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.
Outstanding Rehabilitation & Repair Project
Outstanding Underground Project
Honorable Mention Project
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.
Queens Midtown Tunnel Rehabilitation
The Queens Midtown Tunnel (QMT) is a twin-tube, tolled, four-lane highway that serves over 90,000 vehicles each day.
Oroville Dam Emergency Recovery
Oroville Dam, built in 1957, is an earth-filled embankment dam located on the Feather River, east of the city of Oroville, CA, in the Sierra Nevada foothills east of the Sacramento Valley.
Brattleboro Bridge Project
In August 2016, Superior Gunite was awarded a subcontract from PCL Civil Constructors to shoot and carve eight web fins on the piers of the I-91 Brattleboro Bridge, in Brattleboro, VT.