We are not aware of specific tolerances for shotcrete in swimming pools. Shotcrete is a method of placing concrete and the cover over reinforcing steel should be the same as that for cast concrete. With respect to the depth of the pool, this would be a building code issue, not a shotcrete issue.
Is shotcrete used as a canal liner?
Shotcrete has been used for canal lining throughout the United States. The Bureau of Reclamation published a study on Canal Lining Test Sections constructed in the Bend, OR, area and studies the durability at 5 and 10 years. Shotcrete is a very viable means of placing canal linings. Basically, shotcrete is a method of placing concrete. Care should be taken to ensure that the mixture is designed to withstand the local environmental conditions, such as using air-entraining admixtures to ensure durability due to exposure to freezing and thawing. ACI 506R-05, “Guide to Shotcrete,” contains a lot of useful information in evaluating and using shotcrete in a variety of applications, including canal linings. If liquid-tightness and long-term durability of the canal lining are important, provisions of ACI 350/350R-06, “Code Requirements for Environmental Engineering Concrete Structures and Commentary,” should also be considered in the design of canal lining reinforcement, cover, and joints.
Can shotcrete be used to repair a wall made of cement and fly ash? If so, should the wall be prepared for the shotcrete application?
Structurally sound concrete that contains up to 20% fly ash in the total cementitious materials should not present any problems for subsequent bonding of shotcrete. Concrete with fly ash contents up to 30% have been used in recent years without any reported problems with strength and bond. Although concrete mixtures with higher levels of fly ash (up to 55%) have been proposed, we don’t have direct experience with their bonding characteristics. We suspect it would be fine as long as the base concrete develops adequate compressive and tensile strength. This could be confirmed by a simple bond strength test of shotcrete on the concrete substrate in question.
The existing surface needs to be properly prepared, removing all soft or deteriorated material back to sound concrete. For extensive defects in the existing concrete, chipping hammers may be required. For removal of light surface carbonation or laitance, a strong, high-pressure water blast or sand/bead blasting may be adequate. Depending on the thickness of the shotcrete, reinforcing may be required in the overlay. Specific details of the repair are best developed by an engineer experienced in shotcrete repair.
I am considering the use of shotcrete as an alternative to grouted riprap for slope stabilization. The project involves a basin with varying slopes and easily erodible soils. Water will cascade down the side slopes. I was going to specify shotcrete with welded-wire reinforcement but am now considering fiber-reinforced shotcrete. Is fiber-reinforced shotcrete the better choice and, if so, is 3 in. (76 mm) thickness sufficient?
Structurally, using proper quantities of either welded wire or fibers should work well. If fibers are used, they should be specified by an engineer who has the experience to specify the type of fiber and either performance requirements or dosage levels. The advantage of fibers is that they are uniformly distributed through the section, whereas the welded-wire reinforcement can be difficult to maintain in the proper location within the pavement section. The proper thickness should also be determined by a qualified engineer, as soil and groundwater pressures can impact the required thickness.
We are considering a shotcrete application over a weathered rock outcrop (consisting of sandstone, siltstone, and clay stone) in our backyard to prevent further erosion and unstable conditions. Does the outcrop need to be prepared as described in your previous Shotcrete FAQs (loose material removed, saturated surface-dry [SSD]) if a mesh that is anchored to the outcrop will be used? Also, will the shotcrete need to have fibers in the mixture? Do we need joints?
It is always a good practice to scale off the loose material from the rock face, particularly when dealing with shale or clay stone, as they degrade when exposed to the air. In addition to scaling the rock face, it should be washed down with air and water prior to gunning. As for expansion joints, they are not normally used when gunning over natural rock. The shotcrete is typically gunned continuously across the hillside without any expansion joints, with a natural gun finish following the natural contours of the rock face. With an anchored mesh in place, the use of fibers is not necessary. In many applications, fibers can be used in place of or in addition to mesh.
Is there a recognized standard addressing pass/fail criteria for abrasion testing of cement mortar shotcrete-lined corrugated steel pipe?
