Shotcrete is a placement method for concrete. Shotcrete has been successfully used for over 70 years in thousands of industrial wastewater treatment/storage tanks, as well as replacement lining of sewers and manholes. Thus, exposure of the shotcreted pond to wastewater should be as good or likely even better than the original cast concrete liner. Long term durability of the shotcreted section will be dependent on the concrete mixture design. Many contractors use supplemental cementitious materials (SCMs) like silica fume or fly ash to improve the pumping or shooting characteristics of the mix. These SCMs also help to reduce permeability, increase strength, and thus make the concrete more durable. Fly ash also has the benefit of adding some sulfate resistance that would be beneficial in wastewater exposure conditions. Shotcrete is often shot on geomembranes or directly on the subgrade soils if they are stable enough to hold the impact, and weight of the shotcrete.
I am currently involved with the design of an unreinforced masonry building retrofit. Could you point me toward resources concerning the seismic behavior of a reinforced shotcrete masonry wall? I am interested in learning more about the force (shear) transfer between the masonry/shotcrete interfaces.
Shotcrete is a placement method for concrete. Thus, seismic design for concrete is applicable to shotcrete placement. Here’s a link to an article in the Winter 2009 issue of Shotcrete magazine, titled “Seismic Retrofit of Historic Wing Sang Building,” that details the seismic retrofit of a brick building in Vancouver, BC, Canada: https://shotcrete.org/wp-content/uploads/2020/01/2009Win_SCM01pg08-12.pdf.
A second article from 1999, “Seismic Reinforcing of Masonry Walls with Shotcrete,” also gives some input on the design: https://shotcrete.org/wp-content/uploads/2020/01/1999Fal_Snow.pdf. In general, the structural engineer must evaluate the condition of the existing masonry structure and determine whether the added shotcrete sections will be supplementing the existing capacity or providing the full resistance to seismic loads.
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.”
We have an existing cut slope approximately 328 ft (100 m) high (3:1 vertical:horizontal) with cut benches and need to apply shotcrete onto the slope surface at a height of approximately 230 ft (70 m) from the road level. Is it possible to reasonably transfer and apply shotcrete mixture at such a height from the road level? What type of transfer hoses and equipment is preferable? Is dry-mix or wet-mix shotcrete preferable?
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.