Shotcrete is a placement method for concrete. Thus, all non-destructive 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 though 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 sophisticated tomographic system that says it can test from 50mm to 800mm (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 with the method.
Shotcrete Incorporated into ACI 318-19 Building Code
Since the shotcrete process originated well over 100 years ago, improvements in materials, equipment, and placement techniques have enabled it to become a well-proven method for structural concrete placement. The efficiency and flexibility of shotcrete have been used to great advantage in sizable structural projects, as the high-velocity impact inherent in the process provides the compaction needed to turn low-slump concrete into freestanding vertical and overhead placements with minimal formwork.
Falls Village Penstock #1 Structural Shotcrete Rehabilitation Project
In early November of 2018, Knowles Industrial Services Corporation (KISC) was issued a contract by First Light Power Resources, Inc. (FLP) to perform a structural shotcrete liner within a steel-riveted penstock at the Falls Village Hydro Electric Plant in Canaan, CT. FLP’s request for bids permitted contractors to provide a design-build approach for a structurally self-sustaining system to be built within the penstock interior. The existing 9 ft (2.7 m) diameter, 360 ft (110 m) long penstock was buried in its entire length on a steep bank and crossed underneath a live highway. Penstock replacement by excavation proved to be too costly, as much of the existing penstock beneath the roadway was encased in reinforced concrete requiring significant demolition and interruption to traffic in this area.
Shotcrete in the Urban Jungle
In today’s complex urban jobsite, easy sites to build on are diminishing, while available sites are becoming harder to reach. In addition, the remaining sites often require creative solutions to develop. The project 1395 22nd St. is a complex of five apartment buildings built into one of the many hillsides in San Francisco, CA, and is surrounded by existing buildings and adjacent properties. Several of the tallest buildings for the project have one level below grade, one level at grade, and ten stories above grade. As part of the foundation system of the project, horizontal and vertical grade beams were installed along the hillside for the first five stories of the buildings and then anchored into the hillside with prestressed tiebacks. The excavated slope was up to a 1.5:1 slope, making for very difficult access to each of the
I have a project where we are designing a shotcrete wall. The contractor plans to mix the shotcrete on site and we need to work with them to come up with a mixture design. Do you have any suggested wet-mix shotcrete designs for on-site mixing applications or could you point me where some may be published?
Shotcrete is simply a placement method for concrete. Most wet-mix shotcrete contractors use a 2 in. (50 mm) diameter delivery hose, so maximum coarse aggregate size should be limited to 3/8 in. (10 mm) nominal. Pumpability usually requires a good paste content. Here’s a link to an article, “Understanding Wet-Mix Shotcrete: Mix Design, Specifications, and Placement,” that should answer many of your questions. It is rare to have wet-mix concrete mixed on site from bulk aggregate and cementitious materials. Most site-batched wet-mix uses dry prepackaged materials that have metered water addition to provide a specific water-cementitious materials ratio (w/cm) for the provided bagged mixture. We also see volumetric mixers used on site that can precisely meter the dry concrete materials and water often with needed water-reducing air-entraining admixtures.
We are applying shotcrete in a slope (8 in. [200 m]) to make it stable. The shotcrete wall has a length of 330 ft (100 m). Do we need to specify construction and an isolation joint? In case it is required as well as an isolation joint, does the separation between joints need to be 30 ft (9 m)? Where can I find information about joints for shotcrete?
Shotcrete is a placement method for concrete. Thus, you should place movement joints (contraction, expansion, isolation) as would be required by your local design codes for concrete. Joint spacing will depend on the amount of reinforcement used in the section to resist temperature and shrinkage volume changes. Construction joint spacing can be determined by the contractor depending on their production rates. Properly prepared construction joints will act as monolithic concrete as long as the joints are properly prepared and proper concrete materials, equipment, and placement techniques are used by the shotcrete contractor. Construction joints should be roughened, cleaned, and then dampened to saturated surface-dry conditions before subsequent shotcrete placement.
The U.S. Bureau of Reclamation has a nice summary document that you may find helpful in your design.
I am helping to design radiation shielding for a cyclotron and we do not have much space. The machine produces both neutrons that must be shielded for as well as gamma rays. I have been told that we could get a density of 3.2 g/cc, which helps for gamma rays, but I need to find out more about the water content of the finished product for the neutron shielding part so I can scale between regular 2.4 g/cc concrete and the high-density shotcrete product. Do you have knowledge of this?
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.