Behavior of Concrete Bridge Decks Reinforced with High-Performance Steel

ACI Structural Journal, Jan/Feb 2008 by Seliem, Hatem M, Lucier, Gregory, Rizkalla, Sami H, Zia, Paul

This paper describes the behavior of concrete bridge decks reinforced with newly developed high-performance (HP) steel that is characterized by its high strength and enhanced corrosion-resistance in comparison with conventional ASTM A615-06 Grade 60 steel. The study presented herein included testing of three full-scale bridge decks with a span-depth ratio of 12.5. The first and second decks were constructed with the same reinforcement ratio using HP and Grade 60 steel, respectively. The third deck was reinforced with HP steel using 33% less reinforcement in an attempt to use its high strength. A nonlinear finite element model was used to predict the mode of failure and failure loads. Test results demonstrate that the use of HP steel at a reduced reinforcement ratio is viable as flexural reinforcement in concrete bridge decks. The paper also presents the test results of specially-designed specimens to study the effect of bending of HP steel bars on their tensile strength.

Keywords: bent bars; bridge decks; flexural-shear; punching.

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INTRODUCTION

Bridge decks are frequently subjected to severe environmental conditions that often lead to serious corrosion problems. The use of high-performance (HP) steel could help to mitigate corrosion problems due to its enhanced corrosion resistance. In addition, HP steel has higher strength compared with conventional ASTM A615-061 Grade 60 steel. Therefore, by using HP steel, the amount of required reinforcement could be considerably reduced. Reducing the amount of steel will alleviate reinforcement congestion and improve concrete placement. Steel that conforms to ASTM A1035-072 was selected for this study because of its high-strength and enhanced corrosion resistance in comparison to conventional ASTM A615-061 Grade 60 steel. This paper is a part of a comprehensive study to investigate the structural behavior of HP steel for bridges. The work presented in this paper examined the behavior of bridge deck slabs and the strength of bent bars required for certain details. The experimental program presented in this paper consisted of two phases. In the first phase, three full-scale bridge decks with a span-depth ratio of 12.5 were tested to evaluate the structural performance of bridge decks reinforced with HP steel as main flexural reinforcement in comparison with the use of conventional Grade 60 steel. In the second phase, four specially-designed specimens were tested to assess the effect of bending on the tensile strength of HP steel bars.

RESEARCH SIGNIFICANCE

Recently, many state transportation departments have begun to use HP steel as a direct replacement for conventional Grade 60 steel in concrete bridge decks.3 However, the behavior of concrete bridge decks reinforced with this novel steel is not well defined. This study is an attempt to use the high strength characteristics of HP steel in concrete bridge decks. In addition, the study evaluates the effect of bending on the tensile strength of HP steel bars.

PHASE I: CONCRETE BRIDGE DECKS

Test specimens

A total of three full-scale bridge decks were considered in this study to examine the flexural limit state behavior, including the mode of failure. The three decks were designed to be identical in all aspects except for the type and amount of steel used in each. All three bridge decks consisted of two spans and double cantilevers, supported in composite action by three precast, post-tensioned concrete girders having cross-sectional dimensions of 24 x 10 in. (610 x 254 mm). The overall nominal dimensions of the bridge decks were 21 ft-10 in. x 13 ft-2 in. x 8-5/8 in. (6655 x 4013 x 220 mm) with a span-depth ratio of 12.5. The supporting girders were posttensioned using deformed prestressing bars of 1 in. (25 mm) diameter with an ultimate strength of 150 ksi (1034 MPa). Each girder was prestressed by four bars resulting in a total prestressing force of 360 kips (1601 kN) per girder. Posttensioning was used to prevent the girders from torsional cracking so as to maintain their torsional stiffness throughout the test. The girders were designed so that their torsional stiffness was similar to that of the steel bridge girders of an actual bridge that was built in Johnston County, NC, in 2004 using HP steel.3

The first and third bridge decks were reinforced with HP steel, whereas the second bridge deck was reinforced with conventional Grade 60 steel for comparison purposes. The test matrix is given in Table 1, and the reinforcement details for the three bridge decks are shown in Fig. 1. It should be noted that the reinforcement ratio ρ is calculated using the total slab thickness. The first and second bridge decks were constructed with the same reinforcement ratio using HP and conventional Grade 60 steel similar to that used in the bridge built in Johnston County, NC, in 2004.3 The third bridge deck, however, was reinforced with HP steel using only 2/3 of the reinforcement ratio used for the first two decks. The reduction in the amount of steel is based on a selected yield strength of 90 ksi (621 MPa), which is within the linear behavior of the HP steel and less than the yield strength of 120 ksi (827 MPa) determined according to the 0.2% offset method specified by ASTM A370-07.4 It should be noted that only the transverse steel was reduced because the deck is continuous in this direction where primary bending occurs.