Development of an Economical Ultra-High-Performance Concrete Mix Using Local Materials

Ultra high-performance concrete (UHPC) is an advanced cementitious composite material characterized by an optimized gradation of granular constituents, a water-to-cementitious materials ratio of less than 0.25, and a high percentage of discontinuous internal fiber reinforcement. UHPC exhibits exceptional mechanical properties in comparison to traditional concrete, including a compressive strength greater than 17.4 ksi (120 MPa) and a post-cracking tensile strength greater than 0.72 ksi (5 MPa). Due to these exceptional mechanical properties and durability properties, UHPC is being increasingly utilized in critical infrastructure projects, such as nuclear waste containment facilities, precast structural connection joints, bridge girders, and bridge deck systems. However, the high cost of commercially available, pre-bagged UHPC remains a significant obstacle. This study aims to develop a cost-effective, nonproprietary UHPC mix design using materials that are readily available in Alabama. Thirty-five trial mixes were produced to optimize non-proprietary UHPC formulations for local materials available in the southeast region. The mix design iterations focused on adjusting supplementary cementitious materials (SCMs), water/cementitious material ratio, fines and type, cement type, and high-range water reducer (HRWR) dosage. The study investigated the effects of binder and HRWR type and content, SCMs, sand type, and curing regimes on the compressive strength of the UHPC. An optimal mix that achieved the desired workability and strength was selected for further evaluation through standard ASTM testing methods. The mixes developed in this research exhibited a minimum static flow of 10 inches. The presence of large quantities of cementitious materials can result in high shrinkage strains. The measured shrinkage strains for the UHPC mix developed in this study varied between 400 to 600 𝜇ε. The mechanical propertiesof UHPC evaluated in this study include compressive strengths of approximately 19 ksi (131MPa) and elastic modulus varying from 6000 ksi to 6400 ksi. The stress-strain behavior of the UHPC in tension is evaluated using a four-point bending test and the inverse analysis. The measured tensile strength of the UHPC mixes varied between 1.55 to 1.7 ksi, with localization strains of 0.004-0.006 in./in. The Rebar development length in UHPC was experimentally investigated using modified pull-out tests. It was found that an embedment length of 10db (wheredb = diameter of rebar) in UHPC with a clear cover of 2db is sufficient to yield Grade 60 mild steel reinforcement.