Shotcrete is a placement method for concrete. We have shot a wide variety of concrete mixtures from lightweight to heavyweight to the use of alternative cements. Reviewing ACI 304.3R-96, “Heavyweight Concrete: Measuring, Mixing, Transporting, and Placing,” the key to producing heavyweight concrete is the aggregates used. The document also mentions that more cement paste is needed in heavyweight concrete and that helps to increase pumpability, which is important to use with shotcrete placement. Shotcrete will allow you to minimize or eliminate formwork, so it may have benefits of reducing cost and time for construction. Wet-mix shotcrete generally has a low water-cementitious materials ratio (w/cm) of 0.38 to 0.45. Dry-mix shotcrete tends to be even lower with a 0.35 to 0.40 w/cm. Shotcrete materials achieve this using high-range water-reducing admixtures. As concrete hydrates and gains strength, the available free water in the concrete is consumed, and so may be less of a problem for shielding. Reviewing the 304.3R document, it appears reaching the 3.2 g/cc density is fairly easy, and even higher densities may be achievable. You may want to review the ACI 304.3R document, as it provides a good background of heavyweight concrete mixture design and use.
I have a 24 in. (610 mm) thick shotcrete wall that needs to be scanned for voids. The project has been struggling to locate a local expert who has the capability to scan this thick of a wall. In addition, this new 24 in. thick shotcrete wall was dowelled and made an “as-one-unit” together with an existing 24 in. form-and-pour wall. Any thoughts and ideas would be greatly appreciated.
Shotcrete is a placement method for concrete. Thus, all nondestructive testing (NDT) applicable to concrete walls would be usable on your wall. However, it is difficult to get good results with a scanning system for heavily reinforced concrete walls of your thickness. Impact echo and impulse response are two one-sided techniques that can provide good results for a portion of the 24 in. thickness, although they would likely not be able to scan the entire depth. Ultrasonic pulse velocity is a potential if you can access both sides of the wall. For one-sided investigation at greater depth, you may be able to use a MIRA system. It is a sophisticated tomographic system that says it can test from 50 to 800 mm (32 in.) thickness. There are national consulting firms that provide these investigation systems. Each requires a highly trained, experienced operator—so be sure to verify the firm can document successful experience using the method.
A contractor has proposed using shotcrete to repair the concrete in the elbows of a draft tube. I have not heard of shotcrete being used in a draft tube. Velocities would range from 10.5 to 3.8 ft/s (3.2 to 1.2 m/s). I am concerned about whether the shotcrete would delaminate after time or be abraded away, as there is considerable abrasion present where the concrete cover has been abraded away on the floor of the elbow. Any guidance would be helpful.
Shotcrete has been used in many dam repairs, including large-diameter draft tubes. Shotcrete is high-velocity (60 to 80 mph [100 to 130 km/h]) placement of concrete. When shotcreting with proper concrete materials, equipment, placement, and curing techniques, along with complete surface preparation, you can expect a tensile bond strength of at least 150 psi (1 MPa) between the existing concrete and the newly shotcreted material. Original Portland Cement Association research by Felt from 1956 showed that 200 psi (1.4 MPa) bond shear strength is required for bonded overlays to act monolithically in flexure. Research by Silfwerbrand in 2003 showed that the ratio of bond shear strength to direct tensile bond strength ranges from 1.9 to 3.1. Thus, using the low value of the range with a 150 psi tensile bond strength yields a shear strength of at least 285 psi (2 MPa), well above the 200 psi needed. You may find more information on the bond between concrete and shotcrete layers in the article “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints” that can be found in our article archive.
Regarding the abrasion, shotcrete displays good toughness in a wide variety of demanding applications. Quality shotcrete should have at least a 4000 psi 28-day compressive strength and, with attention to mixture design using silica fume and a low water-cementitious materials ratio (w/cm), can comfortably reach 6000 to 8000 psi (40 to 55 MPa) or more. Shotcrete also can easily use steel or synthetic fiber to significantly increase the toughness of the in-place concrete.
Finally, because shotcrete requires no formwork or bonding agent for a high-quality repair, you will find the shotcrete process provides an economical solution.
An article about a draft tube modification project can be found in our article archive.
With structural shotcrete walls, what is the anticipated shear strength of the sprayed shotcrete to a vertical wall with geotextile fabric applied to a retention system? What is the anticipated dead load of the sprayed shotcrete at the base of the sprayed wall when the shotcrete wall is sprayed to a nominal thickness of 6 in. (150 mm)? Also, what is the maximum wall thickness that can be sprayed in a single pass application with a normal mixture design to achieve 4800 to 5000 psi strength?
Shotcrete is a placement method for concrete. Thus, structural properties are equivalent to those of concrete with specified strength. Shotcrete compressive strengths will normally range from 4000 to 7000 psi at 28 days, so your 4800 to 5000 psi (33 to 35 MPa) is very normal. Density of shotcrete placed concrete is the same as cast concrete ranging from 145 to 150 lb/ft3. Walls can generally be built at any required thickness because we will create the wall by bench shooting building the wall from the bottom to the top in vertical lifts.
I live in NW Florida, Fort Walton Beach. In 1987 a previous owner gutted and substantially rebuilt my house. The exterior brick walls were covered with wire mesh and over 1 in. (25 mm) of coquina shell shotcrete. I bought the house in 1993. I am forever trying to reduce my bills. I would like to find any information I can use to reduce my home insurance. Do you have any links to anyone that has data about my siding being more fire resistant or wind resistant than ordinary masonry/brick veneers? Any guidance you can provide will be greatly appreciated.
