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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StructuralQ I have a client who may be interested in using shotcrete for walls in a radiosurgery unit requiring radiation shielding. Could you please tell me the typical density of shotcrete?
Shotcrete made with normalweight aggregates will have a density of approximately 145 lb/ft3 (2323 kg/m3).
StructuralQ I have a customer who would like to place 2 in. (51 mm) of shotcrete onto our geotextile canal liner, which has been used for many years with 2 to 4 in. (51 to 102 mm) of shotcrete. In all of these previous projects, contraction joints were installed. For this project, the customer is asking whether this is an absolute requirement, as the geocomposite canal liner beneath is the water containment component. Does it make a difference in terms of cracking and joints whether the shotcrete is 2 or 4 in. (51 or 102 mm) thick? What is the typical finishing that is done on canal projects?
Long expanses of concrete canal lining exposed to the sun and weather would experience significant internal tensile drying shrinkage stresses. Regular contraction joints help to relieve the internal tension created by concrete shrinkage. If no contraction joints are provided, shrinkage will still occur and the concrete lining will produce its own contraction joints, better known as “cracks.” Unfortunately, the resulting cracking will be random and can vary significantly in size and length. Thus, contraction joints are a good approach to help induce cracking at regular, controlled locations. If the client doesn’t want contraction joints, they need to understand that cracking will be much more extensive and likely more noticeable.
Theoretically, with the same percentage of embedded reinforcement, cracking between a 2 or 4 in. (51 or 102 mm) should not be substantially different. Of course, the 4 in. (102 mm) thick shotcrete section would require twice the concrete material and twice the embedded reinforcement to maintain the same percentage of reinforcement. A 2 in. (51 mm) thick section could have some difficulty in maintaining adequate cover over embedded reinforcing bars. The designers could also consider using fiber-reinforced shotcrete to help control shrinkage and temperature stresses, although fairly high dosages are needed for effective elimination of reinforcing bars. More guidance on fiber-reinforced shotcrete is available in ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete”. A 2 in. (51 mm) overlay is absolutely the least possible and 3 or 4 in. (76 or 102 mm) is far more normal in practice.
Canals are generally specified to have a natural gun finish, a rough broom finish, or a light broom finish.StructuralQ I have a project where the foundation sub is planning to shotcrete foundations walls instead of pouring them. They’ve submitted all the procedural things necessary to prove their competence and know what they’re doing. For a portion of our foundation, we’re immediately adjacent to an existing building. The sub mentioned today on site that they were not planning to put Styrofoam or anything between our new wall and the existing wall that would resist lateral pressure from the fluid concrete and the question was raised whether this is ok or not (FYI there is still rigid insulation, waterproofing, etc.). The argument is that the concrete is obviously stiff enough to stay in place, thanks to the nozzle-applied admixture, without an interior form that it wouldn’t be exerting any lateral pressure on the adjacent wall. I can follow that logic and almost buy it but I’m wondering if we still need something to resist the force applied from actually shooting the concrete in place?
Shotcrete is a high-velocity placement of concrete. In most thick walls, as I imagine your foundation walls are, the shotcrete contractor will be bench shooting the walls. This means they will be shooting the full wall thickness in 3 to 4 ft (0.9 to 1.2 m) high lifts where most of the impact forces and weight of the shotcrete is carried by the previously shot material. This results in very low impact forces on the back side of the section. When creating a section with a one-sided form, shotcrete contractors have used thin material, like Masonite, pegboard, or even an expanded mesh material, as we just need to have a surface to define the back of the section.
StructuralQ I have a project where we are designing a shotcrete wall. The contractor plans to mix the shotcrete on site and we need to work with them to come up with a mixture design. Do you have any suggested wet-mix shotcrete designs for on-site mixing applications or could you point me where some may be published?