We are not aware of any recognized standard for abrasion testing or acceptance specifically for this application. ASTM International has several abrasion tests for concrete and mortar that include:
- ASTM C418-05, “Standard Test Method for Abrasion Resistance of Concrete by Sandblasting”;
- ASTM C779/C779M-05(2010), “Standard Test Method for Abrasion Resistance of Horizontal Concrete Surfaces”;
- ASTM C944/C944M-99(2005)e1, “Standard Test Method for Abrasion Resistance of Concrete or Mortar Surfaces by the Rotating-Cutter Method”; and
- ASTM C1138M-05(2010)e1, “Standard Test Method for Abrasion Resistance of Concrete (Underwater Method).”
Also, ACI 350-06, “Code Requirements for Environmental Concrete Structures,” Section 4.6, has requirements for protection against erosion.
When replacing welded-wire reinforcement with micro- or macro-synthetic or steel fibers, how is the “equivalent dosage” of fibers determined?
The equivalent dosage of fibers to replace embedded steel reinforcement needs to be evaluated by the design engineer for the specific project or application. Guidance for the designer is available in ACI 506R-05, “Guide to Shotcrete,” and ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete.”
We are constructing a new custom residence on the Gulf Coast of Texas using a Monolite insulated concrete form (ICF) system. The ICF system is basically a “sandwich” system with an expanded polystyrene (EPS) panel with a wire cage and shotcrete on both sides. Because of the storm surge and high humidity of the region, we are looking for a mixture formula for a waterproof shotcrete for the exterior coating to help prevent moisture migration to the interior. What can you suggest?
The insulation itself should provide a vapor barrier. Various additives can be used with the shotcrete to improve its permeable properties, such as silica fume or a commercial waterproofing additive. It is also not uncommon to use a plaster coat over the shotcrete to provide improved water resistance and an architectural finish. The density and uniformity of the shotcrete can be influenced by the competency of the shotcrete applicator. It is always advisable to engage a competent and experienced shotcrete contractor to ensure the best possible results. You can search for a contractor with certified shotcrete nozzlemen from our Buyers Guide at shotcrete.org/products-services-information/buyers-guide/ or submit a bid request through our Online Bid Submittal Tool at www.shotcreteweb.wpengine.com/pages/secured/ProjectBidRequest.aspx.
We are studying a repair to an existing large-diameter corrugated metal pipe. The owner requires that the repair meet the fifth edition of the AASHTO LRFD Bridge Design Specifications with 2010 Interim Revisions. We want the owner to consider shotcrete as opposed to installing a new carrier pipe. I have pipe dimensions, depth, and so on, but need some help deciding if this is practical.
Shotcrete has been used in many cases to repair, rehabilitate, and strengthen pipes, culverts, and tunnels. It is not uncommon to use shotcrete to strengthen a culvert under a highway or roadway section. Shotcrete is a method of placing concrete at a high velocity. The shotcrete placed inside the existing pipe can be designed for strengths from 4000 to 10,000 psi (27.5 to 69 MPa), depending on the amount you are willing to spend on the shotcrete products. We cannot speak to the acceptance by the governing body, but it has been done successfully and often in the past. It is vitally important that the shotcrete contractor be competent and experienced in installing the lining. Your specification should require evidence of similar previously completed projects with current references.
We are currently placing a shotcrete wall in a tunnel. The wall has a minimum thickness of 8.25 in. (210 mm) and is placed against secant piles. Our specs called for a wet cure. To minimize shrinkage cracking, what is the minimum amount of time to allow after shotcrete placement before the wet cure is applied?
There is a difficult balance between wet curing too early or too late. You should not add water too early (before the material sets), as this would increase the water-cement ratio (w/c) of the material on the surface. You also do not want to add water during the finishing process, as this would also work the water into the surface and increase the w/c at the surface. Good practice would be to use an evaporative retardant, which generally also serves as a finishing aid during the finishing process, and then get the wet cure set up as soon as possible.