Portal: General

You are correct in stating that entrained air is necessary in concrete that is exposed to freezing and thawing while critically saturated. Even vertical walls can get critically saturated in places. Because you are working in a part of the country that experiences significant freezing and thawing, it is imperative that you maintain sufficient air content in the shotcrete. Remember, you are going to lose some air content in the placement process so the air content of the shotcrete mixture going into your pump must be higher than the desired in-place air content. It is a wise idea to do some testing in advance of the actual shotcreting to determine how much air content you will lose.

Proper placement is the most important element in achieving good shotcrete results. Most defects that occur in shotcrete are due to poor placement. Shotcrete success depends largely on the skill and actions of the nozzleman. The nozzleman’s goal is to achieve adequate compaction and good encasement of the reinforcement (if present) with no entrapped rebound or hardened overspray. For this reason, it is important to require that the nozzleman be ACI certified for the application. There are specific certifications for both wet and dry processes as well as vertical and overhead applications. If the nozzleman is certified, the probability that you will get the desired results is significantly increased. For more information on certification, visit the ASA website, shotcrete.org, and click on Certification.

Go to the ACI website, www.concrete.org, and click on the Certification tab. A button will appear for the Certified Personnel Directory. Click this button. Using Search Option 2, customize the search by type of certification and location. Please note, ACI will identify the individual by name, city, and state only. The individual address, telephone, or employer is not available from ACI or the ASA. Education for ACI Certification is available through the ASA office. Contact ASA for the roster of ASA Educators. Certification exams are conducted by ACI-approved examiners in strict compliance with ACI certification policies.

Shotcrete is concrete forced or impelled through a hose using a pressurized air system. Therefore, the guidelines for jointing concrete are no different than for concrete placed by other methods.

Shotcrete is an all-inclusive term to describe the spraying of concrete or mortar that may be accomplished through either a dry- or wet-mix process. Gunite refers only to the dry-mix process in which the dry cementitious mixture is blown through a hose to the nozzle, where the water is injected immediately prior to application. Because complete mixing of the water and dry ingredients is not possible in the nozzle, mixing is completed as the material impinges on the receiving surface, through manipulation of the nozzle. This requires a very highly skilled nozzleman, especially in the case of thick or heavily reinforced sections. Large aggregate is seldom used with the dry-mix process. Wet-mix shotcrete involves pumping of a previously prepared mixture, typically ready mixed concrete, to the nozzle. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The mixture usually contains minus 1/2 in. aggregate, although larger-size aggregate has also been used.

The use of the term “shotcrete” first occurred in Railroad Age magazine more than 50 years ago in place of the then proprietary word “Gunite,” and has been used by the American Concrete Institute since at least 1967 to describe all sprayed concrete or mortar.