High-reactivity metakaolin and silica fume in steel fiber reinforced dry-mix shotcrete

Shotcrete as a construction material has been in use since the early part of this century. It is increasingly being used in new construction and in repair and rehabilitation of older structures. Its use spans a broad array of applications such as in tunnel linings, slope stabilization and in the repair of marine, highway and railway structures. Shotcrete is easy to apply, does not require complicated equipment and i f properly applied, it is sufficiently durable. However, one of the major drawbacks with shotcrete in general and dry mix shotcrete in particular, is the inordinately high amount of rebound that occurs during production. In addition to being an economic liability, rebound results in such adverse conditions as high in-situ cement content and low in-situ fiber volume fraction. The high cement content, apart from being detrimental to shotcrete durability is also of particular concern from the environmental point of view. Cement production is highly energy intensive and therefore, in the last couple of decades, the use of industrial by-products as supplementary cementing admixtures has gained popularity. Considerable research on the use of silica fume, fly ash and blast furnace slag in shotcrete has already been carried out and today, most shotcrete in North America contains one or more of these admixtures. In the case of silica fume, it is seen to impart all of its known benefits in cast concrete to shotcrete, but the main attraction of silica fume for use as an admixture in shotcrete is in the dramatic reduction in rebound through significant increase in cohesiveness. High strength cementitious composites suffer from an inherent lack of toughness and silica fume dry mix shotcrete is no exception. Silica fume shotcrete beams show considerable instability under static flexural loading. In addition, all concrete and shotcrete structures having silica fume develop a darker color, which is undesirable from an architectural point of view. In light of above drawbacks witnessed with silica fume addition to dry mix shotcrete, there is a need to search for a substitute to silica fume, which shares all its benefits but none of its demerits. This search narrowed down to a relatively new development in the field of supplementary cementing admixtures namely, High Reactivity Metakaolin (HRM). It is a refined byproduct of the glass manufacturing industry. As it has shown very promising results when used as a cement replacing admixture in cast concrete, it was hoped that the same benefits namely, high strength, better durability, pleasing aesthetics and perhaps most significantly, a higher fracture toughness, would accrue from its use into dry mix shotcrete as well. With this in mind, this research investigates the use of High Reactivity Metakaolin in fiber reinforced dry-mix shotcrete and compares the effects of H RM with those of silica fume. This research looks into the changes in microstructure, fresh and hardened shotcrete properties resulting due to the presence of H RM and silica fume in the matrix. At the dosage levels in this investigation, the portlandite (CH) consumption of H RM is lower than that of silica fume. This is as a result of the higher SiC>2 content in silica fume. It is found that H RM does not reduce the material and fiber rebound as substantially as does silica fume. It appears that silica fume imparts more cohesiveness to the fresh dry mix shotcrete compared to H RM and this not only means lower rebound but also higher buildup thickness. Steel fibers bearing two different types of geometry were included in this study to investigate the flexural toughness of HRM-dry mix shotcrete. However, due to the inherent lack of control on water content in the dry process of shotcreting, it was not possible in this study to examine the effect of H RM on shotcrete toughness vis a vis silica fume. An investigation was carried out on the influence of admixtures, especially of their particle size and shape, on the fresh dry mix shotcrete properties including rebound and build-up. It was found that while the shape of the admixture particles did not significantly affect these properties, size and specific surface area of the admixture particles played a decisive role in affecting rebound and the overhead build-up. Admixtures comprising of finer particles reduced rebound substantially and among the fillers investigated, carbon black, which is finer than silica fume, outperformed all others. In general, it appears that those pozzolans that have spherical particles and higher specific surface area are ideally suited for use as supplementary cementing materials in dry mix shotcrete.Applied Science, Faculty ofCivil Engineering, Department ofGraduat