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 am repairing a concrete masonry unit (CMU) block wall that was partially damaged when a portion of the roof collapsed. The engineer on the project is proposing to apply shotcrete to one side of the wall to help structurally reinforce the wall. I would like to know if there is a way to finish the wall so it is cosmetically pleasing, especially since this is on the inside of an existing building with the other walls being a painted CMU. Also, were can I get some conceptual pricing for applying the shotcrete?
Shotcrete can, and often is, finished to provide nice printable wall surfaces. To be the same general texture of the concrete block wall, you should specify a wood or rubber float finish. You can access the ASA Buyers Guide at https://shotcrete.org/products-services-information/buyers-guide/ to locate organizations regarding budget or conceptual pricing.
RepairQ I am working on a fire re-build project near Napa, CA. The existing building has 12 in. (300 mm) thick shotcrete basement walls. There are many areas where the concrete cover spalled off from the rebar. The rebar appears to be ingood condition with no visible buckling. Is it possible to repair these walls with a 1 1/2 to 2 in. (38 to 50 mm) layer of shotcrete to restore the concrete cover? If so, what sort of surface prep is required? Is a concrete bonding agent required?
Yes, this is definitely a good application for shotcrete. To restore the cover:
- Chip or hydrodemo back to sound concrete. If a reinforcing bar is more than half exposed, chip at least 1 in. (25 mm) back behind the bar.
- Make sure the surface is roughened and clean.
- Bring the concrete surface to 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. 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.
- If chipping out a section do not feather edge. Provide a ¾ to 1 in. (19 to 25 mm)) depth of cut at the edge to provide adequate thickness for the integrity of the shotcreted material at the edge.
This article on the excellent bond between shotcrete provides more detail: https://shotcrete.org/wp-content/uploads/2020/05/2014Spr_TechnicalTip.pdf
RepairQ 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 reinforcement required? The stone is rather friable and I don’t want to attach too much to it for fear of further damaging the stone.
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.
RepairQ 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.
RepairQ I am working on repairing some mildly deteriorated walls in a drinking-water treatment plant. There are no chlorides used in the treatment process. I would like to apply a 1 in.-thick shotcrete layer over the existing concrete utilizing a mix containing silica fume, which will achieve a compressive strength of 5000 psi at 28 days. I am having difficulty formulating a mix to meet those requirements that also has a water soluble chloride content of less than 0.10 % chloride ion concentration by mass of cement. I cannot get the chloride ion concentration below 0.15%. What adjustments can I make to get to my goal of 0.10% or less?
There are areas that have no problem getting values lower than the most stringent ACI requirement of 0.06% for prestressed concrete with no special adjustments. It would be prudent to test each of the proposed shotcrete constituents to determine their soluble chloride ion content. The most likely suspects are the aggregate and water sources. Typically portland cement and silica fume would contribute little, if any, detectable chloride ions. Assuming this would be a dry-process application, the only admixture other than the silica fume might be an air entraining agent, which would not provide any chloride ions. This leaves only the aggregates and water as the sources. At a minimum, the aggregates and water should be tested by a qualified laboratory for soluble chloride ion content. Alternate sources of aggregates and water may be required based on the laboratory results.
RepairQ 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.RepairQ 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.
RepairQ I have a new construction project where I want to apply shotcrete to cast-in-place concrete columns and an elevated, post-tensioned concrete slab as a finish material. The finished application is intended to be in varying depths from 3 to 12 in. (76 to 305 mm) or more. The desired end result is a smooth, curvilinear, sculptural form. Is this type of application achievable?
Shotcrete can and has been used to increase the size of columns and thicken overhead slabs while providing great-looking linear or curvilinear finishes. Examples of curvilinear finishes can be found in past Shotcrete magazine articles. You can search the Shotcrete magazine archives.
RepairQ I have a project wherein some 25,000 sq ft of existing shotcrete is to undergo varying degrees replacement, repair and restoration. It is on slopes varying from 1:1 to 1:10 or so. It is approximately 40 years old in most cases. It is in a fairly arid climate (Southern New Mexico) with little rainfall and typically low humidity. The subgrade is non-plastic gravelly sandy material. It was reinforced with wire mesh (looks like 6x6x10x10). I’m interested in any techniques and/or materials that might be applicable
I recommend reading the following publications in Shotcrete Magazine: “Shotcrete for Ground Support: Current Practices in Western Canada”, by C.Chan, R Heere, & D. R. Morgan, Part I printed in Winter 2002, and Part II printed in Spring 2002. “Soil and Rock Slope Stabilization Using Steel Fiber Reinforced Shotcrete in North America”, by M.Ballou & M Niermann, Summer 2002.
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.