I am searching for criteria/guidelines or ratings on what different profiles are achieved by shotcrete. I am hoping there are installed shotcrete profile requirements with respect to final surface roughness. We manufacture a waterproofing system and are often asked to be installed over shotcrete, to which we have no objections. However, I am hoping there are criteria/guidelines/ratings on achieved profile of the finished surface. For example: The concrete industry often talks about roughness achieved by shotblasting and the surfaces getting to various degrees between CSP-1 to CSP-9. Is there a criteria/guideline/rating system, or something similar with shotcrete? Here is the link to some such guidelines.

Shotcrete is a method of placing concrete. The surface texture of roughness varies considerably depending on the application and the abilities of the installer. The surface can vary from a rough nozzle finish to a smooth trowel finish and many variations between these two extremes. In buildings, the typical finishes are wood float, rubber float, or trowel finish. The owner and the architect determine what finish will be required and generally specify the finish in the construction documents. The documents which might be of help to you are ACI 506R, “Guide to Shotcrete,” ACI 301, “Specifications for Structural Concrete,” and ACI 117, “Specification for Tolerances for Concrete Construction and Materials.”

Is shotcrete applied to hardened cast-in-place concrete considered monolithic by the American Concrete Institute (ACI)? Is shotcrete-to-shotcrete considered monolithic by ACI? Can either of these connections be made watertight? Or at least as watertight as the concrete? Is shotcrete without admixtures truly watertight or waterproof? (My definition of “watertight” would be a measurable amount or more than leaching of moisture and calcium.)

Shotcrete properly applied to a well-prepared existing concrete surface will create an excellent bond and structurally act as a monolithic system without joints or layers. Pulloff testing of shotcrete applied to concrete will often fail in the underlying concrete substrate, and not at the bond interface or within the shotcrete section.
Shotcrete is a method of placing concrete and should have similar watertightness characteristics. These characteristics can be enhanced with admixtures and supplementary cementitious materials such as silica fume. Please refer to the images of cores from shotcrete applied to existing concrete.

Do we need to coat reinforcing steel after sandblasting and prior to placement of product?

The answer is no; shotcrete will bond well to sandblasted reinforcing bar on overhead or vertical applications. Shotcrete, like conventionally placed concrete, can be placed over uncoated black bar or bar that is coated with rust inhibitors. In repair areas where there is heavy scale on the reinforcing bar and spalling of the concrete, the repair can sometimes include some sort of reinforcing bar treatment or inclusion of a rust inhibitor in the shotcrete mixture. It depends on the situation and the assessment of the design engineer as to what is necessary.

Why is there not more extensive use of fiberglass reinforcing bars? It seems like it would be a natural choice for most projects involving shotcrete in wet applications, as well as conventionally placed concrete, especially in the types of jobs we do, such as the rehabilitation of existing concrete channels that usually contain acidic waters. I understand that anything other than steel is more expensive, but isn’t prevention now cheaper than remediation later?

Although similar in dimensions, fiberglass reinforcing has distinctly different structural properties when compared to conventional steel reinforcement. This is a question better answered by the fiberglass reinforcing industry or the structural engineering community. As the American Shotcrete Association, we do not get involved in the engineering design of structural sections. However, it should be pointed out that properly designed and applied shotcrete provides a very corrosion-resistant environment around embedded steel reinforcement, providing excellent long-term durability in normal exposure conditions.

We have a large project involving shotcreting soffits in an underground parking garage. The shotcreting overhead is not the problem; the problem is properly screeding the excess shotcrete from the ceiling leaving a semi-smooth finish.

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.