Mechanisms of placement and stability of dry process shotcrete

The knowledge available today in shotcrete technology usually permits the production of strong and durable shotcretes. Unfortunately, very few research projects have focused on the development of the shotcrete process itself. Consequently, the fundamental knowledge on the properties and behaviour of fresh dry-mix shotcrete is very limited. The objective of this research project is thus to develop methods to evaluate the properties of fresh dry-mix shotcrete in order to better understand its behaviour, or its shootability. Shootability is defined by the ease and efficiency with which a shotcrete mix can be placed; the main parameters which characterise shootability are rebound, maximum buildup thickness and reinforcement encasement. To understand and control these characteristics requires an understanding of the mechanisms involved in the placement and the stability of a shotcrete layer. These mechanisms involve both rheological and mechanical concepts: the dynamic placement of the material, without external vibration, implies high shear rates, whereas the in-place stability of the fresh material is more closely related to plasticity in terms of shear resistance. The consistency of fresh dry-mix shotcrete is considerably stiffer than that of conventional concrete, and therefore special apparatus and new testing procedures had to be created or adapted to evaluate the fresh properties of this unique material. The tests included the penetration test, fresh tensile strength test and fresh shear strength test. These tests along with detailed analysis of the in-place proportions of the mixtures helped to provide an understanding of the mechanisms needed to optimise the shootability of dry-mix shotcrete. Apart from the properties related to shootability, fundamental properties such as the yield value, the cohesion and the angle of friction were also determined. Additional information has also been gathered on the fresh dynamic properties of shotcrete, which are especially important in understanding the factors affecting the sound placement of a layer of shotcrete. Finally, a set of clear recommendations is proposed in view of the knowledge and information gathered concerning future avenues of research and development.Applied Science, Faculty ofCivil Engineering, Department ofGraduat