In the early 1900s, Carl Akeley, devised a novel method of making plaster models. He made a wire frame, and with a special gun he devised, sprayed plaster onto the frame by means of compressed air. The principle of Akeley’s method was to hold a supply of dry material in the gun, convey it to the nozzle with compressed air, and wet it with the proper amount of water as it was blown onto the frame. This resulted in strong, thin coatings that would not slump off the frame and eliminated the problem of the plaster setting up before it was fully placed. The equipment then was used to patch deteriorated concrete with a sand and cement mix. The results were excellent and, in 1911, patents and trade names were issued for the "Cement Gun" which is what the equipment was called, and "Gunite," the material produced by the process. Until the 1960s all shotcrete work was dry-process, using pressure-tank guns. During that decade, the rotary type of gun was developed. Today variations of these two basic designs are used for all dry-process applications. The wet-process also was developed in the 1950s, with pressure tanks being used to force a stiff mortar through the hose. Additional air was added at the nozzle and the process came to be known as Shotcrete. The subsequent development of dependable concrete pumps of various types hastened the advancement of this technology, and today the wet-process is widely used and favored for certain applications.

Pre-blended dry or semi-dampened materials are placed into the shotcrete equipment and metered into a hose. Compressed air conveys material at high velocity to the nozzle, where water is added. Material is consolidated on the receiving surface by the high-impact velocity. The dry-process can be used for any shotcrete application, from the smallest patching and sealing projects up to the largest, such as tunnel linings, pools, canals or reservoirs. The maximum production achievable with dry-process equipment is 10 -12 yd³/hr of dry mix, depending on conditions. To achieve this ultimate capacity, it is necessary to use 2" or 2-1/2" hose and a 900 cfm or larger compressor. Most applications have production rates of 2 - 6 yd³/hr of mix, using 1-1/4" or 1-5/8" hose and a 450-600 cfm compressor. Patching and sealing are done at even lower rates with 1" or 1-1/4" hose and a 250-350 cfm compressor. Rebound (rejected components of a mix) is an inherent part of the shotcrete process. Skill and expertise is required for control of rebound and overspray and must be a consideration when calculating in place volume. What are the advantages of the dry-process? •Easy setup, shutdown and cleanup. On-site control of materials. •Nozzleman can be up to 1,000 ft horizontally or 500 ft vertically from the gun.

The wet-process was developed in the 1960s with pressure tanks being used to force a stiff mortar through a hose. Additional air was added at the nozzle and the process came to be known as Shotcrete. The subsequent development of dependable concrete pumps of various types hastened the advancement of this technology, and today the wet-process is widely used and favored for certain applications. All ingredients, including water, are thoroughly mixed and introduced into the shotcrete equipment. Wet material is pumped to the nozzle where compressed air is introduced to provide high velocity for placement and consolidation of the material onto the receiving surface. Most wet-process shotcreting is done with premixed mortar or small aggregate concrete. The mix design and consistency of supply are very important in order to provide a mix with the workability or plasticity to be pumped through a small-diameter hose. Sand gradation is the same as for the dry process and for coarse aggregate mixes, 20-30 percent of 3/8" aggregate is added. Cement content will vary according to the aggregate gradation and surface area; however most mixes contain approximately 700 pounds or more of cement

Just as in concrete mix designs, the water-to-cement materials ratio is the single most important parameter influencing the compressive strength, shrinkage and overall durability of the material. With gunite, only the amount of water necessary for hydration is added to the sand and cement mixture. So it can be said that gunite is more densely packed than ready-mix concrete. In fact, 14 yards of gunite equal about 10 yards of ready-mix concrete or shotcrete mix. Denseness is an important factor in the possibility for superior strength in gunite and its high resistance to the passage of water. Nozzling technique is also crucial and less forgiving than ordinary ready-mix concrete. Good technique by the nozzleman can mean the difference between a dense high-strength material or one that looks good on the finished surface but actually has underlying sand pockets, voids and poorly encased reinforcing steel in both the “dry-mix” and “wet-mix” processes. Poor application technique will result in a higher probability of cracking and its negative ramifications. Placement of the nozzleman is key as well. Standing on the reinforcing steel can result in it being to shallow or too deep within the material. This could potentially cause rust spots or cracking.

The dry-process was developed in the early 1900's, culminating in patents granted in 1909 and the process being commercialized in 1911.