Multi-million dollar underground stations are currently under construction on Metro and LRT lines in Toronto, Ontario, Canada. Traditionally, the thick, heavily reinforced structural concrete station walls have been constructed using the conventional form-and-pour concrete construction method. This construction method, while widely used, is not without its challenges.
Many of the underground station sites are in congested urban areas with limited spaces for laydown of concrete formwork, and crane access time for handling and installation of formwork is often on a critical path for completion of station construction.
This retaining wall was contracted for a new townhouse development project in Squamish, BC, Canada. Vancouver Shotcrete & Shoring Inc. has serviced many municipalities and private companies over the past 30 years in the fields of shotcrete shoring, pools, and custom rockscapes.
The shotcrete retaining wall was designed to be anchored by Titan 40 self-drilling anchors with a test load capacity of 52,000 lbs (24,000 kg) each, spaced every 6 ft (2 m) on center. The temporary 4 in. (100 mm) shotcrete was reinforced with 4 x 4 x 1 in. (100 x 100 x 25 mm) welded wire mesh, with a 0.8 in. (20 mm) continuous whaler lining to span the load between each anchor. The engineer specified a 5800 psi (40 MPa) shotcrete mix for this purpose.
Shotcrete is routinely used for the repair of deteriorated concrete masonry and brick. Shotcrete should easily have a minimum compressive strength of 4000 psi (28 MPa) when properly shot and cured. According to the National Concrete Masonry Association, current CMU units have a minimum 2000 psi (14 MPa) compressive strength. Older CMU had a lower 1500 psi (10 MPa) minimum. Thus, the shotcrete placement will be significantly stronger and less permeable than the in-place CMU. This should give the wall better resistance to saltwater and freeze-thaw exposures thus extending useful life.
Depending on the depth of the new shotcrete placement, you may consider mechanically tying the shotcreted layer back to the sound CMU with epoxy or mechanically embedded anchors or j-bolts. Thicker sections may also benefit from the use of a steel wire mesh or fibers.
When shooting onto existing CMU sections, the surface must be properly prepared and then shotcreted with proper shotcrete materials, equipment, and placement techniques. Shotcrete placed onto an existing CMU surface will provide an excellent bond IF the following conditions are met:
Make sure the surface is roughened and clean.
The amplitude of roughness should be +/- 1/8th in. (3 mm) or more.
A high-pressure water blaster (5000 psi [34 MPa] or more) or abrasive blasting can help to roughen and clean the surface.
Bring the CMU surface to a saturated surface dry (SSD) condition. This means the surface feels damp, but water is not picked up on a hand.
Make sure the shotcrete placement is properly executed with high-velocity placement and quality materials.
The shotcrete should have a minimum 28-day compressive strength of 4000 psi (28 MPa).
Be sure the shotcrete contractor is using an air compressor able to produce at least 185 CFM (5.2m3/min) for wet-mix and 375 CFM (10.6 m3/min) for dry-mix (gunite) of air flow at 100 to 120 psi (0.7 to 0.8 MPa).
Use of an ACI-certified shotcrete nozzleman is recommended.
No bonding agent should be used. It will interfere with the natural bonding characteristics of shotcrete placement.
Finally, without details on the loading conditions, and structural details of the original construction, we cannot comment on the structural integrity of the repaired wall. You should contact a professional engineer experienced in concrete repair to evaluate the structural integrity of the wall when repaired.
Multi-million-dollar underground stations are currently under construction on Metro and LRT lines in Toronto, Ontario, Canada. Traditionally, the thick, heavily reinforced structural concrete station walls have been constructed using the conventional form-and-pour concrete construction method. This construction method, while widely used, is not without its challenges.
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ASA Outstanding Shotcrete Awards Program
The ASA Outstanding Shotcrete Project Awards Program exists to recognize excellence and innovation on projects in which the application of shotcrete has played a significant role.
ASA’s Annual Outstanding Shotcrete Project Awards Program provides an exciting real-world demonstration of the exceptional advantages of placing concrete via the shotcrete process. Many sustainability advantages are also inherent in the shotcrete process and play a significant role in winning projects as well as the project owner’s ultimate decision to use shotcrete as the method of concrete placement. Projects must be completed between January 1, 2022, through September 1, 2024, and can be submitted in the following areas: Architecture │ New Construction, Infrastructure, International Projects, Pool & Recreational, Rehabilitation & Repair, and Underground.
To assist in your submission, we have provided submission resources to inform you of the submission guidelines, a list of questions, and a copy of the owner release form. Please email any questions to [email protected].
During the summer of 1970, on my 18th birthday, my father woke me up and told me they needed me on a job. I drove out that morning in July to the Crucible Specialty Steel plant in Midland, PA. It was my first experience working on a Gunite job. We were gunning refractory in a vessel, and I was throwing 100 lb (46 kg) bags of pre-packaged refractory into a paddle mixer to pre-dampen the material. We emptied the paddle mixer on sheets of plywood and shoveled the pre-dampened refractory material into the dry-mix shotcrete gun’s hopper. The Jetcreter was a continuous-feed gun, and it was tough for us to keep up with it. It was a very long day for me because it took 15 hours to complete the gunning. I left the house that morning in the dark and returned home in the dark. That was my introduction to “Gunite” (now referred to as dry-mix shotcrete).
Deep in the heart of London’s financial centre, work has been continuing to make one of the world’s largest stations safer and easier for passengers to use. Finding your way around the existing labyrinth of tunnels, connecting five London underground lines, is a task worthy of the most experienced navigator.
