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.
We are considering the use of shotcrete to line a 3600 ft (1097 m) channel that is approximately 15 to 20 ft (4.5 to 6 m) wide. The purpose of the lining is to cap impacted sediments in the channel bottom. What is the suitability of shotcrete for this type of application, and can you provide a conceptual/budgetary estimate for the implementation of this approach?
Shotcrete is a method of placing concrete and therefore the material has the same basic characteristics of concrete. Shotcrete is often used for canal, channel, and ditch lining. It is important with shotcrete (concrete) that the subgrade the material is placed over be compacted and stable. The thickness, strength, and reinforcing needs to be designed and specified by a professional engineer familiar with this type of structure or pavement. For budget numbers, you should contact one of our contractor members, who can be found in the Buyers Guide on the Web site at shotcrete.org.
We are currently working on a tunnel that will cross through a drinking water protection zone in the alluvial aquifer. Do shotcrete technology and materials exist that can be used on groundwater-sensitive areas?
Shotcrete is the same as concrete when evaluated as a material and its exposure to potable water. In the U.S., many admixtures and cements for concrete have been tested and certified to meet the NSF 61 standards for materials exposed to potable water. In my experience, potable water stored in concrete tanks with direct exposure to the concrete (no coatings) has not exhibited any significant rise in alkalinity. Exposure of a tunnel in a groundwater aquifer would seem to have much less contact area per volume of water contained in the aquifer, such that any rise in alkalinity would be miniscule. Because concrete in the U.S. is universally accepted for the storage and transport of potable water, I’d assume that the use of shotcrete in your tunnel would be perfectly acceptable.
Often, steel fiber-reinforced shotcrete (SFRS) linings are applied in underground construction. In some areas of high tensile stresses, it is necessary to use additional ordinary reinforcement (reinforcing bar/mesh). It may be inefficient to switch to non-fibrous shotcrete for these regions. Are the shadowing problems to be expected in that case (SFRS with additional ordinary reinforcement) more severe and how can they be resolved?
It is not uncommon to encapsulate lattice girders or steel sets in fibrous shotcrete. The skill of the nozzleman, the size and density of the reinforcing, and the characteristics of the mixture and the accelerator are the most important factors in achieving good encapsulation of reinforcing bar or these more complicated applications around lattice girders or steel sets. With welded-wire reinforcement, you should have a 4 x 4 in. (100 x 100 mm) or greater spacing. With reinforcing bar, you should use the minimum diameter possible at a minimum spacing of around 6 in. (150 mm). Preconstruction mockups should be considered to prove the competency of the nozzleman and the mixture. Please note that the best nozzleman cannot succeed without a good, workable mixture.
Is it possible to use a penetrating sealer, such as those used on driveways, to make shotcrete repel moisture? If so, will the sealer improve the shotcrete’s freezing-and-thawing performance?
It is possible to use a penetrating sealer on shotcrete in the same manner as cast-in-place concrete. We are not aware of research on the durability of such a sealer and do not know if it would enhance the freezing-and-thawing characteristics. A high-quality shotcrete mixture that is properly placed will exhibit excellent freezing-and-thawing characteristics with or without a sealer.