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.
- 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.
- 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.
- 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).
- 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).
- 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.
- 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.
I have a project wherein some 25,000 sq ft of existing shotcrete is to undergo varying degrees replacement, repair and restoration. It is on slopes varying from 1:1 to 1:10 or so. It is approximately 40 years old in most cases. It is in a fairly arid climate (Southern New Mexico) with little rainfall and typically low humidity. The subgrade is non-plastic gravelly sandy material. It was reinforced with wire mesh (looks like 6x6x10x10). I’m interested in any techniques and/or materials that might be applicable
I recommend reading the following publications in Shotcrete Magazine: “Shotcrete for Ground Support: Current Practices in Western Canada”, by C.Chan, R Heere, & D. R. Morgan, Part I printed in Winter 2002, and Part II printed in Spring 2002. “Soil and Rock Slope Stabilization Using Steel Fiber Reinforced Shotcrete in North America”, by M.Ballou & M Niermann, Summer 2002.
We are going to be using shotcrete for repairs in a parking structure. We have no experience performing this work and will be subcontracting this portion of the job. What should we be watching for when the shotcrete is being applied?
Surface preparation is a critical operation. The substrate must be prepared properly. All deteriorated concrete must be removed. This is generally accomplished with light-duty chipping hammers, scarifiers, or scabblers. The remaining concrete is then sandblasted or waterblasted to remove the concrete “bruised” by the initial removal operation. The objective is to create a clean, sound surface with the proper surface roughness to receive the shotcrete.
After the surface preparation, the substrate must be saturated with clean water and then allowed to dry to a saturated, surface-dry condition immediately prior to shotcreting. Shotcrete should not be applied to a bone-dry surface as the substrate will absorb water in the shotcrete mixture intended for hydration of the cement. Also, a bone-dry surface will tend to allow plastic and drying shrinkage cracks to form. Conversely, a surface that is wet at the time of shotcreting will result in a high water-cement ratio (w/c) at the interface between the substrate and the shotcrete. High w/c at the interface will result in significantly lower bond strengths.
As with all concrete, proper curing and protection is critical. Failure to cure properly will result in lower shotcrete strengths and may cause some delaminations if drying shrinkage causes stresses that exceed early bond strength. Plastic shrinkage cracking and “crazing” may also result from failure to cure and protect properly. Moist curing is the preferred method of curing. If moist curing is not feasible, membrane curing compounds may be used.
Finally, be sure the nozzleman who will be applying shotcrete on your project is certified by the American Concrete Institute (ACI). Certified nozzlemen have been trained and tested on the requirements for proper shotcrete application. Insisting on this certification dramatically increases the probability that you will get the desired results.