Dry mix process and wet mix process are the two methods of shotcrete construction which are based on the time at which the water is added to the mortar or concrete mix. in Dry mix process, water is added at the nozzle, while water is added in the concrete or mortar mix in wet mix process.
The principle, advantages and disadvantages of dry and wet mix shotcrete process is explained below.
Dry Mix Process for Shotcrete
American Concrete Institute (ACI) defines Dry mix shotcrete as ” the shotcrete in which most of the mixing water is added at the nozzle.”
Dry Mix Shotcrete Principle
The dry mix process involves the mixing of cement and wet aggregates at required proportion before supplying it to the shotcreting device. The thoroughly mixed ingredients are then placed on the device hopper. During the shotcreting operation, the mix, under the action of compressed air is taken from the hopper to the nozzle through the delivery hose of the equipment.
Once the dry mix reaches the nozzle, water under high pressure is sprayed to the mix through a perforated ring attached to the equipment. While spraying, the water wets the dry mix. Thus, the required wet concrete or mortar mix is jetted at a higher velocity on the surface to be shotcreted.
The amount of water added for the dry mix shotcrete process is either controlled by the nozzleman or placing operator. The water is controlled by means of a valve present in the device.
The dry mix shotcrete is applied in areas where there requires fewer placements and no or limited vehicle access.
Advantages of Dry Mix Shotcrete Process
The main advantages of dry mix shotcrete process are:
- Increased Bonding ability and bonding strength
- The water content is controlled at the nozzle
- Less waste is created during dry shotcrete
- Low investment
Disadvantages of Dry Mix Shotcrete Process
The main disadvantages of dry mix shotcrete are:
- Experienced nozzleman is required to control the water content varied through nozzle adjustment
- Possibilities of making shotcrete with variable water-cement ratio and water content
- Higher cost
- Spraying operation is tough
- High dust emission
- Water-cement ratio of the mix is dependent on the nozzle operator
- Difficulty in addition of admixtures
Wet Mix Process for Shotcrete
The wet mix process involves the mixing of all ingredients to form mortar or concrete with required water content. Here, the mix to be shotcreted is prepared before placing it in the shotcreting equipment. The delivery equipment used can be either a positive displacement type or a pneumatic -feed.
Wet Mix Shotcrete Principle
The process involves forcing the wet mix to the nozzle through the delivery hose by means of compressed air. The mix is then shot at high velocity on the surface to be shotcreted.
As shown in the figure-2 above, the mixed ingredient is delivered to the shotcrete pump by means of a transit truck.
Here, the nozzleman can only control the rate and amount of compressed air introduced. There is no direct control over the concrete or mortar properties by the nozzleman.
As the wet mix prepared requires transportation facilities, it is employed in large area construction and where the access of vehicle is not a concern.
Advantages of Wet Mix Shotcrete Process
- The water content is controlled more precisely
- Cost of the process is lesser
- Labour required is less
- The spraying operation is easy as the mix is prepared before
- The air consumed is less
- Admixtures can be added as per design requirement
Disadvantages of Wet Mix Shotcrete Process
- Bond strength and tensile strength is less
- Wastage of materials
- Transportation of wet mix will require high control