I have a client with an old, soft-stone masonry building of approximately 150 years of age. The mortar is badly deteriorated and the stone is quite friable. I am advocating the use of shotcrete as an application to the interior face of the walls that will restore both in-plane and out-of-plane strength to the building walls. My client has expressed concern that there may be incompatibility issues between the stone masonry and the shotcrete both from a structural stiffness perspective as well as from a moisture intrusion perspective. (We have successfully used shotcrete over stone masonry in the past.) Do you have any information you can share with me on this topic? Do you have either examples of incompatibility or successful use of shotcrete over stone masonry?

As you have noted, shotcrete has been used extensively to reinforce unreinforced or under-reinforced masonry walls and rock walls. It has been used on the Crater Lake Lodge to strengthen and stabilize a rock wall foundation and any number of other projects. In California, shotcrete has been used to strengthen or repair walls since the 1933 Long Beach Earthquake. It was used to strengthen the California State Capitol (3 ft [0.9 m] thick brick walls) in the late 1970s and all of the older unreinforced masonry walls for the San Francisco School District. To the best of our knowledge, there have been no failures of shotcrete strengthening on the West Coast in the past 80 years.

I am interested in any information or suggestions you may have regarding practical working space requirements for shotcrete applications. Shotcrete is a common approach for sewer pipeline and storm-water culvert rehabilitation projects. My concern relates to the space requirements necessary to best ensure a quality installation—for pipelines, this boils down to the question: What is the smallest diameter pipe that can be used for this method? Technical specifications that I have come across call for a minimum of 3 ft (1 m) between the surface being covered and the application nozzle. To me, this means that pipes that are much smaller than 6 ft (1.8 m) would create some difficulty. Similarly, for applications between vertical walls, how much room does a nozzleman need between the wall receiving the shotcrete and the wall at his/her back? Are there robotic means or other methods in use that would allow shotcrete applications without a hands-on nozzleman? Are there any other workspace limitations or controls that should be considered when determining feasibility of shotcrete application methods?

In the case of installing a lining inside of an existing pipe, there are robotic methods available, such as spin lining, where the cementitious material is cast from a rotating head as the carrier is moved along the pipe. For pipe smaller than 42 in. (1067 mm) diameter, the spin lining is likely the best method.
For pipe larger than 42 in. (1067 mm) and up to 6 or 8 ft (1.8 or 2.4 m) diameter, either hand shotcrete nozzling or spin lining are applicable. For pipes much larger than 6 to 8 ft (1.8 to 2.4 m), hand nozzling is likely the best solution.
In the case of clearance between a wall to be shotcreted and an obstruction, 3 ft (1 m) is a good rule of thumb, but a qualified and experienced shotcrete contractor can use modified equipment to place quality shotcrete in tighter spaces. A recent article on shotcreting in confined spaces can be found here.

I had wallpapered over a cement interior basement wall years ago. Recently, When I removed the wallpaper and the liner beneath it, the shotcrete came off with the paper. Is there any way I can repair these spots? Can the shotcrete process cover a garage floor that is heavily pitted, has a few cracks, and has some dirt and road salt marks? Will it hold up to road salt and prevent further deterioration?

You mention that you had originally wallpapered over a cement interior basement wall. It is not clear that the cement interior wall was placed using the shotcrete process. If it was installed with the shotcrete process, then the application was flawed due to improper surface preparation or application. There are many concrete repair products on the market which could be used to repair the surface. Many of these are troweled on by hand or sprayed. Check with a local building supply company or on the Internet.
With respect to the garage floor, we would not recommend the shotcrete process for a thin overlay on a horizontal surface. Again, there are many products on the market that are designed for resurfacing floor slabs. Check with a local building supply company or on the Internet for potential products.

We are working on an historical renovation project where the existing structure has been reinforced with shotcrete. We need to attach structural studs/furring members to the face of this shotcrete. Is there any difference between standard concrete and shotcrete when it comes to fastener embed depth? We are considering powder-actuated fasteners (Hilti-type) or Tapcons.

The embedment depth of anchors in shotcrete would be the same as it would be for conventionally formed and placed concrete. Shotcrete is essentially a method of placing concrete and the same rules would apply. As with any anchoring system, it is important to make sure that you are anchoring to sound material.

