Formwork in Concrete Construction

Formwork, also known as shuttering, plays a pivotal role in concrete construction. It serves as a temporary mold into which fresh concrete is poured, shaped, and hardened to create structural elements. The choice of formwork material and type depends on the design and the specific structural member being constructed. Formwork is essential for achieving precision, ensuring safety, and optimizing construction efficiency.

Types of Formwork Based on Structural Members

Formwork is classified according to the structural elements it supports:

• Slab Formwork: Used for horizontal slabs in floors and roofs.
• Beam Formwork: Designed for linear members like beams.
• Column Formwork: Specialized molds for vertical columns.

Each type is tailored to maintain the integrity of the structure while ensuring easy removal after the concrete sets.

Cost and Reusability of Formwork

Formwork is a significant contributor to construction costs, comprising 20-25% or more of the total expense. Hence, its economic design is crucial. Temporary formwork structures must balance durability and cost-efficiency. Formwork can be categorized based on reusability:

• Reusable Forms (Panel Forms): Designed for multiple uses, reducing material wastage and cost.
• Non-Reusable Forms (Stationary Forms): Typically used for one-time applications in complex or custom structures.

The process of removing formwork, called stripping, must be conducted carefully to avoid damaging the concrete.

Materials for Formwork

Formwork can be made from various materials, each suited to different scales and conditions of construction:

• Timber: Commonly used for its ease of use and availability.
• Plywood: Preferred for smooth finishes and reusability.
• Steel: Ideal for durability and large-scale projects.
• Precast Concrete: Used in specific designs requiring curved or pre-molded shapes.
• Fiberglass: Suitable for intricate designs or curved surfaces.
• Aluminum: Lightweight and reusable, commonly used in modern construction.

Timber Formwork

Timber is a traditional and versatile material for formwork. It should meet the following requirements:

• Well-Seasoned: Free from excessive moisture to prevent warping.
• Lightweight: Easy to handle and assemble.
• Workable: Can be nailed or sawed without splitting.
• Defect-Free: No loose knots or uneven surfaces.

Common Dimensions:

• Sheeting: 25-40 mm thick.
• Joints: 50 x 70 mm to 50 x 150 mm.
• Posts: 75 x 100 mm to 100 x 100 mm.

While timber is cost-effective for small projects, it has limitations, such as susceptibility to warping and limited reusability.

Plywood Formwork

Plywood formwork consists of resin-bonded sheets attached to timber frames. It offers several advantages:

• Smooth Surface: Reduces finishing costs for exposed concrete.
• Larger Panels: Speeds up installation and dismantling, saving labor costs.
• High Reusability: Can be reused 20-25 times, making it more economical than timber.

This formwork is especially suitable for projects requiring a clean, polished surface.

Steel Formwork

Steel formwork is made of thin steel plates reinforced with angles. It is commonly used for repetitive, large-scale projects. Its benefits include:

• Durability: Resistant to warping, shrinking, and moisture.
• Ease of Use: Can be quickly installed and dismantled.
• Superior Finish: Produces smooth, defect-free concrete surfaces.
• Longevity: Suitable for high-reuse scenarios, reducing long-term costs.

Steel formwork is ideal for circular or curved structures, where precision and durability are critical.

Requirements for Good Formwork

Effective formwork must meet several criteria:

1. Strength: Withstand dead and live loads.
2. Rigidity: Maintain its shape under stress.
3. Leak-Proof Joints: Prevent cement grout leakage.
4. Ease of Removal: Permit dismantling without damaging the concrete.
5. Affordability: Use materials that are inexpensive and widely available.
6. Reusability: Enable multiple uses to reduce costs.
7. Lightweight: Simplify transportation and assembly.
8. Stability: Rest on a firm and level base.

Economy in Formwork

To reduce costs:

• Minimize structural variations to enable reuse.
• Optimize designs for slender, economical sections.
• Reduce material wastage by avoiding unnecessary sawing or cutting.
• Use high-quality formwork for a better surface finish, minimizing additional work.

Construction Process for Concrete Formwork

The construction of formwork involves the following steps:

1. Propping and Centering: Ensuring the formwork is firmly supported.
2. Shuttering: Erecting panels and securing them in place.
3. Camber Provision: Adjusting for deflection during concrete pouring.
4. Cleaning and Treatment: Applying release agents to prevent sticking.

Removal of Formwork

The formwork removal process is critical to maintain structural integrity:

• Sequence:
• Vertical walls, beams, and column sides are removed first.
• Slab soffits and heavily loaded supports are removed last.
• Conditions:
• Concrete must attain at least twice the stress it will bear.
• Rapid hardening cement or warm weather may expedite removal.
• Method: Gradual easing to avoid sudden load transfer.

Time Period for Formwork Removal

The time required for formwork removal depends on the structural element:

1. Walls, columns, and vertical sides of beams: 1-2 days.
2. Slabs with props: 3-7 days.
3. Beam soffits: 7-14 days.
4. Heavily loaded elements: 14-21 days.

Comparison of Steel and Timber Formwork

Types of Formwork

1. Timber Formwork: Suitable for small-scale projects.
2. Plywood Formwork: Cost-effective for smooth finishes.
3. Steel Formwork: Ideal for repetitive and large-scale use.

FAQs

What is formwork?

Temporary molds for shaping concrete.

What are the requirements?

Strength, rigidity, reusability, and cost-efficiency.

How much does formwork cost?

Typically 20-25% of the total structure cost.

By understanding the materials, design considerations, and construction processes of formwork, engineers and builders can optimize costs, ensure quality, and deliver durable structures.