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.
I have three apartment buildings (with three, four, and five units). Their basements were insulated just over 10 years ago with closed cell spray foam with a class one fire rating. Now for some reason, the building inspector says I have to cover the foamed walls of all three large basements with 0.5 in. (12 mm) drywall and build stud walls to hang the drywall for fire protection. This would reduce the width of the basement stairs significantly, and they would become so narrow that they would then be out of compliance. These basement walls are uneven fieldstone foundations, with bumps, protrusions, and even some curves. I was thinking that shotcrete-applied concrete might work much better and be more appropriate than wood and drywall for a sometimes-moist basement. According to the local Building Code, a 2 in. (50 mm) layer of concrete would suffice. How can a shotcrete application be made to adhere to closed cell foam? The wall heights are approximately 7 ft (2 m) plus bond.
Shotcrete is just a placement method for concrete. So shotcreting will provide the fire resistance of concrete. For securing the concrete, you can place anchors through the foam into the original basement wall. You can consult with an engineer experienced with shotcrete on anchor size and spacing required for supporting the shotcrete layer.
I have a project where the foundation sub is planning to shotcrete foundations walls instead of pouring them. They’ve submitted all the procedural things necessary to prove their competence and know what they’re doing. For a portion of our foundation, we’re immediately adjacent to an existing building. The sub mentioned today on site that they were not planning to put Styrofoam or anything between our new wall and the existing wall that would resist lateral pressure from the fluid concrete and the question was raised whether this is ok or not (FYI there is still rigid insulation, waterproofing, etc.). The argument is that the concrete is obviously stiff enough to stay in place, thanks to the nozzle-applied admixture, without an interior form that it wouldn’t be exerting any lateral pressure on the adjacent wall. I can follow that logic and almost buy it but I’m wondering if we still need something to resist the force applied from actually shooting the concrete in place?
Shotcrete is a high-velocity placement of concrete. In most thick walls, as I imagine your foundation walls are, the shotcrete contractor will be bench shooting the walls. This means they will be shooting the full wall thickness in 3 to 4 ft (0.9 to 1.2 m) high lifts where most of the impact forces and weight of the shotcrete is carried by the previously shot material. This results in very low impact forces on the back side of the section. When creating a section with a one-sided form, shotcrete contractors have used thin material, like Masonite, pegboard, or even an expanded mesh material, as we just need to have a surface to define the back of the section.
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. The examples don’t have to be applied to our area; the same conditions may apply to other saltwater areas.
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 Saint Lawrence Seaway (Pointe de la Prairie Lighthouse) that provides a lot of detail on an installation similar to yours, including salt-water exposure in a tidal zone: (shotcrete.org/wp-content/uploads/2020/01/2014Sum_Sustainability.pdf). Plus, this project also has extreme freezing-and-thawing exposure.
Are there published tolerances for shotcrete, specifically wall thickness, plumbness, and irregularities in surface, or should these tolerances be provided on the construction drawings? ACI 117 provides these tolerances for cast-in-place concrete, but specifically states it does not apply to shotcrete.
ACI 117 provides an excellent guide for tolerances for concrete structures. Although shotcrete is concrete, ACI 117 specifically excludes shotcrete because shotcrete’s unique method of placement permits a wider variety of applications and uses than that of form-and-pour concrete. Shotcrete can generally be finished to the tolerances required for the application. For example, lining a channel might not need close tolerance control, while an Olympic luge/sledding track or skateboard park may require very exacting tolerances. ACI 506.2-16, “Specification for Shotcrete,” in the Tolerances section (and the Mandatory checklist item) requires the specifier producing the contract documents to provide the tolerances required for the project. ACI 506R-16, “Guide to Shotcrete,” Section 3.8, Tolerances, provides a more descriptive commentary. Pertinent portions of that section include:
- Tolerances provide an indication of the finished product expected by the owner, but meeting tolerances may require additional effort and cost. Tolerances given by ACI 117.1R, for placement of reinforcing steel, cover over reinforcing steel, and overall alignment of cast-in-place structural members should be generally the same for shotcrete. Tolerances that require distinct values for shotcrete construction are cross-sectional dimensions, cover, and surface finish (or flatness). Therefore, specifying tolerances that can be consistently achieved are needed so that project expectations can be met at a reasonable cost.
- Specified tolerances should be based on use and function and can be the same as concrete, but are typically broader. Some finished surface tolerances may be waived to achieve proper coverage over existing reinforcement.