We are working on a project with a wall that requires additional capacity due to increased loading requirements. We are contemplating shotcrete with additional reinforcing to provide additional thickness for the wall. Is it possible to achieve a composite wall to design for a thicker section for bending, using the bond of the existing concrete and shotcrete along with a reinforcing bar hook anchor epoxied into the existing wall? Any information you can provide would be appreciated.

Shotcrete is often used in similar applications. The question of bending is a structural engineering question. Shotcrete is a method of placing concrete and the properties of shotcrete are similar if not the same as cast concrete. To achieve a composite wall, you must ensure that the existing surface is properly prepared to maximize the potential bond between the overlay shotcrete and the existing wall. Shotcrete placed against a properly prepared existing wall should achieve great bonding strength without the use of bonding agents. Drilled and grouted dowels also contribute to the system, working as a composite wall.

Can shotcrete be used to help seal a leaking pond? We have a 1.5 acre (6070 m²) pond that we are in the process of completing. We spread 90,000 lb (40,823 kg) of bentonite in, but the bentonite washed off the steep banks and now we are stuck with a half-full pond. Would shotcrete be a practical solution for our problem?

Properly designed shotcrete (both concrete materials and reinforcing are important in the design) placed by an experienced shotcrete contractor can certainly be used to provide a somewhat watertight lining for your pond that will be serviceable, durable, and require little to no maintenance for decades to come. We would suggest you consult with an engineer or shotcrete contractor experienced in this type of shotcrete work. You may use our online Buyers Guide to find an ASA corporate member consultant or contractor to assist you.

I have a new construction project where I want to apply shotcrete to cast-in-place concrete columns and an elevated, post-tensioned concrete slab as a finish material. The finished application is intended to be in varying depths from 3 to 12 in. (76 to 305 mm) or more. The desired end result is a smooth, curvilinear, sculptural form. Is this type of application achievable?

Shotcrete can and has been used to increase the size of columns and thicken overhead slabs while providing great-looking linear or curvilinear finishes. Examples of curvilinear finishes can be found in past Shotcrete magazine articles. You can search the Shotcrete magazine archives.

We are working on a renovation of an existing shopping plaza where some of the existing walls are split face block. Would it be an acceptable application to resurface the block with shotcrete to achieve a smooth finish? If so, what is the thinnest we would be able to go?

Shotcrete could be used for this application. The thickness of the overlay would be dependent on the material used. A potential concern would be the lines of the existing block showing on the new surface. We would suggest that you search for and review various ASA Shotcrete magazine articles as well as ACI 506R-05, “Guide to Shotcrete”

We have a client with a 6 1/2 in. (165 mm) thick reinforced concrete roof slab, the underside of which is in need of repair. There are places where the concrete has spalled, exposing reinforcing bar that has a 3/4 in. (19 mm) cover. There is efflorescence, and there is spalling that does not expose reinforcing bar and some at the steel supporting the concrete slab. In addition, there are hairline cracks and rust spots. Is shotcrete a feasible overhead repair for this situation? What holds the shotcrete to the slab? What is the minimum thickness of shotcrete we should specify? Should we specify shotcrete to be used only at the spalls, cracks, and efflorescence or the whole underside of the slab? Do you have a shotcrete repair procedure that we can put in our specification?

This type of repair is commonly done using the shotcrete process. The extent of the repair is an engineering issue, not a shotcrete issue—shotcrete can and is used for patches and overlays. The shotcrete will adhere to the properly prepared existing concrete. It is installed such that the weight of the plastic shotcrete does not;exceed the adhesion to the existing surfaces; if additional material is needed, it is added at the initial layer or layer set up. The minimum thickness is related to the material used for the repair and the need to establish cover on the existing or added reinforcing. Some repair mortars can be placed as thin as 1/2 in. (13 mm).
Please find a link to a paper on “Concrete Repair by Shotcrete Application”.
The success of the shotcrete repair will be highly dependent upon using a qualified shotcrete contractor and doing an excellent job of preparing the surfaces. Where the reinforcing is exposed, you should require that it be chipped out the entire perimeter allowing for a space of 3/4 in. (19 mm) behind the reinforcing bar so that the repair material can completely encase the reinforcing.