We are rehabilitating a limestone-brick masonry storm sewer by lining it with shotcrete. The sewer is approximately 7 ft (2.1 m) tall with an arch ceiling and walls that are 8 ft (2.4 m) apart. The limestone surface is fairly rough, but the brick portions are not. While the existing structure shows no signs of needing to be reinforced for structural support, we are reinforcing to prolong the service life of the culvert. Is there a recommended minimum shotcrete thickness and reinforcement?

Shotcrete has been used to successfully rehabilitate sewers for over 50 years.

The thickness to be used is an engineering issue and beyond the scope of our association. We would recommend a 2 in. (50 mm) minimum thickness reinforced with either polypro­pylene fibers or a light-gauge welded wire reinforcement. The surfaces must be cleaned thoroughly to remove grease, oils, and other substances deleterious to good bond. Bonding to brick is not a problem.

Finish is another consideration. The added liner thickness will reduce the size of the culvert. If capacity is not an issue, it is recommended to leave the new shotcrete lining with a nozzle finish. If capacity might be a problem, then a float or trowel-smooth finish may be necessary.

Can you provide any information or research on the sound absorption performance of shotcrete?

We are not aware of any testing done specifically for the sound absorption performance of shotcrete. Shotcrete is a method of placing concrete and, once hardened, it should have similar parameters as cast concrete. With shotcrete, one has the ability to use many different finishes, which might influence the sound absorption characteristics. A smooth troweled shotcrete wall would be the most similar to a formed cast-in-place wall. On the other end of the spectrum, a nozzle-finished wall would likely absorb far more sound. A recent design of a concert hall at a major university was to be built with oval concrete or shotcrete perimeter walls covered with fabric curtains for sound purposes. The ceiling was to be suspended nozzle-finished shotcrete.

We want to shotcrete a porous rock wall to stop water leakage out of a small pool that is home to an endangered fish. The wall is quite rough and uneven. The pool will be drained to do the work. How long should we allow the concrete to cure before refilling the pool with water? Does this sound like a good application for shotcrete?

This is a great application for shotcrete. Once drained, the existing surfaces should be cleaned by water blasting or sandblasting to provide a good bonding surface. The shotcrete can be submerged within a few hours or upon reaching the final set (hardened). One factor to be concerned about is the chemical reaction and alkalinity of the area around the shotcrete during the curing period. A good solution would be to submerge the pool for a period of at least a week, drain and waste the water, flush the shotcrete surfaces, refill the pool, and test the pH before reintroducing the fish. This should eliminate the danger of the alkalinity to the fish.

What are the requirements for selection of the shotcrete lift height and delay between successive layers? ACI 506R describes only a general approach.

Although some have tried, there are not and should not be specific recommendations for lift height or time between lifts of layers. Shotcrete is a method of placing concrete, and concrete properties vary with many parameters, such as admixtures, ambient temperature, concrete temperature, slump, and age of concrete, to name a few. The lift height is also influenced by the surface on which you are shooting (rough, porous, smooth, dense, and so on); the orientation being applied (vertical, sloped, or overhead); and the size and density of the reinforcing steel, if it is present. Regardless of the period of time between lifts or layers, the receiving surface must be clean and moisture-conditioned to create a good bond between lifts or layers. As you can see, there are too many variables to spell out recommended guidelines or rules of thumb for lift heights or time between lifts or layers. The goal is to place the lifts or layers in heights or thick­nesses that do not slough or sag. The time between lifts or layers is the time required for the initial lift or layer to support the subsequent lift of layer. These decisions must be made on the job on a daily and hourly basis by a properly trained and experienced nozzleman and shotcrete foreman. These decisions may vary during the day to meet the current situation. It is critical that the shotcrete is placed by a shotcrete contractor with trained and experienced crews who is experienced and successful in the type of work being installed.

