What are Construction Joints in Concrete ?
Construction Joints are placed when the mass concreting works are done and cannot be completed on a single stretch. The interface of the past days hard concrete and present days fresh concrete is to be properly done, So as the bonding should be perfect for load transferring and avoiding water leakage. This interface joint is known as ‘Construction Joint’ or ‘Day work Joint’.
Location of Construction Joint
The location of a construction joint is important for the durability and effectiveness load transfer. The following are the points to be remembered while locating the joints.
- Construction joints parallel to the slab span can be placed anywhere except those locations in T- beam construction that rely on a portion of this slab to act with the beam in resisting flexure.
- For slabs and beams the construction joints are located at mid-span or middle third of the span.
- It is convenient to locate horizontal joints at the floor line or in line with window sills.
- For designing concrete members for lateral forces, special design treatment of construction joints may be required.
- In walls, a horizontal length of placement in excess of 40 feet is not normally recommended.
- Joints and Girders shall be at offset of minimum distance of two times the width of intersecting beams.
- In the design of hydraulic structures, construction joints are usually spaced at shorter intervals than in non hydraulic structures to reduce shrinkage and temperature stresses.
- If the placing of concrete is involuntarily stopped for a time longer than the initial setting time of the concrete the old surface is to be considered as a construction joint and treated as such before casting is resumed. However from the point of view of strength of the structure, it is desirable to position construction joints at points of minimum shear.
Quality of Construction Joints
Construction joint quality is significantly influenced by the precision in the concrete placement. Maximum bond strength and water tightness are achieved by using quality concrete that has a low slump. The concrete should allow fair amount of placement and consolidation.
Segregation and bleeding of concrete produces weak surface of low bonding behavior. The concrete surface produced should be clean and structurally reliable. Rough aggregates, protruding portions and slight indentation are not useful or recommended. Surface retardants are frequently utilized to attain a suitable surface.
Advantages of Construction Joints
The advantages of placing construction joint in the concrete are as follows :
- Free horizontal movement of the floor.
- limited vertical movement.
- Optimal load transfer.
- Prevention of cracks formed due to thermal variations.
- Proper joining between two concrete placements.