I am an architect working on a project in which a masonry building was “coated” with a shotcrete or gunite material. It is reported that this was troweled on approximately 20 years ago. We have made test holes and it is adhered very well, except where moisture penetration through the parapet has compromised adhesion. This is installed with five layers approximately 1 in. (25 mm) thick. We are considering leaving the material in place, patching as required, and removing the existing nonpermeable paints (which are peeling). Our thought is to coat the surface with a variegated colored stain, allowing for permeability of vapor. Can the material be patched as necessary? What is the recommended finish? Do we need to be concerned that the material could permit water penetration? Are there certified contractors in the Cleveland, OH, area?

From what you have described, the original work may have been done by the shotcrete process or hand-applied. Typically in the shotcrete process, over a large area at the thickness of 5 in. (127 mm), you would expect to see reinforcing mesh or reinforcing bars.
The material can be patched. If the areas are small, it may be prudent to hand patch. Shotcrete is most effective in larger applications. It is important to select a good repair mortar or mixture design.
ASA does not have a recommended finish. Generally in this type of application, the goal would be to match the existing surfaces as close as reasonably possible.
Many patches are done on bridges and other structures without regard to water penetration.
We do not have certified contractors; however, many of our corporate contractor members have ACI Certified Nozzlemen on their teams and the Buyers Guide on the ASA website can be searched by location and specialty. Also on our website, we have a position paper on “Shotcrete Contractor and Crew Qualifications” with a handy checklist summary that would be useful in qualifying potential contractors for your project.

We are an architectural design firm that specifies extensive amounts of shotcrete to mimic rockwork in our designs for large-scale animal exhibits. We often encounter differing views on the need for galvanized or coated reinforcing bar in our shotcrete work, especially when they are used as pool lining walls containing salt water. Can you tell us your opinion on whether galvanizing or coating of reinforcing bar is necessary if both integral and surface waterproofing are specified?

Shotcrete is a process of placing concrete. The need to use galvanized or any other type of reinforcing is, or should be, based on the needs of the project and the preference of the structural design engineer. For liquid-containing concrete structures, the designer may review ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures,” which has specific concrete material, cover, and reinforcing requirements for concrete exposed to a variety of chemical and saltwater exposures. (The aforementioned ACI Committee 350 publication may be purchased from the American Concrete Institute at www.concrete.org”.)

We are working with an architect in New York City on an unreinforced masonry (URM) building where they want to remove brick to provide a larger storefront opening. I would like to use the remaining walls to resist lateral forces but the brick is insufficient. We would like to remove one width of brick and apply 4 in. (102 mm) of reinforced shotcrete in its place. Can you tell me where I can find applicable code and design guidelines for this application?

Your proposed solution is certainly reasonable and is used regularly. Shotcrete has been used to strengthen both URM and tilt-up panels to accommodate enlarged openings. Shotcrete is a method of placing concrete and the in-place properties would be the same. The applicable code would be the code you would use if you were to strengthen this wall with concrete. Designs using the ACI 318 Building Code and Commentary are fully applicable to shotcrete placement, although compressive tests for acceptance are secured using cores from shotcrete test panels per ASTM C1140/C1140M and C1604/C1604M, rather than cast cylinders.

An artist we have commissioned will be using gunite for the creation of a large-scale geode-inspired sculpture. There is some concern from the community about vandalism, specifically graffiti. Do you recommend sealing or applying anti-graffiti coating to gunite? If so, what brand of sealant or coating do you recommend?

The ability to clean graffiti from the surface will, to some extent, depend on the finish texture. A rough texture will be difficult to coat successfully with a sealer or paint. Commercially available anti-graffiti paints have been used very successfully on shotcrete tanks with relatively smooth float finishes. We do not have any recommendations on the brand of sealer or coating.

We recently stained a shotcrete wall. After we placed the staining on the wall, the stain came out in different shades across the wall, in effect bring out the different curing of the concrete. What can be done to eliminate this inconsistency?

It is not unusual to have variations in the tone of color for shotcrete or concrete walls that have been stained due to variation of the texture or density of the surface being stained. An acid-based stain typically results in more consistent shading. When anticipating that a wall will be stained, extra care needs to be taken in the curing process. It is generally recommended that walls to be stained should be water-cured to avoid any interaction between a curing compound and the stain material. If a curing compound is used, it must be completely removed prior to applying the stain material. Consult the stain supplier for more information.

We are currently in the process of doing a seismic upgrade to one of our parking structures using shotcrete. During this process, the murals that are painted on the interior walls are being removed and will be repainted at a later date. How long do I wait before it is cured enough to begin painting?

The easy answer is that shotcrete material is the same as concrete material and that the same rules or guidelines would apply to shotcrete as to concrete. We usually tell our customers to present this question to the painters. The curing process and chemical reactions are greatest in the first 28 days. Generally a paint or coating is not applied until after the curing of the shotcrete is complete, or mostly so, and the moisture content of the shotcrete is below a point specified by the coating manufacturer.

Is it possible to put a texture on the application side of a shotcrete wall? I understand that I can shoot against a form, but what about the side that gets screeded?

There are many textures that can be applied to the finish surface of the shotcrete. The least expensive is the natural nozzle finish which is rough and tends to absorb light as opposed to reflecting light and standing out. On the other extreme is carved and stained simulated rock as found in zoos and amusement parks. Stamping or rolling also creates a great finish. The broom finish is also very common. Color and textures are options and the owner or designer needs to decide on the value and effect he/she is looking for. Whatever finish, texture, pattern, color, stain, lump, bump, or crease that can be applied to concrete also applies here.