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.

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.

We will be using shotcrete to repair a concrete box culvert that has some minor spalling. Do we need to apply a bonding agent before applying the shotcrete? How should we prepare the surface?

No bonding agent is required. A key to a successful repair is proper surface preparation. The surface receiving the shotcrete must have the deteriorated material completely removed, be thoroughly cleaned, and in a saturated surface-dry condition (SSD) at the time of shotcrete appli­cation. Another key item is proper curing and protection following shotcreting. Details can be found in the Task Force 37 Report “Guide Specification for Shotcrete Repair of Highway Bridges.” The document is available from the American Associ­ation of State Highway and Transportation Officials (AASHTO), Washington, DC.

Tiber Creek Sewer Rehabilitation

As part of the DC Clean Rivers Project ? Division 1 Main Pumping Station Diversions, the Tiber Creek trunk sewer was designated to receive a new structural liner in a 122 ft (37 m) section near the intersection of New Jersey Ave. and Tingey St. in southeast Washington, DC.