Shotcrete Vs Gunite Which Is Best?
Shotcrete Gunite – Both Shotcrete and Gunite open up new methods of concrete construction. Sprayed concrete lets you go vertical with concrete without all the formwork needed for poured in place concrete.
Concrete can be applied to a vertical or overhead surface with a trowel or pneumatically with a shotcrete or gunite process. While the processes are different, some people use the terms interchangeably. This paper reviews the differences in the processes and looks at their inherent advantages and disadvantages. Both systems require a trained crew.
One distinct advantage of either the shotcrete or gunite system over hand-application is the water/cement (W/C) ratio of the in-place concrete. A benefit of shotcrete and gunite is that the in-place W/C ratio is usually less than a mix that was not pneumatically applied. One of the reasons for this is that more water is evaporated during the spraying process than is evaporated with the trowel process.
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 – 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.
Stop and Start – For example, if a gunite machine is stopped, it only takes a few seconds to clear the delivery hose. If a shotcrete machine is stopped, the hose, which may be 3 inches in diameter and may be 300 feet long, can contain over half a yard of wet concrete which will start to set up. Murphy shows up at every jobsite, so procedures have to be in place to clear the shotcrete hose if machinery failures occur.
During a normal work day, either system will need to be stopped and started a number of times. Restarting the shotcrete system requires more energy to get that long column of concrete moving again. If there is a weakness in the shotcrete machinery and delivery hose, it will become evident during one of the re-start occurrences.
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Hazardous Dust – Gunite or shotcrete dust is hazardous. Adequate protection needs to be taken to protect the health of the crew, especially the nozzle man. Gunite can produce much more dust than shotcrete. To control the dust with a gunite machine, typically the mix will be pre-wetted by having 2% water (by weight of cement) added before the blend is put into the gunite rig. Pre-wetting the concrete limits dust, but does not eliminate it. Especially in a confined space, the dust from a gunite process limits visibility more than from a shotcrete process.
Rebound – Rebound is another difference. The amount of rebound from a gunite operation is greater than from a shotcrete operation; however, with a gunite operation, the rebound can be collected and recycled. From a shotcrete operation, the rebound is waste material.
Dragging the Hose – In moving around the work-site, the weight of the hose can become a serious consideration when making the decision to go with gunite or shotcrete. The gunite hose is typically 2-4 inches in diameter and will contain only a few ounces of material per linear foot. The shotcrete house is typically 2-3 inches in diameter and will contain about 7 pounds of material per linear foot.
Hose Leaks – If a gunite hose leaks, a serious dust problem immediately develops. If a shotcrete hose leaks, a small amount of water escapes and the pumped mix requires more energy to pump the column of concrete. If the leak is not fixed, it is likely that the concrete mix will stiffen to the point it cannot be pumped and then the job is shut down until the hose is cleared and the leak is fixed.
Shotcrete Gunite Resources- Although much of spraying concrete can get reduced to selecting equipment, in may ways this can be described as an art-form. It is important to find a mentor that can guide you and share experiences of hard lessons learned. In addition to finding other people are knowledgeable about shotcrete a good resource is the American Shotcrete Association. The ASA has a great magazine thatÂ you will want to read.
If you need advice or an introduction to others that are passionate about shotcrete call or email Nolan Scheid for more information. We are happy to help.