A detailed overview of large formwork construction, summarizing and resolving common problems.

Undoubtedly, large formwork is widely used due to its advantages such as high construction efficiency, low labor intensity, high turnover rate, and convenient installation and dismantling. Despite these numerous advantages, inadequate handover and common defects can still lead to substandard concrete quality during construction. To avoid quality problems, what issues should be considered during large formwork construction? We have compiled a list of potential problems to share with you.

I. Transportation, storage and assembly of large formwork

(1) The cylindrical mold can be transported as a whole by trailer, or it can be disassembled into flat molds and transported horizontally by trailer stacking. When stacking flat molds, the dunnage must be aligned vertically and tied securely.

(2) When storing flat formwork, the self-stabilizing angle required by the local conditions should be met. Two large formwork panels should be stored face to face. For long-term storage of formwork, the formwork should be replaced as a whole. Large formwork stored on the construction floor must have reliable anti-tipping measures. It must not be placed along the outer wall edge and must be stored perpendicular to the outer wall.

(3) Large templates without support or with insufficient self-stabilizing angle should be stored on a dedicated stacking rack or stacked flat. They should not be leaned against other templates or objects to prevent slippage and tipping.

(4) When assembling or dismantling the formwork, the personnel in charge of the dismantling and hooking must stand in a safe and reliable place before they can operate. It is strictly forbidden for personnel to be lifted along with the large formwork.

II. Installation of the large template

Installation procedure: Inspection → Cleaning → Hoisting → Inspection → Sealing → Adjustment.

Installation details:

(1) To ensure the quality of concrete, the installation quality of the formwork should be guaranteed to be flawless. Therefore, before the formwork is installed on the wall, the condition of the sponge strips pasted at the joints of the formwork should be carefully checked; whether the surface is clean; whether the formwork openings are deformed, and if so, they should be leveled; whether the reinforcing bars are misaligned and affect the placement of the formwork; and whether the reinforcing bars, embedded pipes, door openings, etc., affect the passage of tie bolts, and should be dealt with in advance.

(2) After checking the formwork support, before closing the formwork, check whether the wall reinforcement, water and electricity pipelines, embedded parts, door and window opening formwork and through-wall bolts are missing, whether their positions are accurate, whether they are firmly installed, and remove debris from the formwork. Once the concrete is poured, if you want to repair it, you have to destroy the problematic concrete and re-pour it, so you have to do the preliminary work well.

(3) When installing large formwork, it should be hoisted into place in the order of formwork number. Before hoisting the formwork, check whether the hoisting ropes, clamps and lifting rings on each formwork are intact and effective, and remove all temporary supports first. Hoisting can only be carried out after the inspection is completed. When using combined flat formwork, first install the formwork on one side of the horizontal wall. After it is vertically suspended, insert the through-wall bolts coated with release agent, then install the formwork on the other side, and finally install the formwork of the longitudinal wall.

(4) Check the installation accuracy of the formwork at any time to ensure that the formwork is installed in an accurate position and that the facade is vertical. The formwork that is installed first can be checked with a 2m long straightedge, and the formwork installed later can be checked for verticality with a double cross straightedge on the back of the formwork. When installing the external formwork of a fully cast-in-place structure, the cantilever beam must be fixed and the position adjusted accurately before the hook can be removed. After the external formwork is installed, the pins must be inserted and the bolts tightened immediately. Operators installing the external floor slab must wear safety belts.

(5) When filling the gaps of large formwork, after the formwork is fixed, the gaps around the formwork can be filled with 1:3 cement mortar to prevent the wall from having a fence-like structure.

(6) When calibrating the large formwork, the top of the formwork should be fixed with clamps after the calibration is qualified, and the through-wall bolts should be tightened appropriately. After installation, the top edge of the formwork of each wall should be straightened, and the fasteners and bolts should be checked to see if they are tight, the joints are tight, the wall thickness is appropriate, and the connection with the outer wall panel is tight. Concrete can only be poured after the inspection is qualified.

III. Removal of Large Formwork

Demolition sequence: Remove longitudinal wall formwork → Remove transverse wall formwork → Remove corner formwork and doorway formwork → Demolding and lifting.

Demolition details:

(1) For single large formwork, the longitudinal wall formwork should be removed first, followed by the transverse wall formwork and the door opening formwork. For each large formwork, the connecting parts such as turnbuckles, top clips, and through-wall bolts should be removed first and placed in the toolbox. Then, the anchor bolts should be loosened to gradually separate the formwork from the wall.

(2) When dismantling the hinged cylindrical formwork, after removing the connecting parts, rotate the demolding device to lift the formwork off the wall. The cylindrical formwork is heavy and close to the wall on all sides. When lifting it out, the hooks must be firmly attached, the lifting must be stable, and it must not sway to prevent damage to the wall.

(3) When removing the corner formwork, first remove the concrete from the outside of the formwork, and then use a crowbar to pry it out from the bottom. When removing the door opening formwork, prevent the concrete of the door opening lintel from cracking.

(4) Remove the protruding concrete in time and repair the recessed parts or corners with cement mortar in time. For door openings with a span of more than 1m, add support or delay demolding after demolding.

(5) Before lifting the large formwork after demolding, carefully check whether all the through-wall bolts have been removed. Lifting can only be carried out after all bolts have been removed and the formwork has been completely separated from the wall. When removing the outer formwork, the hook should be hung first and the rope tightened before removing the pins and supports.

IV. Prevention of Common Quality Defects

1. Joint defects

Gaps between template panels or errors during hoisting and installation can lead to joint defects. To avoid these defects, gaps between template panels should be minimized, and the width of the assembly adjustment joint between template panels should be appropriately increased. After hoisting and alignment, a joint plate should be installed and wedges driven in. This process aligns the two template panels to the same plane and simultaneously forces the convex rubber strip on the joint plate into the adjustment joint, achieving both leveling and sealing of the templates.

2. Concrete damage

During demolding, concrete can easily be damaged. Installation and other possible reasons can make demolding large formwork difficult. Prying at the top or striking the formwork with a sledgehammer can easily damage it and affect the quality of the concrete.

3. Large formwork hoisting

(1) When hoisting large formwork, it is important not to collide with the wall to prevent cracks in the wall. Large formwork should be moved directly on the floor without touching the ground to reduce the time occupied by the tower crane.

(2) The hook should be perpendicular to the formwork and should not be lifted at an angle to prevent collision with adjacent formwork and walls. When unhooking, do not let go of the hook. You can only let go of the hook when it is lifted above your head. You can only drive or swing the boom when it is above the allowable height above the obstacle.

4. Post-mold removal work

(1) After the large formwork and its supporting formwork are removed, the cement slurry on the surface of the formwork should be cleaned in time and a release agent should be applied for future use. Apply the release agent to the floor to prevent it from splashing onto the reinforcing steel. Failure to clean the large formwork in time may reduce its quality and affect its turnover rate.

(2) After the large formwork is removed, the residual concrete on the formwork should be removed in time and a release agent should be applied. When cleaning and applying the release agent, the formwork should be temporarily fixed. A 50-60cm wide walkway should be left between the formworks with opposite sides. The formwork should be fixed with tie rods at the top.