Comparison of Post-Installed Anchor Types (ACI 318)

1. Overview

This summary compares different types of post-installed concrete anchors, including adhesive, expansion, undercut, and cast-in-place types. Although they follow the same design framework under ACI 318, their performance, failure modes, and required checks vary.

2. Anchor Type Comparison

Anchor Type	Installation Mechanism	Load Transfer Mechanism	Design Implications
Cast-in-Place	Before concrete sets	Mechanical embedment (headed, hooked)	Often governed by steel strength or breakout. Typically well-confined.
Expansion Anchor	Torque-tightened into drilled hole	Friction + bearing (wedge expansion)	Sensitive to edge distance and spacing. Requires torque verification.
Undercut Anchor	Mechanically interlocked cut in concrete	Mechanical interlock	High resistance to pullout. Often better seismic performance.
Adhesive Anchor	Injected adhesive in drilled hole	Bonded to concrete surface	Requires bond strength testing. Sensitive to installation and curing.

3. Key Design Differences by Type

All anchor types use the same failure mode equations in ACI 318, including steel failure, concrete breakout, pryout, and pullout. However, these key factors differ:

• Strength Reduction Factor (φ): May vary depending on testing and ductility of the anchor.

• ψ Modifiers: Adhesive anchors include installation angle, temperature, and hole cleaning effects.

• Required Checks: Adhesive anchors require bond strength data. Expansion anchors require torque testing.

• Seismic Qualification: Must meet ACI 355.2 (mechanical) or ACI 355.4 (adhesive) for seismic design.

4. Example Comparison (Same Inputs)

Anchor Size: 16 mm | hef: 100 mm | f'c = 28 MPa

• Expansion Anchor:

  - Use N_cb, V_cb (empirical), steel strength

  - Sensitive to edge and spacing

  - φ typically = 0.75

• Adhesive Anchor:

  - Use bond strength N_b from data sheet

  - Installation controls quality heavily

  - φ often = 0.65

5. Code References

• ACI 318-19 Chapter 17 – Anchor Design

• ACI 355.2 – Qualification for Mechanical Anchors

• ACI 355.4 / ICC-ES AC308 – Adhesive Anchor Testing and Qualification