I have a seawall with a gunite (dry-mix shotcrete) outer layer. The gunite layer has cracked in multiple locations on the seawall resulting from years of exposure to the harsh environment. The original gunite was not part of a soil nail system. I am considering a re-coat of shotcrete probably 3 to 4 in. (76 to 100 mm) thick with wire mesh and L-anchors on a 2 to 3 ft (0.6 to 0.9 m) grid. I know the importance of surface treatment for bonding, etc., but I am not sure if I should remove the original gunite layer (which is still sound in some places) or apply the re-coat. The new overlay needs to be structurally effective. I know that a soil nail system is the most dependable solution, but cost is a major concern. Do you have any suggestions?

The new shotcrete layer can be added to the existing shotcrete or installed after the existing shotcrete is removed. The decision to remove or not remove the existing shotcrete is beyond the scope of what we can comment on. If the existing shotcrete is left in place and overlayed, it should be thoroughly cleaned and roughened to create a good bonding surface. Because this is in a marine environment and you are considering the use of wire mesh, you need to make the new layer thick enough to have sufficient cover on the reinforcing steel. Alternately you could consider the use of fiber-reinforced shotcrete and silica-fume-enhanced fibrous shotcrete. Please note that there are many types of fibers on the market. We recommend that you review some of the Shotcrete magazine articles on fibrous shotcrete and on shotcrete in a marine environment on the ASA Web site. We suggested two papers for reference. The first is by Gilbride, Bremner, and Morgan on the Port of Saint John, and the other is by Morgan on the use of fibers that cover marine repairs. You mentioned using “L-anchors” at 2 to 3 ft (0.6 to 0.9 m) spacing. The use of grouted anchors with a reasonable embedment is quite common, but the design of such anchors is again beyond the scope of what we can advise.

Our client has a retaining wall that has experienced movement in the precast concrete panels and has asked us to research a product that could be applied to give a smooth look to the retaining wall. Is shotcrete a possible option? I would also like information on the recycled content of shotcrete.

Shotcrete is basically concrete that is pneumatically applied. Shotcrete can be used as an overlay for an existing wall to provide structural strengthening and a smooth look. Again, we suggest that you visit ASA’s Web site and search previous Shotcrete magazine articles for finishes. Before the shotcrete is applied, the wall must be stabilized from any anticipated future movement. Relatively thin layers of shotcrete or concrete will not withstand future wall movements without distress and cracking. The recycled content of most shotcrete mixtures is limited to the substitution of fly ash or other pozzolans for a percentage of the cement in the mixture. To properly place shotcrete, this substitution is generally limited to approximately 25% of the cement content.

I am a civil engineer looking to use shotcrete in a culvert rehabilitation project. Due to flow constraints, we are forced to have a maximum wall thickness of 3 in. (76 mm). For the typical 96 in. (2438 mm) precast concrete culvert, the walls are approximately 9 in. (228 mm). What can I do to obtain a near similar product with only 3 in. (76 mm) of wall thickness? Can shotcrete be applied at higher compressive strengths, 10 psi (0.07 MPa), or is it better to use fiber-reinforced shotcrete? The intent of the retrofit is to at least obtain a 10-year service life to this temporary solution.

This is an engineering question, not an application question. Precast pipe is sized for multiple uses and services. Depending on this service (depth of cover or loads), creative reinforcing bar placement and higher compressive strengths can reduce the wall thickness significantly. For example, success has been realized using elliptical steel to reduce concrete section thickness. Fiber reinforcement is secondary reinforcing and is not a suitable replacement for reinforcing steel. Given the short life required of the culvert, and assuming fairly equal loading on the circumference, a 3 to 4 in. (76 to 101 mm) section with judiciously placed reinforcing bar, and silica fume (8 to 10% of cement for higher strength up to 10,000 psi (69 MPa)]) would be sufficient. The resulting culvert’s life would probably be much longer than 10 years. In the end, an engineering call should be made, but the material will perform.

I am repairing a concrete masonry unit (CMU) block wall that was partially damaged when a portion of the roof collapsed. The engineer on the project is proposing to apply shotcrete to one side of the wall to help structurally reinforce the wall. I would like to know if there is a way to finish the wall so it is cosmetically pleasing, especially since this is on the inside of an existing building with the other walls being a painted CMU. Also, were can I get some conceptual pricing for applying the shotcrete?

