The ASA Technical Questions and Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.
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StructuralQ We are building a new home. Foundation contractors who place traditional basement walls tell us they would never go into a house built with walls constructed using shotcrete. When used for walls, can shotcrete be of equivalent strength as placed concrete?
Shotcrete is a method of building a structure using a concrete mixture. A shotcrete mixture likely would exceed the compressive strength of most mixtures used for placed walls because the application of shotcrete requires a much lower water-cementitious material ratio than commonly found in residential wall mixtures. A shotcrete mixture will have a water-cementitious material ratio of approximately 0.50, yielding a compressive strength of about 4000 psi at 28 days. Poured wall mixtures have ratios of approximately 0.70 and compressive strengths of 2500 to 3000 psi. The lower water-cementitious material ratios of shotcrete mixtures produce other benefits such as reduced shrinkage and lower permeability. Additionally, the greater compaction of shotcrete achieved through the velocity of placement improves compressive strength and durability.
StructuralQ We are building an area of a park that is on an existing pier in Brooklyn, NY. We are researching using shotcrete to form contours on certain areas. Weighting of the pier is an issue. Our question is, can lightweight concrete, or cellular concrete, be used in shotcrete? If so, what are the weights?
Shotcrete is a placement method for concrete. Lightweight concrete usually ranges from 90 to 115 lb/ft3 (1400 to 1800 kg/m3). In wet-mix shotcreting, lightweight concrete should use presoaked aggregate to make the mixture pumpable. When it’s pumpable, as with conventional concrete mixtures, it is then accelerated to a high velocity by air at the nozzle and projected onto the surface. Lightweight aggregates can also be used directly in dry-mix, and there you don’t need to worry about pumpability because the dry materials are conveyed through the delivery hose. Water is added at the nozzle.
Here’s the specific reference on lightweight from ACI 506R-16, “Guide to Shotcrete”:
2.1.3.2 Lightweight aggregates—Lightweight aggregates should conform to ASTM C330/C330M if used in shotcrete. The aggregate should meet one of the gradations shown in Table 1.1.1. Wet-mix shotcrete with lightweight aggregate is seldom used and is difficult to pump because the aggregate absorbs water, which reduces the consistency of the mixture. Presaturating the lightweight aggregate before batching improves pumpability. Lightweight aggregate mixtures have been shot for wall and floor construction. Shotcrete is frequently employed for fireproofing structural steel members using lightweight aggregates in the mixture.We suspect that cellular concrete cannot be shot because it uses injection of a pre-formed foam into a cement slurry and is highly fluid. That would preclude any stacking of material to make a vertical surface, and would instead just be pumped in place like a high slump concrete.
StructuralQ We are concerned about the compressive strengths of shotcrete recently placed on one of our projects. The specification calls for 8000 psi (55 MPa). Test results indicate we are only at 5200 psi (36 MPa) at 28 days. Ambient temperatures are constant at about 45 °F (7 °C) at the point of placement. Should we be considering removal of the shotcrete?
Shotcrete, like any other concrete mixture, will continue to gain strength as long as there are unhydrated cement particles present along with sufficient temperature and moisture. Strength development will generally be quite slow at the ambient temperature reported. The inclusion of supplementary cementitious materials in this mixture is a benefit in this instance as strength will increase as long as calcium hydroxide is available from the hydration of the cement. The specified strength should eventually be attained as long as the ambient temperature does decrease further and some form of moisture is available to the shotcrete.
StructuralQ 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.
StructuralQ We are considering the use of shotcrete to line a 3600 ft (1097 m) channel that is approximately 15 to 20 ft (4.5 to 6 m) wide. The purpose of the lining is to cap impacted sediments in the channel bottom. What is the suitability of shotcrete for this type of application, and can you provide a conceptual/budgetary estimate for the implementation of this approach?
Shotcrete is a method of placing concrete and therefore the material has the same basic characteristics of concrete. Shotcrete is often used for canal, channel, and ditch lining. It is important with shotcrete (concrete) that the subgrade the material is placed over be compacted and stable. The thickness, strength, and reinforcing needs to be designed and specified by a professional engineer familiar with this type of structure or pavement. For budget numbers, you should contact one of our contractor members, who can be found in the Buyers Guide on the Web site at shotcrete.org.
StructuralQ We are constructing a new custom residence on the Gulf Coast of Texas using a Monolite insulated concrete form (ICF) system. The ICF system is basically a “sandwich” system with an expanded polystyrene (EPS) panel with a wire cage and shotcrete on both sides. Because of the storm surge and high humidity of the region, we are looking for a mixture formula for a waterproof shotcrete for the exterior coating to help prevent moisture migration to the interior. What can you suggest?
The insulation itself should provide a vapor barrier. Various additives can be used with the shotcrete to improve its permeable properties, such as silica fume or a commercial waterproofing additive. It is also not uncommon to use a plaster coat over the shotcrete to provide improved water resistance and an architectural finish. The density and uniformity of the shotcrete can be influenced by the competency of the shotcrete applicator. It is always advisable to engage a competent and experienced shotcrete contractor to ensure the best possible results. You can search for a contractor with certified shotcrete nozzlemen from our Buyers Guide at shotcrete.org/products-services-information/buyers-guide/ or submit a bid request through our Online Bid Submittal Tool at www.shotcreteweb.wpengine.com/pages/secured/ProjectBidRequest.aspx.
