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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.
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.
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.
Shotcrete is concrete applied using the shotcrete process. Therefore, any recycling potential that applies to concrete would apply to shotcrete.
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.
We are not aware of any recognized standard for abrasion testing or acceptance specifically for this application. ASTM International has several abrasion tests for concrete and mortar that include:
- ASTM C418-05, “Standard Test Method for Abrasion Resistance of Concrete by Sandblasting”;
- ASTM C779/C779M-05(2010), “Standard Test Method for Abrasion Resistance of Horizontal Concrete Surfaces”;
- ASTM C944/C944M-99(2005)e1, “Standard Test Method for Abrasion Resistance of Concrete or Mortar Surfaces by the Rotating-Cutter Method”; and
- ASTM C1138M-05(2010)e1, “Standard Test Method for Abrasion Resistance of Concrete (Underwater Method).”
Also, ACI 350-06, “Code Requirements for Environmental Concrete Structures,” Section 4.6, has requirements for protection against erosion.
The equivalent dosage of fibers to replace embedded steel reinforcement needs to be evaluated by the design engineer for the specific project or application. Guidance for the designer is available in ACI 506R-05, “Guide to Shotcrete,” and ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete.”
It is possible to use a penetrating sealer on shotcrete in the same manner as cast-in-place concrete. We are not aware of research on the durability of such a sealer and do not know if it would enhance the freezing-and-thawing characteristics. A high-quality shotcrete mixture that is properly placed will exhibit excellent freezing-and-thawing characteristics with or without a sealer.
We are not aware of any testing done specifically for the sound absorption performance of shotcrete. Shotcrete is a method of placing concrete and, once hardened, it should have similar parameters as cast concrete. With shotcrete, one has the ability to use many different finishes, which might influence the sound absorption characteristics. A smooth troweled shotcrete wall would be the most similar to a formed cast-in-place wall. On the other end of the spectrum, a nozzle-finished wall would likely absorb far more sound. A recent design of a concert hall at a major university was to be built with oval concrete or shotcrete perimeter walls covered with fabric curtains for sound purposes. The ceiling was to be suspended nozzle-finished shotcrete.
