This detail is normally only used on very large competition pools on the order of 164 ft (50 m) in length. It takes a lot of skill, technique, and care to properly encapsulate the waterstop and it should only be attempted by a shotcrete contractor with experience in this application. The successful encapsulation of the waterstop is more challenging with the dry-mix process than when using wet-mix shotcrete. The techniques for the proper encapsulation are generally developed by the individual shotcrete contractor and there is no specific “guideline” available for encasing waterstops.
We own a 200-year-old house with a rubble foundation. The foundation is structurally sound, but needs to be repointed, and some of it has no mortar at all. We would like to seal it to make it watertight and keep out radon. Could shotcrete be applied directly to the interior of the rubble wall (which includes small, loose stones; large gaps; and cracks), or would we have to first have the walls repointed and smoothed over?
Yes, shotcrete would be an excellent method to fill the voids, open mortar joints, and gun an overlay over the irregular stone foundation. The use of shotcrete would be dependent on the access and ability of the applicator to safely place the shotcrete. A tight or low crawl space would make it difficult. We would suggest cleaning out loose materials with compressed air and water prior to the shotcrete placement. We recommend installing either a 2 x 2 in. (51 x 51 mm) 12-gauge or a 3 x 3 in. (76 x 76 mm) 11-gauge wire mesh over the stone foundation and gunning the shotcrete in place to fill in the mortar joints, creating a shotcrete overlay over the entire stone surface.
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’m looking for information on the quantity of rebound expected when applying shotcrete against soil. We have a W4 4 x 4 in. (102 x 102 mm) layer of mesh 2 in. (51 mm) from the soil face that is covered by a 4 in. (102 mm) initial layer of shotcrete. Is there a general ballpark figure that can be used, such as a percent of the total shotcrete placed?
Your question does not indicate the orientation of the application. If the shotcrete is being applied to a sloped surface for a channel or slope the rebound should be incidental. If shooting a vertical wall, the amount of rebound is relative to the skill of the nozzleman, the quality or nature of the mixture, the shotcrete process being used (wet-mix or dry-mix), the stability of the wire mesh, and other parameters. The range could easily vary from 5 to 20% on vertical walls relative to the aforementioned listed parameters.
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.
We are developing a tunnel. At the tunnel portal (entrance), we have high walls around the portal about 60 ft (18 m) tall. They will have an inner structural shotcrete layer (4 in. [102 mm]) and outer architectural shotcrete (12 in. [305 mm]). Between the structural shotcrete and rock/soil, we have a drainage system to handle the groundwater. At the same time, we may have water at the top ground surface that will drain from top to bottom of the wall. The owner didn’t want to make the water flow as a sheet over the wall surface. We proposed an inlet and vertical 6 in. (152 mm) pipe drop from the top to bottom and band to a ditch at the base of the wall. Can we locate the 6 in. (152 mm) pipe between the structural shotcrete and the architectural shotcrete?
The Federal Highway Administration’s “Manual for Design & Construction of Soil Nail Walls” should address this issue. Many soil nail wall systems incorporate a drainage ditch at the top of the wall that catches the runoff and takes it to the ends of the wall. Your concept of a catch basin and drain between the layers is not something we have seen in the past and we are not qualified to express an opinion on this. We have seen systems with catch basins at the top of the wall and the drains behind the initial layer of shotcrete requiring notching the subgrade. To answer your question, yes, a 6 in. (152 mm) pipe can be fully encased in shotcrete between the layers. Complete encasement of an embedment of this size needs an experienced shotcrete nozzleman with properly sized equipment, appropriate concrete mixture design, and a trained shotcrete crew. The issue of appropriateness of the approach is better answered by a licensed professional engineer familiar with soil nail systems or retaining walls, and shotcrete/concrete.
We are designing a 41 x 60 ft (12 x 18 m), 2.5 to 4.5 ft (0.8 to 1.4 m) deep swimming pool. The walls and floor are 8 in. (203 mm) thick with No. 4 (No. 13) bars. We are specifying shotcrete to build the pool. What should be the minimum spacing for the expansion joint?
You need to address this question to a professional engineer who is knowledgeable in the characteristics of shotcrete and concrete. Shotcrete is a method of placing concrete and the same parameters used in concrete design apply to shotcrete placements. Many pools of this size are designed and built without expansion joints, but it is beyond the scope of our association to provide further guidance.
My company has been using the gunite process (dry-mix shotcrete) for years now. What I have been finding lately is that a lot more questions are being asked by outside safety services, neighbors to our facility, etc., about the health effects of the shotcrete process. I believe that with the new proposed laws dealing with silica, everyone is paying more attention to products with sand and cement, and shotcrete has both. To try to educate myself and to answer these questions I am on the search for enlightenment and am coming up short. Here is where I am falling short: there is no (or I haven’t found a) general material safety data sheet (MSDS) on shotcrete. Most MSDSs I have found on the Internet are for proprietary mixtures. I have yet to find an MSDS or any safety info on just plain sand and cement mixture.
Shotcrete is a method for placing concrete, so an MSDS for concrete or its constituent components would be appropriate. MSDS sheets for cement, aggregates, and concrete are readily available from cement manufacturers, aggregate suppliers, and concrete producers, as evidenced by a simple web search. This is the type of issue that can be discussed and effectively addressed by networking with other contractors and suppliers in the shotcrete industry. This is a primary benefit of actively participating in ASA—you or your organization should join ASA and attend committee meetings. ASA meetings are held three times a year. Check our Calendar for the next available meeting.
What is the R-value per inch for shotcrete without any integrated insulation?
Shotcrete is concrete, pneumatically applied. So the same R-value fire ratings for concrete would apply to shotcrete. The standard is Joint ACI – TMS 216.1, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”
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.