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 We plan to use fiber-reinforced (polypropylene fibers) shotcrete as a brown coat for stucco (three-coat stucco) over a concrete shear wall. Does a maximum thickness of 1.5 in. (38 mm) of shotcrete require any mechanical anchor/connection, or is the bonding strength of the shotcrete layer to the concrete shear wall substrate sufficient?
The addition of fiber will not increase the bond of shotcrete to the concrete shear wall. A 1.5 in. (38 mm) thick layer of properly designed and applied shotcrete should have adequate bond to a properly prepared concrete substrate without additional mechanical anchors. However, exposure conditions, geometry of the wall, shrinkage potential of the shotcrete mixture, application technique, and curing—as well as the age and quality of the shear wall concrete substrate—may affect the bond. These factors should be considered by an engineer experienced with shotcrete overlays in deciding whether additional anchoring is advisable.
StructuralQ We want to know if it’s possible to apply a 1 in. (25 mm) lift of shotcrete to a berm (2:1 slope). The berm will be treated with emulsion (oil and water) prior to the shotcrete. We are only looking for long-term erosion control. Will the emulsion be required or will it cause a bonding problem with the soil?
Shotcrete is a placement method for concrete. If the berm is composed of granular materials, one wouldn’t expect the shotcrete to actually bond to the soil. Rather, placing a thin shotcrete layer would create a uniform, relatively impermeable layer of concrete to prevent water from penetrating through and washing out the soil underneath. The emulsion may help to stabilize the soil to help withstand the pressure of shotcreting directly against the berm, but many similar soil stabilization projects will shotcrete directly onto the natural soils. Also, when considering using the emulsion, be aware that some oils may contain ingredients (like sulfur) that can attack the concrete over time. You should consult with an engineer or concrete materials specialist to ascertain whether the specific oil you want to use will have a long-term effect on the shotcreted layer.
StructuralQ We would like to apply a 2 in. (50 mm) layer of shotcrete on 10 ft (3 m) diameter steel pipes including wire mesh. Is this practical? If so, how do we do this successfully?
This type of application is very common. Either wet- or dry-process shotcrete can be used successfully. The mixture should contain a minimal amount of coarse aggregate and be rich in cementitious material to minimize rebound. Generally either 2 x 2 in. (50 x 50 mm) 14 gauge or 2 x 4 in. (50 x 100 mm) 12 or 14 gauge welded wire fabric is used. The wire fabric needs to be spaced off the surface of the steel pipe to allow the shotcrete to encase the wire properly. This can be accomplished by welding studs or nuts on the pipe surface and securing the wire to them.
StructuralQ We would like to get approval to use shotcrete on the perimeter walls of an existing laboratory building. We would be shooting against a waterproofing membrane and shoring lagging. The project engineer is concerned that the shotcrete will damage the membrane, resulting in leaking into the occupied space. Are there any examples where this type of shotcrete placement has been used?
This is a commonly used technique in the Western U.S. and Canada, and has been used successfully from Stanley Hall at the University of California at Berkeley, Berkeley, CA, to the Baltimore Hilton Convention Center near Camden Yards, Baltimore, MD. There are a number of suppliers of waterproofing materials to choose from for this application. In selecting a supplier, be sure there is field service available to inspect the project before placement of the shotcrete.
StructuralQ We’re designing two steel stacks to be located in Texas and subject to hurricane winds. The diameters are 9.19 and 4.92 ft (3 and 1.5 m), respectively. Both stacks are 118 ft (36 m) high. To reduce the vortex shredding oscillation effects, we want to increase the mass of the stacks by means of internal gunite lining. Our calculation provides good results considering approximately 2 in. (51 mm) thick interior liner. For this scope, is it better to consider external or internal lining? Would there be any advantage to applying an external liner with respect to the interior solution? Which of these two is the most cost-effective solution?
Shotcrete would work well either for the stack exterior or as an interior lining. The 2 in. (51 mm) thickness could be easily applied on either the outside or inside surfaces.
For the 9 ft (3 m) diameter stack, it would be easier and more cost-effective to scaffold and gun the inside of the stack. The work could be done with less difficulty, from an interior hanging platform suspended with sky climbers. There would be less cost involved with rigging than there would be with scaffolding or rigging around the outside circumference of the stack.
For the 4.92 ft (1.5 m) diameter stack, there is much less room to work inside, so unfortunately it may need to be gunned on the outside.StructuralQ What is the maximum thickness for shotcrete used for shear walls? Can we use more than 12 ft (3.7 m) if we use a double layer of reinforcing?
There is no stated maximum thickness for shotcrete used in shear walls or any other type of wall. Walls have been successfully placed to a thickness of 36 in. (914 mm) for some time. The two main concerns are the heat of hydration and proper encapsulation of the reinforcing steel. Because shotcrete mixtures typically contain more cement per cubic yard or cubic meter than formed and poured placements, there will be more heat generated by the shotcrete mixture. The ability of the nozzleman to encapsulate the reinforcing will be a function of proper mixture design, proper selection of shotcrete equipment, and the skill level of the nozzleman and the crew.
StructuralQ Why is shotcrete not applied to general housing? It could be applied against outer insulation reinforced panels, forming both pillars and walls at the same time. Higher cost of shotcrete should be more than compensated by much shorter time and lower need of cranes.
Shotcrete has been used for residential concrete construction, often for domed or other curved shapes. It has also been used to provide a structural skin over internal insulation panels in more rectangular layouts. Since shotcrete is concrete projected at high speed (between 60 and 80 mph [97 to 129 km/h]) many insulation products do not withstand the impact and abrasion associated with shotcrete impact. In many cases when shooting over an insulating foam panel that cannot withstand the high velocity impact, shotcrete isn’t used, but a low-velocity plaster/grout mix is spray applied over the insulation. Here’s a link to the PCA website with a page on residential housing using a foam inner panel: https://www.cement.org/think-harder-concrete-/homes/building-systems/shotcrete. You can also find similar system information doing a web search for “shotcrete foam panel.”
StructuralQ With structural shotcrete walls, what is the anticipated shear strength of the sprayed shotcrete to a vertical wall with geotextile fabric applied to a retention system? What is the anticipated dead load of the sprayed shotcrete at the base of the sprayed wall when the shotcrete wall is sprayed to a nominal thickness of 6 in. (150 mm)? Also, what is the maximum wall thickness that can be sprayed in a single pass application with a normal mixture design to achieve 4800 to 5000 psi strength?
Shotcrete is a placement method for concrete. Thus, structural properties are equivalent to those of concrete with specified strength. Shotcrete compressive strengths will normally range from 4000 to 7000 psi at 28 days, so your 4800 to 5000 psi (33 to 35 MPa) is very normal. Density of shotcrete placed concrete is the same as cast concrete ranging from 145 to 150 lb/ft3. Walls can generally be built at any required thickness because we will create the wall by bench shooting building the wall from the bottom to the top in vertical lifts.