When plans call to form concrete shaft walls, like those that line an elevator shaft, a concrete slip form system needs to be used. The slip form allows for the shaft walls to be formed and poured in sections without breaking the pour for the entire shaft. Once each section is poured and the concrete has set but not cured, the slip form is pulled up by a crane to form the next section of the shaft. By doing this, the concrete from the two pours can bond together and there is no joint. This is important as most structural plans for shafts require that they be created as a monolithic pour, or without any breaks or joints in the concrete.
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Things you need
- Structural prints
- Tape measure
- Locking slip form system
- Socket set and wrench
- Hammer (if needed)
- Orange spray paint
- 4-steel choke cables
- 2-Lifting machines
- Tag line ropes
- 4-way rigging
- Sledge hammer
- Steel brace beams
- Bolts with washers and nuts
- Form oil
- Chalk line
- Snap ties
Locate an area on site where the slip forms may be assembled. Make sure there is enough room to lay the panels down and that a crane or other type of lifting machine can access the area to help stand and move the forms.
Find the page in the structural prints for the job that gives the details for the shaft dimensions. Look at the page showing the area that the shaft is in and find the shaft on the page. Find the call-out for the shaft -- a circle that is split in two with numbers written in each half. The bottom number in the circle is the page the details for the shaft will be on and the top number refers to which detail on that page to use.
Note the dimensions of each face of the exterior of the shaft. Use a tape measure to select slip form panels that will go together to make each face. Lay the panels smooth side down in the work area keeping each set of panels needed for one face separate from the other groupings of panels.
Lock the panels for each together using the included hardware of the slip form system. Follow the manufacturer's instructions to properly connect the panels to each other. Most systems will require a socket set and wrench for installing the locking hardware. Some will use a pin and lock system, for which a hammer is needed.
Mark the panel sets with orange spray paint to indicate which exterior shaft face they form.
Note the dimensions of each face of the interior of the shaft. Select the slip form panels that will go together to make each face. Lay the panels smooth side down on top of the corresponding exterior face that is assembled on the ground. Lock the panels together and mark them with orange paint.
Attach picking shackles to the picking points on the slip forms for the interior faces of the shaft. The picking points are pre-made holes in the top edge of the individual panel frames. Follow standard rigging guidelines for placing the shackles to ensure safety and ease of handling.
Connect the eye of a steel choke to each shackle. Connect the other end of the steel choke to one of the lifting hooks of the 2-ways attached to the lifting machine -- one choke to each hook of the 2-way. Tie a tag line to the bottom of the slip form by passing the rope through one of the preformed holes in a panel frame.
Stand one of the slip form faces upright. Rig the next interior wall form that will run from the one standing (ie if the North face is standing, rig the East or West wall next). Lift that form up, using a second lifting machine, and guide it over to the first form, using the tag line to maintain control of the panel when it is in the air, until their edges meet and the forms are at a right angle to each other. Make sure both panels are touching the ground on their bottom edge and the smooth side of the forms is facing out.
Connect the two edges of the forms that are touching using the locking hardware included with the slip form system. Release one of the lifting machines by disconnecting the picking shackles from the top of one of the form faces. The shackles should remain attached to the steel chokes.
Rig the next interior slip form. Stand the form so its edge meets the edge of the form already standing and connected that it runs from at a right angle (ie: Rig the South or North wall that remains lying down if planning to connect to a standing East or West wall). Connect the forms. Disconnect the picking shackles and pick, stand and connect the remaining wall.
Use a tape measure and level to square the box that the interior slip forms create when they are all connected. The diagonal measurements of the interior of the box -- from top corner to top corner -- need to be equal for the form to be considered to be in square. Adjust the alignment of the forms as needed by striking the corners with a sledge hammer to move the entire face until the correct measurement is achieved.
Bolt the steel bracing provided with the slip form system between the panels on the interior of the box as detailed in the slip form system instructions.
Attach picking shackles to the four corners of the box and tie a tag line to one of the corners. Connect the 4-way rigging to the picking machine and then to the shackles. Lift and place the interior slip form in the shaft hole. Verify that it is in the correct position by checking the location coordinates provided on the structurals.
Insert the snap ties that will be used to hold the exterior slip form in place through the slip form panels of the interior face using the pre-made slots in each panel. Insert the snap ties from outside of the box and have one worker on the inside to place the locking pins in the ends of the ties.
Snap a chalk line near the top of the forms at the height of the planned concrete pour. Spray the forms with form oil. Form oil serves as a release agent to prevent the concrete from adhering to the form panels. Install all mechanical, electrical and rebar specified by the structurals for that section of the shaft.
Attach the shackles to the picking points of one of the exterior slip forms and lift and place the form in position, smooth side facing the interior face. Make sure the interior and exterior walls match (ie: North with North). Attach the exterior wall to the interior by guiding the ends of the snap ties attached to the interior face through the snap tie slots of the exterior face and locking them in place with the locking pins. Remove the shackles and pick, stand and attach the remaining exterior slip forms. Connect the corners of the exterior forms using the locking hardware that came with the slip form system.
Calculate the concrete needed as would be done with any wall pour and order the concrete. Pour the concrete in between the two faces of the shaft form. Use a concrete vibrator to make sure that the concrete gets around all the mechanical/electrical inserts and rebar inside the form.
Wait for the concrete to set or become firm enough to be free standing. The set time will be dependent on the weather and concrete mix used. Consult with the concrete supply company for the set time to expect. The set time will be shorter than the cure time.
Release and remove the locking hardware on the corners of the exterior forms. Remove the locking pins from the snap ties on both the interior and exterior of the shaft form. Rig one exterior panel at a time and pry the form from the concrete.
Clean the interior forms while it is in place in the shaft area. Check that the box is still in square. Make adjustments as needed and prepare the next section for a concrete pour.
Tips and warnings
- Pre-build the rebar structures required for each section of the shaft and for the entire length of the finished shaft, out-of-place and fly them into position with the crane, just as the panels were assembled, to speed up the installation process.
- Do not deviate from the installation and bracing instructions that came with the slip form system. Slip form systems are engineered and designed to withstand the pressure of concrete and any deviation from the plan could lead to the form breaking apart during the pour.
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