The best information on this subject can be found in ACI 506R-05, “Guide to Shotcrete,” and likely in past articles in Shotcrete magazine. The distance that can be pumped is a function of too many parameters to fit a rule of thumb. The distance that can be pumped is influenced by the equipment being used, the vertical lift, the available compressed air, and other factors. We would suggest that you consult with one of our corporate members (shotcrete.org/BuyersGuide) in the area of the project and get their input.
I have been experiencing slow curing times (early set times). Every year during the wet season, my shotcrete curing times go from 1 MPa (145 psi) in 2 hours to 1 MPa (145 psi) in 8 hours. I believe that there is a change in the materials when the groundwater comes up. I have had water tests done, but I’m not sure what to be looking at. The recycled water that was being used had a pH of 5.7. We changed water, the problem was still there, and the pH is now 9.7. What effects does the pH level have?
A pH of 5.7 is slightly acidic, while 9.7 is quite alkaline. According to PCA’s “Design and Control of Concrete Mixtures,” most inorganic acids have no adverse effect on concrete. Organic acids (such as tannic acid) can significantly reduce strength when present in higher concentrations. Some alkaline materials, such as sodium hydroxide, in higher concentrations may cause a quick set. However, because this occurs in the rainy season, another factor that may have an impact is an increase in dissolved solids. PCA states that solid contents exceeding 50,000 ppm can increase water demand, accelerate set, lower compressive strength, and increase permeability of the hardened concrete. The appropriate test for acceptable non-potable concrete mixing water is ASTM C1602/C1602M, “Standard Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete.”
We own a 200-year-old house with a rubble foundation. The foundation is structurally sound, but needs to be repointed, and some of it has no mortar at all. We would like to seal it to make it watertight and keep out radon. Could shotcrete be applied directly to the interior of the rubble wall (which includes small, loose stones; large gaps; and cracks), or would we have to first have the walls repointed and smoothed over?
Yes, shotcrete would be an excellent method to fill the voids, open mortar joints, and gun an overlay over the irregular stone foundation. The use of shotcrete would be dependent on the access and ability of the applicator to safely place the shotcrete. A tight or low crawl space would make it difficult. We would suggest cleaning out loose materials with compressed air and water prior to the shotcrete placement. We recommend installing either a 2 x 2 in. (51 x 51 mm) 12-gauge or a 3 x 3 in. (76 x 76 mm) 11-gauge wire mesh over the stone foundation and gunning the shotcrete in place to fill in the mortar joints, creating a shotcrete overlay over the entire stone surface.
I’m looking for information on the quantity of rebound expected when applying shotcrete against soil. We have a W4 4 x 4 in. (102 x 102 mm) layer of mesh 2 in. (51 mm) from the soil face that is covered by a 4 in. (102 mm) initial layer of shotcrete. Is there a general ballpark figure that can be used, such as a percent of the total shotcrete placed?
Your question does not indicate the orientation of the application. If the shotcrete is being applied to a sloped surface for a channel or slope the rebound should be incidental. If shooting a vertical wall, the amount of rebound is relative to the skill of the nozzleman, the quality or nature of the mixture, the shotcrete process being used (wet-mix or dry-mix), the stability of the wire mesh, and other parameters. The range could easily vary from 5 to 20% on vertical walls relative to the aforementioned listed parameters.
My company has been using the gunite process (dry-mix shotcrete) for years now. What I have been finding lately is that a lot more questions are being asked by outside safety services, neighbors to our facility, etc., about the health effects of the shotcrete process. I believe that with the new proposed laws dealing with silica, everyone is paying more attention to products with sand and cement, and shotcrete has both. To try to educate myself and to answer these questions I am on the search for enlightenment and am coming up short. Here is where I am falling short: there is no (or I haven’t found a) general material safety data sheet (MSDS) on shotcrete. Most MSDSs I have found on the Internet are for proprietary mixtures. I have yet to find an MSDS or any safety info on just plain sand and cement mixture.
