There is a paper by Denis Beaupre about this issue in the May 1999 issue of Shotcrete magazine. The simple answer to bonding layers of shotcrete is the same as bonding layers of concrete in typical repair applications. Bonding agents are not recommended. The bond strength between shotcrete layers is generally superior to cast interface because of the impact of velocity and the matrices that form at the bond plane and provide a denser, therefore stronger interface. The key in any bonding situation is primarily dependent on the surface preparation before application of the next layer. The surface must be clean and free of latence and any other unsound materials and should be roughened or textured (gun finish is sufficient) to provide sufficient keying or mechanical locking as required. The surface should be SSD and overspray from progressive application should be controlled. ACI International and the International Concrete Repair Institute can provide direction for surface prep. AASHTO/AGC/FHWA Task Group 37 Report, “Guide for Shotcrete Repair of Bridges and Structures” contains spec and procedure information that should be useful.
Do you have any publications on shotcrete curing, specifically in tunneling? How is shotcrete cured in tunnel constructions with the temperature and moisture problems?
All concrete must be cured to ensure full and proper hydration of cementitious components control of shrinkage. Shotcrete is concrete placed pneumatically, therefore must be cured, as all concrete must be. The tunnel environment presents positive and negative conditions. The humidity in an underground space is generally high in humidity and constant in a moderate to cool temperature. Both conducive to slow egress of moisture from the concrete and “natural” curing. The negative in tunnel construction is ventilation air which is generally of high volume and high speed, which tends to dry the surface and “pull” important moisture out of the sprayed concrete. Most tunnels can tolerate extra water in the work space, therefore misting or spraying water onto the concrete surfaces, especially overhead, is the most practical method of curing. Sprayed on liquid membranes are effective as long as their interference with bonding of additional layers of concrete, sprayed or cast, is not an issue. Recommended reading: “Understanding and Controlling Shrinkage and Cracking in Shotcrete” by D.R. Morgan and C.Chan, published in the ASA Shotcrete magazine.
I am interested in constructing my home using shotcrete applied over polystyrene panels. There are several systems for this, but I’m most interested in avoiding “thermal bridging” that occurs when metal reinforcement passes from the inside of the home to the outside through the foam insulation. I am also interested in fabricating the panels myself, if possible. There was a system utilizing metal reinforcement grids on each side of the polystyrene panel connected by plastic components. Can you point me toward a company that offers this system in the US?
ICS, 3-D panels are structurally reinforced styrofoam panels that, in conjunction with properly applied shotcrete, become a superior building system. This is a proven panel with a global track record and much experience among ASA members. They are located in Brunswick, GA.
I would like to get expert opinions regarding a proposal. I am reviewing from a contractor to replace precast concrete wall panels with shotcrete wall. The wall acts as a retaining wall and the precast panels were specified to span between the soldier piles (with tiebacks), driven and anchored into the rock at a spacing of 10 feet. Shotcrete walls over 3-inch wood lagging have been proposed to replace the precast panels and they have been designed exactly the same way as reinforced concrete walls. Using ACI Code working strength design for 4000 psi concrete, and fs= 24000 psi steel, the reinforcing in the shotcrete walls have been determined using value of a = 1.76 . ( As= M / 1.76. d ) I do not feel comfortable accepting the same equations and numbers for a shotcrete wall as for a cast-in-place or precast concrete wall with all the quality controls and rigid specifications per ACI 318 Code concerning mixing, formwork, placement, vibration and curing. Could you please provide an expert opinion on the matter? What would be the reasonable values of coefficient to determine the reinforcing in shotcrete walls?
We often use shotcrete in lieu of cast in place concrete without using different design factors. Shotcrete is simply a method of placing concrete. Properly designed and constructed, the same reinforcing steel used for cast-in-place concrete or precast concrete should be able to be used with shotcrete constructed retaining walls. The only differences would be in the reinforcing detailing, in that the rebars should be tied in a configuration that makes them suitable for proper encapsulation with shotcrete. Avoid bundled bars or other conditions not conducive to proper shotcrete encapsulation. See “ACI 506R-90 Guide to Shotcrete” for guidance, except that it is possible to use much larger diameter bars than indicated in that document, as has been described in several articles. (See for example the article by James Warner on “Dealing with Reinforcing” in the Winter 2001 of Shotcrete magazine.)
Is it possible to put a texture on the application side of a shotcrete wall? I understand that I can shoot against a form, but what about the side that gets screeded?
