Yes, either dry-mix or wet-mix process can and have been used at this height. Because either process can be used in these conditions, you should use an experienced shotcrete contractor who will pick the best method based on their firm’s shotcrete capabilities. Factors such as the shotcrete contractor’s specific shotcrete equipment, material availability, site constraints, remoteness of the location, and crew experience will influence their choice.
We have 18 ft high, 12 in. thick (5.5 m high, 205 mm thick) walls to shotcrete and need horizontal cold joints to place the shotcrete in three pours (three height sections). How do we create the joint?
Shotcrete is routinely used in creating retaining walls or soil-nailed walls in this fashion. Designers and inspectors often confuse placement of multiple layers of shotcrete in building out a section with cold joints experienced in cast-in-place concrete construction. Unlike cast-in-place concrete, shotcrete provides thorough consolidation and densification by high-velocity impact of fresh concrete material on the receiving surface. The high-velocity impact of shotcrete on a hardened, previous shot layer (or existing concrete surface) provides a strong abrasive blast to open up the surface, and then provides an immediate exposure of that hardened surface to fresh cement paste. As a result, shotcrete exhibits excellent bond to concrete and previously shot surfaces. Thus, the structural action between the sections acts as a monolithic section without any weakened planes.
In shotcrete construction, surface preparation between layers to provide full bond is important. ACI 506.2-13, “Specification for Shotcrete,” specifically addresses this in the requirements of Section 3.4.2.1 and 3.4.2.2 that require:
3.4.2.1 When applying more than one layer of shotcrete, use a cutting rod, brush with a stiff bristle, or other suitable equipment to remove all loose material, overspray, laitance, or other material that may compromise the bond of the subsequent layer of shotcrete. Conduct removal immediately after shotcrete reaches initial set.
3.4.2.2 Allow shotcrete to stiffen sufficiently before applying subsequent layers. If shotcrete has hardened, clean the surface of all loose material, laitance, overspray, or other material that may compromise the bond of subsequent layers. Bring the surface to a saturated surface-dry condition at the time of application of the next layer of shotcrete.
An experienced shotcrete contractor should routinely provide proper surface preparation between shotcreted sections, and use skilled crews with ACI certified nozzlemen to place and cure the shotcrete placements.
We are planning on placing shotcrete over rock face and are concerned about water seeping out of the rock and forming ice behind the shotcrete. Could you please offer some insight?
Shotcrete is a functionally impermeable material when properly designed and placed. Thus, if water accumulates between the rock substrate and shotcrete, depending on temperatures and thickness of the shotcrete, it may freeze. As with all concrete, shotcrete material can be air-entrained to enhance resistance to freezing-and-thawing exposure. You should consult with an experienced engineer to determine whether the type of rock, geometry of the sections, and anchoring of the shotcrete (such as with soil nails or rock bolts), along with the bond of shotcrete to the rock, will provide the desired performance.
We are considering sealing off the openings of some existing water tunnels by constructing reinforced concrete walls within the openings. One wall, for instance, is 9.25 ft wide by 12 ft high (3 x 4 m) and is to be 22 in. (559 mm) thick. Other walls will be in the range of 18 to 26 in. (457 to 660 mm) thick. What are the limits as to the maximum thickness/size wall that shotcrete can be used to construct? Also, I noticed that in the latest revision of ACI 506R-05 that the previous recommended limits on the reinforcing bar size have been removed. I believe in the past, the reinforcing bars were limited to the smaller-size bars to reduce the development of sand pockets behind the bars. Is there a recommended limit on the size bars that can be used? Perhaps it was in the ACI standard but I just overlooked it.
Experienced shotcrete contractors with qualified crews have often shot structural concrete sections 24 to 30 in. (610 to 762 mm) thick with reinforcing bars up to No. 11 in size. There is no real limit to how thick you can build a shotcrete “wall.” Though earlier versions of ACI 506R, “Guide to Shotcrete,” did recommend limiting reinforcing bars sizes, modern equipment, concrete mixtures, and shooting techniques have proven that large-diameter steel reinforcing bars can be properly encased on a routine basis by experienced shotcrete crews. Thus, ACI 506R was updated to reflect the successful industry practice.
