27-07-2012, 12:27 PM
Shotcrete and gunite
Shotcrete- Wet Gunning
“Wet gun” means the wet (already mixed) concrete is pumped to the nozzle. Air is added at the nozzle to carry the concrete mix to the target — again pneumatically applied. We call the system “wet gunning.” Here too operator training is important, but not quite as technical for wet gunners. Mistakes in application are usually far less critical than with dry gunning in Monolithic Dome construction. Rebound, or the waste created by sprayed concrete falling to the floor, is usually half as much for shotcrete as compared to gunite.
Equipment
Dry gun equipment is totally different from the wet gun. The air needed to transport the concrete is at least four to six times greater than that needed for the wet gun system. For wet gunning, a pea rock or grout pump is usually utilized. Monolithic has updated and improved the small peristaltic pump in an effort to provide more affordable equipment for wet gunning.
For house-size domes either system will work. But, for the novice, the wet gun is my recommendation.
The dry gun does not have the capacity for large domes — 50 feet in diameter and bigger. Dome Technology once had a five-man crew shotcrete 238 cubic yards of concrete in one day! They can routinely apply 100 plus cubic yards. This is well beyond the capacity of a dry gun system.
Shotcrete – Shotcrete is the process of pumping wet concrete down a hose to apply it to a surface. At the shotcrete nozzle, air is added. The air from the shotcrete nozzle provides several functions, including to propel the mix at a higher velocity and to spread the mix out into a cone-shaped spray pattern. Since the shotcrete is being propelled at a high velocity, compaction occurs as the shotcrete particles accumulate on the application surface. Increasing the air can mean higher shotcrete velocity and better compaction, which leads to denser concrete that exhibits greater compressive strength and less permeability. With shotcrete the W/C ratio is controlled at the batch plant or at the mixing station. The concrete needs to be plastic enough to pump. Since low W/C ratios produce a more durable and water-resistant mix, water-reducing agents are often used. In general, a W/C ratio under 0.5 produces a more durable and waterproof mix. Since the concrete is pumped wet, the process is also known as Wet Mix and Wet Process. Wet process shotcrete is an extension of concrete pumping.
Gunite- Dry Gunning
“Gunite” is a trade name for “dry gunned” concrete, invented and patented by a North Carolina man. The term “gunite” has been used so much that, to most people, it means spray applied concrete. People often talk about a gunite pool — meaning a concrete pool where the concrete is pneumatically applied or sprayed in place using air pressure. Many of us in the concrete spraying business have started using the term “dry gun” to delineate this process.
“Dry gun” means the cement and sand are injected into an air stream conveying it to the nozzle. The nozzle operator then adds the water at the nozzle and has total control of the water-cement ratio. The delivery hose of the mix is generally quite light, as the hose is mostly filled with air containing the mix of cement and sand.
Dry gunning allows concrete to be placed drier than most “wet gunning,” but that is not always a plus for dome building. Often the concrete is stacked at a very heavy thickness. This can be a disaster for an Airformed structure. Operator training is very critical and can mean the difference between success and failure of application. The danger of dry gunning the concrete on a Monolithic Dome increases as the size of the dome increases.
Gunite – Gunite, which started out as a brand name, is also called Dry Process. Typically, a much larger air compressor is used than with Wet Process. A gunite machine will normally be used with a compressor in the 300 to 700 CFM range. The gunite machine feeds controlled amounts of the dry concrete mix into the air stream. The air stream carries the dry concrete material down the hose to the nozzle. At the gunite nozzle the nozzle man adds enough water to the dry materials to enable it to stick on the applied surface and to hydrate. This typically is much less water than is required to pump the wet mix. This gives the nozzle man a much greater control of W/C ratio than with wet mix. Since the gunite can be applied dryer, it is possible to build thicker during a single application. In many cases for overhead work, dry process can be a better choice. An additional piece of equipment called a pre-dampener can add approximately 2% water by weigh of cement to the mix before it is added to the dry gunite rig. This will help with hydration and dust. Applying gunite may be easier technically than spraying wet mix shotcrete but there is a higher level of responsibility of the Nozzleman for a quality job.
Each process has advantages and disadvantages. Knowing the advantages and disadvantages will allow better decision making when selecting the process to use.