I would like advice about spraying shotcrete on the exterior walls of a house I am building. In constructing the exterior walls of the house, I plan to shoot approximately 0.75 in. (19 mm) on Day 1 and shoot 1.25 in. (31 mm) on Day 2 for 2 in. (50 mm) thick walls. I have hung 14-gauge wire mesh spaced at 1 in. (25 mm) over all the walls and am planning to use a 3000 psi (20.7 MPa) mixture. I am greatly concerned about cracking. Is my planned technique a good way to mitigate cracking or are there better approaches? Should I consider upping the strength of the concrete?

In structural applications, most of the impact force from nozzling shotcrete is directed toward compacting the shotcrete in place rather than against the formwork. This was the subject of a study conducted by Marc Jolin of Laval University, Quebec City, QC, Canada, and reported in the Fall 2008 issue of Shotcrete magazine. There is virtually no hydrostatic pressure on the forms from the application using the shotcrete process. A copy of this study can be viewed on the ASA WebIt is fine to place shotcrete in two layers on 2 consecutive days, although simply placing two layers on 2 consecutive days won’t prevent long-term drying shrinkage cracking. For the best bond, the surface of the shotcrete on Day 1 should be given a rough broom finish to provide a rough texture for the Day 2 shotcrete to bond to. On Day 2, before shooting, wet the surface of the Day 1 shotcrete to prevent a hot, dry surface from absorbing water from the fresh shotcrete. Please note that the surface needs to be dampened but allowed to dry to an SSD condition. A surface that is too wet can inhibit good bonding. It is essential to moist-cure the shotcrete as soon as it has finally set to help reduce early-age shrinkage cracking. On a hot, windy day, you may need to fog the surface soon after placement with a pressure washer using a fogging nozzle to reduce the rapid evaporation of water from the surface of the shotcrete. Wet curing with a wetted burlap overlay or drip system for at least 3 days (preferably 7 days) is recommended to help reduce the potential for longer-term drying shrinkage cracking. Using macrosynthetic fibers in the shotcrete mixture will also help reduce early-age shrinkage cracking. Because you are in Florida, unless you are shooting in the dead of winter, you may also want to consider using a concrete mixture with up to 20 to 25% fly ash. This will slow down the hydration of the cement and resultant set time to give you some more time to finish the surface and get proper curing started. Fly ash also helps reduce the concrete permeability and increases the long-term strength and is generally less expensive than portland cement. If you use a concrete mixture with silica fume (also called microsilica), it will increase the water demand of the mixture during hydration and has a greater tendency for early-age plastic shrinkage cracks. Thus, if you use silica fume, you will need to pay close attention to keep the surface wet through fogging and then wet curing as soon as it is practical. As previously mentioned, a 2 x 2 or 3 x 3 in. (50 x 50 or 75 x 75 mm) wire mesh would be preferred to reduce congestion of the reinforcement. Stay away from rolled mesh, as it is very difficult (even nearly impossible) to get to lay flat. Sheets of welded wire mesh are recommended. ASA recommends a minimum 28-day compressive strength for shotcrete of 4000 psi (27.6 MPa). A 3000 psi (20.7 MPa) mixture will have a higher water-cement ratio (w/c); therefore, there is more water in the mixture, which will significantly increase the potential for drying shrinkage cracking in the final surface. A 4000 psi (27.6 MPa) mixture is easily achieved with current portland cements and normal supplemental cementitious products such as fly ash. Finally, you mentioned that you will be shooting the surface of a house. You haven’t provided any details about what you are shooting the shotcrete on, but the substrate must be rigid enough to not vibrate when shotcrete hits the surface. If it is not rigid enough, the vibration of adjacent areas of freshly shot plastic shotcrete could cause cracking. This would be more of a problem in the Day 1 coat of shotcrete, but cracks that form in the Day 1 shotcrete would create a weaker section and increase the likelihood of mirrored cracking in the Day 2 layer. Again, please note: While it is appropriate to wet down the Day 1 shotcrete prior to application of the Day 2 shotcrete, it is important to let the wetted Day 1 shotcrete dry back to an SSD condition before application of the Day 2 shotcrete. If the Day 2 shotcrete is applied to a wet substrate (with liquid water on the surface), it will fail to meet the specified 150 psi (1 MPa) bond pulloff strength requirements for the project.