There are many textures that can be applied to the finish surface of the shotcrete. The least expensive is the natural nozzle finish which is rough and tends to absorb light as opposed to reflecting light and standing out. On the other extreme is carved and stained simulated rock as found in zoos and amusement parks. Stamping or rolling also creates a great finish. The broom finish is also very common. Color and textures are options and the owner or designer needs to decide on the value and effect he/she is looking for. Whatever finish, texture, pattern, color, stain, lump, bump, or crease that can be applied to concrete also applies here.
Can you provide any information on insulating gunite in spa installations?
There are two ways to insulate the outside of concrete spas. The first way is to shotcrete the spa and then glue Styrofoam to the outside of the concrete shell or to spray the insulated foam to the outside surface. The second way is to use the ICF (insulated concrete form). You would only have to use one side of this form system. This system would act as the outside form so that the shotcrete could bond to the foam. This type of system has foam insulation thicknesses from 1 to 4 inches thick. Yes, it can be fitted to form circles. Each ICF system is different, so some research would be needed to see which system would work the best. Since most spas are formed up before they are shot, the ICF system would serve two purposes: forming and insulation in one step.
When used on walls, can shotcrete be of equivalent strength as poured concrete?
Basically, shotcrete is a method of placing concrete that does not require forms. As a matter of fact, shotcrete requires the concrete mix to be proper every time. With formed concrete walls, the ready mixed concrete going in can be substandard and still appear to be okay. Shotcrete also provides a more dense concrete less susceptible to water penetration. The most glaring difference will be the quality of the materials used. Most poured walls are designed for a compressive strength of 2500 to 3000 psi. Typically they are placed with a water/cementitious material ratio of 0.60 and higher. Curing is almost unknown in the poured wall sector. Protection only occurs in the coldest weather. By the very nature of the process, shotcrete will have a much lower w/cm ratio. This will produce a wall with higher compressive strength and have the attributes of lower w/cm ratio concrete, i.e. reduced permeability, less shrinkage, increased durability. With proper curing and protection, the shotcrete mixture will produce significantly better long-term performance. The shotcrete process should allow for easier addition of insulation to the walls as well. This is especially important if the basement is to be used for more than just storage.
What is the minimum thickness that shotcrete can be applied? We are currently using shotcrete on a restoration project and have a concern at the corner locations are returning to tight recessed steel framed windows. There is an exterior wood molding approximately 1 inch from the tight corner that needs to be preserved. Do you have any suggestions as to how we can address this? Do we need to provide caulking between the wood molding and the shotcrete?
Thicknesses depend on the structure and surface (surface prep is the key to proper bonding of shotcrete) the shotcrete is being applied to. Depending on the application 1/4 flash coat to 1 inch thickness can be the minimum. As far as shotcrete up to the steel windows, you have to consider that cracking may occur off of each corner. This can be minimized by adding additional reinforcement at those locations. It is common to tool in a joint around the windows so that we could apply a caulk later. The caulking will assure a waterproof seal between the window and the concrete during temperature changes that may create some expansion and contraction. You do not have to depend on the trim work to create the weather and water tight seal the architect requires.
I am looking for design information for shotcreting a steel sheet pile wall to create a composite structure for a lift station wet well. I can design the sheet piling, which would be driven into the ground in a plan circle of 12 feet diameter, followed by excavation. I need to know the practicality of then applying a layer of shotcrete, primarily as a means of sealing the joints of the sheet pile, protecting the sheet pile from the wastewater, and providing additional wall strength. The lift station will be above the water table during construction, but would be periodically below the water table under groundwater conditions.
There are four common types of sheet pile sealing: 1.) all seams were welded to keep the ground water from seeping in, 2.) the sheet pile surface was sandblasted for bonding, 3.) wire mesh was tack welded to the sheet pile and 4.) rebar was tack welded to the sheet piles. This was done prior to the shotcrete layer. In each case the shotcrete is used as a coating to keep the water from touching the piles and in the third and fourth examples, it is used as a structural coating as well.
Is there any reference that differentiates between temporary shotcrete work and permanent shotcrete work, as far as inspection/testing requirements?
Temporary lagging of shotcrete must meet some standard as it is the shoring holding back the earth. If reinforcing is used in the design of the temporary shoring it must be fully encapsulated to provide the design strength of the lagging as specified in the design. A temporary structure may have a low safety factor but the strength of the rebar and shotcrete must meet the design specifications. Many times it is more important to do good shotcrete for the temporary shoring just because it has a lower factor of safety and therefore less allowance for poor construction practices.