The ASA Technical Questions and Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.
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RepairQ I have a seawall with a gunite (dry-mix shotcrete) outer layer. The gunite layer has cracked in multiple locations on the seawall resulting from years of exposure to the harsh environment. The original gunite was not part of a soil nail system. I am considering a re-coat of shotcrete probably 3 to 4 in. (76 to 100 mm) thick with wire mesh and L-anchors on a 2 to 3 ft (0.6 to 0.9 m) grid. I know the importance of surface treatment for bonding, etc., but I am not sure if I should remove the original gunite layer (which is still sound in some places) or apply the re-coat. The new overlay needs to be structurally effective. I know that a soil nail system is the most dependable solution, but cost is a major concern. Do you have any suggestions?
The new shotcrete layer can be added to the existing shotcrete or installed after the existing shotcrete is removed. The decision to remove or not remove the existing shotcrete is beyond the scope of what we can comment on. If the existing shotcrete is left in place and overlayed, it should be thoroughly cleaned and roughened to create a good bonding surface. Because this is in a marine environment and you are considering the use of wire mesh, you need to make the new layer thick enough to have sufficient cover on the reinforcing steel. Alternately you could consider the use of fiber-reinforced shotcrete and silica-fume-enhanced fibrous shotcrete. Please note that there are many types of fibers on the market. We recommend that you review some of the Shotcrete magazine articles on fibrous shotcrete and on shotcrete in a marine environment on the ASA Web site. We suggested two papers for reference. The first is by Gilbride, Bremner, and Morgan on the Port of Saint John, and the other is by Morgan on the use of fibers that cover marine repairs. You mentioned using “L-anchors” at 2 to 3 ft (0.6 to 0.9 m) spacing. The use of grouted anchors with a reasonable embedment is quite common, but the design of such anchors is again beyond the scope of what we can advise.
RepairQ I have been asked to recommend repairs to a fire-damaged brick wall. The wall is 12 in. (30.5 mm) thick and 14 to 16 ft (4.25 to 4.9 m) high. The fire caused spalled brick—3/8 in. (10 mm) deep—and soft mortar joints. The damaged side of the wall is exposed to weather. I plan to recommend tuck-pointing the mortar joints but am wondering if shotcrete is appropriate to repair the spalled brick. The brick could be cut out and replaced, but shotcrete would seem to offer the advantage of repairing and reinforcing the brick wall.
Shotcrete would be an excellent process to repair or overlay your wall. You are correct in saying that it could not only repair but also reinforce and enhance the strength of the wall system. It is important to remove all deteriorated brick and sandblast or water-blast the surface if you are looking for a good bond between the shotcrete and the existing brick. Dowels epoxied or grouted into the existing brick are often used to mechanically tie the shotcrete overlay to the brick wall and also stabilize the new reinforcing steel in the shotcrete overlay.
RepairQ 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.
RepairQ I want to apply a 3.5 in. (89 mm) veneer of shotcrete over an existing cast-in-place wall. I am concerned about how well the shotcrete will bond to the existing wall. This wall is 50 ft (15.25 m) in height. What are the keys to doing this work successfully?
This is a common use for shotcrete. The key elements are as follows.
- Proper surface preparation. To establish suitable surface roughness, use heavy-duty sandblasting, high-pressure water blasting, or mechanical methods such as scabblers or scarifiers, followed by sandblasting or high-pressure water blasting to remove the “bruised” surface material. Refer to ICRI Guideline No. 03732, concrete surface profile Chip 6 (CSP 6), or greater.
- Provide mechanical connection between the shotcrete and concrete by installing L-bar anchors (epoxy or portland cement grouted) on a systematic pattern, with reinforcing bar (or heavy-duty mesh) spanning vertically and horizontally between the anchor bars. Size and spacing of the bars to be determined by the structural engineer. Position anchors and reinforcing bar to ensure adequate shotcrete cover to them. Nonmechanically connected veneers are not recommended.
- Wash concrete surface with clean water to remove dust or any other contaminants to achieve a good bond and presaturate concrete. Allow concrete to dry back to a saturated surface dry (SSD) condition immediately prior to shotcrete application. If concrete dries excessively, bring back to SSD condition with fogging. (A 3000 psi [21 MPa] water pressure sprayer works well for this purpose).
- Apply the shotcrete from the bottom up, taking care not to entrap rebound/hardened overspray. Use proper shotcreting techniques to encase reinforcing bar and anchors. Use 45-degree construction joints (do not construct long tapered joints).
