Portal: Underground

This has been done in the past to improve hydraulic capacity and provide better wear resistance in the invert. Typically, the minimum cover over the corrugation is 2 to 3 in. (51 to 76 mm) with a welded-wire fabric either welded or otherwise attached to the corrugated pipe. The cover could likely be reduced with the use of structural fibers of either steel or synthetic material. Steel fibers and wire mesh should not be used together. Care must be taken to specify the required finish. This application would likely benefit from a smooth trowel or light broom finish. A light broom finish is preferable from a safety standpoint, as a trowel finish creates a very slippery surface both during construction and for the maintenance at a later date. In addition to the hydraulic and wear characteristics, once shotcreted, the entire pipe will become a composite section with improved structural characteristics. It should be noted that the pipe must be large enough for workers to work in safely and productively. This would mean an absolute minimum of 48 in. (1219 mm) and preferably larger.

A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.

The article “Shotcrete Testing—Who, Why, When, and How” in ASA’s Summer 2011 issue of Shotcrete magazine should help answer most of your questions on testing of shotcrete. ACI 506R, “Guide to Shotcrete,” and ACI 506.2, “Specification for Shotcrete,” also have helpful information on shotcrete testing. Most competent testing labs should be able to test the compressive strength of cores extracted from shotcrete panels or sections, as they are very similar to concrete cylinder tests. If conducting more advanced testing, you may want to consider selecting a lab experienced with shotcrete.

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.

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.

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.

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.

Shotcrete is routinely used to seal shale after excavations. It is typically done as soon as possible after the excavation because the shale will deteriorate when exposed to the air. When shotcreting, it is considered good practice to wet the receiving surface prior to gunning to create a saturated surface-dry (SSD) condition so the substrate will not draw moisture from the newly placed shotcrete. A good SSD condition is where the surface is wet without any standing water on it. Gunning over wet shale should not be a problem unless the water seeping from the shale is moving. If that is the case, we would recommend installing weep holes with plastic pipe at the locations where the water is seeping from and using an accelerator to flash-set the material immediately around the weep-hole pipe. It is also a good idea to install weep holes at regular intervals along the excavation or exposed hillside. It is important to use a qualified shotcrete subcontractor for this or any high-quality shotcrete installation. A qualified shotcrete contractor will use ACI-certified nozzlemen and should provide you with a résumé of similar, successfully installed projects, along with the up-to-date contact information of representatives from the owners or engineers involved in those projects. The ASA Buyers Guide (www.shotcreteweb.wpengine.com/pages/products-services-information/buyers-guide/) is an excellent source of shotcrete contractors.

Fibers are typically added to shotcrete linings for canals, channels, and other water structures in lieu of conventional reinforcing, such as welded wire reinforcement (WWR). For your “large amount of concrete via the shotcreting process,” we assume that you are using the wet-mix shotcrete process.

Temperature/shrinkage reinforcement is typically placed in thin sections governed by the structural concrete provisions of ACI 318 at a rate of 0.15 to 0.18%. Please be aware that if the lining is intended to be liquid-tight and has movement joints spaced at greater than 40 ft (12 m) apart, a reinforcement ratio of at least 0.50% is recommended by ACI 350 for concrete liquid-containing structures. Assuming that the section does not need to be liquid-tight and using the ACI 318 requirements, let’s consider the tensile capacity of a conventionally reinforced section and provide an equal or greater tensile capacity with fibers. Assuming a 3 in. (75 mm) thick lining with an assumed 28-day compressive strength of 4000 psi (28 MPa), using a WWR of 6 x 6 x W2.9/W2.9 in this section provides a percentage of steel of 0.161% and a tensile capacity of 3770 lb/ft (5610 kg/m). (Asfy = 0.058 in.2/ft [0.12 mm] x 65,000 psi [448 MPa] = 3770 lb/ft [5610 kg/m].)

Then, we assume that the direct tensile strength is 75% of the flexural strength (modulus of rupture [MOR]). For 3770 lb/ft (5610 kg/m) in a section 3 x 12 in. (75 x 300 mm), we have 3770/(12 × 3) = 105 psi (0.72 MPa). Then, we need an average residual strength (ARS) (ASTM C1399) of 105/0.75 = 139.6 psi (0.963 MPa) = 140 psi (0.965 MPa).

Using a macrosynthetic fiber, one can achieve these results using 4 to 5 lb/yd3 (2.4 to 3.0 kg/m3) in wet-process shotcrete. Fiber manufacturers will provide exact dosages to meet the ARS requirements.

Using steel fibers, approximately 43 lb/yd3 (25.5 kg/m3) will provide an equivalent area of steel to the WWR of 6 x 6 x W2.9/W2.9 in a 3 in. (75 mm) thick concrete section. Using steel fibers, however, may require a flash coat to cover the fibers that will protrude and rust over time. The corrosion of the fibers will only reach a carbonation depth of 0.05 to 0.10 in. (1 to 2 mm) but may result in staining the lining.

These calculations assume a thickness and strength. You must adjust for your conditions.

There are really two questions here: 1) Underground fiber-reinforced versus mesh reinforced; and 2) slope protection fiber reinforced versus mesh reinforced.

1. Underground fiber reinforced versus mesh reinforced: it is not clear what the alternatives are that you are considering, but sprayed concrete has a good, solid track record for ground support. If it is a simple comparison of steel mesh versus steel fiber reinforcement, then the issue is one of a design approach.
   Wire mesh and bolts have a longer history and are simple to design as a rigid structure. To install mesh and bolts, however, requires working under unsupported ground. Mechanized spraying of concrete is done with the operator under supported ground and therefore is intrinsically safer.The design of fiber-reinforced sprayed concrete as ground support is approached differently. The sprayed concrete is allowed to deform to a certain extent before coming to rest with the ground forces finding a new equilibrium. The extent of this deformation depends on the energy absorption of the sprayed concrete structure, which is provided for by the fibers.Steel fiber-reinforced sprayed concrete is by far faster to place and therefore has economic benefits. As the fibers are discontinuous, there is merit in considering this structure less susceptible to corrosion and consequential durability issues. We recommend consulting ACI 506.1R and ACI 506.5R.
2. Slope protection fiber reinforced versus mesh reinforced: for slope protection, both fiber-reinforced and wire-mesh-reinforced shotcrete work well and are durable, reliable, and cost effective if done properly. Care must be taken with wire mesh reinforcing to ensure that it is maintained in the middle of the section and not on the ground where it is not effective. Wire mesh can also be difficult to install on an irregular surface and require more shotcrete material to cover the area and the mesh. The wire mesh can be an asset to the installer in providing a grid to support a scaffold system. In many applications, the choice of wire mesh or fibers should be left to the installer with the engineer specifying the minimum requirement for each.

Shotcrete is well suited to the application you have described. You need to determine the characteristics that you want from the shotcrete (strength, toughness, freeze-thaw durability) and include these in the specification. The 1 in. (25 mm) seems very thin for a long-term installation. Please be aware that the material costs (in most cases) will be a small part of the total cost of the installation. You should also make sure that you have a good specification for surface preparation. If the existing surface is not properly prepared, the added shotcrete will not bond well and the installation will not last very long.

The ASA Online Buyers Guide (www.Shotcrete/BuyersGuide) is an excellent source to locate members within the field of shotcrete whom are listed as shotcrete consultants.