• Concrete construction requires joint techniques.construction image by Pali A from Fotolia.com
  
  Concrete construction structures can be exposed to many types of external stresses. Temperature induced shrinkage and expansion results in differential movement. Different loading conditions and stresses can cause cracking and scaling of concrete surfaces and under extreme situations, can result in complete failure of the concrete structure. During construction, certain joints are used for expansion control to relieve these stresses and prevent damage or failure of the structure.
  
  

Isolation Joints

• Isolation joints allow the concrete to move without cracking.Concrete mixer image by zalisa from Fotolia.com
  
  Concrete isolation joints are used to separate or isolate structural members that are adjacent to each other. A concrete isolation joint relieves differential pressure movement in the vertical plane when the loading conditions are uneven or settlement occurs. Isolation joints are sometimes called expansion joints as they account for stresses caused by temperature changes. As the temperatures change, concrete structures expand and contract. Since concrete is not a ductile material, these movements could cause failure.
  
  

Control Joints

• Concrete shrinks after it is in place.cement,concrete image by Greg Pickens from Fotolia.com
  
  Concrete control joints are cut into a concrete slab to weaken the plane of stress. When and if the stress forces occur at these designated control joints, the concrete is allowed to move through the joints without cracking. In a slab of concrete, shrinkage can happen quicker at the exposed concrete surfaces. Control joints cut into the surface of concrete prevent the upward curling at the concrete edges. Welded steel wire fabrics can be used in conjunction with control joints to decrease width of a crack and in some cases stop the crack from propagating throughout the concrete structure.
  
  

Contraction Joints

• Cracking can weaken a concrete structure.grey concrete image by amlet from Fotolia.com
  
  Similar to a control joint, contraction joints are placed in concrete structures to address shrinkage. After wet concrete is placed, the water evaporates and the concrete mass tends to shrink. The installation of contraction joints must be done at the proper time usually by saw cutting or a specialized grooving tool. Once the contraction joint is cut, a plastic strip supports the joint until it is cured. Once shrinkage occurs, the contraction joint absorbs the differential movement by creating a plane of weakness in the surface. The concrete mass is allowed to shrink and move without cracking.