I have a 24 in. (610 mm) thick concrete dome that serves as an enclosure to protect extremely sensitive and important equipment that needs to withstand high impact demands such as tornados or missiles. The contractor is proposing to use the shotcrete method with the following sequence: shoot approximately 1 in. (25 mm) (to achieve reinforcing bar cover); let stand for 8 hours; then place a reinforcing mat; then shoot the majority of the dome thickness; let stand for 8 hours; then place the other mat of reinforcing; then shoot the remaining concrete cover. I am concerned that, with an 8-hour duration between concrete placements, the three layers of concrete will not be adequately bonded such that they behave monolithically. In particular, I would be concerned that the aggregate of the concrete that is shot onto a mat of reinforcement will not be able to make its way “behind” the bar’s deformation, thus causing voids. Please let me know your thoughts on the aforementioned concerns, whether it would be reasonable to shoot a 24 in. (610 mm) dome with a minimum of two layers of reinforcement all at once, and whether any of the ACI codes or standards speak to shotcrete joints parallel to reinforcement.

Multi-layer buildout of shotcrete sections is very common and has decades of successful performance in existing structures. Shotcrete applied to a properly prepared, existing hardened concrete substrate (such as a previously shot shotcrete layer) develops an excellent bond. The high-velocity impact of shotcrete on the surface is in effect like sandblasting, and opens up the receiving surface immediately before exposing it to the fresh cementitious paste. Cores taken through multiple layered shotcrete sections exhibit no signs of reduced bond. Often it is nearly impossible to identify where one layer stops and the next starts.
Incremental placement of reinforcing bars in layered application is also common. Proper shotcrete consistency, nozzleman technique, and air velocity will force fresh cement paste around the back of the bar and fully encase the reinforcing bar, even when in contact with the previous hardened concrete surface.
Shooting a 24 in. (610 mm) thickness at one time with two layers of reinforcement in the mostly overhead orientation of a dome would require use of special concrete mixture designs with chemical accelerators, and would be very difficult to execute with consistent quality. Also, depending on the formwork design, unbalanced loading on the dome by shooting very thick sections adjacent to sections not yet shot would be a potential concern.
For more information on the performance of shotcrete in layers, you can review this article from Shotcrete magazine, “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints.”

We have a vertical shaft that is (right now) 70 ft (21.3 m) deep and we do blasting every 5 ft (1.5 m) after applying shotcrete to the vertical surface for protection. My concern is that if we have less than 48 hours between successive blasting, is it allowable? How does one measure if the shotcrete reaches the required percentage of strength?

The best guidance on this subject can be found in ACI 506.5, “Guide for Specifying Underground Shotcrete” (available through the ASA Bookstore), and some articles from past issues of Shotcrete magazine might be of interest:

With properly qualified nozzlemen, a good shotcrete mixture, and high-quality accelerator added at the nozzle, the re-entry time can be minimal—normally 24 hours.

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.

I am currently involved in the design of several long retaining walls. One option under consideration is the use of soil nails with shotcrete reinforced by welded wire fabric (WWF) and the other is the use of mechanically stabilized earth (MSE) reinforcement (geogrids) with shotcrete reinforced by WWF. What is the best method (or product) to anchor each system to the shotcrete, and how are shotcrete-to-shotcrete (gunite) anchors treated in an MSE wall?

There are many ways to attach a shotcrete facing to a soil nail shoring system or an MSE wall system. For the attachment to a soil nail wall system, you could review the Federal Highway Administration’s “Manual for Design & Construction of Soil Nail Walls.” For MSE wall systems, you should consult with the MSE wall system vendors. Shotcrete facing systems are commonly used on both types of walls, but it is beyond the scope of our association to provide further guidance.

I need to find the reference in ACI standards indicating the technical and practical reasons why thermal expansion joints and contraction settings are eliminated in the stabilization of nonstructural slopes covered with shotcrete and steel fiber. Can you help?

