Short Answer:
Formwork for RCC members is designed to shape and support freshly poured concrete until it gains enough strength to support itself. It must be strong, stable, and leak-proof to hold the weight of the concrete and reinforcement without bending or shifting.
The design of formwork includes deciding the type of material (like wood, steel, or plywood), dimensions, spacing of supports, and method of assembly. It must also ensure easy removal after setting, safety for workers, and reusability. Properly designed formwork ensures quality finish, accurate dimensions, and structural stability.
Detailed Explanation:
Formwork designed for RCC members
Formwork is the temporary structure used to mold concrete into the required shape and hold it in position until it sets and becomes strong. In RCC (Reinforced Cement Concrete) construction, formwork plays a critical role because it supports both the concrete and the reinforcement bars during the casting stage. Proper design of formwork ensures strength, safety, surface quality, and economy of the structure.
The formwork must be designed carefully based on the shape, size, and position of the RCC member—whether it’s a slab, beam, column, wall, or footing. It should also consider the concrete pressure, construction load, time of removal, and surface finish required. The design should meet the requirements laid out in IS 14687 and IS 456:2000.
How Formwork is Designed for RCC Members
- Understanding the Structural Requirements
- The designer must know the size, type, and position of the RCC member (e.g., beam, column, slab).
- The weight of fresh concrete, reinforcement, and live loads during casting is calculated.
- Selection of Formwork Material
- Common materials include timber, plywood, steel, aluminum, and plastic.
- Material is chosen based on required finish, reuse cycles, and budget.
- Steel formwork is more durable and gives smooth finishes, while timber is easier to shape for custom work.
- Design of Formwork Components
- Sheathing (face panels): Holds the concrete surface.
- Bearers and runners: Transfer loads from the panels to the supports.
- Vertical supports (props or soldiers): Hold the entire setup upright.
- Load Calculations
- The formwork is designed to resist dead loads (self-weight and wet concrete) and live loads (workers and equipment).
- Lateral pressure from wet concrete must be calculated for each type of RCC element.
- Spacing and Support Arrangement
- Proper spacing of vertical and horizontal members is necessary to prevent deflection or collapse.
- The span and thickness of slabs or beams affect how closely props and beams are placed.
- Fixing and Assembly
- All joints should be tight to prevent leakage of cement slurry.
- Easy-to-assemble systems help save time and reduce labor.
- Safety and Accessibility
- Platforms, ladders, and bracing must be included for worker safety.
- Adequate bracing prevents tilting and collapse of formwork under load.
- Stripping and Reuse
- Formwork should allow easy removal without damaging concrete.
- Stripping time depends on member type (e.g., 24–48 hours for vertical members, longer for slabs/beams).
- Reusability improves economy, so materials are selected for multiple uses where possible.
- Surface Finish and Alignment
- Panels must be straight, clean, and coated (e.g., with oil) to ensure smooth concrete surface and easy removal.
- Proper leveling and alignment ensure dimensional accuracy and reduce finishing work.
- Code Compliance
- IS 456:2000 specifies requirements for formwork strength, removal time, and tolerances.
- IS 14687 provides guidelines for modular and system formwork design.
Conclusion:
Formwork for RCC members is designed by calculating loads, selecting suitable materials, and ensuring proper support and safety. A well-designed formwork ensures structural strength, dimensional accuracy, and surface finish of RCC elements. Following correct design practices as per IS codes ensures safety, quality, and cost-effectiveness in RCC construction.