Wet-Mix silica Fume Shotcrete: Effect of Silica Fume Form
D. R. Morgan and John Wolsiefer, Sr.
Appears on pages(s):
chlorides; compressive strength; durability; permeability; electrical resistance; flexural strength; freeze-thaw durability; scaling; shotcrete; silica fume; Materials Research
Silica fume is commercially available in North America in a number of different forms. The influence of three different forms of silica fume on the plastic and hardened properties of wet-mix shotcrete were investigated. The silica fume forms studied were: uncompacted (as produced), compacted low-density, and compacted high-density. All three forms came from the same silicon metal furnace. In addition, a compacted low-density silica fume from a ferrosilicon furnace was evaluated. This study demonstrated significant improvements in the adhesive and cohesive properties and thickness of buildup before sloughing on vertical and overhead surfaces for all forms of silica fume, when compared to a conventional portland cement wet-mix shotcrete. Similarly, there were substantial improvements in the compressive and flexural strengths of all the silica fume-modified shotcretes compared to the plain portland cement shotcrete. The silica fume from the ferrosilicon furnace was evaluated for freeze-thaw resistance (ASTM C 666, Procedure A), and to resistance to scaling from deicing chemicals (ASTM C 672), and was found to be durable in these aggressive exposure tests. Various other physical properties of the shotcretes, such as air content in plastic and hardened concrete, parameters of the air-void system, drying shrinkage, chloride permeability, electrical resistivity, absorption after immersion and boiling, and volume of permeable voids, were evaluated. This study has demonstrated that, while there are some differences in the relative performance of shotcretes made with the three different forms of silica fume, all three forms can be used to produce high-quality wet-mix shotcrete. Finally, brief case histories are presented to show where wet-mix silica fume shotcretes subjected to numerous cycles of freezing and thawing, in a saturated condition in the field, have displayed good durability.