Short Answer:

Construction joints in RCC structures are provided when concrete placing is stopped and continued later. These joints ensure proper bonding between the old and new concrete without affecting the structural strength. They are carefully planned and located at suitable positions where the stress is minimum.

To provide construction joints, the surface of the old concrete is cleaned, roughened, and moistened before placing the new layer. A bonding agent or cement slurry may also be applied. Proper joint preparation helps in load transfer and prevents cracks or separation between different concrete pours.

Detailed Explanation:

Construction joints provided in RCC structures

Construction joints are intentional separations between different concrete placements in RCC structures. They are used when the concrete pouring cannot be completed in a single stretch, such as in large slabs, beams, or columns. These joints allow work to be resumed without affecting the integrity or strength of the structure.

The main goal of providing construction joints is to ensure continuity and proper load transfer between the already hardened concrete and the fresh concrete. They are different from expansion and contraction joints, which are meant to allow movement. Construction joints must be properly planned, located, and treated to avoid weak zones or cracks.

How Construction Joints Are Provided

1. Planned Location of Joints
   • Construction joints are located at points of minimum shear and bending stress.
   • For slabs, they are usually provided at the middle of spans (between supports).
   • For beams, they are placed at points of minimum moment, generally near one-third the span length.
   • For columns, joints are provided at the base or just above the floor level.
2. Surface Preparation
   • Before placing new concrete, the surface of the old concrete must be prepared properly.
   • It is roughened to expose the aggregates, cleaned of dust, laitance, and loose particles.
   • Surface is then thoroughly moistened but kept free from standing water.
3. Use of Bonding Agent or Slurry
   • A bonding agent such as epoxy or polymer resin may be applied.
   • Alternatively, cement slurry or grout is brushed onto the old concrete just before placing new concrete.
4. Proper Compaction and Curing
   • The fresh concrete is compacted well to avoid air voids at the joint.
   • Proper curing is done after joint concreting to ensure uniform strength development.
5. Use of Dowels or Shear Keys (If Needed)
   • In some structural members, dowels or shear keys are provided to ensure mechanical interlock.
   • These are mostly used in pavements or slabs to prevent relative movement across the joint.
6. Code Guidelines
   • IS 456:2000 provides guidelines for the location and preparation of construction joints.
   • It recommends that construction joints should be made only with the approval of the engineer-in-charge and after careful planning.
7. Avoiding Weak Zones
   • Construction joints should never be located at points of high stress or where bending moments are maximum.
   • Proper alignment of reinforcement across the joint should be ensured to maintain continuity.

Conclusion:

Construction joints in RCC structures are essential when work is interrupted. They are carefully located and treated to ensure a strong bond between old and new concrete. Good surface preparation, use of bonding agents, and proper planning help in maintaining structural integrity and preventing cracks or failure at the joints.