Although some shotcrete structures have been allowed greater tolerances than allowed for concrete, shotcrete structures can be built to the same degree of accuracy and tolerance as cast-in-place concrete.
You should review the entirety of Section 3.8, Tolerances, in ACI 506R-16 to get a complete description of tolerances for shotcrete placements.
We have a client who wants to reline the surface of a brick drying room made from bricks and used to dry bricks. The temperature doesn’t exceed 400°F (200°C). Apart from the usual shotcrete best practice, do you recommend the addition of any additives to make the shotcrete/reinforcement more suitable for the heating and cooling cycles?
Generally, sand/cement in standard concrete mixtures starts breaking down around 400 to 500°F (200 to 260°C) because the portland cement starts to dehydrate. It loses strength with every phase and will have no strength after 1000°F (540°C). Putting it on brick depends on the shape of the brick surface. Uneven surfaces with holes will hold much better than flat brick. If the brick is flat, placing more than a couple of inches (±50 mm) may delaminate and fall off the wall without anchors of some sort. You could probably use a 2 x 2 in. (50 x 50 mm) or 3 x 3 in. (75 x 75 mm) mesh. In refractory, we consider low temperatures at 1700 to 1800°F (930 to 980°C). In the higher temperature refractory, we use calcium aluminate cement and “traprock,” which is a fine crushed limestone aggregate. The mixture ratios would be the same as a typical sand and cement gunning mixture. This is what traditionally has been for coal bunkers and coal dryers or any other sections in the 1700°F (930°C)-plus temperature range.
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).
We want to know if it’s possible to apply a 1 in. (25 mm) lift of shotcrete to a berm (2:1 slope). The berm will be treated with emulsion (oil and water) prior to the shotcrete. We are only looking for long-term erosion control. Will the emulsion be required or will it cause a bonding problem with the soil?
Shotcrete is a placement method for concrete. If the berm is composed of granular materials, one wouldn’t expect the shotcrete to actually bond to the soil. Rather, placing a thin shotcrete layer would create a uniform, relatively impermeable layer of concrete to prevent water from penetrating through and washing out the soil underneath. The emulsion may help to stabilize the soil to help withstand the pressure of shotcreting directly against the berm, but many similar soil stabilization projects will shotcrete directly onto the natural soils. Also, when considering using the emulsion, be aware that some oils may contain ingredients (like sulfur) that can attack the concrete over time. You should consult with an engineer or concrete materials specialist to ascertain whether the specific oil you want to use will have a long-term effect on the shotcreted layer.
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.
We have a backwash tank on a waste water treatment plant which is made by a secant wall. The lower area for this structure will receive a shotcrete liner approximately 12 ft (3.7 m) tall on average, the interior perimeter of the structure includes 104 lineal ft (32 m) of unreinforced and 440 ft (134 m) reinforced sections which are a 12 in (300 mm) minimum thickness. The drawings call for vertical control joints with waterstop approximately every 30 ft (9 m). We don’t believe the control joints are necessary and could achieve the same desired performance with one monolithic installation of the shotcrete. Are the control joints really necessary when you are installing the shotcrete against a solid secant wall which does not contain any control joints?
By control joints, we assume you mean contraction joints. Shotcrete is a placement method for concrete. All normal concrete experiences drying shrinkage that creates a volume change in the hardened concrete. Though shotcrete has a lower w/cm than most form and pour concrete, it will still undergo shrinkage. In being shot on an existing concrete wall the shotcrete liner will be restrained by the bond to the substrate and the restraint of the horizontal volume change from shrinkage can create internal tensile stresses in the concrete. This is likely the reason the designer has specified contraction joints in the section. 30 ft (9 m) spacing between joints is common in new construction of concrete tanks. The question becomes whether the bond of the shotcrete to the existing substrate is high enough to restrain the volume change and prevent cracking along the hundreds of lineal feet of wall you will be lining. The thickness of the lining, the type and duration of curing, the concrete mix design, the strength of the concrete, the strength of the substrate, the quality of shotcrete application, proper surface preparation and exposure to seasonal temperature changes will impact the effect of the volume change of the lining. With the many variables we’ve pointed out you can see there isn’t a clear answer that covers all situations. We recommend you discuss your opinion with the designer or consult with a professional engineer experienced in shotcrete repairs to fully evaluate the specific structural sections you’re shotcreting.