Shotcrete can, and often is, finished to provide nice printable wall surfaces. To be the same general texture of the concrete block wall, you should specify a wood or rubber float finish. You can access the ASA Buyers Guide at https://shotcrete.org/products-services-information/buyers-guide/ to locate organizations regarding budget or conceptual pricing.

We are looking at lining an existing 20 ft (6.1 m) diameter brick sewer with shotcrete that is 15 in. (0.4 m) or more thick and fairly heavily reinforced. Can this be done? The existing sewer is about 3 mi (4.8 km) long and 100 years old. Would shotcrete be a suitable method of rehabilitation? The rehabilitation is not just a liner, but the owner wants the shotcrete designed as a replacement pipe inside the existing brick sewer, designed for all earth and other superimposed loads as though the brick sewer were not there.

Yes, this can and should be done in shotcrete. Shotcrete has been used to successfully line brick sewers for 75 years. Shotcrete has been used to line over $40 million worth of brick sewers in Atlanta alone. Large brick sewers have been lined with shotcrete in most of the major midwestern cities. All of them were designed using the existing sewer as a one-sided form. Properly designed and constructed, shotcrete will provide the owner with a new concrete pipe or permanent tunnel lining and the associated expected longevity.

I am in the process of designing a 6″ shotcrete overlay for an existing wall that is approximately 1,250 square feet. The shotcrete subcontractor has proposed to use a dry-mix shotcrete. What are the advantages and disadvantages to the dry-mix process? The design includes dowels on 24″ centers and 4×4 W4xW4 wire mesh. Can the entire 6 inch thickness be placed at one or will it require a number of different lifts to build up to the 6 inch thickness?

The overlay can be placed successfully with either a dry-mix or wet-mix shotcrete process. The preference of the shotcrete subcontractor is likely related to his/her past experience and what they are best suited doing. The advantages of dry-mix process are beyond the scope of a simple answer. The process is well described in ACI 506R Guide to Shotcrete. The entire 6 inch thickness can be placed in one layer using the bench gunning technique. The number of vertical lifts would depend upon the height of the wall and the nature of the surface that the shotcrete is being placed against.

My firm just completed a 2 in. (51 mm) overlay of shotcrete in an existing storage tank. Almost immediately after the shotcrete was applied, we noticed spider web cracking on almost the entire surface. The weather was very hot during shotcreting, and we suspect this caused the cracking. The project engineer is concerned about permeability and is thinking of having the shotcrete removed. Is removal really required or can we live with this cracking?

Removal is probably not called for in this situation. Spider web cracking usually is an indication of crazing, a form of plastic shrinkage cracking. Crazing generally occurs when the combination of temperature and humidity creates a rate of evaporation at the surface of the concrete that is higher than the rate of bleed water exiting the concrete. Because the surface has very little, if any, tensile strength at this time, crazing cracks start to form. The good news is that crazing is an aesthetic problem. It affects only the very top surface and does not extend deeply into the concrete. Crazing cracks are more apparent when the surface is damp.

To avoid or limit crazing, be conscious of the weather conditions during placement. If there will be high temperature, low humidity, and moderate to high winds, measures such as fogging and/or erection of windbreaks may be required during placement. Synthetic fibers will help inhibit the formation of crazing cracks. Curing must begin as soon as possible, especially in these conditions.

What wire size and opening are recommended for repair of bridge substructures? We realize the mesh would not be for restoring or improving structural capacity, merely to help control cracking.

The inclusion of wire mesh must be considered on a case-by-case basis, depending on the thickness and orientation of the shotcrete. Thin sections may well not have any wire mesh. In aggressive environments, at least 2 in. (50 mm) of shotcrete must cover the mesh. The mesh size should be at least 2 x 2 in. (50 x 50 mm) and preferably 4 x 4 in. (100 x 100 mm) to allow for proper encapsulation. Overhead shotcrete usually includes wire mesh for thicknesses greater than 2 in. (50 mm) in case the shotcrete debonds from the substrate. The mesh must be mechanically anchored.

