Mixture proportioning and freezing and thawing durability of ultra high performance concrete mixtures using local materials

Use of local materials in the fabrication of ultra high performance concrete (UHPC) provides a more affordable alternative to prepackaged UHPC mixtures. However, rigorous testing to quantify strength and assess durability is required to approve new UHPC mixtures. This study expands on previous studies that developed UHPC mixtures by modifying existing mixtures to reduce production cost while ensuring adequate strength and durability.;The first steps in developing more economical mixtures were to reduce curing temperatures to 95°C (203°F), decrease the water-to-cementitious materials (w/cm) ratio, and replace silica fume with Class F fly ash. The cementitious materials were further reduced by increasing the fine aggregate content, which results in more economical mixtures. Fine aggregate top size was also increased to 4,750 gm (0.187 in.). Lastly, the effects of steel fibers on compressive strength were evaluated. Eight mixtures were identified as the most feasible for further development. The selected mixtures produced average seven day compressive strengths that ranged between 118.8 MPa (17,230 psi) and 138.0 MPa (20,020 psi) for 102 mm (4 in.) cube specimens and between 129.8 MPa (18,830 psi) and 139.9 (20,290 psi) for 51 mm (2 in.) cubes.;Since the chemical and physical requirements for local materials may not be as strict as for commercial products, it is imperative that the durability parameters of newly developed UHPC mixtures be assessed. The second part of the study included the assessment of UHPC freezing and thawing durability. Resistance to freezing and thawing was evaluated in accordance with ASTM C 666 Procedure A. A total of 39 UHPC mixtures were selected for this study. Results indicate that UHPC mixtures are highly susceptible to curing practices after heat treatment, with specimens that were dry cured after completion of heat treatment producing greater durability factors (DFs). Also, mixtures proportioned with at least 12.5% fly ash content provided superior DF versus mixtures with 100% silica fume (no fly ash).;Lowering the w/cm ratio also increased DF. Fine aggregate content and top size had small influences on durability of UHPC mixtures. Additional parameters were also studied during freezing and thawing testing. Quality factor, mass, and length measurements were performed, and their changes during freezing and thawing testing were evaluated.;A study using a modified version of ASTM C 666 was also performed to determine if a lower freezing temperature would influence the freezing and thawing durability of UHPC. The minimum temperature was decreased from -17.8°C (0°F) to -26.1°C (-15°F) for the modified testing procedure. An average DF of 99.8 was obtained from testing a UHPC mixture at the lower freezing temperature, only 0.9% below the DF obtained using the standard ASTM C 666 cycle.;The last part of this research included the evaluation of flexural strength of UHPC mixtures. The objective was to quantify flexural strength before and after freezing and thawing testing. Flexural strengths ranged between 6.03 MPa (875 psi) and 14.18 MPa (2,060 psi) for specimens tested at 14-days and between 5.72 MPa (829 psi) and 14.42 MPa (2,090 psi) after specimens had been subjected to 300 cycles of freezing and thawing. A correlation between flexural strength of UHPC mixtures and DF was investigated to determine if DF was a reliable indicator of tensile capacity after freezing and thawing. Results showed that the effects of fiber content, w/cm ratio, fine aggregate content, fly ash content, aggregate top size, and curing regimen on DF of UHPC for the modified mixtures were easier to quantify compared to the effects of the same variables on flexural strength, especially after UHPC specimens had been subjected to cycles of freezing and thawing.