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How to make construction joints? How to construct joints?

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How to make construction joints? How to construct joints?
asked Jun 30, 2015 in Concrete Technology by mfby2k  

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Method of how to make construction Joints
Joints must be carefully designed and properly constructed</strong> if uncontrolled cracking of concrete flatwork is to be avoided. The following recommended practices should be observed:

  1. The <strong>maximum joint spacing should be 24 to 36 times the thickness of the slab</strong>. For example, in a 4-inch [100 mm] thick slab the joint spacing should be  about 10 feet [3 m]. It is further recommended that joint spacing be limited to a maximum of 15 feet [4.5 m].
  2. All panels should be square or nearly so. The length should not exceed 1.5 times the width</strong>. Avoid L-shaped panels.
  3. For contraction joints, the<strong> joint groove should have a minimum depth of 1/4 the thickness of the slab, but not less than 1 inch [25 mm]. Timing of jointing operations depends on the method used: Preformed plastic or hard board joint strips are inserted into the concrete surface to the required depth before finishing. Tooled joints must be run early in the finishing process and rerun later to ensure groove bond has not occurred. Early-entry dry-cut joints are generally run 1 to 4 hours after completion of finishing, depending on the concrete’s setting characteristics. These joints are typically not as deep as those obtained by the conventional saw-cut process,  but should be a minimum of 1 inch [25 mm] indepth.Conventional saw-cut joints should be run within 4 to 12 hours after the concrete has been finished.
  4. Raveling during saw cutting is affected by the  strength of the concrete and aggregate characteristics. If the joint edges ravel during sawing, it  must be delayed. However, if delayed too long, sawing can become difficult and uncontrolled cracking may occur.
  5. Use pre-molded joint filler such as asphalt-impregnated fiber sheeting, compressible foam strips, or similar materials for isolation joints to separate slabs from building walls or footings. At least 2 inches  [50 mm] of sand over the top of a footing will also prevent bond to the footing.
  6. To isolate columns from slabs, form circular or square openings, which will not be filled until after the floor has hardened. Slab contraction joints should intersect at the  openings for columns. If square openings are used  around columns, the square should be turned at 45  degrees so the contraction joints intersect at the  diagonals of the square.
  7. If the slab contains wire mesh, cut out alternate wires,  or preferably discontinue the mesh, across contraction  joints. Note that wire mesh will not prevent  cracking. Mesh tends to keep the cracks and joints tightly closed.
  8. Construction joints key the two edges of the slab together  either to provide transfer of loads or to help prevent  curling or warping of the two adjacent edges.  galvanized  metal keys are sometimes used for inerior  slabs, however, a beveled 1 by 2 inch [25 by  50  mm] strip, nailed to bulkheads or form boards, can  be used in slabs that are at least 5 inches [125  mm] thick to form a  key which will resist vertical  loads and  movements. Keyed joints are not recommended for  industrial floors</strong>. Metal dowels should  be used in slabs  that will carry heavy loads. Dowels  must be carefully  lined up and parallel or they may induce restraint  and cause random cracking at the end of the dowel.
  9. Joints in industrial floors subject to heavy traffic require special  attention to avoid spalling of joint edges. Such joints should  be filled with a material capable of supporting joint  edges. Manufacturer’s recommendations and  performance records should be checked before use.
answered Aug 27, 2015 by fawad  

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