There is not a fixed value that one can use for maximum lift height. The maximum lift height when bench shooting is controlled by the concrete mixture (admixtures, aggregate, slump), concrete temperature, size and layout of reinforcement, substrate being shot against, and ambient temperatures. These factors must be evaluated by the nozzleman during the placement. The maximum height is constrained by the ability of the fresh concrete in the lower portion of the lift to carry the weight of concrete in higher portions without creating sagging or sloughing. Hot weather conditions will allow higher lift heights than cold weather in non-accelerated concrete mixtures.
Are there industry-standard design guides for the design of formwork for shotcrete? I have reviewed the technical questions and found the article “Dynamic Forces during Shotcreting Operations” by Frederic Gagnon and Marc Jolin, but the impact loads in this article are quite small (approximately 90 lb [40 kg] per nozzle application), and I feel uncomfortable using that as the sole design load. However, since there is little to no hydrostatic load from shotcrete, I am having a difficult time coming up with a reasonable design load in lieu of this.
Shotcrete placement produces very localized pressure on the one-sided forms we typically use. The 90 lb (40 kg) is a reasonable figure when shooting directly against the form. If shooting thick walls, we typically use a benching approach for placement that puts most of the force directly on the supporting floor (or earth) and putting little force on the form. Thus, the form is more of a way to define the back surface. You will often see shotcrete forms in lower-height applications using thin forming materials such as Masonite, pegboard, or even a stay-form for the formed surface. Often, the larger load controlling the design on a form may be the wind loads expected during the construction period.
What is the cure time for shotcrete?
Shotcrete is a placement method for concrete construction. Shotcrete-placed concrete should be properly cured to provide desired strength and reduce potential shrinkage. ACI 308.1-11, “Specification for Curing Concrete,” and ACI 308R-16, “Guide to External Curing of Concrete” are excellent reference documents. ASA recommends curing a minimum of 7 days, and prefers curing with water, maintaining a continuously wet surface condition for the 7-day period. If using a curing membrane instead of water curing, ASA recommends applying the curing membrane at twice the curing membrane manufacturer’s recommended application rate, and applying in two layers with the second perpendicular to the first. If applying a coating over the final concrete surface, you should check with the coating supplier to verify the duration and properties required before application of the coating.
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.
Will the new ACI Tech Note for core evaluation (ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation”) replace ACI 506.2-95, “Specification for Shotcrete”? That document has some sections about core evaluation.
ACI 506.2-95, “Specification for Shotcrete,” is the deprecated version, and is no longer published (it isn’t readily available on the ACI website). Thus, the Tech Note along with the current ACI 506.2-13, “Specification for Shotcrete,” is the current industry standard for evaluating cores. An engineer may specify use of the outdated ACI 506.2-95; however, they are opening up their exposure because it isn’t the current standard. This is similar to the exposure if an engineer uses a much older version of ACI 318, and not the current one.
Can we find an appropriate and easy way to evaluate the shrinkage performance of shotcrete?
Shotcrete is a placement method for concrete. So, standard concrete tests for shrinkage are applicable. You will find an article from Shotcrete magazine, “Shotcrete Testing—Who, Why, When, and How,” helpful. The specific section on drying shrinkage tests says:
“Drying shrinkage of the shotcrete can be tested using general provisions of ASTM C157. Because the shotcrete is shot into a large panel and not into the relatively small mold specified by ASTM for the shrinkage test beam, it is recommended that a beam approximately 11.25 in. (285 mm) in length be sawed from a test panel. As most shotcrete uses coarse aggregate less than 1 in. (25 mm), a 3 in. (75 mm) thick panel with a 3 in. (75 mm) wide cut should approximate the ASTM requirements. The A/E should specify in the contract documents drying shrinkage limits that are appropriate for the design of the structure.”
Can we find an appropriate and easy way to evaluate the
Shotcrete compressive strength should be tested using cores from test panels. Shotcrete cannot be shot into closed cylinder forms and be representative of the in-place shotcrete. ASTM C1140-11 and C1604-05(2012) provides the panel configuration and compression testing requirements. However, if using the wet-mix process for shotcreting and one wants to verify the compressive strength, air content, or temperature of ready mixed concrete materials as delivered, rather than as shotcreted in place, samples can be taken from the truck before pumping. These cylinders would follow ASTM C31-15 for making and curing concrete test specimens.
Can we find an appropriate and easy way to evaluate the shrinkage performance of shotcrete?
Shotcrete is a placement method for concrete. So, standard concrete tests for shrinkage are applicable. You will find an article from Shotcrete magazine, “Shotcrete Testing—Who, Why, When, and How,” helpful. Here’s a link to the archived PDF of the article (shotcrete.org/wp-content/uploads/2020/01/2011Sum_Hanskat.pdf). The specific section on drying shrinkage tests says:
“Drying shrinkage of the shotcrete can be tested using general provisions of ASTM C157. Because the shotcrete is shot into a large panel and not into the relatively small mold specified by ASTM for the shrinkage test beam, it is recommended that a beam approximately 11.25 in. (285 mm) in length be sawed from a test panel. As most shotcrete uses coarse aggregate less than 1 in. (25 mm), a 3 in. (75 mm) thick panel with a 3 in. (75 mm) wide cut should approximate the ASTM requirements. The A/E should specify in the contract documents drying shrinkage limits that are appropriate for the design of the structure.”
What is the R-Value per inch of shotcrete?
Since shotcrete is simply a placement method for concrete, the R-value is the same as cast concrete. ACI 122R-14, “Guide to Thermal Properties of Concrete and Masonry Systems,” would be a good reference.
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.