Tuesday, January 21, 2014

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Types of Joints in concrete

Concrete shrinks irreversibly as it cures and dries out. After this initial shrinkage has occurred, concrete expands and contracts reversibly with changes in temperature and moisture content. This movement can cause concrete to crack uncontrollably unless it is reinforced with steel. Although, reinforcement increases the strength of concrete and absorbs the stress of expansion and contraction, it can not prevent cracking altogether – it can only distribute the stresses so that there will be many irregular cracks instead of a few big ones. Irregular cracks are unsightly and difficult to maintain. So, concrete is build with special joints that are designed to control cracking location. Actually, joints are pre-planned cracks.

Following are the some kinds of joints we generally use in concrete:

  • Control/Contraction/shrinkage joint
  • Construction joint
  • Isolation/expansion joint

Control joints:
Control joints are constructed to prevent random shrinkage cracking and to allow the concrete crack in straight line at desired locations.

There are three basic techniques used to add control joints to concrete-

a.      High density fiber board or thin plastic strips can be used to make control joints when concrete is poured.
b.     Making a V-shaped groove at regular intervals in concrete.
c.      In existing concrete, installers can cut joints along the surface at regular intervals. 

Construction joints:
Construction joints are installed wherever a concrete pour is in interrupted for some reasons. It may be the end of a day’s work or it may be that some other work needs to be done before resuming the pouring. Construction joints are usually coated with oil to prevent bond with the next pour and located so that they can also act as control joints.

Isolation joints:
Isolation joints are used to separate new concrete from existing or adjacent construction, which might expand and contract differently or experience different soil settlement or their movement. If the fresh concrete were not separated from these elements by an isolation joint, a crack could form where the two meets.


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