Introduction

Before the introduction of the more controlled shotcrete specifications that are currently being practiced by the M.T.O., M.T.Q. and other specifying authorities, shotcrete or Gunite specifications were often written simply by listing the process and a specified 28 day compressive strength. The quality of shotcrete applications was ensured simply by limiting the eligibility of contractors who could bid the job to those with a pre-determined number of years experience. Mix proportions were often specified by volume and batched using full bags of cement and shovels of bulk sand. The components were then mixed using a mortar mixer and shot into place through a shotcrete machine. Random cores could be taken from the repaired structure to determine if the shotcrete would meet the specified compressive strength. However, few other pro-active measures were taken to ensure an installation of good quality.

Shotcrete specifications have evolved considerably since this time and the introduction of
factory controlled pre-blended materials, pozzolins such as silica fume and fly ash, admixtures such as accelerators and powdered air entraining agents, fibre reinforcement of shotcrete mixes and certification of shotcrete nozzlemen have improved the quality of shotcrete applications and reduced the occurrence of poor quality shotcrete.

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