Project Name:
Surface Reef Silo 4
Location:
North-West Province, South Africa
Shotcrete Contractor:
Shotcrete Africa SCP
Architect/Engineer:
Royal Bafokeng Platinum
Material Supplier/Manufacturer:
FSE Engineering Services
Equipment Manufacturer:
Amsteele Systems
General Contractor:
Shotcrete Africa SCP
Shotcrete is a placement method for concrete. Thus, the fire resistance for shotcrete placement is the same as concrete. The primary reference for fire resistance of concrete is ACI 216.1-14(19) Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.
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.
The Duck Island Clean Water Facility, located in Lowell, MA, is a 32 million gal./day (120 megaliter/day) activated sludge treatment plant. The facility accepts wastewater from several Massachusetts cities and towns, including the City of Lowell and the towns of Chelmsford, Dracut, Tewksbury, and Tyngsborough. The service area includes approximately 220,000 people. The facility has been in the news for the past decade due to its need for massive repairs
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 is a placement method for concrete. Thus, the fire resistance for shotcrete placement is the same as concrete. The primary reference for fire resistance of concrete is ACI 216.1-14(19), “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”
This is a great application for shotcrete placement of high-quality concrete without formwork. If you are merely adding shotcrete to fill out to a uniform surface profile without any structural requirements you may not need reinforcement. However, it may still be advisable to include fibers in the shotcrete mixture to help control plastic shrinkage cracking. Generally, you would want to keep a minimum thickness of 1 in. (25 mm) to provide enough thickness for finishing. If you need the shotcreted sections to be self-supporting and carry loads as structural concrete, you should consult with a structural engineer to determine the appropriate thickness and reinforcement for the expected loads. Shotcrete is a placement method for concrete, so standard reinforced concrete design is appropriate for shotcreted sections.
If you only need color in the exposed shotcrete surface, just putting color in the flash coat should be adequate as long as the flash coat is continuous across the entire area. You may want to specify the final “flash” layer to be thicker, perhaps 1/2 in. (12 mm) to 1 in. (25 mm) thick. This would help assure consistent color and still allow the finishers to produce a consistent final texture. As long as the concrete materials are the same in the production, you wouldn’t need to shoot the final layer all in one day. Also, be sure curing methods are the same for the entire area as varying moisture can sometimes affect the early appearance of the coloring, though it usually balances out over time.
This is a great application for shotcrete placement of high-quality concrete without formwork. If you are merely adding shotcrete to fill out to a uniform surface profile without any structural requirements, you may not need reinforcement. However, it may still be advisable to include fibers in the shotcrete mixture to help control plastic shrinkage cracking. Generally, you would want to keep a minimum thickness of 1 in. (25 mm) to provide enough thickness for finishing. If you need the shotcreted sections to be self-supporting and carry loads as structural concrete, you should consult with a structural engineer to determine the appropriate thickness and reinforcement for the expected loads. Shotcrete is a placement method for concrete so standard reinforced concrete design is appropriate for shotcreted sections.
The ACI 350 Code doesn’t really cover repair in detail because it is more about new environmental structures. Shotcrete is concrete; it provides excellent bond to properly prepared concrete substrates and it inherently creates a composite section with the existing concrete that acts monolithically. The supplemental shotcrete could be considered an additional layer of concrete that acts monolithically and thus provides the needed concrete cover. Reference-wise, you could refer to the article “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints” in the Shotcrete magazine archive, as it discusses the bond issue between layers. There are research papers that discuss a 200 psi (1.4 MPa) bond shear stress is needed for a bonded concrete overlay to act monolithically, and that bond pulloff (tensile) test results should be multiplied by 2 or 3 to represent the bond shear capacity. Because we typically well exceed a 145 psi (1.0 MPa) tensile bond pulloff strength, properly applied shotcrete will easily exceed the 200 psi requirement for the bond to act monolithically.
An additional factor in providing additional cover with shotcrete is that shotcrete has very cement-rich paste, along with low permeability, and thus provides a better alkaline environment to combat corrosion of embedded reinforcement, so is effectively giving even better cover than normal form-and-pour concrete.