Properly screeding and finishing overhead shotcrete is very challenging. The contractors who do this type of work properly have very well-trained and skilled tradesmen throughout the crew, including the nozzlemen, rodman, and finishers. Shotcrete that is not screeded and finished properly will likely suffer bonding and other issues.
Could you provide information regarding the appearance of efflorescence on a newly constructed 2 million gal. (7.57 million L) holding tank? The tank was constructed correctly and has held water for over 6 months. A leak test shows no water loss over a 72-hour period, and no moisture has been seen on the surface, but efflorescence has been noted. The tank was painted after the shotcrete was properly cured. (The applied paint was inspected by a NACE inspector and found to be approximately 7 mils [0.2 mm] and meets the specification.) At what point will this stop and what is the best practice to prevent it from happening again? Would covering the areas where it has occurred with additional paint seal the cracks??
Efflorescence is common on many exposed concrete and cement mortar applications. Generally it is seen when cracks in concrete or mortar are exposed to water rather than accumulating within the crack. The basic mechanism creating efflorescence is when concrete is exposed to water for a long time; excess free lime (calcium hydroxide) in the cement paste goes into solution with water (leaches). Then when that water eventually leaves the crack and dries on the surface, the white residue of calcium hydroxide creates what is termed “efflorescence.”
It is very common to see efflorescence on brick structures where the mortar joints are exposed to rainwater that leaches out the calcium hydroxide and the resulting white efflorescence is highlighted on the dark-colored face of the brick. In concrete tanks, it is often found in cracks that can accumulate water for a sufficient time to leach the calcium hydroxide. The bottoms of vertical cracks or low areas in horizontally oriented cracks often show the greatest buildup of efflorescence. These can be surface cracks that are exposed to rainwater or through wall cracks that are exposed to water contained within the tank.
Although the tank was cured properly to help deal with long-term drying shrinkage, surface cracking on shotcrete often results from early-age plastic shrinkage cracks. These are shallow cracks that form within hours (or minutes, in extreme conditions) of placement due to rapid evaporation of water from the exposed surface of fresh concrete (common in exposed floor slabs or in your case the fresh shotcrete wall surface).
To answer your question regarding when it will stop, the answer is it won’t unless the cracks are sealed, or water is prevented from getting into the cracks. Cement-rich shotcrete has more than enough free lime to continue the leaching for decades. Although surface-applied coatings may initially span small cracks, as the walls of tanks expand and contract due to filling and emptying, and undergoing daily and seasonal thermal changes, the surface cracks will open and close slightly and eventually mirror through the coating. Coatings designed to tolerate moving cracks would likely be much thicker than the 7 mils used on your project. If the cracks are through-wall cracks that are seeping from the contained water, the crack will need to be sealed, most commonly by injection of polyurethane grout or interior surface coatings.
To answer your question on how to prevent this in the future, early-age plastic shrinkage cracks can be reduced by fogging the fresh shotcrete surface to keep the surface humidity high and reduce evaporation of the water at the surface of the concrete. Also, using fibers in the shotcrete can help reduce plastic shrinkage cracking. In hot or windy climates, placing the final layer of shotcrete during the coolest or calmest time of the day may help, too.
To answer the question if additional paint would seal the cracks, simply coating with an additional 7 mil (0.2 mm) coating would provide a temporary seal, but more than likely the crack will mirror through after some period of exposure. A coating designer would need to evaluate the crack widths and potential movement to design a coating system that would provide a long-term seal.
Finally, the efflorescence caused by exposure to rainwater is generally only a visual defect and doesn’t affect the long-term structural integrity or durability of the tank. Many owners tolerate efflorescence on the tanks and simply clean it off when it becomes objectionable.
Can you refer me to the standards for adding water to ready mixed shotcrete?
Wet-mix shotcrete is a placement method for concrete. Ready mixed concrete used for wet-mix shotcrete needs to meet the requirements of ACI 506.2-13, “Specification for Shotcrete.” ACI 506.2 specifies concrete materials shall meet ASTM C94/C94M, “Standard Specification for Ready-Mixed Concrete.” ACI 506.2 also requires you shall batch, mix, and deliver wet-mixture shotcrete in accordance with Specification C94/C94M, or Specification C1116/C1116M if fiber-reinforced. Further guidance may be found in ACI 506R-05, “Guide to Shotcrete,” and ACI 304R-00, “Guide for Measuring, Mixing, Transporting, and Placing Concrete.”
Is there any documentation showing that it is okay to tie off to a man-lift basket? I have never found any. How do other contractors deal with ACI requirements of an air lance, knowing that OSHA has contradicting standards of air wand pressure?
OSHA requires that the personnel in aerial man baskets be tied off with the appropriate harness and lanyard. When you are in a JLG or other type of man lift, the only place to tie off to is to the basket or boom bracket. This question may be better answered by studying current OSHA documents.
We cannot recall any of our members being cited for using an air lance or blow pipe.
We are shooting 5000 psi (34.5 MPa) shotcrete. Because of rising temperatures, the mixture is getting too stiff to pump, and the inspector will not let us add water. What should be done in this situation??
At the point at which concrete/shotcrete temperature is starting to rise and the mixture is stiffening up, adding water should not be allowed. Water should only be added when the mixture is stable and only up to the water specified in the approved mixture design. In warm or hot conditions, retarders, set stabilizing admixtures, or ice may be needed to keep the mixture stable for the period of time to transport and pump the load.
I know that there are many factors that affect the distance that shotcrete can be pumped. For a dry-mix process, is there a rule of thumb for a maximum recommended horizontal pumping distance?
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.