Concrete, while strong and durable, is prone to cracking due to shrinkage, temperature changes, and structural movements. To prevent unsightly and potentially harmful cracks, joints are strategically introduced into concrete structures. These joints allow controlled movement and accommodate stresses that develop over time.
There are two primary types of joints used in concrete construction: construction joints and movement/expansion joints . Each type serves a specific purpose and must be constructed according to strict specifications to ensure the long-term performance and integrity of the structure.
Types and Specifications of Joints
A. Construction Joints
1. Definition
A construction joint is introduced at a point where concreting stops temporarily. It ensures continuity between adjacent placements without allowing relative movement. Proper preparation is essential to achieve good bonding between old and new concrete.
2. Positioning of Construction Joints
Construction joints should not compromise the strength of the structure. Their placement must align with formwork panel sizes to maintain regularity and uniformity in appearance.
3. Forming Exposed Horizontal and Inclined Joints
To create neat and straight lines on exposed surfaces, battens of approved dimensions are used. This helps maintain aesthetic appeal while ensuring functional integrity.
4. Surface Preparation
Before placing fresh concrete, the surface of the existing concrete must be thoroughly prepared:
- Remove laitance (the weak, cement-rich layer on the surface)
- Expose coarse aggregate
- Methods include water jetting, brushing with wire brushes, sandblasting, needle gunning, or using a scaling hammer
This treatment should ideally occur when the concrete has set but not hardened.
5. Alternative Method Using Retarder
A chemical retarder can be applied to delay setting of the surface layer. Afterward, the softened laitance is washed away with a water jet. The surface is then cleaned with compressed air and slightly wetted before placing new concrete.
6. Vertical Construction Joints
Vertical joints must avoid feather-edging by forming against stop-end forms. Kickers or stub-ends should be at least 70 mm high. Surface preparation follows the same method as for horizontal joints.
7. Inspection Before Resuming Concreting
No concreting should commence until the location and condition of all construction joints—both horizontal and vertical—are inspected and approved by the engineer.
B. Movement/Expansion Joints
1. Definition
Movement or expansion joints are designed to allow relative movement between adjacent parts of a structure. They accommodate thermal expansion, contraction, settlement, and seismic activity while maintaining watertightness.
2. Handling and Installation of Waterstops
Waterstops are critical components in movement joints. On-site jointing should primarily use butt joints in straight runs. For intersections or directional changes, special pieces may be made but must pass required tests by the engineer.
3. Installation Requirements
Waterstops must be fully supported within the formwork, free from nails, and clear of reinforcement. Any damaged waterstops must be removed and replaced immediately.
4. Concreting Around Waterstops
Great care must be taken during concrete placement and compaction to prevent bending or distortion of the waterstops.
5. Treatment of Set Concrete Surfaces
Where specified, the surface of set concrete in a movement joint should be painted with two coats of bituminous paint. New concrete can only be placed once the paint has dried completely.
6. Formation of Expansion Joints
Expansion joints are formed using preformed joint filler materials approved for the specific application.
7. Caulking Grooves
Caulking grooves must be provided as shown on drawings. Prior to caulking:
- Clean the groove with a wire brush
- Remove loose material using compressed air
- Allow the groove to dry
- Prime and apply an approved jointing compound
8. Painting at Caulked Joints
To ensure compatibility and sealing effectiveness, two coats of paint with the same base as the caulking compound should be applied to the face of the caulking strip and 50 mm of adjacent concrete on each side.
Conclusion
Proper design and execution of joints are crucial to the durability and functionality of concrete structures. Whether it’s a construction joint , which maintains structural continuity, or a movement joint , which accommodates dynamic forces, adherence to specifications ensures the structure remains safe, crack-free, and serviceable for years.