Many of our members add them by hand at the mixer and have had good success when using an adequate mix time. Another method is to have the mix blended at a bag mix plant with the fibers.
I am interested to know if any shotcrete contractors have shot a magnesium phosphate material (dry-process) before and, if so, could you detail the special requirements necessary in placing such a unique product?
Phosphate-bonded refractory materials were routinely shot in cyclone boilers in the 1970s. These phosphate-bonded materials don’t have a cement bond, but achieve a chemical bond when heat is applied. Without knowing the precise formulation of the mixture and grain sizes involved, we cannot tell you definitively that your specific material can be shotcreted. However, there is a long history of successful past experience with phosphate-bonded refractory materials being shot with the dry-mix process. You may want to consider a field trial before construction to verify your specific mixture works with your dry-mix shotcrete equipment.
What difference would there be in the density of shotcrete before and after shooting? Is there any shotcrete mixture-design software in SI units available? Or any document of shotcrete mixture design in SI units for optimizing shotcrete design?
Shotcrete is simply a placing method for concrete. Thus, the mixture design and material properties are the same as concrete. We are not aware of any software specific to shotcrete in any units. ACI 506R-05, “Guide to Shotcrete”, has guidance on desirable mixture characteristics (aggregate grading, supplemental cementitious material [SCM], and so on) that would be helpful in developing a concrete mixture design for shotcrete placement.
Is there is a manufactured depth gauge that would be glued/nailed to the form to allow the nozzleman to physically see how much concrete is being applied to the surface? We have a condition where there will be two or three applications on the same surface, and I am concerned that the correct depth is not being applied in each pass.
ACI 506R-05, “Guide to Shotcrete,” Section 5.6, on Alignment Control (refer to ASA Bookstore: https://shotcrete.org/bookstore/ProductDetail.aspx?itemid=506R-05) gives specific guidance on proven methods to establish the line and grade of the surface, as well as proper material thickness and cover. Common methods are use of ground wires, guide strips, depth gauges, and depth probes. Please refer to “ASA’s Buyers Guide” (https://shotcrete.org/products-services-information/buyers-guide/) and contact one of our members who provides supplies to the shotcrete industry
Is there a specification with regard to cold joints when using shotcrete?
Generally, the interface between sequentially placed layers of shotcrete is not considered a cold joint because the shotcrete abrasion, velocity of impact, and high paste content make excellent bonding conditions. Cores taken through layers of shotcrete on shotcrete often show that it is virtually impossible to ascertain one layer of shotcrete from the next. Please refer to ACI 506R, “Guide to Shotcrete,” for information on joints in shotcrete.
I would like to know the standard operating procedures for cleaning out shotcrete hoses with air and/or water and, in particular, how to keep the hose from whipping when using air.
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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