Some designers are eliminating wire mesh and relying on synthetic fiber reinforcement for shrinkage crack control. The use of synthetic fiber eliminates the concern over cover and corrosion in aggressive environments. Specific recommendations on the amount and type of fiber should come from the manufacturer.

I want to apply a 3.5 in. (89 mm) veneer of shotcrete over an existing cast-in-place wall. I am concerned about how well the shotcrete will bond to the existing wall. This wall is 50 ft (15.25 m) in height. What are the keys to doing this work successfully?

This is a common use for shotcrete. The key elements are as follows.

  1. Proper surface preparation. To establish suitable surface roughness, use heavy-duty sandblasting, high-pressure water blasting, or mechanical methods such as scabblers or scarifiers, followed by sandblasting or high-pressure water blasting to remove the “bruised” surface material. Refer to ICRI Guideline No. 03732, concrete surface profile Chip 6 (CSP 6), or greater.
  2. Provide mechanical connection between the shotcrete and concrete by installing L-bar anchors (epoxy or portland cement grouted) on a systematic pattern, with reinforcing bar (or heavy-duty mesh) spanning vertically and horizontally between the anchor bars. Size and spacing of the bars to be determined by the structural engineer. Position anchors and reinforcing bar to ensure adequate shotcrete cover to them. Nonmechanically connected veneers are not recommended.
  3. Wash concrete surface with clean water to remove dust or any other contaminants to achieve a good bond and presaturate concrete. Allow concrete to dry back to a saturated surface dry (SSD) condition immediately prior to shotcrete application. If concrete dries excessively, bring back to SSD condition with fogging. (A 3000 psi [21 MPa] water pressure sprayer works well for this purpose).
  4. Apply the shotcrete from the bottom up, taking care not to entrap rebound/hardened overspray. Use proper shotcreting techniques to encase reinforcing bar and anchors. Use 45-degree construction joints (do not construct long tapered joints).
  5. Use shooting wires, guide forms, or other suitable methods (for example, rods with alignment bubbles) to establish proper line and grade. When the shotcrete has stiffened sufficiently, trim it to line and grade with cutting rods and then finish using fresnos or floats to provide the desired surface texture (wood floats for more textured finish, rubber/sponge floats or magnesium floats for intermediate texture finish, or steel floats with steel toweling for smooth finish). Note: very smooth finishes are not recommended as they tend to show imperfections from hand-finishing procedures. Avoid over-finishing of shotcrete or procedures/timing which could pull tears or sags/sloughs/delaminations in the fresh shotcrete.
  6. Cure the freshly placed shotcrete using one of the methods prescribed in ACI 506R-90. Our preferred method is fogging/misting until the shotcrete has reached initial set, followed by wet curing for 7 days using presaturated plastic-coated geotextile fabric (for example, Transguard 4000), which is kept wet with soaker hoses. Curing compounds are a (second best) alternative, but should not be used if a paint or coating is to be applied, unless they are approved by the coating/paint supplier for such purposes.

I am working on repairing some mildly deteriorated walls in a drinking-water treatment plant. There are no chlorides used in the treatment process. I would like to apply a 1 in.-thick shotcrete layer over the existing concrete utilizing a mix containing silica fume, which will achieve a compressive strength of 5000 psi at 28 days. I am having difficulty formulating a mix to meet those requirements that also has a water soluble chloride content of less than 0.10 % chloride ion concentration by mass of cement. I cannot get the chloride ion concentration below 0.15%. What adjustments can I make to get to my goal of 0.10% or less?

There are areas that have no problem getting values lower than the most stringent ACI requirement of 0.06% for prestressed concrete with no special adjustments. It would be prudent to test each of the proposed shotcrete constituents to determine their soluble chloride ion content. The most likely suspects are the aggregate and water sources. Typically portland cement and silica fume would contribute little, if any, detectable chloride ions. Assuming this would be a dry-process application, the only admixture other than the silica fume might be an air entraining agent, which would not provide any chloride ions. This leaves only the aggregates and water as the sources. At a minimum, the aggregates and water should be tested by a qualified laboratory for soluble chloride ion content. Alternate sources of aggregates and water may be required based on the laboratory results.