What Are Construction Joints?

To prevent the surface cracks in concrete with variations in temperature, appropriate arrangements like construction joints in concrete are provided.

Definision of construction joint is A vertical or horizontal surface in reinforced concrete where concreting was stopped and continued later. A separation provided in a building that allows its component parts to move with respect to each other; a joint where two placements of concrete meet. A joint where two successive placements of concrete meet. A separation provided in a building which allows its component parts to move with respect to each other. The cause of such movement may be thermal, seismic, or wind loading.

Construction joints in concrete are basically the gaps in the concrete structure. Concrete, like the other construction materials, has the characteristic to expand and contract due to temperature and moisture changes, and due to concrete strain. These changes in volume may create cracks in the hardened concrete, unless it is suitably controlled by using concrete sealers.

The stress forces that produce cracks are prevented by arranging provisions for changes in volume at selected places. These provisions are called control relief joints. These joints reduce the random cracks on surfaces by the collection and dissipation of stress forces produced by moisture and temperature changes. Inadequate control of joints may create damaging cracks.

If the control joints are to be truly effective, they should be placed and installed properly. Construction joints are a plane surface between two elements of concrete. It is concrete that is placed against concrete already placed. It is fairly hardened to a level that vibration will not produce consolidation. Construction joints may be vertical as produced in a slab, horizontal as in a column, or both as created in a wall. Construction joints are also normally called cold joints. Construction joints may occur in the normal concrete or the lightweight concrete.what are construction joints

How Cracks Occur
The concrete is mixed according to specified proportions, and mixed to produce a homogeneous mixture. This mixture occupies the maximum volume in this initial state. After completion of the curing, the concrete is dried, and loses the uncombined water by evaporation. The moisture loss produces concrete contraction, and a reduction in length. Unless the changes in volume are restricted by using relief joints, cracks may be produced.

When concrete is restrained from contraction, cracks may develop when dried since any restriction in concrete movement may cause production of tensile stress forces. Concrete that is uncontrolled may not crack. All concrete elements have some restraints due to compulsions, like the bonding necessary with the existing walls and the structural links. The restraints that facilitate concentration of the drying stresses can produce cracks in the restrained elements, unless suitable control joints are used.

Control Of Drying Shrinkage
Drying shrinkage can be minimized by several methods as explained below:
  1. The water cement ratio should be reduced to achieve suitable placement and consolidation. 
  2. Quantity of fine aggregate should be closely maintained at a level that will just create sufficient workability and characteristics that produce sound finish. 
  3. The aggregates used should be proper, clean, and of a suitable grade according to the design of structures. 
  4. Suitable reducing agents should be used for reduction of the drying shrinkage. Water reduction will decrease the water cement ratio to favorable limits. 
  5. Low slump should be used for the placement of concrete. 
  6. Concrete should be properly consolidated. Concrete that is cured immediately after the concrete finishing will have greater strength, and less liable to the creation of shrinkage distress cracks. 

Construction Joint Quality
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 trends of concrete encourages weak surface of low bonding behavior. The concrete surface produced should be clean and structurally reliable. Rough aggregate protruding portions and slight indentation are not useful or recommended. Surface retardants are frequently utilized to attain a suitable surface.

Types of Construction Joint
Construction joints are classified into 4 types depending upon the type of joint. They are as follows,

1. Butt Type Construction Joint
Butt-type construction joints are the simplest type of joints formed by a bulkhead and are satisfactory for thin floors that aren’t heavily loaded.

2. Tongue and Groove Construction Joint
Tongue and groove type of construction joint is usually given in slabs. The keyway may be formed by fastening metal, wood, or pre-molded key material to a wood bulkhead. The second placing of concrete later enters the groove to form the tongue and thus allow for shear forces to be transmitted through the joint, In plain slabs on ground this ensures that future slabs will remain level with previously cast concrete.

3. Butt Type Construction Joint with Dowel
This type of joint is done by just adding a metal or steel dowel in between the conventional butt joint. Dowels are provide to transfer load in floors carrying heavier loads.

4. Butt Type Construction Joint with Tie bar
This is same as that of butt type construction joint with dowel, but the dowel used here is a tie bar. The tie bar is a deformed steel rod, which increases the surface area, which in turn increases the friction between the 2 concrete parts.

Location Selection of Construction Joint
The selection of location of a construction joints is important for the durability and effectiveness load transfer. The following are the points to be remembered while locating the joints :
  1. Construction joints parallel to the slab span can be placed anywhere, except those locations in T-beam construction that rely on a portion of the slab to act with the beam in resisting flexure. For slabs and beams it is, therefore, usual to have construction joints at mid span of the middle third of the span. 
  2. 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. 
  3. Joints in girders shall be offset a minimum distance of two times the width of intersecting beams. 
  4. In walls a horizontal length of placement in excess of 40 ft is not normally recommended. 
  5. It is convenient to locate horizontal joints at the floor line or in line with window sills. 
  6. In the design of hydraulic structures, construction joints usually are spaced at shorter intervals than in non hydraulic structures to reduce shrinkage and temperature stresses. 
  7. Designing concrete members for lateral forces may require special design treatment of construction joints.

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