparing behavior of RC members reinforced with different types of high-strength steels are relatively limited. This study aims to fll the gap. Two types of high-strength steels are eval- uated, namely, Grade 100 A1035 (ASTM A1035/A1035M 2011). Both steels have specifed yield strengths of 100 ksi (690 MPa). The relevant research and design guidelines for using A1035 steel are well-documented by ACI Innovation Task Group 6 (2010). In addition to its higher strength proper- ties, A1035 steel features better corrosion resistance due to its low carbon and high chromium composition. The stress- strain relationship for A1035 steel proposed by ACI Innova- tion Task Group 6 (2010) is presented in Eq. (1). Relevant research for using SD685 can be found elsewhere (Aoyama 2001). The typical stress-strain relationship of SD685 steel is presented in Eq. (2) (Wang et al. 2009). The required material properties for A1035 and SD685 high-strength steels are summarized in Table 1 along with the conventional Grade 60 steel conforming to ASTM A706/706M (2009). One of the remarkable differences between the two high- strength steels is that SD685 steel exhibits a distinct yield plateau with a minimum steel strain of 0.014 before the onset of strain hardening (Aoyama 2001), whereas A1035 steel does not display a well-defned yield plateau. For comparison and design purposes, the stress-strain model for Grade 60 steel (Priestley et al. 1996) is presented in Eq. (3). The theoretical stress-strain curves based on Eq. (1), (2), and (3) are illus- trated in Fig. 1. f f s s s s = ≤ ≤ = 29 000 0 0024 170 , . ε ε (ksi) for 0 - + ≤ ≤ = 0 43 0 0019 0 0200 150 . . . ε ε s s f (ksi) for 0.0024 (ksi) for 0.02 ≤ ≤        ε s 0 06 . (1) f f s s s = ≤ ≤ 29,000 (ksi) for 0 ε ε 0 00345 . s s = ≤ 100 (ksi) for 0.00345 ε ≤ = - -       ≤ ≤ 0 01 138 38 0 097 0 087 00 2 . . . . f s s ε ε (ksi) for 0.01 97          (2) f s s s = ≤ ≤ 29,000 (ksi) for 0 ε ε 00 . 0207 60 f s = (ksi) for 0.00207 ≤ ≤ = - -               ε ε s s f 0 008 60 1 5 05 0 12 0 112 2 . . . . . (ksi) for 0.008 ≤ ≤          ε s 0 12 . (3) Flexural responses of RC beam specimens using conven- tional Grade 60, SD685, and A1035 steels as fexural reinforcement were experimentally studied. Five pairs of RC beam specimens were tested under a monotonically increasing gravity-type loading. Test results of all specimens are collectively discussed. Title No. 112-S65 High-Strength Flexural Reinforcement in Reinforced Concrete Flexural Members under Monotonic Loading by Marnie B. Giduquio, Min-Yuan Cheng, and Leonardus S. B. Wibowo ACI Structural Journal, V. 112, No. 6, November-December 2015. MS No. S-2014-126.R5, doi: 10.14359/51688057, received March 4, 2015 and reviewed under Institute publication policies. Copyright © 2015, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published ten months from this journal’s date if the discussion is received within four months of the paper’s print publication.