Shotcrete is a method for placing concrete, so an MSDS for concrete or its constituent components would be appropriate. MSDS sheets for cement, aggregates, and concrete are readily available from cement manufacturers, aggregate suppliers, and concrete producers, as evidenced by a simple web search. This is the type of issue that can be discussed and effectively addressed by networking with other contractors and suppliers in the shotcrete industry. This is a primary benefit of actively participating in ASA—you or your organization should join ASA and attend committee meetings. ASA meetings are held three times a year. Check our Calendar for the next available meeting.
What is the R-value per inch for shotcrete without any integrated insulation?
Shotcrete is concrete, pneumatically applied. So the same R-value fire ratings for concrete would apply to shotcrete. The standard is Joint ACI – TMS 216.1, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”
What are the common standard test methods to measure or assess the permeability of shotcrete?
Boiled absorption and volume of permeable voids testing (ASTM C642) may be required for structures that need enhanced liquid-tightness or resistance to aggressive environmental exposures. The test is sometimes used to provide an overall indication of the quality of the shotcrete mixture, particularly in dry-mix. However, many factors, including admixtures and aggregate, as well as shotcrete placing, can affect the porosity of shotcrete, so it should not be considered an absolute measure of shotcrete quality. When required, the mean average of tests on three specimens from a test panel, or from in-place shotcrete, should be less than or equal to the specified boiled absorption and/or specified volume of permeable voids limits at the specified test age with no single test greater than the specified boiled absorption plus 1%.
The Los Angeles City Bulletin states that no bars over No. 8 (No. 25) shall be used. The structural engineer has No. 10 (No. 32) bars in the columns. I am being told the test panel will get this approved but my City Inspector is balking a little. Is there a publication or code somewhere that allows the test panel to supersede the LADBS Bulletin?
The International Building Code (IBC), Section 1913, allows for larger bars as long as it is demonstrated in a Preconstruction Test Panel. However, the Local Building Code likely takes precedence over the IBC. You may want to present IBC Section 1913, which requires anything over a No. 5 (No. 16) bar to be proven in a Preconstruction Test Panel.
There have been many projects shot in Los Angeles County subject to the LADBS with bar sizes larger than No. 8 bars. ASA is not in a position to give you project references, but perhaps our local members can.
Properly encasing No. 10 (No. 32) bars can be challenging, and should only be attempted by qualified contractors using ACI Certified Nozzlemen who have previous successful experience doing this type of work. You may use our online Buyers Guide to find an ASA corporate member consultant or contractor to assist you.
What is the fire rating information for shotcrete?
Shotcrete is concrete, pneumatically applied. So the same fire ratings for concrete would apply to shotcrete. The standard is ACI 216.1, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”
I have been asked to come up with a 5000 psi (35 MPa) in 24 hours shotcrete mixture, using cement, fly ash, silica fume, and fine aggregate. I need some advice on a mixture.
Design of a concrete mixture to be placed by the wet-mix shotcrete method is essentially the same as normal cast-in-place concrete mix design. The major differences with shotcrete mixtures are:
- The maximum coarse aggregate size is generally limited to about 3/8 in. (9.5 mm);
- They use a fairly low water-cementitious material ratio (w/cm) and slump to allow shooting on vertical surfaces without sloughing;
- The potential to use an accelerator that can be added at the nozzle; and
- The pumpability is an important workability characteristic.
Since you desire a high-early-strength mixture, using fly ash as a supplemental cementitious material (SCM) wouldn’t be recommended because it slows set and strength gain at early ages. Microsilica may be beneficial for early strength gain. Consideration should be given to using accelerator added at the nozzle. There is some guidance on concrete mixture design in ACI 506R-05, “Guide to Shotcrete”; however, because local materials (aggregates, cements, SCMs) can vary significantly, you should consult with an engineer or concrete testing laboratory familiar with shotcrete to produce and test a mixture design to meet your requirements.