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.
We have a pool designed with the cast-in-place concrete construction method in mind. The project has been awarded to a dry-mix gunite contractor. To accommodate the contractor’s placement method, we have been working with him on the details. They are planning on casting the floor and shooting the walls. There are two main areas of concern/questions that we have. First is in regard to the air entrainment and the admixtures that are appropriate for gunite. They have not used air-entrainment admixtures prior to this project. What is the effect to durability without using air? What are the workability effects of adding air entrainment? Which product is recommended? Second, we have specified a hydrophilic waterstop between the cast-in-place floor and the wall. With the walls using a gunite application, what is the best method for preventing water infiltration in the construction joint? Does it hurt the integrity of the joint by installing a hydrophilic waterstop? If the water stop is omitted, what does the surface roughness need to be to provide a monolithic-type connection?
Air entrainment will generally slightly reduce the compressive strength of concrete, but significantly increase the resistance to freezing-and-thawing exposure. Dry-mix shotcrete (gunite) is generally a very paste-rich mixture. With modern cements, the normal 28-day compressive strengths easily exceed commonly specified compressive strengths. ASA recommends a minimum of 4000 psi (28 MPa) for shotcrete, and 4000 psi to 5000 psi (28 to 34 MPa) strengths are routinely specified.
- Air entrainment increases the workability. The small air bubbles act as a form of lubricant to ease internal friction between the concrete mixture components.
- You should contact one of our material supplier members to see what they offer. You can use our Buyers Guide at shotcrete.org/products-services-information/buyers-guide/, and limit your search to “Admixture Sales” with the “Air-Entraining” subcategory.
- Quality shotcrete shot against a properly prepared concrete substrate should produce a watertight interface. The hydrophilic waterstop at the joint could be considered a secondary method of making the joint watertight. Though not necessary, it is kind of a “belt and suspenders” approach with a relatively low cost to place.
- 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 Sections 3.4.2.1 and 3.4.2.2 that state:
“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.”
For more details on bond between shotcrete layers, you may want to refer to an article in the Spring 2014 issue of Shotcrete magazine, “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints.” A PDF of the article can be found at shotcrete.org/wp-content/uploads/2020/01/2014Spr_TechnicalTip.pdf.
I had a concrete pool shell installed using gunite (dry shotcrete method) in July 2013. It was never finished due to unfortunate circumstances and has been exposed to the elements of weather over the last 2 years, mostly filled up with water from rain and, in the colder months, frozen like a pond. We would like to finish the pool but were told by the pool company that the concrete looked odd and we should have it strength tested. We had core samples taken from the walls and floor from a certified testing lab. The results from the six samples ranged from 1700 to 2200 psi (12 to15 MPa). When the pool was blown on July 3, 2013, it was to achieve 4000 psi (28 MPa) compressive strength in 28 days. Is it normal for the shotcrete strength to have weakened so much?
Properly produced concrete material shotcreted in place should gain strength over time, not lose strength. ASA recommends that concrete placed by the shotcrete method have a minimum compressive strength at 28 days of 4000 psi (28 MPa). Coring does damage the sample somewhat, so it is common to require core strength to meet 85% of the specified compressive strength. Cores should be no less than a nominal 3 in. (76 mm) in diameter for representative results because smaller cores (less than 3 in. [76 mm] diameter) are more subject to damage from the core extraction, affecting the reported strength. Thus, at 85% of 4000 psi (28 MPa) the minimum should be 3400 psi (23 MPa). Based on the reported values, and assuming a 3 in. (76 mm) diameter or greater core, the concrete strength is well below ASA’s recommended strength, and the strength you originally specified in 2013.
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.
Our shotcrete mixture needs to be NSF 61 certified. We have been able to obtain certification of all components with the exception of reinforced fiber. Is there or does fiber reinforcing need NSF 61 approval?
Each manufacturer of concrete constituents needs to have their products tested by NSF if they want NSF 61 certification. Whether the fibers need NSF 61 certification is an issue with the local authority having jurisdiction for exposure of components to potable water supply systems in your state. Generally, this is the state EPA-type agency, but maybe a federal agency if on a federal project.
We don’t maintain a database of manufacturer products that meet NSF 61. However, you can readily identify contacts for the fiber manufacturers who are ASA corporate members with our Buyers Guide at https://shotcrete.org/products-services-information/buyers-guide/. When entering the Buyers Guide, you can select “Fiber + Reinforcement Sales” and the fiber type subcategory to get a list of our member fiber suppliers.
I just had a swimming pool built and everything I have read online says that the gunite shell has to be sprinkled with water for several days after the pour. My pool builder says they never do it, and, when I asked why, I’m just told that’s the way they do it. I told them I am worried because every other pool builder says to do that except the one I hired and I can get no answer as to why. Is this an acceptable practice? I am worried that years down the line I may have a problem. I live in Oviedo, FL, and the weather has been in the low 70s and the humidity not particularly high. They did hit the water table and have a pump running—would any of this have an impact?
ASA recommends a minimum of 7 days curing for all exposed shotcrete surfaces. Wet curing is preferred to supply additional water to the concrete surface. If a spray-on curing membrane is used instead of water curing, the material should be applied at twice the manufacturer’s recommended rate for formed surfaces. Curing is important to allow the concrete to develop as much strength as possible and to help resist cracking from internal shrinkage of the concrete. Low humidity, wind, and exposure to sun will increase the need for proper curing. If the site is dewatered, the groundwater is below the concrete work, and not effective in curing the exposed shotcrete surface. The American Concrete Institute (ACI) has an excellent reference, ACI 308R-01, “Guide to Curing Concrete.” It appears your contractor is not following the industry standards as documented by ACI.
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.
I would like to know if there is any parameter for test panel dimensions and inclination of proposed shotcrete surfaces represented by the test panels. I also would like to know if there is any specification for frequency of making test panels during the shotcrete project duration. In general, what is the specified type of panels and number of them to be specified and to what time frequency should a contractor assemble them for quality control purposes?
ACI 506.2-13, “Specification for Shotcrete,” is an excellent resource for answering your questions. ACI 506.2 addresses both preconstruction panels and test panels used for material quality control during construction. ACI 506.2, Section 1.5.1.4, requires that when preconstruction panels are required, the Contractor shall “Construct test panels for each proposed shotcrete mixture, each anticipated orientation, and each proposed nozzleman.” Preconstruction test panels vary in size to adequately represent the embedded reinforcement and section thicknesses in the work to be done.
Test panels shot during construction for evaluation of material properties are covered in ACI 506.2, Section 1.6.3.1, which states: “Construct a test panel for each mixture, each nozzleman, and each work day or for every 50 yd3 (38 m3) placed—whichever results in the most panels. The face dimensions of a test panel shall be a minimum of 16 x 16 in. (406 x 406 mm) with a minimum depth of 5 in. (127 mm). For toughness testing in accordance with ASTM C1550, the face dimension shall be 30.5 in. (775 mm) in diameter and 3 in. (76 mm) thick. Shoot test panels in a vertical orientation only unless otherwise specified.”
ACI 506.2-13 has extensive provisions for the submittals, testing, materials, and execution of shotcrete work by a shotcrete contractor. It cites many of the ASTM testing standards appropriate for shotcrete construction. We recommend you review the document in its entirety to become familiar with current industry standards.
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.