Shotcrete is a method of placing concrete. Fibrous shotcrete will have very similar, if not identical, properties as fibrous cast concrete. Expansion and contraction joints should be similar in shotcrete to those needed in cast concrete. ACI 224.3R-95, “Joints in Concrete Construction,” covers joints in many different applications. The closest relevant document for eliminating joints is ACI 360R-10, “Guide to Design of Slabs-on-Ground,” where, in Section 8.3, it states:
“To eliminate sawcut contraction joints, a continuous amount of reinforcement with a minimum steel ratio of 0.5% (PCA 2001) of the slab cross-sectional area in the direction where the contraction joints are eliminated is recommended.”
This 0.5% reinforcement is consistent with the provisions of ACI 350-06, “Code requirements for Environmental Engineering Concrete Structures and commentary,” for the minimum reinforcement for temperature and shrinkage without contraction joints.
You can refer to ACI 506 series documents regarding shotcrete, and possibly the FHWA SA-96-069R “Manual for Design, Construction, and Monitoring of Soil Nail Walls” for additional guidance. Copies of the ACI 506 series documents are available in the ASA Bookstore.

The Los Angeles City Bulletin states that no bars over No. 8 (No. 25) shall be used. The structural engineer has No. 10 (No. 32) bars in the columns. I am being told the test panel will get this approved but my City Inspector is balking a little. Is there a publication or code somewhere that allows the test panel to supersede the LADBS Bulletin?

The International Building Code (IBC), Section 1913, allows for larger bars as long as it is demonstrated in a Preconstruction Test Panel. However, the Local Building Code likely takes precedence over the IBC. You may want to present IBC Section 1913, which requires anything over a No. 5 (No. 16) bar to be proven in a Preconstruction Test Panel.
There have been many projects shot in Los Angeles County subject to the LADBS with bar sizes larger than No. 8 bars. ASA is not in a position to give you project references, but perhaps our local members can.
Properly encasing No. 10 (No. 32) bars can be challenging, and should only be attempted by qualified contractors using ACI Certified Nozzlemen who have previous successful experience doing this type of work. You may use our online Buyers Guide to find an ASA corporate member consultant or contractor to assist you.

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.

I am a structural engineer and I am supposed to design structures for shotcrete applications. Should I calculate and check its stability by the “working stress method?” Or, could I use the “ultimate limit design?” Are there regulations or specifications about the application of method on ACI? Finally, is elastic coefficient different between normal concrete and shotcrete?

Shotcrete is a method for placing concrete. Thus, the concrete placed by the shotcrete method has the same physical properties as cast concrete with the same mixture proportions. Either working stress or ultimate strength methods used for concrete design are applicable. Local building codes may require a particular design approach.

We have a two-story shotcrete wall enclosing an indoor community pool. We are specifying a board-form finish for the interior and the exterior will have a parge finish coat. Are there any issues with the consistent moisture from the pool that should be addressed in the concrete mixture or topical sealant? How should we deal with the exterior versus interior finishes in regards to water intrusion protection and allowing the green concrete to “dry out” over time?

Shotcrete is a method of placing concrete and the characteristics of shotcrete are those of cast concrete. Although the enclosed swimming pool will increase the interior humidity, the high humidity should have no detrimental effects on the exposed shotcrete, and may even be beneficial in reducing long-term drying shrinkage of the wall.
Both cast-in-place and shotcreted concrete are commonly used for construction of water tanks with constant exposure to water under significant hydraulic pressure. Using good construction techniques with good-quality concrete to build the tank’s walls produces walls with no moisture evident on the exterior face of the tanks. Simply having a high-humidity atmosphere is a much less severe exposure and should not result in interior air moisture being transmitted into and through the shotcrete wall. Any coatings considered for aesthetics should follow the manufacturer’s recommendations for drying time of concrete before application. If there is a concern about the permeability of the shotcrete wall, a premium shotcrete mixture including silica fume might prevent some issues on this application.

I need to specify a shotcrete cover to some structural steel in a coal dump hopper. The idea is to provide abrasion and impact protection to the steel beams. However, the client cannot afford to have the hopper out of service for an extended period. Is there a “high-early-” strength option for shotcrete as there is for cast-in-place concrete?

There are prepackaged materials commercially available for impact and abrasion resistance. Please contact material suppliers from “ASA’s Buyers Guide” for product information: shotcrete.org/Buyers Guide.