Category CONCRETE FORMWORK

The self-raising formwork system has many characteristics that make it versatile in the construction industry. [21]

The quality of the self-raising forms may be attributed to the fact that they are preassembled in a factory-type system. This elimi­nates the uncertainty that often surrounds equipment fabricated on site. The factory setting allows the formwork to be assembled before it is needed, allowing proper quality assurance programs to be implemented.

The speed of construction has assumed an increasing importance in reducing the overall cost of a building project. The rapid and efficient method of stripping, raising, and positioning of formwork through the self-raising forms has increased the rate of construc­tion of the service core and lift shafts. Traditional forming tech­niques typically take four to five days to complete a 30,000 ft2 (2800 m2) floor, but with self-raising forms only two to three days are needed to complete the same floor. The stripping and raising of the forms from one floor to the next typically takes 20 to 30 min­utes. This efficient method of forming results in direct cost reduc­tion is due to reductions in construction time.

The labor force needed for the stripping, raising, and positioning of the forms is greatly reduced from the conventional formwork systems. Typically 10 to 12 workers are needed as compared to 20 or more for conventional or jump forming. The work crews are split into groups of two to three workers, which are then assigned a particular task. The workers usually work in a clockwise manner around the building with one crew working ahead of the others. The self-raising forms are prefabricated and remain connected un­til the top of the building is reached. This reduces the need to assemble and disassemble the forms after each lift.

With the use of built in hydraulic rams to raise the formwork system, the requirements for crane time and crane attendants is greatly reduced. Craning operations are only needed to initially place the formwork system and to disassemble the system once the forming is complete. This allows the crane to be used else­where on the job site, thus possibly increasing productivity else­where since the crane is rarely needed with the self-raising forms.

Cost

The cost for any formwork system is important in controlling the cost of the project. The initial cost of purchasing or renting a form­work system and the number of reuses may determine which system to use. The initial costs for self-raising forms are typically in the range of $1 to $2 million a set. A set of forms are able to pour one complete floor every two to three days. From this initial cost, three-quarters of the cost is toward the purchase of the forms, while the remaining quarter is used to rent the raising equipment. To make the purchase and rental of these forms feasible, the project must have a minimum of 20 floors. Typically a set of forms can be reused for a minimum of 55 floors with little rehabilitation to the forms. The raising equipment on the other hand can be used for three to four jobs with only the simple re­placement of bolts and nuts before each new job. Therefore, with the number of reuses in both the forms and raising equipment, the self-raising forms can be used from job to job for increased savings.

The work cycle is repeated until the desired height is reached. When the form reaches the top of the building it is normally disas­sembled in units and lowered by tower crane, starting with the working scaffold and the outside roll-back forms. Anchorage in­serts hold the inner forms and rams, while the upper beams and platforms are disassembled and lowered. Finally the inner forms and rams are detached and lowered.

Once the concrete has been cast, the ram support bracket is de­tached from the anchors in the first lift, and the ram is retracted. After retraction, the ram is then reattached to anchors in the lift above.

Stripping

Once the concrete has gained sufficient strength, the stripping process is once again done.

The form support brackets are attached to the anchors installed in the concrete near the top of the previous lift. With the outside forms rolled back, installation of rebar, embedments, and box-outs can proceed. Once completed, the outside forms are rolled into position, and the taper ties are installed ready for the next pour. The hydraulic ram usually remains extended and attached to the second lift below during the casting of concrete.

The system is ready to be raised to the next level once the system is supported on the ram and the ram support brackets. A typical work cycle can be seen in Figure 8.3.

Form Raising

Through a central console, the entire form is raised by the hydrau­lic rams to the next elevation. Individual ram controls used to do the fine grading, levels the entire platform, adjusting the

plumbness of the core. The necessary cleaning and oiling of the interior form is done at this time.

Once the forms and ties have been inspected and oiled, the forms are ready for concrete. Minimum concrete strength must be at­tained, usually through cylinder strength tests, before the forms can be raised.

Stripping

Once the concrete in the second lift has attained sufficient strength, the taper ties and other embedded items are removed. The outside forms are then rolled back on the mounted trolleys.

The interior forms, which are attached to the top grid beam, allow for small movement and are released at this time.

The form support bracket at the bottom is also released at this time, leaving the system supported on the rams and the ram support brackets.

Before the second lift can begin, the necessary anchors must be in place and the first lift must be of sufficient strength. Tower cranes are used to assemble and place the form system on the starter wall.

Form and Ram Placement

The inner form and raising rams are attached to the anchors when the first lift has gained sufficient strength. Once the inner form is in place, the outside form is rolled back on the trolley system, permitting the cleaning and oiling of forms, installation of form liners, steel reinforcement, embedments, and box-outs. The out­side forms are then rolled into position, allowing the installation of taper ties.

The self-raising forms can be initially placed on the foundation mat, but the process works better when a starter wall that takes the
shape of the wall foot print has initially been cast. The starter wall should be about 6 ft (1.83 m) tall and have all the necessary anchor­age in place, ready to support the forms and rams for the next level of casting.