Shotcrete is a method of concrete placement, not a product. Therefore, concrete placed by the shotcrete method will exhibit the same characteristics as concrete placed by other methods. Mixture designs and proportions for shotcrete are modified for high-velocity placement. The high velocity provides some performance improvements over conventional cast-in-place methods when properly placed.
Are there specific benefits in using silica fume in shotcrete beyond reduced permeability in the hardened shotcrete?
Shotcrete containing silica fume will tend to be more adhesive (sticking to substrate surfaces) and cohesive (adhesion to itself). This will result in quicker build-up (greater thicknesses per pass) and possibly reduced need for accelerators. Silica fume additions also result in dramatic reductions in rebound, particularly with the dry-mix process.
Is there a U.L. (Underwriters Laboratories) certification for shotcrete?
No. Shotcrete is a method of placing concrete. Therefore, any applicable certifications would apply to concrete regardless of the method of placement.
My firm is a general contracting entity that frequently uses shotcrete subcontractors. When project specifications are not clear on testing, I have been relying on the advice of my shotcrete subcontractors on the frequency of taking tests for compliance with strength requirements. We always shoot a test panel prior to starting construction. How much testing should we be doing during construction?
ACI 506.2, “Specification for Shotcrete,” recommends that a test panel be produced for every 50 yd3 (38 m3) of shotcrete placed or one per day, whichever is less. A minimum of three cores are to be cut from the test panel for compressive strength testing in accordance with ASTM C 42, “Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete.” Testing must be performed in accordance with ASTM C 1140, “Standard Practice for Preparing and Testing Specimens from Shotcrete Panels.” The average of the strength results from the cores must be at least 85% of the specified strength with no individual core less that 75% of the specified strength.
As a specifier, should I specify which process—dry or wet—should be used on my projects? What are the significant differences?
The application of shotcrete can be done successfully with either method. The dry-mix shotcrete process tends to be more favorable for lower volume placements. It is also a more flexible method, allowing for more frequent relocations of equipment. Equipment is more easily cleaned at the end of the placement. The nozzleman must exercise great care in adding the necessary amount of water while shooting.
The wet-mix shotcrete method is more favorable for larger volume placements. Rebound is substantially less than in the dry-mix shotcrete process. The nozzleman does not have to be concerned with controlling the water addition. This method is less efficient when there is a requirement for frequently starting and stopping placements. The wet shotcrete mixture has a limited “pot-life.”
Remember, shotcrete is not a special product. It is a method of placing concrete. All the recommended practices for concrete placed by any other method, such as curing and protection, also apply to shotcrete.
I am trying to find out if there is any research or literature regarding the drying shrinkage of shotcrete. Can you help?
See ACI 506R, Sec. 1.7 (ACI document). Typical shrinkage varies in the range of 0.06 to 0.10 percent after 28 days drying. It is typically slightly higher than similar strength concrete, mostly due to less and/or smaller coarse aggregate in the shotcrete mix.
We are shotcreting our first wall and the contractor tells us that in shotcrete, the lapping of the bars is not done by putting the bars alongside each other as in conventional pouring of concrete but rather a gap is left between the bars in order to avoid voids behind bars bundles. A two-inch gap is being used on our job. Is there a publication that deals with reinforcing steel placement in shotcrete in general and one that deals with bar laps in particular?
The ACI 506R-90 Guide to Shotcrete, Section 5.4.2 is the publication you are looking for. Amongst other things it states: “If the design allows, lapping of the reinforcing splices should be avoided. Lapped bars should be spaced apart at least three times the diameter of the largest bar at the splice”. If laps are not permitted by the design, then it is best to lap the bars one on top of the other (relative to the shooting orientation), rather than side-by-side, to facilitates proper encapsulation with shotcrete.
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.
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.