- Use shooting wires, guide forms, or other suitable methods (for example, rods with alignment bubbles) to establish proper line and grade. When the shotcrete has stiffened sufficiently, trim it to line and grade with cutting rods and then finish using fresnos or floats to provide the desired surface texture (wood floats for more textured finish, rubber/sponge floats or magnesium floats for intermediate texture finish, or steel floats with steel toweling for smooth finish). Note: very smooth finishes are not recommended as they tend to show imperfections from hand-finishing procedures. Avoid over-finishing of shotcrete or procedures/timing which could pull tears or sags/sloughs/delaminations in the fresh shotcrete.
- Cure the freshly placed shotcrete using one of the methods prescribed in ACI 506R-90. Our preferred method is fogging/misting until the shotcrete has reached initial set, followed by wet curing for 7 days using presaturated plastic-coated geotextile fabric (for example, Transguard 4000), which is kept wet with soaker hoses. Curing compounds are a (second best) alternative, but should not be used if a paint or coating is to be applied, unless they are approved by the coating/paint supplier for such purposes.
RepairQ My firm just completed a 2 in. (51 mm) overlay of shotcrete in an existing storage tank. Almost immediately after the shotcrete was applied, we noticed spider web cracking on almost the entire surface. The weather was very hot during shotcreting, and we suspect this caused the cracking. The project engineer is concerned about permeability and is thinking of having the shotcrete removed. Is removal really required or can we live with this cracking?
Removal is probably not called for in this situation. Spider web cracking usually is an indication of crazing, a form of plastic shrinkage cracking. Crazing generally occurs when the combination of temperature and humidity creates a rate of evaporation at the surface of the concrete that is higher than the rate of bleed water exiting the concrete. Because the surface has very little, if any, tensile strength at this time, crazing cracks start to form. The good news is that crazing is an aesthetic problem. It affects only the very top surface and does not extend deeply into the concrete. Crazing cracks are more apparent when the surface is damp.
To avoid or limit crazing, be conscious of the weather conditions during placement. If there will be high temperature, low humidity, and moderate to high winds, measures such as fogging and/or erection of windbreaks may be required during placement. Synthetic fibers will help inhibit the formation of crazing cracks. Curing must begin as soon as possible, especially in these conditions.
RepairQ Our client has a retaining wall that has experienced movement in the precast concrete panels and has asked us to research a product that could be applied to give a smooth look to the retaining wall. Is shotcrete a possible option? I would also like information on the recycled content of shotcrete.
Shotcrete is basically concrete that is pneumatically applied. Shotcrete can be used as an overlay for an existing wall to provide structural strengthening and a smooth look. Again, we suggest that you visit ASA’s Web site and search previous Shotcrete magazine articles for finishes. Before the shotcrete is applied, the wall must be stabilized from any anticipated future movement. Relatively thin layers of shotcrete or concrete will not withstand future wall movements without distress and cracking. The recycled content of most shotcrete mixtures is limited to the substitution of fly ash or other pozzolans for a percentage of the cement in the mixture. To properly place shotcrete, this substitution is generally limited to approximately 25% of the cement content.
RepairQ Our current project is a pier with severe corrosion of reinforcement and obvious spalls. The work will all be overhead with the surface 18 in. (457 mm) above the mean tide level and, for a variety of reasons, dry-mix is not an option. We are looking for a good, dense, wet-mix design for saltwater marine exposure. Compressive strengths need to be in the mid-range of 7000 to 8000 psi (48.3 to 55.2 MPa).
For a potentially suitable wet-mix shotcrete mixture design for marine structure repair, go to the ASA Web site (shotcrete.org). Click on Shotcrete magazine and search for “Shotcrete Classics: Deterioration and Rehabilitation of Berth Faces in Tidal Zones at the Port of Saint John.” This mixture design worked well for over 1.2 miles (2 km) of ship berth face repair over a 10-year period. Note: Because of high freezing-and-thawing exposure, the shotcrete was required to be air entrained. While the original mixture design called for 7% air content as shot, it was subsequently modified to require an air content of 7 to 10% as batched (at the point of discharge into the shotcrete pump) and an air content of 5 ± 1.5% as shot (into an air pressure meter base). Such shotcrete has provided good freezing-and-thawing resistance. You should be aware that your local materials (coarse and fine aggregates and cement) may have different properties in the concrete mixture, however, as compared to the mixture discussed in the article. It is recommended that a local engineer, testing laboratory, or concrete supplier be retained to develop a concrete mixture using local materials that meets the performance requirements of the mixture design mentioned in the article. Also, test panels constructed with the mixture, nozzlemen, and equipment to be used in the shotcreting are highly recommended to verify the strength performance of the shotcrete.
RepairQ 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.
RepairQ 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.
RepairQ 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.