Characterization of optimized low heat high performance concrete

The shaft seals for a deep geological repository may include various materials (bentonite/sand, asphalt-based material, and concrete) with different functions. The Nuclear Waste Management Organization reference concrete is the Low Heat High Performance Concrete (LHHPC) originally developed by the Atomic Energy of Canada Limited (AECL). The LHHPC mix was optimized based on the original reference concrete mix design, using mix ingredients from local and sustainable sources. The purpose of the current project was to obtain further characterization data to confirm the performance of the optimized LHHPC mix. The study includes the following: • Optimized mix ingredients sourcing; • Reference water preparation and testing; • Mix ingredients characterization; • Trial batch qualification; and • Test program. The optimized mix ingredients selected, sourced and characterized are all suitable for LHHPC mixes. In particular, the free silicon content in the silica fume met the performance target for the study. The reference waters prepared for the test program include a reference crystalline rock water (CR-10) and a reference sedimentary rock saline water (SR-270) for saturated hydraulic conductivity tests. Reference waters were prepared by Wood and their chemical composition was tested in a certified external laboratory. It is understood that another reference sedimentary rock saline water (SR-290) will be used for further testing. Optimized LHHPC trial batches were prepared and the LHHPC specimens were cured in a moist environment at 100% relative humidity. The qualification tests were conducted, including bulk density and porosity at 30 days, pH in distilled water at 28 days, 7-day unconfined compressive strength (UCS) and slump and slump flow retention up to 2 hours from the time of mixing. All these test results met the qualification test requirements for the study. The following properties were measured at the appropriate age for the optimized LHHPC mixture following test methods as described in the project test plan: • Chemical and mineralogical composition to approximately the 1 wt% level; • Bulk density; • Porosity; • Unconfined compressive strength and crack initiation; • Split tensile strength; • Creep; iv • Triaxial compression; • Saturated hydraulic conductivity; • Maximum temperature rise at the center of cubic specimens; • Shrinkage rate; • pH; • Slump and slump flow retention; • Rheology of fresh concrete mix to obtain viscosity and yield stress; and • Thermal conductivity. The results indicate that the optimized LHHPC mix met the relevant performance requirements for up to 270-day project parameters. Based on the slump and slump flow retention test results, along with the rheology test results, the following revisions to the performance targets for initial slump and slump flow are recommended to be made: • Slump 220 +30/-20mm provided no bleeding and/or segregation • Slump flow 400 – 650mm provided no bleeding and/or segregation. Based on the trial batch qualification test results, it is recommended to add the temperature rise test to the existing LHHPC trial batch qualification tests. This is the final report. (author)