A control joint in a wall, particularly in concrete masonry, is a specific type of joint designed to minimize cracking due to shrinkage. Here's a detailed explanation:
Understanding Control Joints
Control joints are crucial for managing stress in walls constructed from materials like concrete masonry units (CMU). These joints are strategically placed to allow for movement within the wall, preventing unsightly and structurally compromising cracks.
Key Features of a Control Joint
- Vertical Orientation: Control joints run vertically along the wall.
- Mortar Filled: The joint is filled with mortar for aesthetic and weatherproofing purposes.
- Bond Breaker: Critically, a bond breaker is installed on one side of the joint. This prevents the mortar from bonding to the masonry unit on that side.
- Tensile Stress Relief: By preventing bonding on one side, the control joint ensures that tensile stress (pulling force) cannot develop across the joint. This concentrated movement at the joint prevents random cracking elsewhere in the wall.
How Control Joints Work
Concrete masonry units (CMUs) shrink over time as they dry and cure. This shrinkage creates tensile stresses within the wall. If these stresses exceed the tensile strength of the masonry, cracking will occur. Control joints act as pre-planned cracks, allowing the wall to relieve these stresses in a controlled manner. Instead of a long, meandering, and potentially structurally weakening crack, the movement is concentrated at the joint.
Example
Imagine a long concrete block wall exposed to varying temperatures. The wall expands and contracts. Without control joints, this expansion and contraction would cause stress to build up, leading to cracking. With strategically placed control joints, the wall can move without cracking the blocks themselves. The movement happens at the joint.
Purpose and Benefits
- Crack Prevention: The primary purpose is to prevent uncontrolled cracking due to material shrinkage.
- Stress Relief: They relieve tensile stresses within the wall.
- Controlled Movement: They allow for controlled movement due to temperature changes or settling.
- Structural Integrity: They help maintain the structural integrity of the wall by preventing random cracks.
- Aesthetics: By controlling where cracks occur, they maintain the aesthetic appearance of the wall.
Installation Considerations
- Placement: Control joints should be placed at intervals determined by engineering calculations, based on the specific materials and environmental conditions. Locations where stress concentrations are likely (e.g., near corners or openings) are common placement points.
- Depth: The joint must extend through the entire thickness of the wall.
- Bond Breaker Material: A flexible, non-bonding material, such as polyethylene foam or specialized joint filler, is used as the bond breaker.
Summary
| Feature | Description |
|---|---|
| Orientation | Vertical |
| Filling | Mortar |
| Bond Breaker | Present on one side of the joint |
| Purpose | To prevent tensile stress from developing across the joint and minimize shrinkage-related cracking. |
| Material | Typically used in concrete masonry walls. |