There is a pre-stabilized steep slope (nailing is used along with shotcrete). The client wishes to benefit from the maximum achievable area in plan and is asking for excavation of the stabilized slope to shift back the wall face around a few meters. The new stabilization plan shall include a safe gradual excavation of the existing wall from top to bottom along with the destruction of the existing shotcrete face, reinforcement, and the installed nails. What is the most common destruction method for the existing shotcrete? At the same time, I think pulling out the nails using hydraulic jacks would be applicable.
The existing shotcrete can be removed by many methods, such as using a Hoe-Ram. The existing soil nails could be removed as suggested or left in place and trimmed back to some distance behind the new plane. We would assume that this work would be done from the top down installing new soil nails and shotcrete facing in lifts of approximately 4 to 5 ft (1.2 to 1.5 m).
We are proposing a vertical support of excavation structure using reinforced shotcrete to retain a 10 ft (3 m) high sandy soil. What are the design criteria to choose the reinforcement and the thickness of the shotcrete? Also, what is the minimum reinforcement and shotcrete thickness you would recommend?
FThe shoring design should be done by a competent licensed professional engineer who specializes in earth retention systems. This is not a question that can properly be answered by ASA. You can search for such a professional in our Buyers Guide section of the ASA website shotcrete.org. Another resource is the FHWA Manual for Design and Construction of Soil Nail Walls.
We have a project in the Los Angeles, CA, area that we have designed as poured-in-place concrete. The contractor would like to shotcrete all basement walls. However, we have columns that are integrated with the wall. A City of Los Angeles technical bulletin does not address shotcrete columns in a wall where the bar spacing varies from the wall. Do you have any experience with this issue and is there a way to shotcrete both the wall and the column?
There have been many prior projects in the city of Los Angeles where columns and pilasters are incorporated into perimeter basement walls. This would likely require a preconstruction mockup panel and might require a waiver from the City Engineering Department. It is extremely important to ensure that the work is done by a qualified and experienced shotcrete specialty contractor who has documented experience doing similar work.
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.
I own a home on a very busy street and the house placement borders the street, approximately 75 ft (23 m) from the curb. Traffic flow has increased over the past year, and I have tried all suggested and approved soundproofing wall systems with varied results. It would seem that a shotcrete product sprayed within a wall cavity would work great. My assumption is that I would need to insulate the outward face of the cavity enough to prevent excessive condensation and moisture buildup. This would certainly solve the sound problem (depending on the mass sprayed). Are there any suggestions or references you might be able direct me to?
A properly designed shotcrete composite wall system would certainly create a quiet atmosphere in the interior of the house. There are 3-D shotcrete wall systems on the market which use shotcrete on both the interior and exterior surfaces with a foam material in the center for insulation and vapor barrier. Attempting to do something like this to an existing structure would require a lot of analysis. Shotcrete is a method of placing concrete and adding shotcrete to an existing wall would significantly increase the weight of the wall and could overload the footings or impact other parts of the system. A structural engineer well-versed in residential construction should be consulted before attempting such a modification.
An inspection report on our home indicated there was evidence of past rodent infiltration. An engineering consulting firm recommended that, to prevent rodents from burrowing underneath the foundation, we have a contractor apply shotcrete across the entire crawlspace bottom, then have a 2 oz. (60 mL) vapor barrier installed on top of it. The barrier would be glued or taped up the sides of the crawlspace. As there is some shrinkage of the concrete during the curing process, I would expect creatures could later emerge between the shotcrete and crawlspace sides. Have you heard this type of shotcrete application in a crawlspace as a structural pest barrier? What thickness should the shotcrete be? Is this use of shotcrete effective? Are there any potential drawbacks to using shotcrete in this way, such as possible problems with the house later on?
Shotcrete is a method of placing concrete and the properties of shotcrete are equivalent to those of cast concrete. The type of work you are describing is done in many cases with the shotcrete process and is commonly called “ratproofing.” As you have engaged an engineer, we would suggest you follow his advice and he should determine the thickness required. Providing a row of dowels around the stem wall to tie the shotcrete to the wall should eliminate any significant separation between the shotcrete and the stem wall. Shrinkage of concrete between the walls may cause some minor hairline cracking, but nothing to allow ingress of rodents or insects.