The dry mix method involves placing the dry ingredients into a hopper and then conveying them pneumatically through a hose to the nozzle. The nozzleman who holds the nozzle than controls the addition of water at the nozzle. Cement Content The dry ingredients are typically blended and packaged by the shotcrete supplier into 30 kg bags or larger bulk bags. Dry mix blends typically contain 250-450 kg/m3 of cement per cubic metre of aggregate. This is about 320 kg/m3 of wet concrete. The process of spraying the concrete results in some of the material rebounding and falling on the ground. This so-called "rebound" commonly amounts to 20% of the shot volume. Most of the rebound consists of coarse aggregate and therefore the remaining shotcrete has a higher cement content than the original mix. Water-Cement Ratio Like all concrete, the water:cement ratio of the dry-mix shotcrete is critical for the final quality and strength. Unlike the wet-mix system where the water content can be carefully controlled in the batching process, water for the dry-mix process is added at the nozzle and can be controlled by the nozzleman. Some moisture may also be present in the aggregates or added to the dry ingredients in a pre-dampener before the material is shot. If too much water is present, the shotcrete will not adhere well to the walls and roof of the tunnel. If there is insufficient water, there will be a lot of dust generated, making it difficult to see the shooting surface. In properly shot dry-mix shotcrete, the water:cement ratio is typically between 0.4 to 0.5. Pre-packaged Blends or Blend on Site The dry ingredients can be blended on site or they can be blended at the mine site. If dry materials are blended at the mine site, they should be used within 1-2 hours of being blended. The natural moisture content of the aggregates is an important factor to consider. Too much moisture will result in premature hydration of the cement and crusting of the equipment. If onsite blending of dry ingredients is used, the natural moisture content of the aggregates should be less than 5%. The storage life of the blended ingredients can be extended to 2-3 days with the use of hydration control admixtures. Such admixtures are added during the blending of the dry ingredients to prevent hydration. An activator is then used at the time of application of the shotcrete which counteracts the effect of the hydration control admixture and accelerates the hydration. If pre-packaged dry ingredients are delivered to the site, they will be completely dry and working with these materials can create very dusty conditions, particularly in underground mines. In such cases, it is recommended that a pre-dampener consisting of fine water spray nozzles be used to reduce dust generation. Pre-packaged material delivered to site in paper or bulk bags provide the best flexibility in terms of maintaining supply and providing good quality control. This is offset however by the much greater expense to purchase and transport the dry shotcrete ingredients. Typical Mix Design A typical mix design for dry mix shotcrete is provided in the following table Advantages and Disadvantages of Dry Mix Advantages of Dry-Mix Method | Disadvantages of Dry-Mix Method | Flexible for use on small jobs, where access for mobile mixers is difficult | Expensive cost of materials for packaged shotcrete | Less expensive equipment | Higher equipment wear rate compared to wet mix | Suitable for use where transport distance from batch plant to shotcreting location is great. | Dust formation |
 Introduction
Wet Mix Process Shotcrete Reinforcement: Mesh vs. Fibre Use of Shotcrete Admixtures and Additives Shotcrete Rock Support Design Shotcrete Equipment | 
