fly ash is not as effective, both silica fume and carbon black sub- stantially reduce ... wall is significantly lower than that in the design mixture, and this translates ...
ACI MATERIALS JOURNAL
TECHNICAL PAPER
Title no. 97-M15
Rebound in Dry-Mix Shotcrete: Influence of Type of Mineral Admixture by V. Bindiganavile and N. Banthia The high rebound in dry-mix shotcrete has severe implications both on its long-term durability and the mechanical performance in service. The use of mineral admixtures is often recommended as a way of reducing rebound, but the exact influence of the admixture dosage rate, shape, and mean size of their particles on the rebound is not clearly understood. In the research reported herein, four mineral admixtures—silica fume, fly ash, high reactivity metakaolin, and carbon black— were investigated in plain and fiber-reinforced dry-mix shotcrete for rebound and other properties. These admixtures represent particle sizes ranging from 0.05 to 10 µ m and particle shapes ranging from spherical to plate-like. Results indicate that while fly ash is not as effective, both silica fume and carbon black substantially reduce the rebound. The performance of high reactivity metakaolin lies somewhere in between. Overall, it appears that the mean particle size of the admixture influences the rebound far more than the shape of its particles. Keywords: carbon black; fly ash; shotcrete; silica fume.
INTRODUCTION Shotcrete, which is mortar or concrete pneumatically projected into place, is fast becoming a material of choice for repair and rehabilitation, slope stabilization, and temporary support in mines and in tunneling. Shotcrete may be applied via either the dry-process or the wet process, and both processes are equally utilized if one considers the total volume of shotcrete applied around the world. One primary concern with the dry-process shotcrete, however, is the high rebound where nearly 20 to 40% of material and up to 75% of fiber may be lost through rebound.1 This is in sharp contrast to the wet-mix process where both material and fiber rebound values stay as low as 5 to 10%. Apart from the excessive loss of material, an additional concern with the dry-process shotcrete is that the rebound material comprises primarily of sand and aggregates. This reduces the aggregate content in the in-place shotcrete and, in turn, unduly increases the cement content. With high cement contents, sometimes approaching 700 kg/m 3, and the lack of sufficient aggregate quantity for volumetric stabilization, shotcrete is rendered highly susceptible to cracking due to shrinkage and thermal effects, vulnerable to attack by aggressive agents and predisposed to inferior long-term performance. With an inordinately high fiber rebound, the fiber volume fraction on the wall is significantly lower than that in the design mixture, and this translates into a major loss of fracture toughness, deformability, and the postcrack load-carrying capacity in shotcrete. Reduction of rebound in the dry-process shotcrete is, therefore, often recognized as one of the most pressing challenges. Use of various mineral admixtures in shotcrete has increased several-fold in the last two decades alone. There are several reasons for their popularity: Most of these admixtures are by-products of other industries, and their use in shotcrete appropriately addresses the issue of their safe disposal. The admixtures, when employed judiciously, enhance the performance of shotcrete in service, and perhaps
ACIMaterialsJournal/March-April2000
Table 1—Properties of mineral admixtures investigated Admixture
Mean particle size, µm
Specific surface area, m 2/g
Particle shape
Carbon black
0.05
44
Spherical
Silica fume
0.10
20
Spherical
HRM
