Development of non-proprietary ultra-high performance concrete (UHPC) and determination of its key mechancial properties

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2022-05

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Ultra-High Performance Concrete (UHPC) is a cementitious composite material that constitutes one of the major advances in concrete technology in the past three decades. UHPC exhibits superb mechanical and durability characteristics with compressive strength exceeding 124 MPa (18 ksi) and sustained minimum tensile strength of 5 MPa (0.72 ksi). The main constituents of this composite material are cement, fine sand, mineral fillers, high amounts of supplementary cementitious materials and high strength steel fibers that contribute to the enhanced tensile capacity of UHPC. Because of its increased tensile strength over that of regular concrete, UHPC offers a potential structural advantage by reducing the weight of structural elements and facilitating the construction of longer bridge spans. Nevertheless, several challenges remain that have hindered the widespread use of the material for bridge applications. Some of these challenges are related to the cost per cubic yard of UHPC, the need of a widely accepted test framework to evaluate its tensile properties, and sufficient research evidence regarding the long term behavior in tension (creep) of UHPC. The purpose of this research is to provide answers to some of these challenges. Initially, the focus of this study is the development of a non-proprietary UHPC mix with a target compressive strength of 138 MPa (20 ksi) and minimum sustained direct tensile strength of 5 MPa (0.72 ksi). The total cost of the mix should be reasonably less than that of proprietary mixes and other important aspects will be addressed, such as the total cement content and the careful selection of mineral fillers and supplementary cementitious materials, to produce a mix which is also friendlier to the environment and to those handling the material. In the second stage of this research, the mechanical properties of the selected non-proprietary mix are investigated with a focus on the tensile properties of the material. Finally, the most important aspect of this dissertation is to determine the influence of cast orientation in the direct tensile properties and long-term behavior in tension (tensile creep) of the non-proprietary UHPC mix, which have been identified as the biggest research gaps existing in current literature.

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