Removal of formwork

1. Safety during formwork removal

To ensure stability during the construction phase, in addition to the other construction provisions, the following recommendations apply:
Formwork may be removed at an early age once the concrete compressive strength is greater than 1.5 MPa.
The early-age strength and stability of the external concrete leaf are ensured in particular by the use of plastic connectors, which make it possible to:

• Limit the slenderness of the external concrete leaf to values below those encountered in conventional walls. With plastic connectors distributed at a minimum rate of 1 per m², the effective buckling height of the external leaf is 1 m. For an 8 cm thick external leaf, the resulting slenderness ratio is 13, which is lower than that of a conventional wall.
• Secure the external concrete leaf to the inner wall and thereby resist out-of-plane forces before the stainless-steel tie bars provide their permanent structural connection. Appendix XX presents the verification showing that resistance to wind loads after formwork removal is ensured when the concrete compressive strength is greater than 1.5 MPa.

The plastic connectors therefore play an essential role in the early-age stability of the external concrete leaf and in ensuring worker safety.

Installation of working and safety platforms

The concrete must achieve a design compressive strength of 13 MPa when the working and safety platforms for the upper storey are installed.
The equivalent of an ST25C welded reinforcement mesh, i.e. 2.57 cm²/m in each direction, must be positioned at the mid-depth of the external concrete leaf. This applies for a support load from the working-platform props of 5 kN/m, an opening ratio of 50% and a maximum height of 180 cm between the spacers located immediately below and above the platform support bars. A supporting calculation is provided in the Appendix, detailing how the construction loads generated by the working platforms are assessed and applied to the concrete leaves.
During the temporary construction stage, the stability of the external concrete leaf is ensured by the rigid connection provided by the plastic spacers.

• Install a spacer to form the working-platform support recess when casting the Level 1 gable wall.

• After removing the formwork from the Level 1 gable wall, install the platform support hooks in the recesses at the top of the wall.

• Construct the Level 2 slab.
• Sling the gable working platform.

• Remove the support hooks.

• Raise the platform. Retrieve the support hooks and attach their retaining cables to the platform.

• Reposition the platform on the support hooks at the upper storey.

• Form the gable wall, cast the concrete and repeat the cycle at the next level.

Comments: Always refer to the requirements and instructions issued by the manufacturers of the working platforms and formwork systems. A designated lifting supervisor must give instructions to the crane operator.

2. Removal of formwork

• Install the stabilising blocks for the formwork gang to be used in the next cycle.
• Loosen the formwork end strips.
• Loosen the tie rods.
• Uncouple the formwork panels if required by the available lifting equipment.
• Loosen the base props of the formwork panels.
• The first formwork gang to be stripped is the one that was closed last.
• When a pair of opposing formwork gangs is stabilised using adjustable slider props, stripping and handling are carried out using two-leg slings of equal length. Maximum lifting capacity: 3 t per leg.
• When a pair of opposing formwork gangs is stabilised using a self-stabilising system, stripping and handling are carried out using a spreader lifting beam.
• Remove one external formwork panel first, secure the wall, and only then remove the next panel.

• Ensure that the cast wall has been completely separated from the formwork:

• Ensure that the concrete wall is fully stabilised before the formwork is completely removed.
• Remove the formwork from blockouts and recesses.

3. Behaviour under construction loads

The wall constructed using the GBE System is modelled as a beam with dimensions of 1000 x 80 mm², supported at its base and at the positions of the plastic spacers.
Two configurations are studied in order to determine the maximum bending moment generated by the support of the cantilever working platforms during the construction stage:

• Case A - cantilever working platform supported on a spacer at tie-rod level.
• Case B - cantilever working platform supported on a spacer positioned at the top of the wall.

The clear span is slightly greater in Case B (1.8 m).
The calculation is carried out for concrete with a 7-day design compressive strength F_cd = 13.3 MPa. A safety factor of 1.5 is applied to F_ck = 20 MPa to obtain the design strength.

At ground-floor level, the wall support is considered as a rigid support; from Level 1 upwards, it is modelled as a pinned support. In addition, the horizontal force of 5 kN acting on the concrete leaf is doubled in order to allow for discontinuity in the wall support zone, for example due to a window opening. The force considered in this study is therefore 10 kN.

Fig. 1: Calculation configurations for the external concrete leaf during the construction stage

The calculation data used to determine the loads from the working platforms are taken from the PTE SATECO 2 technical documentation (see Fig. 2 below).
The maximum force applied to the plastic spacers during the temporary construction stage is approximately 12.3 kN (see Fig. 3). The maximum resistance obtained during mechanical testing of the spacers was N_max = 32 kN (see Section 3.1 of the GBE System mechanical test report), giving a design resistance of N_Rd = N_max / 1.5 = 21 kN, which is greater than the applied force of approximately 12.3 kN.

Fig. 2: Working-platform load distribution / Fig. 3: Rx reactions in the external concrete leaf during the construction stage

4. Finishing formwork tie holes

• Remove the end pieces (Figure B - orange) from the spacers, except where the spacers are intended to support the working and safety platforms.
• Protect the wall with Bidim geotextile before installing the working platforms.

• Fill the holes left by the spacers with:
  • Insulation, such as polyurethane foam, to maintain continuity of the thermal insulation.
  • Colour-matched concrete repair mortar on the external concrete leaf.
  • For the external concrete leaf, bonded concrete plugs cast in dedicated moulds to match the colour of the concrete.
  • Mortar or concrete repair mortar on the internal concrete leaf.
• Proceed to the next construction cycle.
• Clean and oil the formwork faces.