Shotcrete placement requires high velocity for full consolidation and compaction of the concrete. Standard shotcrete equipment with a properly sized air compressor will produce a velocity of 60 to 80 mph (95 to 130 km/h). The 400 psi you stated is not a velocity, but a pressure. Sixty psi is a very low air pressure and may not create the velocity needed for proper shotcrete placement. Shotcrete generally uses compressors that create 100 to 125 psi of air pressure. Also, depending on the shotcrete process used (wet-mix or dry-mix), ACI 506R-16, “Guide to Shotcrete,” states you need an air compressor that can produce a flow rate of at least 200 ft3/min (5.7 m3/min) for wet-mix to 600 ft3/min (17 m3/min) for dry-mix for your 1-1/2 in. diameter hose.
Per contract I have to reinforce first-floor walls with 5 in. (125 mm), 4000 psi (28 MPa) concrete. Due to poor condition of the backup wall, conventional form-and-pour is not an option. I have proposed the use of shotcrete to the architect. What type of shotcrete will meet design criteria for this type of work?
Shotcrete is a placement method for concrete. It is routinely used to strengthen existing masonry or concrete walls. The 5 in. thickness can be easily achieved with either dry-mix or wet-mix shotcrete. If using dry-mix, you may want to review available prepackaged bagged dry concrete materials. A comprehensive listing of our ASA member companies supplying bagged dry concrete materials can be found in our online Buyers Guide. If using wet-mix, the local concrete supplier should be able to provide concrete mixture designs that will achieve the 4000 psi compressive strength requirement. Dry-mix and wet-mix processes use different equipment and skills of nozzlemen are somewhat different. We encourage requiring use of an ACI-certified shotcrete nozzlemen in the process to be used on the project. If looking to select a qualified shotcrete contractor, you may want to review our ASA position paper on Shotcrete Contractor and Crew Qualifications.
There is a dam rehabilitation project where stepped reinforced cement concrete (RCC) was used for overflow protection. The surface is spalling rather badly and testing indicated that the RCC was marginal for long-term durability. Is it feasible to place shotcrete over RCC to improve aesthetics and provide additional strength and durability?
Shotcrete has been used for over 100 years for slope stabilization on natural soils. Your self-consolidating concrete (SCC) sounds like it could be considered a high-grade soil, so stabilization and protection of the SCC with shotcrete is certainly a good application for shotcrete. The fresh surface of the shotcrete can receive a wide variety of finishes, from an as-shot finish (rough) to a floated or even stamped or carved look. You should consider appropriate amounts of reinforcement (reinforcing bars and/or fibers) to control potential shrinkage cracking, as appropriate for your exposure and serviceability requirements.
We are a small community in south central Illinois with a deteriorating masonry building issue that drastically needs to be addressed. I have been looking at your shotcrete product and I am thinking that this may be the most efficient and economical way to protect these exposed surfaces from the elements and stabilize these structures. My question to you is: Have any other communities used this product for this purpose, can this product be used in a situation such as ours, or am I just barking up the wrong tree? The wall in question is three stories tall, about 40 ft (12 m); the exposed wall was interior multi-course thick masonry from the late 1800s-era.
Your proposed project is a great application for shotcrete. We’ve seen shotcrete used for enhancing structural integrity of historic masonry structures across the country. Often shotcrete is used on the back side of the wall to add structural strength while preserving the exterior appearance. In effect, we build a structural concrete wall in-place behind the old wall. Shotcrete has the natural advantage of not requiring any formwork, and can create a good bond to the existing wall, letting the structure elements work together. Here’s a link to a past article from Shotcrete magazine documenting the restoration of a historic brick building (shotcrete.org/wp-content/uploads/2020/01/2009Win_SCM01pg08-12.pdf). If you don’t need to preserve the exterior appearance, you can certainly shotcrete the exterior of the wall using the same approach. You can finish the interior (or exterior) surfaces in a variety of ways to provide the architectural appearance you desire. Shotcrete is a placement method for concrete, so using shotcrete will provide a final structure with the strength and durability of cast concrete. By the way, the term “gunite” is the old tradename for what we currently call “dry-mix shotcrete.” Let us know if you have any further questions.
I am currently working on a project that involves repairing the concrete walls of a sanitary sewer interceptor structure and line pipe and was wondering if you had any reference information regarding shotcrete being used to repair similar items and how well it has held up. Any information you send will be greatly appreciated.
Shotcrete, both wet- and dry-mix, have been used to repair and reline sewer structures for many decades and has held up well as a lining or repair material. It is important to have the work done by a qualified shotcrete specialty contractor and to specify a durable concrete mixture design. The following are links to articles which may also be of interest to you:
We’re proposing to install a new shotcrete shear wall against existing 8 in. (203 mm) CMU walls for a project of ours. STRUCTURAL has provided a steel reinforcement cage across each wall plane, which is tied to the existing CMU wall with a series of anchors running horizontally and vertically. Between this new shotcrete wall and existing CMU wall, we are calling for a vapor barrier layer to retard moisture migration from the exterior to the interior. At this point, we are considering going with a PROSOCO Cat-5 fluid-applied vapor barrier system over the CMU but were curious how this might interface with the shotcrete. Have you dealt with this vapor barrier issue before, or seen similar installations? Would the steel reinforcing cage be adequate to support the shotcrete by itself during application, or should we be concerned about the surface of the backup wall?
Shotcrete has been placed against fluid-applied waterproofing and other smooth surfaces, which would likely be similar to the surface you are concerned with. An experienced shotcrete contractor should be able to install this shear wall, taking care to apply the shotcrete in a sequence which inhibits the material from sagging or sloughing. The key is hiring a good, well-experienced shotcrete contractor.
The reinforcing bar size and spacing can help provide support for building up the shotcrete. The spacing should be no greater than 12 in. (305 mm) on center each way and must be rigid.
We are the Architects of Record for a multi-family rehabilitation here in Portland, OR. The existing building is three stories plus a half basement. It is a historic building and is also an unreinforced masonry building. As a result, it requires significant seismic upgrades. Our structural engineer is proceeding with a design that uses several 4 in. (102 mm) thick shotcrete walls as shear elements throughout the building. In most instances, these walls are being constructed adjacent to an existing wall to minimize their impact to the existing floor plans. This seems fine against the exterior masonry walls but we have concerns where the shotcrete is to be installed directly adjacent to a standard architectural partition (gypsum wall board and wood studs). We are planning on plywood sheathing to act as a one-sided form to prevent the shotcrete application from harming the existing stud wall. However, we were curious if we should also include a water barrier so that the application did not allow moisture to migrate into the existing walls during the installation. Is this assumption correct? And if so, is there a performance or product recommendation that you can offer for this purpose?
Shotcrete has been used in similar structural upgrades for decades quite successfully. There are many ways to ensure that the moisture from the fresh shotcrete would not impact the existing partitions. Methods for protection include use of green board (as used in a shower or bath), plywood, painted coatings, moisture-resistant sheeting, and so on.
Shotcrete is placed with a low water-cement ratio (w/c) and the water is needed by the shotcrete to hydrate the cement and harden. As long as the existing surface or treated surface does not actively absorb the moisture, the shotcrete will use the available internal moisture to hydrate the cementitious materials.
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.