Composite Steel-Concrete Construction & Slab Saw Cut / Control Joints

[maejr]

I would like some feedback on controlling shrinkage cracks in an elevated concrete slab when the structural framing system is a composite design using steel beams, shear studs, and composite floor deck. Over the years I have heard many opinions concerning the need for saw cut joints. These opinions range from the use of control joints would defeat the composite action to the joints being required at such a tight spacing that it is cost prohibitive. The other question is to what depth and spacing these joints should be cut if they are used.<br><br>Thank you for your time and assistance.<br>Mark <br><br><br>

 

[Ron]

Shrinkage cracks in composite systems, as with unreinforced concrete, occur at primary restraints.&nbsp;&nbsp;In the case of composite structural systems, the primary restraint is at the beams.&nbsp;&nbsp;Secondly, the restraint from the reinforcing is somewhat continuous, and third, the restraint from the deck corrugations is different in the directions parallel and perpendicular to the longitudinal flute direction.&nbsp;&nbsp;With that in mind, the concrete will shrink above the rebar in the same manner that unreinforced concrete shrinks, so conventional jointing should be adequate.&nbsp;&nbsp;In order of prominence, cut the joints:<br><br>a.&nbsp;&nbsp;at the beams (I am not convinced that this has any effect on the composite action, since the primary composite action is planar)<br><br>b.&nbsp;&nbsp;approximately every 15 to 18 feet perpendicular to the longitudinal flutes<br><br>c.&nbsp;&nbsp;approximately every 12 to 15 feet parallel to the longitudinal flutes<br><br>The joints should be cut to a depth of 20 percent of the cover over the rebar, but not less than 3/4 inch.&nbsp;&nbsp;Sawcuts should be made very early, even more so than slabs on grade, because of the greater restraint concentrations and because of the top and bottom cure/drying conditions.&nbsp;&nbsp;Using Sofcut or wet cutting with diamond saw will allow quicker sawcutting without raveling.

 

[daski]

I don't specify any control joints for slabs-on-deck. I feel the deck flutes and corrugations (composite deck (lok-flor)) prevent any major movement due to shrinkage like you would get on a slab on grade. I haven't gotten any callbacks for cracks.<br><br>I did specify them once on a slab that was going to remain exposed to view, but I don't know if it made any difference.<br><br>I recently saw a 3&quot; form deck (9/16&quot slab that had cracks everywhere. I don't usually specifiy those types of slabs, so I don't know if that is normal or not. In any event it's not a structural problem.<br><br>I don't think saw joints would compromise the composite action, as long as they were located on the column lines, then they would be parallel with the beam and therefore parallel with the direction of compressive stress. <p> <br><a href=mailto:markdaski@aol.com>markdaski@aol.com</a><br><a href= > </a><br>