Behavior of High-Performance Steel as Shear Reinforcement for Concrete Beams (1) قسمت اول

Matthew S Sumpter, Sami H Rizkalla, Paul Zia. ACI Structural Journal. Farmington Hills: Mar/Apr 2009. Vol. 106, Iss. 2; pg. 171, 7 pgs

Abstract
This paper describes the behavior of high-performance (HP) steel as shear reinforcement for concrete beams. HP steel is characterized by enhanced corrosion resistance and higher strength in comparison to ASTM A615-06 Grade 60 steel. The HP steel selected for this research is commercially known as microcomposite multistructural formable (MMFX) steel and conforms to ASTM A1035-07. Nine reinforced concrete beams were constructed using No. 9 (φ 29) longitudinal bars and No. 3 (φ 10) stirrup bars. The main variables considered in this study are the stirrup spacing and type of reinforcing steel material. Test results indicate that using HP steel reinforcement increases the shear capacity and enhances the serviceability in terms of strength gain and reduction of shear crack width. Current design codes can conservatively be used for the design of HP steel using a yield strength of 80 ksi (552 MPa). [PUBLICATION ABSTRACT]
Copyright American Concrete Institute & civil engineering in east structure www.mh.coo.ir Mar/Apr 2009
Copyright civil engineering in east structure Mar/Apr 2009

Matthew S Sumpter, Sami H Rizkalla, Paul Zia. ACI Structural Journal. Farmington Hills: Mar/Apr 2009. Vol. 106, Iss. 2; pg. 171, 7 pgs

[Headnote]
This paper describes the behavior of high-performance (HP) steel as shear reinforcement for concrete beams. HP steel is characterized by enhanced corrosion resistance and higher strength in comparison to ASTM A615-06 Grade 60 steel. The HP steel selected for this research is commercially known as microcomposite multistructural formable (MMFX) steel and conforms to ASTM A1035-07. Nine reinforced concrete beams were constructed using No. 9 (φ 29) longitudinal bars and No. 3 (φ 10) stirrup bars. The main variables considered in this study are the stirrup spacing and type of reinforcing steel material. Test results indicate that using HP steel reinforcement increases the shear capacity and enhances the serviceability in terms of strength gain and reduction of shear crack width. Current design codes can conservatively be used for the design of HP steel using a yield strength of 80 ksi (552 MPa).
Keywords: beams; high-performance concrete; high-strength concrete; shear; steel.

INTRODUCTION

There is a growing interest in the use of high-strength materials to reduce dead loads and consequently increase span length. Not nearly enough research has been conducted on high-performance (HP) steel in comparison with highstrength concrete, however. HP steel conforming to ASTM A1035-071 has higher strength in comparison with conventional ASTM A615-06 Grade 602 steel, which may reduce the amount of required reinforcement. This could potentially reduce the costs of material and labor and relieve reinforcement congestion for future structures. The commercially available microcomposite multistructural formable (MMFX) steel, which conforms to ASTM A1035-07,1 was chosen for this study. The objective of this study was to determine the feasibility of using HP steel as shear reinforcement for concrete members, particularly the member behavior under overload condition with the steel being at high stress levels. The research consisted of evaluating the behavior of beams reinforced with HP steel stirrups and longitudinal reinforcement, determining the ability of current design codes to predict the shear strength, assessing the capability of detailed analysis to predict the shear strength, and making design recommendations.

RESEARCH SIGNIFICANCE

The behavior of concrete members subjected to shear and reinforced with HP steel is not well defined. One concern is whether the high stress levels induced in the reinforcement may cause excessive cracking in the concrete. Another concern is how accurately the current design codes can predict the shear strength. This research complements the ongoing efforts at North Carolina State University and elsewhere to provide design guidelines for the use of HP steel in concrete structures.

EXPERIMENTAL INVESTIGATION

Test specimens

The experimental program consisted of nine reinforced concrete beams divided into three main categories according to the type of reinforcing steel. The type of steel includes conventional ASTM A615-06 Grade 602 steel and ASTM A1035-071 steel. Within each category, the stirrup spacing varied to reflect a minimum, intermediate, and maximum level of shear reinforcement as allowed by ACI 318-05.3 All beams had cross-sectional dimensions of 12 x 16 in. (305 x 406 mm), with a total length of 16 ft (4.88 m). In addition, all beams were provided the same longitudinal reinforcement ratio to keep the effect of dowel action constant. As a result, changes in the observed behavior could be attributed to either the HP steel or to the stirrup spacing. Two layers of three No. 9 ([straight phi] 29) bars were used in the bottom and one layer of three No. 9 ([straight phi] 29) bars was used in the top of the beam, with all stirrups of No. 3 ([straight phi] 10) bar size. The cross section is shown in Fig. 1.

All transverse reinforcement contained closed stirrups with hooks extended by a distance of six bar diameters past the bend, as specified by ACI 318-05,3 Section 7.1. Longitudinal steel in the bottom layers were designed with a 90-degree hook at the end of the bar to ensure proper anchorage. The test matrix given in Table 1 labels beams using three parameters: type of longitudinal steel, type of www.mahdihashemi.blogspot.com transverse steel, and the spacing of the transverse steel in inches. The letter "C" indicates conventional Grade 60 steel and "M" represents MMFX steel. For example, Beam C-M-4 contains conventional Grade 60 longitudinal steel and HP steel stirrups spaced 4 in. (102 mm) center to center. Casting and testing was organized into three beam sets, where each set contains three specimens and differs based on the transverse reinforcement spacing. Within a given beam set, the specimens change based on the combination of reinforcing steel material used. Set 1 beams have a shear span-depth ratio (a/d) equal to 2.62, whereas beams in Sets 2 and 3 have an a/d equal to 3.08.

Material properties

The nine beams were fabricated using normalweight concrete with an aggregate size of 3/8 in. (9.5 mm). The average concrete compressive strengths at the day of testing ranged from 3900 to 4730 psi (27 to 33 MPa). The concrete compressive strength was determined by testing 4 x 8 in. (102 x 204 mm) cylinders, which were cast for each beam set and cured under the same conditions as the beams. Tension coupons of the HP and Grade 60 steels were tested according to ASTM A370-07,4 and the stress-strain relationships are shown in Fig. 2. The ASTM A1035 steel experienced linear behavior until a stress level of approximately 80 ksi (552 MPa), followed by a negligibly small reduction in the elastic modulus up to 100 ksi (690 MPa), and then nonlinear behavior to a maximum strength of 172 ksi (1186 MPa) at 5% strain. The yield strength, per ACI 318-05,3 Section 3.5.3.2, is based on strain at 0.35% and was 97 ksi (669 MPa). It should be noted, however, that the 0.2% offset method by ASTM computed the yield strength of HP steel to be 120 ksi (827 MPa). The yield strength of the Grade 60 steel was determined to be 62 ksi (427 MPa) based on strain at 0.35%.
Test setup and instrumentation.......................!
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