A non-technical description of equipment used in the sprayed concrete industry today
by Tom Norman, Airplaco Equipment Company
In the beginning, man created the dry-mix concrete process…and it was good. Today the dry-mix concrete process and the wet-mix concrete process are both in use. The choice is a matter of much discussion in our industry, and is based on preference, familiarity, application and equipment.
Wet-mix shotcrete involves pumping of a previously prepared concrete, typically ready-mixed concrete, to the nozzle. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface.
This article will focus on equipment for the dry-mix process, in which a dry cement mixture is blown through a hose to the nozzle, where water is injected immediately before application. The concrete is blasted by pneumatic pressure from a gun, hence the term: "gun"-ite.
Dry-Mix Shotcrete Equipment
The main categories of dry-mix shotcrete equipment are as follows:
- Pressure Vessels & Gunite Machines
- Batch Plants & Pre-Dampeners
- Water Pressure Boosters
- Hoses and Nozzles
Pressure Vessels & Gunite Machines
The key piece of equipment for any dry-mix shotcrete setup is the gunite machine, primarily a rotary gunite machine. The key to this piece of equipment is the addition of water at the nozzle--the most distinguishing feature of the dry-mix process. The first gunite machines (known as the "cement gun") were pressure vessels, basically steel tanks affixed with plumbing for air to flow throughout. Modern rotary gunite machines include a hopper where dry or pre-dampened material can be added. These machines are typically powered by an air motor, although hydraulic powered machines are also available. The motor rotates the gunite machine’s material feed system, which continuously directs material from the hopper to the outlet, where the material is propelled by air to the hose and nozzle.
Batch Plants & Pre-Dampeners
Batch plants are essentially mixers that combine cement and sand (aggregate) used in the dry-mix process. Materials used in the dry-mix process can be pre-bagged or mixed onsite. Pre-dampened materials can reduce static electricity, dust and rebound that are common with using "bone-dry" materials. Pre-bagged materials are available from several suppliers in specifications to meet virtually any job requirement. To control costs and delivery of the proportions of your mix, use a batch plant or combination batch plant/pre-dampener. The addition of a spray bar system to the batch plant gives the ability to pre-dampen materials before they are fed into the dry-mix shotcrete/gunite machine.
Water Pressure Booster
Another popular piece of equipment for the dry-mix gunite contractor is a water pressure booster pump. This pump, fairly small and portable, is used to increase the pressure of the water being delivered to the nozzle.
Hoses and Nozzles
Hose and nozzle choices for the dry-mix process are readily available to meet the needs of low to high production job requirements. The most common setups in the dry-mix process are 1 ½” or 2” material hose and nozzle designs. Various nozzle tips can be used to control the spread of material as it is placed on the surface by an ASA (American Shotcrete Association) certified nozzleman.
Putting it all together
A fully self-contained dry-mix shotcrete set-up will include a batch plant with a built in pre-dampening system working to supply a gunite machine, a heavy-duty air compressor, a reliable source for bulk dry materials, material hose, air and water hose, gunning nozzle, water pressure booster pump, and ASA certified nozzleman.
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