Concrete structures rely on formwork to achieve accurate dimensions and desired shapes. Formwork, acting as a temporary mold, plays a crucial role in ensuring the strength and uniformity of structural parts. The accessories used in formwork are equally vital, and among them, form ties are indispensable. These small yet essential components hold the formwork in place against the lateral pressure of freshly poured concrete. This article delves into the role of form ties, their types, and how they contribute to successful construction projects.
What is a Form Tie?
A form tie is a formwork accessory designed to maintain structural integrity during concrete placement. It holds the opposing sides of the formwork together, countering the lateral pressure of wet concrete and ensuring uniform wall thickness.
Components of a Form Tie
- Internal Tension Member: The central element of the tie, which resists tensile forces.
- External Holding Device: Anchors the tie to the formwork.
Form ties come in various types and configurations, with some designed to be removed after the concrete has hardened and others remaining embedded in the structure. Selecting the appropriate form tie depends on the project’s structural and aesthetic requirements.
Types of Form Ties
Form ties are broadly categorized into two types:
- Continuous Single Member Type
- Internal Disconnecting Type
1. Continuous Single Member Type
Also known as one-piece ties, these ties are single units designed to provide lateral support to formwork. Depending on their design, they may be entirely removable, partially removable, or broken off after the concrete hardens. Below are the different types of continuous single member ties:
Flat Tie
- Description: A flat steel plate with holes on either end for locking using wedges and pins.
- Applications: Prefabricated modular panels and light-duty projects.
- Limitations: These are non-removable and must be broken after concrete hardens. A notch near the ends facilitates easy breaking.
- Advantages: Simple and cost-effective for single-use applications.
Loop Tie
- Description: A single metal rod with looped ends for locking.
- Applications: Light-duty projects similar to flat ties.
- Limitations: Non-removable; includes crimped sections to prevent twisting during removal.
- Advantages: Offers slightly more flexibility than flat ties due to the looped ends.
Fiberglass Tie
- Description: Composed of glass fibers and resin, these ties can be easily cut to desired lengths.
- Applications: Projects requiring high load capacity and aesthetic appeal.
- Removal: The ties are cut flush with the concrete surface after curing, leaving no visible marks.
- Advantages: Lightweight, durable, and capable of handling greater loads than flat or loop ties.
Snap Tie
- Description: A rod with enlarged buttons or loops at each end, often combined with cones or washers for water resistance.
- Applications: Medium-duty projects.
- Limitations: Non-removable; weaker cross-sections for easy snapping.
- Advantages: Ensures uniform wall spacing and reduces water leakage risks.
Taper Tie
- Description: A tapered rod with threaded ends secured using wing nuts.
- Applications: Projects requiring precise wall thickness and reusable components.
- Removal: Can be removed entirely after greasing.
- Advantages: Reusable and suitable for high-strength applications.
Threaded Tie
- Description: A fully threaded rod secured with nuts and washers.
- Applications: Heavy-duty projects with high load requirements.
- Removal: Can be removed if encased in a plastic sleeve.
- Advantages: Durable, reusable, and capable of supporting significant loads.
2. Internal Disconnecting Type
These ties include threaded ends that allow external components to be detached without breaking or pulling the inner member. They are further categorized as follows:
She-Bolt Tie
- Description: Consists of an inner tie rod with threaded ends and two she-bolts.
- Applications: Medium- to heavy-duty applications.
- Removal: Inner tie rod remains embedded, while she-bolts are reusable.
- Advantages: Easy to assemble and disassemble, offering reusability and adaptability.
Coil Tie
- Description: Composed of an inner tension member (with helical coils) and external bolts.
- Applications: Medium- and heavy-duty works.
- Variations:
- Two-strut coil for medium loads.
- Four-strut coil for higher loads.
- Removal: External bolts are reusable, while the inner member stays embedded.
- Advantages: Offers flexibility for thick walls and complex designs.
Load Capacity of Form Ties
The load capacity of form ties varies based on their type and material. Below is a table summarizing the safe load ranges:
| Form Tie Type | Safe Load Carrying Capacity (lbs.) |
|---|---|
| Flat Tie | 1,500 – 3,000 |
| Loop Tie | 2,250 – 3,750 |
| Fiberglass Tie | 3,000 – 7,500 |
| Snap Tie | 2,250 – 3,350 |
| Taper Tie | 3,000 – 43,700 |
| Threaded Tie | 10,000 – 32,500 |
| She-Bolt Tie | 3,000 – 38,000 |
| Coil Tie (2-Strut) | 4,500 – 13,500 |
| Coil Tie (4-Strut) | 7,500 – 27,000 |
The selection of form ties should be based on the anticipated load, wall thickness, and project specifications.
Conclusion
Form ties are critical components in ensuring the stability and accuracy of concrete structures. From lightweight flat ties to robust tapered and threaded ties, there are options to suit a range of project requirements. Choosing the right form tie involves understanding the load demands, wall thickness, and the tie’s reusability. Proper installation and removal methods further ensure the integrity of the concrete and the efficiency of the construction process.
By leveraging the appropriate form tie system, construction professionals can achieve both functional and aesthetic success in their projects.