We are removing up to 0.75 in. (19 mm) of the existing scaled concrete on a fire-damaged concrete wall. The architect has asked if shotcrete is applicable for a vertical 0.75 in. (19 mm) application. Also, the walls are circular and the working distance from the wall is no more than 36 in. (0.9 m). Is this enough room to apply shotcrete?

Yes, shotcrete can be applied in a 36 in. (0.9 m) area. Keep in mind, however, that it’s difficult to get as nice a gunning pattern as you would like when you are that close to the receiving surface. When you cannot back off from the wall, there is a tendency for a more irregular gunning surface, which would require more cutting and screeding to get an aesthetically pleasing result.

Can shotcrete be applied on wet shale rock? How well does shotcrete bond to shale?

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.

What is the impact force on formwork resulting from a shotcrete application? I am designing the formwork for a wall to be placed via shotcrete and need to know the forces imposed on the wall forms.

In structural applications, most of the impact force from nozzling shotcrete is directed toward compacting the shotcrete in place rather than against the formwork. This was the subject of a study conducted by Marc Jolin of Laval University, Quebec City, QC, Canada, and reported in the Fall 2007 issue of Shotcrete magazine. There is virtually no hydrostatic pressure on the forms from the application using the shotcrete process. A copy of this study can be viewed on the ASA Web site at www.shotcreteweb.wpengine.com/archivesearch/ArchiveSearch.asp.

We recently contracted with a shotcrete company to install a shotcrete structure for a swimming pool. After the pool was completed and filled with water, rust stains began emerging through the plaster surface. When we broke out a section of the pool structure, we found that there was little to no coverage of shotcrete over the steel reinforcement. The shotcrete company’s excuse is that they shot the pool to maintain the desired finished depths and widths and there was little to no coverage because the steel was set too high (even if that were the case, they never alerted anyone during the installation). This sounds like an excuse to me. Shouldn’t the shotcrete company we hired make sure that the concrete coverage met or exceeded what the structural engineer called for? Is there any credibility to their explanation of why they didn’t cover the reinforcing bar enough? What is the standard practice for shotcrete installation?

In short, the shotcrete contractor is responsible for maintaining proper cover over the reinforcing steel. The reinforcing bar installer should set the steel in the proper location for achieving the required cover corresponding to the final desired shape. If the shotcrete contractor finds that he cannot maintain proper cover with the reinforcing as placed, however, he needs to communicate to the designer/owner/general contractor that the reinforcing needs to be fixed before he shoots the section in place. There is no excuse for placing shotcrete with less than the specified cover, as shooting it with reduced cover will obviously create a section that has much less durability than intended by the designer.

I have been in the swimming pool industry for 30 years and I deal with a lot of different engineers on my commercial projects who want a wet test to verify water tightness before the finish is applied to the pool. In my experience, air-entrained shotcrete tends to be porous and leak. Are there any engineering specifications that state that air-entrained shotcrete is porous and will leak if the surface is not trowel-finished?

Properly added and mixed air-entraining admixture in concrete will actually reduce the permeability of concrete. This is because the small, well-formed air bubbles from air-entraining admixtures are not interconnected as larger, entrapped air bubbles may be in non-air-entrained concrete. Thus, the reported higher permeability of the air-entrained shotcrete is not a material flaw but must be from poor shotcrete application. Air entraining from 4 to 7% air is advantageous for enhanced resistance to the freezing-and-thawing cycles of saturated concrete and should be specified by the designer in areas subject to significant numbers of freezing-and-thawing cycles annually. The reported high permeability and resultant failure to pass a water-tightness test could be investigated by taking cores of the “porous” material and conducting a petrographic analysis of the core. Based on the reported results, I strongly suspect that the in-place shotcrete has major issues with sand pockets, overspray, and rebound.