Flexural Behavior of an Ultrahigh-Performance Concrete I-Girder Benjamin A. Graybeal, Ph.D., P.E., M.ASCE 1 Abstract: The flexural behavior of an ultrahigh-performance concrete UHPC was investigated through the testing and related analysis of a full-scale prestressed I-girder. A 28 ksi 193 MPa compressive strength steel fiber reinforced concrete was used to fabricate an 80 ft 24.4 m long AASHTO Type II girder containing 26 prestressing strands and no mild steel reinforcement. Intermediate and final behaviors, including cracking, flexural stiffness, and moment capacity, were investigated. Test results are compared to predictions based on standard analytical procedures. A relationship between tensile strain and crack spacing is developed. The uniaxial stress-strain response of UHPC when subjected to flexural stresses in an I-girder is determined and is verified to be representative of both the stress and flexural stiffness behaviors of the girder. A flexural design philosophy for this type of girder is proposed. DOI: 10.1061/ASCE1084-0702200813:6602 CE Database subject headings: Concrete structures; Concrete beams; Prestressed concrete; Flexural strength; High-strength concrete; Fibers; Girders. Introduction Ultrahigh-performance concrete UHPC is a new class of con- crete that has been developed in recent years. When compared with high performance concrete HPC, UHPC exhibits superior compressive and tensile mechanical behaviors, as well as excep- tional durability properties. A research program was initiated to characterize many of the behaviors relevant to the use of UHPC in the highway bridge industry Graybeal 2006a,b. Full-scale AASHTO Type II prestressed concrete girders were tested under flexural and shear loadings. The results of the flexural testing program are discussed herein. The term UHPC covers a broad range of cementitious com- posite materials that exhibit sets of mechanical and durability properties far advanced beyond conventional concretes. The French led a significant portion of the early development of these materials in corporate, national, and academic research laborato- ries. This background led to the publication of the Association Française de Génie Civil Interim Recommendations for Ultra High Performance Fiber-Reinforced Concretes Association Française de Génie Civil 2002. This document states that UHPCs tend to have a compressive strength over 21.7 ksi 150 MPa, internal fiber reinforcement to ensure nonbrittle behavior, and a high cementitious materials content. UHPCs also tend to have very low water-to-cementitious materials ratios, minimal or no coarse aggregates, and an optimized gradation of granular materials. The UHPC studied in this research program is the only con- crete of this type currently commercially available in North America. This high cement, high silica fume content concrete has an extremely low water-cementitious materials ratio less than 0.20 and requires high-range water-reducing admixtures to achieve the correct rheology. This concrete contains no coarse aggregate and is internally reinforced by 0.5 in. 13 mm long, 0.008 in. 0.2 mm diameter straight steel fibers that are included at a volumetric ratio of 2%. Further details on the mix composi- tion can be found in Graybeal 2007. The material characterization research program associated with the research discussed herein focused on quantifying the mechanical and durability properties of this UHPC. Table 1 pro- vides a listing of many of the properties that were quantified using standardized tests. Of particular interest in the realm of structural concrete, after the application of a steam curing treatment this UHPC has a compressive strength of 28 ksi 193 MPa, a tensile cracking strength of 1.3 ksi 9 MPa, and a creep coefficient of 0.29. The use of this new class of concrete in structural applications has been limited. One reason for the slow implementation is the perceived complexity of the structural behaviors of UHPC com- ponents as compared to conventional concrete components as well as the lack of full-scale UHPC component test results. Given the increased material cost of UHPC as compared to conventional concrete, practical use of UHPC structural components will likely require optimization of the component to make use of the ad- vanced mechanical and durability properties. Of primary interest, this optimization frequently leads to the use of the tensile prop- erties of UHPC to carry a portion of the loads imparted on the component. Without an understanding of the UHPC tensile prop- erties as exhibited on the structural component level, the optimal design of said components is not possible. UHPC tensile properties are distinct from those of conven- tional concrete due to the increased tensile cracking capacity of the cementitious composite matrix and the crack-bridging behav- ior of the fiber reinforcement. In contrast to fiber-reinforced con- ventional concretes, UHPC can exhibit significant, sustained postcracking tensile capacity prior to crack localization, fiber 1 Research Structural Engineer, Federal Highway Administration, Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, McLean, VA, 22101. E-mail: benjamin.graybeal@fhwa.dot.gov Note. Discussion open until April 1, 2009. Separate discussions must be submitted for individual papers. The manuscript for this paper was submitted for review and possible publication on January 7, 2008; ap- proved on March 6, 2008. This paper is part of the Journal of Bridge Engineering, Vol. 13, No. 6, November 1, 2008. ©ASCE, ISSN 1084- 0702/2008/6-602–610/$25.00. 602 / JOURNAL OF BRIDGE ENGINEERING © ASCE / NOVEMBER/DECEMBER 2008 Downloaded 20 Nov 2008 to 129.2.19.102. Redistribution subject to ASCE license or copyright; see http://pubs.asce.org/copyright