Shotcrete hoses can be cleaned out using either water or air. In many instances, the site conditions make cleaning with water not feasible. When cleaning with air, the free end or discharge end of the hose should be secured to something to ensure that the hose does not whip as the material and cleaning ball or rag discharges.
I would appreciate if you could comment on a city of Los Angeles shotcrete code that requires that shotcrete lifts not exceed 3 ft (1 m) and that 3 hours must pass before the second lift can be applied. First of all, if you waited 3 hours between lifts, you would have full-length cold joints along the whole length of the wall. You’d also have to wash out the pump after every lift or the concrete would harden in the pump and hoses. Second, you can’t leave a 4000 to 5000 psi (28 to 34 MPa) mixture sitting in the truck for 3 hours! Does it make any sense to you?
This provision has been an issue for shotcrete contractors in the region for many years. ASA and ACI Committee 506, Shotcrete, do not endorse the concept stated in the “Los Angeles Bulletin.” Unfortunately, this provision has shown up in other areas around the country.
A good shotcrete practice is to limit lift height to that which can be placed without sloughing or sagging and to place subsequent lifts at such a time that the previous lift is sufficiently firm to support the subsequent lift. ACI 506R-05, “Guide to Shotcrete,” Section 8.5.8, specifically addresses this point. The following is a link to ASA’s bookstore: https://shotcrete.org/bookstore
When was the 4000 psi (28 MPa) standard set for shotcrete?
ASA has taken the position that structural shotcrete is shotcrete that meets or exceeds a compressive strength of 4000 psi (28 MPa). Looking at pertinent ACI Codes related to watertight concrete, as we would expect in a pool, we find ACI 318-95, “Building Code Requirements for Structural Concrete,” introduced a provision in 1995 that required: “Concrete intended to have low permeability when exposed to water shall have a Minimum f´c of 4000 psi (28 MPa)”. Similarly, ACI 350-01, “Code Requirements for Environmental Concrete Structures,” first issued in 2001 required: “Concrete intended to have low permeability when exposed to water, wastewater, and corrosive gases shall have a Minimum f´c of 4000 psi (28 MPa)”. Since ACI 350 is more directly applicable to water-containing structures, the 2001 date is probably the most relevant, though ACI 318 introduced the concept in 1991. We do, however, see shotcrete specified at lesser levels for different types of uses.
What is the standard method for steel fiber-reinforced shotcrete (SFRS) testing? Are you supposed to core test panels or do you only do that for plain shotcrete? Additionally, is round panel toughness testing on SFRS standard today in the United States?
SFRS is routinely cored from shotcrete test panels or in-place shotcrete linings without difficulty. The shotcrete should, however, have a minimum compressive strength of about 10 MPa (1450 psi) at the time of coring.
Round panel testing of flexural toughness of fiber-reinforced shotcrete to ASTM C1550 is often specified and used for quality control (QC) purposes in tunneling and mining projects in North America and elsewhere (for example, mines in Australia) virtually every day of the year.
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Which method of placing concrete provides a longer service life—traditional cast-in-place concrete with two-sided forms or shotcrete?
Shotcrete is a placing method for concrete. Wet-mix shotcrete will be very similar in density to fully consolidated concrete when the concrete mixture designs are similar. Properly mixed and shot, dry-mix shotcrete may have a slightly higher density. Properly designed, placed, and cured, both concrete and shotcrete will give an excellent service life.
We will be blasting close to a shotcrete wall. Can you suggest any precautionary measures or offer lessons learned when blasting next to a recently shotcrete wall?
It is very common in drill and blast operations to blast shortly after the application of shotcrete. There are certainly risks involved, but a knowledgeable and experienced mining crew working with or for a knowledgeable, experienced contractor would not have any problem with this type of application.
A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.
I have a question regarding the oscillator on a shotcrete rig. When applying shotcrete, does the oscillator serve any purpose other than uniform application? I’m searching for the main reason to use an oscillator and am wondering if the integrity of the shotcrete would be compromised if it were disabled?
Assuming this is an oscillator on a robotic arm, it should not be disabled. Good nozzling technique, for either wet or dry, requires the nozzle to be moved in a constant overlapping circular pattern. This allows for better encapsulation of reinforcing bar and produces a more uniform surface; and, particularly for dry process, it is required for final mixing of materials that occurs on the surface.
Without proper nozzle technique, which requires oscillation, you will not get uniform, homogeneous shotcrete.
Can shotcrete be recycled?
Shotcrete is concrete applied using the shotcrete process. Therefore, any recycling potential that applies to concrete would apply to shotcrete.
In placing shotcrete in layers, what is the recommended thickness of each layer?
Appropriate thickness of the shotcrete layers is impossible to generalize because it depends on many factors, including:
- The type of shotcrete (wet- or dry-mix);
- The texture and stiffness of the receiving surface;
- The physical properties of the fresh concrete used, including a) w/cm ratio; b) slump; c) use of accelerator; d) type of supplementary cementitious materials used in the mixture (microsilica, fly ash, and slag); e) fibers used in the mixture; and f) mixture temperature;
- Weather conditions—Is it hot or cold, dry or wet, and/or windy or calm?;
- The shotcrete equipment used: a) type of nozzle; b) distance from the receiving surface; and c) air pressure and air volume;
- The orientation of the shotcreting (vertical/sloped/overhead)
Experienced shotcrete contracting firms using ACI Certified Nozzlemen have a wealth of experience in evaluating all these factors to achieve the proper results. You may consider subcontracting the shotcrete work to an ASA member contractor with experience in this type of work. You can submit your project details for bids from our ASA Corporate Members using the Web form at shotcrete.org. For further reference, ACI 506R-05, “Guide to Shotcrete,” provides some general discussion of the shooting techniques that may be appropriate. Retaining an engineer or shotcrete consultant experienced in shotcrete application may be of value to assist in evaluating your specific factors and recommend the best solution.