If you only need color in the exposed shotcrete surface, just putting color in the flash coat should be adequate as long as the flash coat is continuous across the entire area. You may want to specify the final “flash” layer to be thicker, perhaps 1/2 in. (12 mm) to 1 in. (25 mm) thick. This would help assure consistent color and still allow the finishers to produce a consistent final texture. As long as the concrete materials are the same in the production, you wouldn’t need to shoot the final layer all in one day. Also, be sure curing methods are the same for the entire area as varying moisture can sometimes affect the early appearance of the coloring, though it usually balances out over time.
I am working on a restoration of a small 1870s train station constructed of serpentine stone in the Philadelphia, PA, area. In many areas, the stone has deteriorated, leaving deep “divets” in the exterior wall faces and in some cases, there is no stone at all. Our intent is to build (infill) the walls back to a flush face for stucco treatment for the lower portions of the wall and to repair or replace stone above that point. Is there a minimum amount of treatment recommended for a shotcrete application? If it can be used for such an application, is reinforcing required? The stone is rather friable, and I don’t want to attach too much to it for fear of further damaging the stone. If shotcrete is not an appropriate approach for this repair, can you advise of other repair methods?
This is a great application for shotcrete placement of high-quality concrete without formwork. If you are merely adding shotcrete to fill out to a uniform surface profile without any structural requirements, you may not need reinforcement. However, it may still be advisable to include fibers in the shotcrete mixture to help control plastic shrinkage cracking. Generally, you would want to keep a minimum thickness of 1 in. (25 mm) to provide enough thickness for finishing. If you need the shotcreted sections to be self-supporting and carry loads as structural concrete, you should consult with a structural engineer to determine the appropriate thickness and reinforcement for the expected loads. Shotcrete is a placement method for concrete so standard reinforced concrete design is appropriate for shotcreted sections.
I am looking through Chapter 20 of ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures and Commentary,” and I don’t see any suggested methods for repair—only that the structure (or any repair) must meet specified criteria (strength, load testing, etc.) for serviceability. I am trying to find some reference allowing shotcrete to be used to reestablish the thickness of an existing structure that has inadequate cover over reinforcement. Can you provide any guidance?
The ACI 350 Code doesn’t really cover repair in detail because it is more about new environmental structures. Shotcrete is concrete; it provides excellent bond to properly prepared concrete substrates and it inherently creates a composite section with the existing concrete that acts monolithically. The supplemental shotcrete could be considered an additional layer of concrete that acts monolithically and thus provides the needed concrete cover. Reference-wise, you could refer to the article “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints” in the Shotcrete magazine archive, as it discusses the bond issue between layers. There are research papers that discuss a 200 psi (1.4 MPa) bond shear stress is needed for a bonded concrete overlay to act monolithically, and that bond pulloff (tensile) test results should be multiplied by 2 or 3 to represent the bond shear capacity. Because we typically well exceed a 145 psi (1.0 MPa) tensile bond pulloff strength, properly applied shotcrete will easily exceed the 200 psi requirement for the bond to act monolithically.
An additional factor in providing additional cover with shotcrete is that shotcrete has very cement-rich paste, along with low permeability, and thus provides a better alkaline environment to combat corrosion of embedded reinforcement, so is effectively giving even better cover than normal form-and-pour concrete.
I recently had a project that requires a velocity of ±400 psi (2.8 MPa) to be applied to prepared areas of deteriorated concrete. My questions is: What is the velocity of the material being applied if I am using 50 ft x 1-1/2 in. (15 m x 38 mm) shooting hose with 60 psi (0.4 MPa) supplied pressure? I would also like to know how that is calculated.
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.