The primary distinction between concrete breakout and pull out lies in their fundamental failure mechanisms: concrete breakout involves the concrete itself failing in shear around the fastener, whereas pull out describes the fastener displacing or extracting the concrete.
When designing and installing fasteners in concrete, it's crucial to anticipate various ways in which the connection can fail. Two common and distinct failure modes are concrete breakout and pull out, each characterized by different load responses and visual cues.
Concrete Breakout
According to the provided reference, Concrete Breakout "occurs when the concrete surrounding the fastener fails in shear." This means that under applied loads (typically tensile or shear), a cone or wedge-shaped section of concrete breaks away from the main concrete element, taking the fastener with it.
- Nature of Failure: A larger, visible section of concrete detaches, often in the shape of a cone (for tensile loads) or a half-cone/wedge (for shear loads).
- Cause: Occurs when the strength of the concrete itself is insufficient to resist the forces transmitted by the fastener, leading to a shear failure plane within the concrete mass.
- Factors: Influenced by concrete strength, embedment depth, edge distances, spacing between fasteners, and the type of applied load.
Pull Out (Related to Concrete Pryout)
While "pull out" isn't always explicitly defined as a distinct failure mode in all concrete design standards, it generally refers to the fastener's inability to remain anchored, leading to its extraction from the concrete. The provided reference describes Concrete Pryout as occurring "when the fastener pulls out or displaces the concrete around it." This description is highly relevant to the concept of a pull-out failure, where the fastener effectively tears away or displaces the surrounding concrete due to inadequate embedment, edge distances, or concrete strength.
- Nature of Failure: The fastener itself moves relative to the concrete, often accompanied by localized crushing, displacement, or spalling directly around the fastener's shaft or head. The damage is typically more confined to the immediate vicinity of the fastener.
- Cause: The anchorage mechanism of the fastener (e.g., friction, mechanical interlock, bond) fails to hold it in place, allowing the fastener to extract or "pull out" of the concrete.
- Factors: Primarily influenced by the fastener's embedment depth, diameter, head/thread geometry, the concrete's compressive strength, and the effectiveness of the installation.
Key Differences Summarized
To highlight the distinctions clearly, here's a comparison:
| Feature | Concrete Breakout | Pull Out (Related to Concrete Pryout) |
|---|---|---|
| Failure Mechanism | The concrete surrounding the fastener fails in shear | The fastener displaces or extracts the concrete around it |
| Primary Failure | Failure of the concrete itself | Failure of the fastener's anchorage within the concrete |
| Visual Appearance | A cone or wedge of concrete breaks away | Localized crushing, spalling, or the fastener extracting |
| Extent of Damage | Often involves a larger area of concrete | Confined to the immediate vicinity of the fastener |
| Governing Factors | Concrete strength, embedment depth, edge distance, spacing | Fastener type, embedment depth, concrete strength, installation |
Practical Implications
Understanding these differences is crucial for safe and efficient design of concrete connections. Designers must consider both potential failure modes, ensuring that the fastener system is robust enough to prevent either concrete breakout or pull out under anticipated loads. Proper embedment depth, adequate edge and spacing distances, and appropriate fastener selection are key to preventing these failures.
