What are ties in formwork?
Form Ties - Concrete Society
Form ties, also known as tie bolts, connect the opposing faces of wall formwork to hold back the pressures applied by the fresh concrete. These ties transfer tensile loads between the robust vertical and/or horizontal members linked to the main formwork.
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Below are the principal types of tie rod systems, though not all are available in the UK:
(a) High tensile through ties, reusable ties usually sleeved in the concrete.
(b) High tensile ties with she-bolts, where the tie is lost in the concrete.
(c) Taper ties, reusable through ties not requiring any sleeving in the concrete.
(d) Mild steel ties, generally lost ties.
(e) Coil ties, high tensile wires (2 or 4) welded to a wire coil at each end; these are lost ties with cones at each end serving as spacers.
(f) Snap ties, wire or flat steel lost ties with a stress inducer, allowing the tie's end to be snapped off after concreting; used for building thin walls.
(g) Friction clamp ties, attached clamping devices used with mild steel or high tensile bars.
Types of Form Ties Used in Formwork
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Formwork for concrete structures is crucial for achieving precise dimensions of structural parts, with accessories playing a vital role. Among these, form ties are particularly important. Below, we explore the different types of form ties used in formwork.
What is a Form Tie?
A form tie is a formwork accessory that holds the concrete wall forms securely against the lateral pressure exerted by freshly poured plastic concrete. This ensures equal spacing between the forms, resulting in uniformly thick walls.
A form tie consists of two parts: an internal tension member and an external holding device. Depending on the type of tie, removal may occur after the concrete has fully hardened. Some ties can be removed while others remain embedded. Below are the various types of form ties.
Types of Form Ties
Form ties can be primarily categorized into two types:
- Continuous Single Member type
- Internal Disconnecting type
Continuous Single Member Type
Continuous Single Member ties, or one-piece ties, comprise a single unit. Special holding devices secure these ties when placed in forms. After the concrete hardens, some can be entirely removed, while others are broken off.
Examples of Continuous Single Member ties include:
- Flat Tie
- Loop Tie
- Fiberglass Tie
- Snap Tie
- Taper Tie
- Threaded Tie
1. Flat Tie
- A thin, flat steel plate with holes at each end for locking with a wedge and pin.
- Preferred for prefabricated modular form panels and light-duty purposes.
- One-time use; not removable from hardened concrete.
- Removed by breaking at a predetermined notch.
- Middle holes prevent turning during breaking.
2. Loop Tie
- A single metal bar with looped ends for locking with a wedge and pin.
- Used for prefabricated modular form panels and light-duty purposes.
- Not removable from hardened concrete; contains a weaker cross-section for this reason.
- Crimped in the middle to prevent turning during breaking.
3. Fiberglass Tie
- Made of glass fibers and resin materials.
- Available in longer lengths and easily cut to size.
- Fixed to form panels using a gripper and tie wedge arrangement.
- Cut at the surface level after concrete hardens, making the hole disappear.
- Available in various colors and capable of bearing higher loads than other ties.
4. Snap Tie
- A single rod with enlarged buttons or loops at each end for suitable tie holders.
- Used with plastic cones or metal washers to act as form spreaders and prevent water leakage.
- Not completely removable from hardened concrete; easy to snap due to weaker cross-sections.
- Only snapped after concrete fully hardens to avoid turning inside the wall.
- Certain sections are crimped or weakened to ease snapping.
5. Taper Tie
- Contains a tapered rod, threaded at both ends, locked using wing nuts and washers.
- Standard lengths range from 34 to 60 inches for various wall thicknesses.
- Gradual reduction in diameter from larger to smaller threaded end.
- Completely removable and reusable after hardening.
- Coated with grease for easy removal before installation.
6. Threaded Tie
- A metal rod completely threaded from end to end.
- Fixed to forms using nuts and washers at each end.
- Removable from hardened concrete if surrounded by a plastic sleeve.
- The sleeve bonds with the concrete, allowing the tie to be reused.
- Capable of handling higher loads, ideally using two nuts at each end.
Internal Disconnecting Type
In this type, the tie member has threaded ends, allowing external members to be removed without breaking or pulling. Internal disconnecting ties include:
- She-Bolt Tie
- Coil Tie
1. She-Bolt Tie
- Consists of one inner tie rod and two she-bolt members. Both ends of the inner tie rod are threaded.
- She-bolts feature a normal threaded end and a tapered end with a threaded hole.
- The tapered end is internally placed into the forms, with the tie rod screwed into the she-bolts on both sides.
- External threaded ends of she-bolts secure the forms with a wing nut and washer arrangement.
- After the concrete hardens, the inner tie rod remains in the concrete, and the she-bolts can be removed and reused.
2. Coil Tie
- Consists of an inner tension member and an external holding member.
- The inner tension member has helical coils welded to the ends of two or four steel struts. This part remains in the concrete.
- The external holding member has two bolts screwed into the coils on both ends, supported by a washer and wale arrangement. The bolts can be removed and reused.
- A two-strut coil is used for medium works, and a four-strut coil is used for heavy works.
- Plastic or wood cones can act as form spreaders at the coil ends if needed.
- Two inner members connected by a threaded rod can be used for thicker walls.
Load Capacity Range of Different Form Ties
Safe load capacity ranges for different types of form ties are as follows:
| Form Tie Type | Safe Load Carrying Capacity Range (lbs.) |
|---|---|
| Flat Tie | 1500 – 3000 |
| Loop Tie | 2250 – 3750 |
| Fiberglass Tie | 3000 – 7500 |
| Snap Tie | 2250 – 3350 |
| Taper Tie | 3000 – 43700 |
| Threaded Tie | 10000 – 32500 |
| She-Bolt Tie | 3000 – 38000 |
| Coil Tie | 2-strut: 4500 – 13500 4-strut: 7500 – 27000 |
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