Shotcrete is simply a placement method for concrete. Most wet-mix shotcrete contractors use a 2 in. (50 mm) diameter delivery hose, so maximum coarse aggregate size should be limited to 3/8 in. (10 mm) nominal. Pumpability usually requires a good paste content. Here’s a link to an article, “Understanding Wet-Mix Shotcrete: Mix Design, Specifications, and Placement,” that should answer many of your questions. It is rare to have wet-mix concrete mixed on site from bulk aggregate and cementitious materials. Most site-batched wet-mix uses dry prepackaged materials that have metered water addition to provide a specific water-cementitious materials ratio (w/cm) for the provided bagged mixture. We also see volumetric mixers used on site that can precisely meter the dry concrete materials and water often with needed water-reducing air-entraining admixtures.
RepairStructuralQ I have a project with a segmental retaining wall. The project is located in Maryland. The CMUs of the retaining wall have significantly deteriorated in several areas. The geo-grid fabric appears to be in good condition in the areas where it could be observed. I suspect the deterioration is caused by saltwater runoff during snow events combined with the freeze/thaw cycles. There are some localized areas where the masonry units have completely disintegrated. The largest area being approximately 4 ft2 (0.37 m2). Is it feasible to remove the deteriorated masonry material, down to a sound surface, and shotcrete the face to restore (or exceed) the structural integrity of masonry units? If so, will this restore the structural integrity of the segmental retaining wall?
Shotcrete is routinely used for the repair of deteriorated concrete masonry and brick. Shotcrete should easily have a minimum compressive strength of 4000 psi (28 MPa) when properly shot and cured. According to the National Concrete Masonry Association, current CMU units have a minimum 2000 psi (14 MPa) compressive strength. Older CMU had a lower 1500 psi (10 MPa) minimum. Thus, the shotcrete placement will be significantly stronger and less permeable than the in-place CMU. This should give the wall better resistance to saltwater and freeze-thaw exposures thus extending useful life.
Depending on the depth of the new shotcrete placement, you may consider mechanically tying the shotcreted layer back to the sound CMU with epoxy or mechanically embedded anchors or j-bolts. Thicker sections may also benefit from the use of a steel wire mesh or fibers.
When shooting onto existing CMU sections, the surface must be properly prepared and then shotcreted with proper shotcrete materials, equipment, and placement techniques. Shotcrete placed onto an existing CMU surface will provide an excellent bond IF the following conditions are met:
- Make sure the surface is roughened and clean.
- The amplitude of roughness should be +/- 1/8th in. (3 mm) or more.
- A high-pressure water blaster (5000 psi [34 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
- Bring the CMU surface to a saturated surface dry (SSD) condition. This means the surface feels damp, but water is not picked up on a hand.
- Make sure the shotcrete placement is properly executed with high-velocity placement and quality materials.
- The shotcrete should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
- Be sure the shotcrete contractor is using an air compressor able to produce at least 185 CFM (5.2m3/min) for wet-mix and 375 CFM (10.6 m3/min) for dry-mix (gunite) of air flow at 100 to 120 psi (0.7 to 0.8 MPa).
- Use of an ACI-certified shotcrete nozzleman is recommended.
- No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.
Finally, without details on the loading conditions, and structural details of the original construction, we cannot comment on the structural integrity of the repaired wall. You should contact a professional engineer experienced in concrete repair to evaluate the structural integrity of the wall when repaired.
StructuralQ I have an unfinished (dirt) basement with a stacked stone and mortar foundation. Can I shotcrete the existing dirt walls and floor with shotcrete MS (micro silica enhanced) and have it adhere to the dirt portion of the basement? If so, what method would be best?
Shotcrete would work well for the overlay of the walls. In most cases, floors are placed by a conventional cast-in-place method. Either the wet- or dry-mix procedure would work well for the walls. To ensure good bond of the shotcrete to the walls, the walls should be cleaned and prepared to assure that the shotcrete is bonding to sound material rather than contaminates such as dirt or weathered material. You might also want to consider reinforcing the walls, but you should consult with an engineer on how to do this and with what material. If you were to use the wet-mix process, you could use the same equipment to place the floor as you are using for the walls.