“Coquina shell shotcrete” is not a well-defined concrete material. Though it may have been promoted to you as shotcrete, it may well have been a stucco-like application accomplished with low-velocity plastering equipment, and thus not high-velocity shotcrete placement. Without physical characteristics of the in-place material (strength, density, type, and amount of reinforcement), it is hard to delineate the structural enhancement the coating may provide. You could have an evaluation by an engineer or testing lab to ascertain the characteristics of the in-place material. Then refer to ACI 216.1-14, “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies,” to see whether your composite system has enhanced fire resistance as compared to brick alone. Regarding wind resistance, you would need to have a structural engineer evaluate the composite system for any potential increase in strength against wind loadings.
I have some hairline cracks visible in the outer 3 in. (75 mm) thick shotcrete outer shell of a monolithic dome home. How concerned should I be about them? The outer coat was applied 10 months ago and they seem to be spreading. The shotcrete was applied over chain-link fencing. These cracks are several feet long and most are from doorway or window openings. What is the best/easiest way to repair these?
The design of any concrete structure must consider loadings and environmental conditions. This includes drying shrinkage of the concrete, and daily and seasonal thermal changes that introduce stresses within the concrete sections. Domes are thin-shell concrete structures and stress concentrations are expected around any openings through the shell. Typically, the design engineer will provide additional reinforcing bars around openings and especially at corners of openings to accommodate the buildup of stresses in these locations. Also, chain-link fence is not considered as acceptable concrete reinforcement, as it cannot accept tension in a straight orientation within the concrete. Proper concrete reinforcement is either deformed reinforcing bars or steel mesh with smooth wires laid out in an orthogonal pattern.
You should have a professional engineer with experience in concrete shell design and construction evaluate your dome home for structural integrity. If the cracking is determined to not affect the structural integrity, epoxy or polyurethane grout injection is routinely used to seal cracks in concrete.
Are there cases of shotcrete being sprayed on the underside of metal floor decking for the purpose of sound transfer reduction and dampening between levels? If so, are there special application methods and formulations?
We could not identify any specific cases of shotcreting onto metal decking for sound dampening. However, shotcrete is used in a wide variety of overhead applications, and as long as we can get good bond to the underside of the metal decking, should perform well. Depending on the surface profile of the metal decking, you may want to consider attaching studs to the decking to enhance the overall bond of the concrete. You may also want to consider using lightweight shotcrete since the concrete sounds like it is more for acoustic purposes, than for structural.
We are currently working on a job that requires integral color for a sculpted rock facing, but the plant close to the job does not supply color. The DOT we are working for has informed us they will not approve the use of retarder in the mixture so we can order the colored mixture from a plant that is 50 minutes away from the site because they are worried about long-term strength of the material decreasing. Do you know of any literature that we can provide to the DOT regarding effects of retarder on 28-day strength of shotcrete?
Shotcrete is concrete. PCA’s Design and Control of Concrete Mixtures, 16th Edition states: “In general, some reduction in strength at early ages (one to three days) accompanies the use of retarders. However, increased long-term strength may result from retarding the initial rate of hydration. Excessive addition rates of a retarding admixture may permanently inhibit the hydration of cement.” Thus, if you closely follow the admixture manufacturer’s recommendations for dosage, you may get higher long-term strength than non-retarded mixtures. You may also consider use of the newer hydration control admixtures that essentially stop hydration until activated, and can theoretically put the concrete “to sleep” for up to 3 days.
I have three apartment buildings (with three, four, and five units). Their basements were insulated just over 10 years ago with closed cell spray foam with a class one fire rating. Now for some reason, the building inspector says I have to cover the foamed walls of all three large basements with 0.5 in. (12 mm) drywall and build stud walls to hang the drywall for fire protection. This would reduce the width of the basement stairs significantly, and they would become so narrow that they would then be out of compliance. These basement walls are uneven fieldstone foundations, with bumps, protrusions, and even some curves. I was thinking that shotcrete-applied concrete might work much better and be more appropriate than wood and drywall for a sometimes-moist basement. According to the local Building Code, a 2 in. (50 mm) layer of concrete would suffice. How can a shotcrete application be made to adhere to closed cell foam? The wall heights are approximately 7 ft (2 m) plus bond.
Shotcrete is just a placement method for concrete. So shotcreting will provide the fire resistance of concrete. For securing the concrete, you can place anchors through the foam into the original basement wall. You can consult with an engineer experienced with shotcrete on anchor size and spacing required for supporting the shotcrete layer.
I have a project where the foundation sub is planning to shotcrete foundations walls instead of pouring them. They’ve submitted all the procedural things necessary to prove their competence and know what they’re doing. For a portion of our foundation, we’re immediately adjacent to an existing building. The sub mentioned today on site that they were not planning to put Styrofoam or anything between our new wall and the existing wall that would resist lateral pressure from the fluid concrete and the question was raised whether this is ok or not (FYI there is still rigid insulation, waterproofing, etc.). The argument is that the concrete is obviously stiff enough to stay in place, thanks to the nozzle-applied admixture, without an interior form that it wouldn’t be exerting any lateral pressure on the adjacent wall. I can follow that logic and almost buy it but I’m wondering if we still need something to resist the force applied from actually shooting the concrete in place?
Shotcrete is a high-velocity placement of concrete. In most thick walls, as I imagine your foundation walls are, the shotcrete contractor will be bench shooting the walls. This means they will be shooting the full wall thickness in 3 to 4 ft (0.9 to 1.2 m) high lifts where most of the impact forces and weight of the shotcrete is carried by the previously shot material. This results in very low impact forces on the back side of the section. When creating a section with a one-sided form, shotcrete contractors have used thin material, like Masonite, pegboard, or even an expanded mesh material, as we just need to have a surface to define the back of the section.