StructuralQ We are currently working on a job that requires integral color for a sculpted rock facing, but the plant close to the job does not supply color. The DOT we are working for has informed us they will not approve the use of retarder in the mixture so we can order the colored mixture from a plant that is 50 minutes away from the site because they are worried about long-term strength of the material decreasing. Do you know of any literature that we can provide to the DOT regarding effects of retarder on 28-day strength of shotcrete?
Shotcrete is concrete. PCA’s Design and Control of Concrete Mixtures, 16th Edition states: “In general, some reduction in strength at early ages (one to three days) accompanies the use of retarders. However, increased long-term strength may result from retarding the initial rate of hydration. Excessive addition rates of a retarding admixture may permanently inhibit the hydration of cement.” Thus, if you closely follow the admixture manufacturer’s recommendations for dosage, you may get higher long-term strength than non-retarded mixtures. You may also consider use of the newer hydration control admixtures that essentially stop hydration until activated, and can theoretically put the concrete “to sleep” for up to 3 days.
StructuralQ We are currently working on a Request for Deviation to use shotcrete in lieu of cast-in-place concrete. The engineer is requesting additional information/confirmation. The application locations are structural, using No. 6 and No. 8 reinforcing bars on 1.5 ft (0.5 m) thick walls approximately 40 ft (12.2 m) high. The engineer’s comments refer to detailing construction joints, curing, and plastic shrinkage gaps (work done in July). We have also requested a slump to be reduced to 2 ± 1 in. (51 ± 25 mm) and the use of 3/8 in. (9.5 mm) aggregate. What methods would you suggest to address each issue?
The project as described sounds very feasible for a structural shotcrete application. As we understand, the concerns are:
- Detailing construction joints—Please refer to ACI 506R, “Guide to Shotcrete,” Paragraph 5.7, Joints. Typically, shotcrete joints are beveled to increase the surface area of the bonding surface and reduce the likelihood of trapping rebound. Other considerations for construction joints should follow the principles of cast-in-place concrete. Shotcrete is a method of placing concrete.
- Curing—Shotcrete is concrete consisting of smaller aggregates and generally higher cement content. Good curing practices should be followed as they should be with cast-in-place concrete. Considerations should include the temperature and humidity when evaluating a curing program. High temperatures with low humidity will require significantly more effort than high temperature with high humidity. The key is to ensure that sufficient moisture is available to hydrate the cement during the curing period.
- Plastic shrinkage gaps or cracking—The shrinkage characteristics of shotcrete are similar to cast-in-place concrete. Shotcrete is composed of smaller particles and higher cement but generally places at a low water-cement ratio (w/c) or less than 0.45. Shotcrete is somewhat more prone to plastic shrinkage cracking due to the surface not being protected by a form in its early stages. If the finished surface is subjected to high ambient temperatures, low humidity, or high winds, it will tend to dry quickly on the surface and exhibit more plastic shrinkage cracking. In these environmental conditions, fogging of the exposed shotcrete surfaces soon after shotcreting may help to reduce or eliminate the plastic shrinkage cracks. Plastic shrinkage cracks are generally superficial in nature and can be repaired if necessary.
- Slump to be reduced to 2 ± 1 in. (51 ± 25 mm)—This is a good range if measured and treated properly. It is important to ensure that the shotcrete material has enough slump at the nozzle to properly encapsulate the reinforcing steel and is stiff enough to stay in place without sloughing or sagging. The slump at the nozzle is far more relevant than the slump at the pump.
The important factors influenced by slump are maintaining the proper water-cementitious material ratio (w/cm) and consistency at the nozzle to ensure good placement. The most important consideration is to ensure that you have an experienced shotcrete contractor who has a history of success with similar projects with respect to the size and complexity of the installation. You can locate shotcrete contractors on the ASA online Buyers Guide at shotcrete.org.
StructuralQ We are currently working on a tunnel that will cross through a drinking water protection zone in the alluvial aquifer. Do shotcrete technology and materials exist that can be used on groundwater-sensitive areas?
Shotcrete is the same as concrete when evaluated as a material and its exposure to potable water. In the U.S., many admixtures and cements for concrete have been tested and certified to meet the NSF 61 standards for materials exposed to potable water. In my experience, potable water stored in concrete tanks with direct exposure to the concrete (no coatings) has not exhibited any significant rise in alkalinity. Exposure of a tunnel in a groundwater aquifer would seem to have much less contact area per volume of water contained in the aquifer, such that any rise in alkalinity would be miniscule. Because concrete in the U.S. is universally accepted for the storage and transport of potable water, I’d assume that the use of shotcrete in your tunnel would be perfectly acceptable.
StructuralQ We are looking for the application of shotcrete on tidal waters. We are located on Lower Puget Sound in Washington state and need examples where this has been used and is holding up under the moving tides.
Shotcrete is a placement method for high-quality concrete. Here’s a link to an article of a rehabilitation of a concrete-supported lighthouse in the St. Lawrence Seaway (Pointe de la Prairie Lighthouse) that provides a lot of detail on an installation like yours, including saltwater exposure in a tidal zone. Additionally, this project also has regular freezing-and-thawing exposure (shotcrete.org/wp-content/uploads/2020/01/2014Sum_Sustainability.pdf).
Another project with tidal zone exposure involved repair of bridge pier pile caps on the East Coast of Florida. The article details the project parameters and testing conducted to verify the quality of the shotcrete placement (shotcrete.org/wp-content/uploads/2020/01/2012Win_White.pdf).