StructuralQ I have been hired to design a large concrete pit for a fertilizer plant. The pit will need to be approximately 13 ft (4 m) deep by 55 ft (17 m) long by 15 ft (5 m) wide. The pit will contain water at varying depths and will support grating covering the pit that will support equipment. The state is requiring the pit slab and walls to be a monolithic pour. Could shotcrete be used in this situation and be considered a monolithic pour?
If the directive from the state is to cast (or shotcrete) both the slab and the walls monolithically, this would be a difficult task with either shotcrete or cast concrete. If the directive is to cast the floor monolithically, and then the walls monolithically, shotcrete could certainly be used and would be considered a monolithic placement. Once the state’s intent is clarified, this question should be posed to a shotcrete contractor who might be the actual contractor on the project for their input.
As this is a fertilizer plant, there may be additional considerations due to the potentially aggressive nature of the fluids introduced into this pit.StructuralQ I have reviewed ACI 506R, “Guide to Shotcrete,” and 506.5R, “Guide for Specifying Underground Shotcrete,” but was unable to find specific criteria pertaining to shotcrete protection for reinforcing steel. Would the clear cover then be based on ACI 318 Section 7.7.1 for cast-in-place concrete? For underground structures, would 3 in. (76.2 mm) of clear cover from ground be required?
Shotcrete is concrete, and therefore if designing structures based on the ACI 318 Code, cover for conventional reinforcing steel should be those suggested in ACI 318 for concrete against ground. If the shotcrete is a “temporary” support, with further placement of “final” support, then these requirements do not apply.
StructuralQ I have some hairline cracks visible in the outer 3 in. (75 mm) thick shotcrete outer shell of a monolithic dome home. How concerned should I be about them? The outer coat was applied 10 months ago and they seem to be spreading. The shotcrete was applied over chain-link fencing. These cracks are several feet long and most are from doorway or window openings. What is the best/easiest way to repair these?
The design of any concrete structure must consider loadings and environmental conditions. This includes drying shrinkage of the concrete, and daily and seasonal thermal changes that introduce stresses within the concrete sections. Domes are thin-shell concrete structures and stress concentrations are expected around any openings through the shell. Typically, the design engineer will provide additional reinforcing bars around openings and especially at corners of openings to accommodate the buildup of stresses in these locations. Also, chain-link fence is not considered as acceptable concrete reinforcement, as it cannot accept tension in a straight orientation within the concrete. Proper concrete reinforcement is either deformed reinforcing bars or steel mesh with smooth wires laid out in an orthogonal pattern.
You should have a professional engineer with experience in concrete shell design and construction evaluate your dome home for structural integrity. If the cracking is determined to not affect the structural integrity, epoxy or polyurethane grout injection is routinely used to seal cracks in concrete.
StructuralQ I have three apartment buildings (with three, four, and five units). Their basements were insulated just over 10 years ago with closed cell spray foam with a class one fire rating. Now for some reason, the building inspector says I have to cover the foamed walls of all three large basements with 0.5 in. (12 mm) drywall and build stud walls to hang the drywall for fire protection. This would reduce the width of the basement stairs significantly, and they would become so narrow that they would then be out of compliance. These basement walls are uneven fieldstone foundations, with bumps, protrusions, and even some curves. I was thinking that shotcrete-applied concrete might work much better and be more appropriate than wood and drywall for a sometimes-moist basement. According to the local Building Code, a 2 in. (50 mm) layer of concrete would suffice. How can a shotcrete application be made to adhere to closed cell foam? The wall heights are approximately 7 ft (2 m) plus bond.
Shotcrete is just a placement method for concrete. So shotcreting will provide the fire resistance of concrete. For securing the concrete, you can place anchors through the foam into the original basement wall. You can consult with an engineer experienced with shotcrete on anchor size and spacing required for supporting the shotcrete layer.