Except in small jobs, it is not possible to place concrete in one continuous operation. Joints are also required for functional consideration of the structure. Concrete joints can be classified under following categories:
1. Construction Joints.
2. Expansion Joints.
3. Contraction Joints.
4. Warping Joints.
1. Construction Joints:
These joints are provided where there is a break in construction programme. Concreting operation should be so planned that the work is completed in one operation. If, however, it has to be stopped before completion of entire work, construction joints are provided. Location of construction joints should be such that it interferes minimum with the functional characteristics of the structure. Best locations for construction joints are as following:
i) Beam: Joint may be located at mid-span or over the center of the column in direction at right angles to the length of the beam.
ii) Columns: Joints should be located a few cm below its junction with the beam.
iii) Slab: Joints may be placed at mid span or directly over the center of the beams, at right angles to the slab.
Formwork for construction joint should be placed at the end of each day’s work.
Before new concreting is started, the concrete surface of hardened concrete should be cleaned, roughened, saturated with water, and applied cement grout. This will ensure proper bond between old and new concrete works. New concreting is started before the applied grout on old surface attains initial set.
2. Expansion Joints:
These joints are provided to allow for expansion of the concrete, due to rise in temperature above the temperature during construction. Expansion joints also permit the contraction of the element. Expansion joints in India are provided at an interval of 18 to 21 m. A typical expansion joint is shown in Fig 1. The open gap of this joint varies between 2 cm and 2.5 cm. Sometimes, to transfer load from one slab to the adjacent slab, dowel bars are also used at suitable intervals at these joints.
3. Contraction Joints:
These joints are provided to permit contraction of the concrete. These joints are spaced closer than expansion joints. These joints do not require any load transfer device as it can be achieved by the interlocking of aggregates. However, some agencies recommend use to dowel bars fully bonded in concrete.
Warping joints are provided to relieve stresses induced due to warping effect. These joints are also known as hinged joints.
Tags: Concrete Joints, Joints In Concrete, Construction Concrete Joints.